SEDIMENTOLOGY AND TAPHONOMY OF TWO LATE CRETACEOUS LAG DEPOSITS: SEVERN (BOWIE, MD) AND BLADEN (ELIZABETHTOWN, NC) FORMATIONS

The Severn Formation of Maryland (Maastrichtian) and the Bladen Formation of North Carolina (Campanian), while not strictly contemporaneous, are significant Late Cretaceous vertebrate localities that have produced a profuse and diverse assemblage of marine and terrestrial fossils. The fossils collected for this study came from the Elizabethtown Landfill Annex site (Bladen Formation) and from outcrops along a small unnamed tributary of the Patuxent River in Prince George’s County Maryland (Severn Formation). Both formations have been described as shallow/marginal-marine lag deposits; nevertheless, sedimentological and taphonomic analyses suggest they differ with respect to hydrodynamic energy, water depth, and reworking history.

To compare the taphonomic condition of the chondrichthyan teeth at each locality more than 2,500 teeth were evaluated with respect to smoothing, chipping, and fracturing of the tooth crown, root, and shoulders/cuspet(s). Nearly 10% of the specimens remained in a pristine condition at the Maryland location, suggesting minimal transport or reworking. The taphonomic condition of the teeth was poorer at the Elizabethtown site, with a majority of the fossils in an abraded or broken state. The North Carolina locality also contains a larger proportion of coarse and very coarse sand (>50%) with scattered pebbles, while the Maryland deposit contained primarily fine to medium sands. Finally, the presence of scavenging forms (e.g. Squalicorax spp.) at both locations suggests a shallow-water nearshore depositional environment and the higher ratio of marine reptile teeth (e.g. mosasaur) to marginal-marine reptile teeth (e.g. crocodylian) in the Severn Formation suggests a depositional environment further from shore.

The Bladen Formation represents a marginal-marine or deltaic lag deposit with input of numerous terrestrial vertebrate fossils, including dinosaur teeth. In contrast, while the Severn Formation contains scattered terrestrial forms, it appears to be a transgressive shelf lag deposit. These descriptions are in general agreement with previous reports on the Severn (nearshore continental shelf with input of distal terrestrial forms) and Bladen (fluvially-dominated estuarine with input of proximal terrestrial forms and distal marine forms).