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The Late Pennsylvanian (Missourian) index fusulinid Eowaeringella in the Manzanita Mountains of central New Mexico Bruce D. Allen1 and Spencer G. Lucas2 1New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM 87801; [email protected] 2New Mexico Museum of Natural History, Albuquerque, NM 87104; [email protected] Abstract The boundary between Middle and Upper Pennsylvanian strata in central New Mexico is generally considered to coincide with the contact between the Gray Mesa and the overlying Atrasado formations. This perception was advanced in the Manzano Mountains by D.A. Myers, who reported the early Missourian fusulinid Eowaeringella near the top of the Gray Mesa Formation from a single locality in Gotera Canyon at the northern end of the range. The Gotera Canyon locality was re-examined and the Eowaeringella horizon recovered, together with additional strata 20 to 25 meters higher in the section that contain early forms of Triticites, including T. cf. pygmaeus, T. wellsi, and T. cf. planus. Thus, at Gotera Canyon, the Eowaeringella Zone is present 50 meters above the top of the Gray Mesa Formation, near the base of the Tinajas Member of the Atrasado Formation. The overlying Triticites-bearing strata appear to be common in the lower part of the Tinajas Member, from as far south as the Gotera Canyon locality northward to the Sandia Mountains. Introduction Strata of the Pennsylvanian System in New Mexico consist of marine and marginal-marine deposits that are widely exposed in mountain ranges along the flanks of the Rio Grande rift (Fig. 1). Deposition of these strata occurred during a period of regional, Late Paleozoic orogenic activity, resulting in a complex succession of lithofacies across New Mexico. Pennsylvanian strata are subdivided in central New Mexico into three Figure 1. Map showing location of the Gotera Canyon section in the Manzanita Mountains. formation-rank units. These are the Blue shaded areas indicate the surface extent of Pennsylvanian rocks. Localities mentioned in the text include Barton (B), Cedro Peak (CP) and Kinney Brick Quarry (KBQ). See Figure 4 Sandia Formation, which contains for a 1:24,000-scale map of the Gotera Canyon locality. a comparatively large proportion of sandstone and shale (generally 100 m thick or less), followed by the Gray Mesa Formation, Here, we report on lower Upper Pennsylvanian largely limestone (generally 100 to 300 m thick), and the fusulinid-bearing strata from a locality in the Manzanita Atrasado Formation, consisting of alternating intervals Mountains, Gotera Canyon (Fig. 1), where Myers of slope-forming clastic deposits and limestone-domi- (1969, 1988) reported an early Missourian fusulinid nated ledges and cliffs (generally 200 to 300 m thick). (genus Eowaeringella) from the top of the Gray Mesa The boundary between Middle and Upper Pennsylvanian Formation. The overall lithostratigraphic architecture of strata, corresponding with the Desmoinesian-Missourian the Pennsylvanian sequence in the vicinity of the local- Stage boundary (ca. 306 Ma), is generally considered to ity is typical of exposures encountered in mountains coincide with the top of the Gray Mesa Formation. and uplifts bordering the Rio Grande from Socorro to Fall 2018, Volume 40, Number 2 New Mexico Geology 35 Albuquerque. A measured section through the lower part Conodont data from the Cerros de Amado, approximately of the Atrasado Formation is presented, and the com- 100 km to the south of the Manzano Mountains, also mon forms of fusulinids recovered from the locality are suggest an early Desmoinesian age for much of the Gray described and illustrated. These forms include a species of Mesa Formation, and indicate that the Desmoinesian- the early Missourian genus Eowaeringella, and late-early Missourian boundary there lies within strata of the over- to middle Missourian Triticites from 20–25 meters higher lying Atrasado Formation, at the stratigraphic level of the in the section. Amado Member (Lucas et al., 2009; Barrick et al., 2013). Therefore, Myers’ (1988) report of the early Missourian Background fusulinid Eowaeringella near the top of the Gray Mesa Formation at Gotera Canyon seemed anomalous, and As summarized above, Pennsylvanian rocks are presently merited re-evaluation. assigned to three formations in central New Mexico— the Sandia, Gray Mesa, and Atrasado formations. The Sandia Formation typically rests on Proterozoic basement, The Gotera Canyon section and transitional Pennsylvanian–Permian strata above The base of Myers’ (1969) section in Gotera Canyon is the Atrasado Formation are assigned to the Bursum in strata of the Gray Mesa Formation near the road that Formation. Before the early 2000s, the Gray Mesa and runs along the floor of the drainage, and traverses the Atrasado formations east of the Rio Grande were com- steep hillslope to the northeast. Bedding is inclined a few monly referred to as the lower and upper members of degrees to the east-northeast along the traverse. Near the Madera Limestone (Read