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Stable Carbon Isotopes in Carbonates of the Devonian Lennard Shelf, Canning Basin: Use as a Chronostratigraphic Constraint, Oceanic Indicator, and Limitations

Abstract

Carbon isotope signatures derived from the Upper Devonian reef complexes along the Lennard Shelf in the Canning Basin of Western Australia enable high resolution correlations and the development of chronostratigraphic frameworks in heterogeneous carbonate slope-to-basin systems. In developing the composite carbon curve for the Lennard Shelf, at least five major and three minor positive d13C excursions have been identified that permit robust correlations on a regional scale. The five major excursions are also interpreted to be of global significance as they are correlative to sections in Europe, North America, North Africa, and China. In the Canning Basin, we developed a correlation framework by integrating the carbon isotope chemostratigraphy with biostratigraphy, magnetostratigraphy, and sequence stratigraphy. The resulting framework improved our understanding of carbonate depositional heterogeneity when compared to precursor frameworks constrained by sequence stratigraphic concepts alone. For example, in carbonate slope settings where depositional variability has historically hindered our ability to recognize and correlate systems tracts, stable isotope chemostratigraphy proved to be a useful chronostratigraphic tool because perturbations in primary marine d13C values corresponded to changes in sea level and oceanic chemistry. As such, isotopic trends were used as a proxy for delineating systems tracks in slope settings, which in turn allowed improved correlations through these heterogeneous settings. The application of the d13C tool is limited in shallow platform-top environments due to the effects of meteoric and burial diagenesis, thus we relied more on sequence stratigraphic concepts and magnetostratigraphy for correlation in these settings. In addition to chronostratigraphic utility, the fluctuations in the carbon isotope signal during the Frasnian and Famennian record disequilibrium of the global oceanic carbon pool and its relationship to stressed oceans. Fluctuating ocean chemistry manifested as anoxic-dysoxic events, decreased oceanic turnover of nutrient rich bottom water masses, and numerous extinctions, all of which led up the ultimate biotic demise at the Frasnian-Famennian boundary. These chemical and biotic impacts/stressors ultimately provide predictive sequence stratigraphic capability through use of the carbon isotope tool.