Uppermost Pennsylvanian and Permian stratigraphy and biostratigraphy at Placitas, New Mexico
— Spencer G. Lucas, J. Mark Rowland, Barry S. Kues, John W. Estep, and Garner L. Wilde
A well-exposed and locally representative section of uppermost Pennsylvanian and Permian strata is exposed at Placitas in the SW1/4 sec. 14, T3N, R5E, Sandoval County, New Mexico. Upper Pennsylvanian strata belong to the Wild Cow Formation, and are overlain by the Permian Abo, Yeso, Glorieta, and San Andres formations. Strata of the uppermost Wild Cow Formation are a mixed marine-nonmarine interval 7-20 m thick. This interval consists of ledgy and nodular limestones, some with marine fossils, redbed mudstones, and siltstones, and intraformational limestone-cobble conglomerates that produce bones of fossil vertebrates, including sharks, lungfishes, amphibians, and pelycosaurs. Fusulinids, brachiopods, and bivalves indicate a middle to possibly late Virgilian age for the uppermost Wild Cow Formation at Placitas. The Abo Formation disconformably(?) overlies the Wild Cow Formation, is as much as 112 m thick and is mostly redbed mudstone, troughcrossbedded sandstone and limestone-pebble conglomerate. The Abo contains impressions of the conifer Walchia and other plants, tetrapod footprints of the ichnogenus Dimetropus, conchostracans, insects, and a thin crinoidal limestone conglomerate. However, these fossils are not diagnostic of a precise age, so it is not clear whether the Abo Formation at Placitas is entirely of Early Permian (Wolfcampian) age. The Yeso Formation (about 175 m thick) at Placitas consists of two members, Meseta Blanca and San Ysidro. The Meseta Blanca Member conformably overlies the Abo Formation (their contact is gradational over about 20 m of section), is up to 60 m thick and consists mostly of trough-crossbedded, yellow-to-tan, eolianite sandstone. The overlying San Ysidro Member is up to 115 m thick and mostly massive and ripple-laminated, thin-bedded, fine-grained sandstone, siltstone, and beds of gypsum. The overlying Glorieta Sandstone sharply overlies the Yeso Formation, is as much as 12 m thick and is mostly brown, trough-crossbedded quartzarenite/metaquartzite. Patchy and thin (up to 5 m thick), gray, dolomitic limestone of the San Andres Formation overlies the Glorieta and is disconformably overlain by fluvial red beds of the Middle Triassic Moenkopi Formation. The actual thickness of the Permian section at Placitas thus is 300-320 m, much less than previously published estimates. The Upper Pennsylvanian–Permian section at Placitas is significant for three reasons: (1) this is the singlemost complete, well-exposed Permian section around the Sandia uplift; (2) at Placitas, well-preserved and diverse marine invertebrate assemblages are present; and (3) the youngest Virgilian strata at Placitas include conglomerates that yield a diversity of fossil vertebrates. This is a rare case where a clear cross-correlation of vertebrate taxa to Upper
Pennsylvanian marine biostratigraphy is possible.
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- Lucas, Spencer G.; Rowland, J. Mark ; Kues, Barry S.; Estep, John W.; Wilde, Garner L., 1999, Uppermost Pennsylvanian and Permian stratigraphy and biostratigraphy at Placitas, New Mexico, in: Albuquerque Country, Pazzaglia, Frank J.; Lucas, S. G.; Austin, G. S., New Mexico Geological Society, Guidebook, 50th Field Conference, pp. 281-292. https://doi.org/10.56577/FFC-50.281