EVOLUTION OF LOWER JURASSIC COASTAL-PLAIN AND FAN DELTA SEDIMENTS IN THE BERYL EMBAYMENT, NORTH-SEA

Coastal plain and fan delta sediments accumulated throughout the Sinemurian to late Pliensbachian in the Beryl Embayment area, North Sea. These sediments represent a marginal, clastic fringe along the southern extension of the Boreal Ocean into the northern North Sea. Two informal formations are recognized in these predominantly sandy deposits. The lower formation comprises interbedded sandstones, silstones and coals of minor fluvial channel and associated coastal floodbasin marsh, lake and sheetflood origin. The overlying formation represents a fan delta system, possibly generated as a result of footwall uplift and associated erosion. This formation is dominated by probable channel sandstones, with marine-influenced debris flow deposits developed at a number of levels indicating an active, coastal fan setting. Fan sediments are capped by a marine sandstone, beds of which interfinger with the overlying offshore siltstones. Thickness variations are related to major normal faults, suggesting at least a limited fault control on the deposition and distribution of facies during a time conventionally regarded as tectonically quiescent in the northern North Sea. There is no clear evidence for a eustatic control on sedimentation, although deposition of post-fan delta shallow marine siltstones may have been principally controlled by eustatic sea level rise recording the acme of the early Jurassic expansion of Boreal waters into the Viking Graben.