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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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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
Devils River limestone
Grayson (Del Rio) marl
The Boquillas-Terlingua problem
Lower Boquillas-Terlingua unit
Upper Boquillas-Terlingua unit
Buck Hill volcanic series
Intrusive igneous rocks
Stocks, laccoliths, bysmaliths, plugs, etc.
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
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
1. Petrography of the igneous rocks of the Agua Fria quadrangle, Texas
2. Estimated stratigraphic throw of several major faults
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.