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Home , Absaroka Range, Blaine Formation, Chugwater Formation, Geology of Wyoming, History of Wyoming, Spearfish Formation

A RECONNAISSANCE OF PERMO- REDBEDS OF EASTERN AND SOUTHWESTERN

By W. John Nelson, Illinois State Geological Survey, Champaign

July 14, 2011

INTRODUCTION

This report follows a reconnaissance field trip to the and several areas in eastern Wyoming on June 10-16, 2011. In addition to myself, participants were Bill DiMichele and Dan Chaney from the Smithsonian Institution, Bob Gastaldo from Colby College in Maine, and Sylvie Bourquin from University of Rennes in France. The first two are paleontologists; the second two are sedimentologists. Red rocks of and Triassic age were our focus. These include the Opeche, Minnekahta, and Spearfish Formations around the Black Hills, and the Goose Egg and Chugwater Formations in eastern Wyoming outside the Black Hills. Except for the thin Minnekahta and Alcova , these rocks are virtually devoid of . Specifically, no plants have ever been reported – except in the Popo Agie Member of the Chugwater, which we did not examine. A goal of the field trip was to learn why these formations lack fossil plants, although units of similar age and elsewhere in the western contain plentiful flora. As a guide to further research, this report compiles our own observations, along with those from published maps and reports.

Geologic Setting

Eastern Wyoming and southwestern South Dakota lie within the Great Plains and central geomorphic provinces (Fig. 1). This portion of the Plains is underlain chiefly by sedimentary rocks of through age, mantled in places by younger deposits. The mountains, including the Black Hills, Big Horn1 Mountains, and , are products of the , which took place during through time. Mountains comprise blocks of the crust, including crystalline basement, uplifted along faults under compressive forces. Faulting at depth induced sharp folds in sedimentary cover. Complementary basins, such as the Bighorn and Powder River basins, sank as the mountains rose. Permian and Triassic rocks are exposed along the flanks of mountain uplifts, and commonly are steeply inclined. During Permian and Triassic time, the area of Wyoming and South Dakota lay in the western interior (present directions) of Pangea. Through time, the North American plate drifted from the equatorial region into the northern subtropical belt. All indicators point to a hot, dry climate. According to some accounts, progressively more extreme climate led to Earth’s greatest mass extinction at the end of the Permian Period. A stratigraphic chart (Fig. 2) depicts correlation of units observed on this field trip, as drawn from various published reports.

Previous Geologic Work

1 Mountains and county are properly spelled as two words, river and basin as one word. All refer to the Rocky Mountain bighorn sheep, which inhabits this area (Urbanek 1988).

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Figure 1. Map showing route traversed on this field trip and localities visited.

Geologists and prospectors accompanied Gen. George Custer’s expedition into the Black Hills in 1874. Their discovery of gold touched off a stampede and triggered the war that led to Custer’s demise. The first extensive geologic reports on the region were those of N.H. Darton (1899, 1901, and 1904). The area we toured is well endowed with geologic maps and reports. State geologic maps of South Dakota (Darton 1951) and Wyoming (Love and Christiansen 1985) provide overviews. U.S. Geological Survey maps and reports cover most of the area we traversed in South Dakota (Braddock 1963, Brobst et al. 1963, Gott and Schnabel 1963, Post 1967, Wolcott 1967, Wolcott et al. 1962). The geologic map and comprehensive report of Robinson et al. (1964) encompass a large area of the northwestern Black Hills. Manahl (1985) mapped geology of the area we visited near Shell, Wyoming. Owing to lack of economic incentive and perhaps, lack of fossils, interpretive studies of the redbeds are few. A single M.S. thesis (Sabel 1981) addresses sedimentology of the Spearfish. M. Dane Picard and co-authors wrote the bulk of interpretive studies on the (Picard 1966 and 1993, High and Picard 1969, Tohill and Picard 1966).

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Figure 2. Correlation chart for Permo-Triassic redbeds in the area visited.

UNIT DESCRIPTIONS

Minnelusa Formation

Underlying the Permo-Triassic redbeds throughout the Black Hills is the Minnelusa Formation, a unit of , , dolomite, gypsum, and minor mudstone. Fusulinids and other marine fossils indicate and Early Permian age (Braddock 1963). According to Maughan (1967), the systemic boundary is at the base of the “red marker”, a widespread red mudstone unit that is prominent on outcrops and in the subsurface. Although we were not aware of the “red marker” while in the field, I may have photographed it in Hot Brook Canyon, where Brobst and Epstein (1963, p. 338, citing C.G. Bowles) stated it is present (Fig. 3). Extensive deformed bedding and brecciation is characteristic of the upper Minnelusa throughout the Black Hills. We observed this phenomenon in Hot Brook Canyon (Fig. 4). Maughan stated, “Evaporites in thick beds are common in Permian strata, but they are thin (1967, p. 133) or sparse to absent in the Pennsylvanian. Evaporites have mostly been leached from surface exposures, but their former positions are indicated by sandstone and limestone breccia, which are common in the upper parts of the Minnelusa and Hartville Formations.” Braddock (1963) also documented evaporite dissolution on the western side of the Black Hills, where a drill core penetrated

3 237 feet (72 m) of anhydrite in 430 feet (131 m) of upper Minnelusa strata drilled. Brobst and Epstein (1963) concluded that ground water moving down-dip dissolved away sulfates in the upper part of the formation. This phenomenon accounts for the numerous mineral springs in the region.

