The Lower Permian Abo Formation in the Fra Cristobal and Caballo Mountains, Sierra County, New Mexico
— Spencer G. Lucas, Karl Krainer, Dan S. Chaney, William A. DiMichele, Sebastian Voigt, David S. Berman, and Amy C. Henrici

In the Fra Cristobal and Caballo Mountains of Sierra County, New Mexico, the Lower Permian (middle-upper Wolfcampian) Abo Formation disconformably overlies the Upper Pennsylvanian (Newwellian) Bursum Formation or Middle- Upper Pennsylvanian (Desmoinesian-Virgilian) Bar B Formation and is conformably overlain by the Lower Permian (Leonardian) Arroyo de Alamillo Formation of the Yeso Group. The Abo Formation is 278-309 m thick and can be divided into two members: (1) lower, Scholle Member, 43-75 m thick, mostly mudstone; and (2) upper, Cañon de Espinoso Member, 230-250 m thick, characterized by numerous sheet-like beds of sandstone. In the Fra Cristobal and Caballo Mountains, three complete measured sections of the Abo Formation exemplify this: Red Gap in the Fra Cristobal Mountains and Saddle Tank and McLeod Hills in the Caballo Mountains. The Abo Formation exposed in the Fra Cristobal and Caballo Mountains is a relatively uniform unit that is almost entirely mudstone (71-80% of the measured sections) and sandstone (19-27% of the measured sections). Minor lithologies are shale, siltstone, calcrete and intraformational conglomerate.

Trace fossils from the lower-middle part of the Abo Formation are a uniform ichnoassemblage of rhizoliths, arthropod locomotion and feeding traces and tetrapod footprints of the Scoyenia ichnofacies. The ichnoassemblage of the upper part of the Abo Formation, especially in the Caballo Mountains, is dominated by infaunal backfilled burrows. Fossil plants from the Abo Formation are mostly conifers and belong to two paleofloras: (1) red siltstone assemblages that are of low diversity, almost totally conifers; and (2) green shale/siltstone assemblages, more diverse but still dominated by conifers. Invertebrate body fossils are only known in the green shale, estuarine facies of the Abo Formation in the Caballo Mountains, and are gastropods and diverse bivalves, including euryhaline pectins and myalinids. Tetrapod body fossils are very localized in conglomerates in the Scholle Member and can be assigned to Trimerorhachis, Diplocaulus, Diadectes and Dimetrodon. These are fossils of Coyotean age (late Virgilian-late Wolfcampian), and regional correlations indicate theAbo Formation in the Fra Cristobal and Caballo Mountains is of middle-late Wolfcampian age.                                               

The Abo Formation in the Fra Cristobal and Caballo Mountains is composed of various conglomerate, sandstone, nodular limestone and mudstone lithofacies that can be combined into three principal architectural elements: sandstone sheets formed by amalgamated channels or by relatively unchannelized flow; sandstone lenses and bodies that represent fluvial channels; and siltstone/mudstone with pedogenic limestone that represents deposits of floodplains. In the Caballo Mountains, localized intervals of green shale with interbedded conglomerate, sandstone and limestone yield pectinacean and myalinid bivalves indicative of brackish/marine waters. These are estuarine facies in the Abo Formation, and in the Derry Hills there is a tongue of Hueco Group marine limestone intercalated in the lower part of the Abo Formation. Abo deposition took place on an extensive alluvial plain in which well-defined, bedload river channels within extensive muddy floodplains were succeeded by sandstone sheets formed by low sinuosity river deposits subject to episodic avulsion and sheetflooding. This change in stratigraphic architecture can be attributed to tectonic changes in which falling base level (relatively rapid subsidence) during deposition of the lower Abo was followed by episodically stable base level (slower subsidence) during deposition of the upper Abo.

The presence of marine and estuarine beds in the Abo Formation in the Caballo Mountains mandates a redrawing of current paleogeographic maps so that the northwestern shelf of the Hueco seaway is moved to the northwest into the Derry Hills and closer to the current eastern margin of the Caballo Mountains.