New Mexico Geological Society Annual Spring Meeting — Abstracts

The Red Tanks Member in the Lucero Mesa area west of Los Lunas consists predominantly of alternating marine limestones, calcareous shales and nonmarine deltaic red and green shales and siltstones. Local abundance and relatively high diversity of bivalves in some Red Tanks units provides data concerning the age of the member and the paleoenvironmental preferences of many of the species. Thin, probably brackish, black "paper shale" units immediately above and below a thin coal seam in the lower half of the member contain numerous leaf fragments, smooth-shelled ostracods, and a numerically abundant but poorly diverse bivalve fauna dominated by Dunbarella whitei and Myalinella meeki. Brown to dark gray shales overlying the nonmarine units are dominated by molluscs; conspicuous bivalves include large numbers of the nuculoids Nuculopsis levatiformis and Phestia bellistriata, along with Aviculopecten basilicus, Edmondia aff. E. nebrascensis, Permophorus aff. P. mexicanus, Pteronites nebrascensis and Myalina (Orthomyalina) subquadrata. Occurrence of the last species is mainly restricted to a thin zone representing a dense shell layer that locally covered a nearshore seafloor. More offshore marine units, such as massive gray argillaceous limestones with calcareous shale interbeds near the middle of the Red Tanks, contain a relatively diverse bivalve assemblage (about 20 species) but bivalves are numerically much less abundant than productoid brachiopods. Characteristic bivalve taxa in these brachiopod-dominated units include Septimyalina burmai, Aviculopecten basilicus, Edmondia aff. E. nebrascensis, Pteronites sp., Pseudomonotis hawni and Schizodus alpinus.Myalina (Myalina) aff. M. (M.) wyomingensis occurs in large numbers in some shale beds in this marine sequence where brachiopod abundance is relatively low. Two steinkerns may represent an early occurrence of Eoastarte, reported previously from the Yeao and San Andres formations of New Mexico. Bivalves are sparsely present in the green and maroon shales and concretionary limestones near the top of the Red Tanks; virtually all specimens are myalinids or Permophorus.

As a whole, the Red Tanks bivalve fauna is similar to that of Midcontinent early Wolfcampian units. Although some long-ranging Pennsylvanian species are present in the Red Tanks, such forms as N. levatiformis, A. basilicus, P. hawni, S. burmai, M. (O.) subquadrata, and Eoastarte? sp. are suggestive of an early Wolfcampian (Early Permian) age.