Three-dimensional numerical modeling of coarse-grained elastic deposition in sedimentary basins

A three-dimensional numerical model of elastic deposition in sedimentary ba sins is used to investigate the development of coarse-grained deltas in res ponse to relative sea level changes. The model incorporates fully three-dim ensional sediment delivery and deposition, together with incised channel fo rmation and slope failure. A combined random-walk/steepest-descent algorith m is used to simulate sediment delivery from a drainage basin outlet to the depositional shoreline, together with a nonlinear three-dimensional diffus ion equation for modeling slope-dependent sediment erosion, transport, and deposition. When sediment channels (transport pathways) are out of a specif ied grade, advective erosion occurs, thereby increasing the total sediment supply delivered to the basin. Multiple sediment sources into a basin can b e simulated, together with tectonic subsidence and eustatic sea level chang es. The model results presented display many of the key features observed i n both modern and ancient coarse-grained deltas. During periods of slowly c hanging and high relative sea level (highstands), a broad delta front progr ades approximately radially away from the sediment source. Conversely, inci sed channels are developed during rapid falls of relative sea level, leadin g to localized deposition of deltaic lobes at the mouths of these channels. The incised channels initiate at the exposed delta front and propagate bac kward toward the sediment source. The rate of growth of incised channels is shown to be an important control on delta morphology, sediment distributio n, and lowstand lobe size and location. Lobes are particularly well develop ed during periods of slowly changing low relative sea level (lowstands). Pe riods of rapid rise of relative sea level lead to flooding and infilling of incised channel systems and a rapid landward shift in deposition (transgre ssions). Incised channels and delta lobes are best developed in a model whe re 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 simi lar to that expected for footwall-sourced deltas in such settings. Many of the features developed in the models are, by their nature, three dimensiona l, 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 th e sequence stratigraphic methodology to many sedimentary basins.