STRATIGRAPHIC FRAMEWORK OF THE PILLARA FORMATION AT GUPPY HILLS PROVIDES NEW INSIGHTS INTO THE MIDDLE DEVONIAN (GIVETIAN) REEF COMPLEXES OF THE CANNING BASIN, WESTERN AUSTRALIA

Lithostratigraphic sections combined with coral biostratigraphy and carbon and oxygen isotope chemostratigraphy (δ¹³C and δ¹⁸O) provide new information about the Middle Devonian (Givetian) reef complexes in the Canning Basin of Northwestern Australia. In contrast to the well studied Frasnian and Famennian reef outcrops, older Givetian reefs, which lack the significant microbial influence characteristic of the FF sections, remain poorly understood. Investigation of these older reefs may have significant implications for better understanding the Late Devonian mass extinction since Givetian aged strata contain the only organic rich reef facies, possibly indicative of anoxic or low oxygen conditions, currently known from any outcrop in the Canning Basin. Within the Pillara limestone at Guppy Hills, Givetian reefs are characterized as non-binding, cavity poor systems dominated by solitary and colonial rugose corals, tabulate corals, and stromatoporoids. The Guppy Hills section provides continuous Givetian to early Frasnian strata that is transitional from platform to reef slope facies in a retrogradational stacking pattern and is interpreted to have been deposited during a transgressive interval. The measured section is well preserved, with only minor localized areas of dolomitization, and is mainly comprised of fossiliferous limestone with scattered mixed carbonate-siliciclasitc intervals and abundant recessive partings. The upper limit of the Givetian aged strata is currently and tentatively marked by the localized disappearance of coral fauna and a subsequent back stepping margin, however ongoing chemostratigraphic and biostratigraphic studies will allow independent determination of the Givetian-Frasnian boundary. Preliminary carbonate isotope analyses using bulk rock samples show a large negative excursion near the base of the section as well as two other possible minor negative excursions. Micro-sampling of early marine cements and micrites will be forthcoming and further results and conclusions will be presented at the conference.