Tensleep and Casper Formations

We observed the only in passing through Tensleep Canyon (the type locality) and north to Shell, Wyoming along the west flank of the Big Horn Mountains. The light gray, fine-grained sandstone exhibits large-scale crossbedding and has been interpreted as largely dune sand. Farther south, the Casper Formation also contains eolian sandstone along with shallow marine sandstone, limestone, and dolomite. Fossils are scarce. Both units are correlated with the Minnelusa on the basis

Figure 3. Sylvie and Bob examine the Minnelusa Formation in Hot Brook Canyon near Hot Springs, S.D. Notice that the lower part of the outcrop has intact bedding, whereas the upper part is deformed. The thin claystone about 2 meters above the geologists’ heads may be the “red marker”, a regional key bed that lies close to the Pennsylvanian- Permian boundary. of lateral relationships. Maughan (1967) interpreted the Wolfcampian shoreline as trending more or less north-south through central Wyoming (through Buffalo and Casper), with fluvial plains to the west and ocean to the east. Ancestral Rocky Mountain uplifts served as sediment

4 sources. Along the shoreline, dune fields merged with beach and shallow offshore bars. Eastward, out to sea, beds of limestone, dolomite, gypsum, anhydrite, and halite were deposited. Climate was warm and arid, leading to hypersaline conditions.

Opeche Formation

Darton (1901) named the Opeche, using the Indian name for Battle Creek in Custer County, South Dakota. This unit also has been called Opeche , a misnomer because the Opeche consists of non-fissile mudstone and siltstone. Descriptions in the literature are generally brief. Reddish brown mudstone, siltstone, and fine-grained sandstone predominate. Gypsum or anhydrite layers occur in places,

Figure 4. View in Hot Brook Canyon, with Minnekahta Limestone on the skyline above slope-forming Opeche Formation and cliffs of Minnelusa Formation. Uneven layering in the Minnelusa is interpreted as the product of extensive dissolution of gypsum and anhydrite by ground water. especially in the subsurface (Braddock 1963). Measured thicknesses range from about 70 to 150 feet (21 to 46 m) with abrupt local variations generally attributed to solution collapse in the underlying Minnelusa. An excellent exposure of the Opeche is in a roadcut on U.S. Highway 18 at the summit of the hill just west of Hot Springs, South Dakota (Fig. 5). The 70- to 80-foot (21 to 24 meters) high exposure consists of interbedded sandstone and siltstone in roughly

5 equal proportions. Sandstone is reddish gray and very fine-grained, quartzose and argillaceous, in beds a few cm to a little over one meter thick. Siltstone to silty mudstone is dark brownish red and indistinctly laminated to very thinly bedded. Sedimentary features include planar and wavy lamination, a little low-angle cross-lamination, and halite crystal casts. Aside from indefinite burrows, we saw no evidence of life. The lower contact is just below road level; the upper contact mostly grassed over. Based on fossils in the bracketing limestone units, age of the Opeche is considered to be Leonardian. In noncommittal fashion, Maughn (1967, p. 143) wrote, “The Opeche is probably transitional from continental into marine deposits, although the red color and lack of fossils suggest the possibility of continental origin. A transitional environment is suggested by stratigraphic position; widespread uniformly thick deposits; inclusion of dolomite, anhydrite, gypsum, and halite; gradation upward into marine Minnekahta Limestone; and uniformity of color.”

Figure 5. Roadcut in Opeche Formation on U.S. Highway 16 just west of Hot Springs, S.D. Evenly bedded siltstone and sandstone without gypsum characterizes most of this formation. Darker colored, soft mudstone is at the top of the exposure, with the Minnekahta Limestone concealed on the grassy slope above.

6 Minnekahta Limestone

The name Darton (1901) chose for this unit is the Dakota Indian name for the springs around the present city of Hot Springs, South Dakota. Many of the springs emanate from this limestone. There is no specific type locality, although canyons and active quarries near Hot Springs show the unit very well. Darton (1904) was the first to recognize Permian age of the Minnekahta based on its sparse assemblage of marine fossils. Like the Opeche, the Minnekahta forms a ring around the Black Hills. It is resistant to erosion and typically forms a low escarpment (Fig. 4). Thickness ranges from about 25 to 60 feet (8 to 18 m); 40 feet (12 m) is typical. The limestone (or dolomite) is light to medium gray, commonly weathering reddish to purplish gray. Texture is lithographic to finely crystalline. Some layers are silty, grading to calcareous siltstone. Anhydrite patches occur in some beds. Gastropod and bivalve fossils are locally common, and they indicate Permian age (Braddock 1963, identifications by John Chronic). The only place where we observed this unit up close was at the dam of Cold Brook Lake just north of Hot Springs, South Dakota. Here the Minnekahta is approximately 40 feet (12 m) thick and consists of limestone that is medium gray, weathering purplish to pinkish gray; micritic, and laminated to thinly bedded. included low-angle cross-lamination, planar lamination with possible neap-spring tidal rhythmites, and possible mud cracks. No fossils were found. Also we saw small-scale paleokarst features. Contacts to adjacent units were not cleanly exposed, but appear to be sharp.

