Geology of Agua Fria Quadrangle, Brewster County, Texas

RI0015. Geology of Agua Fria Quadrangle, Brewster County, Texas, by C. G. Moon. 45 p., 4 figs., 1 map, 5 pls. Reprinted from Geological Society of America Bulletin, v. 64, no. 2, 1953.

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ABSTRACT
The 15-minute Agua Fria quadrangle in southwestern Brewster County, Texas, is arid, sparsely vegetated, and includes diverse topographic features that result chiefly from complex structure and variation in rock resistance to erosion. The mountainous and more complicated southern part of the area has suffered much deformation by igneous intrusions and faulting.

The Comanche series is represented by the Devils River limestone, Grayson marl, and Buda limestone. A disconformity separates it from the overlying gradational Gulf series which consists of the Boquillas,Terlingua, and Aguja formations. Because the Boquillas-Terlingua boundary problem is critical and unsettled, lithologic members and paleontologic zones in that section are described in considerable detail. A distinctive 50-foot rock unit, herein named the Fizzle Flat lentil, occurs about the middle of the Boquillas-Terlingua sequence. A widespread angular unconformity separates the Gulf series from the Tertiary Buck Hill volcanic series. Quaternary terrace gravels occur at different levels, and other alluvial deposits have been mapped.

The Tertiary hypabyssal igneous rocks are alkalic and form stocks, laccoliths, plugs, sills, dikes, and bysmaliths or trap-door domes. Several lava flows are preserved in the southwest part of the quadrangle. Metamorphic effects generally are slight.

The area is part of the Big Bend sunken block. Except where influenced by intrusive masses, a pattern of northwesterly normal faults establishes the structural trend of the area. Step faults are common. Most of the major faults are downthrown to the southwest with the huge intervening blocks tilted gently to the northeast. That much of the fault pattern was established during the Laramide revolution and that faulting recurred along the old lines of weakness fairly late in Tertiary time are postulated.

Keywords: Agua Fria, Boquillas, Brewster County, Terlingua, Texas

CONTENTS

Introduction

Field work

Acknowledgments

Previous work

Physiography

Stratigraphy

Stratigraphic units

Comanche series

Devils River limestone

Grayson (Del Rio) marl

Buda limestone

Gulf series

Tentative correlation

The Boquillas-Terlingua problem

Lower Boquillas-Terlingua unit

Upper Boquillas-Terlingua unit

Aguja formation

Tertiary

General description

Buck Hill volcanic series

Quaternary

General description

Gravel

Alluvium

Intrusive igneous rocks

Introduction

Trap-door domes

Stocks, laccoliths, bysmaliths, plugs, etc.

Sills

Dikes

Metamorphic effects

Igneous petrography

Structural geology

General description

Normal faults

Domes

Geologic history

Economic geology

References cited

Figures

1. Index map of the southern part of Trans-Pecos Texas

2. Correlation of Gulf series in Agua Fria quadrangle

3. Trap-door dome, Gray Hill, and domical uplift, Packsaddle Mountain

4. Structural setting of the Agua Fria quadrangle

Plates

1. Geologic map and structure sections of Agua Fria quadrangle, Texas

2. Boquillas-Terlingua sequence

3. Tuffs and topographic forms

4. Buck Hill volcanic series

Tables

1. Petrography of the igneous rocks of the Agua Fria quadrangle, Texas

2. Estimated stratigraphic throw of several major faults


Citation
Moon, C. G., 1953, Geology of Agua Fria Quadrangle, Brewster County, Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 15, 45 p.