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Bulletin 34—Geology of the south Manzano Mountains, New Mexico

By J. T. Stark, 1956, 49 pp., 1 fig., 8 plates, 1 appendix, 1 index.

The South Manzaño Mountains in central NM form a unit of the easternmost of the Basin Ranges and are structurally continuous with the Los Piños to the south and the North Manzano and Manzañita Mountains to the north. Precambrian metaclastics of the area consist of the basal Sais quartzite overlain by the Blue Springs schist and White Ridge quartzite in conformable sequence, followed by 5,000 ft of rhyolite flows and intercalated basic sills. A small outcrop of granite in the northwest part of the area is similar to and correlated with the Ojito stock of the North Manzaño Mountains, which intruded metaclastics older than the Sais quartzite. In the south, the Priest granite and associated pegmatite and aplite dikes intrude all older formations. Vein quartz ranging from thin stringers to massive quartz reefs over 1,000 ft thick are prominent throughout the range and represent several periods of intrusion.

Three periods of Precambrian orogeny are recognized: (1) Early folding of the metaclastics and metarhyolite into an asymmetric syncline, its axis striking northeast across the north-trending range and the steeper eastern limb overturned, with the axial plane dipping steeply to the southeast. At this time, regional cleavage was developed paralleling the axial plane, with small drag folds on the flanks of the syncline. (2) Later, cross, folding, faulting, and small crenellations were developed on the older schistosity. (3) Finally, granites were intruded, with associated dikes and quartz veins, across preexisting structure.

A later disturbance, probably Laramide, formed three major thrust-fault zones, the easternmost bringing Precambrian formations against Paleozoic sediments. A normal fault in Tertiary time is responsible for the block tilting of the range with respect to the Rio Grande valley on the downthrown side. Small scarps in Quaternary gravels of the Tio Bartola pediment indicate recent normal faulting.

The steep western escarpment of the South Manzaño Mountains, a familiar landmark to travelers in the Rio Grande valley in the vicinity of Belen, affords an unusually extensive exposure of the Precambrian or basement rocks of this region. Not only do these rocks appear for a distance of 19 mi along the front, but the 3,000 ft of vertical exposure in ridges and steep-walled canyons provides a particularly favorable opportunity for viewing the structure of these rocks in a third dimensions as well. In view of this favorable situation, as well as the interest of the oil industry in the nature of the Precambrian basement, or granite, as it is commonly called, it appeared that a careful study of the South Manzaño Mountains was especially in order. Furthermore, the North Manzaño Mountains had been mapped by Reiche, and the Los Piños Mountains to the south had been mapped by Stark and Dapples, so that this study completes the continuity of geologic information along the entire front south of Sandia Mountain. The study was confined essentially to the Precambrian rocks, and only their structural relations to the overlying Pennsylvanian rocks was noted.

Though the Precambrian rocks are moderately to thoroughly metamorphosed, a large part may be interpreted as having been a sequence of clastic sedimentary beds, with some sills of basic igneous rock; still another part is interpreted as a series of rhyolite flows. Of especial interest in this area are large masses of quartz, here called quartz reefs, which are interpreted as of intrusive of vein origin. Several masses of granite have intruded the Precambrian rocks and are regarded as of Precambrian age. No mineral deposits of any consequence have been found associated with the quartz reefs, but large quantities of quartzite are excavated and crushed for railroad ballast from extensive pits on the south side of the Atchison, Topeka and Santa Fe Railway track at the south end of the area.

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