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Bulletin 41—Geology of Puertecito quadrangle, Socorro County, New Mexico

By W. H. Tonking, 1957, 67 pp., 3 tables, 9 figs., 1 plate, 1 appendix, 1 index.

Puertecito quadrangle embraces a segment of the Colorado Plateau physiographic province adjacent to the northern and western margins of the Basin and Range province. The center of the area is approximately 21 mi north-northwest of Magdalena, NM. Mapped units include the Abo, Yeso, and San Andres Formations, of Permian age; the Triassic Chinle Formation; Cretaceous Dakota Sandstone, Mancos Shale, and Mesaverde Group; the Tertiary Baca Formation; Tertiary Datil volcanics, minor intrusives, and Santa Fe Group; and Quaternary pediment-veneer gravels, spring deposits, landslides, and alluvium.

The late Miocene (?) Datil volcanics, exposed in the Bear Mountains and the eastern foothills of the Gallinas Mountains, are subdivided into three members: the lower, Spears Member, consists of quartz latite tuff, agglomerate, breccia, and volcanic sandstone and conglomerate; the middle, Hells Mesa Member, consists of semiwelded to welded rhyolite tuff; the upper, La Jara Peak Member, consists of flows and flow breccias of basalt in the lower portions, and basaltic andesite in the upper part. The composition range in the La Jara Peak Member is explained readily by fractionation in the magma chamber. For the most part, dikes and sills of syenodiorite are intrusive only into Permian strata, whereas basaltic dikes, sills, and plugs are largely intrusive into post-Permian strata, including the upper portion of the late Miocene (?) volcanic sequence.

At least two periods of deformation took place in this quadrangle. The first orogeny, which occurred in late-Cretaceous or early-Tertiary time, consisted essentially of eastward directed compressional forces that resulted in the formation of the La Cruz anticline and the Sierra Lucero uplift. The basal portions of the Baca Formation are interpreted as synorogenic, for they were folded with older beds during this deformation. The second orogeny took place in late-Miocene (?) time before the cessation of volcanism. Normal faulting, with a large horizontal component of movement, resulted from this deformation, which apparently was a product of a force couple directed in an east-southeast and west-northwest direction. Dike swarms trend about N. 10 W., parallel to the major trend of the faults, and were intruded during the early stages of faulting. The Santa Fe Group, which overlies the Datil volcanics, has not been faulted or intruded in this area.

Detailed petrographic work is correlated with six chemical analyses that were made of the igneous rocks. The refractive indices of fused samples were plotted against the percentage of silica in the chemical analyses. These data indicate that the syenodiorite intrusions are not consanguineous with the extrusions and basaltic dikes. Application of this silica-refractive-index curve to the volcanic rocks of the Gallinas Mountains suggests that they are chemically similar to those of the Bear Mountains. Several stratigraphic sections were measured and are shown in the appendix.

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