The Upper Jurassic ‘Boulder Beds’ and related deposits: a fault-controlled submarine slope, NE Scotland

Abstract

During the late Jurassic (Kimmeridgian) rifting in the North Sea, large-scale normal faults developed to form relatively deep marine basins. The north-western margin of one of these basins, defined by the Helmsdale Fault, is described to show the nature of sedimentation associated with a submarine fault scarp. Subsidence to the SE of the Helmsdale Fault led to a fault scarp down which various sediment mass flows moved, including debris flow, rockfall, high and low concentration turbidity currents and possible storm surge wash-over deposits. Redeposition by soft-sediment sliding was important as a slope accretion process. Deep channels were incised into the slope and infilled by coarse-grained elastics: contemporaneous, small-scale submarine fans, located farther off in the present North Sea, may have developed as a result of slope-bypassing sediment flows down such channels. The succession is characterized by extreme sediment heterogeneity compared to many deep water environments and it may be part of an ancient fan delta. The main influences upon slope accretion were the synsedimentary movements on the Helmsdale Fault and the nature of the available sediment input, such that the succession represents structurally controlled and sedimentologically modified slope development.