GEOLOGY OF THE SACRAMENTO MOUNTAINS ESCARPMENT, OTERO COUNTY, NEW MEXICO BULLETIN 35 Geology of the Sacramento Mountains Escarpment, Otero County, New Mexico by LLOYD C. PRAY 1 9 6 1 STATE BUREAU OF MINES AND MINERAL RESOURCES NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY CAMPUS STATION SOCORRO, NEW MEXICO NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY Laurence H. Lattman, President NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES Frank E. Kottlowski, Director George S. Austin, Deputy Director BOARD OF REGENTS Ex Officio Toney Anaya, Governor of New Mexico Leonard DeLayo, Superintendent of Public Instruction Appointed Judy Floyd, President, 1977-1987, Las Cruces William G. Abbott, Secretary-Treasurer, 1961-1985, Hobbs Donald W. Morris, 1983-1989, Los Alamos Robert Lee Sanchez, 1983-1989, Albuquerque Steve Torres, 1967-1985, Socorro Published by Authority of State of New Mexico, NMSA 1953 Sec. 63-1-4 Printed by University of New Mexico Printing Plant, February, 1984 Available from New Mexico Bureau of Mines & Mineral Resources, Socorro, NM 87801 Ab str act The Sacramento Mountains constitute a sharply asymmetrical cuesta at the eastern edge of the Basin and Range province in south-central New Mexico. The escarpment rises abruptly for more than a mile above the desert plains of the Tularosa Basin. From the crest, near 10,000 feet, the surface slopes gently to the Pecos River, 100 miles to the east and 6,000 feet lower. The rugged west-facing escarpment has been carved in an uplifted fault block composed largely of sedimentary rocks having an aggregate thickness of about 8,000 feet. Tertiary sills and dikes of intermediate composition are of local importance. The bedrock strata range in age from late Precambrian to Cretaceous, but almost the entire sedimentary section is of Paleozoic age. The strata are largely the product of marine deposition on a stable shelf area. Most of the pre - Pennsylvanian formations are thin and laterally persistent across much of southern New Mexico, and many are separated by disconformities. The Pennsylvanian and Permian units are thicker, and show greater lateral variability as a result of more tectonic instability and diastrophism than prevailed earlier. The oldest rocks exposed are late Precambrian and consist of about 100 feet of slightly metamorphosed quartz sandstone, siltstone, and shale, intruded by diabase sills. These rocks are separated from the Paleozoic strata by an unconformity with an angular discordance of about 10 degrees. The Lower Ordovician(?) Bliss sandstone forms the base of the Paleozoic sequence, and consists of 110 feet of glauconitic quartz sandstone and minor elastic dolomite. The El Paso formation, 432 feet of sandy dolomite of Lower Ordovician age, may be conformable on the Bliss sandstone, and is separated from the overlying Montoya formation of Middle and Upper Ordovician age by a sharp disconformity. The Montoya formation is 190 to 225 feet thick. Dark massive dolomite forms the lower member, and lighter colored cherty dolomite, the upper member. Lithologically distinctive strata composed of 150 to 200 feet of white-weathering, thin-bedded; sublithographic dolomite above the Montoya formation are termed the Valmont dolomite and are of Upper Ordovician age. The Valmont dolomite appears to be gradational with the Montoya formation, but is separated from the overlying darker cherty dolomite of the Fusselman formation by a disconformity. The Fusselman formation is about 70 feet thick along most of the escarp- ment, but thins and is locally absent toward the north. It contains a Silurian fauna somewhat older than is known from the Fusselman formation farther to the southwest in New Mexico and Texas. Strata of Devonian age persist throughout the area but are nowhere 2 NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES more than 100 feet thick. Lithologically, they represent a transition from the underlying dolomites of the lower Paleozoic section to the limestone and shale of the upper Paleozoic. Gray silty dolomite of the Onate formation is overlain by gray to black shales and minor limestone of the Sly Gap formation (Upper Devonian) in the northern half of the escarpment. Black shales tentatively correlated with the Percha formation (Upper Devonian) overlie the Onate in the southern escarpment and fill at least one channel cut in the Sly Gap formation farther north. The basal Mississippian strata throughout the area consist of gray nodular limestone and shale of the Caballero formation (Kinderhookian), which ranges from 15 to 60 feet thick. Abundant crinoidal limestone and many bioherms, some as thick as 350 feet, form much of the Lake Valley formation (Osagian) and record a period of prolific marine invertebrate life. The Lake Valley formation is 200 to 400 feet thick in the northwestern part of the escarpment and thins to the east and south. Dark siliceous limestone of the Rancheria formation (Meramecian) overlies a persistent unconformity of low angular discordance. These