Forced Regressions in a Sequence Stratigraphic Framework: Concepts, Examples, and Exploration Significance

ABSTRACT Sequence stratigraphic concepts suggest that stratal geometries develop and are largely controlled by changes in relative sea level. On the shelf, lowstand deposits, which form during falls and subsequent stillstands of relative sea level, can be recognized by the presence of an unconformity at the base, the isolated and basinward position relative to the previous shoreline, and the abrupt seaward translation of shallow-water and shoreline facies into the basin across an unconformity surface. This seaward translation of facies and shoreline regression in response to relative sea level lowering is termed a “forced regression.” A forced regression is independent of variations of sediment flux and is in contrast with “normal” regressions that occur in response to excess sediment flux relative to space available on the shelf (i.e., accommodation). Forced regressions commonly are associated with a zone of sedimentary bypass, subaerial exposure, and possible fluvial erosion between the newly formed and preceding shorelines. Certain shelf sands, previously interpreted as offshore or mid-shelf sand bodies, thus can be reinterpreted as stranded lowstand shorelines associated with forced regressions. This alternative interpretation has economic significance insofar as it suggests different subsurface correlations and reservoir geometries with the potential for development of new play types and enhanced recovery in older fields. Examples of forced regression can be observed at a variety of scales and ages. Several such examples include the modern East Coulee fan delta and the Lower Cretaceous Viking Formation in Alberta, Canada, the Quaternary Rhône Delta, and the Quaternary Hudson Valley system.