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Bulletin 84—Geology of the Walnut Wells Quadrangle, Hidalgo County, New Mexico

By R. A. Zeller, Jr. and A. M. Alper, 1965, 105 pp, 15 tables, 13 figs., 2 plates, 3 appendices, 1 index.

The Walnut Wells 15-min quadrangle in southern Hidalgo County, NM, lies in the Basin and Range physiographic province and includes a part of the Animas Mountains. Strongly deformed Paleozoic and Cretaceous sedimentary rocks are overlain unconformably by a less deformed thick series of Tertiary rocks. Late elevation of the Animas range was relatively small so that erosion has not proceeded deep enough to expose pre-Tertiary rocks and structures except in one area. Tertiary arching in the east-central part of the quadrangle resulted in exposure of older rocks and structures.

Late Paleozoic formations exposed include Horquilla Limestone, Earp Formation, Colina Limestone, and Concha Limestone, all of which are similar in character to the corresponding formations in the Big Hatchet Mountains 15 mi to the east. The Early Cretaceous Heel-to-Finish, U-Bar, and Mojado Formations described from the Big Hatchet Mountains area are recognized in this quadrangle. A new Cretaceous conglomerate formation, which overlies the Mojado, is named Cowboy Spring Formation.

Layered Tertiary rocks consist largely of quartz latite tuffs but also include rhyolite tuff, latite and andesite flows, conglomerate beds, and fine-grained sedimentary beds. Many of the volcanic formations are welded tuffs. In the northern part of the area the Animas stock intruded the Oak Creek Tuff. The fact that these two rock masses were derived from a common magma which ascended through a common channel is indicated by field evidence, similarity in mineral and chemical composition of the intrusive and extrusive rocks, and similarity of elongation of zircon porphyry of the Walnut Wells plug south of the Animas Quartz Monzonite and the Oak Creek Tuff. This fact suggests origin from the same magma for all three rock masses. The zircons of other extrusive formations in the quadrangle have dissimilar elongations.

Several volcanic centers or source areas are present in the quadrangle. Besides the channels occupied by the Animas stock and Walnut Wells plug, the source areas of the Basin Creek Tuff and the Pine Canyon Formation are known; the source of the Center peak Latite was probably from the area of its greatest thickness.

The Winkler anticline, which is doubly plunging and northeast-trending, is one of the most instructive structures of southwestern NM. It started to develop before deposition of the Lower Cretaceous rocks. Development continued during Cretaceous deposition, reached a maximum during the time between deposition of the Cretaceous and Tertiary formations in the area, and continued during Tertiary deposition. Two significant points are demonstrated by the structure; first, that the region was influenced by an early period of deformation and second, that this old structure continued to develop intermittently during Cretaceous and Tertiary deposition.

Geologic structures include tight and open folds, high-angle faults trending in various directions, a lineament which is the focus of two intersecting unconformities, and normal faults. Evidence of four periods of deformation is found in the area. The first period, during which growth of the Winkler anticline began, is mentioned above. The second period, which was later than early Cretaceous deposition and earlier than Tertiary deposition, included the strongest deformation. The third period, which occurred during Tertiary deposition, included mild deformation which is recorded chiefly in depositional features of Tertiary rocks. The last period, which was after Tertiary deposition, may have been a continuation of the period of Tertiary deformation. During this last period, which is still somewhat active, broad gentle folds were formed, and the Animas Mountains were elevated and tilted eastward along flanking normal faults.

Mineral deposits of the quadrangle include manganese, silver, lead, and fluorspar. Silver and lead were mined commercially from the Gillespie and Red Hill mines many years ago; only manganese has been produced commercially in recent years.

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