New Mexico Geological Society Annual Spring Meeting — Abstracts

A well exposed and locally representative section of uppermost Pennsylvanian-Permian strata is exposed at Placitas in the SW1/4 sec. 14,
T13N, R5E, Sandoval County, New Mexico. Upper Pennsylvanian strata belong to the uppermost Madera Formation and are overlain by the Permian Abo, Yeso, Glorieta and San Andres Formations.

Uppermost Madera strata are bioclastic limestones and limy mudstones culminated by a mixed marine-nonmarine interval 20-25 m thick. This interval consists of ledgy and nodular limestones, some with marine fossils, red-bed mudstones and siltstones and intraformational limestone-cobble conglomerates that produce bones of fossil vertebrates. Fusilinids and brachiopods indicate a middle to possibly late Virgil ian age for the uppermost Madera Formation at Placitas.

The Abo Formation conformably overlies the Madera Formation and is as much as 120 m thick and mostly red-bed mudstone, siltstone, troughcrossbedded sandstone and limestone-pebble conglomerate. The Abo contains impressions of the conifer Walchia and a thin (0.2-m-thick) crinoidal limestone conglomerate 40-50 m above its base. 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 (up to 175 m thick) 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 and is as much a 11 m thick and 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.