THE CIBOLO REEF BLOCK REVISITED

The Permian paleogeography of the Marfa Basin is less well known than that of the adjacent Delaware Basin. Rocks of Wolfcampian through probable Ochoan age are present in the Shafter area, Presidio County, Texas. The Mina Grande Formation (Guadalupian) represents foresets prograded westward from the Capitan shelf edge over the basinal Ross Mine Formation (Wordian), but the only “reefy” Permian facies in the area is exposed along Cibolo Creek. Here a massive, cliff-forming Permian reef block, about 0.5 km² in outcrop, overlies thin-bedded limestone along an approximately horizontal contact. The reef boundstone contains radial cements, algae, and locally abundant fusulinds including Chusenella cibolensis Stewart, indicating an Early Leonardian age (Cibolo Formation). The underlying thin bedded limestone contains Polydiexodina indicating a Guadalupian age (Mina Grande Formation). First to draw attention to this older-over-younger relationship was Skinner, who in 1933, described the contact as a thrust fault. Rix, in his 1953 dissertation, agreed that the contact was a fault, but, instead of a thrust fault, he proposed that the contact represented a reverse fault that was tilted into its present, nearly horizontal position. But most faults in the area, including all of the larger faults, have normal displacement. In addition, the gouge, breccia, abundant slickensides and calcite veins that would have been produced by such a thrusting movement are lacking. A variety of drag folds and truncated beds do occur below the contact, suggesting movement of a slide block.

Slide blocks the size of the Cibolo reef block (and larger) are not uncommon, and slope failure is an important erosional process along carbonate platforms including those of the Permian Basin. Tectonic and seismic activity frequently trigger such slope failures and can result in collapsed platform margins, and the displacement of large blocks that separate along glide planes and travel downslope by rotation or translation. The Cibolo reef block has some disruption, but it contains a general vertical facies change from boundstone upward to grainstone. During Middle Permian time, the proximity of the still young Ouachita/Marathon Mountains to the immediate east would doubtless have produced a good deal of seismic activity along the eastern margin of the Marfa Basin.