--> ABSTRACT: Time-Stratigraphic Correlation of Lower Wilcox Valley-Fill Sequences, Colorado and Lavaca Counties, Texas, by Paul E. Devine and David M. Wheeler; #91029 (2010)

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Time-Stratigraphic Correlation of Lower Wilcox Valley-Fill Sequences, Colorado and Lavaca Counties, Texas

Paul E. Devine, David M. Wheeler

Lake Paleocene and early Eocene lower Wilcox strata in southeast Texas are characterized principally by sandstone-rich deposits of fluvial and deltaic systems that prograded from a stable platform area into an unstable growth-faulted shelf margin setting. In contrast, incised valley systems, initiated during episodes of sea level lowering and filled dominantly with mud during subsequent transgressions, punctuate several lower Wilcox intervals. Further, valley-fill sequences are known to provide seals and/or reservoirs for a number of stratigraphically trapped hydrocarbon accumulations. Time-stratigraphic correlation of lower Wilcox strata provides improved differentiation of stacked valley-fill sequences and thereby more refined interpretation of depositional history and ore accurate mapping for exploration purposes.

Previously published reports have identified three main valley features in the lower Wilcox: the Lavaca, Smothers, and Hallettsville channels. Each valley was recognized and correlated along the erosional surface at the base of the sequence where the facies contrast between valley-fill mudstone and surrounding lower Wilcox sandstones is most dramatic. Through our technique of time-stratigraphic correlation, however, we identify six valley-fill sequences; four have been mapped over a two-county area.

The key to differentiation of individual valleys is to recognize the top of each discrete valley-fill sequence where it has not been truncated by erosion at the base of a subsequent, younger valley. Identification of individual valley-fill sequences is possible where each cycle of downcutting and fill is followed by normal fluvial-deltaic deposition. We establish time-equivalence among these multiple-age valleys over a regional area through the relationships of the valley-fill tops to time-stratigraphic correlations carried in the surrounding and intervening lower Wilcox strata. The tops of successively younger valley-fill sequences are found progressively higher in the succession of markers.

The interpretation and exploration potential of lower Wilcox valley-fill systems may be broadly characterized as two types, based on stratigraphic and sedimentological evidence. Lower in the section, the Hallettsville complex fills a shelf-edge canyon initially developed during a period of sea level lowstand and later modified during transgressive drowning. The extreme thickness of the downdip Hallettsville sequence is closely associated with growth faulting and instability of the narrow shelf located at the margin of the stable platform. Exploration potential exists for channel-fill reservoirs that appear to be preserved most predictably at the intersection of the dip-oriented canyon trend with the strike-oriented trend of the growth-faulted shelf margin. The Smothers channel is iden ified as the updip time-equivalent incised drainage.

Higher in the section, valley-fill sequences like the Lavaca channel show characteristics of estuarine and bay environments associated with shorelines similar to the Holocene Texas Gulf Coast. Relatively deep incisement of lower Wilcox valleys, however, is related to their formation near the seaward end of the progradational deltaic platform. Exploration potential exists where updip convex curvature of mudstone-filled valleys or the intersection of primary and secondary drainages form lateral barriers to hydrocarbon migration. These barriers can form stratigraphic traps or enhance structural closure.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.