To complete seven postings on my 1978 Eagle Mts. adventures (plus one musical interlude) ...
Figure 1. (Looking approx. Southwest)
This is what the southern "half" of the Eagle Mts. looks like from the Allamoore Exit (Exit 129) on Interstate 10, between Van Horn and Sierra Blanca, Texas. I don't know if the dirt road pictured had a name in 1978, but it is currently called "Guest Ranch Road" and to reach the mountains, it crosses Eagle Flat, a subtle intermontane basin ("bolson").
The area is variously classified as part of the Mexican Highlands section of the Basin and Range Province (Underwood, 1980b), the Trans-Pecos Volcanic Field, and the Chihuahuan Desert.
Within Texas, the Basin and Range structural block that includes the Eagle Mts. and the Indio Mts. (to the SE) and Devil Ridge (to the NW) trends roughly NW-SE and is bounded on the west by the Red Light Bolson and on the east by the Eagle Flat Bolson. (Underwood, 1980). South of the Rio Grande (aka Rio Bravo), this block also includes (southward to the Rio Conchos) the Sierra Pilares, Sierra Ventana, Sierra Pinosa, and Sierra de La Parra.
The "chapters" of Eagle Mts. area geology include:
Precambrian
About 5 miles northeast of the Eagle Mts. are the Carrizo Mountains (primarily south of I-10), in which the southernmost Precambrian rocks in the Van Horn region are exposed. Flawn (1953) described the Carrizo Mountains Formation as consisting of 5,000 feet of low-grade metamorphic rocks, e.g., meta-arkose, metaquartzite, schist, phyllite, and limestone.
Within the northeast flank of the Eagle Mts., Flawn (1953) mapped five unnamed Carrizo Mountains Formation units in a small exposure, present as an inlier. The lithologies were similar to those of the Carrizo Mts. proper with an additional amphibolite unit and random Quartz-tourmaline veins.
Permian
Underwood (1980b) described occurrences of the Permian Powwow Conglomerate and overlying Hueco Limestone, along the north flank of the Eagle Mts. These units were deposited during the Permian (Wolfcampian) transgression onto the Diablo Platform.
Cretaceous
In the area of the Eagle Mts., the Cretaceous marine rocks measure around 2,135 meters in thickness and range in age from Aptian through (possibly) late-Turonian and were deposited along the eastern margin of the Chihuahua Trough.
Precambrian
About 5 miles northeast of the Eagle Mts. are the Carrizo Mountains (primarily south of I-10), in which the southernmost Precambrian rocks in the Van Horn region are exposed. Flawn (1953) described the Carrizo Mountains Formation as consisting of 5,000 feet of low-grade metamorphic rocks, e.g., meta-arkose, metaquartzite, schist, phyllite, and limestone.
Within the northeast flank of the Eagle Mts., Flawn (1953) mapped five unnamed Carrizo Mountains Formation units in a small exposure, present as an inlier. The lithologies were similar to those of the Carrizo Mts. proper with an additional amphibolite unit and random Quartz-tourmaline veins.
Permian
Underwood (1980b) described occurrences of the Permian Powwow Conglomerate and overlying Hueco Limestone, along the north flank of the Eagle Mts. These units were deposited during the Permian (Wolfcampian) transgression onto the Diablo Platform.
Cretaceous
In the area of the Eagle Mts., the Cretaceous marine rocks measure around 2,135 meters in thickness and range in age from Aptian through (possibly) late-Turonian and were deposited along the eastern margin of the Chihuahua Trough.
From oldest to youngest, the Comanche Series consists of the Yucca Formation, Bluff Formation, Cox Sandstone, Finlay Limestone, Benevides Formation, Espy Limestone, Eagle Mountains Sandstone, and Buda Limestone. Conformably overlying the Buda Limestone, the Chispa Summit Formation is the only Gulf Series unit (Underwood, 1963).
Laramide Orogeny
Underwood (1980b) made general mention of Laramide thrust-faulting in the area, termed as the Chihuahua Tectonic Belt or Laramide Orogenic Belt, considered to be part of the Western Cordillera.
