Sharp-based shoreface successions reconsidered in three-dimensions: A forward stratigraphic modelling perspective

Abstract

Sea-level fall is commonly inferred to generate a sharp-based shoreface succession that displays an abrupt vertical transition from heterolithic, lower shoreface to sandy, upper shoreface deposits across a marine erosion surface. Three-dimensional, process physics-based, coupled hydrodynamic-morphodynamic models are constructed to compare bedding architecture and facies patterns of wave-dominated delta deposits preserved during normal (static sea level) and forced (falling sea level) regression and then transgression during subsequent sea-level rise. The models suggest that wave-dominated deltas will develop a sandy shoreface inner clinoform dipping from the subaerial delta plain to a relatively flat wave-scoured subaqueous delta top, which is laterally separated from a delta front outer clinoform that dips from the subaqueous delta top edge to the shelf floor. As these systems prograde, deposits of these dual-clinoforms will become vertically stacked and will be separated by a regressive surface of marine erosion. Significant grain-size contrasts between these vertically stacked clinoform deposits reflect differences in sediment-transport directions and sorting under river and wave-driven littoral currents along the coast, and cannot be related uniquely to sea-level changes. The marine erosion surface under a sharp-based shoreface deposit records abrupt facies shift across a kilometres-wide, wave-eroded surface and defines a discontinuity in the preserved vertical succession. The continuity of a regressive surface of marine erosion mapped over many tens to hundreds of kilometres across mid-shelf regions of some stratigraphic sequences reflects a gradual lateral shift in the position of littoral current erosion on a subaqueous delta top. Timelines cross such vertical lithic discontinuities throughout the extent of a prograding deposit, and the regressive surface of marine erosion thus has little chronostratigraphic significance. The results of these models suggest caution in inferring sea-level changes from the character of vertical facies changes observed in individual well logs and isolated outcrop exposures.