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 6000 feet lower.
The rugged west-facing escarpment has been carved in section of sedimentary rocks about 7500 feet thick. This section is almost entirely of Paleozoic age. Geologic maps at a scale of two inches equals one mile of an area 28 miles long and 15 miles wide along the escarpment were prepared during 13 months of field investigations. This report deals with the stratigraphy of the mapped area, which is about 85 per cent of the area of the escarpment of the Sacramento Mountains.
The bedrock strata range in age from late pre-Cambrian (?) to middle Permian, and include representatives of all periods of the Paleozoic era. The strata are largely the product of marine deposition on a stable shelf area. Most formations are thin and laterally persistent across much of southern New Mexico and are separated by disconformities.
The oldest rocks exposed are late pre-Cambrian (?) 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°.
The Upper Cambrian (?) Bliss sandstone forms the base of the Paleozoic sequence, and consists of 110 feet of glauconitic quartz sandstone and minor clastic dolomite. The El Paso formation, 370 feet of sandy dolomite of Lower Ordovician age, is probably disconformable on the Bliss sandstone, and is separated from the overlying Montoya formation of Upper Ordovician age by a sharp disconformity representing all of middle Ordovician time. The Montoya formation is 140 to 250 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 formation (new name) and are of Upper Ordovician (?) age. It appears to be gradational with the Montoya formation, but is separated from the overlying darker, cherty dolomite of the Fusselman (?) formation by disconformity. The Fusselman (?) formation is about 70 feet thick along most of the escarpment, but thins and is locally absent toward the north. It contains a Silurian (?) fauna, somewhat older than is known for the Fusselman formation farther to the southwest in New Mexico and Texas.
Strata of Devonian age persist throughout the area, but are nowhere more than 100 feet thick. Lithologically, the 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 (upper Middle Devonian) is overlain by yellow gray limestone and shale of the Sly Gap formation (Upper Devonian) in the northern half of the escarpment. Black shales of the Percha (?) formation (Upper Devonian) overlie the Onate in the southern escarpment, and lap northward onto the Sly Gap.
The base of the Mississippian strata throughout the area consists of gray nodular limestone and shale of the Caballero formation (Kinderhookian), which ranges from 15 to 50 thick. Abundant crinoidal limestone, and many bioherms, some as thick as thick as 400 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 450 feet thick in the northwestern part of the escarpment, and thins to the east and south. Dark siliceous limestone of the Las Cruces (?) and Rancheria formations (Meramecian) overlies a persistent uniformity 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.
The Pennsylvanian strata, 2000 to 3000 feet thick, record a period of structural unrest in New Mexico, as indicated by abundant coarsely clastic strata derived from nearby areas of uplift and erosion, and by considerable lateral variation of the strata. The Sacramento Mountain area received sediment during most of the Pennsylvanian time. The strata are subdivided into three formations and one member (all new names). The basal formation, the Gobbler, consists of 1200 to 1600 feet of coarse quartz sandstone, argillaceous limestone, and shale of Morrowan (?) to middle Missourian age. As much as 800 feet of gray cherty limestone, the Bug Scuffle limestone member, is prominent in the southwest and northeast part of the escarpment, and grades abruptly into shallow marine and deltaic clastic rocks that trend northwest through the area. The Beeman formation (upper Missourian) consists of 350 to 450 feet of feldspathic sandstone, limestone, and shale. Abundant sandstone in the Gobbler and Beeman formations indicate nearby highland areas of pre-Cambrian rocks, the Pedernal Mountains, were nearby to the east in pre-Virgilian Pennsylvanian time. The Holder formation, of Virgilian age, consists of as much as 900 feet of strata. Discontinuous limestone bioherms mark the base of the formation, and red marls, nodular limestone, and white massive limestone characterize much of the upper strata.
In the northwestern Sacramento Mountains, sedimentation apparently was nearly continuous from Pennsylvanian into Permian time. About 300 feet of alternating marine and non-marine gray and red shale, sandstone, and limestone conglomerate of the lower Wolfcampian Bursum formation directly overlies some of the youngest Pennsylvanian strata in North America. Elsewhere in the area, pronounced uplift and deformation by folding and high angle normal faulting occurred along northerly trends and is the major diastrophism of the Paleozoic Era in this area. It probably began in late Virgilian time, reached a climax before upper Wolfcampian time, and was of declining importance after Wolfcampian time.
