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 3D seismic data provide new insights on c. 2 km thick Cenozoic post-transform slope sediments and fluid-flow phenomena along the Nigerian Transform Margin. The study documents large-scale mass-transport deposits (MTDs), deep-water channels, sediment waves, and a range of fluid-flow phenomena such as pockmarks, pipes, seabed mound and gas hydrates. They are observed from Pliocene-aged sediments and distributed above structural highs, regional faults, and active and relict deep-water channels in the eastern part of the area, closest to the Niger Delta cone. The fluid-flow features are interpreted to be indicative of an active petroleum system in the deeper subsurface and from fluid migration along planes of deep-seated faults. MTDs are mapped at multiple levels and the volume of failed sediments increased through time within the western part of the study area. The repeated and increased volume of MTDs in the area is attributed to an increased rate of sedimentation through time and slope gradient during the late Cenozoic. The presence of repeated MTDs and fluid-flow phenomena on the Nigerian Transform Margin has implications for installations of offshore facilities as they constitute potential geohazards. The study also documents, for the first time, polygonal fault systems offshore Nigeria.

ABSTRACT Mississippian (Tournaisian–Viséan) carbonate mounds in the Compton and Pierson limestones, Ozark region, North America, have been called Waulsortian. However, European Waulsortian mounds contain features such as geopetals with multigenerations of carbonate mud (polymuds) and stromatactis cavity systems that are rare to absent in Ozark mounds. To determine similarities and differences, examine their origins, and clarify nomenclature, mounds in the Compton and Pierson limestones are compared with Waulsortian mounds in the Feltrim Limestone, Ireland. Features considered included mound size, geometry, style of aggradation, composition, depositional setting, and diagenetic history. Mounds in the Compton and Pierson limestones are <10 m (33 ft) thick and form singular knoll-form or aggregates with a strong lateral growth component. In contrast, individual Waulsortian mounds in the Feltrim Limestone range from 5 to > 30 m ( 16 – 100 ft ) thick, but coalesce and vertically aggrade to form complexes that exceed 500 m ( 1600 ft ) . Pierson mounds are crinoidal and grain-rich, whereas Compton and Feltrim mounds are bryozoan-rich and mud-dominated. All mounds have similar cement stratigraphy and diagenetic histories. Mud-rich Compton mounds and Feltrim mounds are interpreted as deeper water than skeletal-rich Pierson mounds. Limited accommodation constrained Compton and Pierson mound size and forced lateral aggradation. Subsidence-driven accommodation in the Dublin Basin allowed Feltrim mounds to grow larger, coalesce, and aggrade vertically. Three types of mounds are recognized: true Waulsortian in the Feltrim Limestone, mud-cored Waulsortian-type Compton and Pierson mounds, and Pierson transported bioaccumulation mounds. Small dimensions of Waulsortian-type Pierson and Compton mounds limit their potential as oil and gas reservoirs, whereas Pierson crinoidal sediment piles are known to form reservoir-size accumulations.