3-D seismic investigation of the diagenesis and deformation of
Cenozoic siliceous sediments on the Eastern Atlantic Margin

Abstract

Three-dimensional seismic data from the East Atlantic Margin are used to investigate the diagenesis and deformation of siliceous sediments. Three themes were tackled.

At the Gjallar Ridge, offshore Norway, seismic and well data indicate that diagenesis of siliceous sediments occurs across a zone ~300 m thick. At the top of the zone circular regions, with relief of ~200 m, are interpreted as regions of preferential diagenesis. Below these, regions with a similar size and distribution are attributed to the same cause. The chemistry of formation water expelled transiently through polygonal faults may play a role in their formation. This is the first recognition from seismic data that silica diagenetic transformation zones can be hundreds of metres thick and heterogeneous, as observed at
outcrop previously.

On the Mauritanian continental margin layer-bound fault systems deform a probable siliceous succession. Where this succession mantles canyons the majority of faults strike perpendicular to the canyon axes due to increased bedding dip and most are antithetic to bedding dip. Where the bedding dip is greater than ~1°, synthetic faults dip more steeply than antithetic faults, which is the exact opposite that would be expected on a dipping margin. It is hypothesised that the layer hosting the faults has been subjected to simple shear of ~20° or more, rotating the fault planes.

Lastly at the Vema Dome, offshore Norway four submarine slides are identified in siliceous sediments. Each covers an area of ~30 km2, is up to 600 m thick, with an upper surface topography consisting of a series of arcuate ridges perpendicular to the dip of the slope. The slides are dominated by fold-and-thrust structures, have short transport distances, and unusually low ratios of length to thickness. These characteristics are attributed to deep
detachments and the shear strength of siliceous sediments at shallow burial depths.