MIDDLE CARBONIFEROUS MILANKOVITCH CYCLICITY: MSEC DATA FROM THREE MIDDLE CARBONIFEROUS SECTIONS INCLUDING THE GSSP AT ARROW CANYON, NEVADA, AND FROM THE LOWER PENNSYLVANIAN PINKERTON FORMATION, COLORADO

We have been using magnetic susceptibility (MS) measurements of marine rocks, in conjunction with biostratigraphic control, for high-resolution chronocorrelation, and call the method MSEC (magnetosusceptibility event and cyclostratigraphy). We have demonstrated that MS in marine sediments is a measure of the concentration of magnetic grains that are dominated by the detrital input of lithogenic material due primarily to eustasy and climate. In those sections we have examined, increasing MS magnitudes correlate well with episodes of regression, while trends of decreasing MS magnitudes correlate with episodes of transgression.

Here we report results from four mainly marine sections, the Middle Carboniferous GSSP (Global Boundary Stratotype Section and Point) located in Arrow Canyon, eastern Nevada, two secondary sections nearby, one in Arrow Canyon and the second in Battleship Wash, ~ 6 km away, and a section of the basal Pinkerton Formation (Lower Pennsylvanian) in Colorado about 20 miles north of Durango. The MSEC signature for all sections shows marked cyclicity, exhibiting both high and low frequency cycles. The three sections from Nevada can be easily correlated using the MSEC results. While in some units there is a correlation between the MS data and changes in lithology, in others there is no obvious correlation.

MSEC data for that portion of the Pinkerton Formation that was collected show four distinct, low frequency cycles (although the upper and lower cycles were not completely collected), two which terminate in coal beds. The high frequency signal exhibits17 cycles (although again, the upper and lower cycles were not completely collected). If one assumes that the MSEC cyclicity represents Milankovitch bands, then both eccentricity and obliquity bands are represented in the data set. The similarity in MSEC cyclicity in all sections sampled suggests that deposition of these Middle Carboniferous rocks was controlled by eustatic fluctuations that were driven by glaciation in Gondwana during that time.