Chemostratigraphy and correlation of the late Pennsylvanian Madera Formation, Cañon de San Diego, New Mexico, U.S.A.

Abstract

Evaluation of unaltered brachiopod geochemistry from the Virgilian Madera Formation of northern New Mexico suggests that strontium isotopes are the proxy of choice for correlating sections. Based on isotopes, a diachronous relationship describes the sequences of the upper Madera Formation exposed in Canyon de San Diego. After consideration of this fact, carbon and oxygen isotopes of unaltered Madera brachiopods correlate well between the two sections in Canyon de San Diego. Late Pennsylvanian unaltered brachiopods from the North American Midcontinent and the European-Russian Platform define a relatively constant trend for87Sr/86Sr during the Missourian (0.708260) with minor oscillations marking the Virgilian (0.708267), possibly heralding the onset of early Permian glaciation. Carbon isotope values support a greater burial rate of organic matter for the Late Pennsylvanian (+3.21‰, PDB) relative to modern times (+1.6‰, PDB) with some regional differences in oceanographic conditions between the North American Midcontinent and the European-Russian Platform. Although regional oceanographic differences characterize the epeiric seas of the North American Midcontinent and the European-Russian Platform they are within natural variations for geographically separate provinces and modern counterparts. Oxygen isotopes of unaltered brachiopods with a global average value of −3.32‰ for the Late Pennsylvanian support the concept of an ice-free Earth, generally warm epeiric sea temperatures (about 17–24°C) with a possible secular shift in seawater-18O composition of about 0.7‰ for the Missourian-Virgilian (Stephanian).