Bedrock Geology of the Principal Dinosaur Fossil Sites in the Northeastern Bighorn Basin

by Edwin S. Robinson, Professor Emeritus, Geosciences Department, Virginia Tech Research Associate, Virginia Museum of Natural History

6676 Blacksburg Road, Catawba, Virginia 24070

May 21, 2009

Contents

Upper Sundance
Morrison,
Lower Cloverly

Coyote Basin

Cedar Creek Area

Bob Simon Area

Two Sisters Area

Petrography

Appendices

Summary and Discussion

Rocks of the Morrison Formation extend around most of the periphery of the Bighorn Basin in northern Wyoming and southern Montana. This formation consists of non-marine sediments widely distributed in western United States, and is the most prolific source of Jurassic dinosaur fossils in the world. In the northeastern part of the Bighorn Basin large numbers of dinosaur fossils have been excavated from seven sites all of which are within close proximity in Big Horn County, Wyoming. The present report is concerned with observations of bedrock geology in the immediate vicinity of these excavation sites which are useful for determining the stratigraphic position of the dinosaur fossils. These sites are situated in the 7km x 13km area, shown in Figure 1, which will be referred to as the area of this study. During recent decades most of the dinosaur fossils have come from five dig sites. Four of these are located in the SW quarter of the Leavitt Reservoir Quadrangle. The Virginia Museum of Natural History (VMNH) pit is at 44º 39' 23"N - 107º 51' 03"W in the Coyote Basin area. Here excavation began in 1999 under the supervision of Dr. Nicholas Fraser. Then in 2003 excavation was begun at a nearby site on the Tri-Moon Butte at 44º 39' 26"N - 107º 50' 41"W in Coyote Basin by Dr. Des Maxwell from the University of the Pacific. Another quarry, operated by Köbi Siber, is at 44º 39' 56"N - 107º 49' 20"W in an area just east of Coyote Basin and bordered on the north by Cedar Creek. Farther to the south is the active quarry at 44º 38' 02"N - 107º 48' 51"W operated by Bob Simon. In 2004 the VMNH began excavations in the NE quarter of the Shell Quadrangle at 44º 34' 19"N - 107º 47' 50"W. The small area surrounding this site is called the Two Sisters area. The effort here was initiated by Dr. Nicholas Fraser. Then in 2008 Dr. Alton Dooley took over supervisory responsibilities for this site and the VMNH Coyote Basin site. In addition to the five recently active sites just described there are two older inactive sites in the Cedar Creek area. One is the "Big Al" Quarry at 44º 40' 10"N - 107º 49' 02"W, which has been inactive for several years. Efforts directed by Dr. Paul Sereno of the University of Chicago have been made to reactivate this site. A much older inactive site is the Howe Quarry at 44º 40' 03"N - 107º 49' 08"W, where excavation was done in 1934 by an American Museum of Natural History expedition under the direction of Barnum Brown.

All of these seven sites are situated in badlands terrane less than five kilometers beyond the steep western front of the Bighorn Mountains.

The Siber Quarry and the Bob Simon Quarry are located on private land, and have been operated for commercial purposes. The other dig sites, situated on public lands managed by the U. S Bureau of Land Management (BLM), have been operated for research purposes. In 2004 a group from Hope College, directed by Dr. Brian Bodenbender, excavated some dinosaur fossils from public land bordering the Bob Simon area on the south as part of an undergraduate educational project.

The stratigraphic section exposed in the SW quarter of the Leavitt Reservoir Quadrangle and the NE quarter of the Shell Quadrangle consists mostly of Mesozoic sedimentary rocks. The five excavation sites are situated in the late Jurassic Morrison Formation, which is the principal focus of this report. Because of controversy about the upper and lower boundaries of the Morrison Formation, some attention is given to the upper part of the underlying Jurassic Sundance Formation and the lower part of the overlying Cretaceous Cloverly Formation. In the study area no invertebrate fossils useful for precise age determination have been found, and no reliable radiometric dates have been obtained. Therefore, the correlations presented in this report are based entirely on physical stratigraphic evidence. For geologic mapping purposes the Mesozoic stratigraphic sequence in this area has been divided into ten mappable units each of which is characterized by physical features easily recognized in the field.

The four small scale bedrock geologic maps have been compiled using data from 207 observation sites located by GPS and transit surveying, stratigraphic sections measured along 23 profiles, and high resolution aerial photographs. One map, approximately 800m X 1000m, includes most of the area known as Coyote Basin, which surrounds the VMNH pit and the newly established Tri-Moon Butte excavation site. Another map, approximately 615m X 880m, covers an area on the south side of Cedar Creek which includes the Siber Quarry, the Howe Quarry, and the "Big Al" Quarry. The third map, approximately 280m X 390m, includes the area surrounding the Bob Simon Quarry. The fourth map covers the 275m x 325m Two Sisters area. The map locations are shown in Figure 1.

To supplement the following discussion a large number of oriented photographs will be presented in a future report. These photos illustrate most of the details used to recognize the different rock units in the field, the various landscape features, and other features used compile the geologic maps and stratigraphic cross-sections. Physical features of hand specimens are described in Appendix 1. Photographs of rock hand specimens are in Appendix 2, and photomicrographs of thin sections are in Appendix 3.

UPPER SUNDANCE, MORRISON, AND LOWER CLOVERLY STRATIGRAPHY

Geologists have been studying the stratigraphy of the Sundance, Morrison, and Cloverly Formations in northern Wyoming for about one century. Much of this work has been summarized by Moberly (1960), Ostrom (1970), and Kvale (1986), each of whom has written his doctoral dissertation on subjects involving late Mesozoic stratigraphy in this area. A review of their efforts is important to understanding how the geologic map units described later correlate with the Sundance, Morrison, and Cloverly Formations.

Consider first the stratigraphic descriptions of Moberly (1960) who begins by expressing the point of view that stratigraphic units selected with regard to “expediency for geologic mapping” are not necessarily best for “historical interpretation of an area”. First, he focuses on the sequence of sediments between the Jurassic Sundance Formation and the Cretaceous Thermopolis Shale which make up the well-known non-marine Morrison Formation, and the non-marine Cloverly Formation. Moberly redefined these two formations, and he proposed the new partly non-marine partly marine Sykes Mountain Formation. For purposes of the present report only the Morrison and lower Cloverly Formations are of interest. Moberly places the base of the Morrison at the contact between the older pale green calcareous and fossiliferous sandstone of the Sundance Formation and the overlying lenticular greenish mudstone. He continues by stating that ”Where both formations are greenish calcareous sandstones and shales the Sundance beds owe their color to a high glauconite content, whereas the similarly colored Morrison rocks have greenish clay matrix around generally lighter colored grains”.

Moberly goes on to describe general features of the Morrison Formation, but he refrains from subdividing it into a sequence of superposed stratigraphic units. Rather, he points out that in the Morrison “the pronounced lithologic changes occurring every few feet vertically and every few dozen yards horizontally result chiefly from the lenticularity of a few repeated lithologies rather than numerous different rock types”. These repeated lithologies include mostly “calcareous, somewhat sandy mudstone or shale, colored shades of green, gray, olive, and yellow.” In part of the Morrison section are “lenses of sandstones…” and “reddish brown calcareous mudstones” which produce a “red banded effect”. Altogether, the Morrison Formation is between 40m and 85m thick with an average of 58m to 61m. Its contact with the overlying Cloverly Formation is defined at “the base of the lowest beds which either show evidence of significant additions of volcanic debris or contain pebbles or granules of black chert”.

In Moberly’s definition of the Cloverly Formation there are three units, all of which may or may not occur at some particular location. Lowermost is the Pryor Conglomerate which is found in the northeastern margin of the Bighorn Basin, but diminishes to a feather edge near Sykes Mountain, which is 50 km north of the area of this report. Farther south are some discontinuous un-named sandstone/conglomerate lenses. Much more widespread is the Little Sheep mudstone unit of the Cloverly Formation. It is mostly a variegated non-calcareous bentonitic mudstone which occurs in shades of gray, purple, pale red, and olive, and “commonly containing some chert-pebble sandstone lenses”. The mudstone weathers to a soft “gumbo” with a “popcorn” textured unvegetated upper surface. In the chert-pebble sandstone lenses “the chief component…is coarse - to fine-grained quartz sand; the chert detritus is generally only a small to moderate, but conspicuous, proportion of the rock.” The thickness of 87m for the Little Sheep Unit of the Cloverly Formation was measured near the place where Cedar Creek flows into Beaver Creek just north of Coyote Basin. Uppermost in the Cloverly Formation is the Himes Unit consisting of claystone colored by iron oxides, lithic arenite and some quartz arenite. In Moberly’s description of the stratigraphy the Pryor Conglomerate does not extend southward into the area of the present report, and the Himes Unit is too high in the Cloverly to be of interest. It is the Little Sheep Unit which is of importance here.

In Moberly’s analysis there is uncertainty about the age of the Morrison/Cloverly contact. It has been established from paleontological data that Morrison rocks are largely Jurassic and that Cloverly rocks are largely Cretaceous. However, it cannot be confidently asserted that the Morrison/Cloverly contact corresponds to the Jurassic/Cretaceous time boundary. The contact may be older, being of very late Jurassic age.

Ostrom (1970) reviewed essentially the same literature that was available to Moberly, as well as Moberly’s work itself, and he also reports the results of 28 Morrison/Cloverly sections he measured in Wyoming and Montana. From this information he proposed a generalized sequence of eight superposed stratigraphic units. He disagrees with Moberly’s philosophy of defining formations on a genetic basis, and prefers to place boundaries where changes in stratigraphy are easy to recognize. With this in mind he proposes a typical Morrison section consisting of Units I, II, and III, and a typical Cloverly section made up from Units IV, V, VI, and VII. Part of the overlying Sykes Mountain Formation is represented by Unit VIII.

In the Morrison Formation Unit I is mostly greenish, calcareous claystone and siltstone, with some red-brown zones, which “rests conformably, and at some places gradationally, on glauconitic sandstone, gray-green shale or limestone containing marine invertebrates.” Unit I ranges between 21m and 52m thick. It is overlain by Unit II, a massive, cross-bedded, white calcareous sandstone, between 3m and 24m thick, which is medium to coarse grained and weathers to pale shades of green, yellow, and tan. Above this is Unit III, a sequence of gray, calcareous, silty, variegated claystone layers between 9m and 37m thick. These layers weather to shades of gray, green, pink, and yellow, and “Locally, red-brown bands occur”. In Unit III illite is the principal clay mineral. The rock breaks into small irregular chips. As regards the appearance of Unit III in the field “In general it resembles Unit I and in the absence of Unit II cannot be readily distinguished from that unit”.

Unit IV, the basal unit of the Cloverly Formation is a massive layer of mostly non-calcareous conglomerate and conglomeratic sandstone. It corresponds to the Pryor Conglomerate in Moberly’s classification. Although Moberly recognized the Pryor only in the area extending north from Sykes Mountain, Ostrom suggests that it may extend farther south based on characteristics of a similar conglomeratic unit found near the Cedar Creek/Beaver Creek junction 50 km to the south, and near Thermopolis, which is another 80 km farther south. Above this is Unit V, a “predominantly unstratified non-calcareous claystone….highly bentonitic… quartz sandstone lenses..”. This unit corresponds to the Little Sheep Mudstone described by Moberly. The uppermost Cloverly Units VI and VII correspond to Moberly’s Himes Unit. Ostrom states thickness ranges of 3m to more than 15m for Unit IV, and between 6m and 61m for Unit V.

Ostrom notes in his introduction that “After years of debate, the age of the Morrison Formation is now generally accepted as Kimmeridgian to Early Portlandian; the overlying non-marine strata are considered as Aptian or Early Albian”. The implication that the Morrison is entirely Jurassic and the Cloverly is entirely Cretaceous was not discussed.

Kvale (1986) has studied the northeastern Bighorn Basin with a focus on paleoenvironments. He made detailed measurements of Morrison/Cloverly sections near the Cedar Creek/Beaver Creek junction in the Leavitt Reservoir Quadrangle, and near Horse Creek in the northeast part of the Shell Quadrangle, and also on Sheep Mountain situated north of Greybull. Kvale defined eleven distinct lithofacies associations, and then used these to describe the physical characteristics of the measured sections. The results were combined to prepare a composite stratigraphic column. The original column in Kvale’s dissertation was redrawn in a different style to obtain Figure 2 where the lithofacies associations are described briefly and identified by letters A through K. In the lowermost part of this section is the glauconitic fossiliferous sandstone (A) of the Sundance Formation, which is overlain by the non-fossiliferous glauconitic lithic arenite (B). Then the lithology changes to silty claystone (C). Here Kvale includes both A and B in the Sundance Formation, and places the contact with the Morrison Formation at the lithologic change from B to C.

Kvale describes the lower part of the Morrison Formation to consist of a 3m thick layer of silty claystone (C) overlain by a 9m sequence which is mostly laminated fine grained sandstone and siltstone (D). Above this is 15m of mostly fine grained channel sandstones (E) with a few seams of silty claystone, and another 8m of the laminated fine grained sandstone and siltstone (D). The channel sandstones are typically 3m to 6m thick and may contain some bone fragments and dark chert pebbles. They tend to be fine grained and fining upward into silt, interfingering with the laminated fine grained sandstone and siltstone.

The upper part of the Morrison Formation described by Kvale consists of 6m of the fine grained channel sandstones (E) overlain by 17m of mostly “an interbedded reddish brown and grayish olive siltstone and claystone lithofacies association” (F) with a few thin zones of sandstone. This siltstone and claystone lithofacies association is the same as the upper part of the Morrison described by Moberly as having a “red banded effect”. Like the lower Morrison, the upper sequence is mostly calcareous.

Kvale provides much more detailed descriptions of the lithofacies associations than are included in the present report regarding sedimentary structures, distribution of dark chert grains, root markings, and burrows. However, he does not propose any formal subdivision of the Morrison Formation into a sequence of stratigraphically superposed units. The total thickness of the Morrison Formation taken from his composite stratigraphic section (Figure 2) is 57m.