and Wood, 1947). Myers the road at the base of the section, in the Gray Mesa (1973), who had been mapping and conducting fusulinid Formation, a several-meter thick channel-fill deposit biostratigraphy in the Manzano Mountains, formally consisting of coarse-grained pebbly sandstone and con- introduced the names Los Moyos Limestone and Wild glomerate is present. Although comparatively thin sand- Cow Formation to replace “lower” and “upper” Madera stone beds are present in the Gray Mesa Formation, the in the area (before that, Myers had used lower and upper thick channel sand near the base of the Gotera Canyon Madera on his maps). Across the Rio Grande to the west, section is unusual and noteworthy. Above the sandstone, in the Lucero uplift, these lithostratigraphic units had the Gray Mesa Formation contains beds of fossiliferous been named the Gray Mesa and Atrasado members of the wackestone, packstone and some carbonate mudstone, Madera Limestone by Kelley and Wood (1946). Following with short covered intervals that for the most part proba- the suggestion of Kues (2001), the names proposed by bly represent fine-grained siliciclastic deposits. Limestone Kelley and Wood in the Lucero uplift are now used to beds are commonly cherty, which is typical of the Gray delineate Pennsylvanian formations over a large portion of Mesa Formation. The steep hillslope levels off somewhat, central New Mexico, replacing lower and upper Madera approximately 75 meters above the road, marking the top as well as the nomenclature proposed by Myers for the of the Gray Mesa Formation and the base of the Atrasado Manzano Mountains (e.g., Nelson et al., 2013a, b). The Formation. The graphic section depicted in Figure 2 Atrasado Formation has subsequently been divided into begins at the top of the Gray Mesa Formation near this eight members, which have been described and discussed change in slope. in detail elsewhere (e.g., Lucas et al., 2009, 2014, 2016). The basal unit of the overlying Atrasado Formation The units of interest to this study are the Bartolo Member (Bartolo Member base) is up to four meters of coarse- at the base of the Atrasado Formation, characterized by grained, arkosic, trough-crossbedded, pebbly sandstone. a relative abundance of siliciclastic deposits, followed by This laterally discontinuous but widespread sandstone at the Amado Member, largely consisting of limestone beds, the base of the Bartolo Member is the Coyote Sandstone and the overlying Tinajas Member, characterized again by Bed (Lucas et al., 2014), a term that dates back to Herrick’s a relative abundance of siliciclastics (Fig. 2). (1900) early discussions of the rocks of the Pennsylvanian In his monograph on fusulinids from the Manzano System in the vicinity of the Manzanita–Sandia Mountains. Mountains, Myers (1988) assigned a late Desmoinesian At Gotera Canyon, the Coyote Sandstone is overlain by an age to the upper part of the Gray Mesa Formation (his Los additional 29 meters of Bartolo Member strata expressed Moyos Limestone), placing it in his assemblage subzone mainly as covered intervals (shale) with a few decimeter- of Beedeina sulphurensis. Rare specimens of the fusulinid to meter-scale exposures of limestone and a single Wedekindellina ellipsoides were reported by Myers (1988, reddish- and greenish-gray sandstone bed. plate 5) from the upper part of the Gray Mesa Formation The overlying Amado Member forms the first prom- near Sol se Mete Peak, approximately 10 km to the north- inent limestone outcrop above the Gray Mesa Formation. east of Gotera Canyon, suggesting correlation of those At Gotera Canyon, the Amado is approximately 18 meters deposits with uppermost lower Desmoinesian (Cherokee thick, consisting of meter-scale beds of both cherty and Group) strata of the Midcontinent (e.g., Wahlman, 2013). non-cherty bioclastic wackestone to packstone. Also pres- In a more recent study of calcareous microfossils from ent are meter-scale intervals consisting of thinner beds Cedro Peak, about 15 km to the north of Gotera Canyon, of wackestone to carbonate mudstone intercalated with Vachard et al. (2013) suggested a late early Desmoinesian thin shale interbeds and partings. The top of the Amado age for the youngest age-diagnostic assemblages they exam- is 1.2 meters of cherty limestone (unit 21 in Fig. 2). Here, ined from the Gray Mesa Formation, although diagnostic the hillslope along the traverse levels off again, forming fossils from the uppermost part of the formation were not a bench that is underlain by interbedded limestone and obtained. Thus, available evidence from the Manzanita shale. Thin carbonate beds in these strata on the gentler Mountains suggests that much of the upper part of the slope above the Amado Member top contain the Gray Mesa Formation may be early Desmoinesian in age. fusulinid Eowaeringella, apparently corresponding to 36 New Mexico Geology Fall 2018, Volume 40, Number 2 lithology m unit 31 Triticites 30 29 mudstone/wackestone with shale breaks Tinajas Mbr.

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