Lower Paleozoic Cavern Development, Collapse, and Dolomitization, Franklin Mountains, El Paso, Texas


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Unconformities and Porosity in Carbonate Strata


The superb exposures of the Lower Ordovician El Paso Group in the Franklin Mountains, El Paso, Texas, provide an excellent opportunity to investigate the effects of unconformities on porosity and permeability of carbonate rocks. Unconformities at cycle, sequence, and supersequence boundaries represent time gaps ranging from thousands to millions of years. Unconformities at cycle and sequence boundaries are marked by tidal-flat facies and reflux dolomitization. No significant karsting is found at these boundaries. A large cavern system was developed in the upper 300 m (1000 ft) of the El Paso Group during the 33 m.y. time gap marked by the supersequence boundary between the Lower Ordovician El Paso Group and the Upper Ordovician Montoya Group. In the upper 75 m (250 ft), the El Paso caverns were tabular and horizontal and were formed near the phreatic-vadose interface. In the lower 225 m (750 ft), the caverns were linear and vertical and were formed in the deep phreatic zone along vertical fractures oriented N20°W and spaced 900 m (3000 ft) apart. A stratiform dolomite unit separated the two cave systems. Collapse was initiated during cave development and continued through Silurian time. Collapse of the El Paso caverns formed large fracture systems and megacollapse breccias 300 m (1000 ft) thick, 450 m (1500 ft) wide, and several kilometers long. Collapse of the cavern roof produced brecciation and fracturing in the overlying Montoya strata. Much of the breccia and adjacent country rock was dolomitized by fluid migrating through the collapsed caverns after Silurian time. Cavern development, collapse, and dolomitization of the El Paso and Montoya groups has completely altered the original porosity and permeability distribution from one controlled by depositional patterns to one controlled by diagenetic processes. Karst-related dissolution resulted in cavernous porosity comprising up to 30% of some intervals. However, infilling sediment and collapse during burial destroyed most of the cavernous porosity by the end of Silurian time; by the end of Pennsylvanian time, much of the fracture and interbreccia-block pore space had been occluded by saddle dolomite. The Ellenburger Group of the Permian basin, the subsurface equivalent of the El Paso Group, produces from fractures and interbreccia-block pore space similar to that found associated with the collapse breccias of the El Paso Group, although the total porosity is only 1 to 3%.

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