Late Pennsylvanian and Early Permian Stratigraphy of the Northern Sacramento Mountains, Otero County, New Mexico
<p>Late Pennsylvanian and early Permian strata in the northernmost Sacramento Mountains of New Mexico were studied in 1951-52 to aid in interpreting the complex depositional history of this time in south central New Mexico and to afford better understanding of the sedimentary and tectonic proc...
| Summary: | <p>Late Pennsylvanian and early Permian strata in the northernmost Sacramento Mountains of New Mexico were studied in 1951-52 to aid in interpreting the complex depositional history of this time in south central New Mexico and to afford better understanding of the sedimentary and tectonic processes involved. For this investigation, which is largely a field study, five months were devoted to mapping a critical area of about 80 square miles, of which 30 square miles were covered in great detail, on a scale of four inches equal one mile.</p>
<p>The map area forms the northernmost extension of the Sacramento Mountain escarpment, which is formed by a block that has been uplifted along a fault on the west and tilted one or two degrees to the east. Prior to this late Cenozoic Basin Range faulting, the area was affected by early Tertiary gentle folding and intrusion of Tertiary (?) sills and dikes of acidic and intermediate composition that cover about five percent of the area. Minor high-angle faults that were largely associated with the boundary fault zone occur in the area and locally complicate the structure. Evidence of late Pennsylvanian and early Permian folding and high-angle faulting is presented in the southeasternmost part of the area of investigation. Various Quaternary stream deposits cover about one-fifth of the area, which is otherwise unusually well exposed.</p>
<p>The evidence indicates that deposition was essentially continuous from late Pennsylvanian (Virgilian) into early Permian (Wolfcampian) time within the area. This is significant in view of the relationships within four miles to the southeast, where a major angular unconformity separates Pennsylvanian and Permian strata. The sediments that are the time-equivalent of part of the hiatus represented by the unconformity have been designated by the author as the Laborcita formation. The lithologic and faunal character of the sedimentary deposits of the Laborcita formation indicate that abrupt lateral transition toward the east and southeast from open marine conditions to terrestrial flood plain environments must have occurred repeatedly within the distance of a few miles. The transition in environments was proved by lateral tracing of strata. One typical lateral succession of contemporaneous deposits was determined to be massive marine limestone; nodular argillaceous fusulinid-bearing limestone; silty limestones, bearing abundant shallow marine forms such as molluscs and brachiopods; dolomitic limestone; green shale; and marine to non-marine red shale and other terrigenous clastics. The lithology encountered in any one bed appears to be a function of the distance from shore line and depth of deposition, and the faunal content probably reflects the depth of deposition.</p>
<p>The cyclic repetition of certain sequences is locally conspicuous and appears to be related to the tectonic instability of this area and to the episodic nature of the deformation in the area to the southeast. From late Pennsylvanian to early Permian time, the deposits indicate a gradual emergence of the area and a transition from marine to non-marine environments, although many fluctuations are recorded and periods of relative stability must have occurred.</p>
<p>Everywhere, the Laborcita formation overlies upper Pennsylvanian marine strata and underlies the red beds of the Abo formation. At the type locality, near the mouth of Laborcita Canyon about 2-1/4 miles northeast of the town of La Luz, the Laborcita formation is predominantly composed of marine beds and is 480 feet thick. About 3-1/2 miles to the southeast, a section of approximately the same thickness consists for about 80 percent of non-marine red mudstones. Within two more miles to the southeast the Laborcita formation thickens to about 1,000 feet. This marked increase in thickness is partly caused by a gradual regression of the Laborcita sea toward the northwest and a successive transgression of time lines of the upper contact of the Laborcita formation.</p>
<p>On the basis of fusulinid occurrences throughout the Laborcita formation, the age has been determined as very late Virgilian and early Wolfcampian. In the lower part of the type section several zones yielded abundant fusulinids, which permitted an accurate location of the Pennsylvanian-Permian boundary about 90 feet above the base of the formation. Preliminary studies of the megafossils by various specialists indicate some disagreement with the Permian age of most of the Laborcita formation. The brachiopods indicate an early Permian age, but the ammonoids from the clay pits east of Tularosa, that occur in a position about 150 feet stratigraphically above fusulinids of distinctly Wolfcampian age, are classified as early late Pennsylvanian. The gastropods also exhibit affinities with Pennsylvanian forms. On the basis of the field studies and detailed stratigraphic control, the author believes that the conflicting paleontologic age-assignments must be resolved by further study and collaboration among the specialists of the different faunal groups.</p>
<p>The non-marine strata overlying the Laborcita formation consist largely of red mudstones, fine sandstone, coarse arkose and minor congolomerate and comprise the Abo formation. In the Tularosa area near the northern boundary of the Sacramento Mountains, the Abo formation intertongues at the base with the upper lower Wolfcampian marine strata of the Laborcita formation and is about 1400 feet thick. Twelve miles to the southeast, in the north central part of the Sacramento Mountains, the Abo ranges from 250 to 500 feet in thickness and overlies with angular uncomformity rocks ranging in age from early Mississippian to late Pennsylvanian. The source of the Abo clastics is considered to be the Pedernal Landmass, a positive are of pre-Cambrian igneous and metamorphic rocks that existed during early Permian time in the northeasternmost part of the Sacramento Mountains, and which probably extended for 100 miles, or farther, to the north.</p>
<p>Pray’s work in the central and southern parts of the Sacramento Mountains indicate that the Abo formation is correlative with the bulk of the Hueco formation. On the basis, Pray considers the age of the top of the Abo formation either latest Wolfcampian or earliest Leonardian. As the Abo formation interfingers at the base with upper lower Wolfcampian marine strata in the map area, it is largely considered to be of middle and late Wolfcampian age.</p>
<p>Thus, the Laborcita and Abo formations in the northernmost Sacramento Mountains indicate that deposition there was essentially continuous from late Virgilian through Wolfcampian time. The deposits indicate a gradual emergence of the area and retreat of the marine waters to the west and northwest, probably as a result of the late Virgilian-early Wolfcampian diastrophism of the Sacramento Mountains in the area to the east and southeast.</p>
<p>In the Sacramento Mountains the Abo formation grades upward into the predominantly marine Yeso formation, that reaches a thickness of about 1300 feet. It consists mostly of limestones, shales, gypsum and sandstones and was not studied in any further detail. The overlying San Andres formation is the youngest Paleozoic formation of the Sacramento Mountains and forms the crest of the range.</p>
<p>In the course of the field investigation a zone of lower Permian algal reefs were discovered northeast of Tularosa in the uppermost Laborcita formation. Detailed studies indicate that a filamentous alga is the main cement-binding and framework-building organism in these reefs, which average a height of about 35 feet, but which locally stood about 60 feet above the level contemporaneous sedimentation.</p>
|
|---|