WALKER, ROGER G. and OLE J. MARTINSEN
Norsk Hydro Research Centre, Bergen, Norway

Abstract: Deep Water Facies and Depositional Environments, Cretaceous, Mid-Norway

Re-examination of facies in the Lysing Formation (Late Cenomanian to Coniacian) in the Haltenbanken area (Smorbukk Field) has given a new perspective on deposition in the former Jurassic rift basin of Mid-Norway. Older models involving sandy debris flows on the slope must be replaced by models of turbidity current deposition closer to the axis of the rift. The Lysing is about 40 - 100m thick and contains three main facies- (1) Sandstone beds 0.1 to over 2.5 m thick which are structureless and/or contain vertical to contorted fluid escape pipes. (2) Sandstones with glauconite grains and granules, and cross stratification in sets up to 20 cm thick. (3) Black mudstones with mm scale siltstone laminations. The mudstones above and below the Lysing are over 100 m thick, and the same facies is also interbedded with the Lysing sandstones. The thickness and lithology of the mudstones suggests deposition in deep water, consistently below storm wave base. This establishes a context for facies 1, and particularly for the unusual cross-stratified sandstones of facies 2. This is NOT a shallow water facies emplaced by slumping onto the slope, as formerly suggested. The ubiquitous fluid escape structures suggest that sand was deposited rapidly from suspension out of turbidity currents, not sandy debris flows. The lateral extent of the turbidites as potential reservoir flow units is partly determined by the interbedded black mudstones - up to 7 m thick and mappable continuously for at least 6.5 km through the sandbody.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas