REINTERPRETATION OF THE SEDIMENTOLOGY AND STRATIGRAPHY OF THE JURASSIC EAST BERLIN FORMATION AT DINOSAUR STATE PARK, ROCKY HILL, CT

The Jurassic East Berlin Fm exposed at Dinosaur State Park (DSP) in Rocky Hill, CT preserves six stratigraphic horizons containing Eubrontes trackways interpreted to be made by a therapod dinosaur like Dilophosaurus. Current interpretations suggest the trackways formed on the margin of a perennial lake along which the dinosaurs moved. A reexamination of the facies and stratigraphy challenges some long held interpretations about dinosaur behavior at DSP.

Laminated black shale beds representing perennial lakes occur in distinct and predictable intervals within the East Berlin Fm that can be correlated over distances of >90 km (S to N) in all available outcrops and cores. They are remarkably uniform in thickness (about 0.5m thick) and lithology over these distances, but may thicken gradually toward the Eastern Border Fault. On a basin scale, the perennial lake facies display aggradational stacking patterns with minimal lateral facies variations (with the exception of a conglomeratic facies immediately adjacent to the border fault). This is predicted for under-filled lake basins, where relatively low sedimentation rates and rapid base level shifts result in transgressions/regressions characterized by rapid vertical, rather than lateral, facies shifts. Laminated black shale beds extend to the border fault, and do not show a lateral transition into lake margin facies.

Track-bearing strata occur in playa mudstone and sandstone between laminated black shale beds. Mudcracks, raindrop imprints, current ripples, trough cross-bedding, pedogenic structures, and possible adhesion structures support the interpretation that the dinosaurs walked on playa sandflats/mudflats that were accreted through episodic sheetflood events. The surfaces of a few cross-bedded units are covered with symmetric ripples indicating occasional periods of ephemeral standing water after the flood events. Bedding surfaces were modified by wave activity in lakes estimated to be <1m deep based on ripple dimensions. There is no evidence that a perennial lake existed contemporaneously with any of the preserved track layers. These data challenge two current interpretations regarding dinosaur behavior at DSP that requires a perennial lake: (1) dinosaurs visited the lake margin to fish, and (2) some tracks were made by swimming dinosaurs.