FLUVIAL STACKING PATTERNS IN RESPONSE TO ACCOMMODATION CHANGES: INSIGHTS FROM TRACKING EXPOSURE SURFACES (AS PALEOSOL PROXIES) IN A NUMERICAL MODEL OF A FLUVIAL-DOMINATED DELTA (Invited Presentation)

Extending sequence stratigraphic concepts into fluvial strata is problematic because key criteria such as geometric observations (e.g., coastal onlap, shoreline trajectory) and surfaces (e.g., marine flooding surfaces) are not present in nor easily inferred from fluvial deposits. Numerical modeling of a fluvial-dominated delta provides a 3D dataset with a known base-level history, clear (apparently “realistic”) geometries, and grain size distributions. Time surfaces can be placed on cross sections and extend from the marine into the fluvial strata. Cumulative exposure time was calculated for each point in the numerical model to track areas of potential paleosol development and their likely maturity. Preliminary results show that base level trends can be seen in many portions of the fluvial strata. Fluvial strata deposited during periods of increasing accommodation are composed of stacked autogenic cycles showing a vertical decrease in exposure time (i.e., decreasing paleosol maturity). Conversely, fluvial strata deposited during periods of decreasing accommodation are composed of stacked autogenic cycles showing a vertical increase in exposure time (i.e., increasing paleosol maturity). However, in some parts of the system the autogenic processes seem to obscure clear evidence of the base-level changes. Although more work is required, this numerical approach may eventually lead to a better understanding of: 1) what the stacking patterns and facies relationships are in the non-marine and their relationship with the corresponding marine deposits, 2) under what conditions and scales autogenic vs allogenic forcing is more likely to be the dominant signal in the rock record, and 3) the geometric scale, time significance, and correlation lengths of depositional and pedogenic features.