Three-dimensional stratal development of a carbonate-siliciclastic sedimentary regime, Northern Carnarvon Basin, Northwest Australia

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Cathro, Donna Louise

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Detailed stratigraphic interpretation of continental margin clinoforms is a necessary first step in understanding the link between this complex stratal architecture and large-scale processes resulting in progradation. Maps derived from a 3D seismic volume (40-55 Hz) nested within a regional 2D multichannel seismic grid (25-35 Hz), and tied to nine hydrocarbon exploration wells, show the detailed morphological evolution of five prograding clinoformal sequences in the Northern Carnarvon Basin (NCB), northwest Australia. Depocenters concentrated along northeast-southwest oriented, linear clinoform fronts are governed by latitudinal variations in sediment productivity. Fronts change from smooth to highly dissected, with intense gullying apparent after the mid Miocene optimum. Bottomsets remain relatively sediment-starved without the development of aprons on the lower slope and basin. Small-scale variability suggests heterogeneous sediment dispersal through these slope conduits. Along-strike sediment transport superimposed on northwest-oriented progradation changes from south-directed in the late Oligocene to north-directed in the late mid-Miocene suggesting a reorganization of circulation in the southeastern Indian Ocean. Prominent seismic discontinuity surfaces represent intervals of shallow paleo-water depth and flooding of the shelf. Exposure surfaces are subordinate. Rather than build to sea-level, progradation occurs with shelf paleo-water depths of 20-200 m. Therefore, onlap onto the clinoform front is not coastal and the sensitivity of the clinoforms to sea-level changes is muted. However, in the midMiocene, partial exposure of the shelf and development of karst topography indicate paleo-water depth falls of 60-180 m across two sub-sequence boundaries. The sequence stratigraphic framework is combined with a twodimensional, forward kinematic and flexural model for deformation of the lithosphere to determine the distribution, magnitude and history of CretaceousTertiary compression-induced inversion across the Dampier Sub-basin in the NCB. Inversion simultaneously creates and destroys accommodation space at different wavelengths superimposed on long wavelength subsidence and eustatic variations that impact the entire margin. Inversion anticlines focused along earlier rift fault systems are small, temporally and spatially variable relative to the total accommodation space created in the sub-basins during rifting and thermal subsidence. Santonian inversion represents ~4 km shortening, whereas the four modeled events in the Miocene each represent ~200-400 m of shortening across the modeled section.