Abstract: High-resolution, time-significant correlations are integral to meaningful stratigraphic frameworks in depositional systems but may be difficult to achieve using traditional sequence stratigraphic or biostratigraphic approaches alone, particularly in geologically complex settings. In steep, reefal carbonate margin-to-slope systems, such correlations are essential to unravel shelf-to-basin transitions, characterize strike variability, and develop predictive sequence stratigraphic models—concepts that are currently poorly understood in these heterogeneous settings. The Canning Basin Chronostratigraphy Project integrates multiple independent data sets (including biostratigraphy, magnetostratigraphy, stable isotope chemostratigraphy, and sequence stratigraphy) extracted from Upper Devonian (Frasnian and Famennian) reefal platform exposures along the Lennard Shelf, Canning Basin, Western Australia. These were used to generate a well-constrained stratigraphic framework and shelf-to-basin composite reconstruction of the carbonate system. The resultant integrated framework allows for unprecedented analysis of carbonate margin-to-slope heterogeneity, depositional architecture, and sequence stratigraphy along the Lennard Shelf. Systems tract architecture, facies partitioning, and stacking patterns of margin to lower-slope environments were assessed for six composite-scale sequences that form part of a transgressive-to-regressive supersequence and span the Frasnian–Famennian (F–F) biotic crisis. Variations are apparent in margin styles, foreslope facies proportions, dominant resedimentation processes, downslope contributing sediment factories, and vertical rock successions, related to hierarchical accommodation signals and ecological changes associated with the F–F boundary. We present these results in the form of carbonate margin-to-basin sequence stratigraphic models and associations that link seismic-scale architecture to fine-scale facies heterogeneity. These models provide a predictive foundation for characterization of steep-sided flanks of reefal carbonate platform systems that is useful for both industry and academia. This study emphasizes the utility of an integrated stratigraphic approach and the insights gained from better-constrained facies and stratal architecture analysis, insights that were not achievable with traditional sequence stratigraphic or biostratigraphic techniques alone.

ABSTRACT Making reliable correlations and sequence stratigraphic interpretations can be challenging in depositionally complex settings due to depositional heterogeneity and data-set limitations. To address these issues, the Canning Basin Chronostratigraphy Project documented the development of a high-resolution, chronostratigraphic correlation framework across different depositional environments in the Upper Devonian (Frasnian–Famennian) of the Lennard Shelf, Canning Basin, by integrating stable isotope chemostratigraphy, biostratigraphy, magnetostratigraphy, and sequence stratigraphy. This integrated data set allows for a rare, detailed look at the carbon isotope record, and specifically its potential as a sequence stratigraphic interpretation tool and its application to improve correlation capabilities, both of which have implications for better understanding of the depositional history of the Lennard Shelf. For platform-top settings, a sequence stratigraphic framework was constructed using stacking pattern analysis constrained by the paleomagnetic reversal record. In slope settings, where depositional variability and a lack of platform-top control have historically hindered our ability to recognize and correlate systems tracts, carbon isotope chemostratigraphy (in conjunction with conodont biostratigraphy and magnetostratigraphy) proved to be a useful chronostratigraphic tool because primary marine δ 13 C values were well preserved. Using the paleomagnetic reversal record, with additional age control from walkout correlations to key outcrop sections, we were able to confidently correlate from the platform-top into the slope. Evaluation of the slope isotope record, within the projected sequence stratigraphic framework from the platform-top, revealed that variations in δ 13 C values corresponded to changes in sea level. Using this relationship, isotopic trends were used as a proxy for delineating systems tracts in slope sections without direct platform-top control. In turn, this improved correlations through heterogeneous slope facies and also allowed for a refined sequence stratigraphic interpretation of Famennian strata in the Canning Basin. Results from this work also allowed us to develop a model that attempts to explain the observed relationships among global carbon cycling, sea-level fluctuations, and paleoceanographic conditions during the Late Devonian.

Abstract: High-resolution chronostratigraphic correlation using elemental chemostratigraphy in platform carbonates is typically difficult to achieve. Here, elemental chemostratigraphy is used to correlate between two platform-top, carbonate-dominated field sections from the narrow Lennard Shelf that existed on the NE margin of the Canning Basin, Western Australia, during the mid-late Frasnian. The correlation, constrained by magnetic polarity reversals and physical ground truthing, is based on recognition of distinctive cyclical “stacking patterns” defined by changes in concentrations of the trace element zirconium (Zr). Zr concentrations are controlled by the amount of the heavy mineral zircon in the sediments, which is derived from a terrigenous source and is diagenetically very stable. The stacking patterns in the lower part of the study sections display gradually upward-increasing values of Zr to a maximum, followed by an almost immediate fall to a minimum. In the upper part of the study interval, the cycles are more symmetrical, with both gradually increasing and decreasing portions. The point at which the change in Zr stacking pattern occurs in the two sections is synchronous and occurs in association with a supersequence maximum flooding surface. The correlation based on maximum and minimum Zr values throughout the two sections is demonstrated to be chronostratigraphic by comparison with correlations based upon paleomagnetism and physical ground truthing. When element ratios commonly used as provenance and paleoclimate proxies are plotted, the variations between closely spaced samples are greater than any systematic variations throughout the study intervals. Therefore, no isochemical chemozones can be defined, implying that during deposition of the study intervals, there were no long-lived changes in sediment provenance or paleoclimate that the elemental chemistry can detect. The work presented here shows that the standard approach of defining isochemical chemozones for chemostratigraphic correlation is not always appropriate. However, an approach using cyclical changes in elemental variables for chemostratigraphic correlation between two closely spaced sections is chronostratigraphically valid. The greater challenge is in application of the same approach to more widely spaced sections, potentially in different facies of a carbonate setting.

Analysis of the distribution of conodonts in sample sets measured down discrete beds exposed in the depositionally steeply dipping marginal-slope facies of the Upper Devonian reef complex of the Canning Basin has been undertaken in order to test the Seddon and the Druce interpretations of conodont biofacies. Each of the 14 sample sets was taken at measured intervals down single beds. The maximum difference in elevation between top and bottom samples was 65 m, indicating the water depth of the bottom sample to have been in excess of 65 m. Analysis of conodont distribution in the 53 sample points of the 14 sample sets does not indicate any clear depth segregation of conodont species that would correspond to the biofacies recognized by Seddon and Druce.