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.

ABSTRACT Multisensor track measurements are a nondestructive method to produce continuously measured high-resolution physical property data sets that are a great asset to a wide range of research, including geotechnical studies and paleoceanography. Interpretation of these physical property data can be challenging because they are typically influenced by multiple variables. This paper specifically focuses on the interpretation of gamma-ray attenuation (GRA) data (a proxy for sediment bulk density) in biosiliceous sediments. The Bering Sea is a basin dominated by biosiliceous sediment, and the late Pleistocene to present core record of Sites U1340 and U1339, drilled during Integrated Ocean Drilling Program (IODP) Expedition 323, has subtle meter-scale changes in the concentration of fine-grained siliciclastic sediment that produce lithologic alternations between diatom ooze and diatom mud. We produced a detailed sedimentologic data set that combined smear slide petrography, scanning electron microscopy, and grain-size analysis for both Sites U1340 and U1339 and correlated it to shipboard GRA bulk density measurements. Results show that bulk density is negatively correlated with diatom abundance and positively correlated with the fragmentation of diatom valves. This study argues that diatom abundance and fragmentation influence sediment packing and drive down-core variability in GRA bulk density. Therefore, denser diatom mud is a result of tightly packed, highly fragmented diatom valves, and diatom ooze is a less dense sediment dominated by whole and less fragmented diatom valves. We suggest that GRA data can be used as a proxy for diatom abundance and an indicator of diatom fragmentation. We include a discussion of how these results may impact the interpretation of ancient bedded siliceous rocks.