St. Marys Formation

St. Marys Formation
Name	St. Marys Formation
Type	Formation
Period	Pleistocene
Lithology	Sand, clay, marl
Namedfor	St. Marys River
Region	Mid-Atlantic United States
Country	United States
Unitof	Chesapeake Group
Underlies	Talbot Formation
Overlies	Choptank Formation

St. Marys Formation is a late Pliocene to early Pleistocene marine sedimentary unit of the Atlantic Coastal Plain known for its fossiliferous sands and clays in the Mid-Atlantic United States. The formation features siliciclastic deposits that record transgressive–regressive cycles tied to global Pleistocene sea-level fluctuations and regional tectonics associated with the passive margin of the eastern North American Plate. It is a key stratigraphic marker within the Chesapeake Group that has informed interpretations of paleoceanography, paleoclimate, and faunal turnover across the Neogene–Quaternary transition.

Introduction

The St. Marys Formation crops out and is subsurface along the Delaware River, Chesapeake Bay, and inland along the Potomac River drainage, connecting notable outcrops near Calvert Cliffs, St. Marys County, Maryland, and parts of Virginia. It has been correlated with other Atlantic Coastal Plain units studied by investigators from institutions such as the United States Geological Survey, Smithsonian Institution, and regional universities including the University of Maryland and Virginia Tech. Its recognition as part of the Chesapeake Group links it to research on the Chesapeake Bay impact crater and regional sedimentary responses to eustatic change documented by researchers affiliated with the National Oceanic and Atmospheric Administration.

Lithology and Stratigraphy

Lithologically, the unit consists predominantly of fine- to medium-grained glauconitic sand, interbedded shelly marl, and sandy clay with abundant phosphatic concentrations; these lithologies resemble neighboring units like the Choptank Formation and Talbot Formation. Stratigraphic relationships place the St. Marys Formation above the Choptank Formation and beneath the Talbot Formation, with bounding unconformities correlated to regional sequence stratigraphic surfaces recognized by the U.S. Geological Survey mapping programs. Key marker beds include phosphatic shell beds rich in Ostrea and concentrations of reworked Miocene carbonate clasts interpreted during mapping campaigns led by researchers from the Maryland Geological Survey and the Virginia Division of Mineral Resources.

Paleontology and Fossil Content

Fossil assemblages document a diverse marine fauna including bivalves, gastropods, echinoids, foraminifera, and vertebrate remains such as cetacean and shark teeth attributed to genera studied by paleontologists at the Smithsonian National Museum of Natural History, George Washington University, and the Natural History Museum of Los Angeles County. Notable invertebrate taxa include species akin to those recorded from the Yorktown Formation and Pollack Formation, enabling biostratigraphic correlations with collections curated at the Carnegie Museum of Natural History and Yale Peabody Museum. Vertebrate occurrences of interest include teeth referable to genera comparable to Carcharodon, Otodus, and mysticete whales that have been examined in collaborative projects involving the American Museum of Natural History and regional museums.

Age and Depositional Environment

Chronostratigraphic assessment places the St. Marys Formation near the Neogene–Quaternary boundary, with biostratigraphic indicators and paleomagnetic data tying portions of the unit to late Pliocene and early Pleistocene stages; these interpretations have been refined through studies by teams at Columbia University and the University of North Carolina at Chapel Hill. Depositional environments vary from inner shelf to estuarine settings influenced by transgressive pulses associated with Milankovitch-scale sea-level change; sedimentological and faunal evidence parallels analyses from the Sangamon and Illinoian stage research carried out by investigators at the University of Michigan and Indiana University.

Geographic Distribution and Extent

The formation extends along the Atlantic Coastal Plain from southern Delaware through Maryland into Virginia, with the thickest successions preserved in subsurface borings and outcrops along Calvert County and St. Marys County, Maryland. Subsurface correlations drawn from borehole logs held by the USGS and state geological surveys show lateral continuity with equivalents in the Cape Charles and Virginia Beach areas, and ties to offshore seismic interpretations conducted by teams at the Woods Hole Oceanographic Institution and Rutgers University.

Economic Significance and Uses

Phosphatic concentrations within the St. Marys Formation have been evaluated for their potential as local fertilizer sources historically mined in the Atlantic Coastal Plain; assessments were reported by the U.S. Bureau of Mines and state mineral resource programs. The unit’s sands and gravels have seen limited use in aggregate and construction where accessible near coastal communities such as Annapolis and Frederick County. Additionally, ground-water studies addressing aquifer frameworks in the region have utilized St. Marys stratigraphy in resource assessments by the USGS, Maryland Department of the Environment, and Virginia Department of Environmental Quality.

History of Investigation and Nomenclature

Named for exposures along the St. Marys River and adjacent stratigraphic work initiated in the early 20th century by geologists associated with the Maryland Geological Survey and the USGS, the formation’s nomenclature and boundaries have been revised through systematic mapping and micropaleontological studies conducted at institutions such as the Smithsonian Institution and University of Delaware. Key contributions include stratigraphic syntheses published by researchers from Johns Hopkins University and collaborative regional reports that integrated paleontological, sedimentological, and geophysical data to refine correlations within the Chesapeake Group.

Category:Geologic formations of the United States