Remagnetization along the Permian-Triassic disconformity in central New Mexico and remanence acquisition in the Moenkopi Formation
Roberto S. Molina-Garza and John W. Geissman
Sites collected from carbonate rocks and red beds near and at the disconformity between Permian and Triassic strata of the San Andres and Moenkopi formations, respectively, in central New Mexico, contain secondary magnetizations carried by goethite (G) and hematite (H). Paleomagnetic field tests show that normal polarity, north directed and steep magnetizations (Gn and Hn) are likely to be of recent origin, but that reverse polarity, south directed and steep magnetizations (Gr and Hr) are ancient magnetizations. In Triassic strata, Hr magnetizations are superimposed on an early acquired ("primary") magnetization. The primary magnetization of the Moenkopi Formation was locked-in during or soon after deposition, but acquisition of a shallow, low unblocking temperature, magnetization continued long after that. In carbonate rocks of the San Andres Formation, remagnetization is more pervasive, and Hr magnetizations have been identified in facies containing hematite/goethite replacements of evaporite nodules and goethite pseudomorphs of pyrite crystals. Magnetizations with similar directions are observed near the unconformity over a broad region of central and western New Mexico, suggesting that a regional tectonic or diagenetic event may be associated with acquisition of these magnetizations. Hr magnetizations cluster significantly better after correcting for structural tilt related to Rio Grande rift extension. Gr magnetizations are better grouped before tilt correction and give a paleopole indistinguishable from the Late Tertiary–Quaternary dipole field. Clasts of carbonate rocks and hematitic sandstones in the Eocene Baca Formation, lying disconformably above Triassic strata, carry random magnetizations and place a minimum limit on the time of acquisition of the Hr magnetizations. The mean pole calculated for 12 sites from four localities (Lat. = 71.4°, Long. = 157.9°E; A95 = 5.1°) is indistinguishable from poles derived from the Upper Jurassic Morrison Formation from the Colorado Plateau and northeastern New Mexico.
- Molina-Garza, Roberto S.; Geissman, John W., 1999, Remagnetization along the Permian-Triassic disconformity in central New Mexico and remanence acquisition in the Moenkopi Formation, in: Albuquerque Country, Pazzaglia, Frank J.; Lucas, S. G.; Austin, G. S., New Mexico Geological Society, Guidebook, 50th Field Conference, pp. 125-132.