Geologic Unit Descriptions

The following are geologic descriptions taken from the Emory Peak-Presidio Sheet of the Geologic Atlas of Texas (Brown and others, 1979). The table below shows the map symbol, the number of grid cells for the unit, the grid value used for the unit, the geologic unit name, geologic age, and a brief description.

Symbol	Npoints	Grid Value	Formation	Geologic Age

Ehh	2346	2	Hannold Hill Formation	Early Eocene	The Hannold Hill Formation comprises a series of fluvial sandstones, conglomerates, and mudstones of early Eocene age.

Ei	17611	3	Eocene intrusive rocks	Eocene	stocks, laccoliths, sills, and dikes. Major rock types--basalt, hawaiite, mugearite, trachyte, quartz trachyte, rhyolite, phonolite, latite, trachyandesite, and their coarser grained equivalents, e.g. monzonite, syenite

Ev	2028	5	Eocene volcanic rocks	Eocene	igneous rocks described as extrusive or intrusive igneous rocks of Tertiary age.

IPh	14	9	Haymond Formation	Pennsylvanian	Shale and sandstone, dark gray, poorly exposed beds a few inches thick.

IPd	905	10	Dimple Limestone	Pennsylvanian	Limestone with black chert pebbles interbedded with green shale.

IPMt	1700	11	Tesnus Formation	Lower Pennsylvanian and Upper Mississippian	Sandstone and shale; sandstone, fine grained, in part quartzitic, thin bedded, greenish brown, weathers rusty brown to olive gray; shale, hard, fissile, chloritic, green, red, brown, black; some arkose, chert, and basal conglomerate.

Kag	55304	14	Aguja Formation	Cretaceous	Clay, sandstone, and lignite. Upper part continental deposits up to 880 feet thick; sandstone, argillaceous, platy, ripple marked, crossbedded, various shade of yellow and brown; clay, in part calcareous with calcareous nodules, greenish gray to yellowish brown and purple; freshwater limestone scarce; a few lignite beds; vertebrate fossils and petrified wood common.

Kbd	21674	16	Buda Limestone and Del Rio Clay, undivided	Cretaceous	Buda Limestone, divisible into three parts; upper unit, limestone, microgranular, porcelaneous, hard, conchoidal fracture, grayish white; middle unit, limestone, argillaceous, marly, nodular, weathers to a lumpy surface; lower unit, similar to upper unit. Del Rio Clay, mostly clay with some interbedded, flaggy, siliceous limestone, friable sandstone, and thin beds of ferruginous clay; clay, soft, bluish to greenish gray, weathers yellow to light brown.

Kbo	97051	21	Boquillas Formation	Cretaceous	Consists of four units: upper unit, shale silty, medium gray, interbedded with limestone, nodular to laminar, granular, brownish gray; followed downward by shale, silty, medium gray, Interbedded with limestone, granular, yellowish gray; shale, silty dark gray, interbedded with limestone, laminated, grading upward to silty limestone; and a lower unit of limestone, clastic, in thin mostly cross-laminated beds, interbedded with siltstone, light yellowish gray to grayish orange, and shale, black.

Kgr	2706	30	Glen Rose Formation	Cretaceous	Alternating resistant limestone ledges and soft marls that weather to form a characteristic stairstep topography.

Kp	30749	33	Pen Formation	Cretaceous	Equivalent to upper part of Austin Chalk. mostly clay, gray, yellow, calcareous, and sandstone beds in upper part. lower 50 ft contains inch-thick chalk beds, concretions throughout, marine fossils throughout, Exogyra ponderosa common.

Kpt	15566	36	Sue Peaks Formation, Del Carmen Limestone, and Telephone Canyon Formation, undivided	Cretaceous	Limestone and shale. Sue Peaks Formation is gray aphanitic limestone and interbedded yell-gray shale thickness 75 -250 ft. Occupies slope between overlying Santa Elena Limestone and underlying Del Carmen Limestone. Del Carmen is gray limestone aphanitic to fine grained, massive, chert nodules, weathers brown; rudistids and milliolids common. thickness 350-475 ft. Telephone Canyon Formation is yellow-gray, marly, nodular limestone, marine fossils common thickness about 75 ft in Big Bend National Park.