To describe the Cloverly Formation Kvale separates the Little Sheep Mudstone unit, originally proposed by Moberly, into three lithofacies associations: a sandstone and conglomeratic sandstone (G); a dusky red to light olive gray argillaceous siltstone and silty claystone (H); and a dark to brightly colored non-calcareous bentonitic claystone (I). Most of the Little Sheep Mudstone consists of the non-calcareous bentonitic claystone (I). Along the eastern margin of the Bighorn Basin the sandstone and conglomeratic sandstone (G) is least important in terms of volume, but locally includes “lenticular deposits of thick, stacked, multistorey sequences of black chert bearing conglomeratic quartz arenites and/or fine to medium grained quartz arenites.” Regarding the Morrison/Cloverly contact, Kvale accepts Moberly’s definition that it is at the base of the Pryor Conglomerate in the area north of Sykes Mountain. But he also shares Moberly’s conclusion that the Pryor Conglomerate does not extend south of Sykes Mountain, and he believes the conglomerates found farther to the south are occurrences of lithofacies association G within the Little Sheep Mudstone. He accepts Moberly’s criterion that presence of volcanic debris (including bentonite) is one basis for distinguishing between Cloverly and Morrison strata, but he points out that presence of dark chert fragments is not useful because they exist in both formations.

In Kvale’s composite stratigraphic section (Figure 2) the Cloverly/Morrison boundary is placed where the calcareous red banded sediments (F) come in contact with the overlying non-calcareous bentonitic gray mudstone (I). Above the contact is 34m of this mudstone with only a few thin seams of sandstone or siltstone. This is capped by 15m of sandstone and conglomeratic sandstone (G) and argillaceous siltstone and silty claystone (H). Overlying this sequence is 29m of bentonitic claystone (I) of the Little Sheep Mudstone and a 23m section of the Himes Member. Altogether, the composite stratigraphic section shows the Cloverly Formation to be 92m thick.

GEOLOGIC MAP UNITS

The principal reason for mapping the bedrock geology and measuring the stratigraphic sections described in this report is to establish the stratigraphic position of the seven dinosaur fossil excavation sites within the 7km x 13km area of the NE Bighorn Basin shown in Figure 1. To achieve this goal it is necessary, insofar as possible, to separate the sedimentary rocks exposed here into a sequence of superposed stratigraphic units. The ten units listed in Table 1 serve that purpose. They were selected on the basis of physical characteristics easily recognized in the field. They complement the earlier descriptions of Moberly (1960), Ostrom (1970), and Kvale (1986), and they provide the additional detail needed to determine more specifically the stratigraphic position of the dinosaur fossils in this particular area. In addition to these units are unconsolidated Cenozoic deposits.

In the area of the Coyote Basin geologic map most of the Morrison stratigraphic section and the lower part of the Cloverly section are exposed, as seen in Figure 3. The Cedar Creek geologic map area includes the upper part of the Sundance Formation, the entire Morrison section and an entire Cloverly section. The stratigraphic section in the Bob Simon area includes the upper part of the Sundance Formation, the Morrison Formation, and the lower part of the Cloverly Formation. In the Two Sisters area rocks from the upper Sundance Formation, and all but the uppermost part of the Morrison Formation are exposed.

Lowest of the stratigraphic units used for geologic mapping is Unit Jsd, a prominent and widely exposed sandstone and arenaceous coquina, pale greenish yellow in color, glauconitic, and calcareous. This unit represents the upper part of the Sundance Formation. Its upper boundary is at the termination of a significant fraction of glauconite. Unit Jsd is exposed in the northern margin of the Bob Simon area, in the southeastern corner of the Cedar Creek area, and near the center of the Two Sisters area. There are no exposures within the Coyote Basin map area, but Unit Jsd does crop out just beyond its southern and western boundaries.

Next in the section is Unit Jml consisting mostly of mudstone with a smaller proportion of very fine grained sandstone. These are non-glauconitic, calcareous rocks with color variations of green, gray, tan, pale red, and pale yellow, which make up the lower part of the Morrison Formation. The generally friable sandstones display parallel laminations, cross laminations, and thin bedding. The Jsd/Jml contact, which corresponds to the Sundance/Morrison boundary, is at the change from glauconitic sandstone to overlying non-glauconitic rocks. At some places in the southwestern part of the Leavitt Reservoir Quadrangle it is difficult in the field to identify exactly where the source of green coloring in the rock changes from glauconite to other minerals. In this study where the presence or absence of glauconite was questionable, thin sections were examined to verify the mineralogy. Exposures of Unit Jml in the Coyote Basin, Bob Simon and Two Sisters areas reveal a few reddish brown zones alternating with gray zones producing what Moberly referred to as a “red banded effect”. In the Cedar Creek area Unit Jml was in a place of subdued relief mostly covered by regolith colored in shades of purple and lavender, as well as tints of red.

Overlying Unit Jml is the middle part of the Morrison Formation, which is designated Unit Jms. Its distinguishing characteristics vary from place to place. In the Cedar Creek area near the Siber, Howe, and “Big Al” quarries it consists of a thick section of sandstone with some interbedded mudstone. Here the sandstone makes up approximately 70%, and mudstone seams make up about 30% (Wilborn, 2002) of Unit Jms. Similarly in the Bob Simon area thick beds of sandstone are prominent in this intermediate zone of the Morrison Formation. However, in Coyote Basin Unit Jms is a much thinner, but markedly distinctive, and easily recognized sheet of calcareous sandstone with an overlying layer of dark gray siltstone in some places. The sandstone part of this unit is light gray to tan in color, and weathers to shades of red and brown. It is a pervasive marker bed which caps several small buttes and is the most durable unit of some small thrust sheets, even though it is only about 1m thick. In various places the sandstone displays ripple marks, cross beds, and parallel bedding. Finally, in the Two Sisters area the Jms lithology is completely different. Here it consists of a distinctive yellow silty claystone rather than sandstone.

The upper part of the Morrison Formation includes a distinctive sequence of very fine grained calcareous sandstone interbedded with mudstone. Fresh rock surfaces are light gray, pale green, and tan in color, but the exposed surfaces weather to darker shades of red and reddish brown which produce the “red banded effect” described by Moberly (1960) and giving the appearance of a sequence of “red beds”. The best exposures of red banding are seen in the Coyote Basin where the sandstones form a series of hogbacks near the western margin of the area, and in the Two Sisters area. In the Cedar Creek and Bob Simon areas Unit Jmrb was mostly covered by regolith consisting of fine grained loose sediment and some larger rock fragments. Bedrock exposures are sparse, but the ground displays a reddish tint with a more muted indication of red banding.

The uppermost part of the Morrison Formation is the elusive Unit Jmy consisting of discontinuous deposits of calcareous sandstone. Some are medium to coarse textured, and coarsely cross-bedded with occasional pebble zones. Elsewhere very friable rock is found in small mounds of very fine grained yellow sand. The best exposures were found in the Coyote Basin area. One very good exposure of Unit Jmy occurs 45m to 75m ENE of the VMNH pit. Here it is a medium to coarse textured, pale green to pale yellow sandstone, 7m thick, and extending approximately 60m in the strike direction and beyond is either missing or concealed. Then, approximately 300m NNE are more exposures of Unit Jmy consisting of light gray, calcareous sandstone, medium grained with a few coarse grains and a few dark minerals, and overlain by mounds of loose, yellow sand. On a large hill in the southeast corner of the Coyote Basin map area at location 44º 39' 08"N - 107º 50' 41"W Unit Jmy occurs as a friable, light gray to buff, calcareous sandstone, which weathers to loose very fine grained yellow sand. Elsewhere in the Coyote Basin area are occasional mounds of yellow sand and a few large chunks of very friable coarse textured rock suggestive of Unit Jmy, situated stratigraphically above Unit Jmrb. No rock exposures were located in the Cedar Creek area, but low ridges partly covered with loose yellow sand suggest that Jmy deposits may be present above Unit Jmrb in that area. Unit Jmy is clearly exposed in the Bob Simon area, but no outcrops are present in the Two Sisters area.

In the area of this study the seven dinosaur fossil excavation sites are all situated in Unit Jms, giving this middle zone of the Morrison special importance. This is the zone designated as Unit II by Ostrom, who describes it as typically “a prominent, massive, white sandstone ranging from 3m to 24m in thickness, but normally about 14m thick…..medium to coarse grained, locally with pebbles….weathers to pale greenish yellow, light tan or brilliant white….massive 10 to 20 foot, cross-laminated beds…”. This description may be applicable over the larger part of the Bighorn Basin, but it differs from the description presented above for Unit Jms. Somewhat similar is the sandstone exposed in the Cedar Creek area where Unit Jms is 22m thick, and in the Bob Simon area where Unit Jms is 21m thick. However, in these areas Unit Jms contains secondary, but significant fractions of interbedded mudstone, a feature not mentioned in Ostrom’s description of Unit II. In Coyote Basin the Jms section consists of a single prominent and pervasive, very fine-grained sandstone layer about 1m thick with some overlying gray siltstone. And in the Two Sisters area Unit Jms consists of silty claystone and gray mudstone. Therefore, it is important to emphasize that Unit Jms, regardless of thickness, lithology, and texture, is a stratigraphic zone which clearly separates the underlying Unit Jml and the overlying Unit Jmrb.

Ostrom (1970) states that his Units I and III would be difficult to distinguish from one another in the absence of Unit II. In the present study these two units correspond to Units Jml and Jmrb respectively. But in the Coyote Basin, Cedar Creek, Bob Simon, and Two Sisters areas there is a distinct intermediate zone, Unit Jms, which clearly separates Units Jml and Jms regardless of its lithologic differences from place to place. However, in the absence of Units Jml and Jmrb it might be difficult to correlate the exposures of Unit Jms between these four areas because of these lithologic and thickness differences. Ostrom does not distinguish any features corresponding to the discontinuous Unit Jmy either in, or above his Unit III. It should be noted that within Unit Jmy a few dinosaur fossils have been found in the Coyote Basin and the Bob Simon areas.

The large hill in the southeast corner of the Coyote Basin area is capped by a prominent exposure, designated as Unit Kp, which consists of coarse grained, cross bedded sandstone with lenses of pebble conglomerate. This unit is thickly bedded and non-calcareous, and consists mostly of quartz grains and pebbles, but includes some dark chert. Here it rests unconformably on Unit Jmy. Elsewhere, about 1 km north of the Two Sisters area a pebble conglomerate interpreted to be Unit Kp directly overlies Unit Jmrb. Otherwise, Unit Kp was not found in direct contact with the other units in any of the geologic map areas. Unit Kp is in the same stratigraphic position, and has physical characteristics similar to the Pryor Conglomerate as described by Moberly (1960), Ostrom (1970), and Kvale (1986). Therefore, the unconformable contact between Units Jmy and Kp in Coyote Basin, and the Jmrb/Kp contact north of the Two Sisters area are interpreted to mark the Morrison/Cloverly boundary.

A dark gray non-calcareous mudstone, designated as Unit Kgm, directly overlies Unit Jmrb except for the few places where the discontinuous Unit Jmy occurs. Hand specimens of Unit Kgm break into chunks with brownish polished surfaces showing striations. Shortly after removing a fresh sample of dark gray rock from the ground it changes to lighter gray as the moisture evaporates, suggesting the presence of bentonite. The weathered surface of this unit is colored mostly in shades of gray but with some zones tinted by pale reds, all being lighter than the dark gray fresh rock. The weathered zone is a soft cover several centimeters to more than ½ meter thick which typically displays an outer “popcorn” textured surface. Relatively steep slopes of Unit Kgm have almost no vegetation. Typical of Unit Kgm exposed on these steep slopes are one or more zones, nearly black in color, and consisting of a large proportion of black chert fragments. Above this unit in what appears to be conformable contact is Unit Kws, a bright white, very fine grained quartz sandstone with no obvious sedimentary structures. Within Unit Kws about 1½ m above the base is a zone of coarse textured, cross-bedded sandstone less than ½ m thick, above which there is more white sandstone.

Overlying the white sandstone is Unit Kc which consists of pebble conglomerate, medium to coarse textured sandstone with some black chert, and other dark grains. The exposures of Unit Kc are weathered to shades of tan and brown. This unit is mostly non-calcareous, and displays some cross bedding. It is the uppermost part of the stratigraphic section exposed in Coyote Basin. Its occurrence in the Cedar Creek and Bob Simon areas is discontinuous. In those areas it is overlain by a multicolored, non-calcareous, bentonitic mudstone, which is designated as Unit Kvm.

Because of the sparse occurrences of Units Kp and Jmy, Units Kgm and Jmrb appear to be in direct contact over much of the area of Coyote Basin, and this contact is marked by a dramatic color change well illustrated in Figure 3. In the Cedar Creek area Unit Jmrb is mostly covered by regolith of fine grained loose sediment and some vegetation. Bedrock exposures are sparse in this hummocky terrane, however the ground displays a reddish tint which together with the vegetation makes a distinct contrast with the bordering gray unvegetated slopes of Unit Kgm. The contrast is not as clear in the Bob Simon area where rubble from quarrying operations as well as loose sediment make it difficult to recognize a clear contact between Units Kgm and Jmrb. Indistinct gradational changes between calcareous sediment and non-calcareous sediment indicate the approximate location of the contact in some places.

In the area of this report Units Kgm, Kws, Kc, and Kvm appear to be sufficiently widespread to divide the Little Sheep Mudstone (Ostrom’s Unit V) into four superposed parts. The combination of Unit Kws and Unit Kc, where it exists, appears to be one of the sandstone and pebble lenses within the Little Sheep Mudstone mentioned by Moberly, Ostrom, and Kvale. Unit Kws appears to be pervasive in the Coyote Basin and Cedar Creek areas, and a somewhat similar unit is found farther away in the Bob Simon area, which may, or may not be a separate lens of coarse sandstone and pebble conglomerate. Because of its distinctive variegated color, Unit Kvm is not easily confused with the underlying gray claystone of Unit Kgm. The stratigraphy above the Little Sheep member is not discussed in this report.

Parts of all four geologic map areas are covered with a thin veneer of rounded boulders, cobbles, pebbles, and other unconsolidated sediment of Quaternary age. These deposits consist of a large variety of igneous, sedimentary, and metamorphic lithologies, and are collectively designated as Unit Qd.

The thickness ranges given in Table 1 for the Mesozoic map units are based upon 9 measured sections in the Coyote Basin area, trigonometric analysis of points located by GPS on the contacts of the different map units in the Cedar Creek area as well as two measured sections in Wilborn (2001), two measured sections in the Bob Simon area, and twelve measured sections in the Two Sisters area.

