Memoir 36—Geology of Organ Mountains and southern San Andres Mountains, New Mexico

By W. R. Seager, 1981, 97 pp., 8 tables, 88 figs., 2 appendices, 4 oversize sheets (includes geologic map at scale 1:31,250), microfiche.

This study integrates results of recent exploration activity, geophysical studies, and radiometric dating into the geologic framework of the Organ and southern San Andres Mountains. Additionally, previously unmapped portions of the area that contain features of structural significance were mapped on a greatly improved 7½-min topographic base map. Sections of Paleozoic rocks not previously described in detail or subdivided into up-to-date stratigraphic units were described and the relationships of the Organ batholith and volcanic rocks in the southern Organ range were investigated.

The Organ and southernmost San Andres Mountains in south-central New Mexico are part of a 150-mi long, west-tilted fault block extending from El Paso, Texas, northward to central New Mexico. The Organ Mountains tower nearly a mile above the floor of the Tularosa Basin on the east and the Rio Grande valley on the west. Oldest rocks exposed are Precambrian granite, overlain by as much as 8,500 ft of mostly marine Paleozoic and Cretaceous strata. In Laramide time, these rocks were deformed along the faulted margins of a basement-cored block uplift which was ancestral to the modern Organ-San Andres range. Lower Tertiary fanglomerate, at least 2,000 ft thick, records the erosional "unroofing" of the Laramide uplift and was sharply deformed as uplift progressed. Magmatism profoundly affected the area in middle Tertiary time. Intermediate-composition lava and mudflow breccia of late Eocene or early Oligocene age buried the deeply eroded Laramide uplift.

The Organ batholith is interpreted to be an exposure of the upper, outer part of a magma chamber whose silicic cap erupted as pyroclastic flows and lavas 33–33.7 m.y. ago. The pyroclastic rocks, approximately 2 mi thick, subsided to form the Organ cauldron, only a piece of which is exposed in the Organ Mountains today. The pyroclastic rocks also formed the roof beneath which the batholith crystallized. Compositional zoning within the volcanics indicates that the erupted magma volume was also zoned, ranging from 77% SiO2 at the top to about 65% at lowest levels. The batholith represents deeper, less silicic (56–68%) levels of the magma chamber, and late-stage mineralized plutons within the batholith are interpreted to be products of the progressive crystallization of the magma chamber. Undated tristanite-trachybasalt flows interbedded with fanglomerate may indicate onset of late Tertiary black faulting in the area. Subsequent uplift of the modern ranges involved an early stage of closely spaced faulting associated with moderate west tilting or, locally, east downwarping. Some faults, initially steep, were rotated into low-angle positions as tilting progressed. The more recent Stage of uplift is distinguished by development of the modern, widely spaced range-boundary faults and their associated horsts, west-tilted blocks, and grabens. Movement on the eastern range-boundary fault of the Organ and southern San Andres fault block has persisted to within the last 4,000-5,000 yrs. Several generations of alluvial fans, as old as middle Pliocene, are products of repeated movement on these faults. Mineral deposits related to the Organ batholith include base-metal deposits as replacements in limestone and dolomite, and as disseminations, vein fillings, and pegmatite minerals in the batholith.

The Organ Mountains, one of the most picturesque and rugged mountain ranges in the Southwest, form the skyline approximately 10 mi east of Las Cruces, NewMexico, in southern Doña Ana County. The row of jutting, fluted, bare-rock pinnacles known as the Needles-the backbone of the range-can be seen on a favorable day from nearly 100 miles away, making them probably the most familiar landmark in the region. Their stark, sawtooth profile, their challenging slopes and changing moods have made the Needles a favorite of artists, photographers, and mountain climbers, as well as a daily pleasure to the people who live within their view. From their summit a broad expanse of southern New Mexico, Texas, and Mexico spreads out below-from the Magdalena Mountains on the north, to far into Mexico and from the Arizona border to the Guadalupe Mountains. Yet the Needles are just the crown of a jumbled range of stark peaks, rugged ridges, and deep canyons that stretches for 150 miles from El Paso, Texas, to central New Mexico.

The lofty central peaks of the Organ Mountains gradually become less imposing northward, finally giving way to the high cuestas and deep canyons of the San Agustin and southern San Andres Mountains, also described in this report. The San Augustin Mountains extend northward from San Augustin Pass to Quartzite Mountain, and from there the San Andres Mountains continue for nearly 100 mi. Only the southernmost part of the San Andres Mountains was made a part of this study, specifically the area encompassing Bear Canyon, the southernmost east-west drainage. To the south of the Organ Mountains, the group of outlying hills and ridges known as the Bishop Cap hills is also an important part of this report. Still farther south lie the Franklin Mountains and considerable reference is made to them although they were not mapped.