Repeated degradation and progradation of a submarine slope over geological timescales (103-104 Myr)
Submarine slopes prograde via accretion of sediment to clinoform foresets, and degrade in response to channel or canyon incision, or mass-wasting processes. The timescales over which slope progradation and degradation occur, and the large-scale stratigraphic record of these processes, remain unclear due to a lack of age constraints in subsurface-based studies, and areally limited exposures of exhumed systems. We here integrate 3D seismic reflection and borehole data to study the geometry and
... the geometry and origin of ancient slope canyons developed within Late Mesozoic strata of the Måløy Slope, offshore western Norway. Slope degradation and canyon incision commenced during the late Kimmeridgian, coincident with the latter stages of rifting. Subsequent periods of incision and canyon formation (Aptian-to-Albian and Albian-to-Cenomanian) occurring during post-rift subsidence. The canyons are straight, up to 700 m deep and 9 km wide on the upper slope, and die-out downdip onto the lower slope. The canyons trend broadly perpendicular to and crosscut the majority of the rift-related, Late Jurassic normal faults, although syn-incision fault growth locally controlled the depth of basal erosion. The headwalls of the oldest, late Kimmeridgian canyons are located at the fault-controlled shelf edge, whereas the younger Aptian-to-Albian and Albian-to-Cenomanian canyons overstep this fault, which had become inactive by this time, and extend further landward into the immediate hangingwall of a larger, basin-bounding fault system. Further downslope, the younger canyons are superimposed on and truncate older, underlying canyon-fills. Boreholes indicate that the canyons bases are defined by sharp, erosional surfaces, across which we observe an abrupt upward shift from shallow marine to deep marine facies (i.e. late Kimmeridgian canyons), or deep marine to deep marine facies (Aptian-to-Albian and Albian-to-Cenomanian canyons). Missing biostratigraphic zones indicate the canyons record relatively protracted periods (c. 2-17 Myr) of structurally enhanced slope degradation and sediment bypass, separated by >10 Myr periods of deposition and slope accretion. The trigger for slope degradation is unclear, but likely record basinward tilting of this tectonically active rifted margin, enhanced by incision of the slope by erosive sediment gravity-flows. The results of our study have implications for the timescales over which slope aggradation and degradation occur, and the complex geophysical and geological (i.e. stratigraphic) expression of related features in the rock record. We also demonstrate that canyon formation caused a major reorganisation of rift-related drainage patterns and sediment dispersal, resulting in an abrupt change in syn-rift facies distributions not predicted by existing marine rift-basin tectono-stratigraphic models.