ABSTRACT

The Austin Chalk-Taylor (Ozan) Marl contact marks the Santonian-Campanian boundary which is a regional unconformity in north-central Texas. This distinctive surface has been examined at three localities where it records evidence of complex sedimentologic and diagenetic histories. This surface is highly irregular and has been stained pervasively with limonite. The most conspicuous aspect of the uppermost Austin discontinuity surface is the abundance of phosphatized and pyritized nodules and bioclasts (gastropods, bivalves, corals, coprolites and Baculites sp.). Fish teeth and bone fragments are also present. These nodules and bioclasts have been penetrated by small-diameter chalk-filled borings. Close examination reveals the presence of abundant trace fossils, especially Rhizocorallium jenenese. These trace fossils have been infilled with reddish-brown clay piped downward from the overlying Ozan Marl. Phosphatic nodules and glauconite also occur in these clay-filled burrows. Well-preserved chalk-filled Rhizocorallium jenenese are present locally. The occurrence of Rhizocorallium sp. in the uppermost Austin Chalk is noteworthy because Rhizocorallium has not been recorded in other Cretaceous chalks of North America or Europe. The exquisite preservation of Rhizocorallium in the uppermost Austin Chalk is indicative of a firm ground paleosubstrate. Based on sedimentologic, mineralogic, and paleontologic data, the Austin Chalk-Taylor Marl contact is interpreted as a condensed horizon (omission surface). This phosphatized condensed horizon overlies the Rhizocorallium firm ground. This thin stratigraphic interval records a complex history of soft-sediment bioturbation, early marine lithification, submarine erosion, and mineralization. The amount of erosion along the Austin-Taylor contact in the study area appears to have been relatively minor. Elsewhere in north-central Texas the Austin-Taylor boundary has been subjected to significant erosion. The Austin-Taylor discontinuity surface formed in a relatively shallow marine (inner to mid-shelf) paleo-environment.