ABSTRACT -- During the Middle and Late Jurassic, the Twin Creek Limestone and Sundance Formation were

laid down in open shelf, lagoonal, and littoral environments. Local thinning and pinching out of strata and abrupt

vertical changes in lithology and biota within these formations are the result of deposition under variable shallow

water conditions and, contrary to previous interpretations, do not represent regional unconformities.

In Bajocian times, three bivalve associations existed. The Cam tonectes stygius fauna inhabited the outer

shelf in westernmost Wyoming in an area of rapid lime-mud deposit*k and Rich Members of the Twin

Creek Limestone). The Pleuromya subcompressa fauna lived nearer to shore and the Trigonia americana fauna

fauna occupied lagoons along the eastern shoreline, sites of Gypsum Spring ~ormationdepositlon.

During Early and Middle CalIovian, the Gryphaea nebrascensis fauna was restricted to an area of normal

marine conditions (attested by the abundance an- of foraminifera and ostracods) northwest of the Sheridan

Arch, which acted as a marine barrier on the Wyoming shelf. The Meleaginella fauna was restricted to

brackish lagoons southeast of the arch (attested by the absence of foraminifera and abundance of ostracods). During

this time, the Leeds Creek Member of the Twin Creek Limestone, the "Lower Sundance, and Stockade Beaver

Shale were deposited northwest of the arch, and the Canyon Springs Sandstone, Stockade Beaver Shale, Hulett

Sandstone, and Lak redbeds were deposited southeast of the arch.

In Late Callovian, the Camptonectes bellistriatus fauna, ostracods, and foraminifera ranged across the

entire shelf. The water was deeper and the abundance of arenaceous formainifera suggests that the water was

cooler than earlier in the Jurassic. During this time, the Pine Butte, Redwater Shale, and Windy Hill Sandstone

Members of the Sundance Formation were deposited in central and eastern Wyoming; the "Upper Sundance" in

northern Wyoming; and the Preuss and Stump Sandstones in western Wyoming.

Ordordian regression ended Jurassic marine deposition in Wyoming and South Dakota.

University of Michigan Deep Blue - digital version.

Publisher: University of Michigan

Year of Publication: 1973

Location: Ann Arbor, MI

Volume Number: 2

# of Pages: 54