Diagenesis and Dewatering of Clay-Rich Sediments, Barbados Accretionary Prism

Abstract

Physical properties measurements of samples from the toe of the Barbados accretionary prism were combined with data from an earlier Barbados drilling study. Examination of porosity and velocity profiles in the light of both empirical and theoretical relationships leads to several conclusions: 1. The initial stages of deformation at the tip of the accretionary prism do not result in a substantial loss of porosity for the accreted sediments. Within 5 km of the toe of the accretionary prism, porosity profiles are similar to profiles at reference sites seaward of the deformation when variations in lithology are considered. Further from the prism toe, within the accretionary wedge (15-20 km), porosity profiles exhibit a loss of approximately 5% porosity relative to the reference sites. 2. The decollement and most of the other major thrusts penetrated in the prism are associated with Miocene sedi­ mentary sections because of the higher porosity of those sediments and not because of some diagenetic change associ­ ated with cementation. The relatively high porosities correlate with abundant smectite derived from volcanic ash altera­ tion. 3. The Miocene lithology appears to compact more slowly than the sediments above and below them. This suggests that the porosity, and probably strength, difference between adjacent units may become greater with depth, resulting in an ever increasing tendency for the Miocene sediments to be the locus of slip in bedding-parallel faulting events as com­ paction proceeds.