Use of Bed-Form Climb Models to Analyze Geometry and Preservation Potential of Clastic Facies and Erosional Surfaces

Abstract

Based on a combination of Walther's Law of Facies and bed-form climb theory, we propose a model that explains how erosion surfaces and vertical sequences of clastic strata are preserved where deposition occurs in channelized or locally erosional environments including fluvial and submarine-channel deposits, barred beaches, and transgressive coastlines. The model considers both lateral and vertical migration of a scour surface and its associated depositional products. As in studies of bed-form climb, we recognize subcritical, critical, and supercritical climb of scour surfaces relative to adjacent depositional forms. For a given erosion surface, the amount of underlying material removed is dictated by the climb angle of the erosion surface. Preservation of sequences also d pends on whether conditions were relatively variable or invariable. One important conclusion of this study is the recognition of the bed-form climb angle's control on the thickness of a sedimentary facies. Estimates of paleochannel depth or paleowater depth based on facies thickness can be made only if the climb angle is zero. By combining depositional models of fluvial, nearshore, and offshore facies, it is theoretically possible to model depositional systems such as deltas and passive continental margins. Potential applications are limited by several factors, including influence of sea level, climatic fluctuations on sedimentation, and variable migration of facies (or nonideal behavior or sedimentary environments). In addition, negative climb angles or climb angles less than horizontal can result in the removal of large amounts of material. Such negative climb angles may be caused by drops in eustatic sea level and lowering of base levels of drainage systems. We believe that this heuristic model has considerable potential in helping sedimentologists and stratigraphers predict the consequences of facies mi ration.