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: 8:00 AM-4:45 PM

Geobiology of the Late Paleoproterozoic Duck Creek Dolomite, Western Australia

WILSON, Jonathan P.1, JOHNSTON, David T.2, KNOLL, Andrew H.2, FISCHER, Woodward W.3, GROTZINGER, John P.4, WALTER, Malcolm R.5, SIMON, Melvin6, ABELSON, John6 and 2007 AGOURON ADVANCED GEOBIOLOGY SUMMER COURSE, The Members of The7, (1)Earth and Planetary Sciences, Harvard University, Botanical Museum 51, 26 Oxford St, Cambridge, MA 02138, (2)Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, (3)Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, (4)Earth and Planetary Sciences, Caltech, 1200 E. California Ave, Pasadena, CA 91125, (5)Earth and Planetary Sciences, Australian Centre for Astrobiology, Macquarie Univ, New South Wales, 2109, Australia, (6)Division of Biology, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, (7)Ed Stolper – Caltech, Roger Summons – MIT, Abby Allwood – JPL, Miriam Andres – Chevron, Crystal Gammon – Caltech, Sky Rashby – Caltech, Maia Schweizer – Caltech, Wes Watters – MIT, jpwilson@fas.harvard.edu

The Duck Creek Formation, Western Australia, preserves a record of carbonate platform deposition more than 1800 million years ago. A 950 m measured section records two upward deepening stratigraphic sequences, providing iterative geochemical and paleobiological records that permit us to distinguish the influences of time and environment. Each sequence begins with peritidal carbonates rich in laminated precipitation structures. These deepen to wave-dominated grainstones and intercalated stromatolitic bioherms. The transition to basinal facies is marked by hematite-rich ironstone and early diagenetic black chert nodules. Basinal facies consist of stylolitic calcimicrites and dolomitic turbidites deposited below storm wave base. X-ray diffraction shows consistent iron (ankerite) enrichment in carbonates, save for basinal limestones in the upper part of the formation. Carbonate C-isotopes do not record a statistically significant depth gradient, however ironstone siderites preserve strongly negative (to -8‰) values. These isotopic depletions are consistent with diagenetic carbonate precipitation within sediments characterized by Fe-respiration of organic matter. Because early diagenetic cherts occur in basinal facies, Duck Creek fossils record microbial communities ecologically distinct from those preserved in peritidal Proterozoic cherts. In concert, sequence stratigraphy, geochemistry, petrology, and paleontology illuminate marine environments just before the regional transition from ferruginous to sulfidic waters below the mixed layer.