Kse	83039	38	Santa Elena Limestone	Cretaceous	Cliff-forming limestone unit that overlies Sue Peaks Formation. Santa Elena Limestone is light-gray to white, fine-grained to microgranular, massive, beds to 10 ft thick, some marl interbeds, chert nodules and silicified rudistids common in massive beds. Thickness about 350-450+ft.

Kst	20227	40	Santa Elena Limestone, Sue Peaks Formation, Del Carmen Limestone, and Telephone Canyon Formation, undivided	Cretaceous	Santa Elena Limestone forms cliffs over Sue Peaks Formation. Santa Elena Limestone is light-gray to white, fine-grained to microgranular, massive, beds to 10 ft thick, some marl interbeds, chert nodules and silicified rudistids common in massive beds. Thickness about 350-450+ft. Sue Peaks Formation is gray aphanitic limestone and interbedded yell-gray shale thickness 75 -250 ft. Occupies slope between overlying Santa Elena Limestone and underlying Del Carmen Limestone . Del Carmen Limestone is gray limestone aphanitic to fine grained, massive, chert nodules, weathers brown; rudistids and milliolids common. thickness 350-475 ft. Telephone Canyon Formation is yell-gray, marly, nodular limestone, marine fossils common thickness about 75 ft in Big Bend National Park.

Ksu	82	41	Sue Peaks Formation	Cretaceous	Sue Peaks Formation is gray aphanitic limestone and interbedded yellow-gray shale thickness 75 -250 ft. Upper part is mostly ls., nodular, gray some interbedded shale; Lower part mostly shale, marly, some thin interbeds of limestone. Occupies slope between escarpments of the overlying Santa Elena Limestone and underlying Del Carmen Limestone .

Kt	4097	42	Maxon Sandstone and Glen Rose Limestone, undivided	Cretaceous	Limestone, clay, and sandstone. Maxon Sandstone, fine grained, calcareous, well cemented to friable, massive to bedded; limestone nodular to bedded, hard to soft. thickness of Maxon Sandstone 114 ft in area of Santiago Mountains, feathers out east and south short of Big Bend Natl Park and Black Gap area.

Kwfr	5251	43	Washita and Fredericksburg Groups, undivided	Cretaceous	Washita Group - alternating thick clay units and thin limestone units; clay, calcareous, commonly sandy, dark bluish gray and black; limestone in lower part hard, bluish gray to whie, in upper part soft, thickbedded; marine megafossils; thickness 200 ft. Fredericksberg Group --Limestone,dolomite,chert, marl. Limestone nodular, aphanitic, marly, gray, yellow, white, pink; dolomite, find grained, gray; chert, in thin layers and nodules; marl locally gypsiferous, gray. abundant Exogyra texana some beds made almost entirely of Gryphaea sp..

MD	5602	46		Mississippian and Devonian	Undivided sedimentary deposits.
MDO	191	48	Ordovician, Mississippian and Devonian	Ordovician, Mississippian and Devonian	Undivided sedimentary deposits.

Mb	3658	49	Black Gap area volcanic rocks	Miocene	Extrusive igneous rocks east of Big Bend National Park

O	521	52	Ordovician, undivided	Ordovician	Includes Woods Hollow Shale, Fort Pena Formation, and upper part of Marathon Limestone northwest side of El Solitario.

OC	955	53	Woods Hollow Shale, Fort Pena Formation, Alsate Shale, Marathon Limestone, and Dagger Flat Sandstone, undivided	Ordovician and Cambrian	Woods Hollow Shale is a greenish clay shale with interbedded laminated gray to yellowish sandy limestone and limy sandstone; some beds of coarsely granular conglomeratic limestone crowded with fragmented fossils. Fort Pena Formation has alternating thick layers of bedded limestone that is partly sandy, with bedded bluish and purplish chert; some thin shale partings; near base one or more beds of coarse conglomerate. Alsate Shale is a hard green shale with thin limestone beds common and conglomerate locally at base. Marathon Limestone is a flaggy dark-gray to gray-black limestone, weathers ashen gray, and has shale intervals and partings, intraformational conglomerate, and some sandstone beds. Dagger Flat Sandstone is a medium grained, massive saccharoidal sandstone with interbedded shale, some calcareous beds in upper part; brownish yellow to yellowish brown, weathers light brown.

