Browsing Theses and Dissertations by Subject "Paleosol"
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Local variability in early Oligocene paleosols as a result of ancient soil catenary processes, Brule Formation, Toadstool Park, NebraskaPaleopedology is often employed in paleoenvironmental reconstructions because the features of paleosols are affected by changes in climate, ecology, topography, and lithology over time. These changes cause small-scale variations in the morphology and apparent development of paleosols and influence the degree to which certain soil features are preserved in the rock record. When drawing inferences about paleoenvironments based on paleosols, care must be taken to ensure that as many of the soil forming factors as possible are understood. Whereas climates can be ignored over small areas, and lithology represents the medium of soil preservation, topographic relief can vary dramatically over local scales, thereby influencing the partitioning of plant communities and contributing greatly to the development of soils. In this study, paleo-geomorphological relationships were investigated along a paleovalley sequence in the Early Oligocene Orella Member of the Brule Formation in the White River G
PALEOPEDOLOGY AND PALEOGEOMORPHOLOGY OF THE EARLY OLIGOCENE ORELLA AND WHITNEY MEMBERS, BRULE FORMATION, WHITE RIVER GROUP, TOADSTOOL GEOLOGIC PARK, NEBRASKAUnderstanding local and regional reactions to the global Eocene-Oligocene climate transition is a continuing challenge. The White River Group in the North American midcontinent preserves dynamic fluvial, volcaniclastic and lacustrine facies that yield to aeolianites. To test whether this shift in sedimentation style was driven by climate change, 20 paleosols from 8 profiles were analyzed from the fluvial-aeolian Orella Member through the aeolian-dominated Whitney Member of the earliest Oligocene Brule Formation at Toadstool Geologic Park, NE. Paleosol morphology and geochemistry were used to assess the balance of aeolian vs. alluvial sedimentation at key stratigraphic intervals and lithologic transitions. Significant loess deposition began at least as early as the lower Orella Member but is masked in most settings by concomitant fluvial deposition. As fluvial influence on landscapes waned across the Orella-Whitney Member boundary, loess deposits predominated and became more recognizable. Paleosols follow different pedogenic pathways in direct response to depositional setting. Whereas all paleosols formed through top-down pedogenesis in alluvial settings, paleosols in aeolian-dominated settings formed though pedogenic upbuilding during aggradational phases and through top-down pedogenesis during depositional hiatuses. The disparity between each style of pedogenic development creates fundamentally different pedogenic associations that must first be understood before climatic interpretations can be drawn from macroscopic paleosol morphology alone. Microscopic analysis of loessic and alluvial paleosols indicates that pedogenic features do not greatly change across the Orella-Whitney Member boundary. Furthermore, results of climofunction calculations from five paleosol Bw and Btk horizons show mean annual temperature (ca. 9.0-10.5 °C) and precipitation (ca. 650-800 mm/y) do not significantly vary across the Orella-Whitney Member transition. Clay mineralogy and the presence of pedogenic carbonate and translocated clay corroborate paleoclimate estimates. However, geochemical paleosol profiles are uniform and do not reflect observed vertical associations of pedogenic features. Constant additions of aeolian sediment, which replenishes base losses through leaching, explain this phenomenon. Interpretations of paleovegetation from root trace morphology and paleosol taxonomy indicate that predominantly open canopy to savanna habitats were in place in the lower Orella Member and continued into the Whitney Member. Evidence for riparian partitioning exists in the lower Orella Member but disappears as fluvial deposits wane in the Whitney Member. Lacking evidence of climate change from paleosol analysis, changes in sedimentation style and vegetative biomes are most likely a reaction to increased aeolian deposition.