Memoir 43—Geology of the Florida Mountains, southwestern New Mexico

By R. E. Clemons, 1998, 113 pp., 10 tables, 61 figs., 5 plates, 5 appendices, 5 sheets, scales 1:24,000 and ca. 1:8,000.

The Florida Mountains are an eastward-tilted Basin and Range fault block approximately 24 km (15 mi) southeast of Deming. The mountains are surrounded by a broad bajada that slopes gently into the Mimbres Basin. Sediments conceal the range-bounding faults except at the northwest end. Ephemeral streams are continuing the erosive processes that have reduced the exposed bedrock area to less than one-half of its initial size.

The oldest rocks exposed in the Florida Mountains are Precambrian hornblende and granitic gneisses exposed only north of Capitol Dome. An Upper Cambrian pluton intruded an andesitic to basaltic volcanic sequence producing the hornblende and pyroxene hornfels common in the western and southern parts of the mountains. The alkali-feldspar plutonic rocks are granite at the northern and southern ends of the range and syenite and quartz syenite in the central part. These shallow plutonic rocks and hornfels were unroofed before deposition of a diamictite that, in turn, was mostly eroded preceding deposition of the Bliss Sandstone in Early Ordovician time. Approximately 4,100 ft of Paleozoic rocks that crop out in the southeastern Florida Mountains include, in ascending order: Bliss Sandstone, El Paso Formation, Montoya Formation, Fusselman Dolomite, Percha Shale, Rancheria Formation, and Hueco Formation. No Mesozoic rocks are present except possibly the basal beds of the Lobo Formation, the bulk of which was deposited penecontemporaneously with Laramide deformation during Paleocene and early to middle Eocene times. Extensive andesitic to rhyolitic volcanism from middle Eocene to early Miocene times accounted for the thick Rubio Peak volcaniclastic section forming Florida Peak, as well as the ash-flow tuff, air-fall tuffs, flow-banded rhyolite, basaltic andesite, and dacite in the Little Florida Mountains. Thick rhyolite fanglomerates in the Little Florida Mountains and alluvial conglomerates forming an apron around the mountains have been deposited as the mountain block was uplifted approximately 213 m (7,000 ft) since early Miocene.

The south Florida Mountains fault is a northwest-trending, high-angle reverse fault that places Upper Cambrian granite against rocks as young as basal Lobo Formation. Multiple, small thrust faults exhibit younger-over-older rocks relations and produce tectonic elimination of strata. A few show older-over-younger relations. Current studies demonstrate that Laramide deformation in the Florida Mountains is not a continuation of the Cordilleran overthrust belt. Evidence suggests that the deformation resembles the basement-cored uplifts of the Rocky Mountain foreland, with possibly some strike-slip components.

Hydrothermal alteration and low-grade mineralization are widespread in the Florida and Little Florida Mountains. Relatively limited activity has produced manganese, zinc, lead, silver, copper, barite, and fluorite ores. Most production of copper, zinc, lead, and silver ores was from shallow oxidized veins, but small amounts of chalcopyrite accompany fluorite and barite in deeper veins. The age of mineralization is believed to be late Tertiary.     Although the mountains had been mapped before (by Darton in 1916 and by Corbitt in 1971), questions about interpretations of several aspects of the geology of the Florida Mountains had not been settled: Are the plutonic rocks of Precambrian, Ordovician, Mesozoic, or some other age? Are the plutonic rocks related, or were they formed by multiple episodes of plutonism? Are the gabbros and diorites (hornfelses) older or younger than the granites and syenites? What is the origin of the south Florida Mountains fault and associated thrust faults? Are they part of the Cordilleran overthrust belt? Clemons spent nearly two decades mapping in the Florida Mountains trying to answer these (and other) questions.

Rather than hold all the data until completion of mapping and laboratory studies, several progress reports and four geologic maps (GM-52, Geology of Florida Gap quadrangle, 1982; GM-58, Geology of Gym Peak quadrangle, 1983; GM-56, Geology of Capitol Dome quadrangle, 1984; and GM-59, Geology of South Peak quadrangle, 1985) were published previously. Terminology, interpretation, and ideas in this report represent revisions from Clemons' earlier publications.