OCe	48570	54	El Paso Formation and Bliss Sandstone, undivided	Ordovician and Cambrian	Bliss Sandstone is the older unit and consists of fine to coarse grained sandstone overlying precambrian rocks. The El Paso Formation consists of limestone, dolomite, and sandstone.

Ocm	3508	55	Chinati Mountains caldera volcanic rocks, including Chinati Mountains Group, Mitchell Mesa	Oligocene	Chinati Mountain caldera volcanic rocks are about 1 km in thickness and include collapse breccias, rhyolitic to basaltic lavas, and a rhyolitic ash-flow tuff. Silicic to intermediate rocks are the largest volume of rocks. A large quartz monzonite intrusion domes the west side of the caldera; many dikes of peralkaline rhyolite intrude caldera-fill volcanic rocks at the north side of the caldera. According to Cepeda and Henry (1983) eight units make up the Group, from bottom to top: collapse agglomerate; lower trachyte, middle trachyte, nonporphyritic domes and flows, Cieneguita dome and flows, lower rhyolite, upper trachyte, upper rhyolite. On the basis of major and trace element chemistry, Cameron and Cameron (1986) recognized 6 groups within the Chinati Mountains caldera rocks--basalts, andesites, main series dacites to low-SiO2 rhyolites, low-high-field-strength (low HFS) cation group, high-SiO2 metaluminous rhyolites and high-SiO2 peralkaline rhyolites. Mitchell Mesa Ignimbrite (or Rhyolite) is the most voluminous and widespread ash-flow tuff of Trans-Pecos Texas and its eruption led to collapse of the Chinati Mountains caldera; it is a multiflow, single-cooling-unit, ash-flow tuff of high-silica rhyolite (77% SiO2); Mitchell Mesa unit is as much as 255 ft thick, averages about 45 ft. Petan Basalt (Trachyte) (also Jones Formation of southern Davis Mountains) is a sequence of porphyritic trachyte lavas that overlie the Mitchell Mesa Ignimbrite north and west of the Infiernito caldera. It contains 25-30 percent phenocrysts and glomerocrysts of plagioclase, clinopyroxene, orthopyroxene, magnetite, ilmenite, zircon, and apatite.

OEg	17892	60	Devils Graveyard volcanic rocks	Eocene-Oligocene	Presidio and Brewster Counties, southwest Texas. Bandera Mesa member (upper member) basalt flow that underlies Mitchell Mesa Rhyolite. Tuffaceous sediment, air-fall tuff, and basalt flows.

Oi	17836	62	Oligocene intrusive rocks	Oligocene	Stocks, laccoliths, sills, and dikes. Major rock types--basalt, hawaiite, mugearite, trachyte, quartz trachyte, rhyolite, phonolite, latite, trachyandesite, and their coarser grained equivalents.

Opf	1302	65	Perdiz Conglomerate, Tascotal Formation, and tuffaceous sediments of Fresno Formation	Oligocene	Perdiz -- weakly cemented fanglomerate of variable composition. Tascotal -- tuffaceous sediments that overlie the Mitchell Mesa Rhyolite including fan apron and eolian facies. Upper part sandstone, tuffaceous sandstone, and conglomerate; Fresno -- interbedded sedimentary materials and flows; includes ash-flow tuff, eolian tuff, eolian tuffaceous sandstone.

Or	2848	66	Bofecillos volcano volcanic rocks, including units 1-8 of Rawls Formation and lava flows in upper part of Fresno Formation	Oligocene	Lava flows in upper part of Fresno Formation are mostly trachybasalts ranging to basaltic trachyandesite; units 1-8 of Rawls Formation are the older part, conglomerate, sandstone, tuff, and basalt.

