LOCAL VARIABILITY IN EARLY OLIGOCENE PALEOSOLS AS A RESULT OF ANCIENT SOIL CATENARY RELATIONSHIPS, BRULE FORMATION, TOADSTOOL PARK, NEBRASKA

Paleopedology is often employed in paleoenvironmental reconstruction since the features of paleosols are affected by changes in climate, ecology, topography, and lithology over time. These changes cause very small-scale lateral 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 making claims about the past using paleosols, care must be taken to ensure that as many of the soil forming factors as possible are understood. While climates and biomes have certain breadth that allows for local variations to be essentially ignored, and the lithology of an area is usually still present as the medium of soil preservation, topographic relief can vary over local scales, and can contribute greatly to the development of soils. Paleo-geomorphological relationships were investigated along a paleovalley sequence in the Early Oligocene Orella Member of the Brule Formation in the White River Group of Toadstool Park, NE. A series of three separate, but laterally contiguous, soil profiles exhibit variation in the type and degree of apparent pedogenesis. Changes in the presence, abundance, and size of multiple soil features such as root traces, soil structure, and slickensides were observed over a distance of ~350 meters. The effect of paleotopographic control on pedogenesis along this paleovalley tends to follow general trends according to the physiographic relationship of soils to one another along modern hill-slopes. The processes of erosion, deposition, and the through flow of water change in a predictable way along landscapes, which are indicated by changes in slope, and back-and-forth transitions from convexity to concavity. Further geochemical analysis should reflect the processes driven by geomorphology. Soils forming on different landscape positions, even though they are adjacent to one another, develop strikingly different morphologies, and without knowledge of their topographic association these soils would be interpreted as having formed under vastly different environmental factors. Reconstruction of the paleoenvironment in and around the Eocene-Oligocene Boundary has significant implications for the temporal and spatial evolution of paleoecology at this time.