Seismic character of bottom simulating reflectors: examples from the mid-Norwegian margin

Three classes of bottom simulating reflectors (BSR) cross-cut the post-breakup sediments of the mid-Norwegian margin. The first class is caused by free gas at the base of the pressure- and temperature-dependent gas hydrate stability zone. The second class of BSR is caused by the diagenetic transition from opal A to opal CT. The third class of BSR is always observed underneath the opal A/opal CT transition, but heat flow data and the amplitude characteristics of this arrival exclude one of the known silicate diagenetic transitions or gas hydrates as the explanation for this reflector. ODP Site 643 drilling results from the Voring Plateau suggest two possible processes as the reason for this third BSR: (a) smectite illite conversion or (b) a sudden increase in the abundance of authigenic carbonates. The genesis of both is pressure- and temperature-dependent and could potentially result in a cross-cutting seismic reflector. The data are not conclusive as to which process is causing the third class of observed BSR.