| Spearfish Formation Stratigraphic range: Permian–Triassic | |
|---|---|
 | |
| Type | Geological formation |
| Underlies | Sundance Formation, Gypsum Spring Formation, Piper Formation |
| Overlies | Minnekahta Limestone |
| Thickness | 350–800 feet (110–240 m) [1] |
| Lithology | |
| Primary | Siltstone, sandstone |
| Other | Shale |
| Location | |
| Region | Black Hills and nearby states |
| Country | United States |
| Type section | |
| Named for | Spearfish, South Dakota |
| Named by | Darton, 1899 |
The Spearfish Formation is a geologic formation, originally described from the Black Hills region of South Dakota, United States, but also recognised in North Dakota, Wyoming, Montana and Nebraska. [2] It is a heterogeneous red bed formation, commonly with siltstone and gypsum low in the formation and sandstone and shale higher up. [3] Other rock types include claystone, conglomerate, dolomite, and oil shale. [4] It is typically regarded as Permian–Triassic in age, [4] although its original description [5] included Jurassic rocks. [6]
The Black Hills are a small and isolated mountain range rising from the Great Plains of North America in western South Dakota and extending into Wyoming, United States. Black Elk Peak, which rises to 7,244 feet (2,208 m), is the range's highest summit. The Black Hills encompass the Black Hills National Forest. The name "Black Hills" is a translation of the Lakota Pahá Sápa. The hills were so-called because of their dark appearance from a distance, as they were covered in trees.
South Dakota is a U.S. state in the Midwestern region of the United States. It is named after the Lakota and Dakota Sioux Native American tribes, who compose a large portion of the population and historically dominated the territory. South Dakota is the seventeenth largest by area, but the fifth smallest by population and the 5th least densely populated of the 50 United States. As the southern part of the former Dakota Territory, South Dakota became a state on November 2, 1889, simultaneously with North Dakota. Pierre is the state capital and Sioux Falls, with a population of about 183,200, is South Dakota's largest city.
The United States of America (USA), commonly known as the United States or America, is a country composed of 50 states, a federal district, five major self-governing territories, and various possessions. At 3.8 million square miles, the United States is the world's third or fourth largest country by total area and is slightly smaller than the entire continent of Europe's 3.9 million square miles. With a population of over 327 million people, the U.S. is the third most populous country. The capital is Washington, D.C., and the largest city by population is New York City. Forty-eight states and the capital's federal district are contiguous in North America between Canada and Mexico. The State of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west. The State of Hawaii is an archipelago in the mid-Pacific Ocean. The U.S. territories are scattered about the Pacific Ocean and the Caribbean Sea, stretching across nine official time zones. The extremely diverse geography, climate, and wildlife of the United States make it one of the world's 17 megadiverse countries.
The Spearfish Formation is interpreted as representing shallow marine to coastal terrestrial deposition, like the modern Persian Gulf. [7] Depositional environments may have included restricted marine bodies and ephemeral lakes (gypsum), hypersaline waters (limestone), hypersaline microbial mats (oil shale), and sabkhas (dolomite). The marine waters of the shallow continental sea retreated during the deposition of the formation, reflected in a change from dominantly nearshore marine to coastal terrestrial deposition over time. [4]
The Persian Gulf, is a mediterranean sea in Western Asia. The body of water is an extension of the Indian Ocean through the Strait of Hormuz and lies between Iran to the northeast and the Arabian Peninsula to the southwest. The Shatt al-Arab river delta forms the northwest shoreline.
Sabkha is a phonetic translation of the Arabic word used to describe any form of salt flat. Geographically, sabkhas can be sub-divided into two broad categories.
The Spearfish Formation is sparsely fossiliferous. So far, only stromatolites, casts of bivalves, and trace fossils have been found. [4]
Stromatolites or stromatoliths are layered mounds, columns, and sheet-like sedimentary rocks that were originally formed by the growth of layer upon layer of cyanobacteria, a single-celled photosynthesizing microbe. Fossilized stromatolites provide records of ancient life on Earth. Lichen stromatolites are a proposed mechanism of formation of some kinds of layered rock structure that are formed above water, where rock meets air, by repeated colonization of the rock by endolithic lichens.
A trace fossil, also ichnofossil, is a geological record of biological activity. Ichnology is the study of such traces, and is the work of ichnologists. Trace fossils may consist of impressions made on or in the substrate by an organism: for example, burrows, borings (bioerosion), urolites, footprints and feeding marks, and root cavities. The term in its broadest sense also includes the remains of other organic material produced by an organism — for example coprolites or chemical markers — or sedimentological structures produced by biological means - for example, stromatolites. Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or mineralization.
