Paleosol Morphology and Geochemistry as Indicators of Changing Paleoenvironments in the Triassic-Jurassic Hartford Basin, Connecticut, USA

The Hartford Basin in Connecticut, U.S.A., contains basalt flows interbedded with fluvial-lacustrine sedimentary rocks that originally accumulated in a half graben formed during Triassic - Jurassic rifting along the nascent Atlantic Ocean. Three of the sedimentary formations within this basin contain paleosol intervals that indicate shifting depositional conditions and paleoclimatic and hydrologic influences on sedimentation and soil formation during the rifting event. The paleosols are found within the massive mudrock facies of the New Haven Arkose (NHA), the Shuttle Meadow Formation (SMF), and the East Berlin Formation (EBF) (lowermost sedimentary formations). Sedimentologic analyses of these formations indicate that the NHA depositional sequences occurred under a more humid climate or wetter hydrology than for the SMF and EBF. The morphology and geochemistry of associated paleosols confirm this interpretation. The NHA pedotype (the earliest paleosols) have very weak pedogenic development and poor aggregation. Few trace fossils and little difference in pedogenically active elements such as Ca, P, Mg, Ti and Zr, from adjacent basalts suggest that “horizons” are actually stacked fine-textured alluvial deposits, and the soils formed in a wet pedogenic environment with limited pedogenesis. The SMF pedotype is similar to modern vertic suborders, with crumb aggregation at the uppermost portion over weakly slickensided planes filled with secondary carbonates. The geochemistry of the SMF pedotype shows slight depletions of nutrient elements such as K and P relative to the NHA pedotype and parent basalt, and an incipient pattern of gilgai morphology, suggesting adequate rainfall with rare seasonal drying during active soil genesis. The EBF pedotype formed in a much drier paleoclimate or hydrology, suggested by the presence of pervasive surface cracks grading to significant slickensides at 50-90 cm depth. Geochemical trends indicate pedogenesis occurred in a drier regime with more seasonal precipitation and longer periods of pedogenic stability.