New Mexico Geological Society Annual Spring Meeting — Abstracts

The late Virgilian to early Wolfcampian Red Tanks Fonnation (Madera Group), an equivalent of the Bursum Fonnation, is well exposed at Carrizo Arroyo, Valencia County, NM. The formation is ~100 m thick and composed of varied nonmarine and shallow marine siliciclastic and carbonate lithologies (Kues & Kietzke, 1976, NM Geol. Soc. Spec. Paper). Based on the occurrence of fossiliferous limestone horizons indicating transgressive events, the Red Tanks Fm. is divided into six depositional sequences. Each sequence consists dominantly of mudstone/siltstone, which constitute ~81 % of the total thickness of the formation. Purple-reddish mudstones/siltstones, locally containing abundant caliche nodules, formed in a continental environment, probably on a coastal plain. Greenish-grey mudstones/siltstones with
several horizons containing abundant fossils (plants, insects, myriapods, conchostracans, freshwater and brackish ostracods) indicate deposition in a lacustrine environment (marsh) on the coastal plain. In the middle of depositional sequence 2, is a thin coalbed, underlain by
fossiliferous siltstone (plants, ostracods), and overlain by marly mudstone containing marginal marine molluscs and plant debris. Carbonate conglomerates occur at the base of depositional sequence 3 and 4, probably representing upper shoreface deposits, and as thin layers in the upper part of depositional sequence 4, representing small channel fills. Sandstones are exposed at the base (shoreface deposits) and in the upper part (fluvial channel fills) of depositional sequence 1, and in depositional sequence 6 (thin fluvial channel fill deposits).

The top of each sequence is formed by thin fossiliferous gray limestones or gray mudstonellimestone interbeds. Limestones of the lower three sequences are characterized by abundant bioclasts indicating deposition in a shallow, open marine environment. Dominant microfacies are bioclastic wackestones containing fusulinids (Triticites; sequence 1), foraminiferal wackestones with abundant calcivertellids (sequence 2) and bioclastic wackestones containing abundant fragments of brachiopods, molluscs, smaller foraminifers, echinoderms, ostracods, bryozoans, rare trilobites, Tubiphytes, the problematical alga Nostocites and Palaeonubecularia (encrusting bioclasts and forming small oncoids). Limestones of sequence 2 and 3 yielded conodonts. Biota of the limestones on top of sequences 4, 5 and 6 indicate a restricted marine environment. Typical microfacies are ostracod wackestones (sequence 5) and bioclastic mudstones and wackestones containing gastropods and bivalves, some ostracods and rarely smaller foraminifers (sequence 6).

The sequences of the Red Tanks Fm. indicate that the coastal plain environment, represented by mudstones/siltstones, was repeatedly interrupted by short transgressive events, causing deposition of fossiliferous, shallow marine limestones during relative sea-level highstands. The facies of the Red Tanks Formation are transitional between the dominantly carbonate, shallow marine facies of the underlying Atrasado Fm. and the continental red-bed facies of the overlying Abo Fm.