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Abstract: Discordant Austin-Taylor (Upper Cretaceous) Contact, Southern Ellis County, Texas

Donald F. Reaser, William C. Dawson

The disconformable Austin-Taylor (Santonian-Campanian) contact exposed near Italy, Texas, is marked by a channel-like sedimentary feature. This feature, more than 12 m wide, is poorly exposed along a short gully tributary to Hog Creek. There, the "knobby" upper surface of the Austin Chalk has been disrupted by thalassinoidean biogenic structures that extend up to a meter into the underlying chalk. Depressions (up to 5 cm deep) between the white to very pale orange "knobs" are partly filled with hematitic clay, inoceramid shells and individual prisms, glauconite peloids, and foraminifers with glauconitic fillings. This burrow-filling sediment has been "piped downward" from the overlying Taylor. Some depressions have a discontinuous rim of "plumose" calcite that suggests the influx of m teoric water. Whole-rock isotopic analyses revealed that Austin Chalk samples from this contact have significantly depleted ^dgr18O compositions (-7.7 to -9.9^pmil PDB).

The overlying lower Taylor (Ozan) consists of very thin-bedded (0.8 to 2.5 cm) inoceramite with a moderate brown to dusky red iron-oxide matrix. These iron oxides have been derived from the weathering of glauconite; rounded to elliptical cavities are abundant in the matrix and impart a scoria-like appearance to the weathered surface. The simple planar, oxidized beds dip from 13° to 16° northeast and discordantly overlie the Austin Chalk. Faunal elements include abundant Inoceramus prisms, small oysters, fish centra, and shark teeth.

This oxidized channel-like feature appears to represent the updip limit of the southeast-trending "Waco channel," a post-Austin regional erosional surface in north-central Texas, which has been described and illustrated by Durham (1990, 1991). The Waco channel is probably a submarine erosional feature that has been modified diagenetically by downdip infiltration of meteoric waters.

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