The effects of lateral tectonics on a fluvio-deltaic system : an application to the Ganges Brahmaputra Delta

Abstract

Deltaic systems have long been recognized for their socioeconomic impacts as well as their high potential to trap and store hydrocarbons. The Sediment Transport and Earth-surface Process (STEP) basin at the University of Texas at Austin has the ability to create large 3D physical experiments, designed for nurturing new understanding of these systems and the parameters that influence their evolution. We explored how a laterally tilting basin influenced a prograding fluvio-deltaic system. The tilting occurs along a rotational axis, bisecting the model’s basement and allowing the delta to experience uplift in one half of basin and subsidence in the opposite half. After six experiments with a range of tilting rates, we observed that varying rates of tilting changed progradation patterns as well as the resultant stratigraphy. The tectonic tilting forced a continuous change in topset slope, which accounts for the evolving behavior of the fluvial system with regards to channel occupation and thus shoreline asymmetry. When slow tilting was applied, the delta advanced faster in the direction of uplift due to the relative decline in basin water depth. This created truncated stratigraphic intervals dominated by active channel cut and fill with thin but laterally linked channel bodies depositing finer material. Behavior was significantly different on the subsidence side of the delta; shoreline migration was stunted while the delta became primarily aggradational, depositing thicker, alternating packages of sands. During higher rates of tilting, deposition at the uplift end was quickly abandoned and instead focused on stacking conformable sequences of delta lobes in the area of increased subsidence, resulting in a complete lack of progradation in any direction. Progressively greater rates of tilting yielded more dramatic steering of channelized flow toward the area of greatest subsidence. Comparing characteristic tectonic and channel timescales proves to be a good predictor of shoreline symmetry along with sediment distribution due to differential subsidence. In this study, we tested the hypothesis that differential subsidence acting on the Ganges-Brahmaputra (G-B) system is responsible for delta asymmetry. The asymmetry in planform shoreline geometry and subsurface stratigraphy of the G-B delta system are extensively similar to the experimental results.