Response of Early Jurassic Paleosols to Environmental Changes in the Hartford Basin, Connecticut

The Early Jurassic Portland Formation was deposited in a rift basin (Hartford Basin, Connecticut) and contains depositional facies and paleosols that record changing tectonic accommodation, alluvial environments, and climate. The lower Portland Formation, exposed in the central part of the basin, is composed of alternating playa, alluvial plain, and lacustrine strata deposited during a period of relatively high accommodation. It contains paleosols that are similar to those preserved in the underlying East Berlin Formation, and are characterized by distinct intervals containing abundant deep mudcracks, highly disrupted bedding, isolated carbonate nodules, soil porosity, and/or rare evaporite molds. These paleosols are interpreted to have formed under relatively arid climatic conditions. Distribution of depositional facies and a scarcity of preserved evaporite minerals suggest they were formed primarily under closed hydrologic conditions with respect to surface water (e.g. when lake level was low), and open hydrologic conditions with respect to groundwater flow (recharge playa).

Cores from the central part of the basin show that facies higher in the Portland Formation are similar to those in the lower Portland, but paleosols become progressively thinner and less well-developed. Intervals containing some isolated carbonate nodules and only shallow mudcracks indicate a general transition to more open or more wet basin hydrology. The upper Portland Formation exposed in outcrops along the eastern margin of the Hartford Basin is dominated by braided river strata. Mudrock deposits within the braided river system contain limited evidence of soil development that includes burrows and greenish-gray/reddish-brown mottled textures. Paleosols are completely absent in the uppermost outcrops of the Portland Formation where thin mud layers occur between thick amalgamated channel complexes. These observations suggest a wet basin hydrology and limited time for paleosol development. Vertical and lateral variations in Portland Formation paleosols reflect progressive changes in hydrology as the rift basin evolved.