3-DIMENSIONAL INTERPLAY OF ADVECTIVE AND DIFFUSIVE PROCESSES IN THE GENERATION OF SEQUENCE BOUNDARIES

Quantitative stratigraphic models have been generated using either geo metric constructs or diffusion algorithms to simulate the development and timing of sequence boundaries and how they vary in response to tec tonics, eustasy, climate, and sediment supply. While diffusion models provide a physical basis for the generation of clinoforms, the resulti ng model predictions appear to do little more than track the transgres sive and regressive movements of the shoreline. On the other hand, geo metric models do a relatively good job in simulating the observed stra tigraphy but offer little in actually ascertaining the physics control ling the processes of deposition and erosion. All these stratigraphic models fail to predict the most fundamental response of depositional s ystems, namely, the development of a downward shift in coastal onlap d uring a fall in relative sea level. This basinward shift and the devel opment of an onlap surface are the critical observations that define c hronostratigraphic sequences and are not simply a consequence of shore line transgressions and regressions. A major assumption in these two-d imensional models is that the observed stacking patterns are solely th e consequence of deposition processes and sediment transport within th e modelling section, in marked contrast to the observation that the ma jor sediment transport direction along margins is sub-parallel to the margin. We present a variation on the diffusion theme in which across- margin transport and deposition are dominated by advection-type proces ses and along-margin erosion and deposition processes are dominated by diffusion. With this combination of advection and diffusion it is pos sible to generate a lowstand system tract and sequence boundaries (i.e ., onlap surfaces). In particular, the generation of the lowstand syst em tract is, in large part, the result of depositional processes opera tive along the margin (i.e., out of the section).