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