Late Devonian Carbonate Margins and Foreslopes of the Lennard Shelf, Canning Basin, Western Australia, Part B: Development During Progradation and Across the Frasnian–Famennian Biotic Crisis

Abstract:  Many studies document the occurrence of carbonate progradational successions that are driven by overall accommodation-limited conditions and represent periods when substantial volumes of sediment are stored in slope settings. However, few have reported on the detailed internal anatomy and components of prograding clinothems, and furthermore, little is known about how environmental parameters affect the development of carbonate margins and slopes when superimposed on the long-term, low-accommodation signal. Upper Devonian (Frasnian and Famennian) carbonate outcrop exposures along the Lennard Shelf, northeast Canning Basin, Western Australia, offer the examination of reefal margin and foreslope development in a variety of settings, one of which is long-term (> 10 Ma) progradation during the late Frasnian and Famennian. In the early phases of progradation, a global biotic crisis and faunal overturn occurred around the Frasnian–Famennian (F-F) boundary, and indications of global climatic cooling are interpreted at points throughout the Late Devonian. These extrinsic and intrinsic controls provide ecologic context and a stratigraphic framework to assess carbonate margin and foreslope architectures and facies distributions in order to better characterize and predict their complex heterogeneity. The outcrop dataset collected includes measured sections tied to interpreted photomosaics and detailed mapping using aerial photographs from the South Lawford Range, Windjana Gorge, and Dingo Gap areas. The outcrop observations highlight distinctive changes in margin style, foreslope composition, stratigraphic packaging, and strike variability within the contexts of the F-F biotic crisis, long-term highstand (HST) progradation, and Late Devonian climatic changes. Intervals before and after the F-F boundary exhibit notable increases in downslope calcimicrobial boundstone encrustation and evidence for waning foreslope contributions from other, previously flourishing, carbonate factories, reflecting stressed ecological conditions and consequent calcimicrobial opportunism during gradual buildup and recovery periods that bracket the extinction. The Famennian slope (post-effects related to the F-F crisis) can be subdivided into a calcimicrobial boundstone-dominated upper slope with a highly complex downdip transition into a mixed debris- and grain-dominated middle slope, and a silt-dominated lower slope to toe of slope, and exhibits a highly complex lateral heterogeneity associated with multiple contributing sediment factories and temporal partitioning of resedimentation. A repeated stacking pattern is observable in the Famennian middle-slope setting, consisting of upward successions of debris followed by a gradation from silt-dominated deposits into grain-dominated deposits; we here interpret these as the expression of high-frequency sequences superimposed upon the longer-term progradation, and a function of transitional (greenhouse to icehouse) climates. These findings offer predictive relationships that link the internal depositional heterogeneity of prograding slope systems to the interplay of long-term accommodation drivers, ecological conditions affecting carbonate sediment factories, and climatic conditions that control the development of high-frequency sequences.