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| Chesapeake Formation (Miocene) | |
|---|---|
| Name | Chesapeake Formation |
| Period | Miocene |
| Type | Formation |
| Region | Atlantic Coastal Plain |
| Country | United States |
| Namedfor | Chesapeake Bay |
| Subunits | Calvert Formation; Yorktown Formation |
Chesapeake Formation (Miocene)
The Chesapeake Formation is a Miocene stratigraphic unit of the Atlantic Coastal Plain of the United States noted for its rich assemblages of marine fossils, distinctive glauconitic sands, and economically significant phosphate deposits. Exposures associated with the Chesapeake Bay impact crater and coastal terranes have made the unit a focal point for studies by institutions such as the Smithsonian Institution, United States Geological Survey, and numerous universities along the Mid-Atlantic, drawing comparisons with contemporaneous units in the Mid-Atlantic United States and southern New England.
The name of the unit derives from Chesapeake Bay, reflecting exposures and borehole records obtained during regional mapping by the United States Geological Survey and state geological surveys. Historical usage of the term has been linked with work by geologists from Johns Hopkins University, University of Maryland, and the Virginia Geological Survey. Nomenclatural treatment has varied: early 20th-century authors associated the unit with the Calvert Formation and Yorktown Formation nomenclature used by the Geological Society of America-affiliated stratigraphers, while later syntheses by researchers at the Virginia Museum of Natural History and the Maryland Geological Survey emphasized a composite “Chesapeake” designation tied to lithofacies related to the Chesapeake Bay impact crater.
Stratigraphically the Chesapeake unit occupies parts of the middle to late Miocene succession within the Atlantic Coastal Plain, interfingering with units mapped as the Calvert Formation, Choptank Formation, and Yorktown Formation. Lithologies include glauconitic greensands, phosphatic shelly sands, clayey silts, and locally cemented phosphorite horizons observed in cores drilled by the USGS Coastal and Marine Geology Program and industry partners such as Anadarko Petroleum. The presence of indurated phosphate nodules, shell hash, and shell beds parallels descriptions from type sections documented by researchers affiliated with Williams College and the College of William & Mary.
Biostratigraphic control from microfossils—especially benthic and planktonic foraminifera—and macrofossils from collections at the Smithsonian Institution and university museums constrain the Chesapeake unit to the Miocene Epoch, principally the Langhian to Serravallian substages as defined by the International Commission on Stratigraphy. Correlations have been made with the Calvert Formation of Maryland and Virginia and with equivalent phosphatic units along the Southeastern United States Atlantic margin. Isotope stratigraphy and magnetostratigraphic tie points developed in collaboration with the Scripps Institution of Oceanography and the Paleontological Society support mid-Miocene ages for much of the succession.
Sedimentological and paleontological evidence indicates deposition in inner to middle continental shelf settings influenced by episodic transgressions linked to eustatic events recognized by the International Union for Quaternary Research community. Facies analyses conducted by teams from Duke University and Rutgers University interpret the glauconitic sands and phosphatic concentration as products of low sedimentation rates, winnowing, and periodic phosphogenesis associated with oxygen-minimum conditions similar to models proposed by researchers at the Woods Hole Oceanographic Institution. The spatial distribution of facies around the Chesapeake Bay impact crater has been used to reconstruct paleogeography by investigators from the Virginia Institute of Marine Science and the Maryland Geological Survey.
The Chesapeake unit is renowned for abundant vertebrate and invertebrate fossils curated by the Smithsonian Institution National Museum of Natural History, Calvert Marine Museum, and university collections. Marine mammals including pinnipeds and cetaceans, elasmobranchs such as Carcharodon-like odontaspidids, bony fishes, and diverse mollusks (bivalves and gastropods) have been described in monographs published by the Paleontological Research Institution and researchers at Georgetown University. Microfossils—planktonic foraminifera and ostracodes—provide paleoenvironmental data used by paleontologists affiliated with the American Museum of Natural History and the Natural History Museum, London for broader Miocene comparisons. Iconic finds from Chesapeake-area borings contributed to portfolios assembled by the Virginia Museum of Natural History and inspired popular exhibits at the National Museum of Natural History.
Phosphate-rich horizons within the Chesapeake unit have been of economic interest and have been examined by commodity geologists at the United States Geological Survey and companies such as International Minerals. Phosphorite nodules, glauconite minerals, and heavy-mineral assemblages including zircon and monazite have been documented in sediment samples archived by the Geological Society of America and regional state surveys. Studies on resource potential and environmental considerations have involved collaboration with the Environmental Protection Agency and regional planning agencies.
Investigation of the Chesapeake unit dates to 19th- and early 20th-century surveys by the United States Geological Survey and academic teams from Johns Hopkins University and the College of William & Mary. Systematic paleontological collecting and drilling accelerated during mid-20th-century coastal engineering and naval construction projects involving the United States Army Corps of Engineers, which yielded the borehole data later synthesized by researchers at the Virginia Department of Environmental Quality and the Maryland Geological Survey. Type localities and reference sections are commonly associated with coastal exposures and boreholes around Chesapeake Bay and its tributaries, with stratigraphic frameworks refined through work published by the Geological Society of America and regional museum monographs.
Category:Geologic formations of the United States Category:Miocene geology Category:Atlantic Coastal Plain