PALEOHYDROLOGIC INTERPRETATION OF A JURASSIC PLAYA SYSTEM IN THE EAST BERLIN AND LOWER PORTLAND FORMATIONS, HARTFORD RIFT BASIN

Basin hydrology plays a fundamental role in controlling sedimentation in playa systems and the proper interpretation of paleohydrology in ancient playa systems is critical to distinguishing between climatic and tectonic depositional controls. The Jurassic East Berlin and lower Portland Formations of the Hartford Basin (Connecticut) record deposition in a series of alternating playa and perennial lake environments which are interpreted to have been deposited in a balanced (periodically open and closed) hydrology with respect to surface water, and a more open hydrology with respect to groundwater flow.

Direct evidence of evaporite minerals, mainly dolomite and magnesite, is uncommon in the these two formations (although evaporite minerals may be dissolved after deposition). These minerals typically occur as thin laminae or spherical crystal balls in perennial lake deposits or as pedogenic nodules. There are no preserved evaporite layers in these strata, and only limited evidence of displacive evaporites and possible saline crusts. However, abundant evidence exists for surface flow across the alluvial plain to sandflat feeding into the playa system. The dominant facies include thin cross-bedded sandstone beds and red siltstone to mudrock containing current ripple cross-lamination, interpreted as sheetflood deposits originating from the mountains bordering the rift basin. Common mudcracks imply frequent wetting and drying across the playa system. Abundant evidence for surface flow into the basin and limited evidence for groundwater discharge into the basin suggest that the sediments of these two formations acted primarily as a recharge playa system during the early Jurassic, and discharged groundwater only periodically to the surface.

These formations contain stratal patterns, lithologies, and sedimentary structures most consistent with those of fluctuating profundal facies associations. For example, they contain mudrock- and siltstone-dominated lithologies, shoaling/aggradational stratal patterns, mudcracks, and fine laminae in the lake facies. Depositional sequences with thick, well-developed transgressive systems tracts and thin lowstand and highstand systems tracts are interpreted to represent balanced-fill lakes, with open surface hydrology during the highstand systems tracts and closed hydrology during lowstands.