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Shelf-Transiting Shoreline Systems and Generation of Stratigraphic Sequences in Shelf-Break vs. Ramp Basins

Ron Steel, University of Texas, Austin, TX

Basins that develop a shelf break and fill by means of clinoform accretion appear to require at least 150-200 m of water depth. Ramp basins have shallow water throughout.

The landward reaches of both types of basins form a shelf platform across which shorelines make regressive (deltas, strand plains) and transgressive (estuaries, barrier-lagoon systems) transits. Numerical simulations of modern deltas on varying shelf width and gradient suggest that (2) the shoreline is more likely to reach the shelf edge (or far into the basin in the case of a ramp) if relative sea level is stable or falling rather than rising, because of the phenomenon of auto-retreat, and (2) regressive shelf-transit time only rarely exceeds 100 k.y. for even the widest shelves. The latter has some consequences for the time scale of fundamental stratigraphic sequences.

Although high-sediment supply deltas may be somewhat insensitive to minor sea level changes and may remain at the shelf margin for prolonged periods, small and medium-size deltas make repeated regressive and transgressive transits in response to accommodation changes on the shelf. There is some evidence of process-regime changes during such shelf transits, depending on whether regressive shoreline trajectories are slightly rising or are slightly falling. In the former cases, deltas are remarkably wave dominated throughout the transect, whereas in the latter case wave-influenced deltas often become tide dominated (ramp basins) or fluvial dominated (shelf-break settings). There is increasing evidence that some turbidite populations on the shelf margin and basin floor derive from the river-generated hyperpycnal flows at the shelf-edge. Transgressive coasts also have a clear tendency to be strongly tidally influenced.

A new generation of sequence-stratigraphic models will be stronger and more predictive when clothed with process data.

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