Poster Number 85
Sunday, 5 October 2008
George R. Brown Convention Center, Exhibit Hall E
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.