RE-EVALUATING DEPOSITIONAL ENVIRONMENTS WITHIN JURASSIC STRATA OF THE HARTFORD BASIN, CT: REFINING THE PLAYA DEPOSITIONAL MODEL

Over the past decade, data collected from several key outcrops and cores within the Hartford Basin have resulted in refined interpretations for sedimentary processes that operated within the basin during deposition of the Early Jurassic East Berlin and Portland Formations. First is the recognition of decimeter- to meter-scale fining-upward successions within playa strata. Each succession begins with light colored trough cross-bedded or current rippled siltstone to sandstone that grades upward into reddish-brown mudstone which can be structureless, contain mudcracks, or preserve evidence of pedogenic alteration. Wave ripples are common on the surface of the current-rippled lower portion. Each succession represents deposition by a flood event that brings the coarser sediment into the basin and creates temporary ponds on the playa surface evidenced by wave ripples. The reddish-brown mudstone is deposited from suspension. Between flood events, the playa experiences long periods of exposure during which mudcracks and pedogenic features formed. The thickest of these successions can be correlated over distances of several hundred meters.

A second development is the recognition of cyclic changes within playa mudstone beds on the Van Houten cycle scale that can be used for correlation purposes and to better understand basin paleohydrology. Principle among these is the common occurrence of carbonate nodules in a position below certain black shale intervals. Their presence supports the interpretation of arid climates and closed surface hydrology, and can improve understanding of groundwater paleohydrology. Their widespread and predictable occurrence is useful for correlation between sections separated by several kilometers.

A third development is the recognition of microbial mats on the track-bearing surfaces of strata exposed at Dinosaur State Park (DSP). Microbially induced sedimentary structures on bedding planes at DSP include sand shadow structures, sand cracks, gas escape structures, and mat desiccation structures. Intervals of microbially modified wavy/crinkly laminae occur within cores and outcrops. These mats preserve evidence of moist but exposed surfaces on the edge of the Hartford Basin, and the sediment-binding nature of the mats likely helped to preserve dinosaur tracks once registered.