COYOTE BASIN GEOLOGY

Geologic field observations were made in Coyote Basin during June and July of the years 2000 through 2004. The approximate boundaries of the study area are latitudes 44° 39’ 05” N and 44° 39’ 35” N, and longitudes 107° 50’ 35” W and 107° 51’ 05” W. Field observations were made continuously along eight profiles and indirectly along a ninth profile, and at 96 points distributed throughout this area. Positions were determined by GPS and transit surveying. Observations include descriptions of lithology, 40 strike and dip measurements, and 49 oriented photographs, and collection of representative rock specimens. These data were used with an aerial photograph to compile a bedrock geologic map at a scale of 1:2000 covering the entire area, and geologic cross sections along the nine profiles. Rocks in the area include lower, middle, and upper units of the late Jurassic Morrison Formation, and basal units of the early Cretaceous Cloverly Formation.

Photo Base Map

A photo base map was prepared from an aerial photograph. The entire area of the map is included in the northeast part of the aerial photograph 8-12-98 BLM 6 WY98AC 1-1-9, which is available from the U. S. Bureau of Land Management in Denver, Colorado. The scale of this 23cm X 23cm air photo is close to 1cm = 67.8m. This scale factor was determined by comparisons of distances between objects identifiable on the photo and on the Leavitt Reservoir Topographic Quadrangle Map, for which the scale was known. Also used were distances between objects measured on the ground with a steel surveying tape.

Digital scanning of the aerial photograph was done at the BLM National Science and Technology Center with a density of 357.14 pixels/cm. Using these data Neffra Matthews at BLM prepared a photo base map for this project by digital correction of photo distortion and enlargement of the Coyote Basin data to a scale of 1:1200 (1cm = 12m). She also provided the digital data file on a CD, and it was subsequently processed using Adobe Photoshop 7 to add latitude and longitude lines and additional notation. The final photo base map is shown in Plate 1. This photo base map was used in day to day field work, and in the compilation of the bedrock geologic map (Plate 2) and a topographic map (Plate 3) of the northwestern part of the area.

The BLM digital data also included a Universal Transverse Mercator (UTM) reference grid. Tick marks on the margins of the scanned area of the air photo indicate the locations of UTM grid lines. UTM coordinates, as well as latitude and longitude coordinates, were obtained in the field by GPS for all of the geologic observation sites. A discrepancy was found where the base map coordinates are compared with a grid of coordinates interpolated from the GPS positions. The BLM grid appears to be offset approximately 24m north relative to the grid based upon GPS field measurements. That is, where the GPS coordinates of an object are plotted using the grid on the BLM photo base map, the location point is placed about 24m north of where the object is seen on the map. In the present report a latitude/longitude grid interpolated from GPS measurements is used.

With the aerial photograph scale of 1cm = 67.8m and scanning at 357.14 pixels/cm one pixel represents an area on the ground approximately 0.2m in diameter. Therefore, objects less than a meter in diameter, such as rocks, sage bushes, and other small geologic features, can be recognized on the enlarged photo base maps. This makes it possible to locate oneself very accurately while making observations in the field.

Topographic Map

With a high resolution photo base map at a scale of 1:1200 it is possible to undertake a transit survey without the necessity of using a stadia rod or target prism for making transit sightings. An optical transit with conventional cross hairs can be used to sight directly on objects of interest which can also be identified on the photo base map. The horizontal component (H) of the distance between that object and the transit position can then be measured directly from the map. Using H and the horizontal angle from a reference base line, the position of the object can be plotted. The difference in elevation (Δh) can be found from the product of H and the tangent of the vertical angle (ø):

Δh = H tan ø.

This eliminates the need for a second person to carry the stadia rod or target to the points of interest, so that a survey can be done by only one person operating the optical transit.

Such a transit survey was done in the northwest part of Coyote Basin where the VMNH pit is located. The positions and relative elevations were determined for 64 points in this area using a transit manufactured by C. L. Berger and Sons. Four transit locations were occupied. The distances between them were measured over the ground with a Lufkin steel surveying tape. Lines between these transit positions were the reference base lines used for measuring horizontal angles. A few objects were observed from more than one transit position. Discrepancies suggest that relative elevations are accurate within better than 1m. The relative elevations were adjusted to an elevation of 1536m above sea level determined for a survey marker near the south edge of the VMNH pit from ties to BLM benchmarks made by Brian Roach in June, 2000.

The topographic contour map shown in Plate 3 was compiled from these data. Elevation above sea level can be found by adding the value of 1524m to numbers on the contour lines. For the purpose of drawing contour lines the elevation points were plotted on tracing paper at the scale of 1:1200, and this plot was placed over the 1:1200 photo base map on a light table. The contour lines were then drawn with consideration of landscape features seen on the photo image as well as the elevation measurements. This accounts for the many flexures along the contour lines that could not be justified from the relatively few elevation points alone. Later the topographic map was digitally reduced to a scale of 1:2000 (Plate 3) to be compatible with other maps of this area.

Geologic Map

The geologic map shown in Plate 2 of the entire area was drawn on a sheet of tracing paper placed over the 1cm = 20m (1:2000) photo base map (Plate 1) on a conventional light table. Contacts between the different stratigraphic map units were identified from color changes and escarpment shadows. The contact between Kc and Kws is indicated by an obvious color change. The lower part of Kws forms a nearly vertical escarpment, and its contact with Kgm was placed close to the base of this escarpment. Few clear contacts between Kgm and Jmy were found. Over most of the area the contact was estimated from a relatively abrupt change in the slope of the land surface, locations of scattered weathered exposures of Jmy, vegetation patterns, and from subtle color variations seen in a few places. Likewise, the lower contact of Jmy with Jmrb could only be estimated from faint color gradations. Elsewhere, clear color contrasts between Jmrb and Kgm above, and Jms below are evident on the photo base map. The hard sandstone in Unit Jms produces steep escarpments, so the contact between this unit and the underlying Jml was placed near the base of these escarpments. In some places additional perspective gained from examination of oriented photographs was useful for recognizing contacts.

Along the western margin of the map area several superposed sections of Jms were observed, which are also obvious on the photo base map (Plate 1). They indicate the succession of thrust faults, which were drawn on the geologic map between the repeated exposures of Unit Jms. The VMNH pit is situated in the uppermost section of Jms in this sequence.

In the southeast corner of the area the conglomerate and coarse sandstone of Unit Kp overlies Unit Jmy. This is the only place in the geologic map area where Unit Kp occurs. Here it forms the land surface, and no exposures of Unit Kgm were found overlying it. Elsewhere in the area Unit Kgm directly overlies Unit Jmy or Unit Jmrb. Dinosaur fossils were discovered in Unit Jmy just below the contact with Unit Kp at a site visited in the summers of 1999 and 2000 by a group directed by Marilyn Fox from the Peabody Museum of Yale University. A fossil bone discovered near this site by the author in 2000 was eroded along its upper surface, which is in direct contact with the base of Unit Kp, suggesting an unconformable contact.

The oldest rock in the area is a very fine grained sandstone exposed in the main dry stream channel in the southern part of Coyote Basin. This rock is similar to the other sandstone layers found in the Morrison Formation. It is overlain by mudstone, and appears to be an intermediate facies in Unit Jml. The base of the Morrison Formation is not exposed in the map area.

The 1:24000 scale Geologic Map of the Leavitt Reservoir Quadrangle (Noggle-Perrin,1989, Geological Survey of Wyoming, Map Series 29) shows the entire area of the present map as part of an allochthonous block overthrust to the west. Steeply dipping strata are found near the terminus of this thrust zone, which lies just beyond the west edge of the present map. The western margin of the thrust zone has been folded into an anticline. Evidence of this is found in the change from east to west dipping strata close to the coordinates of 44° 39’ 25”N and 107° 51’ 05”W. In the eastern part of the map area strata are nearly horizontal, dipping less than 10 degrees mostly toward the east.

Geologic Sections

To obtain more detailed information about the stratigraphy in Coyote Basin, and especially the northwest area close to the VMNH pit, geologic sections were measured along nine profiles. From their locations shown on Plate 2 it can be seen that Profiles 1, 2, and 3 focus upon Units Jmrb and Jmy, Profile 4 crosses Kgm, Kws, and Kc, and Profile 5 provides additional information about Kws and Kc. Profile 6 crosses the upper part of Unit Jml and the hard sandstone of Unit Jms. Profile 7 extends from the top of Unit Jms to the base of Unit Kc in the northwestern part of the area. In the southeastern corner of the area Profile 8 reaches from the top of the Jml sandstone facies in the dry stream channel in the lowest part of the map area to the top of the bordering hill where Unit Kp is exposed. Finally there is Profile 9 situated close to the southern boundary of the map area, and extends from the top of the Jml sandstone facies in the dry stream channel up through the Jms sandstone exposed on the bordering hillside.

Much of the land surface along these profiles is covered by regolith consisting of relatively soft silt and rock fragments which conceal the solid bedrock. From place to place solid bedrock was exposed near ridge crests. The directions of the profiles were chosen along lines with greater rock exposure rather than lines in the dip direction. Profiles 4, 5, and 6 follow accessible paths over good exposures rather than dip lines which were too steep to traverse safely. Profiles 8 and 9 cross nearly horizontal strata in directions chosen with regard to the amount of exposed bedrock. The directions of the nine profiles are: Profile 1-N53W; Profile 2-N54W; Profile 3-N77W; Profile 4-N75W; Profile 5-N72W; Profile 6-N60W; Profile 7-N76W; Profile 8-N84E (lower part) and N40E (upper part);Profile 9-N83E.

All of the sections, except along Profile 7, were measured using a Lufkin steel surveying tape 91.4m long. The procedure was to drape the tape over the land surface, then record the distances of features including bedrock exposures, ridge crests, ravines between ridges, and all significant changes in the slope of the land surface. Also, the inclination angle (µ) of the tape on the land surface along each interval between the recorded distances was measured using a Brunton pocket transit inclinometer, and the character of the surface material was noted.

Each interval (D) between the adjacent recorded distances along a profile was adjusted to account for the inclination (µ) of the surveying tape and the angle (φ) between the profile direction and the dip direction. The horizontal component (H) of the inclined interval length (D) is:

H = D cos µ,

and the component of H in the dip direction is:

X = H cos φ.

Combining these expressions gives:

X = D cos µ cos φ.

Also, the change in elevation (Δh) along the interval is:

Δh = D sin µ.

A topographic profile adjusted to the dip direction can be obtained by plotting sequentially the values of X and Δh. This was done for each of the geologic sections, and then the stratigraphy observed along the profile was drawn at the average angle of dip determined from measurements near the profile. The representative values of strike and dip are as follows: strike N14E dip 31E for Profiles 1 and 2; strike N23E dip 22E for Profile 3; strike N14E dip 24E for Profiles 4 and 5; strike N40E dip 9E for Profile 6; strike N14E dip 14E for Profile 7; strike N60E dip 6SE for Profile 8; and strike N40E dip 2SE for Profile 9. The results are presented in Plate 4, and represent the stratigraphy as if the observations had been made along lines crossing the sections in the dip direction.

After measuring Sections 1, 2, 3, and 4 transit sightings were made on one or more points along each profile for elevation control. The topography along the profiles was then adjusted to the control points. These data were included with the transit data in compiling the topographic map in Plate 3. Also, transit sightings on the upper and lower ends of Profile 8 confirmed the elevation difference of 28m calculated from the surveying tape inclination and distance increments.

Stratigraphic columns displaying the thicknesses and descriptions of the rock strata observed along the nine profiles are presented in Plate 5. The accuracy of the thicknesses depends upon the accuracy of the dip angle and the inclination angle of the surveying tape. For the range of angles measured in this survey an error of 1 degree would introduce an error of approximately 3% in the value of thickness. In view of the difficulty in finding suitable surfaces for measuring dip in this area a subjective judgment is that the uncertainty in calculations of thickness is in the range of 15%.

Data along Profile 7 were obtained in a different way. This profile extends 112m in a S76E direction, which is the dip direction, from the lower contact of Unit Jmrb on the west end to the Kws/Kc contact on the east end. No field measurements were made on the ground along this profile. However, transit sightings were made on the points where this profile intersects the lower contacts of Units Jmrb, Kgm, Kws, and Kc. Using the horizontal distances and the differences in elevation between these points, and a dip angle of 24 degrees, thickness values for Units Jmrb + Jmy, for Unit Kgm, and for Unit Kws were calculated from the measurements along Profile 7. These values are presented in the following discussion together with the results from Sections 1 – 6, 8, 9.

A principal objective in measuring sections is to find the thicknesses of different stratigraphic units. For the lowest part of the Morrison Formation, Unit Jml, this was not possible because the contact with the underlying Sundance Formation is not exposed in the area. The upper mudstone part of Unit Jml, which overlies the sandstone facies exposed in the dry stream channel, is 11m thick along Section 6, 6.5m thick along Section 8, and 6.1m thick along Section 9. Outside of the map area there are exposures of Unit Jsd approximately 470m S and 370m WNW of the west end of Profile 8, but differences in dip at these locations preclude estimation by extrapolation of the depth to the Jml/Jsd contact near this profile.

The hard sandstone part of Unit Jms is 1.2 m thick in Section 6, and close to this value elsewhere in the area. Near the VMNH pit between 2m and 3m of gray siltstone overlies the 1.2m thickness of sandstone. The total thickness for Unit Jms, then, is between 3m and 4m.

Along Profiles 1 and 2 the lower boundary of Jmrb is clearly exposed. However on Profile 1 the Jmrb/Jmy contact is covered, and its position was estimated within ± 8m from a gradational color change in the unconsolidated sediment on the land surface. The position of the upper contact of Jmy with Kgm, which is based on a change in the slope of the land surface, is judged to be accurate within ± 5m. Approximate thicknesses are 33m for Jmrb and 5m for Jmy on Section 1. There is no evidence of Unit Jmy in Section 2, and Unit Jmrb, which is 33m thick, appears to be in direct contact with Unit Kgm.

Along Profile 3 the contacts between Units Jms, Jmrb, Jmy, and Kgm are quite distinct, so that the thicknesses of 24m for Unit Jmrb and 7m for Jmy shown on Plate 5 would be accurate within a meter if the dip angle of 22 degrees is correct. Uncertainties related to dip are discussed above.

