A DEEP-WATER PERMIAN OUTCROP BELT IN THE CHINATI MOUNTAINS OF WEST TEXAS AND ITS PALEOGEOGRAPHIC IMPLICATIONS

In the Chinati Mountains of West Texas, an outcrop belt of Permian strata extends for more than 15 miles from Cibolo Creek in the southeast to Pinto Canyon in the northwest. The Alta, Pinto Canyon, and Cibolo Formations compose most of the outcrop, and they have a combined thickness of over 5,000 feet. In the Pinto Canyon area, the Alta Formation contains laminated, organic-rich siltstone beds that are sporadically interbedded with 6–12-inch beds of fine-grained, normally graded sandstone. The overlying Pinto Canyon Formation contains blocks of contorted limestone suspended in a siliceous mudstone matrix. Fold hinges within the blocks suggest mass transport deposition from southeast to northwest. Outcrops at the Cibolo Creek area expose a vertical succession of thinly interbedded, organic-rich siltstone and sandstone units, with sandstone increasing in proportion upward through the Alta Formation. The overlying Cibolo Formation, which is time-correlative with the Pinto Canyon Formation to the northwest, contains carbonate-filled channel bodies with inclined surfaces and boulder conglomerates.

We propose that the Cibolo Creek outcrops in the southeast part of the Chinati Mountains were deposited in an upper slope setting along the margin of the paleo Marfa Basin, whereas those to the northwest at Pinto Canyon were deposited on the basin floor. This reconstruction supports the idea that the Marfa Basin formed in a foreland position northwest of the Marathon-Ouachita orogenic belt, and that it likely received sediment from Gondwana. In addition, data from well penetrations suggest that basement-rooted fault blocks on the Laurentian side of the orogen influenced sediment routing and the position of depocenters.

The Permian section in the Chinati Mountains is overlain by more than 3,000 feet of Cretaceous carbonate strata and more than 5,000 feet of Eocene to Miocene volcaniclastic rocks. Each package is bound by an unconformity representing unknown amounts of missing section. A network of Laramide, Basin and Range, and Rio Grande Rift faults, along with several large laccolith intrusions, crosscut the Permian outcrops in the Chinati Mountains, further complicating subsurface correlations and paleogeographic links between nearby basins.