The Abo formation of upper Wolfcampian age was deposited over a marked uniformity, and rests on truncated strata as old as the lower Lake Valley formation. The Abo formation ranges from 250 to 550 feet thick, and consists of continental red mudstone and coarse arkose in the northern part of the area. Toward the south, the middle part of the Abo formation grades into brackish to marine limestone and shale of the Culp tongue (new name) of the Hueco formation. Upper and lower tongues of Abo red beds persists far to the south, and probably correlate with red strata in the Hueco formation of Texas.
The Yeso formation of Leonardian age is gradational with the Abo formation and consists of about 1300 feet of red beds, yellow and gray shale, limestone, silty quartz sandstone, and evaporates of gypsum and some halite. It records the fluctuating conditions of a shallow back-reef or lagoonal area. Carbonate rocks are more abundant toward the open seas to the southeast, and red beds and evaporates more prevalent toward the shore areas to the north.
The resistant crest and eastern slope of the Sacramento Mountains are formed by the limestone and dolomitic limestone of the Glorieta (?) and the San Andres formations of the Guadalupian age. The Yeso, Glorieta (?), and San Andres formations appear to represent essentially continuous deposition. A total of about ten feet of several thin beds of clean, fine to medium grained quartz sandstone occur in the lower 120 feet of the carbonate strata. This section forms the Glorieta (?) formation. The sandstone beds thicken away from the crest of the mountains. Erosion has removed more than half of the 1400 feet of limestone of the San Andres formation from the crest of the Sacramento Mountains.
Quaternary deposits consist of at least three levels of terrace gravels, recent alluvium spring deposits, and large amounts of slum material. The alluvium of the Tularosa Basin is a minimum of 1800 feet thick, and in part may be Tertiary age.
The Sacramento Mountains are a fault block range, tilted one to two degrees to the east, and bounded on the west by a gravity fault zone near the base of the present escarpment. Piedmont scarps and numerous sympathetic minor faults near the edge of the mountain block, the truncation of sculpture and internal structure of the range, the available stratigraphic evidence, and the regional tectonic pattern all are evidence in favor of faulting as the dominant mechanism for the uplift of the range. The estimated minimum displacement along the boundary fault is of the order of 7000 feet for a distance of 20 miles along the escarpment, and decreases to about half this amount at the northern and southern limits of the mapped area.
The internal structure of the mountain block was largely developed by folding and high-angle faulting during late Pennsylvanian and earliest Permian time. Most of these pre-Abo structures trend north-south. The intensity of the deformation appears to increase from west to east throughout the length of the escarpment, toward the area of maximum pre-Abo uplift and erosion. Anticlines are more sharply defined than synclines, and domes are common along the anticlinical crests. The deformation is believed to be controlled by both vertically acting forms and by lateral compression.
Gentle folds and some high-angle faults in the Permian strata indicate later mild deformation. If a dissected erosional surface in the highest part of the mountains represents the exhumed basal unconformity of former Cretaceous rocks, some of this later structural development is of Mesozoic age. Some of these later structural development is of Mesozoic age. Some of these later structural features may have developed during the intrusion of sills and dikes of trachyandesite of early Tertiary age, that are common in the lower Pennsylvanian and Permian shales.
The major uplift of the range is believed to be of late Cenozoic age, and may be continuing at the present time. A zone of monoclinal folding and gravity faulting along the Sacramento River in the southeast part of the area has formed in late Cenozoic time, and is an echelon with the boundary faults of the Sacramento and Guadalupe Mountains.
Some of the problems of major interest for further study in the Sacramento Mountain region include the bioherms of the Mississippian and Pennsylvanian formations, detailed studies of the lithofacies and biofacies of the Gobbler and Bursum formations, and detailed stratigraphic examination of the Paleozoic strata of adjacent mountain areas in order to permit proper regional analysis of the Paleozoic history of southern New Mexico. Careful collection and study of the invertebrate faunas of the rock units of the lower Paleozoic section of the Sacramento Mountains could result in a major contribution to the stratigraphy of New Mexico and other western areas.
