Three‐dimensional numerical modeling of coarse‐grained clastic deposition in sedimentary basins

A three-dimensional numerical model of clastic deposition in sedimentary basins is used to investigate the development of coarse-grained deltas in response to relative sea level changes. The model incorporates fully three-dimensional sediment delivery and deposition, together with incised channel formation and slope failure. A combined random-walk/steepest-descent algorithm is used to simulate sediment delivery from a drainage basin outlet to the depositional shoreline, together with a nonlinear three-dimensional diffusion equation for modeling slope-dependent sediment erosion, transport, and deposition. When sediment channels (transport pathways) are out of a specified grade, advective erosion occurs, thereby increasing the total sediment supply delivered to the basin. Multiple sediment sources into a basin can be simulated, together with tectonic subsidence and eustatic sea level changes. The model results presented display many of the key features observed in both modern and ancient coarse-grained deltas. During periods of slowly changing and high relative sea level (highstands), a broad delta front progrades approximately radially away from the sediment source. Conversely, incised channels are developed during rapid falls of relative sea level, leading to localized deposition of deltaic lobes at the mouths of these channels. The incised channels initiate at the exposed delta front and propagate backward toward the sediment source. The rate of growth of incised channels is shown to be an important control on delta morphology, sediment distribution, and lowstand lobe size and location. Lobes are particularly well developed during periods of slowly changing low relative sea level (lowstands). Periods of rapid rise of relative sea level lead to flooding and infilling of incised channel systems and a rapid landward shift in deposition (transgressions). Incised channels and delta lobes are best developed in a model where tectonic subsidence is similar to that expected for hanging-wall-sourced deltas in extensional half-graben settings. In contrast, both channels and lobes are less well developed in a model where tectonic subsidence is similar to that expected for footwall-sourced deltas in such settings. Many of the features developed in the models are, by their nature, three dimensional, and two-dimensional analysis of the model results can lead to erroneous interpretations of the causes of along-strike variability. These aspects of sequence variability have important implications for the application of the sequence stratigraphic methodology to many sedimentary basins.