REINTERPRETATION OF THE EARLY CRETACEOUS ARUNDEL CLAY (POTOMAC FORMATION) DEPOSITION OF THE M-NPPC DINOSAUR PARK

The M-NCPPC Dinosaur Park in Prince George’s County, Maryland is host to one of the largest Early Cretaceous fossil collections from the Arundel Clay of the Potomac Formation from the mid-Atlantic region. However, relatively little recent work has been done to reexamine these fossils, nor the environments in which they were deposited. Historically, Dinosaur Park has been interpreted as an oxbow lake deposit due to its fine-grained clay composition representing overbank filling, prevalence of lignite, and its fossiliferous nature. Recent observations have prompted a reevaluation of this interpretation. Firstly, the clay matrix contains poorly sorted, angular, pebble-sized clay fragments, not typical of a lake environment. Additionally, the vertebrate fossil material found to date has been mostly disarticulated with minimal abrasion, indicating minimal transport. The fossil assemblages themselves are interesting due to the marked absence of amphibians, which could indicate increased salinity, as well as the presence of aquatic taxa such as hybodont sharks and a stingray, that indicate a brackish, if not marine, influence. Finally, lignite that is pervasive throughout the outcrop is comprised of various sized pieces and preservation types (solid wood vs. charcoal), all with no clear orientation, nor sorting at all stratigraphic levels. This evidence has led to the reinterpretation of Dinosaur Park deposition as a possible fluvial log jam or debris flow occurring after a forest fire event evident from the preserved charcoal and wood fragments within the deposit. Further oxygen isotope analysis of vertebrate material will be conducted to examine if marine signals are present in taxa at the site, which would provide additional evidence against an oxbow lake interpretation.