MSEC VALUES FOR THE MIDDLE PERMIAN GLOBAL BOUNDARY STRATOTYPE, GUADALUPE MOUNTAINS NATIONAL PARK

The basal Guadalupian Series boundary exposed in Stratotype Canyon, Guadalupe Mountains National Park, has been chosen as the Middle Permian GSSP (Global Boundary Stratotype Section and Point). We have sampled this GSSP boundary interval at ~0.04 m intervals (224 samples), and have sampled a corresponding section in Colleen Canyon in the near by Delaware Mountains. Our purpose was to (1) establish the MSEC (Magneto-susceptibility Event and Cyclostratigraphy) signature for the GSSP, and (2) compare it with the Colleen Canyon signature as a regional correlation test for the GSSP interval.

We have been using magnetic susceptibility (MS) measurements of marine rocks in conjunction with biostratigraphic control for high-resolution chronocorrelation. MSEC data composites the MS record in marine strata with the coeval biostratigraphic record. MSEC chronozones are determined to have isochronous boundaries. MS in marine sediments is a measure of the concentration of magnetic grains that are dominated by detrital input of lithogenic material due primarily to eustasy and climate. In sections we have examined, increasing MS magnitudes correspond well with episodes of regression, whereas trends of decreasing MS magnitudes correspond with episodes of transgression.

Within the basal Guadalupian GSSP, MSEC variability is relatively low. Unit 3 limestones, containing the GSSP, exhibit high frequency cyclicity with MS values generally < 1 x 10-8 m3/kg. At 0.4 m above the GSSP, a 0.2 m shale unit has MS values to 2 x 10-8 m3/kg. Immediately above the shale, MS values in Unit 3 limestones fall to levels ranging from 2 to 4 x 10-9 m3/kg. These low values extend into the basal shales of Unit 4, where MS values again reach 2 x 10-8 m3/kg. These low values occur in the El Centro Mbr of the Cutoff Fm, long recognized as the major eustatic flooding event of the mid-Permian. In the Colleen Canyon section, MS values in Unit 3 limestones are very low, generally <1 x 10-9 m3/kg, but Unit 4 shales have MS values that more closely match Unit 4 values in the GSSP. We correlate the two sections between the shale in Unit 3 in Stratotype Canyon with an MS high in Unit 3 within the Colleen Canyon section. This high does not represent a shale unit in Colleen Canyon. We also correlate the two sections based on the consistent lithologies and MS values at the base of and within Unit 4.