Sundance Formation

Members: In ascending order: Canyon Springs Sandstone, Stockade Beaver Shale, Hulett Sandstone, Lak, Pine Butte, Redwater Shale, and Windy Hill Sandstone (IMLAY 1980).

Chronostratigraphic age: Middle Bathonian to Middle Oxfordian (IMLAY 1980).

Geographic distribution: Western South Dakota, southeastern, central and northwestern Wyoming.

Nomenclatorial history: The Sundance Formation was named by DARTON (1899) after outcrops in the vicinity of the town Sundance in Crook County in northeastern Wyoming.

He assigned all of the marine strata between the red beds of the Triassic Spearfish Formation and Late Jurassic continental deposits to the Sundance Formation. Since DARTON (1899) did not assign a type section for the formation, IMLAY (1947) defined a reference section north of the town of Spearfish/Lawrence County in South Dakota. He further defined the five formal members: Canyon Springs Sandstone, Stockade Beaver Shale, Hulett Sandstone, Lak, and Redwater Shale. PIPIRINGOS (1968) described the formation in southeastern Wyoming and added two members: the Pine Butte between the Lak and Redwater Shale and the Windy Hill Sandstone between the Redwater Shale and the Morrison Formation (see Figure 2-3).

Westward from the Black Hills the members lose their distinct character. In the subsurface of the Powder River Basin the members are not recognizable. In the Bighorn Basin the informal stratigraphic subdivision by NEELY (1937) into the “lower” and the “upper”

Sundance Formation is still in use by geologists. IMLAY (1956) divided the “lower”

Sundance into three lithologic units: a basal, a middle and an upper member, coeval with the stratigraphic units in the Black Hills area.

Measured sections: Black Hills: Minnekatha (MIN), Elk Mountain (EM), Spearfish (SF), Thompson Ranch (TR), Hulett (HU), T cross T Ranch (T-T), Stockade Beaver Creek (SBC). Bighorn Basin: Red Lane (RL), Red Rim Ranch (RR), Hampton Ranch (HR), Hyattville (HY). Central and southeastern Wyoming: Alcova Reservoir (AR), Freezeout Hills (FH), Squaw Women Creek (SWC).

Thickness: 63 m at section Squaw Women Creek (SWC) to 101 m at section Red Lane (RL).

Lithology: The Canyon Springs Sandstone Member extends as a lithologic unit westward from the Black Hills/South Dakota into the Bighorn Basin/Wyoming and from north-central Colorado to the Sheep Mountain southeast of Lander/Wyoming. The member ranges in thickness from 0 to 28 m. It consists mainly of large-scale cross-bedded, ripple marked, yellowish-brown to white or salmon colored, partly oolitic, fine- to medium-grained sandstones (IMLAY 1980). In the Bighorn Basin, the member is 0,5 to 8,0 m thick and consists of fine- to medium-grained sandstones intercalated with occasional limestone beds. Near the top of the Canyon Springs Sandstone Member a previously unknown dinosaur tracksite was discovered in 1997 along the west flank of the Bighorn Mountains.

The presence of dinosaur tracks in this stratigraphic interval can be traced in outcrops from 25 km north of Greybull/Wyoming approximately 100 km southward, toward the town of Ten Sleep/Wyoming (KVALE et al. 2001). According to KVALE et al. (2001), this tracksite is one of two most extensive Middle Jurassic dinosaur tracksites currently known in the United States. The member was named after an outcrop northwest of Horton in the Black Hills area of Wyoming.

The Stockade Beaver Shale Member in the Black Hills consists of greenish-gray, olive-green and gray, calcareous, fissile, silty shales. At the sections Stockade Beaver Creek (SBC), Elk Mountain (EM) and Minnekatha (MIN), limestone nodules were found

throughout the member. The thickness ranges between 12 and 25 m at the measured sections. In central and southeastern Wyoming, the lithology of the member is comparable to the conditions in the Black Hills, while the thickness ranges between 10 and 30 m. In the Bighorn Basin and the Powder River Basin, the member consists of soft, olive-green, gray and greenish-gray, calcareous shale with silty to sandy interbeds. The most distinctive feature is the local abundance of Gryphea sp. shells in the member. According to IMLAY (1956), the occurrence of this genus in the Stockade Beaver Shale Member is uncommon in the Black Hills. IMLAY (1947) named the member after outcrops on the west side of Stockade Beaver Creek (see Figure 2-9), northeast of Newcastle in eastern Wyoming.

