Indo Mountains Research Station

GEOLOGY OF THE INDIO MOUNTAINS

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The Geology of the Indio Mountains is described in Underwood (1962). This small mountain range is comprised of two main sequences of rocks and has been shaped by two different deformational events. Most of the rocks in the Indio Mountains are late Cretaceous sedimentary rocks deposited on the northern shoreline of the Chihuahua Trough. The oldest unit, the Yucca Formation, is over 600 meters thick and was deposited on land in a fluvial or alluvial fan environment (Smith, 1991). The basal chert-pebble conglomerate grades upwards into a quartz sandstone reflecting a rise in sea level and transition to beach environment. Continued transgression led to deposition of the Bluff Mesa Formation, a fossiliferous shallow marine limestone (Underwood, 1962). Subsequent sea level fluctuations led to deposition of an alternating sequence of limestones and sandstones (Bluff Mesa Formation, Cox Sandstone, Finlay Limestone, Benevides Formation, and Espy Limestone) as the shoreline migrated in and out of this area.

During the late Cretaceous and early Tertiary, these rocks were folded and thrust southwestward, producing the general NE dip of bedding. This deformation occurred on the northern margin of the Chihuahua Tectonic Belt (often correlated with the Laramide orogeny), a boundary which roughly coincides with the edge of the older Chihuahua Trough (Henry and Price, 1985).

Explosive volcanism and extensional faulting followed this deformational event. There is no volcanic center in the Indio Mountains, but nearly 200 meters of volcanic tuff in the Flat Top area (Allberg, 1996) are derived from the nearby Eagle Mountains and Van Horn calderas (30-44 my; Henry and Price, 1984). Recent work (Smith, 1996) shows that the Indio Fault, a large NW-trending normal fault was forming at the same time as volcanism, but continued after volcanism ceased. Small amounts of copper mineralization accompanied volcanism; hydrothermal fluids flowed upwards through the fractured Yucca formation but were stopped by the overlying, less permeable Bluff Mesa Formation, so copper minerals are concentrated along this contact.

References

Allberg, C., and Frances Julian. 1996. Tertiary volcanic stratigraphy of Flattop Mountain, Indio Mountains, West Texas. Geological Society of America, south-central section.

Henry, C.D., and J.G. Price. 1984. Variations in caldera development in the Tertiary Volcanic Field of Trans-Pecos Texas. Journal of Geophysical Research, Vol. 89, pp. 8765-8786.

Henry, C.D., and J.G. Price. 1985. Summary of the tectonic development of Trans-Pecos Texas. Bureau of Economic Geology, Misc. Map 36.

Smith, D.E., and F.E. Julian. 1991. Stratigraphy of the Yucca Formation, Indio Mountains, West Texas. Geological Society of America, south-central section.

Smith, W.T., and F. Julian. 1996. Sedimentation and tectonic development of the Indio Mountains, West Texas. Geological Society of America, south-central section.

Underwood, J.R. 1962. Geology of Eagle Mountains and vicinity, Trans-Pecos Texas. Doctoral Dissertation, University of Texas.

Wallace, A.B. 1975. Mineral deposits of the Indio Mountains, Hudspeth County, Texas. M.S. Thesis, University of Texas at El Paso.

Web Resources

Indio Mts. Mines and Minerals. Listing of mines in the Indio Mts. and minerals recovered from those mines.

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Last Update, 20 Sep 2010