Gatuna Formation (late Cenozoic), Pecos Valley, New Mexico, and Trans-Pecos Texas
— Vincent C. Kelley

The Gatuna Formation was identified and named in 1938 by Lang from exposures in Gatuna Canyon (fig. 1, Gc) and SW 1/2 sec. 35, T. 19 S., R. 30 E. along Nimenium Ridge, Eddy County, New Mexico. Lang described the unit as "an assemblage of rocks of various kinds that were laid down in the Pecos Valley in post-High Plains time and apparently after the completion of the maximum cycle of erosion in this valley." He did not describe a type section. The dominant lithology is fine red sand and silt largely derived from nearby Late Permian and Triassic rocks. Although sand is dominant, the Gatuna section locally contains mudstone, conglomerate, limestone, gray shale or very locally, gypsum. The most typical color is reddish orange, but gray, yellow, black or purple occur locally. Lang (1938) reported as much as 90 m in Cedar Canyon (fig. 1, Cc), T. 24 S., R. 29 E., and I (1971) described a section nearby at Pierce Canyon Crossing (fig. 1, Pc) of the Pecos River dipping 35 to 50 degrees, with neither top nor bottom exposed, also about 90 m thick. The section there is dominantly friable reddish-brown or tan-brown siltstone, sandstone and claystone. Several 0.3 m-thick, pebbly conglomerate or conglomeratic sandstone beds occur in the sandstone, a character which clearly sets them apart from the associated Permian beds. Vine (1963) measured three sections in close proximity along the north bluff of Pierce Canyon (sec. 22-24, T. 24 S., R. 29 E.) which range from 11 to 24 m thick. The individual sections bear little resemblance to one another as to the sequence of sandstone and siltstone, and the eastern section contains blocks and small fragments derived from underlying Permian rocks. Bachman (1976) measured and designated a reference section about 17 m thick just north of Gatuna Canyon (SW'/4 SW 1/4 section 36, T. 19 S., R. 30 E.). Bachman (1976) concluded that the presence locally of Ogallala-like caliche fragments in the Gatuna indicates a post-Ogallala (Pleistocene) age. One of the more significant descriptions of early Pecos Valley fill is that of Maley and Huffington (1953) for the area around Pecos, Texas and northward into adjoining New Mexico. They suggested that the deeper parts of the thick fill would likely be Tertiary in age.

During recent field review of the Gatuna Canyon outcrops by several geologists, a 10-cm long fish was found in laminated shale in the upper part of the exposure near the center of sec. 1, T. 20 S., R. 30 E. (John Hawley and J. P. Bradbury, verbal communications, 1980). The fish has been identified as threadfin shad, Dorosoma petenense (Gunther) by Robert R. Miller.