--> Coupling of Basin-Floor Fan Behavior With Shelf-Margin Processes: Maastrichtian Washakie Basin, Wyoming

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Coupling of Basin-Floor Fan Behavior With Shelf-Margin Processes: Maastrichtian Washakie Basin, Wyoming

Abstract

The linkage between relative sea-level change, architecture, and spatio-temporal evolution of Maastrichtian basin-floor fans in Washakie Basin, Wyoming has been investigated at the scale of beds, lobes, lobe complexes, and submarine fans using 3 cores and 630 wireline logs. The bottomset deposits of basin-scale clinothems 9 and 10 form lobate shapes on toe-of-slope and basin floor. The fan lobe complex is about 28 km long, 15 km wide, and 21 m thick. The lobe complex is weakly developed during the early period indicating minor sediment volumes delivered to deep-water. During the main depositional phase, the lobe complexes of Clinothem 9 aggraded without strong lateral migration via fixed slope-channels in concert with a highly aggradational shelf-edge during relative sea-level rise. In contrast, the deep-water lobe complexes of Clinothem 10 prograded continuously coeval with shelf-edge progradation and slight sea-level fall. The depocenters of lobe complexes were laterally switched by compensational stacking and slope-channel avulsions. During the late development of both clinothems, the deep-water lobe complexes became smaller or retreated concurrent with shelf flooding. At lobe scale, amalgamated channels, channel-levees, and muddy deposits are recognized in the axis, fringe, and distal fringe of lobes respectively. The channels are identified from the gamma-ray motifs, channel-levees from the repetition of thin and thick beds, and the muddy distal fringe of lobes from the high gamma ray. The core study shows that channelized lobe axes have amalgamated structureless or weakly flat-laminated sandstone beds with erosional surfaces and mud clasts, whereas levees are composed of thin ripple cross-laminated sandstone beds, interlayered with silt and mud. Mud with silt laminae and hybrid beds with soft deformation are deposited in distal fringe of lobes. Washakie Basin deep-water fan-lobe complexes 1) are composed of amalgamated channel-levee systems connected to slope-channels; 2) evolve through stages of initiation, aggradation, progradation, and retreat. The lobe complex stages and switching of deep-water depocenters are linked to coeval shelf-edge trajectory changes between successive maximum flooding events on the shelf. Linkage of lobe-complex stacking pattern with shelf-edge behavior was possible because Washakie Basin had constantly high sediment discharge to deep-water fans despite significant sediment reworking of shelf-edge deltas by waves and tides.