The thrust-faulted Cretaceous rocks of the Laramide Orogenic Belt (closest to the modern-day Rio Grande) include the western edges of El Paso, Hudspeth, Culberson, Jeff Davis, and Brewster Counties. Before this terrane was block-faulted beginning in the late-Miocene Basin and Range tectonic event, it was the terrain upon which the Oligocene volcanic centers erupted.
Great Ignimbrite Flareup
Laramide Orogeny
Underwood (1980b) made general mention of Laramide thrust-faulting in the area, termed as the Chihuahua Tectonic Belt or Laramide Orogenic Belt, considered to be part of the Western Cordillera.
The thrust-faulted Cretaceous rocks of the Laramide Orogenic Belt (closest to the modern-day Rio Grande) include the western edges of El Paso, Hudspeth, Culberson, Jeff Davis, and Brewster Counties. Before this terrane was block-faulted beginning in the late-Miocene Basin and Range tectonic event, it was the terrain upon which the Oligocene volcanic centers erupted.
Great Ignimbrite Flareup
McAnulty (1976) recognized six Oligocene (36 to 31 m.y.) western Trans-Pecos, Texas volcanic centers as being "Resurgent Cauldrons" as defined by Smith and Bailey (1968), including the Eagle Mts. He used the following Smith and Bailey (1968) criteria as being applicable to the six volcanic centers:
1. "Presence of cauldron facies volcanic sequences, i.e., circular to elliptical outcropping sequences of silicic volcanic rocks, substantially thicker than, and not correlatable with, adjacent more widespread volcanic rocks."
2. "Central plutons, ring dikes, and ring plugs intruding or adjacent to cauldron facies volcanic rocks."
3. "Doming of pre-volcanic and volcanic rocks and presence of large-scale structural sags."
The roughly oval cauldron-ring margins of the Eagle Mts. igneous core trend NNW and measure approximately 10 miles long and 5 miles wide (McAnulty, 1976; Underwood, 1963). Gillerman (1953) mapped the igneous rocks of the central Eagle Mts. and differentiated three major volcanic units, from oldest to youngest, the "Lower Rhyolite", the "Trachyte Porphyry", and the "Upper Rhyolite" and the Eagle Peak Syenite, which intrudes all three volcanic units.
Tieh, et al (1975) performed 21 chemical analyses on the three major volcanic units and the Eagle Peak syenite stock, providing evidence of two eruptive cycles.
Legett (1979), Verrillo (1979), and Zamzow (1983) renamed and redefined the volcanic units as:
Lower Rhyolite - Carpenter Canyon tuff - 25 - 330 meters thickness. Panther Peak member welded tuff with well-developed eutaxitic texture overlying the Indian Springs member, composed of poorly-welded ash flow and air-fall tuffs.
Trachyte Porphyry - Frenchman Canyon rachyte lavas, overlying the Lower Unit composed of densely-welded tuff.
Upper Rhyolite - High Mill tuff - 25 to 330 meters thickness. Eagle Bluff member composed of lithic welded tuff, overlying the Cottonwood Canyon member (Cottonwood Canyon tuff, composed of densely-welded tuff, overlying Silver Eagle tuff, composed of moderately-welded tuff, overlying a basal Epiclastic crystal tuff.
Zamzow (1983) concluded that there were two caldera-forming eruptions from zoned magma chambers fed from the same source. The first caldera cycle erupted the Carpenter Canyon Tuff and then the Frenchman Canyon Trachyte, while the second caldera cycle erupted the High Mill tuff, followed by the Eagle Peak Syenite intrusion. Caldera subsidence occurred during both cycles, but the evidence doesn't suggest any resurgence of either caldera.
Basin and Range/Rio Grande Rift Block Faulting
As is typical of the Basin and Range Province, the structural block of the Eagle Mountains and Indio Mountains is most likely bounded by high-angle normal faults.
Outflow Facies
From the accumulated investigations of the Eagle Mts., there seem to be no detected outflow facies. Several publications mention volcanics in the southern Indio Mts. and in the southern Quitman Mts., but to my knowledge, there hasn't been any connections established between the Eagle Mts. and these outflow units.
References:
Flawn, P.T., 1953 Geology and mineral resources of Precambrian rocks of the Van Horn area, Texas. [incomplete/confusing citation given.]