Spearfish Formation

N.H. Darton (1899) named the for the city of Spearfish in the northern Black Hills, which was settled in 1877, one after the Battle of Little Bighorn. The city is on Spearfish Creek, which probably took its name from Indian fishing activities. Although Smith (1903) extended the Spearfish into the Hartville uplift of eastern Wyoming, today the term “Spearfish Formation” is generally restricted to the Black Hills, including the Belle Fourche valley near Devil’s Tower. The Spearfish forms a broad valley that encircles the Black Hills and is commonly called the “red valley” (Fig. 6). The formation erodes to a hummocky landscape of rolling hills, suggesting widespread landsliding and/or dissolution of gypsum beds. Outcrops are numerous, but no long, continuous sections were seen or reported in the literature. Roadcuts and other man-made excavations provide the best views in the southern Black Hills. From Newcastle, Wyoming northward the upper part of the Spearfish, more resistant to erosion, forms extensive bluffs and breaks. Published values for total thickness range from 330 to 550 feet (100 to 168 m), with considerable uncertainly resulting from incomplete exposures and internal deformation attributed to a variety of causes. Geologists who mapped the Spearfish in the southern and western Black Hills commonly divided the formation into three informal units:

 A lower unit of siltstone, 50 to 100 feet (15 to 30 m) thick and containing little or no gypsum  A middle unit of mudstone and siltstone alternating with gypsum layers as thick as 50 feet (15 m). Thickness of the middle unit is 100 to 220 feet (30 to 67 m).

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 An upper unit 140 to 300 feet (43 to 91 m) thick composed of siltstone and sandstone, generally coarsening upward, and containing little or no gypsum.

Figure 6. “Red valley” underlain by Spearfish Formation, looking west from summit on U.S. Highway 16 just west of Hot Springs, S.D. Unusually heavy rains this spring turned the valley largely green. Wooded slopes to south (left) are Sundance and younger units overlying the Spearfish. The core of the Black Hills is to the right, out of the picture.

Lower siltstone member. We observed the lower member on a bluff just east of the dam in Cold Brook Canyon north of Hot Springs, S.D. Estimated thickness is about 20 m with the lower part of the unit poorly exposed. Two-thirds to ¾ of the interval is reddish-brown silty mudstone that is massive to weakly fissile. Interbeds of siltstone to very fine sandstone mostly lack definite sedimentary structures. Near the top of the lower member is friable, very fine sandstone that is at least 3 m thick and displays wavy lamination and crossbedding. This is succeeded in turn by a 3-meter covered interval and ledge-forming gypsum at least 6 m thick, marking the base of the middle member. The lower member has similar characteristics in roadcuts on State Highway 71 near Cascade Spring, in Sec. 20, T8S, R5E, Fall River County. Veins and stringers of satin spar a few cm thick are numerous. The lowest thick gypsum layer varies from 50 to 150 cm thick and has a sharp, planar lower contact and a mounded upper surface.

8 Another outcrop showing part of the lower siltstone is along Custer County Highway 769 (near NE corner, Sec. 14, T5S, R1E). Reddish brown, non-fissile silty mudstone 9 to 12 m thick is sharply overlain by gypsum at the base of the middle unit. Nearly uniform in lithology, the mudstone may be a weakly developed paleosol (Entisol or Inceptisol). It lacks definite soil features, such as peds, slickensides, root traces, and nodules.

Middle gypsiferous member. We examined the middle member of the Spearfish at several sites around the city of Hot Springs, in the roadcuts at Cascade Spring, and at several points between Hot Springs and Sundance, Wyoming. The unit consists of brownish red silty claystone to siltstone and thick layers of gypsum in roughly equal proportions. Mudstone is massive to blocky, lacking definite sedimentary structures. Green reduction spots are common and there are a few thin, gray to greenish-gray mottled layers. Veins, lenses, and stringers of satin spar are pervasive. We saw gypsum beds as thick as 6 m; other authors report thicknesses as great as 15 m. Much of the gypsum is white (alabaster) to gray, pink, and greenish gray, and displays wavy or crenulated lamination that suggests algal mats (Fig. 7). Many gypsum beds are discontinuous and sharply folded (Fig. 8) as a result of dissolution and/or phase changes between anhydrite and gypsum. Some gypsum has highly crusted, brecciated appearance, most likely due to partial dissolution. Thin layers or lenses of black, highly organic rock occur near the base of the middle unit in two of the three roadcuts at Cascade Springs. This substance has a dull luster, very low density, lacks lamination, and shows no trace of plant material. It is not . The black rock occurs along with thin dolomite stringers and greenish gray siltstone near the base of the middle unit. Post (1967) and Wolcott (1967) both allude to this material in their reports. Sabel (1981) logged 11 cm of “black organic marlstone” in his Trout Haven Ranch section, about 23 km north of Cascade Spring. Sabel reported, “Approximately 63 percent of this bed is calcite, 14 percent silicate minerals, and the remaining 23 percent organics. Shiney black ovoids, often rich in vitronite (sic), are scattered along the bedding planes, which are distorted around them. This relationship suggests the ovoids were deposited as clasts.” Microscopic examination failed to reveal microfossils, such as spores and pollen. Sabel suggested that the organic marlstone was deposited “in bodies of still water where organic matter was preserved in the reducing conditions that existed on the bottom.” In the Custer County Highway 769 roadcut, a bed of gypsum about 1 meter thick marks the base of the middle member. At the base of the gypsum are thin lenses of light gray, microgranular dolomite. The horizon can be identified precisely with reference to the geologic map and text of Braddock (1963), who reported that the dolomite is widespread and ranges up to 15 feet (4.5 m) in thickness. The dolomite, like other Spearfish strata, is devoid of fossils.

Upper siltstone and sandstone member.

Landslides and talus largely conceal the upper part of the Sundance in the southern Black Hills. One area where the upper unit is well exposed is on the south side of Pilger Mountain Road, west of Hot Springs (Sec. 10, T7S, R3E, Fall River County). Laminated, brownish-red siltstone 15 to 18 m thick bears green reduction spots and lacks gypsum. Grain size and layer thickness increase upward to the unconformable upper contact.

