Diminished Amplitude Reflections (Fadeout) in Bermuda Rise Stratification

摘要

Single and multichannel deep tow seismic records collected on the southwesternBermuda Rise reveal areas within the acoustic layering where the intensity of reflectors fade or even disappear over lateral distances of 20-60 m. The fadeouts are conspicuously present in a highly stratified interval consisting of lower to middle Eocene siliceous turbidites and thin, cherty layers interbedded with hemipelagic/pelagic clays and oozes. Acoustic scattering at rough interfaces, gas pockets, and changes in geoacoustic properties do not readily account for the diminished amplitudes of the reflections. Detailed examination of the wave coda from selected fadeout areas reveal changes in reflector geometry due to faulting of the sediments. Strong reflections within the stratified sequence are thought to result from the thin, high impedance chert beds that occur within the turbidite layers. Secondary reflections (multiples) generated by the hard, chert interfaces are thought to interact constructively with primary reflections to generate strong interference reflections, but also destructively to cause fadeouts where the faulting has changed the layer configuration. Alternatively, the fadeouts may result from the dissolution of chert beds due to the upward migration of silica deficient pore fluids along faults and fractures in response to excess pore pressures.