Unsung Pioneers of Sequence Stratigraphy: Eliot Blackwelder, Joseph Barrell, Amadeus Grabau, John Rich and Harry Wheeler
Authors: Pemberton S. George , Bhattacharya Janok P. , MacEachern James A. , Pemberton Erin A. L. 
Published: January 2024

Few facies analysts or stratigraphers would argue against the contention that the transition to facies-driven sequence strati- graphic correlations represents one of the most important paradigm shifts in modern stratigraphy. What is less commonly appreciated is that most of the fundamental underpinnings of sequence stratigraphy were derived from a relatively small number of pioneers in the early to mid-1900s; most of them largely unknown or underappreciated by the current generation of sequence stratigraphers. Chief among these are Eliot Blackwelder, Amadeus Grabau, Joseph Barrell, John Rich, and Harry Wheeler. Blackwelder was perhaps the first to point out the presence and significance of regional unconformities in the packaging of strata in North America, which would later come to form the basis of the well-known cratonic megasequences of Larry Sloss. However, Blackwelder is best appreciated as the mentor and colleague of not only these other pioneers, but also of key workers who themselves ultimately mentored some of the most renowned sequence stratigraphers of the 1970s and 1980s. Amadeus Grabau focused his attention on expounding the law of the correlation of facies of Johannes Walther and bringing facies-driven correlations into stratigraphy; an approach at odds with the then prevailing view of lithostratigraphy as the principal physical stratigraphic framework. Grabau recognized that the rock record contains numerous temporal gaps that partition facies succes- sions, for which he defined the term “hiatus”. His Pulsation theory was used to explain the cycles of deposition and hiatal erosion/bypass, and although the mechanism is different, the net outcome of such “oscillations” is broadly similar to the effects of eustacy and tectonism that we assign to such changes today. Joseph Barrell developed the concept of base level and explored its role in controlling erosion versus deposition. Like Grabau, Barrell insisted that sedimentation was not continuous, leading to a stratigraphy that is replete with breaks of varying durations. For these he coined the term “diastems” for the small but more numerous breaks, which could be contrasted with those reflecting longer breaks which he referred to as “discontinuities”. John Rich evaluated depositional topography, for which he coined the terms clinoform, undaform and fondoform. He recognized that such depositional bodies necessarily link genetically related associations of sediment deposited from the shoreline to the basin center; what today we regard as the depositional system. The resulting architectures bounded by these surfaces form the underpinning of all systems tracts in modern sequence stratigraphy. Harry Wheeler formalized the concept of time-stratigraphy, demonstrating that temporal gaps in stratigraphy are as important in understanding the rock record as the rocks themselves. His novel approach of developing stratigraphic cross-sections with time on the vertical axis pioneered the concept of chronostratigraphy through the development of what are now referred to as Wheeler Diagrams. His resurrection of the base-level concept expounded by earlier workers was instrumental in defining sequences as unconformity-bounded units, re-establishing the concepts pio- neered by Blackwelder himself. In concert, these stratigraphic visionaries erected a stratigraphic framework focused on the understanding of the stratigraphic record as opposed to its simple lithostratigraphic mapping or biostratigraphic dating. These unsung pioneers put into place virtually all components of modern sequence stratigraphy more than two decades before its popularization in the scientific commubity.