Strontium Isotope Stratigraphy: Case Studies of Upper Carboniferous and Lower Permian Successions from Southeast Utah and Northeast Nevada

Abstract

Strontium Isotope Stratigraphy (SIS) is a valuable tool in correlating sedimentary sequences. Whole rock, conodonts and brachiopods can be studied. Here, conodonts and some whole rock samples were analyzed. Samples were dissolved, and strontium isotope compositions were analyzed using a thermal ionization mass spectrometer. The Early Permian interval exhibits a well-known 87Sr/86Sr secular trend, herein called the McArthur curve, that could provide well-constrained correlations in combination with detailed biostratigraphy. Three Upper Carboniferous to Lower Permian sites were studied, including Valley of the Gods in SE Utah, Carlin Canyon in north-central Nevada and the Rockland section in northeast Nevada. Results for the lower Cutler beds of SE Utah showed all four limestone units within an otherwise red-bed succession tested as latest Carboniferous, and the CPB was interpolated to occur just below the Cedar Mesa Sandstone. In Carlin Canyon, base-Artinskian conodont samples from the Buckskin Mountain Formation showed lower 87Sr/86Sr ratios than whole rock samples. However, all samples were more radiogenic than the expected base-Artinskian value. Rockland samples from the Pequop Formation were considerably more radiogenic than the expected values for the base-Kungurian, in contrast to results from Sakmarian to lower Artinskian strata in the same section, which generally yielded expected values. The variability in SIS results from these three studies necessitated an in-depth review of the McArthur curve. It was found that most North American data had higher 87Sr/86Sr values when compared with published data from Russia and China. This discrepancy was particularly apparent in data from mid-Artinskian to latest Kungurian. Potential reasons for this discrepancy include diagenetic overprint, basin isolation, and reduced mixing caused by the closure of the Uralian Seaway and the Artinskian Warming Event. More studies are needed in order to understand the cause fully. A new LOESS curve was developed to represent the combination of data from this study with previous published data. It was then proposed that two LOESS curves, one for North American data and one for Chinese and Russian data, best represented the regional differences in strontium isotopic signatures. Overall, this study highlights that Lower Permian 87Sr/86Sr values show significant variability, necessitating cautious application.