Gillerman, E., 1953 Fluorspar deposits of the Eagle Mountains, Trans-Pecos, Texas; United States Geological Survey Bulletin 987, 98 pp.
Hoffer, J.M., Leggett, Bob D., Verrillo, D.E., 1980 Tertiary volcanics of the western Eagle Mountains, Hudspeth County, Texas, in Dickerson, P.W., Hoffer, J.M., and Callender, J.F. (eds.) Trans Pecos Region (West Texas), New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, pp. 237 - 240.
Leggett, B.D., 1979 Tertiary volcanics of the northwest Eagle Mountains, Hudspeth County, TX, (M.S. Thesis); University of Texas, El Paso, 100 pp.
McAnulty, W.N., Jr., 1976 Resurgent cauldrons and associated mineralization, Trans-Pecos, Texas, in Tectonics and Mineral Resources of Southwestern North America; New Mexico Geological Society Special Publication 6, pp. 180 - 186.
Smith, R.L. and Bailey, R.A., 1968 Resurgent cauldrons: Geological Society of America Memoir 116, pp. 613 - 662.
Tieh, T.T., Nelson, R.A., and Eggler, D.H., 1975 A geochemical study of igneous rocks of the Eagle Mountains, Trans-Pecos, Texas, in Trans-Pecos Permian Basin Guidebook, Society of Economic Paleontologists and Mineralogists, Publication 75-15, pp. 120 - 128.
Underwood, J.R., 1963 Geology of Eagle Mountains and vicinity, Hudspeth County, TX, Texas Bureau of Economic Geology, Geologic Quadrangle Map 26, 32 pp. of text.
Underwood, J.R., 1980a Eagle Spring Station on the San Antonio - San Diego Mail Line, in Dickerson, P.W., Hoffer, J.M., and Callender, J.F. (eds.) Trans Pecos Region (West Texas), New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, pp. 311 - 314
Underwood, J.R., 1980b Geology of the Eagle Mts., Hudspeth County, TX, in Dickerson, P.W., Hoffer, J.M., and Callender, J.F. (eds.) Trans Pecos Region (West Texas), New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, pp. 183 - 193.
Underwood, J.R., 1980c Physiographic features, Trans-Pecos region, in Dickerson, P.W., Hoffer, J.M., and Callender, J.F. (eds.) Trans Pecos Region (West Texas), New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, pp. 57 - 58.
Verrillo, D. E., 1979 Geology and petrography of the Tertiary volcanic rocks in the southwest Eagle Mountains, Hudspeth County, Texas (M.S. Thesis); University of Texas, El Paso, 139 pp.
Zamzow, C.E., 1983 Tertiary volcanics of the eastern Eagle Mountains, Hudspeth County, Texas (PhD Dissertation); University of Texas, El Paso, 209 pp.
1. "Presence of cauldron facies volcanic sequences, i.e., circular to elliptical outcropping sequences of silicic volcanic rocks, substantially thicker than, and not correlatable with, adjacent more widespread volcanic rocks."
2. "Central plutons, ring dikes, and ring plugs intruding or adjacent to cauldron facies volcanic rocks."
3. "Doming of pre-volcanic and volcanic rocks and presence of large-scale structural sags."
The roughly oval cauldron-ring margins of the Eagle Mts. igneous core trend NNW and measure approximately 10 miles long and 5 miles wide (McAnulty, 1976; Underwood, 1963). Gillerman (1953) mapped the igneous rocks of the central Eagle Mts. and differentiated three major volcanic units, from oldest to youngest, the "Lower Rhyolite", the "Trachyte Porphyry", and the "Upper Rhyolite" and the Eagle Peak Syenite, which intrudes all three volcanic units.
Tieh, et al (1975) performed 21 chemical analyses on the three major volcanic units and the Eagle Peak syenite stock, providing evidence of two eruptive cycles.
Legett (1979), Verrillo (1979), and Zamzow (1983) renamed and redefined the volcanic units as:
Lower Rhyolite - Carpenter Canyon tuff - 25 - 330 meters thickness. Panther Peak member welded tuff with well-developed eutaxitic texture overlying the Indian Springs member, composed of poorly-welded ash flow and air-fall tuffs.
Trachyte Porphyry - Frenchman Canyon rachyte lavas, overlying the Lower Unit composed of densely-welded tuff.