Os	5041	67	South Rim Formation from Pine Canyon Caldera	Oligocene	Pine Canyon Rhyolite--light brown to gray, densely welded multiple cooling units of peralkaline rhyolitic ash-flow tuff. Boot Rock Member; rocks lying between the Pine Canyon Rhyolite and Lost Mine Member. Caldera fill is varicolored breccia having densly welded glassy matrix containing subangular fragments quartz trachyte and rhyolite. The outflow facies spilled over rim of caldera and is poorly to moderately welded, 10-20 m thick and contains lithic and vitric clasts only in basal surge deposit. Lost Mine Member is package of quartz trachyte ash-flow tuffs and local lava flows. Wasp Spring Member is coarse, well-bedded tuff. Burro Mesa Rhyolite has lower unit of highly welded blue-gray ash-flow tuff with rheomorphic folds, ramp structures. The upper unit has basal black vitrophyre, clasts of tuffaceous sediment and porphyritic quartz trachyte and common pumice lumps

Pabp	1322	71	Black Peaks Formation	Paleocene	Sandstone and clay; sandstone at base, conglomeratic, crossbedded, gray to grayish-white, some cannonball concretions split into platy layers; clay, mottled gray and deep dark-red; vertebrate fossils common; thicknesses measured range from 284 to 866 feet, crops out in northern part of Big Bend National Park.

Pakj	24107	72	Javalina Formation	Paleocene	Clay and sandstone; clay, bentonitic, mostly structureless, nodular calcareous concretions common, dull gray, olive green, deep dark-red, dirty brown, weathers into rounded topographic forms; sandstone, lenticular bodies, crossbedded, ripple marked, vertebrate fossils and petrified wood common; thicknesses measured range from 244 to 936 feet, crops out in Big Bend National Park and northward.
Pobo	13213	79	Bolson Formation	Pliocene	Clay, silt, sandstone, and conglomerate; mostly clay, silt, and gypsiferous fine-grained sandstone, coarsens toward margins where lenses of pebble to boulder conglomerate are common, mostly pale red to light brown

Pof	1996	80	Fingers Formation	Pliocene
Qal	21156	87	Alluvium	Quaternary	Alluvium and low terrace deposits along streams, sand, silt, clay, and gravel. Thickness variable.

Qao	70000	88	Older Alluvium	Quaternary	Alluvium, colluvium, and caliche on surfaces dissected by modern drainage in Trans-Pecos, Presidio, Van Horn, and El Paso areas; pebbles, cobbles, boulders up to 4 ft in size, and sand; unconsolidated to partly consolidated by caliche cement; composed of chert, quartzite, limestone, and volcanic rocks of vesicular, aphanitic, and porphyritic textures.

Qf	103384	89	Alluvial fans	Quaternary	Colluvium and fan deposits, may include older Quaternary deposits in some areas

Qls	5046	90	landslide deposits	Quaternary	Displaced bouldery masses of rock

Qt	256	93	Terrace deposits	Quaternary	In upland regions unit includes fluvial terrace deposits, undivided. Light-brown, reddish-brown, gray, or yellowish-brown, gravelly quartz and lithic sand and silt to sandy gravel. Deposits become increasingly fine grained on Coastal and Nueces Plains. Locally, calcium carbonate-cemented quartz sand, silt, clay, and gravel intermixed and interbedded. Low terraces of major rivers are capped by 2-4 m of clayey sand and silt. Sandy gravel on higher terraces varies somewhat in composition from river to river. Gravel along the Rio Grande is subrounded clasts of locally derived limestone and chert and rounded clasts of basalt, volcanic porphyry, quartzite, milky quartz, and banded chalcedony derived from the west.

QTb	3870	95	Bolson Formation	Quaternary and Tertiary	Clay, silt, sandstone, and conglomerate; mostly clay, silt, and gypsiferous fine-grained sandst. in central part of Presidio bolson, coarsens toward margins where lenses of pebble to boulder conglomerate are common, mostly pale red to lt. brown; thickness up to 2,000 ft.

Qu	6230	97	Undivided	Quaternary	Undivided sedimentary deposits.

Ti	13069	98	Tertiary intrusive rocks, undivided	Tertiary	McKinney Hills laccolith intrusion is augite-hornblende granite or microgranite with variable amounts of iron-rich olivine that in some samples forms as much as 5 percent of the rock. At Roy's Peak the intrusion is about 950 ft thick. Most abundant type of rock is fayalite microgranite (Maxwell and others, 1967, p. 186-188).

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