The lower contacts of Units Kgm and Kws along Profiles 4 and 5 are quite distinct, but the upper contact with Kc is harder to recognize. The thickness of 19m for Unit Kgm shown in Plate 5 should be accurate within a meter if the dip angle of 24 degrees is correct. For Unit Kws thicknesses of 8m were determined for Section 4 and 6m for Section 5. More credence is placed on the 6m value because the Kws/Kc contact is more distinct. Values of 23m for Unit Kgm and 6m for Unit Kws were calculated from the transit measurements along Profile 7. The base of Unit Kgm was more clearly exposed on Profile 6 than on Profile 7, so the thickness of 19m for Unit Kgm is judged to be the more reliable measurement.

In the northwestern part of the area near the VMNH pit combined thickness values for the upper part of the Morrison Formation including Units Jmrb and Jmy are 38m in Section 1, 33m in Section 2, and 31m in Section 3. An additional value of 34m was obtained for Section 7. Note that the smallest thickness value for Unit Jmrb, 24m, and the largest thickness value for Unit Jmy, 7m, were both measured along Profile 3 where exposures are especially clear. The combined thickness of 31m is reasonably close to the combined thicknesses of Jmy + Jmrb measured for Sections 1, 2, and 7. So it appears that in Section 3 the thick deposit of Jmy sandstone fills a locally deep channel scoured into the upper surface of the older Jmrb deposits.

Using the data in Sections 1-7, the thickness of the Morrison Formation exposed in the mapped area of Coyote Basin, as defined by Moberly (1960), Ostrom (1970), and Kvale (1986) is between 45m and 53m. This includes 11m of the upper part of Unit Jml, between 3m and 4m of Unit Jms, and between 31m and 38m of Units Jmrb+Jmy.

In the southeastern corner of the geologic map area (Plates 1 and 2) is a prominent hill where the terrane and the stratigraphic section appear to be significantly different. This is the only place where Unit Kp occurs. Furthermore, beneath Unit Kp in Section 8 the thickness of 13m for the combination of Units Jmy and Jmrb is only 40% of the average thickness of this combination in the northwestern part of the area. This anomaly invites further study.

The ranges of thickness for the geologic map units in Coyote Basin, based upon the data described above, are presented in Table 1 where they can be compared with the results from the Cedar Creek, Bob Simon, and Two Sisters areas which are described below.

CEDAR CREEK AREA GEOLOGY

A reconnaissance of the bedrock geology was carried out in an area situated on the south side of Cedar Creek about 2km ENE of Coyote Basin. The approximate boundaries of the area are 44º 39' 52"N to 44º 40' 12"N and 107º 49' 00"W to 107º 49' 40"W. The Howe Quarry, the “Big Al” Quarry, and the Siber Quarry are located in this area. Field observations were made during June and July of Years 2000, 2001, and 2003 at 31 positions, determined by GPS, which include descriptions of lithology, locations of contacts, 8 strike and dip measurements, and 13 oriented photographs. Information from these positions was used together with vertical air photos to compile a bedrock geologic map of the area, and to calculate thickness estimates for Units Jml, Jms, and the combination of Jmrb and Jmy. The map compilation involved preparation of a photo base map, and tracing contacts which could be identified on the photo base map and verified from field observations.

The Photo Base Map

A photo base map was prepared from vertical air photographs BLM 6 WY98AC 1-3-13, and 1-4-11 which are available from the U. S. Bureau of Land Management in Denver, Colorado. The scale of the air photos was estimated to be 1cm = 68.4m (1:6840) from calculated distances between GPS positions measured at locations recognizable on the photos. These air photographs were scanned at a density of 138 pixels/cm using a Scanmaker 4800 flatbed scanner, and then were combined using Adobe Photoshop 7 to prepare the photo base map of the Cedar Creek area, which is shown in Plate 6. No adjustments were made to eliminate photographic distortion, which is indicated by irregularities in the latitude and longitude lines.

Geologic Map

The geologic map shown in Plate 7 was drawn on a sheet of tracing paper placed over the photo base map on a conventional light table. Contacts between the different stratigraphic map units were identified from escarpments and changes in surface slope, color and vegetation, together with data from the 31 ground observation sites which are indicated on the map. The base of the Morrison Formation is indicated in the southeast corner of the photo base map (Plate 6) by the distinctive contrast between the shades of purple indicative of Unit Jml and the greenish tan color of the Sundance Formation, Unit Jsd. The purple coloring of Unit Jml also contrasts with the shades of yellow, pale orange, and gray characterizing the overlying Unit Jms. In many places the position of the Jml/Jms contact is blurred by debris eroded from Jms, which is exposed at higher elevation. However, observation site 126 lies directly on the Jml/Jms contact, and it passes close to site 120. The color change between Unit Jms and the reddish brown shades of Unit Jmrb is less obvious on the photo base map, but can be located on the ground at several places. Observation sites 116 and 130 lie on this Jms/Jmrb contact. The “red banded effect” which characterizes Unit Jmrb is clearly evident in the Cedar Creek area even though very little solid bedrock is exposed. The Jmrb terrane consists of low hummocks covered by weathered rock fragments and loose sediment which preserve color bands of different shades of red and brown. Bordering the reddish brown Jmrb terrane are a few low mounds with comparatively sparse vegetation, tentatively identified as Unit Jmy. These mounds consist of light gray mudstone, some traces of yellow sand. Otherwise, the uppermost part of Unit Jmrb is a ½ m thick layer of calcareous sandstone which weathers to a light brown color. This calcareous sandstone layer is interpreted to mark the upper boundary of the Morrison Formation in this area.

Overlying the calcareous sandstone is the gray bentonitic mudstone of Unit Kgm. At this Jmrb/Kgm contact there is a relatively abrupt increase in the steepness of the hillslope. The steep slope is almost devoid of vegetation, and is characterized by bands of different shades of gray with discontinuous tints of pale red or purple. In the upper part of this unit there are two distinct bands of black chert. The definitive red/gray color change seen in Coyote Basin at the Jmrb/Kgm contact is not obvious in the Cedar Creek area.

The gray mudstone of Unit Kgm contrasts with the hues of red, brown, purple, and yellow which characterize the overlying mudstone of Unit Kvm. Separating these two units is the white sandstone of Unit Kws which forms steep escarpments, and also is easily recognized by color contrasts. On the geologic map the upper boundary of Unit Kgm is shown. The overlying Units Kws and Kvm are not shown separately, rather, they are labeled collectively as Kws/Kvm. The coarse sandstone and conglomerate of Unit Kc is not evident in this area. This unit extends eastward from Coyote Basin, but disappears a few hundred meters west of the Cedar Creek area.

The Geologic Map of the Leavitt Reservoir Quadrangle (Noggle-Perrin, 1989) indicates that the area of this report is part of the allochthon which also includes all of Coyote Basin. That map also shows the Morrison Formation in the Cedar Creek area to be situated about midway between the hinge of a north trending monocline on the east, and the axis of a broad northeast trending syncline on the west. Five strike/dip measurements shown on that map as well as six of the eight strike/dip measurements made for the present report indicate somewhat irregular local variation in rock attitude with strike directions in the northeast quadrant. The other two measurements, made at observation sites 51 and 133, show anomalous strike directions in the northwest quadrant which were measured close to the fault near longitude 107º 49' 15"W. Other evidence of this fault and another fault just west of longitude 107º 49' 20"W are apparent offset ridges seen on the photo base map (Plate 6).

Unit Thicknesses

Thickness estimates for the units of the Morrison and lower Cloverly Formations were made using positions and elevations of observation sites located on the unit contacts. First, representative values of strike and dip for the area near the Howe and Siber Quarries were chosen in the following way. At observation sites 46 and 55 in the Siber Quarry and sites 52 and 137 in the Howe Quarry measured dip values of 14, 11, 19, and 16 degrees respectively were averaged to obtain the representative dip angle of 15 degrees. Note that the anomalous measurements at sites 51 and 133 were not used in the averaging. Next, it can be seen on the geologic map that the Jml/Jms contact and the Jms/Jmrb contact both trend in the approximate direction of N45E. From the Leavitt Reservoir 7½ Minute Topographic Quadrangle Map the average downward inclination of the land surface along these contacts is 4 ½ º toward the southwest. Therefore, the true strike direction differs from the N45E direction of the contacts by the angle (θ), which depends upon the dip angle of 15º and the 4 ½ º inclination of the land surface:

θ = arcsin ( tan 4.5 º / tan 15º ) = 17º.

This result is used to calculate the true strike direction of N28E. Note that a very close value of N27E can be obtained simply by averaging the strike directions measured at sites 46, 52, 55, and 137.

Using the representative strike/dip values of N28E, dip 15º NW the thickness of any particular unit can be calculated from the positions and elevations of two points, one somewhere on its upper boundary and the other somewhere on its lower boundary. In the following calculations all elevations were interpolated from contours on the Leavitt Reservoir 7½ Minute Topographic Quadrangle Map. Consider first the thickness of Unit Jml. Near the Siber Quarry observation site 126 is situated on its upper contact with Unit Jms at an elevation 1610m. This site is approximately 275m in a N63W direction, which is the dip direction, from the lower Jml/Jsd contact, which is at an elevation of 1640m. The distance was measured on the photo base map (Plate 6). The thickness (t) can be calculated using the dip angle (15º), the horizontal component of distance between the two points (275m), and the difference in elevation (30m):

t = [ 275m – ( 30m / tan 15º ) ] sin 15º = 42m for Unit Jml.

Next, consider the thickness of Unit Jms. Near the Siber Quarry observation site 126 can be used again, this time to mark the lower boundary at elevation of 1610m, and observation site 130 is on the upper boundary at elevation 1620m. The horizontal component of the distance between these sites is 57m, and the direction of the line connecting them is N32W. The thickness (t) is calculated from the angle between the line connecting the sites and the strike direction (28º + 32º = 60º), distance (57m), the elevation difference (10m), and the dip angle (15º):

t = [ 57m sin 60º + 10m / tan 15º ] sin 15º = 22m for Unit Jms.

Finally, with observation site 130 on the lower boundary of Unit Jmrb at elevation 1620m and site 131 on the upper boundary of Unit Jmrb at elevation 1625m the thickness of this unit can be calculated. The line connecting them is 140m long in the N34W direction. Using this distance, the angle between this direction and the strike direction (34º + 28º = 62º), the 3m elevation difference, and the 15º dip angle, the combined thickness is:

t = [ 140m sin 62º + 5m / tan 15º ] sin 15º = 36m for Units Jmrb + Jmy.

The results of these calculations suggest a total thickness of 100m for the Morrison Formation near the Siber Quarry. The accuracy of this estimate is also questionable owing to uncertainties about strike, dip, elevation, and distance. Recalculation of the thicknesses of the units using different values for these parameters indicates that a) an error of 1 degree for the dip angle produces an error of approximately 5% in the thickness value; b) an error of 10 degrees in the strike direction produces an error of approximately 7% in the thickness value; and c) an error in the elevation difference produces and equal error in the thickness value, d) an error in the distance between observation sites of 6m produces errors of between 4% and 7% in the thickness value for the range of distances used in the above calculations, the error being larger for shorter distances. Assuming uncertainties of 3 degrees for dip measurements, 20 degrees for strike measurements, 6m for elevation differences, and 6m for distances, the combined uncertainty for the thickness values is estimated to be approximately 30%.

Other thickness data from the Cedar Creek area reported by Wilborn (2001) include sections measured by the conventional Jacobs staff method. A section of gray claystone, identified as Unit Kgm for purposes of this report, was measured along the profile extending in a S83E direction from observation point 59, and the thickness was found to be 28m. Another section, identified as Unit Jms for purposes of this report, was measured in the Howe Quarry, and the thickness was found to be 21.7m. This result is very close to the thickness of 22m which was obtained in this report from calculations using points on the upper and lower contacts of Unit Jms near the Siber Quarry.

BOB SIMON AREA GEOLOGY

Field observations in the Bob Simon area were made on June 28, 29, 2002 at 13 positions located by GPS. More observations were made on July 1, 2003, and again on July 5, 2004 at 15 additional positions located by GPS. Altogether, these observations include identification of bedrock lithology, measurement of strike/dip at five locations, and oriented photographs from 14 locations. Also, stratigraphic sections were measured along the two profiles, which combine to cross the entire Morrison Formation from its lower contact with the Sundance Formation near the northern border of the area to its upper contact with the Cloverly Formation in the southern part of the area.

During June and July of 2004 geological field work commenced in the Bob Simon area by a group from Hope College under the supervision of Dr. Brian Bodenbender. This project was sponsored by the National Science Foundation to engage undergraduate students in research involving dinosaur paleontology and geologic mapping. As part of this effort Dr. Tim Demko and Mr. Justin Scott were responsible for geologic mapping. They made available some useful information included in this report.

The field observations were used together with information from an air photo and the Leavitt Reservoir 7 ½ Minute Topographic Quadrangle Map to compile a provisional bedrock geologic map of the area. The 28 locations and the 2 profiles are plotted on the photo base map and the geologic map.

Photo Base Map

A photo base map shown in Plate 8 of the area was prepared by enlarging a portion of Air Photo 8-12-98 BLM 6 WY98AC 1-4-4 which is available from the National Science and Technology Center, U.S. Bureau of Land Management, Denver, Colorado. The scale of the air photo was estimated to be 1cm = 69.6m (1:6960) from comparisons of distances between road intersections measured on the air photo and on the Leavitt Reservoir Quadrangle Map for which the scale is known.

The portion of the air photo covering the study area was scanned at a density of 288 pixels/cm using a Microtek Scanmaker 4800 scanner to obtain a digital image for enlargement. For the air photo scale of 1cm = 69.6m each pixel then represents an area on the ground with a diameter of 0.3048m. The scanned image was then processed with HP Photo Printing software and printed at a density of 49.2 pixels/cm using a Hewlett Packard Deskjet 845c color printer. This produced an enlargement of 4.63X to a scale of 1cm = 15m.

The part of the Leavitt Reservoir Topographic Map corresponding to the study area was scanned, and the digital image was enlarged to a scale of 1:1500 to be compatible with the photo base map and the geologic map. Superposition of the enlarged topographic map over the photo base map on a light table revealed latitude and longitude discrepancies. For features on the photo base map positions measured by GPS plot on the topographic map 12m north and 46m west of the positions of these same features on the topographic map. In the preparation of the maps in this report the latitude/longitude grid is placed to be consistent with the GPS observations.