Figure 2-9: Sundance Formation at section Stockade Beaver Creek (SBC). This is the type section of the Stockade Beaver Shale Member. (1) Spearfish Formation, (2) Gypsum Spring Formation, (3) Stockade Beaver Shale Member, (4) Hulett Sandstone Member (5) Lak Member, (6) Redwater Shale Member. Note that the Canyon Springs Sandstone Member is absent at this location.

The Hulett Sandstone Member in the field area consists of light-gray, yellowish-brown, light greenish-gray, calcareous, locally fossiliferous, thin- to thick-bedded, fine- to medium-grained sandstones. Locally, the sandstone is slightly glauconitic or/and oolitic and the degree of bioturbation (burrows, tracks, trails) is high. Further, a wide range of sediment structures like ripple lamination, cross-bedding and planar stratification can be observed.

Often thin (mud drape-size to 10 cm) layers of gray to greenish-gray, soft shale are interbedded. In the Black Hills the thickness ranges from 10 to 25 m, in the Bighorn Basin and western Powder River Basin from 5 to 35 m, in central and south-eastern Wyoming from 10 to 20 m. The lower and the upper contact of the Hulett Sandstone Member are conformable and gradational. The member was named by IMLAY (1947) for outcrops on the north side of Bush Canyon, north of the town Hulett in northeastern Wyoming.

The Lak Member is composed of orange-red or maroon, massive siltstone or fine-grained sandstone. Locally, gypsum beds near the base of the unit are present as at section Alcova Reservoir (AR) (see Figure 2-10). Fossils are not known from this unit. Sediment structures, if observable, are very poorly developed. The thickness ranges between 10 and 28 m. The member occurs in the Black Hills and in central southeastern Wyoming.

According to IMLAY (1980), the member pinches out in the Powder River Basin and north of Lander in the Wind River Basin. It is absent in the Bighorn Basin, Bighorn Mountains, northern Wind River Basin, and northwestern Wind River Mountains. If the absence of the member in parts of Wyoming is related to erosion during origin of the J-4 unconformity or to non-deposition can not be answered. PETERSON (1954) reported a lateral gradation into the oolitic sandstones and limestones in the upper part of the “lower” Sundance Formation in the western Powder River Basin/southeastern Bighorn Mountains near the town of Kaycee/Wyoming. The member, also known as “Sundance red”, was named by IMLAY (1947) after the Lak reservoir, close to the L.A.K. Ranch northeast of Newcastle/Wyoming.

Figure 2-10: Lak Member at section Alcova Reservoir (AR). At this location a gypsum bed is exposed at the base of the Lak Member and overlies the greenish-gray beds of the Hulett Sandstone Member. Length of Jacob stick 1,5 m.

The Pine Butte Member consists of greenish-gray, light-green to gray, thin-bedded, calcareous, glauconitic, fine-grained sandstone interbedded with thin shale and siltstone layers. Sediment structures are faint planar bedding and ripple lamination. Otherwise the member lacks primary sediment structures and bioturbation. JOHNSON (1992) reported the occurrence of furrowed trails. Often bivalve fragments and crinoids are found. The

thickness ranges between 0 and 15 m. The Pine Butte Member is sharply overlain by the Redwater Shale Member and is locally truncated by the latter. IMLAY (1980) placed the J-4 unconformity at this contact. The member was introduced by PIPIRINGOS (1968) and named for outcrops in southern Wyoming.

The Redwater Shale Member consists of greenish-gray, olive-green to gray, calcareous shales and interbedded thin, coquinoid siltstone, sandstone and limestone layers. The most striking feature is the abundance of worn belemnites (Pachyteuthis densus) and fragments of the oyster Camptonectes bellistriatus, which is the most common bivalve in the coquinas (WRIGHT 1973). In southeastern Wyoming, the member shows varying amounts of limestone nodules and four lithologic units of alternating siltstone and shale layers can be distinguished (PIPIRINGOS 1968, ANDERSON 1978; 1979, IMLAY 1980).

