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January 2015 Bulletin

Subsurface Evidence for Unconformity at Base of Carrizo Formation, Atascosa and Wilson Counties
Texas Richard N. Hargis, Independent Geological Consultant
 
Introduction
 
The surface presence of a regional unconformity at the base or just above the base of the Carrizo Formation has been documented in geological literature since the early days of surface geology in Texas. It has been described at numerous Carrizo outcrop exposures extending from northern Mexico to Louisiana. Stenzel (1951) stated: “It is now known to be traceable throughout the outcrop area of the Carrizo sand in Texas as a continuous regional disconformity”. It was primarily on the basis of the presence of this unconformity that Plummer (1932) placed the Carrizo Formation stratigraphically in the Claiborne Group and not in the Wilcox Group. Hargis (1985) proposed a stratigraphic classification of the Wilcox Group of South Texas into Upper Wilcox, Middle Wilcox and Lower Wilcox Subgroups (Figure 2). As is now know from subsurface studies of the Wilcox Group, the Carrizo Formation is an updip, non-marine facies of the Upper Wilcox Subgroup that belongs as a depositional sequence of the Wilcox Group and not the Claiborne Group (Hargis, 1985, 2010), (Hamlin, 1988). An unconformity at the base of the Massive Carrizo was documented in the shallow subsurface of East Texas in Madison, Leon and Houston Counties by C.M. Jones (1986) where it truncates the lower Carrizo and Sabinetown. Jones states: “it is best developed in Leon, Freestone and Anderson Counties and locally in parts of Lee and Milam Counties”. In South Texas, the surface unconformity has never been recognized in the subsurface in geological literature. Hamlin (1988) recognized “basal Carrizo scouring” in a well in Atascosa County (Killam #1 Schraeter) where he states: “several tens of feet of missing section at the top of the middle Wilcox were estimated from well-log correlations”, but he attributed this to fluvial-channel scouring rather than a regional unconformity. However, he did state “that additional undiscovered basal downcutting may exist elsewhere near outcrop”. His study was based on limited shallow well control and he did not realize how extensive this scouring into the middle Wilcox interval was within the shallow subsurface of Atascosa and Wilson Counties. In studies made for the Evergreen Underground Water Conservation District (UWCD), this author did stratigraphic studies of the Carrizo-Wilcox interval (Wilcox Group) in northern Atascosa County in 2008 and in Wilson County in 2010. These two studies, which revealed the evidence for this paper, included the adjacent parts of Bexar and Guadalupe Counties. The two study areas are outlined in Figure 1 and referred to hereafter as the “Atascosa area” and the “Wilson area” or together as “both or the two study areas”. In the Atascosa study, a total of 538 well logs were correlated and in the Wilson study, a total of 605 well logs, that identify the “major transgressive shales” (maximum flooding surfaces) present in each well (Hargis, 1996, 2010) as well as the stratigraphic intervals defined by these shales (Figure 2). The updip limit of each of the shales present was determined and structural and isopach maps were made over the areas covered and intervals defined by the shales. Multple cross sections were constructed with the five dip sections extending from the base of the Carrizo outcrop to the downdip limit of both study areas being pertinent to this paper (Figure 3). Selected logs from four of the dip sections, covering only the lower Carrizo-Middle Wilcox interval, are shown herein; A-A’ (Figure 4), F-F’ (Figure 5), H-H’ (Figure 6) and D-V’ (Figure 7). The larger circles on the dip section locations in Figure 3 indicate the location of the selected well logs. The outcrops of the Carrizo and Middle Wilcox are outlined in Figure 3. The Carrizo outcrop was plotted from the Bureau of Economic Geology (BEG), Geologic Atlas of Texas, Seguin Sheet, 1974 and San Antonio Sheet, 1974. The Middle Wilcox (Sabinetown) outcrop was not separated from the overall Wilcox outcrop on the BEG maps and is estimated by combining the BEG base of Carrizo outcrop with Plummers (1932) description of the Sabinetown outcrop at Losoya. The base of Carrizo outcrop also represents the surface
South Texas Geological Society January 2015 39 location of the unconformity and the eroded top of the Middle Wilcox in the two study areas. Based on the evidence gathered from these two Evergreen studies, it became evident that the surface unconformity at the base of the Carrizo extends into the shallow subsurface of Atascosa and Wilson Counties and is present at the base of the Carrizo throughout most, if not all, of both study areas. This paper presents and discusses the evidence for this conclusion.
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