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

Abstract

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.

Keywords

• falling‐stage
• forced regression
• sequence stratigraphy
• sharp‐based shoreface