Novel discrete element modelling of Gilbert‐type delta formation in an active tectonic setting—first results

Abstract

Gilbert deltas are now recognised as an important stratigraphic component of many extensional basins. They are remarkable due to their coarse‐grained nature, large size and steep foresets (up to 30–35°) and may exhibit a variety of slope instability features (faulting, slump scars, avalanching, etc.). They are also often closely related to major, basin‐margin normal faults. There has been considerable research interest in Gilbert deltas, partly due to their economic significance as stratigraphic traps for hydrocarbons but also due to their sensitivity to relative base level changes, giving them an important role in basin analysis. In addition to field studies, numerical modelling has also been used to simulate such deltas, with some success. However, until now, such studies have typically employed continuum numerical techniques where the basic data elements created by simulations are stratigraphic volumes or timelines and the sediments themselves have no internal properties per se and merely represent areas/volumes of introduced coarsegrained, clastic and sedimentary material. Faulting or folding (if present) are imposed externally and do not develop (naturally) within the modelled delta body itself. Here, I present first results from a novel 2D numerical model which simulates coarse‐grained (Gilbert‐type) deltaic sedimentation in an active extensional tectonic setting undergoing a relative base level rise. Sediment is introduced as packages of discrete elements which are deposited beneath sea level, from the shoreline, upon a pre‐existing basin or delta. These elements are placed carefully and then allowed to settle onto the system. The elements representing the coarsegrained, deltaic sediments can have an intrinsic coefficient of friction, cohesion or other material properties appropriate to the system being considered. The spatial resolution of the modelling is of the order of 15 m and topsets, foresets, bottomsets, faults, slumps and collapse structures all form naturally in the modelled system. Examples of deltas developing as a result of sediment supply from both the footwall and hanging‐wall of a normal fault, and subject to changes in fault slip rate are presented. Implications of the modelling approach, and its application and utility in basin research, are discussed.