Interaction of Gravity Flows With a Rugged Mid-Ocean-Ridge Seafloor: An Outcrop Example from Macquarie Island

Abstract Rugged bathymetry along slow-spreading mid-ocean ridges has the potential to impinge a strong control on gravity flows derived from oceanic fault scarps. Sedimentary lithofacies from the Macquarie Island ophiolite supports this hypothesis by displaying systematic variations that correspond with volcanic substrate differences and proximity to rift-related faults. Pillow-basalt terranes are associated with tightly confined bedrock corridors that funnel gravity flows into one direction. Vertical lithofacies variations formed from high-density to low-density turbidity currents record successive fill stages of the corridor axis. During initial stages tight confinement in the axis suppressed flow dilution and fluid turbulence. With continued corridor-axis filling, more dilute gravity flows predominated and formed lateral gradations from axial coarse-grained turbidites into thinly interbedded overbank lithofacies along corridor margins. These gravity-flow lithofacies converge into very thin muddy condensed intervals along inter-corridor highs where significant bottom-current reworking occurred. Conversely, partly confined tabular-basalt-floored basins promoted lateral expansion and dilution of gravity flows throughout the duration of basin filling. Variable paleocurrent-indicator directions record multiple reflections of single gravity-flow events off basin-bounding fault barriers. Coarsening-upward trends in these partly confined basins from thinly interbedded pelagic chert and ripple-bedded sandstones up into stacked turbidites preserves the ponding of sediment-starved submarine fans. In general, these ponded basins preserve the fine-grained distal ends of a gravity-flow continuum from coarse-grained fault-proximal en masse failures and cohesionless debris flows into medial high-density turbidity flows and distal dilute turbidity flows.