The Riepetown Formation is a geological formation of Permian age located primarily in White Pine County, eastern Nevada, United States. It is a distinctive unit within the regional stratigraphy, characterized predominantly by carbonate rocks and chert, indicating a marine depositional environment.
Overview The Riepetown Formation constitutes a significant part of the Permian stratigraphic sequence in the eastern Great Basin. It generally overlies the Pennsylvanian-Permian Ely Limestone and underlies the Permian Arcturus Formation. Its outcrops are notably present in areas such as the Egan Range and near the town of Ely. Geologists use the Riepetown Formation to understand the paleogeography, depositional history, and tectonic evolution of the western North American continental shelf during the Permian period. Its fossil content aids in biostratigraphic correlation and dating.
Etymology/Origin The formation is named after Riepetown, a former mining settlement located approximately 5 miles (8 km) west of Ely in White Pine County, Nevada, where the formation is well-exposed and was first formally described. The Riepetown Formation was first defined and described by James Gilluly in 1937 in his U.S. Geological Survey Professional Paper 189-A, "Geology and ore deposits of the Shoshone Mountains, Nevada," which covered the geology of the Ruth quadrangle.
Characteristics The Riepetown Formation is typically composed of dark-gray to black, cherty limestone and dolomitic limestone, often containing abundant dark chert nodules or beds. Some sections may include minor interbeds of shale or siltstone. The chert is often distinctive and can comprise a substantial portion of the rock unit. The formation's thickness can vary but is generally in the range of 300 to 700 feet (90 to 215 meters). Fossil content within the Riepetown Formation commonly includes marine invertebrates such as brachiopods, fusulinids (a type of foraminifer), crinoid columnals, bryozoans, and corals. These fossils indicate deposition in an open marine shelf environment, likely under normal marine salinity conditions. The presence of dark, cherty carbonates often suggests deposition in somewhat deeper or restricted shelf settings where silica-rich fluids or biological activity contributed to chert formation.
Related Topics
- Ely Limestone: The underlying formation, primarily composed of massive marine limestones.
- Arcturus Formation: The overlying formation, characterized by interbedded limestones, siltstones, and sandstones, reflecting a transition to a more detrital-influenced shelf environment.
- Permian Period: The geological time period during which the Riepetown Formation was deposited, approximately 298.9 to 251.9 million years ago. The formation is specifically considered Leonardian in age (early Middle Permian).
- Great Basin: The broader physiographic and geological province where the Riepetown Formation is found, part of the Cordilleran miogeosyncline/shelf sequence.
- Paleozoic Era: The broader geological era to which the Permian period belongs.