Hyperpycnal-Flow Evolution from the Shelf Edge to the Lower Slope as Evidenced by Slope-Channel Deposits in the Tertiary Central Basin of Spitsbergen

Petter, Andrew L.¹, Piret Plink-Bjorklund², Ron Steel³ (1) Jackson School of Geosciences, The University of Texas-Austin, Austin, TX (2) Colorado School of Mines, Golden, (3) The University of Texas at Austin, Austin, TX

Early Eocene slope channels in the Central Basin, Spitsbergen, display distinctive spatial facies variations that reflect changes in flow velocity, sediment concentration, and degree of confinement. The observed facies are interpreted to the product of quasi-steady turbidity currents, debris flows, and transitional flows generated from hyperpycnal outflow. Hyperconcentrated density- and debris-flow deposits overlie the deepest level of basal erosion and consist of medium-grained, mostly structureless sandstones with occasional thick mud-clast lags. Large, shale-filled scours are sometimes present. Turbidites overlie shallower, broader scours, and are dominated by plane-parallel laminated and current-rippled beds of fine-grained sandstone. Internal scours are sand filled and shallow.

Gullies at the shelf edge shoaled and broadened downslope into depositional channels because scour was greatest where momentum from fluvial outflow was accentuated by acceleration across the gradient break at the shelf-edge delta front. Mud-clast debrites and sandy debrites are typically found in the lower sections of shelf-edge gullies but transition to turbidites downslope in the depositional channels. The upper sections of slope gullies were less confined, and are consequently filled with turbidites. The transition between gullies and channels is characterized by sandy debrites at the base of scour in the channel axis and turbidites at slightly elevated channel margins. Large, shale-filled scours overlie the channel-axis debrites and separate channel-margin turbidites into distinct, bar-like sand bodies on both sides of the channel. Channel-margin ‘bars' coalesce downslope into continuous sand bodies that extend over the entire channel. Downslope facies changes therefore reflect a downslope decrease in confinement and sediment concentration.