Slump and Debris-Flow Dominated Upper Slope Facies in the Cretaceous of the Norwegian and Northern North Seas (61-67øN): Implications for Sand Distribution

Abstract

A regional sedimentological study of Cretaceous sequences in the Mid-Norway region (Norwegian Sea) and in the Agat region (Agat field area, northern North Sea) reveals that these sequences were predominantly deposited in an upper continental slope environment by slumps and debris flows. Examination of nearly 500 m of core from 14 wells shows eight distinct lithofacies: facies 1 (contorted conglomerate and pebbly sandstone) represents deposits of sandy slumps and debris flows, possibly in a channel setting; facies 2 (contorted sandstone) is the most widespread and is the product of sandy slumps and debris flows; facies 3 (contorted mudstone) indicates deposition from muddy slumps and debris flows; facies 4 (rippled sandstone) suggests bottom-current reworking; facies 5 (gr ded sandstone) represents turbidity-current deposits and is very rare; facies 6 (laminated mudstone) is a product of pelagic or hemipelagic deposition; facies 7 (cross-bedded sandstone) is indicative of tidal processes; and facies 8 (laminated sandstone) represents delta-front and shelf deposits. These facies and their association suggest a shelf-edge delta to upper slope environment of deposition. Existing core data document deltaic facies only in the Mid-Norway region. The proposed shelf-edge delta and upper slope model has important implications for sand distribution. (1) This model provides an alternative to the conventional submarine-fan model previously applied to these sequences. (2) Although slump and debris-flow emplaced sands are usually discontinuous and unpredictable, highly amalgamated slump and debris-flow sands may develop thick reservoirs. (3) By using the Eocene Frigg Formation as an analog, it is predicted that externally mounded seismic facies in the study area may be composed of sandy slumps and debris flows.