Wheeler's confusion and the seismic revolution: how geophysics saved stratigraphy

The leading stratigrapher in his day was the late Harry Wheeler, who was one of the first stratigrapher’s to use the term “sequence” to refer to unconformity-bounded rock units, although his concepts did not catch-on at the time because of an unwieldy insistence that all stratigraphic units be defined on the basis of arbitrary vertical cutoffs and diachronous facies boundaries. The net result was a nearly impenetrable proliferation of different names for the same genetic, correlative and descriptive lithofacies. Stratigraphy was largely phased out of University curricula and by the mid-1970’s was replaced by an emphasis on process-based facies models and petrographic studies. The facies-oriented stratigraphic cross sections of the time bore little resemblance to real stratigraphic architecture and were not amenable to the analysis of reservoir-seal pairs, critical in delimiting flow units in subsurface reservoirs.

At around this time, Peter Vail, and other geophysical pioneers within the hydrocarbon industry, recognized that the stacking of offset seismic traces enhances the subtle, but nevertheless sharp and highly coherent lithological contrasts across beds. They also realized that seismic data was unable to image the arbitrary facies boundaries and imaginary arbitrary vertical cutoffs upon which so much lithostratigraphy was based. This led Peter Vail to make the revolutionary statement that “primary seismic reflections are generated by stratal surfaces which are chronostratigraphic, rather than by boundaries of arbitrarily defined lithostratigraphic units.” Seismic data allowed geologists to observe unconformable lapout boundaries and to extend these into the areas where strata became conformable. This allowed a revolutionary reformatting and reevaluation of the stratigraphic record into sequences bounded by their unconformities and correlative conformities. This, in turn allowed for a vastly superior correlation of genetically related stratigraphic units and enabled a re-evaluation of the forcing parameters that control distribution of sediments both locally and globally. Reflection seismic data thus provided the key technological breakthrough that provided continuous cross sectional views of stratigraphic basin fills. This fresh approach has fundamentally revitalized the science of stratigraphy.