Late Devonian Carbonate Margins and Foreslopes of the Lennard Shelf, Canning Basin, Western Australia, Part A: Development During Backstepping and the Aggradation-To-Progradation Transition

Abstract:  Carbonate reefal margin and foreslope settings are characteristically heterogeneous and difficult to predict due to a spectrum of sediment source factories, resedimentation processes, resultant deposit types, and controlling parameters. In particular, the effects of changes in long-term accommodation on the composition, architecture, and sediment distribution patterns of carbonate margin–foreslope–basin systems are poorly understood. Upper Devonian (Frasnian) outcrop exposures along the Lennard Shelf, northeast Canning Basin, Western Australia, were investigated to assess the development of reefal margin and foreslope settings during long-term 1) platform backstepping with aggradational pulses, and 2) across the transition from platform aggradation to progradation. Measured sections tied to interpreted photomosaics and detailed mapping using aerial photographs were collected from the South Lawford Range and Windjana Gorge areas. The exposures reveal distinctive differences in foreslope grain composition, deposit characteristics and proportions, margin morphology, and stratigraphic expression 1) during platform evolution between backstepping events, and 2) depending on position within the long-term accommodation setting. Between backstepping events, aggradational margins can be classified as “growth escarpments” with associated grain-dominated, onlapping foreslope deposits. Margins across the long-term transition from aggradation to progradation evolved from erosional escarpments with onlapping debris deposits to accretionary, interfingering configurations. Development of growth escarpments between backstepping events was a function of vertical reefal growth from sustained high accommodation conditions during the long-term transgressive systems tract, coupled with a Frasnian reefal assemblage that responded to light and tracked relative sea level. This net vertical reefal growth also resulted in relative margin stability and the deposition of grain-dominated foreslopes. Conversely, margins were highly unstable and underwent repeated failure across the long-term aggradation-to-progradation transition, reflecting a lack of underlying substrate to support basinward advance, and resulting in debris-dominated foreslopes. These observations provide relationships that predict margin and foreslope associations of facies-scale heterogeneity and seismic-scale geometry within a low-frequency sequence stratigraphic framework.