--> Abstract: The origin of the Austin Chalk "Waco Channel" in the Greater Giddings Field Area, by M. Podell, K. Lewallen, and L. F. Krystnik; #90994 (1993).

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MARK PODELL, KYLE LEWALLEN, and LEE F. KRYSTINIK, Union Pacific Resources Company, Ft. Worth, TX

ABSTRACT: The origin of the Austin Chalk "Waco Channel" in the Greater Giddings Field Area

The 100 mile long, 50 mile wide "Waco Channel" which trends northwest to southeast from McLennan to Brazos County, Texas has in the past been interpreted to be an erosional feature with up to 225 feet of missing middle Austin section, when the top of the Austin or the Pecan Gap is used as a datum for stratigraphic correlations an incised channel appears to be present within the Austin. Correlation of approximately 1,200 electric logs and the isopach maps created from them indicate that the observed geometry of the apparent "Waco channel" is the result of changes in chalk sequence geometry in close proximity to the San Marcos Platform and not channel confined erosion.

The Austin Chalk can be broken into three general depositional regimen being the platform margin, slope and ocean floor. The platform margin (west flank of the Waco Channel, eastern Burleson County) is characterized by the highest rates of carbonate deposition. The slope (the area previously interpreted as post-depositional erosion, central and eastern Burleson County) is subject to minor deposition, depositional bypass or even syndepositional erosion. On the ocean floor (east flank of the Waco channel, Brazos County and east) sediment accumulated as toe of slope deposition. Consequently, the Austin Chalk thickness varies based on changes in relative sea level through time and the lateral distance from the San Marcos Platform. The Austin Chalk will be thickest in platform margin areas. Slope areas are thin and occasionally eroded. The Austin chalk thickens again at the toe of slope on the sea floor.

Correlation within portions of the lower Taylor demonstrates onlapping of the Austin Chalk surface on the eastern flank of the San Marcos Arch and downlapping on the Gober in Eastern Brazos County. This interpretation suggests that the Austin Chalk - Lower Taylor contact may be an example of a drowning unconformity associated with a sea level rise and subsequent highstand.

This work is in its early stages. The main purpose of this poster session is to solicit opinions and suggestions for future research.

AAPG Search and Discovery Article #90994©1993 AAPG Southwest Section Meeting, Fort Worth, Texas, February 21-23, 1993.