Oligocene Volcanism and Multiple Caldera Formation in the Chinati Mountains, Presidio County, Texas

RI0135. Oligocene Volcanism and Multiple Caldera Formation in the Chinati Mountains, Presidio County, Texas, by J. C. Cepeda and C. D. Henry. 32 p., 15 figs., 3 tables, 2 pls., 1983. ISSN: 0082335X: Print.

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ABSTRACT
The Chinati Mountains caldera, which lies in Trans-Pecos Texas in the southern Basin and Range Province, was formed by eruption of the Mitchell Mesa Rhyolite. The caldera (30 km by 20 km) is part of a larger Oligocene volcanic province that includes Trans-Pecos Texas and extends to the Sierra Madre Occidental of western Mexico.

Volcanism in the Chinati Mountains area began several million years before formation of the Chinati Mountains caldera. Rocks of the Morita Ranch Formation, Infiernito caldera, and Shely Group ring the caldera on the south, east, and north. After its collapse, the caldera was filled by rhyolitic to trachytic lava flows and an ash-flow tuff of the Chinati Mountains Group. These include, from oldest to youngest, the lower trachyte, middle trachyte, lower rhyolite, upper trachyte, and upper rhyolite (ash-flow tuff). The Chinati Mountains Group was then intruded by the West Chinati Stock, the resurgent dome of the caldera.

Three cycles of rhyolitic to trachytic magmatism, all derived from a zoned magma chamber, are represented by (1) Mitchell Mesa Rhyolite to lower and middle trachytes, (2) lower rhyolite to upper trachyte, and (3) upper rhyolite to West Chinati Stock. Dominant caldera collapse followed eruption of the Mitchell Mesa Rhyolite, but collapse is also associated with rhyolitic eruptions in the second and third cycles. The entire sequence erupted between 32 and 33 mya.

The Chinati Mountains area is the site of one major, inactive silver mine and numerous prospects for silver, lead, zinc, copper, molybdenum, uranium, and fluorite. The Shafter silver district produced 31 million ounces of silver from Permian dolomitic limestones just south of the southern boundary of the caldera. Major prospects are associated with a quartz-monzonite porphyry intrusion (copper-molybdenum) just west of Shafter and with the West Chinati Stock (silver, lead, zinc, copper, and fluorite). All mineralization is probably genetically related to the caldera.

Keywords: ash-flow tuffs, calderas, igneous rocks, K/Ar, mineral deposits, Oligocene, Presidio County, Texas, Trans-Pecos, volcanism

CONTENTS

ABSTRACT

INTRODUCTION

REGIONAL SETTING

ACCESS

HISTORY

PREVIOUS WORK

FIELD METHODS

NOMENCLATURE

STRATIGRAPHY

PRE-TERTIARY ROCKS

TERTIARY VOLCANIC ROCKS

Morita Ranch Formation

Infiernito caldera sequence

Shely Group

Mitchell Mesa Rhyolite

Chinati Mountains Group

Collapse agglomerate

Lower trachyte

Middle trachyte

Lower rhyolite

Upper trachyte

Non-porphyritic domes and flows

Upper rhyolite

Gonzales rhyolite

Tascotal Formation

Perdiz Conglomerate Intrusive rocks of the Chinati Mountains caldera
Red Hill

West Chinati Stock

Plagioclase trachyte intrusions

Dikes and sills

Flow-banded dikes and plugs

GEOCHEMISTRY

STRUCTURAL GEOLOGY

LARAMIDE-TYPE FOLDING

CALDERA COLLAPSE

Chinati Mountains caldera

lntracaldera collapse zones

BASIN AND RANGE FAULTS

WEST-TRENDING STRUCTURES

ECONOMIC GEOLOGY

WATER GEOTHERMAL POTENTIAL

BASE AND PRECIOUS METALS

URANIUM

GEOLOGIC HISTORY

PRE-TERTIARY GEOLOGIC HISTORY

MID-TERTIARY VOLCANIC ACTIVITY

Morita Ranch Formation

Infiernito caldera sequence

Shely Group

Chinati Mountains caldera

LATE CENOZOIC EVENTS

SUMMARY

ACKNOWLEDGMENTS

REFERENCES

Figures

1. Approximate distribution of Tertiary volcanic rocks in Mexico and adjacent regions of West Texas, New Mexico, and Arizona

2. Known calderas and the Bofecillos volcano of Trans-Pecos Texas and adjacent Mexico

3. Generalized geologic map of the Chinati Mountains area

4. Index map showing selected previous study areas in Presidio County, Texas, and adjacent Chihuahua, Mexico

5. Known and suspected stratigraphic relations among Tertiary rocks near the Chinati Mountains caldera

6. Generalized stratigraphy of the Morita Ranch Formation near Shafter

7. Generalized stratigraphy of the Shely and Infiernito Groups in the Infiernito caldera area, northern Chinati Mountains

8. Distribution of the Mitchell Mesa Rhyolite, Texas and Mexico

9. Volcanic agglomerate (mud-flow deposit), consisting dominantly of lower rhyolite,in matrix of finer fragments

10. Massive cliffs of lower rhyolite, overlain by upper trachyte, along western front of Chinati Mountains

11. Numerous flows of upper trachyte composing main mass of Chinati Peak

12. a. Flow-banded, rhyolitic dome along Oso Creek south of Aguja Peak. b. Domes of non-porphyriticrhyolite along presumed secondary collapse zone

13. Distinctly layered cliffs of upper rhyolite within secondary collapse area in central Chinati Mountains

14. Distribution of the Perdiz Conglomerate and Tascotal Formation

15. Structural features of the central Chinati Mountains

 

Tables

1. Nomenclature of the Chinati Mountains Group

2. K-Ar ages of rocks of the Chinati Mountains caldera

3. Comparison of Smith and Bailey caldera model with events in the Chinati Mountains caldera

 

Plates (in pocket of book)

1. Geologic map of the Chinati Mountains caldera

2. Geologic cross sections of the Chinati Mountains caldera


Citation
Cepeda, J. C., and Henry, C. D., 1983, Oligocene Volcanism and Multiple Caldera Formation in the Chinati Mountains, Presidio County, Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 135, 32 p.