NUMERICAL MODELLING OF DEPOSITIONAL SEQUENCES IN HALF-GRABEN RIFT BASINS

Abstract

A three-dimensional numerical model of sediment transport and deposition in coarse-grained deltas is used to investigate the controls on depositional sequence variability in marine half-graben extensional basins subject to eustatic sea-level change. Using rates of sea-level change, sediment supply and fault slip reported from active rift basins, the evolution of deltas located in three contrasting structural settings is documented: (1) footwall-sourced deltas in high-subsidence locations near the centre of a fault segment; (2) deltas fed by large drainage catchments at fault tips; and (3) deltas sourced from drainage catchments on the hangingwall dip slope. Differences in the three-dimensional form and internal stratigraphy of the deltas result from variations in tilting of the hangingwall and the impact of border fault slip rates on accommodation development. Because subsidence rates near the centre of fault segments are greater than all but the fastest eustatic falls, footwall-sourced deltas lack sequence boundaries and are characterized by stacked highstand systems tracts. High subsidence and steep bathymetry adjacent to the fault result in limited progradation. In contrast, the lower subsidence rate settings of the fault-tip and hangingwall dip-slope deltas mean that they are subject to relative sea-level fall and associated fluvial incision and forced regression. Low gradients and tectonic tilting of the hangingwall influence the geometry of these deltas, with fault-tip deltas preferentially prograding axially along the fault, creating elongate delta lobes. In contrast, broad, sheet-like delta lobes characterize the hangingwall dip-slope deltas. The model results suggest that different systems tracts may be coeval over length scales of several kilometres and that key stratal surfaces defining and subdividing depositional sequences may only be of local extent. Furthermore, the results highlight pitfalls in sequence-stratigraphic interpretation and problems in interpreting controlling processes from the preserved stratigraphic product.