Ron Steel¹, Donatella Mellere², Piret Plink-Bjorklund¹, Szczepan Porebski³, Maija Schellpeper¹

(1) University of Wyoming, Laramie, WY
(2) Padova University, Padova, Italy
(3) Polish Academy of Sciences, Krakow, Poland

ABSTRACT: Shelf-Edge Deltas: Driver for Shelf-Margin Accretion

Large volumes of sand are dispersed out across the shelf-slope break both where delta complexes dominate the lowstand shelf margin, and where a large single river dominates. Datasets illustrating the latter scenario commonly show incised river valleys on the shelf that link directly to a slope canyon, a conduit taking the sand budget directly out onto the basin floor. Relatively little of the sand is thus stored on the slope, and there is little slope growth unless there are intra-slope basins. In cases where a delta complex dominates the shelf edge, in contrast, the sand budget is dispersed more broadly along the strike of the slope, there are few slope canyons to funnel sand much beyond the base-of-slope, but there is significant accretion of the shelf margin. A third scenario, a storm-wave regime at the shelf edge, also appears to be largely negative for sandy shelf-margin accretion. A dataset where hummocky cross-stratified sands accumulated thickly on the outer shelf shows that the shelf-edge trajectory in such a regime is commonly fairly steep and aggradational, causing the slope to be accretionary but muddy.

We argue that sandy shelf-margin accretion is driven optimally by fluvial-dominated, shelf-edge delta complexes. The shelf-edge accretionary trajectory in this case is near horizontal because of stable or slightly falling sea level, and there is no significant development of a backing coastal plain during regression. The sand-prone aspect of the shelf edge is further enhanced by the appearance of sandy estuaries during the transgressive phase. Examples of all 3 regime types at the shelf edge illustrate their relative contributions to shelf-margin accretion.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado