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Perspective |
1 Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712-0254, U.S.A.; present address: Department of Earth Sciences, Chiba University, Chiba, 263-8522, Japan; shuji{at}faculty.chiba-u.jp
2 Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712-0254, U.S.A.
3 Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, K7L 3N6, Canada
Existing sequence-stratigraphic models for shoreline deposits commonly assume a constant process regime throughout the relative-sea-level (RSL) cycle over a wide range of timescales (third-order cycles as well as high-frequency, fourth- and fifth-order cycles), with the possible exception that tidal processes may be more important during transgressions. However, the dominant process affecting the coastal zone is a function of multiple interdependent factors and can change at any time during high- or low-frequency RSL cycles; indeed, changes are possible on timescales as short as 1,000 years. Thus, the relative intensity of wave and tidal processes may change gradually or abruptly on a regional scale because of changes in bathymetry or coastal morphology caused by rising or falling water levels, and/or changing shelf width.
The specific nature of the response varies as a function of the physiographic and tectonic setting because the attenuation or amplification of wave and tidal action is strongly dependent on local and regional bathymetry and coastal morphology. Fluvial energy may also vary with sea-level change, as a result of climate change.
Moreover, whereas most facies and sequence-stratigraphic reconstructions are based on river-, wave- or tide-dominated end-member environmental models, the majority of real-world environments are of mixed-energy character where these processes coexist in subequal proportions. In such mixed-energy coastal environments, changes in the relative intensity of the depositional processes on a local scale can also cause stratigraphic variations in the nature of the deposits as the environments migrate laterally.
Reconstructions of regional or local process changes are complicated by the fact that changes in the grain size delivered to the coast, as a result of systematic variations in fluvial accommodation and the degree of bypass, may cause product changes without any change in the processes. Fine and very fine sand, such as is dominantly delivered to the transgressive and highstand coastlines favors the preservation of wave-generated hummocky and swaly cross stratification, whereas medium and coarse sand, as is delivered to the falling stage and early lowstand coastlines, permits the development of current-generated (including tide-driven) cross stratification. Future sequence-stratigraphic models need to incorporate and be more sensitive to such process and product changes.
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