Palaeontology and Stratigraphy

At Carrizo Arroyo southwest of Albuquerque, New Mexico, USA, an approximately 105-m-thick section of upper Paleozoic clastic and carbonate rocks yields extensive fossil assemblages of marine and nonmarine origin. Most of the section at Carrizo Arroyo belongs to the Red Tanks Member of the Bursum Formation,~100m thick and mostly variegated shale, mudstone and siltstone of nonmarine origin, intercalated with some beds of limestone and shale of marine origin. Red Tanks Member fossils include palynomorphs, charophytes, plant megafossils, non-fusulinid foraminifers, fusulinids, bryozoans, brachiopods, gastropods, bivalves, nautiloids, eurypterids, ostracods, syncarid crustaceans, conchostracans, insects and some other arthropods, echinoids, crinoids, conodonts, fish ichthyoliths and bones of amphibians and reptiles. At stratigraphic levels 43m and 68m above the base of the section are Lagerstätten of plants, insects, crustaceans, eurypterids and other fossils that form unique Late Paleozoic nearshore arthropod assemblages. Most of the fossil groups from the Red Tanks Member have been used to support diverse placements of the Pennsylvanian-Permian boundary at Carrizo Arroyo. We review previous placement of the Pennsylvanian-Permian boundary in the Carrizo Arroyo section and present newly collected insect and conodont data. The insects indicate that the two Lagerstätten in the Red Tanks Member are of early Asselian age. The new conodont data include the presence of Streptognathodus virgilicus in the uppermost part of the Atrasado Formation, which constrains its age to the middle to upper part of the Virgilian and to a comparable position in the Gzhelian. The only biostratigraphically-significant conodont assemblage in the Red Tanks Member comes from a marine horizon near the middle of the member, and the assemblage is probably equivalent in age to the Midcontinent Streptognathodus nevaensis Zone, of early to middle Asselian in age. A significant amount of latest Pennsylvanian to earliest Permian time apparently is not represented by rock record at the Carrizo Arroyo section, most likely at a major disconformity at the top of the Atrasado Formation and smaller ones at the bases of depositional sequences in the lower part of the Red Tanks Member. Conodont biostratigraphy provides compelling evidence that Bursum Formation deposition was not simply driven by glacio-eustatic cyclicity, but in this area it was partly overprinted by local tectonics..

Upper Pennsylvanian (Missourian) strata of the Tinajas Member of the Atrasado Formation in the Cerros de Amado, Socorro County, include an unusual lacustrine deposit, the Tinajas locality. This locality yields a diverse fossil assemblage of plants, crustaceans, insects, a cnidarian, molluscs and fishes from laminar black shale that contains a prolific conchostracan assemblage. A lacustrine origin of the black shale is supported by the abundance of conchostracans; presence of darwinuloid (freshwater) ostracods, land plants and insect wings; lack of marine indicators; and the lithology and geometry of the deposit. Overlying gray shales are full of Dunbarella, which indicates the onset of nearshore, marginal marine conditions. Indeed, the stratigraphic succession at the Tinajas locality is analogous to a classical cyclothem. Thus, fluvial/deltaic deposits grade upward into a coastal lake that is then transgressively overlain by marginal marine and fully marine sediments. The preservation of the lacustrine sediments at the Tinajas locality was a local, unique event on the Late Pennsylvanian coastline in central New Mexico, as no other occurrence of a similar paleoenvironment is known. FIGURE 1. Measured stratigraphic section at the Tinajas locality. See text for description of lithographic units.