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Regionally correlatable regressions, recorded in Upper Cretaceous strata from the United States Western Interior, represent low-order cyclic progradations of the epeiric seaway shoreline. Stratigraphic intertonguing of coastal sandstone deposits and marine shales developed in response to higher order depositional cycles where episodic transgressions interrupt regressions. A simple model based on Walther's Law of Facies has most often been used to interpret these deposits, including channeled estuarine sandstones commonly found at the top of the sequence. The regressive phase history is presumed to reflect progradation of a barrier island and its associated environments, whereas transgressions are presumed to be mainly erosive.
I propose an alternative explanation for these deposits based on time-stratigraphic architecture and component facies distributions of the Point Lookout Sandstone in the northwestern San Juan basin, New Mexico. Interpretations of outcrop measured sections and well-log correlations suggest an individual parasequence is actually the product of a depositional couplet that includes a regressive phase and a transgressive phase. Strand-plain progradation during the regressive phase produced upward-coarsening, shoreline deposits. Deposition during the transgressive phase occurred mainly in a zone landward of a reworked shoreline trend and comprises a lagoonal-estuarine system.
Specific examples of transgressive backbarrier deposits are described from outcrops of the Point Lookout Sandstone. These facies include an areally extensive, fine-grained, variably stratified lagoonal deposit and an upward-fining, cross-bedded sandstone deposited in highly migratory tidal channels. The channeled estuary developed by infilling of the backbarrier lagoon. The deposition of these backbarrier facies during episodic transgressions is significant for reinterpreting the typical vertical lithologic succession in cyclic regressive stratigraphic sequences. The backbarrier deposits are separated from the strand-plain deposits by a flooding surface and do not represent environments that translated seaward during shoreline progradation.
Transgressive estuarine deposits appear to be an important potential reservoir facies in the Point Lookout section. Each estuarine sandstone is developed as a discrete reservoir unit at the landward limit of a parasequence. These sandstones can be correlated into the subsurface and their distribution mapped parallel to depositional strike. The distribution of channeled estuarine reservoirs can be predicted and effectively exploited based on the depositional couplet model.
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