9706115 Kraus One of the most important advances in fluvial sedimentology has been the development of alluvial architecture models. An important component of these models is channel avulsion, and two recent studies provide new and more realistic insights on the influence of avulsion on alluvial architecture. The new model of Mackey and Bridge (1995) predicts "avulsion sequences" in which major sandstones become progressively thinner up-section. Heller and Paola (1996) have presented a simple geometric model that explores how the relationship between avulsion frequency and sedimentation rate can influence alluvial architecture. Despite their potential for interpreting the rock record, the validity and usefulness of the models depend upon field testing in both modern and ancient alluvial sequences. Consequently, the research proposed here is to examine the alluvial architecture of Eocene alluvial deposits in light of the new models. This research is innovative in that the alluvial architecture and the intrinsic and extrinsic controls on the fluvial system will be established by integrating the analysis of floodplain paleosols with the more typical analysis of sandstone body characteristics. This project has two major goals: (1) to field test the new concepts of avulsion and alluvial architecture and provide field data for refining these new models and (2) to further develop paleosols as an integral part of alluvial architecture studies (both field and quantitative studies). To focus on only the effects of subsidence rate and avulsion frequency on alluvial architecture, time-equivalent upstream and downstream cross-sections in an exceptionally well-exposed ancient example will be examined. The stratigraphic unit to be analyzed, the Willwood Formation, is especially appropriate for this study because it has excellent biostratigraphic and paleomagnetostratigraphic records that can be used for time correlation and calculating sediment accumulation rates. Previous studies of Willwood paleosols in other parts of the basin provide a framework for analyzing the paleosols. Field study will focus on describing the major sandstone bodies and the paleosols surrounding them. The external geometry, internal characteristics, and interconnectedness of the major sandstones in the two study areas will be described. Paleosols will be described and further studied in the lab to determine the different the different kinds of paleosols and their maturity. The analyses of the sandstone bodies and the paleosols will be integrated to test and refine the computer models. The two cross sections will be examined for up-section changes in sandstone body size and interconnectedness and in paleosol maturity to test for the presence of avulsion sequences predicted by Bridge and Mackey (1995). The alluvial architectures in the two study areas will be compared and contrasted to see what effect increasing rates of sediment accumulation had on alluvial architecture and to test how applicable the results of Heller and Paola (1996) are to the stratigraphic record. Both the new alluvial architecture models and the integration of paleosol analysis into alluvial architecture studies have important implications for the way alluvial processes and the resulting deposits are understood. Together they provide a fundamental new assessment of how alluvial deposits can be described and interpreted, and they offer enormous potential for better prediction of alluvial architecture in subsurface resources such as aquifers and oil and gas fields.
