The Sundance Formation is a western North American sequence of Middle Jurassic to Upper Jurassic age Dating from the

Bathonian In the geologic timescale the Bathonian is an age and stage of the Middle Jurassic. It lasted from approximately 168.3 Ma to around 166.1 Ma (million years ago). The Bathonian Age succeeds the Bajocian Age and precedes the Callovian Age. Str ...

to the Oxfordian, around 168-157 Ma, It is up to 100 metres thick and consists of marine shale, sandy shale,

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

, and

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

deposited in the

Sundance Sea The Sundance Sea was an epeiric sea that existed in North America during the mid-to-late Jurassic Period of the Mesozoic Era. It was an arm of what is now the Arctic Ocean, and extended through what is now western Canada into the central west ...

, an inland sea that covered large parts of western North America during the Middle and early Late Jurassic.

Geology

The Sundance Formation underlies the western North American Morrison Formation, the most fertile source of

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

fossils in the Americas, and is separated by a

disconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

from the underlying Middle Jurassic

Gypsum Springs Formation The Gypsum Springs Formation is a stratigraphical unit of Middle Jurassic age in the Williston Basin. It takes the name from Gypsum Springs in Wyoming, and was first described in outcrop in Freemont County by J.D. Love in 1939.Love, J.D., 1939. ...

Fossils

The Sundance Formation is known for fossils of an extinct species of marine cephalopod, the

belemnite Belemnitida (or the belemnite) is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone. The parts are, from the arms-most ...

'' Pachyteuthis densus'', as well as several extinct species of

oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not ...

, including ''Deltoideum'', ''Liostrea'', and ''Gryphaea nebrascensis''. Fossil dinosaur 'footprints' on an ancient ocean shoreline are preserved in the formation and protected at the Red Gulch Dinosaur Tracksite, located in the Bureau of Land Management Red Gulch/Alkali National Back Country Byway, near Shell in

Big Horn County, Wyoming Big Horn County is a county in the U.S. state of Wyoming. As of the 2020 United States Census, the population was 11,521. The county seat is Basin. Its north boundary abuts the south boundary of Montana. History Big Horn County was created by ...

.BLM−Bureau of Land Management, Wyoming Office: "Red Gulch Dinosaur Tracksite" website
info, maps, photo gallery, accessed 8.21.2015

Paleobiota

Vertebrates

Invertebrates

Fish

References

Jurassic geology of South Dakota Jurassic geology of Wyoming Jurassic System of North America Upper Jurassic Series Geology of the Rocky Mountains Geologic formations of Montana Geologic formations of Wyoming Paleontology in Wyoming {{Wyoming-geologic-formation-stub Middle Jurassic Series Bathonian Stage Callovian Stage Oxfordian Stage