LITHOSTRATIGRAPHIC AND PETROGRAPHIC ANALYSIS OF UPPER CRETACEOUS THROUGH LOWER PALEOGENE SEDIMENTS AND HARDGROUNDS IN THE NORTHERN SALISBURY EMBAYMENT: CABIN BRANCH, MARYLAND – MID-ATLANTIC COASTAL PLAIN

The stratigraphic boundary between the Upper Cretaceous Severn Formation and the lower Danian Brightseat Formation has historically been challenging to reconcile in the field, despite there being a hiatus separating them. As part of the Atlantic Subsurface Stratigraphic Initiative, outcrop and core material were examined to determine regional lithologic and stratigraphic boundaries in the Salisbury Embayment. Grain size and petrographic analysis of Upper Cretaceous and Paleogene sediments and hardgrounds from a continuous section in Maryland were analyzed to provide regional context for Paleogene sedimentation. The Cabin Branch locality is in Prince Georges County and consists of variably exposed outcrop in a 0.4-mile section of the creek. The contact between the Severn and Brightseat formations is disconformable and characterized by a discontinuously exposed carbonate-cemented nodulose hardground in the stream bed. This hardground is indicative of a 5.6 m.y. hiatus at the K-Pg boundary. A disconformity also exists between the Brightseat Formation and the overlying Aquia Formation and is marked by a second carbonate-cemented hardground containing phosphate pebbles and glauconite-filled burrows. Three additional stratigraphically distinct, intraformational hardgrounds that lie above a highly burrowed interval, were also sampled in the Aquia Formation. These hardgrounds share similarities to the overlying hardgrounds in mineral and bioclastic composition but with varying percentages. A zone of the articulated bivalve Cucullaea gigantea, also lies above a highly burrowed interval. Observational petrography indicates that the dominant bioclasts in all five hardgrounds include silicified mollusks, benthic and planktic foraminifera, gastropods, crinoids, and bivalves. Grain size analysis of the Aquia Formation sediments indicates an overall coarser lithology composed of 45-60% medium sand and less than 10% clay. Sediment composition is predominately sub-rounded to rounded quartz and glauconite grains with minor phosphate and is fossiliferous. The lithostratigraphic and petrographic features identified in this investigation will be examined for potential use in correlation across the Salisbury Embayment.