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Memoir 11—Geology of part of the southern Sangre de Cristo Mountains, New Mexico

By John P. Miller, A. Montgomery, and P. K. Sutherland, 1963, 106 pages, 1 map. Companion to Memoir 27.

Precambrian metasedimentary and metaigneous rocks, Paleozoic sedimentary rocks, and Cenozoic sedimentary deposits and igneous rocks are exposed in the Tesuque-Velarde-Tres Ritos-Cowles thirty-minute quadrangle. The mapped area includes the main central and western parts of the southern Sangre de Cristo Mountains in north-central New Mexico and a part of the Rio Grande depression to the west.

Precambrian rocks, mostly exposed in the highest ridges as well as the deepest canyons of the mountains, consist of the older, metasedimentary Ortega Formation and the younger, chiefly metaigneous Vadito Formation, both intruded by the Embudo Granite. The Ortega Formation includes a lower 10,000-foot-thick quartzite, overlain by staurolite schist and muscovite-quartz-biotite phyllite, with a distinctive black carbonaceous phyllite at the top. The Vadito Formation, unconformable on the Ortega, contains minor conglomerate near its base, metarhyolite felsites, and minor muscovite-quartz-biotite phyllite as well as higher, thick-bedded, hornblende-andesine amphibolites. Pegmatites, quartz veins, and widespread hydrothermal mineralization are related to the intrusion of the Embudo Granite into the Ortega and Vadito rocks, and postdate the dynamothermal metamorphism of these two formations. Weak metallic mineralization, mainly of copper, occurs widely and is related to the hydro-thermal metamorphism. The only major mine is the once-productive Pecos Mine near Tererro which yielded zinc, lead, and copper.

The Precambrian rocks are isoclinally folded along general east-west-striking axes, with the axial planes dipping steeply southward and overturned to the north. Major divergences from this regional trend are the result of drag along the Picuris-Pecos fault and in part also may have been caused by injection of granite magma. Seven major folds were mapped in the Precambrian rocks. There has been a 23-mile, right-lateral, strike-slip separation of Precambrian units along the 53-mile-long north-south-trending Picuris-Pecos fault. This strike-slip faulting probably is Precambrian in age, but later episodes of vertical movement took place along the fault zone.

Paleozoic strata crop out in the eastern half of the mapped area, unconformable upon the Precambrian rocks, and include the Del Padre Sandstone of undetermined age (0 to 750 feet thick); The Espiritu Santo Formation of undetermined age (0 to 55 feet thick); the Mississippian Tererro Formation (0 to 86 feet thick); the Pennsylvanian La Pasada, Flechado, and Alamitos formations (total thickness 2200 to 6500 feet); and the Sangre de Cristo Formation of Pennsylvanian and/or Permian age.

The Del Padre and Espiritu Santo formations were deposited sometime during the interval from Cambrian to early Mississippian time. The Del Padre Sandstone is thickest east of the Picuris-Pecos fault near the Truchas Peaks area and thins away from that area primarily by lateral interfingering with the limestones of the Espiritu Santo Formation. The Tererro Formation of Meramecian age unconformably overlies the older units in most areas and is divided into three members, in ascending order: the Macho Member of limestone-boulder conglomerate, the Manuelitas Member of light-gray limestone and limestone pseudobreccia, and the Cowles Member of silty, cross-bedded limestone.

The Pennsylvanian strata rest unconformably on all older rocks and show marked lateral changes in thickness and lithology from a thin shelf facies in the south (Pecos shelf) to a thick elastic trough facies in the north (Taos trough). In the south, the La Pasada Formation (1000 feet thick) consists of alternating limestone, shale, and thin sandstones, ranging from Morrowan to middle Desmoinesian in age, and is overlain conformably by the Alamitos Formation (1200 feet thick) which is characterized by arkosic sandstone, gray to red shale, and limestone. It ranges from late Desmoinesian to Virgilian in age and is overlain by arkosic sandstone and red shales of the Sangre de Cristo Formation, of Virgilian and/or Permian age in the Pecos area. North of Jicarilla Peak, the La Pasada Formation grades laterally into the Flechado Formation (2500 feet thick) which consists of sandstone and shale alternating with thin limestones. In the north, the Flechado is overlain by the Alamitos Formation (4000 feet thick), there made up of arkosic sandstone, conglomerate, and shale with limestone and siltstone in the upper part. The Alamitos Formation in the north is of upper Desmoinesian age and is overlain, apparently conformably, by the Sangre de Cristo Formation which is believed to be upper Pennsylvanian in age in that area and appears to grade southward into the upper part of the Alamitos Formation of the Pecos region.

Pennsylvanian terrigenous sediments were derived primarily from the west and northwest, from the southern part of the Uncompahgre highland, and reflect two major periods of uplift of the west side of the Picuris-Pecos fault. The first period began in Morrowan time and provided a predominantly metasedimentary rock source; the second period began in late Desmoinesian time and, in addition, exposed feldspar-rich metaigneous rocks to erosion.

Cenozoic deposits rest unconformably on various Paleozoic and Precambrian units in the western and northern parts of the mapped area. The Cenozoic units, in ascending order, are the Picuris Tuff, the Tesuque and Ancha formations of the Santa Fe Group, the Servilleta Formation (possibly equivalent to the Ancha Formation), glacial deposits, stream terraces, and floodplain alluvium.

Present major structural features are related to Laramide deformation but were controlled in their development by Precambrian structures. Vertical dip-slip movement occurred along the Picuris-Pecos fault (believed to be part of a regional geofracture) during Laramide time with no rotation of the fault plane from the vertical attitude established during Precambrian time. Precambrian rocks on the west, upthrown side were brought in contact with Pennsylvanian rocks on the east. East of the Picuris-Pecos fault are a major south-plunging syncline and an eastward-adjacent anticline of Laramide age. These folds form the southern terminus of the Sangre de Cristo Mountains about ten miles south of the mapped area where their plunge carries the resistant Precambrian crystalline rocks beneath the surface.

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