^^' "XC-V"^'' :V:^P-' L - V "XV DESCRIPTION OF THE EL PASO DISTRICT. By GL B. Richardson. INTRODUCTION. throughout the region is of the desert type. The lowlands and The Salt Flat is one of the prominent bolsons of the trans- most of the uplands are bare of trees and only the highest Pecos country. It has the prevailing northwest-southeast LOCATION. mountains support a stunted forest growth. trend, is more than 150 miles long, averages possibly 15 miles The El Paso quadrangle extends from latitude 31° 30' to The rocks of trans-Pecos Texas reveal a long and varied in width, and is a closed basin with no drainage outlet. It 32° 00' and from longitude 1060/to 106° 30', occupying an area history, which began in pre-Cambrian time. Almost all the sys­ occupies a structural trough, and in Texas it is bounded on the of 1014 square miles in western Texas .and the adjacent part of tems from the Algonkian to the Quaternary are represented by east by the Guadalupe and Delaware mountains and on the Mexico. The Texas-New Mexico boundary forms the north­ sediments, and locally this area has been at different times the west by the Sierra Diablo. The center of the basin is 3600 ern limit of the quadrangle and the Rio Grande, flowing across seat of igneous activity by which a variety of molten magmas feet above sea level. Low, marshy areas, commonly floored the southwestern quarter of the area, defines the international were intruded into preexisting rocks or extruded on the surface. with gypsum, occur near the State boundary, where there is boundary. The Mexican portion, occupied chiefly by the river also a salt deposit of commercial importance, but the greater valley, has not been surveyed. To complete the description of OUTLINE OF THE TOPOGRAPHY AND GEOLOGY OF TEANS-PECOS part of the basin is underlain to an unknown depth by gravel, the Franklin Mountains a tract about 6 miles wide just west of TEXAS EAST OF THE EL PASO QUADRANGLE. sand, and clay derived from the adjacent highlands. the quadrangle is included in the maps and descriptions of this Light can be thrown on the general -geology of the El Paso Southwest of the Salt Flat and south of the Sierra Diablo, folio and the whole area will be referred to as the El Paso district by briefly reviewing the conditions in a zone across the about 100 miles southeast of El Paso, there is an area of early district. The surveyed portion of the district has an area of northern part of trans-Pecos Texas, immediately south of the Paleozoic and pre-Cambrian rocks which outcrop in low hills 894 square miles. It is situated in the extreme northwestern Texas-New Mexico boundary. (See fig. 2.) This zone is char- near the Texas and Pacific Railway. The rocks of pre- part of trans-Pecos Texas. TKANS-PECOS TEXAS. Trans-Pecos Texas, that part of the State which lies west of FIGURE 2. East-west sketch section across the northern part of trans-Pecos Texas. Scale: 1 inch = approximately 23 miles. Pecos River, is distinctly different from other parts of Texas in p , pre-Cambrian quartzite and porphyry; COS, Cambrian, Ordovician, and Silurian limestone and sandstone; Ch, Hueco limestone (Pennsylvanian); Cdm, Delaware Mountain formation (Permian?); topography, climate, and geology. The greater portion of the gs, gypsum (probably Permian); K, Cretaceous limestone and sandstone; gr, intrusive granite, post-Carboniferous; Qb, Quaternary bolson deposits. State is occupied by plains, but west of the Pecos the plains are acterized by highlands of northwest-southeast trend separated Cambrian age are separable into two distinct formations; one succeeded by mountains, which mark the boundary between the by parallel belts of lowlands. Pecos River, which rises in the consists of metamorphic rocks including quartzites, slates, and Great Plains and the Cordillera. The portion of the Cordillera southern Rocky Mountains and flows southward and southeast­ various siliceous schists of sedimentary origin, with some altered included in trans-Pecos Texas is the southern continuation of ward parallel to the Cordilleran front, south of the New basic intrusives, and the other is composed of fine-textured red the central mountainous area of New Mexico, and is character­ Mexico-Texas line has a meandering course through the midst sandstone, limestone and breccia. These rocks have been much ized by an assemblage of diverse topographic forms which of a broad plain known as Toyah Bolson. This basin includes disturbed and are unconformably overlain by 500 feet of con­ individually resemble features of the Rocky Mountain province that part of the Pecos Valley which extends southward from glomerate and coarse red sandstone of probable Cambrian age; on the north, the Basin Range province on the west, and the the State boundary as far as the escarpment of the Stockton these