Depositional systems and tectonic/eustatic history of the Oligocene Vicksburg episode of the northern Gulf Coast
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Regional depositional systems analyses combining surface and subsurface geological and geophysical data provide the framework for a sequence stratigraphic study of the Lower Oligocene Vicksburg Formation of the Gulf Coastal Plain. The results describe the eustatic history of the Vicksburg stratigraphic unit. The two primary Texas depocenters, the Houston embayment and the Rio Grande embayment, were separated by a deep-rooted structural nose: the San Marcos arch. A barrier / strandplain intervened between the Louisiana deltaic depocenter and the Houston embayment. Within the embayments, deltaic complexes merged along strike with barrier / strandplains. Contemporaneous growth faulting controlled deltaic depositional patterns in the Rio Grande embayment and, to a lesser degree, in the Houston embayment. Smaller wave-dominated delta complexes interspersed with barrier / strandplains extended across the San Marcos arch. Updip of the paralic depocenters, fluvial systems traversed coastal plain units. Seaward of the paralic systems, sand and mud deposits prograded across and built up over the relict Jackson shelf and shelf margin. The contact between the Vicksburg Formation and the underlying Jackson Group marks the position of the Eocene - Oligocene boundary within the Gulf Coastal Plain section. On regional dip-oriented well-log cross sections there is a distinct, abrupt, seaward shift in the paralic facies at the Jackson - Vicksburg boundary; this contact corresponds to an Exxon-model Type 1 unconformity. The unconformity is related to the development of an Antarctic ice sheet in the earliest Oligocene. During middle Vicksburg time, a minor transgression (genetic stratigraphic sequence boundary) flooded the coastal plain. Overlying the progradational Vicksburg Formation, the lower Frio Formation accumulated in an aggradational mode; this switch of depositional modes corresponds to an Exxon-model Type 2 sequence boundary. Construction of genetic stratigraphic sequence diagrams and comparison to Exxon's coastal onlap curves across different areas of the Oligocene coast show that the effects of local depocenters (sediment influx) may mask eustatic effects. Only truly regional events, such as the middle Vicksburg transgression and the basal Vicksburg seaward shift in coastal position, correlate across the coastal plain and may result from a eustatic change.