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Figure 7. Uneven lamination in bedded gypsum layer of middle Spearfish Formation in a roadcut on State Highway 585 south of Sundance, Wyoming. The crenulated laminae that pinch and swell suggest algal activity.

Figure 8. Bill DiMichele with small in same gypsum layer shown in previous photo.

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From Newcastle, Wyoming northward the upper non-gypsiferous unit becomes thicker and more resistant to erosion, commonly forming escarpments. The upper part contains sandstone in thick, tabular, rhythmic layers. Although not accessible, the exposure on Red Butte (Fig. 9) appears typical. We examined upper Spearfish up close behind the Ohlson ranch house near Beulah, Wyoming. Sandstone is very fine-grained and generally lacks lamination or other structures. Several upward-fining sequences, about 2 to 5 m thick, comprise sharp-based sandstone grading upward to siltstone and silty mudstone.

Figure 9. Red Butte, along U.S. Highway 85 north of Newcastle, Wyoming. Capping the butte is bedded gypsum, the Gypsum Spring Formation. Remainder is Spearfish Formation, which coarsens upward overall.

The contact between the Spearfish and overlying Jurassic rocks is unconformable. We saw the contact south of Pilger Mountain Road (County Highway 12) en route from Hot Springs to Sundance (SE ¼ NE ¼, Sec. 10, T7S, R3E, Fall River County). Laminated red siltstone of the Spearfish is overlain by massive, orange-weathering sandstone, the Canyon Springs Sandstone Member of the (Gott and Schnabel 1963). The contact is knife-sharp and truncates layering in the Spearfish (Fig. 10). In places, rounded and polished chert pebbles as large as 5 cm occur at the base of the Sundance (Fig. 11). Wolcott (1967) observed chert pebbles in the Hot Springs quadrangle and interpreted the Canyon Springs as eolian and shallow marine sediments. Braddock (1963) described the same relationships, including chert pebbles, in Custer County north of the outcrops we examined.

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Figure 10. Erosional contact between orange sandstone of Sundance Formation, above and red siltstone of Spearfish Formation, below. Steps on contact to left are small faults.

Figure 11. Rounded chert pebbles at base of Sundance Formation widely mark this unconformable surface. Locality same as above.

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North of Newcastle, Wyoming at Red Butte (east side U.S. Rt. 85, Sec. 34, T47N, R61W, Weston County), massive gypsum of the Jurassic Gypsum Spring Formation sharply overlies a pinnacle of red Spearfish siltstone and sandstone (Fig. 9). The same contact is accessible in a roadcut on State Rt. 24 one mile south of Hulett, Wyoming (SW ¼, Sec. 13, T54N, R65W, Crook County). Interbedded limestone and mudstone of the Gypsum Spring truncates Spearfish bedding with about 5 feet (1 ½ m) of relief. The upper part of the Spearfish in northeastern Wyoming closely resembles the upper Red Peak Member of the Chugwater in central to east-central Wyoming. I suspect that in the southern Black Hills, the sub-Jurassic cut more deeply, removing the upper sandy beds of the Spearfish.

Goose Egg Formation

Burk and Thomas (1956) selected this name for a succession of red, gypsiferous mudstone and siltstone that previously had been called by a variety of awkward names. As Urbanek (1988, p. 83) relates, the name originated with the Goose Egg Ranch, founded by the Searight brothers, who trailed in 27,000 head of cattle from Texas and settled on Poison Spider Creek southwest of Casper. “Cowboys found a nest of wild goose eggs, and brought them to the cook; this gave the owners an idea for a brand and a ranch name.” A post office later was established, but this is no longer extant. At its type locality (Fig. 12), the Goose Egg is 380 feet (115 m) thick and composed of dominantly red mudstone, siltstone, fine sandstone, gypsum, and limestone. Burk and Thomas named six members, including the Opeche Shale at the base (70 feet, 21 m) and the Minnekahta Limestone (10 feet, 3 m). On this reconnaissance trip, we did not spend much time examining the Goose Egg. We viewed the type section from the roadside; it is on private property and we did not seek access. Roadcuts and stream banks south of Barnum were inspected briefly (Fig. 13). More Goose Egg exposures were seen from the car in passing around Alcova Reservoir, around Roughlock Hill southwest of Kaycee, and along a gravel road that winds between Shell and Hyattville, Wyoming. At all of these places the Goose Egg erodes to low, rounded hills and hummocks commonly capped by gypsum. The formation is incompetent and subject to landsliding and tectonic deformation. Long, continuous exposures are not common, but with some effort, good composite sections probably could be assembled in the areas mentioned. The prevalent rock type is reddish brown silty mudstone and siltstone, non-fissile to weakly fissile, alternating with layers of gypsum commonly 10 feet (3 m) or thicker. Gypsum beds tend to be discontinuous, perhaps due to dissolution. No fossils were seen.

Correlation. Based on lithology, the Goose Egg correlates with lower and middle parts of the Spearfish Formation. Burk and Thomas (1956) regarded the Goose Egg as directly correlative with the Phosphoria and Dinwoody Formations of the Wind River Mountains of western Wyoming. The latter two units are composed largely of carbonate rocks, tongues of which extend eastward into the Goose Egg. Burk and Thomas (p. 9) wrote, “The [Phosphoria] formation has yielded beautifully preserved diversified faunas which indicate Middle Permian age – Guadalupian for the greater part, and possibly Leonardian for the lower part.” The Dinwoody “carries a marine mollusk and brachiopod fauna which dates it as earliest Triassic (Newell and Kummel, 1942). An unconformity of uncertain magnitude, possibly encompassing Ochoan time, separates Phosphoria from Dinwoody and may extend into the Goose Egg.

