Saturday, 15 July 2006

Interpreting Sedimentation Patterns and Paleoclimate from Paleosol Sequences of the Triassic-Jurassic Hartford Basin Rift Valley, Connecticut, USA.

Cynthia Stiles, Univ of Wisconsin, Dept of Soil Science, 1525 Observatory Drive, Madison, WI 53706 and Elizabeth H. Gierlowski-Kordesch, Ohio Univ, Dept of Geological Sciences, 316 Clippinger Labs, Athens, OH 45701-2979.

Rifting during the opening of the Atlantic Ocean during the Triassic to Jurassic periods (from 250 to 175 Myr ago) produced rift valleys on both sides of the nascent Atlantic ocean. The Hartford Basin in Connecticut, U.S.A., contains sedimentary rocks that originally accumulated as sediments within one of these Triassic-Jurassic rift valleys. The sedimentary fill of this ancient valley reflects the source area materials from the surrounding highlands and contains paleosol sequences that contain imprints of the climate of the region. Detailed description of the paleosol horizons from three of the formations within this rift basin highlights the depositional paleoenvironments as well as the climatic influences on sedimentation and soil formation during the Triassic and Jurassic rifting event. The continental fill of the Hartford Basin contains four sedimentary formations with three basalt flows (all dated as approximately 200-202 Myr) separating them. These formations, the New Haven Arkose (NHA), the Shuttle Meadow Formation (SMF), the East Berlin Formation (EBF), and the Portland Formation (in order from youngest to oldest) are intercalated with the Talcott, Holyoke, and Hamden Basalts, respectively. The three lowermost formations are composed of breccias, conglomerates, sandstones, siltstones, mudrocks, and limestones. The paleosols are found within the massive mudrock facies. Analyses of sedimentation patterns and lithologies in the basin indicate that New Haven Arkose depositional sequences occurred under a more humid climate than for the Shuttle Meadow and East Berlin Formations. The morphology and geochemistry of several paleosols found within the sequences confirm this interpretation. The NHA paleosols, the earliest sequences, have very weak pedogenic development and poor aggregation – occurring as loose flakes with faint root traces along aggregate faces. There are no large root casts to indicate surface stability and boundaries in paleosols “horizons” are actually stacked fine-textured alluvial deposits. The bulk geochemistry of these paleosols indicates very little difference in pedogenically active elements such as Ca, K, P, and Mg from adjacent basalts. Zirconium contents are similar to basalt contents throughout the profiles. These paleosols can be classified as paleo-Inceptisols, formed in a wet pedogenic setting that limits pedogenesis. The SMF paleosols have stronger crumb-like aggregation at the uppermost portion of the sequences with very weak slickenside planes filled with secondary carbonates. The geochemistry of these paleosols, which can be discerned to occur in Vertisol-like paleo-gilgai with microhighs and microlows, show slight depletions of plant nutrient elements such as K and P relative to the NHA paleosols and parent basalt. These paleosols can be classified as Vertisol-like paleosols, because of their weak development in claystone to mudstone, but little pedogenesis, indicating that there is still a good deal of rainfall occurring with rare seasonal drying during paleosol formation. The EBF paleosols are interpreted to have formed in a much drier time period than either NHA or SMF paleosols, which is corroborated by the morphology of the paleosols. The presence of relatively pervasive cracks at the surface grading to significant slickensides at about 50-90 cm depth indicates that these are paleoVertisols. Upper crumb aggregation and gilgai microtopography had been truncated by basalt flows, but slickenside planes indicated direction of clay deformation. The geochemistry of the EBF paleosols – with lower total Ca, K, and P, elevated Ti at depth, and Zr contents concentrated in the upper portion of the sequences – indicates that these were soils formed in a drier climate with more seasonal precipitation and longer periods of pedogenic stability. Thus the tectonic/depositional model is supported by paleosol information in the Hartford Basin.

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