Shelf-Edge Sand Belts
It has been suggested on theoretical grounds (Porebski & Steel, 2006; Yoshida et al., 2007) that deltas on cross-shelf transits, in basins with an open shelf break, tend to show stronger tidal influence on inner-shelf reaches (increased tidal prism; shallower water) compared with stronger wave influence as they approach shelf edge reaches. In the latter setting, shelf-edge deltas are subject to frequent large, open-ocean swells and storm waves, and except in front of river fairways and segments where the shelf edge is invaginated, the resulting deposits should be thoroughly storm-wave dominated and occur in elongate, along-strike sand belts. Despite many exceptions, these scenarios are likely to be reasonable base cases. Examples of such shelf-edge, storm-wave generated sandbelts are shown for the Pliocene paleo-Orinoco shelf edge (Trinidad), and for the Maastrichtian Fox Hills shelf edge (Washakie/Great Divide Basin, S Wyoming). These data suggest a further proviso, namely that sediment supply should be high and that deltas reach or remain at the outer shelf with rising relative sea level (forced regression would cause coastal irregularity, increased tidal influence, wave shelter). This is also likely to be the case for many of the Gulf of Mexico’s Paleogene shelf-edges, where storm-wave activity is well documented (Galloway, 2001). In the paleo-Orinoco and Fox Hills cases, such wave-dominated sandbelts can extend along strike for 100km and are frequently 75-150m thick. Ironically, these huge shelf-edge sand reservoirs do not easily bypass the shelf break or connect to deepwater except where there is river breakthrough or canyon capture.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009