Tertiary Volcanism of the Bofecillos Mountains and Big Bend Ranch State Park, Texas: Revised Stratigraphy and 40Ar/39Ar Geochro

The Bofecillos Mountains and Big Bend Ranch State Park contain a complex sequence of volcanic, volcaniclastic, and intrusive rocks that erupted from sources both within and outside the area. Detailed geologic mapping and stratigraphic, petrographic, geochemical, and 40Ar/39Ar and K-Ar studies justify substantial revision of previously recognized relationships. Magmatism occurred in several distinct episodes between 47 and 18 Ma ago, spanning the complete range of activity in Trans-Pecos Texas. Major groups of rocks erupted at approximately 32 to 31 Ma, 29 Ma, 28 to 27 Ma, and 25 to 18 Ma from sources within the Bofecillos Mountains. Older volcanic rocks in the Bofecillos Mountains and state park erupted from sources mostly outside the area. The Chisos Group of lavas, tuffs, and tuffaceous sediments crops out in the southeast and south parts of the state park. Volcanic rocks erupted from sources to the south and east in two distinct pulses at 47 and 34 to 33 Ma. The first pulse consisted solely of the 47-Ma-old Alamo Creek Basalt, one of the oldest volcanic rocks of Trans-Pecos Texas. The second pulse consisted of the Bee Mountain Basalt, the Tule Mountain Trachyandesite, and the Mule Ear Spring Tuff. Volcanic rocks in the northwest panhandle of the state park erupted from several local sources, as well as from sources in and around the Chinati Mountains caldera to the northwest. These include the 37?- to 32-Ma-old Morita Ranch Formation, which consists of rhyolite and basalt lava; the 32.8-Ma-old Mitchell Mesa Rhyolite; and the 32.7-Ma-old Cienega Mountains Rhyolite, a peralkaline rhyolite lava dome. The Mitchell Mesa Rhyolite, the most voluminous and widespread ash-flow tuff of Trans-Pecos Texas, erupted from the Chinati Mountains caldera and spread throughout what is now the state park. The Solitario laccolith-caldera system, immediately east of the Bofecillos Mountains, formed by (1) a complex sequence of early sill, laccolith, and dike injection; (2) doming during intrusion of the main laccolith; (3) ash-flow eruption; (4) caldera collapse; (5) intracaldera volcanism and sedimentation; and (6) late intrusion. These events occurred in three distinct stages at 36.0,35.4, and 35.0 Ma. This report greatly revises the Fresno and Rawls Formations, which consist of lavas and tuffs that erupted from a series of volcanoes within the Bofecillos Mountains and which blanket the area. The Fresno Formation is composed of two major parts: quartz-normative trachyte that erupted 32 to 31 Ma ago (Rancherias Lavas Member) and nepheline-normative basalt that erupted about 29 Ma ago (Campo Javelina Basalt Member). The Rawls Formation is redivided into six members. The Las Burras Lavas Member consists of aphyric and porphyritic basalt emplaced between 27.8 and 27.3 Ma. The Leyva Canyon Lavas Member consists of quartz trachyte to rhyolite lavas and minor tuffs that erupted in two pulses at 27.3 and 27.1 Ma. The Sauceda Lavas Member consists of nepheline-normative basalt to trachyte lavas. The Segundo Lavas Member consists of a heterogeneous mix of quartz trachyte lavas and related ash-flow tuffs, nepheline-normative trachyte, and conglomerate. The Rancho Viejo Tuff Member is a nepheline-normative trachyte ash-flow tuff. The Alazan Lavas Member consists of basaltic to trachytic lavas nearly identical to the Sauceda lavas. The four younger members have indistinguishable 40Ar/39Ar ages of about 27.1 Ma, a time of voluminous and diverse magmatism in the Bofecillos Mountains. Two major ash-flow tuffs, the 30.5-Ma-old San Carlos Tuff and the 27.8-Ma-old Santana Tuff, erupted from the Sierra Rica caldera complex in Chihuahua and flowed into the Bofecillos Mountains, where they are interbedded with the Fresno and Rawls Formations. The Closed Canyon Formation, formerly member 9 of the Rawls Formation, consists of nepheline-normative basalt, coarse sedimentary rocks, and rare rhyolite ash-flow tuff that were emplaced contemporaneously with initial basin-and-range faulting in the region. The volcanic rocks erupted from widely scattered vents throughout the Bofecillos Mountains between 25 and 18 Ma ago, which is indicative of some of the youngest volcanism in Trans-Pecos Texas. Major- and trace-element geochemical data of basalts indicate a change from lithospheric-dominated sources before about 30 Ma to asthenospheric-dominated sources afterward. This change is consistent with interpretations of structural and tectonic data indicating that pre-30-Ma magmatism was part of a continental volcanic arc related to subduction off the west coast of North America and that post-30-Ma magmatism is related to regional, basin-and-range extension.
Christopher D. Henry
Linda L. Davis
Michael J. Kunk
William C. McIntosh

Henry, C. D., Davis, L. L., Kunk, M. J., and McIntosh, W. C., 1998, Tertiary Volcanism of the Bofecillos Mountains and Big Bend Ranch State Park, Texas: Revised Stratigraphy and 40Ar/39Ar Geochronology: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 253, 74 p.

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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation

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