Summary of Sequence Stratigraphy
Research Group Meeting
AAPG Annual Meeting, San Antonio, April 21, 2008
The research group discussion focused on whether or how we can use stacking patterns for sequence stratigraphic interpretations, and what is the significance of realizing that formation of stratigraphic surfaces may have have a lag time in relation to the actual change in the controlling factors.
Andy Petter from UT Austin presented a talk on Aggrading alluvium during base-level fall, with lowstand shoreline detachment from the alluvial river.
Physical and numerical modeling of fluviodeltaic depositional systems, conducted under constant conditions of relative sea-level fall, sediment discharge, and water discharge, demonstrates that “sustained” alluvial aggradation is the inherent stratigraphic response of alluvial rivers that are steeper than the receiving basin. High alluvial gradients are primarily a consequence of high sediment supply relative to available water discharge (low rate of terrestrial diffusion). During the course of the experiments, fluvial grade was never reached, fluvial incision was never observed, and aggradation occurred independently of shoreline position. The experiments revealed detachment of the shoreline from the alluvial river as an autogenic response of certain fluviodeltaic systems to steady relative sea-level fall, creating a “non-deltaic” alluvial-river system that continues to aggrade beyond shoreline detachment. We term this behavior “autodetachment”. Autodetachment is a consequence of increasing partioning of sediment supply into the ever-enlarging alluvial profile that occurs to the detriment of delta-front deposition prior to shoreline detachment. Thus, as relative sea level fell, the length of the delta front decreased to zero. Simultaneously, alluvial aggradation also led to decreasing rates of delta progradation. Therefore, as the delta front disappeared, the rate of shoreline regression exceeded the rate of progradation, producing a “bedrock” river between the detached alluvial river and the shoreline. Alluvial aggradation during relative sea-level fall due to low shelf gradients has been envisioned by previous researchers, but this model is the first to realize the full implications of this behavior. Our model demonstrates that deltas with these geometric parameters become poorly supplied during regression, and large lowstand deltas should not be expected for these types of systems. The results of this study complement existing autostratigraphic theory, and provide an alternative to allogenic drivers for geomorphic and architectural evolution of fluviodeltaic wedges. Extensive alluvial aggradation has been generally interpreted as the result of relative sea-level rise, but we show that analysis of relative sea-level changes from ancient successions need to account for geometric parameters of the fluviodeltaic wedge. Additionally, our model may provide an explanation for anomalous sequences, such as the lower Castlegate Formation of the Book Cliffs, that have been argued to have been deposited by terminal rivers. Finally, these behaviors may be significant for lacustrine stratigraphy due to coupling between discharge and lake levels.