2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 4
Presentation Time: 2:20 PM

High-Resolution U-Pb Calibration of Carboniferous Glacigenic Deposits, Paganzo Basin, Northwest Argentina:a New Perspective on the Timing of Late Paleozoic Glaciations

GULBRANSON, Erik L.1, MONTANEZ, Isabel2, SCHMITZ, Mark3, LIMARINO, Carlos4, MARENSSI, Sergio A.4 and CROWLEY, Jim5, (1)Department of Geology, University of California, Davis, CA 95616, (2)Geology, University of California, Davis, CA 95616, (3)Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725, (4)Geology, University of Buenos Aires, Buenos Aires, 1428, Argentina, (5)Department of Geosciences, Boise State University, 1910 University Dr, Boise, ID 83725-1535, elgulbranson@ucdavis.edu

Recent studies of the Late Paleozoic Ice Age (LPIA) reveal a more dynamic history of glaciation than previously considered, including multiple short-lived glaciations in southern Gondwana. The timing and geographic extent of such discrete pulses of glaciation, however, are poorly constrained reflecting the paucity of radiometric ages for glacigenic deposits in southern Gondwana. Here we report 9 high-precision (age uncertainties of < 0.1%) U-Pb radiometric ages for the Permo-Carboniferous Paganzo Group, NW Argentina. These ages redefine the late Paleozoic stratigraphy throughout the basins of this region, and for the first time assign high-resolution constraints for the onset of glaciation. An age of 319.60 ± 0.08Ma (2σ) calculated for an ash immediately above the first post-glacial transgression in the Upper Carboniferous Guandacol Formation indicates a minimum earliest Bashkirian or latest Serpukhovian age for the Mid-Carboniferous glaciation in Argentina. A 17 Myr unconformity revealed by new U-Pb ages records an earlier major pulse of glaciation of mid-Viséan age, thereby marking two major pulses of glaciation (upper Mississippian and mid-Carboniferous boundary) in the western region of southern Gondwana. Seven additional ages through the Paganzo Group delineate a latest Moscovian age for two additional Carboniferous glacial events in Gondwana, and redefine the onset of widespread aridity to the latest Moscovian in marked contrast to the established Permian age. This has significant implications not only for regional sequence stratigraphic interpretations, but also for global-scale paleoclimatic reconstructions. The U-Pb calibrated stratigraphy thus indicates a several million-year interval of aridity in the latest Moscovian-early Kasimovian for the high-latitude (~45ºS) Panthalassic margin, coeval to dryland system environments of the paratropics. This inferred internal reorganization of atmospheric moisture transport coincides with a major change in floral diversity.