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The lower Tuscaloosa Formation in east-central Louisiana and southwestern Mississippi consists of a southward-thickening wedge of coarse to fine-grained sandstones and shales. The Lower Cretaceous carbonate-shelf margin is a natural boundary separating the updip fluvial depositional system from the downdip deltaic depositional system. The lower Tuscaloosa Formation thickens from a minimum thickness of 120 ft (37 m) updip to a penetrated thickness of 2,800 ft (850 m) downdip. The tectonic setting is one of regional uplift to the north, local uplifts within the alluvial valley, and growth faulting south of the Lower Cretaceous shelf margin.
Based on analyses of core, lithologic logs, and a series of sandstone isolith maps constructed from electric logs, the lower Tuscaloosa is interpreted to have been deposited in 3 major depositional systems: (1) a fluvial system, (2) a deltaic system, and (3) a barrier-island system. Seven subenvironments of these depositional systems include: (1) fluvial channels, (2) floodplains, (3) deltaic distributary channels, (4) distributary natural levees, (5) crevasse splays, (6) shallow-marine bays, and (7) accretion ridges.
The deltaic depositional system is composed of several fluvial-dominated, wave-influenced deltas. The distribution of deltaic depositional subenvironments was influenced by the tectonics of the basin and by the near-shore wave energy. This type of delta persisted throughout most of the lower Tuscaloosa deposition.
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