Forward stratigraphic modelling of forced regressions: evidence for the genesis of attached and detached lowstand systems

Abstract

AbstractA complex series of interactions between subsidence, eustasy and sediment supply determine whether a forced regressive shoreface will be physically attached to underlying sandy deposits, or detached and encased in marine mudstone. Using a Shell-developed and proprietary forward stratigraphic modelling system, these interactions of controls on clastic depositional geometries can be simulated. Upper Cretaceous subsurface and outcrop data from the Western Interior Basin of Canada form the basis of this study. The outcrop data suggest that high frequency (200 000 years), relatively low amplitude (8 m) relative sea-level changes occurred during deposition of the studied succession. When these parameters are convolved with the Haq third-order eustatic sea-level curve and a constant rate of subsidence and sediment supply, a series of attached lowstand, forced regressive shorefaces are generated by the forward stratigraphic modelling system. In order to generate a detached lowstand shoreface a relatively large magnitude, high frequency eustatic fall or a high frequency tectonic uplift is required. The modelling studies also suggest that (i) forced regressive deposits preferentially develop as attached lowstand systems, (ii) detached lowstand systems in most cases initially develop as attached lostand systems which are subsequently detached by transgressive-regressive wave erosion, (iii) if the sequence boundary is picked below the sharp-based, forced regressive shoreface (attached and detached lowstand systems tract interpretation) or above it (falling-stage systems tract interpretation) it must remain in that position relative to the correlative down-dip, sharp-based shoreface sandbodies regardless of whether those bodies are attached or detached and (iv) the role of sediment supply as a controlling parameter in the generation of forced regressions appears to be a secondary one.