No corrections for photogrammetric distortion have been made on the photo base map of the Bob Simon area. Topographic relief in this 280m X 390m area exceeds 40m, which is sufficient to produce differences of several meters when comparing apparent distances between features on the photo base map with distance values between these same features calculated from their GPS positions. Therefore the crosses marking intersections of latitude and longitude lines were placed on the photo base map to be consistent with the GPS positions of the nearby observation sites. Because of photo distortion, these intersections to not align to produce a perfectly orthogonal grid of equally space lines. However, this slightly irregular grid should be more useful to an individual visiting the area to locate features seen on the photo base map.

Geologic Map

In the Bob Simon area there are exposures of the upper Sundance Unit Jsd, the lower Morrison Unit Jml, a thick section of the middle Morrison Unit Jms and the upper Morrison Unit Jmrb, and a large exposure of Unit Jmy. The Cloverly Formation Units Kgm and Kws are clearly exposed. In the southern ¼ of the area there is a relatively steep slope of variously colored mudstone which is interpreted to be Unit Kvm. This upper part of the Cloverly section was not measured, so there are no data on the thickness of the variegated mudstone. Distribution of these stratigraphic units is seen on the geologic map in Plate 9.

Recognition of the contacts between these stratigraphic units proved to be more difficult here than in the Coyote Basin and Cedar Creek areas. The exception is the contact between the upper Sundance, Unit Jsd, and the overlying non-glauconitic mudstone and siltstone at the base of Unit Jml, which is exposed almost continuously in the northern part of the area. Otherwise, the cover of regolith produced by weathering and soil development, wash from erosion, and excavation debris obscure the contacts over much of the area. The red banded character of Unit Jmrb is evident in a few places, but the colors tend to be more in shades of muted brown tinted by red, and do not provide the dramatic contrast with Unit Kgm which is seen in Coyote Basin. Near the contact with Jmrb the noncalcareous and bentonitic character or Unit Kgm is confused by the erosion and the downslope wash of calcareous debris from the Morrison Formation. After the aerial photography was done in 1998 excavation of dinosaur fossils over much of the central part of the area has been so extensive that many smaller land forms evident on the photo base map were no longer recognizable when the area was visited in the summers of 2002, 2003, and 2004.

For these reasons, as well as the limitation of time for this reconnaissance survey, a clear exposure of the Jml/Jms contact was found only at the base of a small escarpment surrounding Site 11 at an approximate elevation of 1500m. Here there is a distinct change from the alternating mudstones and fine grained, laminated sandstones of Unit Jml to the coarsely cross bedded Jms sandstone. Higher in the section near Site 13 the upper surface of Unit Jms is exposed on a broad ledge at an interpolated elevation of 1490m. Beginning about 10m south of Site 13 the Jms sandstone is overlain by a sequence of alternating mudstones and thinly bedded sandstone layers characteristic of Unit Jmrb. This is the only clear Jms/Jmrb contact observed in the Bob Simon area. Finally, a clear Kgm/Kws contact was found at Sites 2, 4, and 28, which are situated in two ravines near latitude 44º37’55”N. Here the gray, non-calcareous, bentonitic mudstone of Unit Kgm is ovelain by very friable white sandstone, Unit Kws, containing a “salt and pepper” sandstone zone with some pebbles a few meters above the contact, as is also seen in the Coyote Basin and Cedar Creek areas. There is not enough evidence to conclude if these exposures are continuous with the exposures of Kws in the Coyote Basin and Cedar Creek areas, or if they indicate a separate lens of coarse sandstone and conglomerate in approximately the same stratigraphic position. In the preceding descriptions the elevations were interpolated from topographic contours on the Leavitt Reservoir 7 ½ Minute Quadrangle Map.

In the oldest part of the section the Jsd/Jml contact near the northern border of the area was walked out in the field for its entire length, and was observed to follow the south side of the intermittent stream bed in the ravine. This is a very well defined contact. For the overlying stratigraphic units there are very few places where the actual contacts can be clearly identified. Lacking other information positions of concealed contacts were interpolated using strike/dip measurements and the enlargement to a scale of 1:1500 of the part of the Leavitt Reservoir Topographic Map covering the study area. In this area the average strike is N60W, and the dip decreases from 19º near the northern border to 11º near the southern border. Each contact was interpolated by first drawing on a copy of the topographic contour map one strike line passing through the location where the contact was observed. Then, assuming a plane surface inclined at an angle interpolated between the positions of the 19º and the 11º dip measurements, other parallel strike lines were drawn at elevations on the hypothetical inclined surface corresponding to the contour levels. Intersection points of the strike lines and contours of the same elevations indicate the position of the concealed contact. This procedure was used to interpolate the Jml/Jms/Jmrb contacts. Although Unit Jmy is clearly exposed in the area of Sites 5 and 15, no exposures of its upper and lower contacts were found. Its upper boundary with Unit Kgm is assumed to follow the ravine where Sites 4 and 28 are located., and its lower boundary is speculative.

Even though there are few places where the contacts were well exposed, elsewhere in the area there were many good bedrock exposures within the different units. Along the ridge immediately south of the Jsd/Jml contact are many exposures of the fine grained sandstone/siltstone constituents of Unit Jml, however considerable digging was required to find mudstone concealed by regolith. This is also true for the mudstone constituents in the overlying stratigraphic units. Most of the dinosaur fossils are in Unit Jms, so a large amount of this bedrock has been exposed by excavation. There are also many natural exposures which display coarsely cross bedded sandstone. Unit Jmrb is less well exposed. However several thin sandstone layers were found along the profile between Sites 2 and 13. On the steep slopes below Sites 2, 3, 4, and 28 the dark gray non-calcareous bentonitic mudstone, Unit Kgm, can be found beneath ½ meter of regolith. The white sandstone with the thin “salt and pepper” sandstone and pebble zones, characteristic of Unit Kws, is found in the ravines near these sites.

There are excellent exposures of white, calcareous sandstone, identified as Unit Jmy, near Site 5 and the small pit seen approximately 12m southwest of this site on the photo base map. Its stratigraphic position is just below the contact with Unit Kgm, and dinosaur fossils were found in the small excavation. Nearby at Site 15 the Hope College group has been excavating dinosaur fossils.

The northeast trending fault seen on the eastern side of the geologic map was discovered by Dr. Tim Demko and Mr. Justin Scott working with the Hope College group in June of 2004. They provided explicit descriptions of locations where clear evidence of this fault is exposed, including drag folding near the fault surface at Site 16 and truncated strata at Site 21 (located approximately 130m east of the northeast corner of the geologic map). Much of the trace of this fault appears to be concealed beneath Cenozoic unconsolidated deposits (Qd).

The topographic base map was aligned over the photo base map on a light table to examine how well the contours represented the visible topography. In aligning the roads and streams in this overlay the latitude/longitude discrepancy, discussed above, became evident. The elevation contours enlarged 16X from a 1:24000 scale topographic map and overlaid on a photo base map enlarged 4.63X from an air photo cannot be expected to exactly represent the topography on the photo base map. However, the match is close enough to reveal how the geologic contact patterns relate to the stream drainage pattern. Red Canyon Creek is a perennial stream which flows south along the western margin of the Bob Simon area. This area, then, is situated on the eastern side of the stream valley. Since the strata dip more or less toward the south, the contacts would be expected to display a southward V-shaped pattern across this valley. The generally northeast trend of the Mesozoic strata in the Bob Simon area form the east side of this V-shaped pattern. However, the secondary ephemeral stream drainage is from east to west, producing easterly headward eroding ravines into the gently westward sloping land surface of this small area. That should produce a pattern of contacts zig-zagging east and west superposed on the regional northeast trend, which is the pattern seen on the geologic map (Plate 9).

Measured Sections

Two stratigraphic sections were measured using a modified Jacob’s staff procedure along profiles shown on the photo base map (Plate 8) and the geologic map (Plate 9). One of these profiles, 152m long and bearing southward through Sites 18, 10, 11, and 12, extends from the base of the Morrison Formation to the upper surface of Unit Jms. The other profile, 159m long and bearing S17E, extends between Sites 13, 4, and 2 from the upper surface of Unit Jms into Unit Kws. Results are presented in Plate 5.

The conventional method for measuring a section with a Jacob’s staff is to proceed along a profile oriented in the direction of true dip with the instrument set at the true dip angle. Then, each observation increment along the profile accounts for an increment of thickness equal to the length of the Jacob’s staff. For the present survey the procedure was modified. The profile direction was chosen to maximize the exposure of bedrock, and for convenience the instrument was set at an arbitrary angle (θ) which is reasonably close to the average dip for the entire length of the profile. Later, adjustments are made to account for the angle (Φ) between the profile direction and the true dip direction, and the difference between the true dip angle (β) along some increment of the profile and the arbitrary dip angle (θ) at which the instrument is set. Then, for each observation increment along the profile the corresponding increment of thickness is not equal to the length of the Jacob’s staff. Rather, this thickness (Δt) must be calculated:

Δt = L ( cos Φ tan β / tan θ )

where L is the length of the Jacob’s staff. For both of the measured sections in the Bob Simon area the instrument was set at θ = 17º. For the profile crossing the lower Morrison section between Observation Sites 10 and 12 the true dip value is β = 19º, and for the profile over the upper Morrison and lower Cloverly section between Sites 13 and 2 the true dip value is β = 15.5º. These values were used in the equation above to calculate the thickness increments in Plate 5.

The results of these measurements indicate that in this area there is a complete section of Unit Jml, which is 32m thick. It consists of several thin fine grained, calcareous sandstone and siltstone layers interbedded with mudstones which are mostly concealed by regolith.

The 21m Jms section is much thicker than than the 3m section in Coyote Basin, and is very prominent as a marker bed and a source of dinosaur fossils. Here, the thickness of the Jms section is close to the value found in the Cedar Creek area, where it is also prominent as a marker bed and a source of dinosaur fossils. The clam fossils found in the Bob Simon Quarry in the upper part of Unit Jms have not been found in Coyote Basin or in the Cedar Creek area.

In the Bob Simon area the “red banded” Unit Jmrb and Unit Jmy have a combined thickness of 30m, which is close to the thicknesses measured in the other two areas. However, here Unit Jmrb beds weather to shades of brown rather than the brighter red tints found in Coyote Basin, and it does not display any conspicuous “banded” appearance.

The section of Unit Kgm is only 11m thick in the Bob Simon area, which is much thinner than the corresponding sections in the other two areas where it is exposed. Otherwise, the rock is the same dark gray, non-calcareous bentonitic mudstone. The overlying section of white sandstone with a zone of “salt and pepper” sandstone and pebbles approximately 2m above the base is similar to the lower part of Unit Kws found in the other areas. No effort was made to find its upper contact, so its thickness was not measured here.

The total measured thickness of the Morrison Formation in the Bob Simon area is 83m, which is the sum of the thicknesses of 32m for Unit Jml, 21m for Unit Jms, and 30m for Units Jmrb and Jmy. The lower part of the Cloverly Formation in this area includes Unit Kgm, which is 11m thick, and the lower part of Unit Kws. The value of dip is important in the calculation of true bed thickness from measurements made on the land surface. For a dip angle near 17º an error of 1 degree would introduce an error of approximately 1.7% in the calculated value of bed thickness. In this area the error in dip is subjectively estimated to be less than 5º.

TWO SISTERS AREA GEOLOGY

For many years a few local residents in Shell, Wyoming have known of the existence of dinosaur fossils in the central part of the Shell Quadrangle. Two of these residents are the sisters Mary Clucas and Sylvia Schaeffer, who guided Dr. Nicholas Fraser to a fossil area in June of 2003. In appreciation of their help this area is now referred to as the Two Sisters area. Shortly after this initial visit a one day reconnaissance of the area was done on June 30, 2003 by Dr. Fraser, Brooke Wilborn, and the author. Encouraged by the discovery of some dinosaur fossil vertebrae, plans were made to begin excavating fossils in the following year, and a permit was obtained from the U. S. Bureau of Land Management (BLM) to undertake this work.

The location of the Two Sisters area is shown on the map in Figure 1. During the summer of 2004 dinosaur fossils were obtained in this area from two nearby pits at the following locations: Pit No. 1 at 44º 34‘ 19.6“ N, 107º 47‘ 50.6“ W; and Pit No. 2 at 44º 34‘ 18.6“ N, 107º 47‘ 49.8“ W. The locations of these pits are indicated on the photo panorama in Figure 4.

There are no improved roads leading to the area. Initially, it was reached by turning east from Beaver Creek Road at 44º 34‘ 30“ N, 107º 49‘ 17“ W, then driving approximately 4 km, first following a barely discernable four-wheel drive trail, then overland to within 200m of the excavation pits, and finally walking this remaining distance. After repeated visits the approach to the area became easier to follow.

A geological survey of the area was done to establish the stratigraphic position of the excavation sites. Five days were spent in the field during late June and early July of 2004 in addition to the one day reconnaissance in 2003. Altogether, data were obtained from 50 observation sites located by GPS, and along 12 profiles in the area. These data include descriptions of lithology, 19 measurements of strike/dip, 17 oriented photographs, 24 transit measurements of relative elevation, and collection of representative rock specimens. Using these sources of information together with black & white and infrared aerial photo imagery, a 275m x 325m bedrock geologic map, and 12 geologic cross sections and corresponding stratigraphic columns were compiled to describe the physical stratigraphy in the area.

Photo Base Maps

Photo base maps of the Two Sisters area were prepared from information prepared by the U. S. Geological Survey (USGS) which can be obtained on-line from the Wyoming Geographic Information Center (www.wygisc.uwy.edu). For the purposes of this report digital orthophoto quarter quad (DOQQ) maps were downloaded for the northeast quarter of the Shell 7.5 Minute Quadrangle. The DOQQ maps were compiled by the USGS by assembling digitized aerial photos covering the area, and applying adjustments to remove effects of relief displacement. A grayscale map was made from conventional black and white aerial photography done in 1994, and a color map was made from infrared aerial photography done in 2002-2003 as part of the National Aerial Photography Program (NAPP). Note that the infrared colors are different from the natural colors seen on conventional color photographs.The digitizing interval is close to 1m for the grayscale map, and 0.82m for the infrared color map.

