Late Pennsylvanian and Early Permian stratigraphy of the northern Sacramento Mountains, Otero County, New Mexico
Authors: Otte Jr. 
Published: March 2023

Late Pennsylvanian and early Permian strata in the northernmost Sacramento Mountains, NM, were studied to interpret their complex deposition in the south-central NM area, and to clarify the sedimentary and tectonic history. A critical area of about 80 sq mi was mapped on a scale of 4 inches equal to 1 mi, including 30 sq mi covered in detail.

The map area covers the northern part of the Sacramento Mountains escarpment, which is a block that was uplifted in late Cenozoic time along a fault on the west and tilted to the east. Prior to this basin-and-range faulting, the rocks were gently folded in early Tertiary time and were intruded by Tertiary sills and dikes of acid and intermediate composition. Minor high-angle faults, largely associated with the boundary fault zone, occur in the area and locally complicate the structure. Late Pennsylvanian and early Permian folding and high-angle faulting occurred in the southeasternmost part of the map area. Quaternary stream deposits cover about one-fifth of the area, which otherwise is unusually well exposed.

Deposition was essentially continuous from late Pennsylvanian into early Permian time within the area. This contrasts with the major angular unconformity that separates Pennsylvanian and Permian strata 4 mi to the southeast. The sediments that are the time-equivalent of part of the hiatus represented by the unconformity are named the Labrocita Formation. The lithologic character and faunas of the Labrocita Formation indicate that abrupt lateral transition toward the east and southeast from open marine to flood plain environments occurred repeatedly within a few miles. The transition in environments was deduced by lateral tracing of strata. A typical lateral succession of contemporaneous deposits is: massive marine limestone; nodular argillaceous fusulinid-bearing limestone; silty limestone, bearing abundant shallow marine forms such as molluscs and brachiopods; dolomitic limestone; green shale; and red shale and other terrigenous clastic rocks. The lithology and faunal content of any bed appear to be related to distance from the shore line and the depth of deposition.

Cyclic repetition is locally conspicuous and has been related to tectonic instability of this area and to episodic deformation 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 nonmarine environments, although many fluctuations are recorded and periods of relative stability occurred.

The Labrocita Formation overlies Upper Pennsylvanian marine strata and underlies the Abo red beds. At the type locality, near the mouth of Labrocita Canyon, about 2.25 mi northeast of the town of La Luz, the Labrocita Formation is composed predominantly of marine limestones and is 480 ft thick. About 3.5 mi to the southeast, a section of similar thickness is about 80% nonmarine red mudstones. Within 2 more miles to the southeast, the Labrocita Formation wedges out into an unconformity. About 7 mi northwest of the type locality, near Tularosa, the Labrocita Formation thickens to an estimated 1,000 ft. This marked increase in thickness was caused in part by a gradual regression of the Laboricita sea toward the northwest and by a successive transgression of time lines by the upper boundary of the Laboricita Formation.

Fusulinids occurring throughout the Laboricita Formation determine the age as very late Virgilian and early Wolfcampian. Fusulinid zones accurately located the Pennsylvanian-Permian boundary about 90 ft above the base of the formation. Preliminary studies of the megafossils by specialists indicate some disagreement with the Permian age of most of the Laboricita Formation. The brachiopods indicate an early Permian age, but the ammonoids occurring about 150 ft above fusulinids of distinctly Wolfcampian age, are classified as early late Pennsylvanian. The gastropods also exhibit affinities with Pennsylvanian forms.