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Peter R. Vail1, Gerald R. Baum1
(1) Rice University, Houston, TX

Abstract: Historical development of the coastal onlap chart and eustatic sea level cycles

In the 1950’s, Larry Sloss proposed major tectonic-stratigraphic sequences used for regional correlations. During the late 1950’s, local onlap charts, mostly Paleozoic and Mesozoic, were compiled from the regional studies performed at Exxon’s affiliates. In the early 1960’s, as seismic acquisition and processing techniques became more sophisticated, these onlap cycles could be readily recognized and dated paleontologically, especially in the Cenozoic. Additionally, the seismic patterns and geometries observed could be related to stratal surfaces seen in geological data. This work was released in 1977, AAPG Memoir 26, laying the foundation for seismic stratigraphy.

The late 1970’s-early 1980’s witnessed several seminal changes in research direction. A series of applied seismic projects began to reveal repetitive patterns of deposition, including incised-valleys feeding lowstand wedges and the enigmatic detached fans. The latter were quite different from previously recognized turbidite deposits. Simultaneously, outcrop studies in Alabama more accurately defined and dated sequence boundaries and flooding events. In addition, a two-component system used rates of sea level change and subsidence to provide a graphic solution to sea level. For the first time, qualitative sea level changes were differentiated from coastal onlap, and flooding events (condensed sections) became as significant as unconformities. In an iterative process between seismic, wireline logs, cores and outcrops, the technology of sequence stratigraphy evolved and was published formally in 1988.

The controversy of eustasy still remains, particularly for the Mesozoic. Oxygen isotope records, a proxy for climate and/or sea level change, suggest eustasy is the primary mechanism in creating sequence boundaries.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana