INTERACTIONS BETWEEN DELTAIC SEDIMENTATION AND SUBSTRATE DEFORMATION OBSERVED IN PHYSICAL EXPERIMENTS (Invited Presentation)

Physical (flume) experiments were undertaken to observe the feedbacks between sedimentation on model deltas composed of sand and loading-induced migration of deformable substrates consisting of mixtures of kaolinite and water. The purpose was to provide insights into natural systems in which the weight of deltaic deposition mobilizes undercompacted marine muds and, as a consequence, the deltas become captured within rapidly subsiding, growth-faulted compartments. In 1-D experiments, the deltas were observed to prograde over the deformable substrate which simultaneously migrated from below the clinoform rollover to the region basinward of the delta foreset through internal shearing. Consequently, the deltas experienced subsidence, faulting, and stratal growth at their shorelines and uplift of a decollement-based, internally sheared critical-taper wedge beyond their toes. A 2-D experiment in the St. Anthony Falls Laboratory Delta Basin facility was designed so that half of the basin contained the deformable substrate mixture, while the other half of the basin had a non-deformable substrate. Loading-induced subsidence due to deposition at channel mouths on the deformable half of the basin caused the channels to be relatively stable. Avulsions and lateral migration of channels were suppressed because elevation of the channel bed by deposition, a primary driver of channel instability, was offset by the loading-induced subsidence. Consequently, channels remained on the deformable half of the basin for a longer fraction of the experimental run-time and a greater fraction of the total sediment input was delivered and stored on this half of the basin. Elongate delta lobes were also constructed during periods of fast subsidence as the stabilized channels built out into the basin for extended intervals. The results of this study have implications for the interpretation of upper crustal and surficial conditions from growth-faulted stratal successions at the scale of both deltas and continental margins, as well as for the improvement of predictive land-building models used in coastal restorations. The subsidence generated by substrate migration influences not only the mass requirements for deltaic land building, but also may impact the behavior of the lowland river on the delta plain.