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Figure 12. Looking west from Goose Egg type locality. Jurassic rocks cap the ridge above red cliffs of Chugwater, with pale red and gray Goose Egg strata at base.

Figure 13. View northeast across Middle Fork Powder River, south of Barnum, Johnson County, Wyoming. “Red wall” of Chugwater Formation is in the distance and interbedded redbeds and gypsum of Goose Egg Formation are in the canyon closer to the camera.

14 Lane (1973) came to essentially the same conclusions as Burk and Thomas, after constructing a series of cross sections across Wyoming using outcrop and well records. Lane neither cited fossil evidence, nor broke down the Permian and Triassic into series or stages. Counter to Burk and Thomas, Lane showed that limestone members of the Goose Egg in eastern Wyoming do not carry into the western part of the state as tongues of Phosphoria and Dinwoody. There are areas in central Wyoming where the Goose Egg lacks carbonate beds. Aside from supporting earliest Triassic age for the Dinwoody and its correlation with the upper part of the Goose Egg, Picard (1993) had little to say about age and correlation of the Permo-Triassic redbeds. The correlation chart in Snoke et al. (1995) does not break down the Permian into smaller units.

Environment of deposition. Referring to the Permian part of the Goose Egg and Spearfish Formations, Maughan (1967, p. 146) wrote that the unit “seems to have been deposited in a shallow sea during a warm arid period.” Maughan named the Ancestral Rocky Mountains (to the south, in southern Wyoming and ) as the chief contributor of sediment, perhaps largely residual soil that was already red. This is a reasonable theory, because the Pennsylvanian-age Ancestral Rockies are flanked by Pennsylvanian strata (e.g. the west of Denver) that are dominantly red. Given near absence of sand in the Permian, very little sediment was recycled from the Minnelusa, Tensleep, and Casper Formations.

Chugwater Formation

This curious name was taken from Chugwater Creek in Platte County, Wyoming. The stream in turn took its name from Chug Springs. “An Indian chief called ‘The Dreamer’ was too lazy to hunt buffalo the hard way; he is supposed to have thought up the idea of stampeding them over chalk cliffs that break abruptly away; because of the sound of falling waters, it was known as ‘water at the place where the buffalo chug.’ Whites adopted the Indian name.” (Urbanek, 1988, p. 39.) Before European contact, Indians indeed killed buffalo by running them off cliffs and into sinkholes (e.g. the Vore Buffalo Jump east of Sundance, where we stopped briefly). Once horses and firearms became available, Indians abandoned this wasteful method of killing bison. Darton (1904) named the Chugwater Formation for the creek, but did not designate a type section. As originally described, the Chugwater contained beds now assigned to the Goose Egg. The first member to be named was the Alcova Limestone (Lee 1927). He took the name from the community (present population 20), as the dam and reservoir were not completed until 1938. There is no stratotype, although the exposure we saw along the west lake shore road would serve admirably. Working with outcrops in western Wyoming, Love (1939) added Red Peak, Crow Mountain, Popo Agie and Gypsum Spring Members of the Chugwater. The last named was later found to be Jurassic age and revised to a separate formation. Beginning in the late 1960s, some geologists classified the Chugwater as a group divided into formations, whereas others continued to call it a formation split into members. We inspected Chugwater outcrops on the north side of Horse Creek near Shell, Wyoming on June 15. Manahl (1985) mapped geology of this area. His description of the Chugwater reads, in its entirety, “”Thin carbonate member [Alcova?] overlying reddish brown calcareous siltstone. Thickness 600 feet [183 meters].”

Red Peak Member. J.D. Love (1939) named this unit for Red Peak in the southern , northwest Wyoming. In most places the bulk of the Chugwater

15 belongs to the Red Peak Member, commonly 200 to 250 m thick. Lithology is dominantly brownish red non-fissile mudstone, siltstone, and very fine sandstone (Figs 14 and 15). A few layers weather gray, greenish gray, or yellowish gray. Picard (1993) reported that the red color is a product of diagenesis, not sediment that was originally red or coated with hematite. The Red Peak has similar lithology and is correlative, at least in part, with the of and Arizona as well as with the upper Spearfish in South Dakota. We took opportunities for fairly detailed study of Red Peak strata at four localities. These were (1) north side of Horse Creek near Shell in Big Horn County, (2) roadsides near Barnum in Johnson County, (3) Red Wall and Rough Lock Hill in northern Natrona County, and (4) west side of Alcova Reservoir in southern Natrona County. All of these outcrops displayed similar features and closely matched published descriptions by Picard (1993) and other authors. Overall, the Red Peak Member coarsens upward. The lower part is slope-forming, non-fissile mudstone and laminated siltstone, whereas the upper part contains ledge- and cliff-forming layers of siltstone to very fine sandstone that become thicker and more numerous upward. Bedding is tabular and has great lateral continuity. Driving past long escarpments such as the Red Wall between Barnum and Rough Lock Hill, we could follow individual layers for many km.

Figure 14. Chugwater Formation on the west side of Alcova Reservoir. Nearly continuous exposures along roadway provide an excellent reference section. The Alcova Limestone Member caps the ridge.

