THE DRUMIAN (CAMBRIAN) GLOBAL BOUNDARY STRATOTYPE SECTION AND POINT (GSSP): APPLYING HIGH RESOLUTION GEOPHYSICAL TECHNIQUES, MAGNETIC SUSCEPTIBILITY (MS) AND GAMMA RAY (GR) SPECTROSCOPY, AS SUPPLEMENTAL STRATIGRAPHIC DATA SETS USED IN INTERPRETING CRITICAL GEOLOGICAL SEQUENCES

Historically, one of the principal barriers to effectively matching stratigraphic positions on separate, Early Paleozoic paleocontinents, was heavy reliance on the biostratigraphy of the most abundant fossils on the platforms. These fossils have a tendency to be endemic taxa. As a result, the lower two series of the Cambrian provide few internationally agreed-upon tie points. One strategy for breaking through historic barriers to defining intercontinental tie points in the two lower series of the Cambrian is to apply a combination of stratigraphic methods. Already, the δ¹³C curves for this time interval shows hope for defining tie points between paleocontinents. However, additional tools are needed as independent checks against the δ¹³C data sets and biostratigraphic interpretations of environments. MS and GR logs for marine sediments have the potential to provide additional paleoenvironmental and correlation tools.

Preliminary work from a high-resolution profile of samples collected at ~5 cm intervals across the Drumian Stage GSSP in the Drum Mountains, Utah, shows a coincidence of positive deflections in MS and GR emissions, both measured on the same samples in the laboratory. MS and GR data sets are highly correlated and reflect the detrital sediment input to the site, as well as a relatively long-term sea-level regression event known to occur at the time. There is a significant change in the character and magnitude of the MS and GR data sets, with major increases in both, beginning at the base of the Drumian Stage. A set of paired MS and GR deflections precisely matches the high δ¹³C 'DICE' excursion that also occurs just above the beginning of the Drumian stage. Thus, after adding in the FAD of the agnostoid trilobite Ptychagnostus atavus, we have four nearly coincident criteria that potentially can be used for correlating the base of the Drumian Stage on a global basis. Studies in other parts of the Phanerozoic section have shown that such correlations can be carried intercontinentally, and are essentially lithology-independent; tied to climate cyclicity and eustacy. In other words, within a constrained stratigraphic interval (defined using the biostratigraphy), MS and GR studies can provide a means of high-precision (and high-resolution) stratigraphic correlation on a global scale.