Paleopedology of paleo-wetland and barite-bearing, hydric paleosols in the Morrison Formation (upper jurassic-lower cretaceous), North Central Wyoming, USA : a multianalytical approach.
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Jennings, Debra S.
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Whereas lakes and wetlands are very sensitive to changes in climate, high-resolution data in these archives are lost unless it is possible to differentiate these deposits from other "floodplain" deposits. Synthesis of micromorphological and geochemical data with facies relationships is critical to accurate interpretations of ancient wetland-lacustrine deposits. A proposed hydrogeomorphological classification and appropriate genetic terminology that is useful in the rock record is presented. Eight Morrison Formation subenvironments, including barite-bearing paleosols, within lacustrine depositional successions from north central Wyoming were delineated using micromorphology and geochemistry, combined with facies relationships. The distribution of wetland pedotypes indicates that evapotranspiration remained high, suggesting that a stable, semi-arid to arid climate dominated in this area throughout Morrison time. This study also characterizes the micromorphology, elemental trends, and stable isotope geochemistry of sulfates in a barite-bearing Morrison paleosols and a modern analog soil (Lufkin Series) using optical microscopy, XRD, ICP-MS, and stable S and O isotope data. Data indicate that barium-rich parent material provides adequate barium for barite precipitation. Barium is mobilized and concentrated in Btg horizons ~100-160 cm below the surface. The presence of humic acids in profiles lower on the landscape prevents barite precipitation and drives the precipitation of gypsum between saturated, anoxic conditions and drier, more oxic conditions. Barite precipitation is a slow, punctuated process that begins first along evacuated macropores and then in the adjacent matrix. The modern analog displays similar catenary relationships, redox features, and micromorphological characteristics compared to the Morrison paleosols, suggesting that similar pedogenic processes led to barite precipitation. Synthesized data reveal that conditions favorable to barite-bearing soil formation are low-gradient basins that have received feldspar-rich sediments (i.e. volcanically influenced basins), soils that developed near salt domes, soils that developed in exposed wetland or lacustrine sediments, and coastal plain deposits. Combined with regional paleosols, barite-bearing paleosols may document temporal changes in drainage, surface stability, and accommodation consistent with sequence boundaries/maximum flooding surfaces, and climate changes.