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2006 Philadelphia Annual Meeting (22–25 October 2006)

Paper No. 8
Presentation Time: 8:00 AM-12:00 PM

HIGH-FREQUENCY SHIFTS IN ATMOSPHERIC CIRCULATION OF WESTERN TROPICAL PANGAEA BASED ON DETRITAL ZIRCON GEOCHRONOLOGY AND WHOLE-ROCK GEOCHEMISTRY


SOREGHAN, Michael J.1, SOREGHAN, Gerilyn S.1 and HAMILTON, Michael A.2, (1)Geology & Geophysics, University of Oklahoma, 100 East Boyd Street, Norman, OK 73019, (2)Geology, Univ of Toronto, Toronto, ON M5S 3B1, msoreg@ou.edu

Detrital zircon geochronology of eolian deposits can provide information regarding regional atmospheric circulation. Within the lower Permian Abo-Tubb interval of northeastern New Mexico, loessite deposits are interbedded with minor fluvial deposits and intercalated with pedogenic horizons. Combined U-Pb geochronology, sedimentology, and whole-rock geochemistry of a single loessite-paleosol couplet buttressed with additional geochronological data record high-frequency shifts in atmospheric circulation within tropical western Pangaea. Trends in whole-rock geochemistry and magnetic susceptibility from loessite to paleosol are consistent with a relative increase in humidity that accompanied the inferred transition from glacial to interglacial states. Furthermore, distinct differences in age spectra of detrital zircons between loessite and paleosol document a significant shift in provenance and hence wind directions. The loessite exhibits a strong age mode between 1800-1600 Ma (Yavapai-Mazatzal terrane) and a significant fraction of very young zircons (near 300 Ma) both of which reflect westerly sources. The paleosol contains smaller and fewer zircons with ages of 1800-1600 Ma, and no very young zircons, but contains a large number of grains between 370-440 Ma and a large number of Grenville-age grains (1300-1000 Ma) reflecting easterly or southeasterly sources. A second sample set of paleosol and loessite, not of the same couplet, show identical detrital zircon age spectra. Given that the Abo-Tubb interval consists of over 120 m of these meter-scale couplets, it appears that regional atmospheric circulation patterns changed at a high frequency; with strong westerly winds blowing during loess deposition switching to weaker (south) easterly winds during pedogenesis. These changes in atmospheric circulation, in turn, appear to track independently derived changes in rainfall patterns. We interpret these consistent and repetitive shifts in atmospheric circulation to reflect glacial-interglacial changes in the intensity of northern-hemisphere-summer monsoonal circulation of western Pangaea between the time of loess deposition (glacial), and pedogenesis (interglacial).

Session No. 207--Booth# 137

T138. Using Detrital Zircon Geochronology to Answer Geologic Questions We Formerly Could Not Ask (Posters)
Wednesday, 25 October 2006: 8:00 AM-12:00 PM
Geological Society of America Abstracts with Programs. Vol. 38, No. 7, p.504
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