RE-EXAMINATION OF UPPER CRETACEOUS AND PALEOGENE FOSSILS AND SEDIMENTS AT THE CABIN BRANCH OUTCROP LOCALITY, MARYLAND: IMPLICATIONS FOR REGIONAL GEOLOGY

Differentiation between the Upper Cretaceous Severn Formation and the lower Danian Brightseat Formation has historically been difficult to do in the field, despite a hiatus separating them. Similarities in sediment, a paucity of exposed outcrop material, and weathering have all contributed to this difficulty. As part of a new U.S. Geological Survey project, Upper Cretaceous and Paleogene deposits from classic localities in Maryland and Virginia, as well as from new cores, are being examined with the goal of providing a regional context for Paleogene sedimentation in the Salisbury Embayment, especially as it pertains to aquifer studies across state boundaries. The Cabin Branch outcrop is located in Prince Georges County (MD) and consists of disjunct exposures along an approximately 0.4-mile section of the creek. The oldest sediments of the Severn Formation consist of 4.5 ft (1.4 m) of silty sands that are composed of sub-rounded to rounded quartz, glauconite, mica, bioclastic fragments, and minor oxides. The Severn Formation is placed in calcareous nannofossil Zone CC25a based on the absence of two marker species; Reinhardtites levis, which defines the top of Zone CC24, and Lithraphidites quadratus, which defines the base of Zone CC25b. The Severn is overlain by approximately 18 ft (5.5 m) of silty quartz and glauconitic sand of the Danian Brightseat Formation (calcareous nannofossil Zone NP3). The K-Pg boundary is a nodular hard ground exposed in the stream bed and represents a hiatus of at least 5.6 m.y. A firmground containing phosphate pebbles and glauconite-filled burrows marks the disconformity between the Brightseat and the overlying Aquia Formation. This hiatus represents at least 2.0 m.y. The Aquia is approximately 14 ft (4.3 m) thick, highly fossiliferous, and is composed of quartz sand rich in bioclastic debris with <20% glauconite. Its basal section is placed in calcareous nannofossil Zone NP5 based on the presence of Fasciculithus tympaniformis. Three distinct hard beds and a zone of the articulated bivalve Cucullaea gigantea are situated above a highly burrowed interval.The boundary between the C. gigantea bed and the burrowed zone is distinct and coincides with a change in the nannofossil assemblage. These features will be examined for possible use in correlation across the embayment.