New Mexico Geological Society Annual Spring Meeting — Abstracts

Diverse marine invertebrate assemblages occur above and below a unit containing vertebrate bones in the upper 20-25 m of the Madera Formation at Placitas, near the north end of the Sandia Mountains, and thus constrain the age of the vertebrates. At the base of the section, massive gray nodular limestone grades upward into splintery gray calcareous shale and greenish-gray mudstone. The limestone contains abundant brachiopods (Composita subtilita, Derbyia spp.), fenestrate and rhomboporoid bryozoans, crinoid debris, and fusulinids. These fusulinids (Triticites coronadoensis, T. whetstonensis. T. cr. bensonensis) are of middle Virgilian (very late Pennsylvanian) age (Wilde, 1990, zone VC-2). The overlying mudstone unit (bed 1A) contains a mixed brachiopod/bivalve/fenestrate bryozoan assemblage (500+ specimens collected). Among the 16 + species of brachiopods, C subtilita (32%), Phricodothyris perplexa (27%), Neospirifer dunbari (10%),
Hystriculina wabashensis (8%), and Cancrinella boonensis (6%) are the most abundant species. Of the bivalve specimens, 80% are Paleolima retifera and Acanthopecten carboniferus. Of special interest is the presence of trilobites (Ditomopyge sp.) and flattened, pyramidal skeletons of conularids (an extinct group of probable cnidarians). Most of these invertebrate taxa are long ranging (Pennsylvanian to Early Permian), but P. perplexa is not known above the middle Virgilian, supporting the fusulinid-based mid-Virgilian age for these lower units. A thin, ledge-forming limestone (bed 3) about 1.6 m above the shale unit contains some of the same faunal elements, as well as sparse large bivalves tentatively identified as Myalina (Orthomyalina) slocombi, another species that ranges no higher than middle Virgilian.

The highest fossiliferous Madera unit is a thin gray bioclastic limestone near the top of bed 9, about 3 m above the top of the vertebrate-bearing horizon. The diverse assemblage (based on 1200+ specimens collected) from this limestone is quite different from the lower assemblage. The upper limestone lacks fusulinids, all five of the dominant brachiopod species, and one of the two most abundant bivalve species of the lower shale. The abundance of gastropods plus bivalves about equals the abundance of brachiopods; fenestrate, rhomboporoid and fistuliporoid bryozoans, crinoid and echinoid fragments, and the trilobite Ditomopyge are also important constituents of this assemblage. Among the brachipods of the upper assemblage, Neochonetes granulifer and its wider morph N. "transversalis" (53%), Juresania aff. nebrascensis (15%), Linoproductus prattenianus (13%), and Derbyia spp., (mainly D. texana, 12%) are the most abundant. Gastropods are represented by large numbers of Amphiscapha muricata (53%) and bellerophontids (e.g., Retispira tenuilineata, Knightites, Euphemites, 40%). Bivalves include Acanthopecten carboniferus (28%), Aviculopecten spp. (21%), and ?Volsellina sp. (17%), together with smaller numbers of such genera as Pernopecten. Pseudomonotis, Clavicosta, Aviculopinna, Edmondia, Astartella, Permophorus, and Leptodesma.

Many of the taxa in the upper limestone range from the Pennsylvanian into the lower Wolfcampian (lowest Permian) in the Midcontinent and/or Texas; some are present in mid-Virgilian strata at the top of the Madera Formation in the Jemez Springs area, 60 km northwest of Placitas. Based on well documented Midcontinent/Texas ranges, the bivalve A. carboniferus and the brachiopods Echinaria semipunctata and Derbyia texana from Placitas bed 9 occur through the Virgilian but do not survive into the Permian. On the other hand, the abundant gastropod A. muricata occurs in the upper 12% of the Virgilian sequence (latest Pennsylvanian) and the lower one third of the Wolfcampian in the Midcontinent. Keeping in mind that the ranges of species in the Midcontinent and Texas regions may not exactly coincide with their ranges in New Mexico strata, the fauna of the upper limestone most strongly suggests a middle to (possibly) late Virgilian age. An early Wolfcampian age is much less likely for the youngest Madera marine invertebrates in the Placitas area.