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Abstract

In the Val Verde basin area, the Lower Ordovician Ellenburger Group represents two third-order sea-level fluctuations as determined from facies architecture, stacking patterns, and accommodation plots based on subsurface core studies. These third-order cycles are superimposed on part of a second-order sea level rise and fall of the upper Sauk Sequence. Ellenburger Group deposition ceased in response to a major sea-level fall represented by a second-order unconformity at the end of the Sauk Sequence. This upper bounding surface is characterized by an extensively developed karst profile, indicative of prolonged subaerial exposure.

Carbonate dissolution and cave formation were most significant at and along major block boundaries and resulted in the generation of vugs, caverns, caves, and solution-enlarged fractures and joints. The roofs of larger caves were brecciated as the caves were buried and subjected to static loading by flooding of the platform and deposition of the Simpson Group. The fracture and breccia porosity found in the cave roof portions of these karst profiles accounts for much of the regionally significant porosity developed within the Ellenburger Group.

A detailed upper Ellenburger Group isopach of the Brown-Bassett/JM fields area illustrates a linear trend of isopach thins coincident with the crest of the present structural trend indicating that, not only were the structures active during Ellenburger time, but that the entire structural trend was regionally high. A well-developed paleokarst system was described from cores taken from the Ellenburger interval in this area. This karst system has a distinctive log signature characterized by elevated gamma ray response that has been interpreted to represent more radioactive clay-rich sediment deposited as cave-fill material. Correlation and isopach mapping of the cave-fill portion of the cave zone, shows the main portion of the paleo-cave network to extend across the entire Brown-Bassett/JM trend in a west-northwesterly direction, paralleling the principal bounding faults. The caves are thickest (up to 70') and best developed adjacent to, but not necessarily coincident with the crests of the structures. It is interpreted that the maximum cave development was localized along the main basement fault zones which acted as secondary conduits for fluid flow.

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