Controls on shelf-margin architecture and sediment partitioning during a syn-rift to post-rift transition: Insights from the Barrow Group (Northern Carnarvon Basin, North West Shelf, Australia)

Abstract The Barrow Group was deposited in the Northern Carnarvon Basin from the latest Tithonian to the Late Valanginian. This moderately deep-water shelf-margin is composed of ~ 100–500 m high clinoforms that prograded during a syn-rift to post-rift transition. Integration of well data with extensive 2D and 3D seismic data was used to constrain the stratigraphic evolution of the Barrow Group in seven 3 rd order seismic sequences (calibrated to dinocyst zones) across four main depocentres. Five shelf-margin categories were recognized based on stratal stacking patterns, the trajectory styles and angles ( T se ), and the progradation/aggradation ratios ( P se / A se ) that were interpreted in terms of rates of accommodation creation and sediment supply ( A / S ratio). Following the uplift of the Southern Carnarvon Basin (sediment source), the stratigraphic evolution of the Barrow Group developed in three stages. During the first stage (late syn-rift I; 148–143.5 Ma), the shelf-margin prograded in a period of tectonic quiescence with relatively limited subsidence. During the second stage (late syn-rift II; 143.5–138.2 Ma), the shelf-margin was affected by increasing rates of accommodation and high sediment supply, which reflects an active period of rifting triggering both tectonic subsidence in the basin, and active uplift in the hinterland. During the third stage (early post-rift I; 138.2–135.4 Ma), the uplift of the continental shelf, following continental break-up, provided a new local source of sediment supply to the Barrow Group that then developed as a passive margin. The Lower Barrow Group (late syn-rift I and II) mainly developed under supply-dominated conditions. However, lateral variations in subsidence regime and shifts in sediment supply led to significant variations in shelf-margin architecture along-strike, directly impacting sediment partitioning between the shelf and the deep-water areas. Flat shelf-edge trajectories were associated with sediment bypass and increase in bottomset thicknesses, whereas rising shelf-edge trajectories were linked with sediment storage on the shelf. In contrast, the Upper Barrow Group (early post-rift I) developed in low-supply conditions with slow thermal subsidence, reflecting the passive context of the margin at this time. The Barrow Group provides a unique example of how rift tectonics can control the stratigraphic architecture of a regressive margin and reciprocally, how studying shelf-margin architecture can help constraining the dynamics and timing of rifting around the break-up stage.