Depositional history of the Wilcox Group, east-central Louisiana



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The Wilcox Group in east-central Louisiana consists of a variable sequence of fine-grained sand, mud, and lignite units with a composite thickness of about 3,000 feet. Studies of sand-body geometry, lithologic composition, and facies relationships indicate that the Wilcox Group consists dominantly of deltaic plain deposits (the Holly Springs and an overlying, unnamed delta system) which filled the Mississippi trough during late Paleocene and early Eocene times. Depositional history of these deposits is divided into four phases: (1) a basal progradational phase, characterized by thick bar-finger and upper deltaic plain sequences indicative of delta construction onto a deep and muddy shelf; (2) a thick transgressive deltaic phase including several shoal-water delta lobes with many distributaries separated by destructional phase units; (3) an upper deltaic phase characterized by small shoal-water delta lobes; and (4) a fluvial-transgressive phase consisting of a massive sand unit of coalescing fluvial deposits capped by a veneer of glauconitic, transgressive sands. Deltaic deposits of the lower part of the Wilcox Group closely resemble corresponding facies of the Recent Mississippi River delta system. The deltaic mass of the upper part of the Wilcox Group differs from both in several significant lithologic and geometric parameters, including: (1) an increase in carbonate accumulation; (2) a decrease in thickness and width of the channel sand and related facies; (3) a decrease in sand percentage; and (4) a decrease in the amount of lignite. A shift in paleodrainage from south to southeast accompanies these changes.