Using conventional photo printing software, the small part of each of the two DOQQ digital map files covering the Two Sisters area was selected to produce grayscale and infrared photo base maps of this area. Knowing the pixel dimensions of these small files, each was then plotted on an area chosen to produce a map at the scale of 1cm = 15m (1:1500). The grayscale photo base map is shown in Plate 10, and the infrared photo base map is shown in Plate 11.

It is stated by the USGS that the digitizing interval used to represent the original air photos imposes a limit on the scale of enlargement of approximately 1:3000 for obtaining a sharply focussed image. Beyond this limit the rectangular pixel zones become evident, and the image appears to lose focus. This effect is clearly seen in Plates 10 and 11. The excessive enlargement was done for subsequent use in preparing a geologic map at this scale. Because other more precise survey data were also used in making the geologic map, the enlargement to a scale of 1:1500 is justified for that map, if not for the photo base maps.

The photo base maps all have cross marks indicating the latitude and longitude grid intersections at 5“ intervals, and the UTM grid intersections at 100m intervals. The grid intersections were located by interpolation using the GPS positions of features which can be clearly recognized on the photo base maps. After plotting the grid positions they were used as reference points for plotting other observation points not marked by easily recognized features.

Topography and Elevation

Relative elevation measurements were made for 24 observation sites using a C. L. Berger & Sons optical transit. The position of each of these sites was determined by GPS, and at each site there was some feature which could be identified on one or another of the photo base maps. From one or more of three transit positions a sighting was made on that feature, and the vertical and horizontal angles were measured. The horizontal component of distance was measured from a photo base map for which the scale is known, and was also calculated from the GPS data. The site position and elevation relative to the transit position were then calculated from the measured angles and the distance. From considerations of the resolution of the photo imagery and the accuracy of GPS the distance measurements are estimated to be accurate within 3m. The accuracy of the relative elevations is estimated to be 1m on the bases of differences in the measurements made from different transit positions and the 3m uncertainty for distance measurements. Note that relative elevations were measured for only the observation sites indicated by red points on the infrared photo base map (Plate 11), and not for sites indicated by green points, which were not visible or could not be identified on the photo base maps.

Additional relative elevation measurements were made along 12 profiles indicated by lines on the infrared photo base map, as part of the procedure used to measure the geologic sections along these profiles. This was done by draping a steel measuring tape on the land surface along a profile. The distances along the tape at which significant changes occurred in the slope of the land surface were recorded, and the angle of inclination of the land surface (µ) between these distance points was measured using a Brunton compass inclinometer. For each distance increment (D) along the land surface the horizontal component is H = D cos µ, and the change in elevation is ∆h = D sin µ. A topographic profile was then drawn by plotting the values of H and ∆h for the sequence of intervals (D) along the profile. The 12 topographic profiles are included as part of the geologic cross sections, which are discussed in a later part of this report.

The small part of the Shell 7.5 Minute Topographic Quadrangle Map corresponding to the Two Sisters area was scanned, then digitally enlarged to a scale of 1cm = 15m. A working copy of the infrared photo base map was placed over the enlarged topographic map on a light table, and the elevation contours were traced. These contours were then modified to be consistent with relative elevation data described above. The contours were also modified qualitatively to account for stream channels, steep slopes, and small ravines which could be seen on the photo base maps.

In some places the original unmodified contour lines closely represented the terrain indicated on a photo base map. By comparing relative elevation values at these places with contour values the set of relative elevation data was adjusted to the sea level reference. This somewhat inaccurate adjustment was used because no nearby bench marks were located for this purpose. It is subjectively judged to be accurate within about 3m. The modified contour lines are shown on the geologic map, which is discussed later in this report, and is presented in Plate 12. Elevation above sea level in the Two Sisters area ranges between approximately 1330m in stream beds near the center of the southern margin and 1362m in the northwestern part at the top of the highest butte in the area.

Landscape Characteristics

Some principal features of the landscape are identified on the grayscale photo base map (Plate 10), and can also be seen in the photo panorama in Figure 4. One feature revealed in these illustrations is a low lying region opening toward the south, and surrounded by steep slopes on the east, north, and west. The low lying region is mostly covered by unconsolidated sediment eroded from the bordering ridges which forms a land surface of low relief. Here a network of intermittent stream channels is clearly evident. This will be referred to as the L1 surface (Plate 10). It is not level, but slopes downward from an elevation of approximately 1350m near the eastern margin to elevations closer to 1340m on the western side.

At higher elevation, and separated from the L1 surface by steep slopes, is another surface of low relief. It will be designated as the M1 surface (Plate 10). Outlines of the M1 surface seen on Plate 10 show that it lies in the northeast part of the Two Sisters area, and also on the western side near the center of the area. This surface is inclined westward from an average elevation of about 1355m on the east to approximately 1345m where it is exposed on the west.

Rising above the M1 surface in the northwest part of the area is one large irregular butte shown in Figure 5, and there are several smaller buttes south of it, all consisting of the mudstone and sandstone layers of Unit Jmrb. The smaller buttes are all capped by a durable sandstone layer 20 cm to 30 cm thick, which also forms wide ledges on the sides of the large irregular butte. The upper surface of this sandstone is referred to as the RB1 surface. The large butte is capped by another durable sandstone layer about 30cm to 40cm thick. The upper surface of this sandstone is called the RB2 surface, and it reaches the highest elevation, 1362m, in the Two Sisters area.

The L1 and M1 surfaces sustain a relatively dense cover of vegetation. Where a thin soil zone exists on the RB1 and RB2 surfaces there is a sparse cover of vegetation. Elsewhere, these latter surfaces as well as the steeper slopes of the buttes, and the slopes separating the L1 and M1 surfaces are almost devoid of vegetation.

The Geologic Map

In the Two Sisters area the stratigraphic section consists of the uppermost part of the Jurassic Sundance Formation and all but the uppermost part of the Jurassic Morrison Formation. The characteristics of the “red banded” sandstones and mudstones of Unit Jmrb (Figure 5), the gray and red mudstones of Unit Jml, and the greenish glauconitic sandstone of Unit Jsd, and are very similar to the the characteristics of these units farther north in the SW quarter of the Leavitt Reservoir Quadrangle. However, the lithology of Unit Jms in the Two Sisters area is significantly different. Here this unit consists predominantly of silty claystone rather than sandstone. This rock breaks into irregular equidimensional fragments a few centimeters in diameter, and bedding surfaces are indistinct. The calcareous tan silty claystone weathers to distinctive shades of yellow. Enlargement of Figure 4 reveals that below the N28W direction mark a single layer of Jms yellow claystone on the east separates into two layers with an interbedded zone of gray mudstone on the west. It is in this gray mudstone that the dinosaur fossils have been found.

A geologic map of the Two Sisters area was compiled at a scale of 1cm = 15m using numerous positions on contacts between different stratigraphic units together with patterns seen on the infrared photo base map (Plate 11). The positions on contacts include the GPS positions as well as measurements of the relative positions by transit surveying and by steel tape along 12 profiles, which were later adjusted to GPS control points.

The contact between the glauconitic sandstone of Unit Jsd and the gray mudstone of Unit Jml is clearly exposed at 44º 34’ 18.5” N, 107º 47’ 40.5” W and 44º 34’ 19.0” N, 107º 47’ 41.9”W near the base of the steep slope bordering the east side of the L1 surface. Unit Jsd is also exposed in an intermittent stream channel on the L1 surface at 44º 34’ 20.4” N, 107º 47’ 46.5” W. Otherwise the Jsd/Jml contact is mostly concealed. In much of the Two Sisters area the contact between Units Jml and Jms is evident from clear color contrasts and easily recognized changes in lithology widely exposed on the steep slopes separating the L1 and M1 surfaces. The weathered regolith of Unit Jms is exposed everywhere on the M1 surface except on the eastern side of the area where it is covered by a veneer of Unit Qd. The contact between Units Jms and Jmrb almost everywhere is marked by a halo or fringe of white regolith at the base of the buttes rising above the M1 surface.

It is possible, to some extent, to correlate the three stratigraphic units of the Morrison Formation with colors on the infrared photo base map (Plate 11). The low lying central part of the area, the L1 surface, is colored in shades of dark gray. Rising from this surface are the steep unvegetated slopes where the mudstone of Unit Jml is indicated by shades of tan and pale brown on the infrared image. Higher on these steep slopes the silty claystone of Unit Jms appears pale blue. However, on the M1 surface above these steep slopes the Jms regolith with its cover of vegetation is colored in darker shades of gray on the infrared image. The white halos and fringes at the base of the buttes formed from Unit Jmrb are clearly seen on the infrared image, and also on the grayscale photo base map (Plate 10). Otherwise, on the infrared image the lower parts of these buttes are colored darker shades of blue and gray, and the higher parts, situated between the RB1 and RB2 surfaces, are tan and pale brown.

On a working copy of the infrared photo base map all GPS, transit, and steel tape positions were plotted, as well as the elevation data. Then, geologic contact lines were drawn using this information together with the infrared color patterns. These contact lines, elevation contours, and principal stream channels were then traced to produce the geologic map shown in Plate 12.

The 19 strike/dip measurements in the Two Sisters area indicate considerable local variation. This is to be expected for the bedding surfaces of the continental sedimentary strata which exhibit some irregularity. Strike measurements separated by less than 10 meters can differ by a few 10s of degrees. The average strike direction for the area is N7W, and the significance of this direction value can be judged from the sums of strike directions in different 30º azimuth ranges, which are listed in Table 2.

These groupings confirm the suggestion that regional strike is slightly west of north. The angles of dip range from high values of 14º to low values of 2º with an average of 6º W. The local strike/dip variations may be the result of small differences in the inclination of the surface at the time of sediment deposition, and differences in post depositional compaction. The decline in elevations of the L1 and M1 surfaces in a westerly direction, described earlier in this report, is consistent with the average dip of 6º in the Two Sisters area.

The 1:24000 scale Geologic Map of Shell Quadrangle, Wyoming (Manahl, 1985) does not indicate any exposures of the Sundance Formation near or in the Two Sisters area. The discovery in 2004 of glauconitic, calcareous, sandstone clearly establishes the occurrence of the uppermost Sundance strata in this area, mostly concealed by unconsolidated sediment, but exposed in a few places.

Geologic Cross Sections and Stratigraphic Columns

To illustrate in more detail the stratigraphy in the Two Sisters area geologic cross sections and stratigraphic columns were prepared from measurements made along 11 profiles shown on the geologic map (Plate 12), and along a 12th profile north of the area near 44º 34’ 41” N, 107º 47’ 53” W where the Morrison/Cloverly contact is exposed. Two of the profiles, ‘03-8 and ‘03-9, were measured in June of 2003, and the remaining ones, A, B, CF, D, E, G, H, L, and M, were measured in June and July of 2004. Because the strata are nearly horizontal in this area no effort was made to orient the profiles in the direction of dip. Rather, they were laid out to take maximum advantage of exposed bedrock along lines where the steep slopes could be traversed safely.

Along each profile a Lufkin steel measuring tape 91.4m (300 feet) long was draped on the land surface. Inclination of the land surface was measured along increments of the profile for the purpose of describing the topography, as discussed in an earlier part of this report. Also, the tape distances of the boundaries of individual stratigraphic layers were recorded. These layer boundaries were identified by changes in lithology and color changes on the weathered land surface, and the characteristics of the layers were briefly observed. Some thin sandstone layers interbedded with mudstone were clearly exposed along profiles crossing Unit Jmrb. There may be other thin lenses of less durable sandstone within this unit which are concealed by weathered debris. For Unit Jml no thin sandstone lenses interbedded with the mudstone were found in the Two Sisters area. Time constraints on completing this reconnaissance survey of the area precluded excavation of the weathered slopes, consisting mostly of mudstone, except at a few locations.

To prepare each geologic cross section first the topography along its profile was plotted using the horizontal (H) and vertical (∆h) components discussed earlier. Then each layer boundary was drawn beginning at its tape distance along the topographic profile. For each profile the layer boundaries are inclined at an apparent dip (∂) calculated from the angle (φ) between the bearing of the profile and the average of nearby measurements of true dip direction, and the true dip (ß):

∂ = arcsin ( sin ß cos φ).

The geologic cross sections are presented in Plate 13. Note that because of the 2:1 vertical exaggeration in these cross sections, the apparent dip angles are also exaggerated.

A stratigraphic column corresponding to each geologic cross section was prepared by projecting the layer boundaries along the topographic profile onto a vertical column. The results with descriptions of each layer are shown in Plate 14.

Consider now the thicknesses of the stratigraphic units of the Morrison Formation. The complete section of Unit Jml is exposed only along Profile H. Here a thickness of 10.1m was measured. For Unit Jms the thicknesses measured along Profiles A, B, C, G, H, and ‘03-9 range between 3.8m and 5.0m with an average of 4.4m. Four of these sections consist entirely of distinctive yellow silty claystone, and for sections B and C upper and lower yellow claystone zones are separated by a gray mudstone zone containing dinosaur fossils. Moving 150m farther south along Profile M both the claystone and mudstone zones thicken considerably, and Unit Jms reaches a thickness of 11.5m. The lower section of Unit Jmrb between the M1 and the RB1 surfaces (Figure 5) was measured along Profiles D, E, and ‘03-8, and thickness values range from 3.5m to 7.5m with an average of 5.4m. The middle section between the RB1 and RB2 surfaces (Figure 5) is 4.2m along Profile 2 and 7.0m along Profile ‘03-8. These results indicate that the thickness of Morrison Formation strata exposed in the Two Sisters area ranges between 22m and 29m with an average of 26m.

The uppermost part of the Morrison section, which extends from the RB2 surface to the Morrison/Cloverly contact is missing in the Two Sisters area. However, this part of the section is exposed approximately 800m farther north at a location indicated on the infrared photo map in Figure 6, which also shows the profile of Stratigraphic Section L. Here the uppermost part of the Morrison section, the interval between the RB2 surface and the Morrison/Cloverly contact, is 1.7m thick. In Section L the 6.7m interval between the RB2 and RB1 surfaces, and the 7.6m interval between the RB1 and M1 surfaces are comparable with corresponding values in the Two Sisters area, suggesting that less than 2m is missing from the uppermost part of the Morrison section above the RB2 surface seen in Figure 5.

