The control of paleo-geomorphology on the depositional architecture of a Late Jurassic axial submarine fan, North Sea, UK

The depositional architecture of submarine fans in rift basins is significantly controlled by complex geomorphology created by widespread normal faults, presenting a key challenge in deep-water sedimentology. Although extensive previous studies have established depositional architectures and sand body distribution patterns of transverse submarine fans controlled by graben boundary faults and interbasin transfer zones, research on how axial submarine fan architecture responds to intra-graben slope gradients and evolving transverse confinement remains inadequate. This study takes the Upper Jurassic B4 oil group in the North Sea X Oilfield as an example. By using 3D seismic data, cores and logging data to restore paleo-geomorphology and dissect depositional architecture, and further reveal the controls of intra-graben paleo-geomorphic variations on axial submarine fan depositional architecture. Our analysis shows that the paleogeomorphology in the study area features axial stepped slope breaks and phased evolution in transverse confinement. As the axial slope transitions from extremely steep slope segments to steep slope segments, slope transition zones and gentle slope segments, turbidity currents evolve from supercritical to subcritical states through hydraulic jumps, while generating divergent flows. This progression drives architectural transformation from sediment bypass, incised channel-overbank systems, distributary channels and channelized lobes to lobes. Concurrently, phased transverse confinement evolution controls vertical stacking characteristics of individual channel-lobes: early asymmetric stages produce lateral migration stacking; middle symmetric stages develop unordered compensational stacking; and late locally confined stages form deflected retrogradational stacking. We propose a dynamic submarine fan depositional architecture response model, which emphasizes how evolving paleo-geomorphology directly controls spatiotemporal configurations of architectural elements by altering gravity flow pathways and energy distribution, and is further modified by feedbacks where the deposits themselves become influencing topographic elements. It provides a new perspective for deep-water depositional models and reservoir prediction in areas with similar geomorphic settings.