MAGNETOSTRATIGRAPHY OF THE UPPER TRIASSIC CHINLE GROUP IN NEW MEXICO: 40 YEARS OF ANALYSIS

In New Mexico, Upper Triassic strata of the Chinle Group are nonmarine, siliciclastic red beds with an extensive fossil record of late Carnian-late Norian age, about 230-205 Ma. Magnetostratigraphic studies of the Chinle Group in NM began in the 1970s and have produced complete Chinle magnetostratigraphies in east-central NM (Tucumcari basin) and north-central NM (Chama basin), and partial Chinle magnetostratigraphy in other parts of the state. However, despite the number of studies, the resultant magnetostratigraphic data are generally of low reliability and do not allow unambiguous correlations based on pattern matching, even within the Chinle outcrop belt in NM. Particularly problematic is recently published Chinle magnetostratigraphy in the Chama basin in which the sampling interval (2-5 m) is not well constrained, some lithostratigraphic correlations are incorrect and biostratigraphic datum points are ignored. If we correlate the NM Chinle Group using established lithostratigraphy and biostratigraphy, the magnetostratigraphic data do indicate that the late Carnian is a normal multichron, the early-middle Norian is of mixed polarity and the late Norian is a normal multichron, which is consistent with the global pattern. Furthermore, Chinle Group magnetostratigraphy does not contradict vertebrate biostratigraphy, both for local and more global correlations. This is clear if the pre-Sonsela/Poleo/Trujillo part of the Chinle in NM is assigned to the late Carnian (Adamanian) and a late Norian (Apachean) age is assigned to the Rock Point/Redonda interval, the conclusion based on the vertebrate biostratigraphy. Although Chinle magnetostratigraphy has been conducted in NM for over 40 years by various laboratories and researchers, the results provide few or no correlations locally or regionally that improve those achieved by lithostratigraphy and biostratigraphy. Chinle Group magnetostratigraphy in NM has also made little contribution to development of a global polarity timescale for Triassic time. Indeed, the methodological mistakes and unreliability of the data have actually more confused than aided correlation. Clearly, very variable sedimentation rates and numerous depositional hiatuses make the Chinle Group a poor prospect for magnetostratigraphic analysis.