Discussion

In the Two Sisters area the Morrison Formation with an approximate thickness of 26m is considerably thinner than the corresponding section in the Leavitt Reservoir Quadrangle area which is between 50m and 100m thick. In both areas the calcareous mudstones of the lower Morrison Unit Jml are very similar. The upper “red banded” Unit Jmrb has the same general characteristics in these areas. The big difference is the lithology of Unit Jms. In most of the Bighorn Basin prominent sandstone layers with some interbedded mudstone occur in the middle part of the Morrison Formation (Ostrom, 1970). The thickness of this unit in the southwestern part of the Leavitt Reservoir Quadrangle ranges from as little as 3m to as much as 22m. In the Two Sisters area the middle part of the Morrison Formation consists of distinctive yellow silty claystone with some interbedded gray mudstone rather than the tan and gray sandstone and interbedded gray mudstone found farther north in the Leavitt Reservoir Quadrangle. Despite this difference in lithology these middle Morrison strata in the Two Sisters area are designated as Unit Jms. Here the thickness of Unit Jms varies between 3.8m and 11.5m. In both areas almost all of the dinosaur fossils occur in the zones of gray mudstone within this unit. There is clear evidence of the contact of the Morrison Formation with the underlying Sundance Formation in the Two Sisters area.

“Bald Ridges of Cloverly”

Within the NE quarter of the Shell Quadrangle approximately one kilometer NNW of the Two Sisters area are some irregular ridges (Figure 6) where Cloverly sediments are exposed in the higher parts. The highest ridges stand out as being devoid of vegetation where they are capped by lower Cloverly sediments. The area was visited briefly in June, 2001 by Brooke Wilborn and the author. Another visit was made in July, 2004, which is described above in the discussion of the Two Sisters area Here, a layer of pebble conglomerate approximately 1m thick was observed at the base of the Cloverly section. It is designated as Unit Kp. Medium to coarse textured non-calcareous sandstone overlies the conglomerate. The field characteristics of the conglomerate and sandstone were essentially the same at the four observation sites marked in Figure 6.

Earlier, during the summer of 2000, a group directed by Marilyn Fox from the Peabody Museum of Yale University came to this area in search of dinosaur fossils. A few bones were discovered in the exploration pit indicated in Figure 6. Based upon our limited observations in 2001 the stratigraphic position of this exploration pit appears to be in Unit Jms.

PETROGRAPHY

As part of the geologic mapping effort in the Leavitt Reservoir and Shell Quadrangles 20 rock specimens were collected at locations in the Coyote Basin, Bob Simon, and Two Sisters areas. The locations are shown in Figure 7, and the position coordinates of the specimens are listed in stratigraphic order in Table 3. Specimens were obtained from Units Jsd through Unit Kws. The petrography of these specimens including physical characteristics and modal mineralogy was done by examination of hand specimens and thin sections. Sampling of the stratigraphic section includes well-indurated specimens from projecting ledges of relatively durable rock from 17 sites, and less durable mudstone specimens from 3 sites.

Specimen Identification

The rocks include 20 hand specimens and thin sections made from 17 of these specimens. Each specimen is identified by a label with three parts, as illustrated by the example 7-Jml-CB. First is a number which is also used to concisely identify the specimen on the index map (Figure 7) and the composition diagram (Figure 8). Next, the mapping unit is identified, which is Jml in this example. Finally, the area from where the specimen was collected is identified by CB for Coyote Basin, BSA for the Bob Simon area, and TSS for the Two Sisters area. The specimen number gives an indication of the position of the specimen in the stratigraphic sequence relative to the other specimens, 1 being oldest and 20 being youngest, with the following exceptions: specimens 8, 9, 10, and 11 are from approximately the same stratigraphic level in Unit Jms at different locations; and specimens 14, 15, and 16 are from the same stratigraphic level in Unit Jmy at different locations.

Petrographic Descriptions

Description of the rock specimens includes features of texture and composition, which were determined from examination of rock exposures in the field, hand specimens, and thin sections. The grain size-, shape-, and sorting criteria, and the rock classification system used in this report are those presented by Blatt and Tracy (2nd edition, 1996), which include the grain size scale proposed by J. A. Udden and C. K. Wentworth and the mineral composition diagram of E. F. McBride.

The physical properties of each specimen are presented in Appendix 1. The macroscopic characteristics of color, and of the bedding and lamination features were observed in hand specimens and rock exposures in the field. Because of the generally small grain size, the important features of texture including grain size, shape, sorting, and packing, and the relative proportions of grains and cement were determined from examination of thin sections. Photographs of hand specimens are in Appendix 2.

Modal mineral compositions are presented in Table 4 for the specimens of sandstone and arenaceous siltstone, and in Table 5 for specimens of argillaceous silty claystone. These mineral proportions were determined from point counts done on the thin sections of 17 specimens using a Swift Automatic Point Counter Model E mounted on a Leitz Wetzler polarizing microscope. The number of counts per slide ranged between 222 and 375. The two most abundant minerals are quartz and orthoclase. It is well understood that in thin sections these two minerals can be very similar in appearance. Therefore, 8 thin sections were stained to distinguish between quartz and orthoclase. Chert and chalcedony are grouped together as other SiO2 minerals. The occasional occurrences of biotite, opaque minerals, zircon, and others are grouped collectively without further specific identification. No analysis was done to distinguish the specific clay mineralogy of specimens except to distinguish between generalized brown clay, clay-sized SiO2 particles, and sparkly cryptocrystalline CaCO3 consisting of bright clusters and threads of clay-sized elements. Photomicrographs from all thin sections are in Appendix 3.

For the sandstone specimens the relative proportions of the detrital grains consisting of quartz and SiO2 minerals (Q), feldspars (F), and the other mineral grains (O) are compared on the triangular composition diagram in Figure 8, which shows the results obtained from thin sections stained to reveal orthoclase.

Discussion

The ages of these rock specimens, which come from the uppermost Sundance, Morrison, and lower Cloverly Formations, range from Oxfordian to early Neocomian time (Moberly, 1960, Kvale, 1986). The specimens display some unusual features, which are remarkably consistent through this time interval of at least 15 million years.

All of the thin sections from sandstone specimens display poikilitic calcite cement. In these thirteen thin sections the fraction of poikilitic calcite is between 19% and 51% of the total rock volume, and averages 40%. This cement consists of interlocking “crystals” generally larger than the embedded detrital grains. Photomicrographs made from the thin section of 13-Jmrb-CB viewed with crossed polars at different magnifications are shown in Figure 9. They illustrate a typical zone of calcite cement approximately 0.5mm in diameter (a) embedding several mineral grains, and which (b) reaches extinction by rotation of the microscope stage. This clearly indicates that the crystalline structure of the cement has a stronger influence on the coarseness of the rock texture than the detrital grains. Several examples of hand specimens shown in Appendix 2 have the macroscopic appearance of medium-grained sandstone. However, in thin section their detrital fraction is seen to consist mostly of very fine sand grains and coarse silt grains. In these specimens the poikilitic crystals of the cement are considerably larger than the mineral grains. The poikilitic calcite produces a cement supported fabric in which the loosely packed detrital grains are embedded. Features of texture seen from megascopic examination aided by a low magnification hand lens are produced by the cement, and provide almost no information about the detrital grain fraction of the rock.

Sandstones can be classified according to modal mineral composition. The widely used system proposed by E. F. McBride (Blatt and Tracy, 2nd edition, 1996) is illustrated in Figure 8b, and is based on the relative proportions of detrital SiO2 grains (Q), feldspar grains (F), and other fragments (O). This system can be used to classify the eight specimens which were stained to distinguish between orthoclase and quartz. The results for the compositions listed in Table 4a, and displayed in Figure 8a are presented in Table 6. The arkose and subarkose specimens, which are from the Morrison and Sundance Formations, are somewhat unusual to the extent that arkosic composition is commonly associated with more coarse-grained sandstones. The only quartz arenite (19-Kws-CB), which is from the Cloverly Formation possesses no feldspar, but is similar to the other sandstone specimens in regard to texture and poikilitic cement.

Rock composition is an important factor in locating the Morrison/Sundance contact in the northeastern Bighorn Basin. In particular, glauconite is central to recognizing the boundary between the marine sediments of the Sundance and the continental sediments of the Morrison. In the Cedar Creek, Bob Simon, and Two Sisters areas the uppermost glauconitic sandstone of the Sundance Formation can be clearly distinguished from the basal, non-glauconitic mudstone of the Morrison Formation. Sandstone specimens 1-Jsd-BSA, 2-Jsd-TSS, and 3-Jsd-TSS from these areas all possess significant proportions of glauconite determined from point counts of, respectively, 311 points, 244 points, and 337 points. For these specimens the glauconite proportions of the total detrital grain volume are, respectively, 11%, 1%, and 1% (Table 4). Glauconitic minerals are produced only in marine waters mainly at depths of more than 60 meters, and their occurrence in shallower marine sediments results mostly from reworking of older glaucony facies (Odin and Fullagar, 1988). Specimens 2- and 3-Jsd-TSS are at approximately the same stratigraphic level, higher in the section than 1-Jsd-BSA17, and close to the Sundance/Morrison contact. These specimens were all produced during a time of marine regression. The diminished glauconite content in specimens 2- and 3-Jms-TSS compared with 1-Jsd-BSA may be the result of shallowing water shortly before emergence of the land surface at the beginning of Morrison time.

In the Coyote Basin map area specimens 4-, 5-, 6-Jml-CB represent the stratigraphically oldest exposed rock, which is pale green very fine grained sandstone. These specimens appear similar to specimens 2- and 3-Jsd-TSS except that only traces of glauconite are present. Are these traces indicative of a final stage marine environment, or are they transported grains from a former marine environment? Moberly (1960) suggested that in places where pale green sandstone rather than silty claystone makes up the base of the Morrison, the greenish color can be attributed to other greenish clay minerals rather than glauconite. However, specimens 4-, 5-, and 6-Jml-CB possess no greenish clay. Additional point counts were undertaken in an effort to more clearly define the glauconite content, which can be quickly recognized in thin sections under plane polarized light and counted using the Swift mechanical point counting attachment. Of 858 points counted on the specimen 5-Jml-CB and 472 points counted on 6-Jml-CB there were no direct hits by the crossed hairlines, although a few “off hairline” glauconite grains were seen. These traces are similar to the traces of glauconite which were seen but escaped the point counting in the thin sections from specimens 9, 10, 13, and 15 situated higher in the Morrison section. The conclusion is that these traces of glauconite are transported grains not indicative of a marine environment of deposition. The specimens 4-, 5-, and 6-Jml-CB are, therefore, believed to come from the lower Morrison Formation.

The three specimens of argillaceous silty claystone, 7-Jml-CB, 8-Jms-TSS44, and 18-Kgm-CB, consist predominantly of a matrix of clay minerals and cryptocrystalline calcite with a much smaller fraction of detrital silt particles (Table 5). Photomicrographs (Appendix 3) made from thin sections of these specimens indicate another feature of texture. Within the matrix of clay-sized particles are sand-sized fragments also consisting, similarly, of silt grains in a clay matrix. They are recognized from subtle as well as obvious color differences. They may be the result of early induration of zones within the larger mass of sediment which were subsequently crushed, producing fragments that became embedded in a surrounding softer matrix having the same composition, as lithification progressed. It should be noted that the VMNH dinosaur fossil excavation site in the Two Sisters area is situated in the silty claystone represented by specimen 8-Jms-TSS.

Some mention should be made of specimen 12-Jms-CB. It represents the rock in which the VMNH dinosaur fossil excavation site is situated within Coyote Basin. This is the only specimen of true siltstone. It represents the upper part of Unit Jms which also includes an underlying sandstone layer represented by specimen 11-Jms-CB. The siltstone has some interstitial calcite cement, which does not display the poikilitic structure so characteristic of the sandstone specimens. There is a smaller proportion of interstitial clay, which appears to consist mostly of SiO2 fragments rather than clay minerals.

Thin sections were not made from three coarse textured specimens from three separate sandstone units. Lowest in the section is 14-Jmy-CB, which is at the same stratigraphic level as 16-Jmy-CB and has similar macroscopic characteristics. Progressively higher in the Cloverly part of the section are specimens 17-Kp-CB and 20-Kc-CB, which are both non-calcareous mixtures of coarse- and medium-sized light and dark colored sand grains with scattered granules and pebbles. In large outcrops lenses of pebble conglomerate are exposed, interbedded with the coarse sandstones represented by these two specimens.

In the present study no X-ray or chemical analyses were done to determine the particular clay minerals in the argillaceous mudstone specimens. This has been done by others including Moberly (1960) and Kvale (1986), so the prevalence of illite in the Morrison mudstones and montmorillonite in the Cloverly mudstones was already known. Supporting field evidence comes from Specimen 18-Kgm-CB, which was dark gray when first removed from the ground, but turned light gray after drying in the sun for about one hour, a color change expected because of evaporation of moisture from the montmorillonite. Similar color changes were observed for samples of Unit Kgm removed from the ground in the Cedar Creek area and the Bob Simon area. This evidence and weathering characteristics presented earlier in this report confirm the existence of montmorillonite in Unit Kgm. Similar field evidence of montmorillonite was not found for the argillaceous mudstones in the Morrison Formation.

On the basis of X-ray diffraction measurements Moberly (1960) found evidence that illite is the principal clay in the argillaceous mudstones of the Morrison Formation, and that montmorillonite is predominant in the Little Sheep mudstone unit of the lower Cloverly formation. He also found small concentrations of kaolinite and traces of montmorillonite in the Morrison and the uppermost Sundance Formations. Kvale (1986) also made X-ray diffraction measurements which revealed the predominance of montmorillonite in the Little Sheep mudstone, and also provided evidence of both illite and mixed layered clays with illite and montmorillonite throughout Morrison and uppermost Sundance strata in the northeastern Bighorn Basin.

SUMMARY AND DISCUSSION

The present study was undertaken to define more clearly the stratigraphic position of seven dinosaur fossil excavation sites situated in the northeastern Bighorn Basin (Figure 1). For this particular area a superposed sequence of ten stratigraphic units is proposed to represent the Upper Sundance, Morrison, and lower Cloverly sedimentary rocks which are exposed here. These units are consistent with the earlier descriptions of Moberly (1960), Ostrom (1970), and Kvale (1986). In addition they include details specific to this study area, which may or may not apply elsewhere in the Bighorn Basin.