strata are about 300 feet thick to the southeast, where they overlie the Caballero formation, and thin by overlap to the northwest. The Helms formation (Chesterian) consists of about 60 feet of limestone and shale, and is restricted to the southern part of the escarpment. Pennsylvanian strata form a complex assemblage of sandstone, shale, and limestone units. In most places the more complete Pennsylvanian sections are about 2,000 feet thick, although 3,000 feet occurs in the northwestern part of the escarpment. Lateral and vertical facies changes, many of which are cyclic, are common. No disconformities have been recognized within the Pennsylvanian strata, although local diastemic breaks are abundant. The Pennsylvanian section is subdivided into three formations and one member. The basal formation, the Gobbler, consists of 1,200 to 1,600 feet of quartz sandstone, limestone, and shale, and is of Morrowan (?) to middle Missourian age. Light-gray cherty limestone forms a distinctive facies as thick as 1,000 feet within the Gobbler formation and is termed the Bug Scuffle limestone member. It grades abruptly into an equivalent thickness of shale and sandstone. The Beeman formation, largely of upper Missourian age, consists of 350 to 450 feet of feldspathic sandstone, limestone, and shale. The deposits grade from a shallow shelf sequence on the east to those of more basinal aspect on the west. The Holder formation of Virgilian age consists of 300 to 900 feet of cyclic repetitions of limestone, shale, and minor sandstone. Algal limestones are abundant; they form massive bioherms along the basal contact and biostromal layers in the overlying part of the Holder formation. The Pennsylvanian strata record increasing tectonism during the period, with progressive differentiation of a positive area (shelf or emergent) in the east and a more negative area, the Orogrande basin, to the west. SACRAMENTO MOUNTAINS ESCARPMENT 3 In the northwesternmost Sacramento Mountains, sedimentation was essentially continuous from Pennsylvanian into Permian time. Sedimen tation was interrupted in most of the area in latest Pennsylvanian and earliest Permian time owing to pronounced uplift accompanying major deformation by folding and faulting. Lower Wolfcampian strata of the Bursum (Laborcita) formation grade from a largely marine sequence of limestone and shale in the northwesternmost part of the escarpment southeastward into thinner nonmarine red and gray shales and limestone conglomerates that unconformably overlie the Holder formation. Bursum strata pinch out into the major unconformity at the base of the Abo formation in the north-central part of the area. The Abo formation of middle and upper Wolfcampian age overlies a marked angular unconformity in all but the northwestern part of the escarpment, and locally was deposited on truncated strata as old as the basal Mississippian deposits. The Abo formation ranges from 250 to 550 feet thick in most of the area. In the northern part the Abo consists of terrestrial red mudstone and coarse arkose, a facies that thickens rapidly toward the northwest. Toward the south, the middle part of the Abo formation grades into brackish to marine limestone and shale of the Pendejo tongue of the Hueco limestone. Upper and lower tongues of Abo red beds persist southward beyond the mapped area and probably correlate with the red beds at the base and in the upper two-thirds of the Hueco limestone. The Yeso formation of Leonardian age consists of about 1,300 feet of red beds, yellow and gray shale, limestone, silty quartz sandstone, and gypsum. Halite and anhydrite occur in the subsurface. Poor exposures prevented detailed study of the Yeso in most of the escarpment area. The deposits record the fluctuating conditions of a shallow marine back-reef or lagoonal area of regional extent. Carbonate rocks are more abundant toward the south, and red beds and evaporites more prevalent toward the presumed shoreward area farther north. Most of the crest and high eastern slope of the Sacramento Mountains is capped by the resistant carbonate rocks of the San Andres formation. Marine limestone and dolomitic limestone form most of the rock unit. The San Andres formation has a maximum thickness of about 700 feet in the mapped area, but thicknesses of only a few hundred feet remain beneath the general level of the summit peneplain along most of the crest. Several units of clean, fine- to medium-grained quartz sandstone with an aggregate thickness of 10 feet or less occur within the lower 120 feet of the San Andres. This basal unit is the Hondo member of the San Andres formation. No unconformity was recognized within the Yeso or San Andres formations. The only known Mesozoic strata of the Sacramento Mountains escarpment occur as a small outlier along the crest of the range near its northern limit, where about 150 feet of quartz sandstone overlies the San Andres formation and underlies thin marine shale of Cretaceous 4 NEW MEXICO BUREAU OF MINES R MINERAL RESOURCES age.