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Depositional-Process Changes in a Sequence-Stratigraphic Context: An Ingredient in a New Generation of Sequence-Stratigraphic and Basin Models

Shuji Yoshida1, Ron Steel2, and Robert Dalrymple3
1 Chiba University, Chiba, Japan
2 The University of Texas at Austin, Austin, TX
3 Queen's University, Kingston, ON

Existing sequence-stratigraphic models commonly assume a constant energy setting throughout the relative sea-level (RSL) cycle. However, the dominant process affecting the coastal zone is a function of interdependent factors and can change at any time during the cycle. Thus, the relative intensity of wave and tidal processes may change on a regional scale as a result of bathymetric changes caused by rising or falling sea level, changing shelf width, and/or tectonic events. Fluvial energy or discharge seasonality may also vary synchronously with RSL change, as a consequence of climate change.

Moreover, whereas most facies and sequence-stratigraphic reconstructions are based on river-, wave- or tide-dominated end-member environmental models, most real-world environments are mixed-energy settings where these processes coexist in sub-equal 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 may cause product changes without any change in the processes: fine and very fine sand, such as is delivered during TST and HST, favors the preservation of wave-generated hummocky cross stratification, whereas medium and coarse sand, as is delivered during FRST and early LST, permits the development of current-generated cross bedding. Future sequence-stratigraphic and basin models need to incorporate such process and product changes.