Bruce Gunnar Langhus
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Ph. D. Thesis
Yazoo Foraminifera and Depositional History, Northeastern Gulf Coast.
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The Upper Eocene Jackson Group displays striking vertical and horizontal facies changes throughout the Northern Gulf Coast. The Yazoo Formation of the Jackson Group and its eastern calcareous equivalent, the Crystal River Formation, clearly exhibit these changes in a series of ten outcrop sections 150 miles along strike between eastern Mississippi and central Alabama. Modal analyses of lithologic components from 309 samples and percentage values of microfossil components from 137 samples indicate horizontal and vertical changes in age, sediment regime and water depth.
Computerized factor analysis coupled with relative entropy mapping generates a facies breakdown of the Yazoo data into discrete variable groupings with interpretable, realistic two-dimensional distributions: Facies I is very similar to modern prodelta muds; Facies II resembles certain Pleistocene reefoid deposits which have been flooded by shelf waters; Facies III is analogous to muddy shoreline sands; and Facies IV strongly suggests modern continental slope muds.
Planktonic foraminifera and calcareous nanoplankton allow the outcrop sections to be correlated with published biochronologies and serve to divide the Yazoo into early, middle and late Late Eocene zones.
Age designations of the outcrops and facies analysis combine to provide a logical depositional history: During early Late Eocene time, several medium-sized deltas supplied sediment to the western and central parts of the area while the eastern part received a mixture of terrigenous and biogenetic detritals. Toward the end of this time, the shoreline retreated southward along part of its length. The subsequent marine transgression left muddy shoreline sands lying directly upon prodeltaic sediments.
Further transgression in the middle Late Eocene produced sediments of a thoroughly mixed nature. During the latter part of the middle Late Eocene, conditions stabilized and persisted into the Latest Eocene when deposition was characterized by deep-water muds in the west indigenous carbonates in the east.
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Supervisor: Franco Medioli