Abstract: Austin Chalk (uppermost Santonian) Discontinuity Surface, North Central Texas

William C. Dawson, Donald F. Reaser

The Austin Chalk-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 (micro-relief 0.5-3.0 cm) and has been stained with iron oxides. 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 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 occurs 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 interesting because Rhizocorallium has not been recorded in other Cretaceous chalks of North America or Europe. Rhizocorallium is excavated below the sediment-water interface, hence their occurrence on the uppermost Austin Chalk surface is indicative of submarine erosion. The exquisite preservation of Rhizocorallium her is indicative of a firm ground paleosubstrate. Based on sedimentologic, mineralogic, and paleontologic data, the Austin Chalk-Ozan Marl contact is interpreted as a condensed horizon.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas