Variation in Depositional Systems Along Shoreline Embayments--Modern and Ancient Examples: ABSTRACT

Abstract

The stratigraphic record of strandline depositional environments shows a systematic change along shoreline embayments in response to changes in the ratio of wave-energy flux to tidal-energy flux. Waves diminish in size and tidal ranges increase from the entrances to the heads of such embayments. A depositional model for shoreline embayments emphasizing sand bodies shows the following: embayment entrance--wave-dominated deltas, microtidal barriers, abundant washover fans, and flood-tidal deltas in lagoons; mid-zone--mixed-energy deltas, mesotidal barriers, numerous inlets, back-barrier tidal-channel sands; and embayment head--tide-dominated deltas, offshore tidal sand ridges, no barriers, extensive marsh/tidal flat systems. Two ancient shoreline embayments, along the Carboniferous shoreline of the southern Appalachians and the Late Cretaceous shoreline of Wyoming and Colorado, illustrate the model. Both examples illustrate a change in sand-body geometry from micro-tidal, wave-dominated barriers at the entrances to mesotidal, inlet-dominated barriers farther inside the embayments. Thus, subsurface exploration for sand bodies containing economic deposits should focus on strandline-parallel sands with lagoonward building washovers and flood-tidal deltas at embayment entrances, and strand-perpendicular tidal sands at embayment heads. Exploration in the mid-zones of the embayments would be the most difficult, because of the complexity brought about by the migration of tidal inlets at the shoreline and tidal channels in the back-barrier area. End_of_Article - Last_Page 580------------