Abstract

The Exxon sequence models of Posamentier and coworkers incorporate two concepts which I question. They suggest that rivers grade to a "bayline", but evidence from modern major rivers indicates that they do not. They also proposed that fluvial sedimentary accomodation space is generated by the seaward shift in equilibrium profiles during sea-level fall, but this is too simplistic a model, based on the use of schematic continental margin configurations which do not incorporate real slope and relief data. In most cases, a fall in base level leads to deltaic progradation and upstream fluvial incision. Most coastal plain fluvial wedges result from two processes: sea-level rise, with the drowning and filling of estuaries, and active source-area tectonism. Major tectonic events adjacent to foreland basins, such as terrane collisions, lead to the development of sequences of third-order type (duration approximately 1-15 m.y.). Changes in intraplate stress regimes resulting from changes in regional plate kinematics have a similar episodicity and can be shown to be responsible for some third-order cycles on extensional continental margins. Examination of the chronostratigraphic basis of the Mesozoic-Cenozoic Cycle Chart of Haq and coworkers reveals internal inconsistencies and an unrealistic downplaying of the built-in potential for error. This chart is not an approved global standard, and it should be used with extreme caution.

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