The characteristics of Unit Jsd are the same as those presented in these earlier studies to describe the upper part of the Sundance Formation. The superposed sequence of Units Jml, Jms, Jmrb, and Jmy represents the Morrison Formation. Although Moberly and Kvale do not subdivide the Morrison into such a specific superposed sequence, their descriptions of Morrison lithology appropriately describe the compositions of these units. Ostrom did present a superposed sequence of Units I, II, and III. The characteristics of Unit Jml and Unit Jmrb are essentially the same as he described for Units I and III. Features of Unit Jms in the Bob Simon and Two Sisters areas are similar to Ostrom’s Unit II, but are substantially different in the Coyote Basin and the Two Sisters areas. Because of these differences, Unit Jms should be viewed as a zone with characteristics that provide a clear division between Units Jml and Jmrb. The seven dinosaur fossil excavation sites are situated in Unit Jms.

In the present study the non-calcareous coarse sandstone and pebble conglomerate exposed in the SE corner of Coyote Basin is designated as Unit Kp. It directly overlies the Morrison putting it at the same stratigraphic level as the Pryor Conglomerate and Ostrom’s Unit IV. A similar conglomerate directly overlying the Morrison is exposed in the “Bald Ridges” of Cloverly approximately 1km north of the Two Sisters area. Evidence in the area of this study indicates that Unit Kp is discontinuous, and is missing from the Cloverly section in the Bob Simon, Cedar Creek, and all but a small part of the Coyote Basin areas.

In the area of this report the Little Sheep Mudstone is divided into four superposed parts: Unit Kgm, a distinctive gray, non-calcareous, bentonitic claystone; Unit Kws, a white, very fine grained quartz arenite with some thin seams and zones of coarse grains; Unit Kc, a non-calcareous pebble conglomerate directly overlying Unit Kws in some places; and Unit Kvm, a colorfullly variegated, non-calcareous, bentonitic claystone. The combination of Unit Kws and Unit Kc, where it exists, appears to be one of the sandstone and pebble lenses within the Little Sheep Mudstone (Ostrom’s Unit V) mentioned by Moberly, Ostrom, and Kvale. Unit Kws appears to be pervasive in the Coyote Basin and Cedar Creek areas. However, there is no compelling evidence for direct correlation with the somewhat similar unit identified as Kws in the Bob Simon area, which may, or may not be a separate lens of coarse sandstone and pebble conglomerate. Because of its distinctive variegated color, Unit Kvm is not easily confused with the underlying predominantly gray claystone of Unit Kgm.

Columns representing the late Jurassic – early Cretaceous stratigraphy, represented in terms of the units proposed in this report (Table 1), are compared in Plate 15 with columns representing the subdivisions of Moberly, Ostrom, and Kvale. In the Moberly, Ostrom, and Kvale columns their subdivisions are labeled by notation and letters, and the author’s interpretation of how these units correlate with the units in Table 1 is indicated by the different colors. The columns representing the Coyote Basin, Cedar Creek, Bob Simon, and Two Sisters areas are arbitrarily aligned so that the lower boundaries of Kvale’s lithofacies F and Unit Jmrb from this report are at the same level. Also, the Moberly and Ostrom columns are aligned so that the Himes/Little Sheep Mudstone boundary and the UnitVI/Unit V boundary are arbitrarily placed at the same level as Kvale’s H/J lithofacies boundary. These different boundary alignments are necessary for the following reasons: 1) Moberly did not subdivide the Morrison; 2) the upper boundary of Unit Kvm was not included in any of the sections measured in the present study; 3) a significant difference between the description of the Little Sheep Mudstone based upon the work of Moberly and Kvale and the description of the corresponding Unit V presented by Ostrom. This third reason needs further explanation.

The original type section for the Cloverly Formation, which is near the Leavitt Reservoir in the NW ¼ Sec. 24, T54N, R92W, Big Horn County, Wyoming, was first examined by Darton (1906). When Moberly redefined the Cloverly section, he found that it could be described more clearly by combining information from the Darton area with information from exposures approximately 3km northeast in the NW ¼ Sec. 17, T54N, R91W, which is near the junction of Beaver Creek and Cedar Creek. Therefore, Moberly proposed a compound type section for the Cloverly which included both of these locations. Later, Ostrom and Kvale also examined this compound type section. Their reports do not state exactly where measurements were made, but the locations in Sec. 17 appear to be somewhere on a prominent butte shown in Figure 10, which surrounds position 44º39’51”N, 107º50’06”W. Uppermost in their Cloverly sections is the Himes member, which is 28m thick by Moberly’s measurement. Corresponding to this are lithofacies G, H, and I with a combined thickness of 27m indicated in Kvale’s column, and Ostrom’s Units VI and VII with a combined thickness of 28m. Below this part of the section is Moberly’s Little Sheep Mudstone, Ostrom’s Unit V, and Kvale’s lithofacies I, H, and G, which all refer to the same part of the Cloverly Formation. The thickness of 76m reported by Moberly (1960, p.1146, Fig. 3), and the combined thickness of 82m for lithofacies G, H, and I shown by Kvale (1986, p. 25-26, Fig. 3) are reasonably close, but differ markedly from the thickness of 30.4m for Unit V in Ostrom (1970, p. 175-176). Underlying Unit V is a 5.5m layer of non-calcareous sandstone and pebble conglomerate, the lowermost part of the Cloverly section described by Ostrom, which he identifies as Unit IV (Pryor Conglomerate). Beneath this he reports 27.7m of calcareous claystone with thin sandstone lenses, identified as Unit III of the Morrison Formation. Ostrom does not say if this section of Unit III was measured at the same location as the overlying Units IV and V.

It is seen that Moberly, Ostrom, and Kvale report approximately the same thickness and lithology for the Himes member and the upper part of the Little Sheep Mudstone. However, Ostrom has identified as Unit IV a layer of sandstone and pebble conglomerate which Moberly and Kvale interpret as an intermediate lens within a much thicker section of non-calcareous bentonitic claystone. Referring to Figure 10, the sandstone and conglomerate are represented by Units Kws and Kc, which are approximately the same thickness as Ostrom’s Unit IV. The underlying gray, non-calcareous, bentonitic claystone, identified as Unit Kgm, is the same as the lower 17m thickness of lithofacies I in Kvale’s column. In Figure 10 the base of Unit Kgm is concealed, but this unit can be traced in the field ½ km south into Coyote Basin where the base is clearly exposed (Figure 3), and thickness values in the range of 19m to 23m were measured. Furthermore, underlying Unit Kgm is approximately 30m of the calcareous, non-bentonitic Unit Jmrb which correlates closely with the 27.7m thickness of Unit III reported by Ostrom. All of these comparisons indicate that the lithologies and thicknesses presented by Ostrom correlate closely with data from Moberly, Kvale, and the present report with one exception. Ostrom does not include the gray, non-calcareous, bentonitic claystone in the lower part of the Cloverly section represented by lithofacies I in Kvale’s column, and by Unit Kgm in this report. This evidence suggests that Unit IV be reinterpreted to be a lenticular conglomerate/sandstone zone rather than an exposure of Pryor Conglomerate. Then, by inserting a claystone layer equivalent to Unit Kgm the Ostrom column in Plate 15 would correlate in all regards with the columns of Moberly and Kvale, which were also compiled from measurements made near the junction of Beaver Creek and Cedar Creek.

Earlier discussions of Morrison petrography by Moberly and Kvale refer to the existence of quartz arenites. In the present study rock specimens from throughout the Morrison section were examined in thin section, and no quartz arenites were identified. The thin sections stained to reveal K-feldspar were found to contain significant fractions of orthoclase. Without the appropriate stain this mineral is almost indistinguishable from quartz, so that rocks of more arkosic composition appear very similar to quartz arenite. Another important feature of all specimens examined in thin section is the ubiquitous poikilitic calcite cement. It is the crystalline structure of this cement rather than the detrital grains that determines the texture of the rock. All but one of the thin sections revealed grain fractions consisting of very fine sand and coarse silt particles embedded in much larger calcite cement crystals.

Darton (1906) defined the Cloverly Formation to describe strata overlying the Morrison Formation in the Bighorn Basin. This has given rise to an ongoing debate about where to place the boundaries of these formations. There has been very little disagreement about placing the lower boundary of the Morrison at the contact with the underlying glauconitic sandstone of the Sundance Formation, and the upper boundary of the Cloverly at the contact with the Thermopolis Shale. Much more controversial is the Morrison/Cloverly contact. In Moberly’s review of stratigraphy in the Bighorn Basin he compares 29 interpretations published between 1904 and 1960. Sixteen include the Little Sheep Mudstone in the Cloverly, and ten include it in the Morrison, and there are three with other alternatives. In a similar comparison of 30 interpretations, several of which were also in the Moberly review, Ostrom shows fourteen which place Unit V in the Cloverly, and fourteen placing it in the Morrison, and two alternatives. In later work Kvale’s interpretation of the section near the junction of Cedar Creek and Beaver Creek follows Moberly’s definition, placing the Little Sheep Mudstone in the Cloverly. Geologic maps published by the Geological Survey of Wyoming by Love and Christiansen (1985), and Ver Ploeg (1992) state that bentonitic claystone is a characteristic of the Cloverly, but do not specifically mention it as a characteristic of the Morrison. Manahl (1985) makes no specific mention of bentonitic claystone in his descriptions of these two formations. However, Noggle-Perrin (1989) places variegated bentonitic claystone in her description of the Morrison, but does not make any specific reference to it in describing the Cloverly. Much farther south of the Bighorn Basin descriptions of the Morrison Formation in Colorado (Peterson and Turner, 1998), Utah (Bilbey, 1998, and Aubrey, 1998), and New Mexico (Anderson and Lucas, 1998) include an upper unit, the Brushy Basin member, which contains a significant proportion of claystone rich in bentonite. Peterson and Turner argue that the bentonitic claystone assigned to the Cloverly Formation by Moberly’s definition should be reassigned to the upper part of the Morrison Formation.

In spite of these different opinions about the Morrison/Cloverly boundary there appears to be no disagreement about the characteristics of strata exposed in the Bighorn Basin. The superposed sequence of units listed in Table 1 can be distinguished by features clearly evident in the field. This sequence does not contradict other descriptions of the stratigraphy in this area. The evidence indicates that the seven principal dinosaur fossil excavation sites are in the same stratigraphic position within the local Morrison/Cloverly section.

ACKNOWLEDGEMENTS

The fieldwork for this report was done during five late June through early July trips to Wyoming (2000 – 2004) when I was a volunteer participant in the dinosaur fossil excavation program of the Virginia Museum of Natural History. I appreciate the many lively discussions I had with Brooke Wilborn, my field companion on an almost daily basis during the 2000, 2001, and 2003 field seasons, and with Nicholas Fraser, the director of the program. I also appreciate the help in the field on many occasions from several others, museum staff people and volunteers.

Rock specimens used in this study were collected during fieldwork done near Shell, Wyoming. Thin sections were prepared by Spectrum Petrographics, Inc. Photomicrographs of all thin sections were made with the assistance of Mr. Clayton Loehn and Dr. Robert Tracy using the Nikon Optiphot-Pol Q imaging Micropublisher polarizing microscope, which was made available by the Va. Tech Geosciences Dept. On several occasions Dr. Richard Law examined the thin sections, and pointed out petrographic features of importance, and arranged for use of the point counting apparatus. Dr. Fred Read and Dr. Kenneth Eriksson also examined several of the thin sections and provided helpful discussion of certain features.

REFERENCES

Anderson, Orin J. and Spencer G. Lucas, 1998. Redefinition of Morrison Formation (Upper Jurassic) and related San Rafael Group strata, southwestern U. S., in Special Issue (K. Carpenter, D. J. Chure, J. I. Kirkland, eds.), Modern Geology, v. 22, n. 1-4, p. 39 – 70.

Bilbey, Sue Ann, 1998. Cleveland-Lloyd Dinosaur Quarry – age, stratigraphy, and depositional
environments, in Special Issue (K. Carpenter, D. J. Chure, J. I. Kirkland, eds.), Modern Geology, v. 22, n. 1-4, p. 87 – 120.

Blatt, Harvey, and Robert J. Tracy, 1996. Petrology, igneous, sedimentary, and metamorphic, Second edition, W. H. Freeman and Company, New York, 529p.

Darton, N. H., 1906. Geology of the Bighorn Mountains, USGS Prof. Paper 51, 129 p.

Kvale, Erik Peter, 1986. Paleoenvironments and tectonic significance of the upper Jurassic  Morrison/lower Cretaceous Cloverly formations, Bighorn Basin, Wyoming, PhD
Dissertation, Iowa State University, Ames, Iowa, 191p.

Love, J. D., and Ann Coe Christiansen, 1985. Geologic Map of Wyoming, U. S. Geological Survey.

Manahl, Kenneth A., 1985. Geologic map of Shell Quadrangle, Wyoming, Map Series 17, The Geological Survey of Wyoming, Laramie, Wyoming.

Moberly, Ralph, 1960. Morrison, Cloverly, and Sykes Mountain Formations, northern Bighorn Basin, Wyoming and Montana, Bulletin of the Geological Society of America, v. 71, p.1137-1176.

Noggle-Perrin, Karin, 1989, Geologic map of the Leavitt Reservoir Quadrangle, Wyoming Map  Series 29. The Geological Survey of Wyoming, Laramie, Wyoming.

Ostrom, John H., 1970. Stratigraphy and Paleontology of the Cloverly Formation (lower
Cretaceous) of the Bighorn Basin area, Wyoming and Montana, Bulletin 35,
Peabody Museum of Natural History, Yale University, New Haven, Connecticut, 234p.

Peterson, Fred and Christine E. Turner, 1998. Stratigraphy of the Ralston Creek and Morrison Formations (Upper Jurassic) near Denver, Colorado, in Special Issue (K. Carpenter, D. J. Chure, J. I. Kirkland, eds.), Modern Geology, v. 22, n. 1-4, p. 3-38

Wilborn, Brooke K, 2001. Two new dinosaur bone beds from the late Jurassic Morrison Formation, Bighorn Basin, Wyoming: an analysis of the paleontology and stratigraphy, M. S. Thesis,Virginia Polytechnic Institute and State University, Blacksburg, Va.