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Discordant Austin-Taylor Contact, Upper Cretaceous, Southern Ellis County, Texas
Donald F. Reaser (1), William C. Dawson (2)
The disconformable Austin-Taylor (Santonian-Campanian) contact exposed near Italy, Texas, is marked by a channel form. This feature, more than 3.7 ft (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 3.3 ft (1 m) into the underlying chalk. Depressions (up to 2 inches [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 meteoric water. Whole-rock isotopic analyses revealed that Austin Chalk samples from this contact have significantly depleted 18O compositions (-7.7 to -9.9 PDB), suggesting recrystallization in meteoric water containing soil gas CO2.
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° NE and discordantly overlie the Austin Chalk. Faunal elements include abundant Inoceramus prisms, small oysters, fish centra, and shark teeth.
This oxidized channel feature appears to represent the updip limit of the southeast-trending "Waco channel," a post-Austin regional erosional surface in North-Central Texas that 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.
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