The Pennsylvanian section at Chaves Box, Rio Arriba County, New Mexico

Abstract

—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. FIGURE 1. Location map and geologic map (after Muehlberger, 1968) of Chaves Box in northern New Mexico. 130 LUCAS, KUES, ZEIGLER, AND KRAINER crystalline quartz is rare. Detrital feldspars and rock fragments are lacking. Accessory minerals are zircon and tourmaline. The matrix is stained dark brown–black by iron hydroxides and hematite. Petrographic sample 3 (Figs. 2, 3A) is a coarse-grained, moderately sorted, angular to subangular arkosic arenite composed of abundant monocrystalline quartz, many polycrystalline quartz grains and detrital feldspars. Most feldspars appear fresh (unaltered) and are broken in-situ. Untwinned potassium feldspars, perthitic feldspars, microcline and rare polysynthetic plagioclases are present. Rock fragments are rare and consist of feldspar and quartz. The sandstone also contains a few detrital micas (biotite and muscovite) and chert grains. The pore space is filled with reddish-brown matrix. Locally, detrital quartz grains display authigenic overgrowths. We also examined two characteristic limestone samples. Petrographic sample 4 (Fig. 3E-F) is a bioclastic wackestone composed of gray, bioturbated micritic matrix that contains relatively few bioclasts, including large fragments (up to several cm) of bryozoans, bivalves, brachiopods, fusulinid tests, small gastropods, echinoderm fragments (partly silicified), echinoderm spines, ostracods, very rare smaller foraminifers and abundant calcified spicules. Petrographic sample 5 (Fig. 3D) is a bioclastic wackestone/packstone (spiculite) composed of abundant, locally densely packed spicules that are partly calcified. Locally smaller foraminifers, mostly tubular species, are also abundant. Ostracods and a few small echinoderm fragments are present. The bioclasts are cemented by calcite; locally small amounts of gray micrite are present. The rock is bioturbated. For comparison, we examined samples from the Cutler Group in El Cobre Canyon. El Cobre Canyon is about 60 km southeast of Chaves Box and would have been in a similar paleogeographic position during the Pennsylvanian. It exposes red bed siliciclastic sediments of the Pennsylvanian-Permian Cutler Group. These are sediments shed from the Uncompahgre uplift, so this comparison establishes the likelihood of an Uncompahgre source for the petrographically similar upper sandstones at Chaves Box. Sandstones in the lowermost part containing plant fossils (Fig. 3C) and are greenish-gray, fine-grained and are moderatelyto well-sorted. Grains are angular to subangular and consist of monoand polycrystalline quartz, abundant fresh and altered detrital feldspars (mostly untwinned potassium feldspars, many microclines, perthitic feldspars and a few polysynthetic twinned plagioclases). Some of the detrital feldspars are strongly altered to clay minerals. Detrital grains of greenish biotite are numerous, constituting about 5-10% of all grains; subordinate muscovite is present too. Rock fragments of quartz and feldspar, some containing micas and brownish chert fragments are rare. The sandstone is cemented by coarse, poikilotopic calcite, which replaces detrital quartz and feldspar grains. In the Cutler Group at El Cobre Canyon, the red sandstones are fineto coarse-grained, moderately to well sorted, angular to subangular subarkoses to arkoses containing large amounts of detrital feldspars. Grains are monoand polycrystalline quartz (granitic and schistose-metamorphic), detrital feldspars (mostly untwinned potassium feldspars, some perthitic, microclines, rarely plagioclases displaying karlsbad and polysynthetic twins). Some feldspars occur as large grains. Many detrital feldpars are partly altered to clay minerals. Rock fragments are composed of quartz and feldspar. Some are fine-grained, schistose and composed of micas and quartz (reworked phyllitic rock fragments). Detrital micas are abundant in the lower part (greenish biotite, muscovite, rarely chlorite), and rare in the higher part of the section (muscovite). Some silty chert grains are present. Some sandstone beds contain sedimentary rock fragments composed of reddish-brown, fine-crystalline carbonate, some of which contain a few small grains of angular quartz, feldspar and micas (reworked caliche crusts). Volcanic rock fragments composed of recrystallized volcanic glass and inclusions of volcanic quartz are rarely present. Sandstones are cemented by coarse, poikilotopic calcite which randomly replaces detrital quartz and feldspar. In many sandstones most grains display thin hematite coatings which seem to prevent replacement of quartz and feldspar by calcite. Locally, small amounts of black-stained matrix (hematite) occur. A few detrital quartz grains display thin authigenic overgrowths. Arkosic sandstones of the Cutler Group indicate a dominantly granitic source, as well as subordinate metamorphics (schists, phyllites) and acid volcanics that have also been reworked. Sedimentary rock fragments are reworked intraclasts derived from eroded caliche horizons which locally are preserved within the Cutler Group. Arkosic sandstones of the Cutler Group are very similar in composition and textural maturity to sandstone 3 of the Chaves Box section, and this implies a common source in the Uncompahgre uplift. FIGURE 2. Stratigraphic section of Pennsylvanian strata at Chaves Box showing stratigraphic levels of petrographic samples and fossil locality. 131 THE PENNSYLVANIAN SECTION AT CHAVES BOX FIGURE 3. Petrographic thin sections of selected rocks at Chaves Box (see Figure 2) and from the Cutler Group at El Cobre Canyon, New Mexico. Sample 3.A, Coarse-grained, arkosic arenite composed of angular – subangular quartz grains and abundant potassium feldspars (crossed nicols). Chaves Box, sample 3.B, Mediumto coarse-grained, poorly sorted, angular-subangular quartzarenite composed mostly of monocrystalline quartz crossd nicols). Chaves Box, sample 2.C, Coarse-grained, subarkosic-arkosic arenite containing angular subangular quartz and feldspar grains cemented by calcite (crossed nicols). Cutler Group, El Cobre Canyon (lower part).D, Spiculite composed of abundant, partly calcified spicules and some gray micrite (plane light). Chavez Box, sample 5.EF, Bioclastic wackestone composed of gray micrite and relatively few larger fossil fragments including bivalves, brachiopod fragments, echinoderms (partly silicified), fusulinids, bryozoans, ostracods and rare smaller foraminifers (plane light). Chaves Box, sample 4. 132 LUCAS, KUES, ZEIGLER, AND KRAINER