Chronostratigraphic and Paleoenvironmental Re-Evaluation of Wilcox/Carrizo Outcrops, Bastrop County, Texas: Implications for Sediment Bypass to Shelf Edge Delta and Deepwater Plays

Abstract

Re-evaluation of Wilcox/Carrizo outcrops in Central Texas, using sedimentology, ichnology and palynology, has provided new insights into up-dip to down-dip relationships with shelf-edge deltas and deepwater deposits.

Lower Wilcox formations remain poorly known. Macrofossils indicate marine conditions during deposition of the Solomon Creek and Caldwell Knob. The Hooper has marine trace fossils and tidal sedimentary structures in its lower part, with carbonaceous siltstones and lignites in the upper part marking a shift to predominantly non-marine conditions. In the overlying Simsboro, basal rip-up clasts indicate an erosional lower contact, with tidal sedimentary structures and rare glauconite above. In the upper Wilcox, older literature emphasizes non-marine deposition for the Calvert Bluff, but along the outcrop belt the upper part of the Calvert Bluff consists of tidal flats, inclined tidal heterolithics, and tidal channels. Ophiomorpha can be locally abundant. A transgressive lag forms the base of the succeeding Sabinetown; it consists of several siltstone-dominated parasequences. A bioturbated siltstone erosionally overlying the Sabinetown yields common to abundant A. homomorphum, a dinocyst influx marking the PETM. A basal Carrizo Glossifungites surface, siltstone rip-ups draped on sigmoidal cross-beds, robust Ophiomorpha, and tidal heterolithics indicate marine deposition.

Overall, the Wilcox/Carrizo is mainly nearshore, shallow marine, with widespread evidence of mesotidal influence. Both the Simsboro and Carrizo are likely the products of tidal deltas. These were not fluvial channel complexes and not conduits for sediment bypass to reservoir sandstones in shelf edge deltas and deepwater turbidites. Various stratigraphic breaks point to the fragmentary nature of this up-dip succession, which probably represents only parts of late highstand and transgressive systems tracts. Although the duration of most breaks remains to be resolved, the time gap at the base of the Carrizo is estimated at over four million years. The thin Simsboro, Sabinetown and Carrizo marine progradational units probably do not extend far basinward, and are not coeval with, nor sediment sources for, paralic shelf edge deltaic deposits and deepwater turbidites. The only continuous link between the outcrop belt and offshore deepwater turbidites is the PETM, which provides a biostratigraphically constrained surface between the lower Wilcox below and upper Wilcox above.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018