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Stratigraphic Signature of Coastal Transgressions

Ron Boyd, Shea Penland, John R. Suter

Coastal transgression can produce a range of continental shelf sand bodies. Four categories of transgression can be recognized, each of which produces a characteristic shelf stratigraphy. (1) Rapid erosional transgression generates a discontinuous stratigraphy of overstepped coastal barriers separated by a thin transgressive veneer. (2) Slow erosional transgression generates a thin transgressive veneer overlying an unconformity. (3) Slow erosional transgression followed by stillstand generates a thin transgressive veneer terminated updip by a fully preserved coastal sequence. (4) During slow depositional transgression, a variable component of the coastal sequence is incorporated into the shelf stratigraphic record. These four categories are not distinct, but represent intervals of transgressive spectrum that is governed by sediment supply, regional gradient, relative sea level, and coastal oceanography. The preservation potential of transgressive stratigraphy is primarily governed by the translation pa h of the shoreface. A simple coastal stratigraphy may consist of a preexisting substrate, fine-grained back-barrier deposits, sandy barrier deposits, and a shelf mud. Horizontal translation of a deep shoreface incises an erosional unconformity into the preexisting substrate, which in turn is overlain by a sandy lag veneer and a shelf mud. Horizontal and vertical translation of a shallower shoreface leaves the preexisting substrate intact, preserving (1) part of the back-barrier deposits, (2) all back-barrier deposits, or (c) all back-barrier deposits plus part of the barrier deposits, overlain by an erosional unconformity, a sandy lag veneer, and shelf mud. Abrupt horizontal and vertical shoreface translation cannot rework the entire shelf-retreat path of the coastal zone. Therefore, par ly reworked segments of the coastal sequence are incorporated into the shelf stratigraphic sequence results from horizontal and vertical translation of a shallow shoreface receiving active sediment supply. Under these circumstances, the coastal stratigraphy may be preserved intact, and erosional unconformities may be absent.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.