Abstract The Salisbury embayment is a broad tectonic downwarp that is filled by generally seaward-thickening, wedge-shaped deposits of the central Atlantic Coastal Plain. Our two-day field trip will take us to the western side of this embayment from the Fall Zone in Washington, D.C., to some of the bluffs along Aquia Creek and the Potomac River in Virginia, and then to the Calvert Cliffs on the western shore of the Chesapeake Bay. We will see fluvial-deltaic Cretaceous deposits of the Potomac Formation. We will then focus on Cenozoic marine deposits. Transgressive and highstand deposits are stacked upon each other with unconformities separating them; rarely are regressive or lowstand deposits preserved. The Paleocene and Eocene shallow shelf deposits consist of glauconitic, silty sands that contain varying amounts of marine shells. The Miocene shallow shelf deposits consist of diatomaceous silts and silty and shelly sands. The lithology, thickness, dip, preservation, and distribution of the succession of coastal plain sediments that were deposited in our field-trip area are, to a great extent, structurally controlled. Surficial and subsurface mapping using numerous continuous cores, auger holes, water-well data, and seismic surveys has documented some folds and numerous high-angle reverse and normal faults that offset Cretaceous and Cenozoic deposits. Many of these structures are rooted in early Mesozoic and/or Paleozoic NE-trending regional tectonic fault systems that underlie the Atlantic Coastal Plain. On Day 1, we will focus on two fault systems (stops 1-2; Stafford fault system and the Skinkers Neck-Brandywine fault system and their constituent fault zones and faults). We will then see (stops 3-5) a few of the remaining exposures of largely unlithified marine Paleocene and Eocene strata along the Virginia side of the Potomac River including the Paleocene-Eocene Thermal Maximum boundary clay. These exposures are capped by fluvial-estuarine Pleistocene terrace deposits. On Day 2, we will see (stops 6-9) the classic Miocene section along the ~25 miles (~40 km) of Calvert Cliffs in Maryland, including a possible fault and structural warping. Cores from nearby test holes will also be shown to supplement outcrops.

Abstract Miocene strata exposed in the Calvert Cliffs, along the western shore of the Chesapeake Bay, Maryland, have a long history of study owing to their rich fossil record, including a series of spectacular shell and bone beds. Owing to increasingly refined biostratigraphic age control, these outcrops continue to serve as important references for geological and paleontological analyses. The canonical Calvert, Choptank, and St. Marys Formations, first described by Shattuck (1904), are generally interpreted as shallowing-up, from a fully marine open shelf to a variety of marginal marine, coastal environments. More detailed paleoenvironmental interpretation is challenging, however, owing to pervasive bioturbation, which largely obliterates diagnostic physical sedimentary structures and mixes grain populations; most lithologic contacts, including regional unconformities, are burrowed firmgrounds at the scale of a single outcrop. This field trip will visit a series of classic localities in the Calvert Cliffs to discuss the use of sedimentologic, ichnologic, taphonomic, and faunal evidence to infer environments under these challenging conditions, which are common to Cretaceous and Cenozoic strata throughout the U.S. Gulf and Atlantic Coastal Plains. We will examine all of Shattuck‚s (1904) original lithologic “zones” within the Plum Point Member of the Calvert Formation, the Choptank Formation, and the Little Cove Point Member of the St. Marys Formation, as well as view the channelized “upland gravel” that are probably the estuarine and fluvial equivalents of the marine upper Miocene Eastover Formation in Virginia. The physical stratigraphic discussion will focus on the most controversial intervals within the succession, namely the unconformities that define the bases of the Choptank and St. Marys Formations, where misunderstanding would mislead historical analysis.