Controls on Forced Regression Stratal Architecture

Henry W. Posamentier and William R. Morris

Forced regression is defined as the process by which a shoreline migrates seaward in direct response to falling relative sea level. The stratal architecture associated with this process can be highly variable and is a function of 1) the physiography of the freshly exposed sea-floor, 2) the ratio of sediment flux to the rate of sea level fall, 3) the "smoothness" of the falling limb of the sea level curve, 4) the variability of sediment flux during relative sea level fall, and 5) the changes of dominant sedimentation process during relative sea level fall as the depocenter is forced seaward.

When sea level falls, progressive down-stepping of the coastal plain commonly accompanies forced regression. This can occur in discrete steps if the rate of sea level fall and/or sediment supply is discontinuous. These steps form during short-lived sea level stillstands superimposed on a period of overall sea level fall, or during periods of rapid shoreline regression accompanying an abrupt increase in sediment flux at the shoreline. If forced regression is associated with a constant sediment flux and a constant rate of relative sea level fall, it is possible that down-stepping of the coastal/alluvial plain will comprise many small indiscernible steps rather than several marked down-steps.

As forced regression proceeds, progressive cannibalization of earlier-formed deposits causes changes in the sediment supply. With successive periods of sea-level fall, lowering of stream profiles results in erosion of potentially sand-rich sub-environments such as distributary mouth bars, shorefaces, and point bars. This addition of a coarser-grained fraction in a downstream location may result in a shift toward dominance by bedload. This change may result in predictable changes of stratal architecture and delta type; suspension and inertia-dominated deposition at the river mouth will give way to traction deposition.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California