Wonsuck Kim, PhD, University of Texas (Austin)
Tectonic influence on deltas has long been recognized for its importance in morphodynamic and stratigraphic development. In the 1styear of the project we explored the control of lateral tectonic tilting on a prograding fluviodeltaic system through six laboratory experiments with a range of tilting rates. Tilting was applied along a rotational axis that bisects the center of the experimental delta, which forced uplift in one half of basin and subsidence in the opposite half. Results from these experiments have given insight into the surface and stratigraphic responses of deltas.
1) Introducing lateral tectonic tilting into an experimental fluviodeltaic system, where uplift occurs on one side of the delta and subsidence is occurring on the other, changes the dynamics of the system with regards to surface processes, resulting in an asymmetric shoreline. The deltaic shoreline position progrades faster under active uplift (relative base level fall) than a shoreline undergoing subsidence (relative base level rise). Despite the ability of subsidence to steer channel and distribute more sediment to the subsidence side, shoreline rates still do not increase due to an active rise of relative base level, as seen in the two strongest tectonic runs.
2) A delta undergoing differential subsidence will preferentially route sediment to the area of maximum subsidence. If the difference in subsidence is great enough, the fluvial system will starve and eventually abandon the portion of the delta that is undergoing relative uplift and dedicate all of the sediment and water discharge to the portion of the delta undergoing the most subsidence. If the difference in subsidence is comparable to the surface reworking, the fluvial channels slightly erode the uplift side and transport larger amount of sediment through the shoreline in the uplift side.
3) Implications of lateral tectonic tilting can be seen in large-scale basin stratigraphy. When the tectonic tilting rate is large enough to induce changes in sediment volume dispersal, it is also large enough to change the stratigraphic stacking patterns of the deposit. Under no tilting and the minimum rate of tilting, there is little different in basin stratigraphy exhibited laterally across the basin. As tilting increases and the timescale ratio nears T* = 0.5, it begins to show amalgamated channels where uplift occurs, and slightly aggradational stacking of channel sand bodies. Overland floodplain deposits representing fine material are still present throughout, however sediment partitioning begins, preferentially routing more material towards the area of maximum subsidence. As the tectonic timescale converges with the channel timescale (T* ≥ ~1), the uplift portion of the delta is re-worked, removing finer material and leaving thin, laterally linked amalgamated channel sands in place while depositing a higher percentage of the sediment volume to the area where more subsidence is occurring. The further relative base level rise (T*≥ ~10), along with the relative increase in sediment discharge in this area, results in an aggradational system where thin amalgamated channel bodies on the uplift portion shift laterally into stacked sandy channel belts separated by fine overland flood deposits.
4) The Ganges-Brahmaputra (G-B) delta system serves as a compelling analogue to the series of lateral tectonic tilting experiments performed in the STEP basin. Differential subsidence (less subsidence or possible uplift in the western portion versus increases subsidence in the eastern portion) results in geomorphic and stratigraphic changes in the same deposit laterally and vertically through geological time. We found that the asymmetry seen in the shoreline of the G-B delta is less likely to be entirely attributed to tidal effects, and instead can be better explained by the differential subsidence observed along the delta due to subduction along the Burma Arc. The current G-B delta has T* = 0.4 – 0.8 similar to the low and medium rotation runs. This is backed by asymmetry in the stratal architecture, where sandier channel belt deposits are found in the western delta versus an increased prevalence of flood deposits in the eastern portion. Shoreline asymmetry, stratigraphic differences, and the theoretical timescales all point towards the comparable tectonic movement to the fluvial activity as being the domineering factor shaping the G-B delta.