Forty years of sequence stratigraphy
The principles and practice of sequence stratigraphy are of ancient heritage. In North America, the separation of the Carboniferous into two systems and the identification of Ozarkian and Comanchean and other indigenous chronostratigraphic entities were efforts toward making the segmentation of the geologic column more representative of stratigraphic observations, particularly on cratons and their margins. Growing awareness of the influence of tectonics on sedimentation, as well as recognition that unconformity-bounded sedimentary packages identified in Montana were craton-wide, led to this writer9s 1948 GSA paper which formally applied Native American tribe names to sequences as lithostratigraphic units. Acceptance of the sequence philosophy beyond the Northwestern campus lacked early manifestations of fervor and enthusiasm, although isolated pockets of true believers were known to exist. Except for papers by me, my students, and the late, lamented H. E. Wheeler, sequences did not appear in the public prints for more than a decade—and then through misappropriation by the U.S. Geological Survey. The list of Indian-tribe sequences was emended and completed at another GSA presentation in 1959. Meanwhile the sequence concept was alive and well in a research facility of a company later to be known as Exxon. Here, Peter Vail and a cohort of preconditioned colleagues seized upon the stratigraphic imagery made available by multichannel, digitally recorded, and computer-massaged reflection seismography to establish the discipline of seismic stratigraphy . The "Vail curve," recording relative change of coastal onlap, defines successive "sequences" which are the third-order bottom rungs of an elaborate hierarchy that is topped by "megacycle sets," themselves subdivisions of the ancestral sequences. The taxonomy of sequence stratigraphy is not at issue (although it has been complicated by the sanctification of "synthem" by the International Subcommission on Stratigraphic Classification). Interpretation is important, however; the Exxon people, their alumni, and adherents see sea-level change as the "be-all" and "end-all" of coastal onlap and its erosional and depositional concomitants. No one will question the influence of sea-level change on many of the observations that make sequence stratigraphy viable, but there are distinctions among and within major-scale unconformity-bounded stratal packages that can be explained only by tectonic change. Whether tectonic influence can be extended to Exxonian sequences remains to be demonstrated. Meanwhile, it is worth considering the proposition that sea-level change is a second- or third-order response to some more significant global phenomenon.