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PM Geos Ltd, Suite 3, 343 Union Street, Aberdeen AB1 2BS, UK
Three cored sandstone megabeds of Tertiary (late Palaeocene) age from two quadrants in the Central Graben of the North Sea are discussed. The megabeds are upwards-fining units 3–9 m thick, underlain by gravelly mudstones (1–1.4 m thick) which are interpreted as debrites. The megabeds are divided into five units (M1–M5) based on grain size and sedimentary structures, which are organized into a type vertical sequence. The basal unit M1 comprises a clast-supported mudclast conglomerate. Unit M2 comprises very coarse- to medium-grained, occasionally gravelly, sandstone which is horizontally laminated to cross-stratified. Unit M3 is the thickest unit (2–4 m) and is a laminated, upwards-fining, medium- to fine-grained sandstone with occasional cross-lamination. Unit M4 is a fine- to very fine-grained sandstone characterized by disturbed lamination and water-escape structures. Unit M5 is an upwards-fining silty sandstone to siltstone characterized by discontinuous, irregular, and locally overfolded laminae. The megabeds are interpreted as having been deposited by turbidity currents. There are some similarities to Bouma sequences and high-density turbidite sequences, but massive sandstone is notably absent. The megabeds are associated with mudstones and debrites and only occasional thin turbidites, and as such they are not part of an organized sequence of turbidites. The beds are notably much thicker than those above and below, and lithologies are distinct from those of the surrounding sediments. These features are comparable to those of ‘megaturbidites’ as defined by Bouma. The debrite which immediately underlies the megabeds is significant in that this relationship is also a common feature of megaturbidites. The debrite is thought to represent slope failure which immediately preceded megabed deposition. A hypothesis is presented of earthquake-induced catastrophic slope failure, debrite deposition and associated megaturbidite generation. Such sandstone megabeds are thought to be widespread sheet-like deposits. They are exploited as a hydrocarbon reservoir in two of the wells discussed.
