INTERPRETATION OF A PLAYA SYSTEM FROM A JURASSIC HOTHOUSE CLIMATE, EAST BERLIN AND LOWER PORTLAND FORMATIONS, HARTFORD BASIN, CONNECTICUT

Sediments in the rifted Hartford Basin (Connecticut) record deposition in playa and perennial lake environments. Basin hydrology plays a fundamental role in controlling sedimentation in playa systems and the proper interpretation of paleohydrology of ancient playa systems is critical to interpreting past climatic conditions. The Jurassic East Berlin and lower Portland Formations are composed of alternating layers of reddish-brown mudstone (playa environments) and dark gray/black shale (perennial lake environments). Sedimentological, mineralogical, and sequence stratigraphic interpretation of these strata indicate that they had a balanced to underfilled (periodically open and closed) hydrology with respect to surface water, and a more open hydrology with respect to groundwater flow.

The lake strata contain aggradational depositional sequences with thick, well-developed transgressive systems tracts and thin lowstand and highstand systems tracts. They are interpreted to represent balanced-fill lakes, with open surface hydrology during the highstand systems tracts and closed surface hydrology during lowstands. Direct evidence of evaporite minerals, mainly dolomite and magnesite, is uncommon and typically occurs as thin laminae in perennial lake deposits (indicating slightly saline lake conditions) or as pedogenic nodules. There is no direct evidence of evaporite crusts in these strata, and only limited evidence of displacive evaporites (empty molds). Abundant sedimentological evidence exists for surface flow across the playa flats. The dominant playa lithologies include thin cross-bedded sandstone beds and red siltstone containing current ripples that are interpreted as sheet flood deposits originating from the mountains bordering the rift basin. The abundant evidence for surface flow into the basin, and limited evidence for groundwater discharge at ground surface suggest that the Hartford Basin acted primarily as a recharge playa system.

All these data can be used to identify climate cycles within an overall warm, arid climate. Saline lakes formed during wetter climates when enough water was available to collect in the basin. During the drier playa phases, there was not enough water to collect on the surface, but there was enough to prevent the accumulation of displacive evaporites.