Abstract

In only a few decades sequence stratigraphy has become one of the cornerstones of modern stratigraphy. Although the sequence is the principal stratigraphic unit, parasequences are the fundamental composite building blocks. By definition, parasequences are typically upward-shoaling successions that are bounded by flooding surfaces that form in response to relatively rapid rises of relative sea level. In that definition little provision is made for significant transgressive deposition during the time of flooding, but transgressive deposits do exist in the geological record. Because these deposits lie above the flooding surface and derive much of their sediment from transgressive erosion of the underlying progradational succession, should the parasequence boundary not be moved from the underlying flooding surface upward to the top of the transgressive unit? By doing so, the underlying progradational unit and the overlying transgressive unit would constitute one complete regressive-transgressive succession. Nonetheless, although appealing, the typical inability to easily, accurately, and consistently identify the uppermost surface of the transgressive unit makes it an untenable surface for stratigraphic purposes. As suggested by earlier workers, therefore, the flooding surface, a readily identifiable surface that caps the progradational package and consistently underlies transgressive deposits, is the most appropriate surface to bound the parasequence. In any event, to ignore transgressive deposits is to ignore an important component of the geological record--a record that not only provides insight into transgressive depositional processes but also is one that can be economically important. As a result, transgressive deposits need to be included in the parasequence definition, possibly only implicitly, but certainly not explicitly excluded. Furthermore, the notion that sequences typically indicate progradational deposition excludes examples where upward-fining transgressive deposits (related to upward deepening) are present and lie below the upward-shoaling (progradational) part of the same parasequence.

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