High velocities and seismic anisotropy in Pleistocene turbidites off Western Canada

Summary. The Winona Basin is a young deep water basin along the base of the continental slope off central British Columbia which is filled primarily with turbidite sediments. Seismic reflection profiles in the basin indicate that in places the sediment thickness is greater than 4.0 s two‐way travel time, and simple interpretations of seismic refraction data indicate that velocities up to 6.0 km s −1 exist within the sediment section. Because the turbidite sediments are acoustically layered with large velocity contrasts between layers, the sediments are probably highly anisotropic. A simple model to account for this has been used to estimate the errors involved in using refraction data to determine layer thicknesses and average velocities. If the effects of anisotropy are ignored, as they usually are, values for thickness and velocity may be overestimated by as much as 10–20 per cent. Even the corrected velocities are still high, however, and the inferred high densities and high degrees of lithification are remarkable when the youth of the sediments is considered. Nearly the entire depositional history is contained within the Pleistocene, and the age of the sediments whose velocity exceeds 4 km s −1 is estimated to be only 1.0 Myr old. Corrected velocities in the deepest part of the basin, where the sediment section is over 6 km thick, exceed 5 km s −1 . The nature of samples dredged from the basin (mudstones to coarse‐grained sandstones and conglomerates) suggests a mechanism for initial diagenesis: The mudstones are moderately porous, 40–60 per cent, and possess little carbonate, whereas the coarser grained rocks are all well cemented with calcium carbonate, with carbonate contents ranging up to 37 per cent. Measured porosities are as low as 8 per cent, but porosities calculated on a carbonate‐free basis range from 35 to 55 per cent. It appears that the fine‐grained sediments are first consolidated primarily by gravitational and/or tectonic compaction, and the coarse‐grained sediments are cemented by carbonate which is precipitated from pore fluids escaping through the highly permeable coarse‐grained layers. As lithification deeper in the basin continues, pore fluids must continue to escape; tectonic fracturing may facilitate this.