16 Finer grained parts of the Red Peak consist of silty mudstone and fine siltstone. Mudstone is mostly massive to blocky, but a few intervals are fissile and can be called shale. Siltstone displays horizontal and wavy lamination, including climbing ripples, and small-scale cross lamination. We also saw contorted lamination, small load casts and ball-and-pillow features, mud cracks, and rare indistinct vertical burrows. Fining-upward packages 1 to 3 m thick are fairly common. Each has a sharp lower contact, and coarse laminated siltstone in the lower part, grading upward to mottled, blocky mudstone. Although these mudstone units lack definitive soil features, they may represent weakly developed soils (Inceptisols or Entisols). The bold ledges of the upper Red Peak are composed of coarse siltstone to extra-fine sandstone that has calcite cement. Picard (1966) classified these rocks as arkose, subarkose, and impure arkose ranging from well-sorted to poorly sorted. This rock is largely massive, in layers as thick as 5 m. Contacts may be sharp or gradational. One ledge-forming unit in the Red Wall has an erosive lower contact that exhibits local relief of a meter or two, and low-angle accretionary bedding. This layer is an exception to the general pattern. Cavaroc and Flores (1990) mirrored some of our observations as they described upward-fining packages with erosional lower contacts. These authors attributed such sand bodies to braided fluvial channels that supplied sediments to deltas. “The mottling and rootlike turbation are evidence that the top of the Red Peak is, at least locally, a paleosol,” they wrote (p. E9). The indistinct burrows that we saw are among the rare bits evidence for life during Red Peak deposition. M. Dane Picard, who devoted his doctoral dissertation and much of his professional career to the Chugwater, observed, “There are small burrows, but they are not definitive as to maker or environment and have not been studied in any detail.” (Picard 1993, p. 220). Beyond these, Picard (1993) and Lucas (1994) tallied a few reptile footprints, some fish scales, and a single pelecypod. “Little variation in water depth occurred over the Wyoming shelf,” wrote Picard (1993, p. 220). “A geologist standing on eastern Wyoming in the as the tide went out might have been tempted to walk to the west, persuaded by the flatness of the tidal plain and the tide’s disappearance that it would not return.” Picard attributed the Red Peak to paralic (that is, along the seacoast just above tide), tidal-flat, and near- shore marine settings. “I believe that large amounts of wind-blown silt may have fallen on the Wyoming shelf, beginning with Red Peak deposition,” he wrote (1993, p. 22). “Silt rained down on the land adjoining the sea, on islands, and in the shallow sea from great dust storms carried by northeasterly trade winds. Waves and currents from the remnant sea sorted the grains and formed sedimentary structures and bedding.” Based on my brief exposure to the Red Peak, I concur with Picard’s judgment. This was a hostile environment and probably devoid of vegetation. Except in small tracts of subtropical deserts, there are no modern analogues. I do not favor the fluvial and deltaic systems that Cavaroc and Flores (1990) proposed for the Red Peak. Massive character of coarse layers, scarcity of channel features, and absence of upward-coarsening packages rule against such settings.

Alcova Limestone Member. Lee (1927) named the Alcova Limestone Member of the Chugwater for the community of Alcova in Natrona County, southwest of Casper. No type section was designated, but exposures near Alcova Reservoir would serve admirably. The Alcova is a widely persistent and distinctive marker unit, both on outcrop and in the subsurface. “On a clear day you can see the Alcova Limestone forever,” wrote Picard (1993, p. 223). This unit has been mapped across more than 50,000 square miles or 130,000 square kilometers.

17 Along the west shore road at Alcova Reservoir, the member is about 25 feet (7½ m) thick and both contacts are sharp. The light gray to greenish gray, microgranular limestone and dolomite contains interbeds of greenish and purplish gray siltstone in the lower part. Carbonate rocks are laminated to thinly bedded; distinct algal laminations (stromatolites) occur in the upper part. No other fossils were found. Near Rough Lock Hill the Alcova is thinner, about 8 feet (2.4 m), and again displays stromatolites. Picard (1993, p. 224) wrote, “The undistinguished Alcova biota includes molluscan, reptilian, and algal components.” The sparse pelecypods and gastropods are not useful indicators of either age or environment of deposition. Lucas (1994) attributed a reptile fossil to Corosaurus, a primitive nothosaur. Only stromatolites are plentiful and prominent. Sedimentary structures also provide no useful clues as to depositional setting. Some authors favored marine settings, others lacustrine. According to Picard (1993, p. 224), carbon and oxygen isotope values tip the balance in favor of seawater. Cavaroc and Flores (1990) invoked a marine transgression that drowned the Red Peak deltas. However, algae and reptiles indicate the sea was extremely shallow, no deeper than during much of Red Peak deposition. Some factor other than sea-level rise may have shut off the silt influx, allowing carbonates to precipitate. Perhaps the Alcova formed a relatively wet interlude, when the great Triassic dust storms temporarily abated.

Figure 15. View east at Red Wall, northern Natrona County, Wyoming. Red cliffs comprise upper Red Peak Member, capped by thin Alcova Limestone. Crow Mountain Member can be seen in distance below “Gray Wall” of Jurassic Sundance Formation.

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Crow Mountain Sandstone Member. J.D. Love (1939) named this unit for a peak near the southeastern end of the Absaroka Range. The Crow Mountain is the only part of the Chugwater that has economic importance, serving as reservoir rock in several Wyoming oil fields. The unit is distinctive from a distance, as its lower sandstone forms smoothly rounded light red to orange outcrops; whereas the upper Crow Mountain erodes to stepped ledges that are mostly darker red, like the Red Peak Member (Fig. 16). We inspected Crow Mountain outcrops rather hastily on a roadside east of Barnum and along the west shore of Alcova Lake. On first impression the smooth, sculptured outcrops of the lower sandstone call to mind eolian units such as the Entrada and Navajo . However, the scale of crossbedding in the Crow Mountain is much smaller, sets generally being 30 cm or thinner. The sand is dominantly very fine- grained, but scattered fine to medium grains occur in the Barnum outcrop. The upper part of the Crow Mountain is dominantly red ledge-forming siltstone and sandstone that resembles parts of the Red Peak Member.

