THE PARASEQUENCE DEFINITION - ARE TRANSGRESSIVE DEPOSITS INADEQUATELYADDRESSED

In only a few decades sequence stratigraphy has become one of the corn erstones of modern stratigraphy. Although the sequence is the principa l stratigraphic unit, parasequences are the fundamental composite buil ding blocks. By definition, parasequences are typically upward-shoalin g successions that are bounded by flooding surfaces that form in respo nse to relatively rapid rises of relative sea level. In that definitio n little provision is made for significant transgressive deposition du ring the time of flooding, but transgressive deposits do exist in the geological record. Because these deposits lie above the flooding surfa ce and derive much of their sediment from transgressive erosion of the underlying progradational succession, should the parasequence boundar y 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 complet e regressive-transgressive succession. Nonetheless, although appealing , the typical inability to easily, accurately, and consistently identi fy the uppermost surface of the transgressive unit makes it an untenab le 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 transgress ive deposits, is the most appropriate surface to bound the parasequenc e. In any event, to ignore transgressive deposits is to ignore an impo rtant component of the geological record - a record that not only prov ides insight into transgressive depositional processes but also is one that can be economically important. As a result, transgressive deposi ts need to be included in the parasequence definition, possibly only i mplicitly, but certainly not explicitly excluded. Furthermore, the not ion that parasequences typically indicate progradational deposition ex cludes examples where upward-fining transgressive deposits (related to upward deepening) are present and lie below the upward-shoaling (prog radational) part of the same parasequence.