Fluid-escape Features Due to Silica Diagenesis in the Faeroe-Shetland Basin

Conversion of opal-A to opal-CT in biosiliceous sediment during burial expels water expulsion by porosity reduction and dehydration. This release of water occurs over large tracts of sedimentary basins within discrete diagenetic reaction zones at burial depths in the range 200-800 m. Seismic data from the Faeroe-Shetland Basin provide evidence for a variety of fluid conduits which terminate upwards in roughly circular erosional depressions at an Early Pliocene unconformity, interpreted as the contemporaneous seabed. These features are indicative of water expulsion and focused fluid flow emanating from the opal-A to opal-CT reaction zone. The sediment immediately below the reaction zone contains, on average, ~30% opal-CT by weight. The estimated volumetric rate of water expulsion per unit surface area during the Quaternary has averaged ~6 m3 My-1 per square meter, which is greater than the vertical flux of water at the same depth from compaction of the deeper basin fill, and amounts to a volumetric rate of ~120 km3 My-1 across the whole basin. In late Miocene times, the calculated rate of water expulsion was around six times greater, which explains why the fluid-escape features terminate at the Early Pliocene unconformity.