Stable Isotope Variations in Upper Devonian Slope Settings of the Lennard Shelf, Canning Basin, Western Australia: Implications for Global Environmental Changes and Impacts on Carbonate Development

The outcrop-based Canning Basin Chronostratigraphy Project (CBCP) aims to develop a high-resolution chronostratigraphic framework for the Lennard Shelf, Canning Basin in Western Australia, where extensive, well-preserved exposures of Middle and Upper Devonian (Givetian-Famennian) reefal platforms illustrate the natural complexity inherent to carbonate systems. Carbon isotope geochemistry may help unravel the temporal and spatial complexity of slope deposits otherwise challenged by traditional chronostratigraphic correlation methods such as sequence stratigraphy. This study seeks to validate the use of secular variations in the carbon isotope stratigraphy as a chronostratigraphic tool for regional and global correlations. The δ13C values are derived from powders of bulk carbonate rock, as well as from individually drilled carbonate muddy-micritic matrix, allochems, grains, and cements. Results from carbon isotope analysis of the drilled components and facies from time-equivalent yet physically-disconnected sections show that secular variations in the carbon isotope stratigraphy are independent of the depositional facies. Therefore, the local depositional environment has a negligible effect on the C-isotope values, which allows carbon isotope stratigraphy to be used as a proxy for global changes to the marine carbon pool and as a chronostratigraphic correlation tool.

Chronostratigraphic correlations observed between measured sections also document strong geographic variability of depositional rock types along strike, which has significant implications for understanding the variability of reservoir properties in subsurface analogs. Previous studies on coeval sections elsewhere in the world have used prominent marker beds, such as the organic-rich back shales of the Upper and Lower Kellwasser events, to make chronostratigraphic correlations. In the Canning Basin, the black shale horizons are absent, but C-isotope values from the carbonate alone identify the Kellwasser Events in the upper rhenana and linguiformis conodont zones. The correlations of the carbon isotope values from the Canning Basin with globally significant events are proving to be regionally and globally robust chronostratigraphic tools that provide insights for understanding Upper Devonian oceanic environments and chemistry, steep carbonate slope variability, and subsurface correlation workflows utilizing core and cuttings.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California