The U. S. Geological Survey topographic sheet of the Santiago Peak quadrangle, Texas, covering 275 square miles, includes the southwestern part of the Marathon uplift, trans-Pecos Texas, where post-Cretaceous folds of the Santiago Mountains cross Paleozoic structural trends.

Paleozoic sediments deposited in the Llanoria geosyncline aggregate 15,000 feet and were deformed in Pennsylvanian time by orogenic movements directed from the southeast.

The Cretaceous sea, which advanced over beveled Paleozoic strata, deposited 2500 feet of sediments, mostly limestones. The Glen Rose limestone is the oldest Cretaceous formation. The Maxon sandstone may be Fredericksburg in age. The Devils River limestone is divided into four members, two of which can be correlated, at least in part, with the Edwards and the Georgetown formations of central Texas. The Aguja is the youngest Cretaceous formation.

Post-Cretaceous doming of the Marathon uplift did not extend west of the Santiago Mountains, in which the chief structural feature is a monocline of Cretaceous strata with the limb overturned to the west. In places the lower flexure of the monocline is broken by a thrust fault, with thrusting from the east. The folding and faulting are believed to have been related to orogeny in the Sierra Madre Oriental 100 miles to the southwest. Large normal faults, trending north-south and northwest-southeast, produced blocks now tilted westward.

Post-Cretaceous sills of alkalic syenite occur west of Santiago Mountains.

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