Investigating Upstream to Downstream Evolution of Ancient Fluvio-Deltaic Channels using Quantitative Seismic Geomorphology

Abstract:

Reconstructing autogenic processes and ancient fluvial landscapes provides an important for understanding intrinsic rivers dynamics under different climate, tectonic, and land-cover conditions through Earth history. While outcrops are commonly used for reconstructing ancient fluvial landscapes, 3D seismic surveys also provide an avenue for understanding intrinsic river dynamics, including lateral migration and avulsion. Deltas are some of Earth’s most populated, dynamic natural systems, and understanding they evolve morphologically from upstream to downstream is important coastal restoration, hazard mitigation, and natural-resource management. Backwater hydrodynamics that result from a river running into a standing body of water significantly impact channel morphology and channel mobility, even many 10s-100s of km upstream of the shoreline. Many recent studies have documented how fluvial morphology and mobility are influenced by backwater hydrodynamics; however, the full magnitude and upstream extent of those changes is still unclear.

Here we evaluate autogenic dynamics in a continental scale fluvio-deltaic system and explore how channel geometry, sinuosity, bifurcation and avulsion behavior changed from proximal (upstream of the backwater zone) through the backwater zone in the Mungaroo Formation (Triassic, Carnavon Basin, offshore Northwest Australia. We mapped >200 channel bodies observed in eight 3D seismic volumes spanning a downstream distance of over 180 km and measured channel-deposit thicknesses in >20 well logs throughout the study area. In general, we found that channels upstream of the estimated backwater zone were smaller and more sinuous and recorded more evidence for both lateral channel migration and avulsion that channel deposits in the backwater zone. We use a channel-centerline-based river-meandering model to assess the role of floodplain controls on lateral migration of Mungaroo channels and evaluate the degree to which changes in channel curvature and meandering correspond to proximal-to-distal changes in bulk grain size observed in well logs. We show how ancient fluvial morphodynamic interpretations extracted from subsurface seismic data can help answer outstanding questions about channel-floodplain processes in modern systems.