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Bulletin 35—Geology of the Sacramento Mountains escarpment, Otero County, New Mexico

By L. C. Pray, 1961, reprinted 1984, 144 pp., 34 figs., 15 plates, 1 addenda, 1 index.

The Sacramento Mountains constitute a sharply asymmetrical cuesta at the eastern edge of the Basin and Range province in south-central NM. The escarpment rises abruptly for more than a mile above the desert plains of the Tularosa Basin. From the crest, near 10,000 ft, the surface slopes gently to the Pecos River, 100 mi to the east and 6,000 ft 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 ft. 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 NM, 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 ft 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 10º .

The Lower Ordovician (?) Bliss Sandstone forms the base of the Paleozoic sequence, and consists of 110 ft. of glauconitic quartz sandstone and minor clastic dolomite. The El Paso Formation, 432 ft 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 190–225 ft thick. Dark massive dolomite forms the lower member, and lighter colored cherty dolomite, the upper member. Lithologically distinctive strata composed of 150–200 ft 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 ft thick along most of the escarpment, but thins and is locally absent toward the north. In contains a Silurian fauna somewhat older than is known from the Fusselman Formation farther to the southwest in NM and TX.

Strata of Devonian age persist throughout the area but are nowhere more than 100 ft 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 Oñate Formation (Upper Devonian) is overlain by gray to black shales and minor limestone of the Sly Gap Formation in the northern half of the escarpment. Black shales tentatively correlated with the Percha Formations (Upper Devonian) overlie the Oñate 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, which ranges from 15 to 60 ft thick. Abundant crinoidal limestone and many bioherms, some as thick as 350 ft, form much of the Lake Valley Formation and record a period of prolific marine invertebrate life. The Lake Valley Formation is 200–400 ft thick in the northwestern part of the escarpment and thins to the east and south. Dark siliceous limestone of the Rancheria Formation overlies a persistent unconformity of low angular discordance. These strata are about 300 ft thick to the southeast, where they overlie the Caballero Formation, and thin by overlap to the northwest. The Helms Formation consists of about 60 ft of limestone and shale, and is restricted to the southern part of the escarpment.

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