Aspects of the stratal architecture of forced regressive deposits
Forced regression refers to the process of seaward migration of a shoreline in direct response to relative sea-level fall. Recognition criteria for forced regressive deposits include: (1) presence of a significant zone of separation between successive shoreface deposits, (2) the presence of sharp-based shoreface/delta front deposits, (3) the presence of progressively shallower clinoforms going from proximal to distal, (4) the occurrence of long-distance regression, (5) the absence of fluvial and/or coastal plain/delta plain capping the proximal portion of regressive deposits, (6) the presence of a seaward-dipping upper bounding surface at the top of the regressive succession, (7) the presence of increased average sediment grain size in regressive deposits going from proximal to distal and (8) the presence of ‘foreshortened’ stratigraphic successions.
The principal factors driving the stratal architecture of forced regressive deposits include: (1) the gradient of the sea floor progressively exposed by falling relative sea-level, (2) the ratio of the sediment flux to the rate of relative sea-level fall, (3) the ‘smoothness’ of relative sea-level fall, (4) the variability of sediment flux and (5) the changes of sedimentary process that occur as sea-level falls and progressively more of the shelf is subaerially exposed.
Forced regressive deposits are grouped into attached v. detached, and smooth-topped v. stepped-topped. Attached deposits are defined as successive downstepped stratigraphic units whose shoreface/delta front deposits are generally in contact with each other. In contrast, detached deposits are defined as successive downstepped stratigraphic units whose shoreface/delta front deposits are generally not in contact with each other. Rather, in this instance a zone of sedimentary bypass exists. Stepped-top forced regressive deposits are characterized by a succession of horizontally topped though downstepping stratigraphic units. In contrast, smooth-topped forced regressive deposits are characterized by a seaward-dipping, albeit smooth, upper bounding surface. The bounding surfaces of forced regressive deposits commonly are expressed as a ravinement surface at the top and an unconformity to correlative conformity at the base.
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An increasing number of studies in recent years have demonstrated that significant progradation of shallow marine systems occurs under conditions of base-level fall. These new data are forcing many sedimentary geologists to critically re-evaluate many aspects of sequence stratigraphy relating to erosion and deposition during base-level (lake- or relative sea-level) fall, and the intrinsic link made between stratal geometries and base-level change. For the first time, this volume brings together a collection of articles that focus solely on forced regressions, providing a more complete picture of the development, formation, variability and preservation of the surfaces and deposits generated during base-level fall.
The results of the studies published here will be of interest to all geologists attempting to understand the relationship between changes in base-level and stratigraphy, and to all who use sequence stratigraphy as a method of stratigraphic correlation and interpretation at outcrop and in the subsurface.