Tertiary Volcanism of the Bofecillos Mountains and Big Bend Ranch State Park, Texas:

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.

CONTENTS

Abstract

Introduction

            Regional Setting

Geochemical, 1sotopic-Age, and Paleomagnetic Data

            K-Ar and ⁴⁰Ar/³⁹Ar Dating

          ⁴⁰Ar/³⁹Ar and K-Ar Data and General Interpretation

            Paleomagnetic Data

Prevolcanic Rocks

Chisos Group: Contrabando Lowland and Southern Bofecillos Mountains

            Bofecillos Mountains

                        Alamo Creek Basalt (Tcac)

                        Sedimentary Strata of the Undifferentiated Chisos Group (Tc)

                        Bee Mountain Basalt (Tcbm)

                        Mule Ear Spring Tuff (Tcm)

                        Tule Mountain Trachyandesite (Tctm)

                        Influence of the Terlingua Uplift on Distribution of the Chisos Group

                        Devils Graveyard Formation (Tdg)

The Solitario

                        Introduction

                        Predome Sills

                        Central Intrusions

                        Needle Peak Rhyolite (Tir3)

                        Doming and Contemporaneous Intrusions

                        Ash-Flow Eruption and Caldera Collapse

                        Filling of the Caldera

                        Late Intrusions

                        Erosion of the Solitario

                        Terneros Creek Rhyolite (Tpr)

Cienega Mountains Area: Volcanism Related to the Chinati Mountains

                        Morita Ranch Formation (Tm)

                        Mitchell Mesa Rhyolite (Tmm)

                        Cienega Mountains Rhyolite (Tcr)

                        Tascotal Formation (Tta)

                        Perdiz Conglomerate (Tp)

Volcanism of the Bofecillos Mountains

            Fresno Formation (Tf)

                        Rhyolite Lava Dome (Tfr)

                        Rancherias Lavas Member (Tft and Tftp) and the Fresno Volcano

                        Peralkaline Rhyolite Lava Dome (Tfpr)

                        Campo Javelina Basalt Member (Tfb)

                        Tuffaceous Sediments of the Fresno Formation (Tfs)

            Sierra Rica Caldera Complex

                        San Carlos Tuff (Tsc)

                        Santana Tuff (Ts)

            Rawls Formation and the Bofecillos Volcano

Las Burras Lavas Member (Trb; formerly member 1)

            Leyva Canyon Member (Trl; formerly members 2 and 3)

                        Lavas (Trlr, Triq1, Trlq2, and Triq3)

                        Ash-Flow Tuffs (Trla1 and Trla2)

                        Sedimentary Deposits (Trid and Trlc) 34

                        Sauceda Lavas Member (Trs; formerly members 4 and 5)

                        Erosion Following Sauceda Lavas

                        Segundo Lavas Member (Trg; formerly parts of member 7)

                        Rancho Viejo Tuff Member (Trr; formerly member 7at)

                        Alazan Lavas Member (Tra; formerly member 8)

                        The Bofecillos Vent

                        Intrusions

                        The "Bofecillos Volcano"

            Closed Canyon Formation (Tm; formerly member 9)

                        History and Geometry of Faulting

                        Sedimentary Rocks (Tmg, Tmsg, and Tmsc)

                        Basalt (Tmb)

                        Ash-Flow Tuff (Tma)

Geochemistry of Magmatism of the Bofecillos Mountains

            Pre-31-Ma Rocks

                        Bee Mountain Basalt

                        Rancherias Lavas Member of the Fresno Formation

            Post-31-Ma Rocks

                        Campo Javelina Basalt Member of the Fresno Formation

                        Las Burras Lavas Member of the Rawls Formation

                        Sauceda and Alazan Lavas Members of the Rawls Formation

                        Silicic Rocks of the Rawls Formation

                        Closed Canyon Formation

                        General Characteristics of Post-31-Ma Basalts

Tectonic Implications

Acknowledgments

References

Appendices

1. Chemical analyses of igneous rocks. Big Bend Ranch State Park

2. Petrography of igneous rocks. Big Bend Ranch State Park

Figures

1. Location map of Big Bend Ranch State Park

2. Map showing calderas and other volcanic centers of southern Trans-Pecos Texas and adjacent Chihuahua, Mexico

3. Satellite image of Big Bend Ranch State Park and adjacent areas

4. Diagrammatic stratigraphy of Tertiary rocks around the Solitario

5. Diagrammatic stratigraphy of Fresno and Rawls Formations in the Bofecillos Mountains

6. Histogram of ⁴⁰Ar/³⁹Ar and K-Ar ages determined on volcanic rocks of the Bofecillos Mountains area

7. Age spectra

8. Stereonet plot of paleomagnetic directions of volcanic rocks of the Bofecillos Mountains area

9. Photograph of the Chisos Group in the west face of South Lajitas Mesa

10. Oblique aerial photograph of the Solitario, looking northeast across the dome

11. Cross sections showing stages in evolution of the Solitario laccolith and caldera

12. Graphs showing total alkalis versus SiO₂ in volcanic rocks of the Bofecillos Mountains

13. Photograph of south face of Santana Mesa, viewed from FM 170 west of Big Hill

14. Photograph, looking southwest toward thick Leyva Canyon lavas

15. Photograph, looking west toward rocks of the Rawls Formation near Cuevas Amarillas

16. Photograph, looking east over the high plateau that makes up the eastern Bofecillos Mountains

17. Photograph, looking north toward Three-Dike Hill along FM 170 in the southwest part of the state park

18. Vertical aerial photograph of the Bofecillos vent

19. Generalized map of basin-and-range faults, contemporaneous basalts of the Closed Canyon

Formation, and basin-fill deposits of southern Trans-Pecos Texas and adjacent Chihuahua, Mexico

20. Photograph of surface of west-northwest-striking basin-and-range fault along FM 170 approximately 4 km west of Lajitas

21. Graph showing saturation index versus MgO in volcanic rocks that have sources in the Bofecillos Mountains, including Bee Mountain Basalt and Fresno, Rawls, and Closed Canyon Formations

22. Element-normalization diagram of volcanic rocks of the Bofecillos Mountains

23. Element-normalization diagram normalized to mean OIB composition

24. Plot of Zr/Nb ratios versus age of volcanic rocks, Bee Mountain Basalt and Fresno, Rawls, and Closed Canyon Formations

Tables

1. Revised volcanic stratigraphy of the Bofecillos Mountains and Big Bend Ranch State Park

2. ⁴⁰Ar/³⁹Ar and K-Ar data, Big Bend Ranch State Park

3. Paleomagnetic data, Big Bend Ranch State Park

Map (two plates)

Geologic map of Big Bend Ranch State Park, scale 1:50,000


Citation
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.