Local variability in Early Oligocene paleosols as a result of ancient soil catenary processes, Brule Formation, Toadstool Park, Nebraska.

摘要

Paleopedology 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 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 soil features, such as root traces and soil structure, were observed over a distance of ∼350 meters. The morphology and mineralogy of the soils along this paleovalley varied according to those processes which are influenced by paleotopography. Geochemical proxies of soil processes, as well as soil micromorphology, indicate only weak pedogenesis. This limited the utility of geochemical climofunctions used in the study, which were not found to differ significantly between geomorphological positions.