A giant (5.3x10(7)m(3)) middle Miocene (c. 15Ma) sediment mound (M1) above the Siri Canyon, Norwegian-Danish Basin: Origin and significance

A large-scale enigmatic mound structure (M1) has been discovered in middle Miocene strata of the Norwegian-Danish Basin, c. 10km east and updip of the Central Graben. It is located about 1km beneath the seabed and clearly resolved by a 3D seismic data set focused on the deeper, remobilised, sand-filled Siri Canyon. M1 comprises two culminations, up to 80m high and up to 1400m long, constituting a sediment volume of some 5.3x10(7)m(3). It is characterized by a hard reflection at the top, a soft reflection at the base, differential compaction relative to the surrounding sediments, and 10ms TWT velocity pull up of underlying reflections, indicating a relatively fast mound fill, attributed to the presence of sand within the mound. Internal seismic reflections are arranged in an asymmetric concentric pattern, suggesting a progressive aggradation to the NW, downstream to a mid-Miocene contour current system. Numerous elongated pockmarks occur in the upper Miocene succession close to the mound and indicate that the study area was influenced by gas expulsion in the mid- and late Miocene. The reflection configuration, velocity, dimensions, regional setting, and isolated location can best be explained by interpreting the mound as a giant sand volcano extruded > 1km upward from the Siri Canyon during the middle Miocene (c. 15Ma). The likely causes of this remarkable structure include gas charge and lateral pressure transfer from the Central Graben along the Siri Canyon reservoir. While this is the first such structure described from this part of the North Sea, similar-aged sand extrudites have recently been inferred from seismic observations in the North Viking Graben, thus suggesting that the mid-Miocene was a time of widespread and intense sediment remobilization and fluid expulsion in the North Sea.