in turn are succeeded by a great thickness of limestone Mexican Plateau province on the southwest. Topographically and Edwards plateaus and lies between the Staked Plains on whose lower part is Ordovician and upper part Carboniferous the trans-Pecos region is a transition area adjoining these the east and the highlands, presently to be described, on the (Pennsylvanian), the intervening systems being absent. There provinces. west. The basin is underlain by several hundred feet of gravel, is abundant evidence here of profound pre-Pennsylvanian sand and clay, in part at least of Quaternary age. West of the erosion. Pecos, the elevation of which in this area is between 2500 and North of this area the Sierra Diablo rises abruptly almost 3000 feet, the plain rises at rates between 15 and 30 feet to the 3000 feet above the Salt Flat. This range constitutes the dis­ mile, along the Texas and Pacific Railway, to the base of the sected southeastern escarpment of the Diablo Plateau, a rather Cordilleran foothills, which are here approximately marked by flat-topped upland having an area approximating 2500 square the boundary between Reeves and El Paso counties. Low miles. The surface of this plateau is not flat over wide areas; .outlying hills, composed of horizontal limestones and shales it slopes gently eastward in the western part and westward in belonging to the Washita group of the Comanche series (Lower the Sierra Diablo, yet the plateau nature of the area as a whole Cretaceous), rise above the unconsolidated debris at the western is distinct. It is underlain by flat-lying or gently inclined border of the basin. strata of upper Carboniferous and Lower Cretaceous age. The The dominant topographic feature of the eastern Cordillera eroded escarpments of the Diablo Plateau are known by in this latitude is the highland belt comprising the Guadalupe different names. North of the Sierra Diablo, adjacent to the and Delaware Mountains, which extend southward from New State boundary, the northeastern border of the plateau is Mexico and separate the lowlands of Toyah Bolson on the east marked by the Cornudas Mountains and the Sierra Tinaja front Salt Flat on the west. These mountains constitute an Pinta, which are groups of isolated igneous peaks and lava- FIGURE 1. Relief map of part of the trans-Pecos country, Texas and New eastward-sloping monocline and present a steep scarp to the capped mesas flanked by Carboniferous and Cretaceous sedi­ Mexico. Salt Flat, above which they rise from 1000 to almost 5000 ments. The western border of the Diablo Plateau north of the The region is one of mountains and intermontane plains. feet. The Guadalupe Mountains extend across the State Texas and Pacific Railway is known as the Finlay Mountains, The dominant topographic trend is northwest and southeast boundary about 45 miles west of Pecos River, where they are and farther north, adjacent to the Texas-New Mexico boundary, except near the New Mexico-Texas boundary, where a north- 10 miles wide, but they converge in a wedge-shaped form, and as the Hueco Mountains. These two mountainous areas are south trend is developed. This region lies in the lowest belt of about 10 miles south of the boundary terminate abruptly in a separated by an abrupt escarpment, about 500 feet high and 20 country extending across the interior of the continent. Paisano, precipitous cliff known as Guadalupe Point. El Capitan Peak, miles long, capped by horizontal limestone of Lower Cretaceous the highest pass on the " Sunset route " of the Southern Pacific one-fourth of a mile north of Guadalupe Point, has an elevation age. At the south end of the. escarpmen t the rocks are bent into a system, has an elevation of 5082 feet, and the altitude of of 8690 feet and is thought to .be the highest point in Texas. rude dome which has been dissected into a group of hills known Allamore, at the summit of the Texas and Pacific Railway, is The Delaware Mountains are the southern continuation of the as\he Finlay Mountains. In the center of the dome upper Car­ 4603 feet. Only two peaks rise above 8000 feet and the Guadalupe Mountains. They extend southeastward uninter­ boniferous limestone and shale outcrop and are unconformably lowlands commonly Tange between 3500 and 4500 feet in ruptedly for about 40 miles, but beyond this stretch are overlain by Lower Cretaceous sandstones and limestones that elevation. In general the trans-Pecos highlands lack continuity considerably dissected and form an irregular highland mass extend far to the north and cover a considerable part of the and exhibit a variety of forms, including isolated peaks, groups that reaches almost to the Texas and Pacific Railway. These southwestern surface of the Diablo Plateau. The Finlay Moun­ of peaks, plateaus, narrow tilted blocks, and broad monoclines.