GEOLOGY OF THE LOS PINOS MOUNTAINS, NEW MEXICO

Abstract

The Los Pinos Mountains form the southern part of a northward-trending mountain group along the Rio Grande Valley in central New Mexico. The pre-Cambrian core, the area of greatest relief, is composed of the Sias quartzite (oldest), Blue Springs muscovite schist (originally siltstones), White Ridge quartzite, and Sevilleta rhyolite, all intruded by the Los Pinos (pre-Cambrian) granite. The pre-Cambrian bedded rocks exceed 12,000 feet in thickness, of which over 4500 feet is rhyolite flows. All dip westward (average 50°), and schistosity parallels the dip. Schistosity of the Los Pinos granite (N. 20° E.) parallels that in other pre-Cambrian rocks. Granitization is pronounced along its borders, particularly along the contacts with the White Ridge quartzite. The pre-Cambrian rocks have been subjected through a long period of time to numerous environmental changes giving rise to several types of metamorphism, commonly superimposed upon one another. Early regional metamorphism is partly obliterated in thermally metamorphosed aureoles around the granite intrusions. In many zones widespread dynamic metamorphism has completely destroyed the effects of earlier regional and thermal metamorphic changes. Pennsylvanian and Permian strata unconformably overlie the pre-Cambrian rocks; they form cuestas on the east and south sides of the uplift, whereas, on the western flank, a normal fault downthrown on the west is suggested. The Pennsylvanian rocks consist of nonmarine sandstone and shale grading upward into marine limestone and shale. The latter commonly grade upward into nonmarine sandstone. Permian “red beds” (Abo sandstone) appear to lie conformably upon the Pennsylvanian. The stratigraphy is described in detail, and measured sections are given of the Derry, Des Moines, Virgil, Missouri, and Aqua Torres series. Pre-Cambrian rocks have been thrust eastward overturning the Paleozoic sediments. Such faulting (Laramide) consists of two closely spaced north-south-trending thrust slices (40–60-degree dips), the Montosa and Paloma. Some bedding-plane movements and normal faulting followed the thrusting.