Figure 16. Crow Mountain Sandstone north of road east of Barnum, Wyoming. Lower pink unit has small to medium-scale crossbedding; upper bedded unit resembles Red Peak Member.

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Working in the southern , Tohill and Picard (1966) found that the lower sandstone attains a maximum thickness of 119 feet (36 m) and is composed dominantly of very fine-grained subarkose, with a sprinkling of fine to medium quartz grains. The larger grains are better rounded and some are frosted. Trough and planar crossbedding in thin to medium sets is prominent. The upper sandstone and siltstone unit (thickness not stated, illustrations indicate 5 to 15 meters) displays ripple marks, horizontal and cross-lamination, mud cracks, clay rip-up clasts, and a variety of simple burrows. These authors inferred that the primary sediment sources lay north and east of the Bighorn basin, and that the Crow Mountain was deposited during an episode of marine regression following the Alcova transgression. The lower crossbedded sandstone is a shallow shelf deposit that contains a few wind-blown sand grains; the upper silty unit records shoaling onto tidal flats and beaches.

Popo Agie Member. Variously classified as a member and as a formation, the Popo Agie takes its name from a river near Lander, west-central Wyoming. The word is a Crow Indian name for “head water” and is pronounced Po-pó-zha (Urbanek 1988). As formalized by Love (1939), the Popo Agie is about 300 feet (90 m) thick and comprises claystone and shale of various colors interbedded with white to red sandstone, gypsum, limestone, and conglomerate. The Popo Agie has yielded a variety of reptile remains (Lucas 1994), lungfish, bivalves, coprolites, and fossil plants. The latter include a large Equisetum, cycadophytes, and wood fragments. We did not closely inspect any outcrops of Popo Agie and therefore, can add nothing to what has been published. Picard (1993) concluded that the Popo Agie was deposited in a lake that covered more than 70,000 square miles (180,000 square km), more than twice the size of Lake Superior.

Gypsum Spring Formation

Originally named as a member of the Chugwater (Love 1939), the Gypsum Spring now is generally considered a formation in its own right, and assigned Jurassic age. include bedded gypsum, limestone (some of which is algal), red shale or claystone, and minor sandstone. Thickness varies from a featheredge to more than 50 m. We observed in passing outcrops of Gypsum Spring overlying Spearfish Formation in the northwestern Black Hills (Fig. 9). The unit also is present in central Wyoming, but we did not inspect any outcrops closely. A moderately diverse invertebrate fauna indicates shallow-marine deposition and early age (Picard 1993).

Sundance Formation

The Middle Jurassic Sundance Formation unconformably overlies older units. Briefly, the Sundance consists of shale, siltstone, sandstone, and impure limestone representing a variety of shallow marine environments. Colors are largely gray and greenish gray, forming a “gray wall” above red Spearfish and Chugwater outcrops (Fig. 15). In places, the basal Canyon Springs Sandstone Member fills channels eroded into Triassic strata. This was probably the case in the outcrops we viewed near Pilger Mountain Road (Figs. 10 and 11).

20 CONCLUSIONS

Although assigned different stratigraphic names, Permo-Triassic redbeds of the Black Hills and eastern Wyoming are basically similar. Silty mudstone, siltstone, and very fine sandstone predominate. Substantial deposits of bedded gypsum and (in the subsurface) halite occur in the Permian portion (lower Spearfish and Goose Egg Formations. Above this level, the sediments overall coarsen upward. Two units of thinly layered limestone and dolomite, the Minnekahta and Alcova, have great lateral continuity. This region was tectonically stable and virtually flat throughout Permo-Triassic time. Deposition fluctuated between shallow subtidal, intertidal, and paralic or supratidal regimes. Climate was hot, arid, and windy. Dust storms carried vast amounts of silt into the shallow sea, where it was redistributed by waves and currents. Under such hostile conditions, very little life braved the region. A few reptiles and fishes, along with rare mollusks, have turned up in the redbeds. Assigning carbonate units to marine transgressions is easy, but may be incorrect. The sea remained as shallow during Minnekahta and Alcova time as during redbed deposition, as evident by mud cracks and stromatolites. Some factor – most likely a climate change – temporarily shut off the dust factory. Wetter interludes might have fostered vegetation fringing the seaway, stabilizing the soil and clearing the waters. Middle and Upper Permian rocks of this study area are broadly similar to those of north-central and west Texas. During Early Permian time, Texas redbeds (Wichita and Clear Fork) supported abundant flora and fauna. These beds gave way to interbedded gypsum, shallow marine limestone and dolomite, and loess-like silt layers of the . A solitary occurrence of terrestrial plants in the Blaine occupies a small channel lateral to a dolomite bed (DiMichele et al. 2004). Although dolomite requires high evaporation rates, it represents the wettest part of the climate cycle compared to halite, gypsum, and wind-blown silt.

ACKNOWLEDGMENTS

Scott Elrick prepared Figures 1 and 2 and helped lay out the illustrations in this report. For that, we owe him a place on our next field trip.

