Two-dimensional (2D) seismic reflection data reveal two silica diagenetic boundaries located in the North Sakhalin Basin, offshore Sakhalin Island. These boundaries form distinctive high-amplitude seismic reflections that predominantly cross-cut Miocene stratigraphy. The boundaries represent the opal-A to opal-CT (cristobalite and tridymite) transition and the subsequent deeper opal-CT to quartz transition and cover an area of c . 107 000 km 2 . The reflections represent diagenetic reaction fronts that advance through silica-rich host sediments during burial. The oldest strata within which the diagenetic fronts have been identified are of Miocene age and the fronts commonly show a parallel relationship to a Late Miocene unconformity. These diagenetic fronts develop a variety of morphological forms. In the North Sakhalin Basin these front geometries are probably the result of the interference of the diagenetic fronts with inclined and folded stratigraphy together with lateral changes in the composition and, therefore, depth of transformation of the sediment. We define criteria for the recognition of these 2D cross-sectional morphological elements and by so doing provide the basis for the objective description and categorization of diagenetic front geometry.

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 data sets from basins in the Northern Hemisphere provides geophysical evidence for a variety of fl uid conduits and roughly circular erosional depressions at the contemporaneous seabed. We interpret these features as indicative of water expulsion and focused fl uid fl ow 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; therefore, 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 m 3 My -1 per square meter, which is greater than the vertical fl ux of water at the same depth from compaction of the deeper basin fi ll. The average volumetric rate of water expulsion is ~120 km 3 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 identifi ed, they should be factored into sediment compaction and fl uid-fl ow histories.

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