Magnetostratigraphy of the Upper Triassic Chinle Group in New Mexico: an Appraisal of 40 Years of Analysis
نویسندگان
چکیده
In New Mexico (NM), Upper Triassic strata of the Chinle Group are nonmarine siliciclastic red beds containing an an extensive fossil record that demonstrates a late Carnian to early Rhaetian age, about 230-203 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 some 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 is inadequate and poorly constrained (2-5 meters), some lithostratigraphic correlations are demonstrably 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. 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. Very variable sedimentation rates and numerous depositional hiatuses make the Chinle Group a poor prospect for magnetostratigraphic analysis.
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