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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 30-min quadrangle. The mapped area includes the main central and western parts of the southern Sangre de Cristo Mountains in north-central NM 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-ft 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 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 hydrothermal 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-mi, right-lateral, strike-slip separation of Precambrian units along the 53-mi-long north-south-trending Picuris–Pecos fault. This strike-slip faulting is probably 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; the Espiritu Santo Formation of undetermined age; the Mississippian Tererro Formation; the Pennsylvanian La Pasada, Flechado, and Alamitos Formations; 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.

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