Ichnology and Depositional Environments of Upper Cretaceous Chalks, Southwestern Arkansas (Annona Formation; Chalk Member, Saratoga Formation): ABSTRACT

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Both Annona and Saratoga chalks were deposited during transgression of middle to outer-continental-shelf environments. The basal contact of each chalk is a discontinuity surface underlain by marl. At these contacts chalk was piped across the discontinuity into marl through omission and postomission-suite burrows. Taxonomic composition of chalk-filled burrows below the Annona basal contact changes along a 60-km outcrop belt from dominance by Thalassinoides in the northeast to dominance by Zoophycos, Planolites, and Chondrites in the southwest. Such change indicates that this discontinuity surface was formed on a relatively steep slope which dipped southwest. Conversely, taxonomic composition of chalk-filled burrows below the Saratoga basal contact is unchanging (primarily ne species of Thalassinoides) over a 90-km outcrop belt, thus indicating that this discontinuity was formed on a relatively shallowly sloping surface. The basal 0.2 to 2.0 m of each chalk is a condensed bed rich in phosphate nodules and glauconite. Saratoga chalk was deposited at shallower depths, is generally much coarser grained, and contains more terrigenous detritus than Annona chalk. Saratoga quartz silt and sand may have been transported from a more shoreward area by poststorm seaward-directed currents. The trace-fossil assemblage within the Annona, consisting primarily of Planolites, Zoophycos, and Chondrites, is similar to that documented from modern deep-sea carbonate oozes. Bioturbation within the Saratoga chalk is preserved only as mottles. Thixotropic preservation and large size burrows in each unit indicate that during deposition these chalks had very soft substrates and that oxygen content was not limiting. End_of_Article - Last_Page 422------------