STRATIGRAPHIC FRAMEWORK OF THE TRACK-BEARING BEDS AT DINOSAUR STATE PARK, ROCKY HILL, CONNECTICUT

The Early Jurassic East Berlin Formation (EBF) exposed along the ramp from the Berlin Turnpike onto Route 9 N in East Berlin, Connecticut, preserves cycles of alternating lake (carbonaceous black and structureless gray mudstone) and playa (rippled, mudcracked, and disrupted reddish-brown mudstone and cross-bedded sandstone) strata that are interpreted to represent 20,000-year Milankovitch cycles. These cycles, particularly the carbonaceous mudstone layers, have been used to correlate EBF strata across the Hartford Basin. One such layer occurs beneath the dinosaur track-bearing layers at Dinosaur State Park (DSP) in Rocky Hill, Connecticut. DSP is the most visited geological site in Connecticut and displays over 750 tracks that are important in understanding the paleoecology of Jurassic rift lake systems in the state. The lake cycles that contain the dinosaur tracks at DSP can be correlated to the EBF at the East Berlin outcrop.

The EBF in the DSP area appears to be much thicker (over 300m) than the section measured at East Berlin (about 170m) and elsewhere, making correlation of similar looking lake cycles difficult. To resolve this, three approaches were taken: (1) comparison of stratigraphic facies patterns, (2) measuring the distance down from the top of the formation, and (3) comparing relative positions of the track-bearing strata within the EBF at DSP to the East Berlin outcrop. The stratigraphic facies patterns at DSP most closely resemble the third lake cycle down from the top of the EBF at the East Berlin outcrop based on the occurrence of a unique thin carbonaceous mudstone bed a few meters above the main one. This interpretation is consistent with correlation using the distance down from the top of the formation and the relative stratigraphic position of the lake beds within the EBF, and with previous basin-wide investigations that demonstrate that the number of lake cycles is the same across the basin.

These observations lead to two primary conclusions: (1) the track-making dinosaurs at DSP occupied the Hartford basin when the lakes were absent or at a lowstand between two episodes of expanded lake levels, and (2) thickening of the entire EBF in the DSP area is likely related to increased accommodation and sediment supply at the basin edge near the Eastern Border Fault system.