In this area, the thickness ranges between 25 and 37 m. In the Black Hills, the lithologic character of the member resembles the previously described conditions. In the Bighorn Basin, Bighorn Mountains and in central Wyoming, above a sharp contact, the upper portion of the member is composed of an impure, light-green, greenish-gray to brownish-gray, cliff-forming, glauconitic, calcareous, fine- to medium-grained sandstone. This sandstone suite is equivalent to the upper two siltstone-shale units of the Redwater Shale Member in southeastern Wyoming (LOVE et al. 1945, PIPIRINGOS 1968, WRIGHT 1973, IMLAY 1980). The thickness of the Redwater Shale Member ranges in the Black Hills between 32 and 55 m, in central and southeastern Wyoming between 27 and 42 m and in the Bighorn Basin between 50 and 80 m. In some areas the member truncates underlying strata (JOHNSON 1992). The contact to the overlying Windy Hill Sandstone Member is sharp, generally unconformable and related to the J-5 unconformity (PIPIRINGOS 1968, PIPIRINGOS & O’ SULLIVAN 1978, IMLAY 1980). If the member is overlain by the Morrison Formation the contact seems to be gradational and conformable (JOHNSON 1992). IMLAY (1947) named the member after outcrops near Redwater Creek, northwest of Spearfish, South Dakota.

The Windy Hill Sandstone Member consists of calcareous, yellowish-brown, light-brown, gray fine- to medium-grained sandstones. Bedding planes are often rippled. IMLAY (1980) reported specimen of Ostrea sp. and Camptonectes sp. The member ranges in the Black Hills in thickness between 2 and 5 m, in southeastern Wyoming between 2 and 10 m. The lower contact of the Windy Hill Sandstone Member is marked by the J-5 unconformity (PIPIRINGOS & O’ SULLIVAN 1978, IMLAY 1980), the upper contact with the Morrison Formation is reported to be conformable and locally the two units intertongue (IMLAY 1980, JOHNSON 1992). The member is not known in northwestern Wyoming.

PIPIRINGOS (1968) named the member after outcrops in the Windy Hills in the Freezeout Hills area in southeastern Wyoming (see Figure 2-11).

The major unconformity (J-5) that separates the Windy Hill from the underlying Redwater Shale Member of the Sundance Formation and the intense interfingering with the Morrison Formation indicate that the member is genetically much closer related to the Morrison Formation than to the Sundance Formation. This relation caused PETERSON, F. (1994)

and CURRIE (1998; 2002) to include the Windy Hill interval as lower member into the Morrison Formation. Under genetic aspects this approach is logic and comprehensive.

However, a formal Jurassic standard stratigraphy comparable to the European standard stratigraphy is lacking for the western United States. A widely accepted standard is published by IMLAY (1980). To avoid further irritations in the already confusing stratigraphic Jurassic nomenclature the stratigraphic standard of IMLAY (1980) is followed in this study and the Windy Hill interval for formal reasons is included as member in the Sundance Formation.

Figure 2-11: Sundance Formation at section Freezeout Hills (FH). (1) Nugget Sandstone, (2) Canyon Springs Member, (3) Stockade Beaver Shale Member, (3) Hulett Sandstone, (4) Pine Butte Member, (5) Lak Member, (6) Redwater Shale Member, (7) Windy Hill Sandstone Member.

Biostratigraphic range: Comprehensive investigations of the paleontology and stratigraphy providing biostratigraphic information of the Sundance Formation are published by WRIGHT (1973; 1974), IMLAY (1947; 1954; 1956; 1957; 1980), PETERSON (1954; 1957a; 1958), CAPARCO (1989), KVALE et al. (2001).

Stratigraphic contacts: The contact of the Sundance Formation with the underlying formations is unconformable and is represented by the J-2 and J-2a unconformities. The upper contact with the Morrison Formation is considered to be the erosional surface of the J-5 unconformity. This locally occurring surface lies at the base of the Windy Hill Sandstone Member (PIPIRINGOS & O’ SULLIVAN 1978). In the Bighorn Basin and Powder River Basin, where the Windy Hill Sandstone Member is absent (IMLAY 1980), evidence for the unconformity is weak (JOHNSON 1992). According to PIPIRINGOS &

O’ SULLIVAN (1978), thickness variations of the Swift Formation between 1 to 75 m might indicate the development of an erosional relief. Other workers like IMLAY (1980) and UHLIR et al. (1988) doubt the existence of an unconformable contact at this stratigraphic level. Therefore, the nature of this upper contact is still discussed.