Rapid glacial sedimentation and overpressure in oozes causing large craters on the mid-Norwegian margin: integrated interpretation of the Naust, Kai and Brygge formations

Abstract

Abstract Along continental margins with rapid sedimentation, overpressure may build up in porous and compressible sediments. Large-scale release of such overpressure has major implications for fluid migration and slope stability. Here, we study if the widespread crater-mound-shaped structures in the subsurface along the mid-Norwegian continental margin are caused by overpressure that accumulated within high-compressibility oozes sealed by low-permeability glacial muds. We interpret 56 000 km 2 of 3D and 150 000 km 2 of 2D-cubed seismic data in the Norwegian Sea, combining horizon picking, well ties and seismic geomorphological analyses of the crater-mound landforms. Along the mid-Norwegian margin, the base of the glacially influenced sediments abruptly deepens to form 28 craters with typical depths of c. 100 m, areal extents of up to 5130 km 2 and volumes of up to 820 km 3 . Mounds are observed in the vicinity of the craters at several stratigraphic levels above the craters. We present a new model for the formation of the craters and mounds where the mounds consist of remobilized oozes evacuated from the craters. In our model, repeated and overpressure-driven sediment failure is interpreted to cause the crater-mound structures, as opposed to erosive megaslides. Seismic geomorphological analyses suggest that ooze remobilization occurred as an abrupt energetic and extrusive process. The results also suggest that rapidly deposited, low-permeability and low-porosity glacial sediments seal overpressure that originated from fluids being expelled from the underlying high-permeability and high-compressibility biosiliceous oozes.