Repeated degradation and progradation of a submarine slope over geological timescales

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.

Syn‐rift sediment gravity flow deposition on a Late Jurassic fault‐terraced slope, northern North Sea

“…We have interpreted these valley‐like erosion features as submarine canyons based on the geomorphology, their relationship to other seismic facies and the regional context. The seismic expression of SF4 correlates well with submarine canyon described on the Måløy Slope, northward of the Lomre and Uer Terraces (Jackson et al., 2008, 2019; Sømme et al., 2013). The dendritic plan‐view expression to the E is interpreted as relating to the canyon heads in SF4, hence the terminology of ‘proximal’ submarine canyon.…”

“…The northern Horda Platform shows first signs of rotation and drainage reversal on the large 6‐ to 15‐km‐wide, E‐dipping fault blocks during SU4 time, with dip‐slope fans overlying the late Oxfordian prograding sequences (Dreyer et al., 2005; Whipp et al., 2014). Canyons 1, 2 and 3 are part of a regional unconformity complex with canyon features extending for more than 150 km along the basin margin to the N, which has been inferred to be caused by basinward tilting of the rift margin due to increased subsidence in the rift axis areas (Jackson et al., 2008, 2019; Koch et al., 2017; Sømme & Jackson, 2013; Sømme et al., 2013), which is likely amplified by high seasonal runoff rates during the humid Kimmeridgian climate (Armstrong et al., 2016). The following subsections will summarize generic observations from the Late Jurassic submarine gravity flow deposits on the Lomre and Uer terraces, with focus on sediment routing and depositional architecture throughout the rift phase.…”

Syn‐ to post‐rift alluvial basin fill: Seismic stratigraphic analysis of Permian‐Triassic deposition in the Horda Platform, Norway

“…The upper sequence boundary (Base Statfjord Gp) marks a turnaround from fining-upwards into coarseningupwards in well-logs (Nystuen et al, 2014) (Figure 5), although it is not particularly pronounced in seismic, and therefore constrained by careful seismic-well-tie. A truncation of the surface in the Smeaheia Fault Block (Figure 8) may be related to a significant unconformity in the greater North Sea, resulting from Middle Jurassic to Early Cretaceous erosion (Goldsmith et al, 2003;Jackson et al, 2008Jackson et al, , 2021Sømme & Jackson, 2013). The Triassic succession is conformably overlain by Lower Jurassic marine mudstone and shale.…”

Fill‐and‐Spill, Tilt‐and‐Repeat (FaSTaR) cycles: Stratigraphic evolution above a dynamic submarine stepped slope

“…The thickening direction of multiple fill-and-spill cycles to the NE, together with the persistent migration of the submarine channels and lobes in the same direction, orientated transverse to the stepped slope profile, supports syn-sedimentary lateral tilting. This contrasts with basinward tilting where submarine conduits tend to have fixed positions (e.g., Jackson et al, 2021). Without invoking a lateral tilt, the documented thickening direction and stacking pattern would need to be explained by the turbidite system initiating on the higher part of the steps (SW) and working their way progressively to lower elevations to the NE.…”

“…In cases where fill-and-spill cycles are vertically stacked, the stratigraphic cyclicity is attributed to the interaction of sediment input and vertical accommodation rejuvenation created by dynamic seafloor deformation induced by fixed structural elements (e.g., Booth et al, 2003;Brooks et al, 2018;Hay, 2012;Spychala et al, 2015). However, accommodation patterns can be spatially variable during the lifespan of a turbidite system, for instance due to basinward tilting (Jackson et al, 2021), lateral tilting (Kane et al, 2010(Kane et al, , 2012 and the emplacement of masstransport complexes (Wu et al, 2020). These factors drive spatially variable topographic configurations and hinder vertical stacking across sediment supply cycles, suggesting that alternative stratigraphic models to the traditional fill-and-spill model need to be developed.…”