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ABSTRACT: Sedimentologic Characteristics of Upper Cretaceous Selma Chalk in Mississippi

HEYDARI, EZAT , Mississippi Office of Geology, P.O. Box 20307, Jackson, MS 39289

Upper Cretaceous (Cenomanian-Maastrichtian) of Mississippi consists (in ascending order) of Tuscaloosa, Eutaw, Mooreville, Demopolis, Ripley, and Prairie Bluff formations. This study concentrates on sedimentologic and petrographic characteristics of the Campanian age Mooreville and Demopolis chalks (Selma Group). These two formations were cored extensively as a part of a feasibility study for a waste disposal site in Noxubee County.

These shallow shelf "chalks" are composed primarily of nannoplankton. However, they contain considerable amounts of invertebrate fossils and clays. Both formations consist of massive chalks with intervals of marls to argillaceous chalk. They are characterized by extensive bioturbation. The most common trace fossils include Thalassinoides, Planolites, and Chondrites. Thalassinoides occurs as large (1-2 cm in diameter), primarily horizontal burrows. It appears to be the earliest burrow to form, evidenced by its reworking by both Planolites and Chondrites. Planolites forms circular to elliptical burrows of up to 1 cm in diameter. Chondrites occurs as small (1-2 mm) burrows that cut across the other two burrows, suggesting that it is the latest burrowing event.

Numerous ash layers occur in these chalks, indicating extensive volcanism in the vicinity. Ash layers contain abundant biotite crystals, up to 5 mm in diameter. Geochronological studies of biotite grains promise to produce absolute ages of the nannoplankton zones and the rate of chalk sedimentation. Due to the lack of deep burial, both chalk units are highly porous and permeable.

 

AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas