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ABSTRACT

Three-dimensional seismic analysis of a submarine palaeo-translational slope failure on the northeast Atlantic margin indicates that it was ‘primed’ and probably ‘triggered’ as a result of diagenesis at a silica chemical reaction front, where biogenic silica (opal A) is being converted to opal CT (Cristobalite/Tridymite). Conversion of opal A to opal CT is a thermochemical dehydration reaction that causes rapid compaction. It therefore is a potential overpressure generation mechanism, usually once sediment has been buried to depths of 300–800 m below the contemporaneous seabed. The overpressure reduces the sediment shear strength, making it susceptible to failure. In this example, the translated succession (volume of 25 km3 and area of 110 km2) was coherent and rigid but the detachment unit was a liquified sediment mass. After failure, the translated succession broke up into a series of faulted-bounded blocks, which differentially subsided into this underlying sediment-fluid mass. Sediment-fluid intrusions utilized the faults bounding the blocks, intruding 200–400 m of the overburden stratigraphy to expel a fluid–sediment mix into the water column and onto the palaeoseabed. Pore pressure decreased and sediment strength within the detachment unit was re-established. Key factors for the initiation of this failure mechanism are (a) the rate of the reaction front advancement (ROFA), (b) the magnitude of the porosity reduction at the reaction front, (c) the sealing capabilities of the overburden and (d) the low shear strength of opal A. Given that the reaction front normally forms at depths of 300–800 m, the mechanism is more likely to induce deep and therefore large volume detachments, which should be more common in high latitude and equatorial regions where biogenic silica production is high.