Abstract

Poor outcrop exposures have limited the numbers of detailed studies of the Upper Cretaceous Demopolis Chalk. This report presents sedimentological investigations of the Lower and Middle members of this unit revealed in a continuous conventional core from Noxubee County, Mississippi.

The Lower Member (the Tibbee Creek Member) conformably overlies the Arcola Limestone Member of the Mooreville Formation, and consists of numerous redox cycles. Each cycle is composed of a basal dark, laminated section, grading into a gray, wispy laminated interval, and capped by a white, bioturbated, carbonate-rich layer. The basal laminated section indicates sedimentation under anoxic bottom water conditions, hostile to burrowing organisms. The wispy laminated interval indicates that bottom waters became slightly oxygenated and organisms tolerant of low oxygen conditions were sparsely present. The bioturbated layer at the top of each cycle is indicative of deposition under fully oxygenated conditions. The cycles reflect climatically controlled variations in the influx of terrestrial organic matter via major regional rivers.

The Middle Member is a light-colored, massive, highly bioturbated, argillaceous chalk. It contains abundant trace fossils including Thalassinoides, Planolites, Chondrites, Teichichnus, and Zoophycos. Thalassinoides occurs as large (1-3 cm), primarily horizontal and vertical burrows. Most Planolites form primarily horizontal, circular to elliptical burrows up to 1 cm in diameter. Teichichnus forms vertical to subvertical burrows with distinct spreiten fabrics. Zoophycos forms horizontal burrows with spreiten fabric. Chondrites occurs as small (1-2 mm) burrows that crosscut all other burrow types. A very shallow burrowing event generated a background-mottled texture apparent in all samples. Thalassinoides was one of the earliest burrows to form, as evidenced by its reworking by other trace fossils. It was followed by Planolites, Teichichnus, and Zoophycos. Chondrites reworked all other trace fossils and was the last bioturbation episode to have influenced the chalk.