Abstract: Dynamic Depositional and Early Diagenetic Processes in a Deep-Water Shelf Setting, Upper Cretaceous Austin Chalk, North Texas

S. D. Hovorka, H. S. Nance

The Austin Chalk of north Texas was deposited during a worldwide Coniacian and Santonian sea level highstand on a deep-water shelf north of the San Marcos platform. Transgressive (lower lower Austin Chalk), highstand (upper lower Austin Chalk), and regressive phases (middle and upper Austin Chalk) of cyclic chalk and marl sedimentation are recognized in core and outcrop and on log cross sections.

Examination of sedimentary structures in excellent exposures in excavations and tunnels created in Ellis County for the Superconducting Super Collider provide new evidence of sediment transport during Austin Chalk deposition. During transgression, bottom currents syndepositionally reworked nannoplankton oozes, creating incised channels as much as 30 m across and 2 m deep. Weakly burrowed channel fills with preservation of fine lamination document rapid infilling. Channel fills are composed of pyritized and carbonized wood and Inoceramus lag deposits, pellet and globigerinid grainstones, and coccolith ooze.

During maximum highstand, bottom reworking was suppressed. Detrital content of highstand marls is low (<20%); organic content is high (1.4-3.5%). Coccoliths preservation is excellent because of minimal diagenetic alteration.

Regression is marked by resumed channel cutting and storm-bed winnowing in the middle and upper Austin Chalk. The suppressed resistivity log response and recessive weathering characteristics of the middle Austin Chalk are not primarily related to depositional environment but to increased input of volcanic ash during the accumulation of this interval. Early stabilization of ash produced clay-coated microfabrics in sediments that are otherwise similar to the transgressive deposits.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994