ABSTRACT: Aspects of Sequence Stratigraphy: Recent and Ancient Examples of Forced Regressions
Henry W. Posamentier, David P. James, George P. Allen
Sequence stratigraphic concepts suggest that stratal geometries develop and are largely controlled by changes in base level (i.e., relative sea level). On a ramp margin, lowstand deposits which form during falls of relative sea level can often be recognized by the presence of an unconformity at their base and by the abrupt seaward translation of shallow-water and shoreline facies into the basin. This translation of facies in response to base level lowering is termed a forced regression. Examples of forced regression can be observed at a variety of scales and ages. Two such examples are the recent East Coulee fan-delta and the Lower Cretaceous Viking Formation, both from Alberta, Canada.
The East Coulee delta, which formed in a roadside ditch as water level rose and then fell, provides a good small-scale example of many aspects predicted by the sequence stratigraphic models. In response to changes in accommodation, several clearly recognizable systems tracts formed, including highstand, transgressive, and lowstand systems tracts. The lowstand deltas and accreted shorelines within the lowstand systems tracts comprise a forced regression. In each case, the shoreline has stepped abruptly basinward and is separated from the immediately preceding shoreline by a zone of sedimentary bypass.
The Lower Cretaceous (Albian) Viking Formation was deposited in a variety of marine depositional environments and is characterized by a number of forced regressions. Most are manifested by isolated shoreface units encased in offshore shales. The similarity between the stratal geometries characterizing lowstand units within the Viking Formation and the East Coulee delta suggests that the processes involved and the stratigraphic response to relative sea level change are essentially scale and time independent.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990