Striving to Attain Formalized “Sequence Stratigraphic” Nirvana

Art D. Donovan
BP, Houston, TX

Since the earliest days of stratigraphy, there has been a great deal of interest in identifying stratigraphic units that cross traditional lithostratigraphic boundaries in order to delineate coeval sedimentary facies (depositional environments). Early attempts to develop this methodology were based on defining and correlating stratal surfaces that a few insightful workers observed crossing traditional lithostratigraphic boundaries. This correlation methodology blossomed with the advent of seismic stratigraphy and the observation that seismic reflections followed stratal surfaces and unconformities. Thus the informal chronostratigraphic methodology of correlating stratal surfaces is actually the cornerstone of sequence stratigraphy.

Over the last 25 years, there have been numerous attempts (allostratigraphic units/synthems) to formalize sequence stratigraphy. Unfortunately, these attempts have not focused on defining stratal-bounded units, but on defining “Unconformity-bounded” Units. Because unconformity-bounded units exist only to the limits of their bounding discontinuities, and other commonly defined stratal surfaces are excluded as boundaries, these units serve limited practical purpose and have not gained a wide acceptance.

If sequence stratigraphy is to transition from an informal to more formal methodology under the stratigraphic code, then a paradigm shift and bigger tent is also needed. Perhaps a shift from “discontinuity-bounded” to more flexible “surface-bounded” (Epi-stratigraphic) units could provide the framework and space to allow the large variety of differing “sequence stratigraphic” methodologies to live, prosper, and function under the stratigraphic code. Under the proposed framework, a user-defined “Episequence”, derived from the Greek work Epipoles meaning enclosing or bounding surface could be the fundamental stratal-bounded unit in a formal classification.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005