Repeated degradation and progradation of a submarine slope over geological timescales

Abstract

ABSTRACT Submarine slopes prograde via accretion of sediment to clinoform foresets and degrade in response to channel or canyon incision or to mass-wasting processes. The timescales over which progradation and degradation occur, and the large-scale stratigraphic record of these processes, remain unclear due to poor 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 origin of ancient slope canyons developed in late Mesozoic strata of the Måløy Slope, offshore Norway. Slope degradation and canyon incision commenced during the late Kimmeridgian, coincident with the latter stages of rifting. Later periods of canyon formation occurred during the Aptian to Albian and the Albian to Cenomanian, during early post-rift subsidence. The canyons are straight, up to 700 m deep, and 10 km wide on the upper slope and die out downdip onto the lower slope. The canyons trend broadly perpendicular to and crosscut most of the rift-related normal faults, although syn-filling fault growth locally helped to preserve thicker canyon-fill successions. The headwalls of the oldest (late Kimmeridgian) canyons are located at a fault-controlled shelf edge, where younger canyons overstep this fault, which was inactive when they formed, extending across the paleo-shelf. Downslope, Aptian to Albian canyons either erode into the older, late Kimmeridgian to Barremian canyon fills, forming a complicated set of unconformities, or in the case of the Albian to Cenomanian canyons, die out into correlative conformities. Boreholes indicate that the canyon bases are defined by sharp, erosional surfaces, across which we observe an abrupt upward shift from shallow- 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 it likely reflects basinward tilting of this tectonically active margin, enhanced by incision of the slope by erosive sediment gravity flows. The results of our study have implications for the timescales over which large-scale slope progradation and degradation may occur on other tectonically active slopes, and the complex geophysical and geological record of these processes. We also show that canyon formation can cause large volumes of margin-derived sediment to bypass proximal sub-basins within rifted terranes, an important process not currently captured by marine rift-basin tectono-stratigraphic models.