HR: 08:00h
AN: V31G-01 INVITED    [Abstracts]
TI: Re-Os Geochronology of Organic-Rich Shales; Placing Absolute Time Pins in Ancient Sedimentary Basins
AU: * Creaser, R A
EM: robert.creaser@ualberta.ca
AF: University of Alberta, Dept Earth & Atmospheric Sciences, 126 ESB, Edmonton, AB T6G 2H1, Canada
AU: Kendall, B
EM: bkendall@ualberta.ca
AF: University of Alberta, Dept Earth & Atmospheric Sciences, 126 ESB, Edmonton, AB T6G 2H1, Canada
AB: Methodological advancements in the past 5 years have allowed the ¹⁸⁷Re-¹⁸⁷Os isotope system to be used to reliably determine depositional ages for organic-rich shales. Clastic sedimentary rocks, in general, do not yield depositional ages using other radioactive isotope methods. In some cases, Re-Os ages with ± 1% uncertainty (2σ, derived from isochron regression analysis) can be determined. This method of directly dating shales is cross-calibrated to ages derived from volcanic rocks using the U-Pb zircon geochronometer to within 0.5%, and is widely applicable within Phanerozoic and Proterozoic sedimentary basins. Within Proterozoic sedimentary basins, the lack of available biostratigraphic control for correlation and determination of relative geologic age highlights the importance of the Re-Os method as an absolute timekeeper. Using the Re-Os method, we have been able to place precise, absolute age constraints within several thick Proterozoic sedimentary basins to better constrain the timing of important global events and processes. The last 200 million years of Proterozoic time was marked by a series of critical events in Earth history, including multiple large scale glaciations that were followed by the appearance of Ediacaran macroscopic metazoans . Though significant advances have been made in our ability to correlate sedimentary rocks of this time period using chemostratigraphy, the global database of high-precision geochronologic data is still limited. Re-Os dating of four separate black shale horizons from three Australian Proterozoic basins shows that the oldest known episode of Neoproterozoic glaciation (the Sturtian) terminated shortly prior to 645-660 Ma. These results show that the type Sturtian glaciation in Australia is significantly younger than putative global counterparts to which it has been correlated, and demonstrates diachronous deposition of these glacial units, which could not result from a global glacial event. In middle Proterozoic time, Mo isotopes in black shales record the nature and extent of Proterozoic deep ocean anoxia. The Velkerri Formation of northern Australia contains three mature organic-rich shales that are the source of prokaryotic cyanobacterial-rich and eukaryotic-poor biomarkers, bitumens and fluid inclusion – hosted oils in nearby rocks, but the age of the Velkerri Formation remains poorly constrained. Our Re-Os dates of 1361 ± 21 Ma and 1417 ± 29 Ma directly constrain the depositional age of the uppermost and lowermost (containing live oil) organic-rich intervals of the Velkerri Formation, respectively. Functionality of the Re- Os shale geochronometer requires some oxygenation of seawater coupled with suboxic/anoxic bottom water conditions for reductive capture of the elements. Currently, the oldest demonstrably accurate Re-Os shale age we have determined is 2500 Ma, suggesting that some oxygenation of Earth's oceans was underway by this time.
DE: 1110 Sidereal geochronology
DE: 1115 Radioisotope geochronology
DE: 1165 Sedimentary geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting