Fluid flow due to the advance of basin-scale silica reaction zones

Abstract

The conversion of biogenic silica (opal-A) to opal-CT (cristobalite and tridymite) in biosiliceous sediment causes increased rates of water expulsion because of the reduction in sediment porosity and dehydration of the amorphous opal-A phase. This release of water occurs over large tracts of sedimentary basins during sediment burial within discrete diagenetic reaction zones. Analysis of two-dimensional and three-dimensional seismic datasets from basins in the northern hemisphere provides geophysical evidence for a variety of fluid conduits and roughly circular erosional depressions at the contemporaneous seabed. We interpret these features as indicative of water expulsion and focused fluid flow emanating from opal-A to opal-CT reaction zones at burial depths within the range 200—800 m. The rate at which water is expelled depends upon the degree of porosity reduction and the weight fraction of bound water at the reaction zone as well as the rate of advance of the reaction zone. Where the reaction is actively taking place within homogeneous biosiliceous sediment, the rate of water expulsion is independent of the reaction rate. This is because water is released across the entire reaction zone, so slow reaction rates are compensated for by expulsion of water across wider reaction zones. We calculate the rate and volume of water expulsion for the Faeroe-Shetland Basin, where 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 at the present day is ~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. The average volumetric rate of water expulsion is ~120 km3 My-1 across the whole basin. Biogenic silica is particularly rich in Neogene successions in high latitude and equatorial regions, and where silica reaction zones are identified they should factored into sediment compaction and fluid flow histories.