Seismic reflectors and unconformities at passive continental margins

Abstract

Vail and co-workers^1–3 have recently suggested, based on seismic reflection data available to Exxon, that more than 25 global unconformities occur in Mesozoic–Tertiary age passive continental margins around the world. These data have been used to subdivide the stratigraphical record into several cycles, each of which is controlled by oscillatory changes in sea-level. The resulting scheme has been used as a basis to predict the stratigraphy of passive margins where there is little or no well control⁴ or where few reflector terminations can be found⁵. We report here four difficulties with this scheme: (1) during times of minimal continental ice cover changes in the relative rate of sea-level are generally not sufficient to cause unconformities, except in old, slowly subsiding, margins; (2) seismic and biostratigraphical resolution restricts the identification of unconformity-bounded stratigraphical sequences to relatively few in continental shelves; (3) sequence boundaries appear to be best defined on slopes or in regions of active tectonic tilting; and (4) limitations in the sequence analysis technique preclude its use in predictive stratigraphy, especially in shelf and slope regions of margins.