--> ABSTRACT: Depositional Sequences and Their Impact on the Stratigraphic Level of Planktonic Foraminiferal Zone Boundaries in Upper Cretaceous and Paleogene Strata of the Eastern Gulf Coastal Plain, by Ernest A. Mancini, Berry H. Tew; #91020 (1995).

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Depositional Sequences and Their Impact on the Stratigraphic Level of Planktonic Foraminiferal Zone Boundaries in Upper Cretaceous and Paleogene Strata of the Eastern Gulf Coastal Plain

Ernest A. Mancini, Berry H. Tew

The Upper Cretaceous and Paleogene strata of the eastern Gulf Coastal Plain can be divided into depositional sequences that are associated with cyclic changes in eustatic sea level. The stratigraphic level of planktonic foraminiferal zone boundaries within a given depositional sequence is strongly influenced by the presence of unconformities, and associated biostratigraphic discontinuities resulting from nondeposition or erosion, the presence or absence of lowstand systems tract (LST) strata, the differential amounts and rates of sedimentation associated with paleobathymetry and/or distance from the shoreline at various depositional sites, differential subsidence within the depositional basin, and paleoenvironmental conditions associated with a particular sequence. The zo e boundaries in these strata correspond to the basal surfaces of marine transgressions. In sequences lacking recognizable initial transgressive systems tract (TST) strata, that is, where late transgressive or condensed section (CS) deposits directly overlie HST strata of an underlying depositional sequence, the zone boundaries occur within CS deposits. In these cases, the maximum flooding surface, the initial transgressive surface, and the sequence boundary generally coincide. This relationship is common where the terrigenous sedimentation rate is very low at the site of deposition and where relative sea level rise overwhelms the preexisting HST due to a combination of subsidence and eustasy. In updip (that is, landward) areas, the sequence boundary, initial transgressive surface, and su face of maximum sediment starvation are in close proximity in a stratigraphic sense because there is less accommodation for accumulation of sediment and that the transgressive and CS deposits tend to be rather thin. Therefore, when a particular biostratigraphic datum is traced basinward relative to a physical correlation surface, the datum is elevated in the stratigraphic section due to the increased thickness of the intervening strata. In this context, downdip would include the areas of more rapid subsidence (greater accommodation) and greater rates of sedimentation. In general, occurrences of age diagnostic planktonic foraminifera necessary for biozonal assignments arc confined to glauconitic sand and marl beds of the TST and associated CS of the depositional sequences.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995