The Pennsylvanian section at Chaves Box, Rio Arriba County, New Mexico
— Spencer G. Lucas, Barry S. Kues, Kate E. Zeigler, and Karl Krainer

At Chaves Box, southeast of Chama (T29N, R4E, Rio Arriba County), a 130-m-thick section of Pennsylvanian strata crops out between Proterozoic quartzite and the Upper Triassic Chinle Group. These strata can be assigned to a lower interval A of quartzose sandstone and an upper interval B of arkosic sandstone and marine limestone. The marine limestone yields a moderately diverse but poorly preserved normal-marine invertebrate fossil assemblage that consists of 11 species of brachiopods, of which Composita subtilita and Anthracospirifer cf. curvilateralis chavezae are by far the most abundant; an unidentified hexactinellid sponge; crinoid fragments, bryozoans, including Prismopora; the gastropod Retispira tenuilineata; the bivalves Aviculopecten, Myalina and Polidevcia; and rare trilobites. The fusulinids Beedeina haworthi (Beede) and Wedekindellina euthysepta (Henbest) indicate a middle Desmoinesian age, as do the brachiopods. The Pennsylvanian section at Chaves Box can be assigned to the Hermosa Group, probably as a clastic facies equivalent to part of the Paradox Formation. The Pennsylvanian strata at Chaves Box document the changeover from quartzose deposition during final erosion and collapse of the San Luis uplift to initial deposition of arkosic clastics shed from the Uncompahgre uplift. This event, which occurred across the early-middle Desmoinesian boundary, is accompanied by a major regional marine transgression that extended marine deposition as far north and east as Chaves Box.