Upper Rhyolite - High Mill tuff - 25 to 330 meters thickness. Eagle Bluff member composed of lithic welded tuff, overlying the Cottonwood Canyon member (Cottonwood Canyon tuff, composed of densely-welded tuff, overlying Silver Eagle tuff, composed of moderately-welded tuff, overlying a basal Epiclastic crystal tuff.
Zamzow (1983) concluded that there were two caldera-forming eruptions from zoned magma chambers fed from the same source. The first caldera cycle erupted the Carpenter Canyon Tuff and then the Frenchman Canyon Trachyte, while the second caldera cycle erupted the High Mill tuff, followed by the Eagle Peak Syenite intrusion. Caldera subsidence occurred during both cycles, but the evidence doesn't suggest any resurgence of either caldera.
Basin and Range/Rio Grande Rift Block Faulting
As is typical of the Basin and Range Province, the structural block of the Eagle Mountains and Indio Mountains is most likely bounded by high-angle normal faults.
Outflow Facies
From the accumulated investigations of the Eagle Mts., there seem to be no detected outflow facies. Several publications mention volcanics in the southern Indio Mts. and in the southern Quitman Mts., but to my knowledge, there hasn't been any connections established between the Eagle Mts. and these outflow units.
References:
Flawn, P.T., 1953 Geology and mineral resources of Precambrian rocks of the Van Horn area, Texas. [incomplete/confusing citation given.]
Gillerman, E., 1953 Fluorspar deposits of the Eagle Mountains, Trans-Pecos, Texas; United States Geological Survey Bulletin 987, 98 pp.
Hoffer, J.M., Leggett, Bob D., Verrillo, D.E., 1980 Tertiary volcanics of the western Eagle Mountains, Hudspeth County, Texas, in Dickerson, P.W., Hoffer, J.M., and Callender, J.F. (eds.) Trans Pecos Region (West Texas), New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, pp. 237 - 240.
Leggett, B.D., 1979 Tertiary volcanics of the northwest Eagle Mountains, Hudspeth County, TX, (M.S. Thesis); University of Texas, El Paso, 100 pp.
McAnulty, W.N., Jr., 1976 Resurgent cauldrons and associated mineralization, Trans-Pecos, Texas, in Tectonics and Mineral Resources of Southwestern North America; New Mexico Geological Society Special Publication 6, pp. 180 - 186.
Smith, R.L. and Bailey, R.A., 1968 Resurgent cauldrons: Geological Society of America Memoir 116, pp. 613 - 662.
Tieh, T.T., Nelson, R.A., and Eggler, D.H., 1975 A geochemical study of igneous rocks of the Eagle Mountains, Trans-Pecos, Texas, in Trans-Pecos Permian Basin Guidebook, Society of Economic Paleontologists and Mineralogists, Publication 75-15, pp. 120 - 128.
Underwood, J.R., 1963 Geology of Eagle Mountains and vicinity, Hudspeth County, TX, Texas Bureau of Economic Geology, Geologic Quadrangle Map 26, 32 pp. of text.
Underwood, J.R., 1980a Eagle Spring Station on the San Antonio - San Diego Mail Line, in Dickerson, P.W., Hoffer, J.M., and Callender, J.F. (eds.) Trans Pecos Region (West Texas), New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, pp. 311 - 314
Underwood, J.R., 1980b Geology of the Eagle Mts., Hudspeth County, TX, in Dickerson, P.W., Hoffer, J.M., and Callender, J.F. (eds.) Trans Pecos Region (West Texas), New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, pp. 183 - 193.
Underwood, J.R., 1980c Physiographic features, Trans-Pecos region, in Dickerson, P.W., Hoffer, J.M., and Callender, J.F. (eds.) Trans Pecos Region (West Texas), New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, pp. 57 - 58.
Verrillo, D. E., 1979 Geology and petrography of the Tertiary volcanic rocks in the southwest Eagle Mountains, Hudspeth County, Texas (M.S. Thesis); University of Texas, El Paso, 139 pp.
Zamzow, C.E., 1983 Tertiary volcanics of the eastern Eagle Mountains, Hudspeth County, Texas (PhD Dissertation); University of Texas, El Paso, 209 pp.
No comments:
Post a Comment