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21 Darton, N.H., 1899, Jurassic formations of the Black Hills of South Dakota: Geological Society of America Bulletin, v. 10, p. 383-396. Darton, N.H., 1901, Preliminary description of the geology and water resources of the southern half of the Black Hills and adjoining regions in South Dakota and Wyoming: U.S. Geological Survey, Annual Report 21, part 4, p. 489-599. Darton, N.H., 1904, Comparison of the stratigraphy of the Black Hills, , and Rocky Mountain : Geological Society of America, v. 15, p. 379- 448. Darton, N.H. (compiler), 1951, Geologic map of South Dakota: U.S. Geological Survey, 1 sheet, scale 1:500,000. DiMichele, W.A., Hook, R.W., Nelson, W.J., and Chaney, D.S., 2004, An unusual Middle Permian flora from the Blaine Formation (Pease River Group: Leonardian- Guadalupian Series) of King County, west Texas: Journal of Paleontology, v. 78, no. 4, p. 765-982. Gott, G.B. and Schnabel, R.W., 1963, Geology of the Edgemont NE quadrangle, Fall River and Custer Counties, South Dakota: U.S. Geological Survey, Bulletin 1063- E, p. 127-190 and 5 plates, map scale 1:24,000. High, L.R., Jr. and Picard, M.D., 1969, Stratigraphic relations within upper Chugwater Group, Wyoming: American Association of Petroleum Geologists Bulletin, v. 53, no. 5, p. 1091-1104. Lane, Donald W., 1973, The Phosphoria and Goose Egg Formations in Wyoming: Geological Survey of Wyoming, Preliminary Report No. 12, 24 p. Lee, W.T., 1927, Correlation of geologic formations between east-central Colorado, central Wyoming, and southern Montana: U.S. Geological Survey, Professional Paper 149, 80 p. Love, John D., 1939, Geology along the southern margin of the Abasaroka Range, Wyoming: Geological Society of America, Special Paper 20, 134 p. Love, John D. and Christianson, A.C. (compliers), 1985, Geologic map of Wyoming: U.S. Geological Survey, 3 sheets, scale 1:500,000. Lucas, Spencer G., 1994, The beginning of the age of in Wyoming: Wyoming Geological Society Guidebook, 44th Annual Field Conference, p. 105-113. Manahl, K.A., 1985, Geologic map of Shell quadrangle, Wyoming: Geological Survey of Wyoming, Map Series 17, 1 sheet, scale 1:24,000. Maughan, Edwin K., 1967, Paleotectonic investigations of the Permian System in the United States, Chapter G, Eastern Wyoming, , and the Dakotas: U.S. Geological Survey, professional Paper 515-G, p. 127-152. Newell, N.D. and Kummel, B., 1942, Lower Eo-Triassic stratigraphy, western Wyoming and southeast Idaho: Geological Society of America Bulletin, v. 53, p. 937-996 and 3 plates. Picard, M. Dane, 1966, Petrography of Red Peak Member, Chugwater Formation (Triassic), west-central Wyoming: Journal of Sedimentary Petrology, v. 36, no. 4, p. 904-926. Picard, M. Dane, 1993, The early : in Snoke, A.W., Steidtmann, J.R., and Roberts, S.M., editors, : Geological Survey of Wyoming Memoir No. 5, p. 210-248. Post, Edwin F., 1967, Geology of the Cascade Springs quadrangle, Fall River County, South Dakota: U.S. Geological Survey, Bulletin 1063-L, p. 443-504 and 3 plates, map scale 1:24,000. Rich, Fredrick J. (editor), 1981, Geology of the Black Hills, South Dakota and Wyoming: Geological Society of America, Field Trip Guidebook, Rocky Mountain Section, 1981 Annual Meeting, 292 p.

22 Robinson, C.S., Mapel, W.J., and Bergendahl, M.H., 1964, Stratigraphy and structure of the northern and western flanks of the Black Hills uplift, Wyoming, Montana, and South Dakota: U.S. Geological Survey, Professional Paper 404, 134 p. and 5 plates, map scale 1:96,000. Sabel, J.M., 1981, The sedimentology of the Spearfish Formation: M.S. thesis, South Dakota School of Mines and Technology, Rapid City, 70 p. and chart. Smith, W.S.T., 1903, Description of the Hartville quadrangle, Wyoming: U.S. Geological Survey, Geologic Atlas of the United States, Hartville Folio, no. 91, 6 p. Snoke, A.W., Steidtmann, J.R., and Roberts, S.M. (editors), 1995, Geology of Wyoming: Geological Survey of Wyoming, Memoir No. 5, two volumes of 937 pages and 10 maps and charts. Tohill, B. and Picard, M.D., 1966, Stratigraphy and petrology of Crow Mountain Sandstone member (Triassic), Chugwater Formation, northwestern Wyoming: American Association of Petroleum Geologists Bulletin, v. 50, no. 12, p. 2547- 2565. Urbanek, Mae, 1988, Wyoming place names: Mountain Press Publishing Co., Missoula, Montana, 233 p. Wolcott, Don E., 1967, Geology of the Hot Springs quadrangle, Fall River and Custer Counties, South Dakota: U.S. Geological Survey, Bulletin 1063-K, p. 427-442 and 2 plates, map scale 1:24,000. Wolcott, D.E., Bowles, C.G., Brobst, D.A., and Post, E.V., 1962, Geologic and structure map of the Minnekahta NE quadrangle, Fall River and Custer Counties, South Dakota: U.S. Geological Survey, Mineral Investigations Field Studies Map MF- 242, 1 sheet, scale 1:24,000.

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