THE SLOSS REVOLUTION

Laurence L. Sloss (Northwestern University) in 1949 conceived sequence stratigraphy for interregionally extensive packages of strata bounded by unconformities, thus creating a new physical basis for stratigraphy, which did not conform to the traditional, paleontologically-defined stratigraphy. He first distinguished four such packages in Paleozoic strata of the northern plains region of the U.S. and Canada. He soon traced these across the entire craton and in 1963 defined two more, younger sequences. Three former Sloss students employed by Exxon soon applied the concept to subsurface exploration for petroleum. By the 1970s, high-resolution seismic reflectors revealed apparently-correlative, unconformity-bound sedimentary packages on several continental margins, which suggested a worldwide cause. The new seismic stratigraphy modified the mentor’s concept by defining a proliferation of cycles with an order of magnitude higher frequency than his original six sequences. The Exxon disciples chose eustatic fluctuations to explain their "Vail Curve of third-order cycles" presumably caused by the oscillation of continental glaciers. But there was no independent evidence for glaciation to match many of their sequences, Sloss pointed out, and stubbornly appealed to his "counter-vailing view" of tectonics to alter continental freeboard rather than simple eustasy. Debate continues.

H.E. Wheeler (University of Washington) also conceived of a sequence approach and stressed the importance of base level of erosion at the unconformities. Both he and Sloss were graduates of Stanford University in the 1930s and doubtless had been influenced by the prescient ideas of professors H.G. Schenck and S.W. Muller, who were grappling with the distinction of abstract time versus preserved rock and unconformities. Independently, petroleum geologist A.I. Levorsen was preparing paleogeologic maps of major, buried unconformity surfaces, which complemented the new unconformity-defined stratigraphy. As Sloss once observed, “the growing geographic scope of stratigraphic investigations and, most importantly, the emergence of subsurface data made it [sequence stratigraphy] inevitable.”