Deltas vs. Rivers on the Shelf Edge: Their Relative Contributions to the Growth of Shelf Margins and Basin Floor Fans (Barremian & Eocene,Spitsbergen)

Steel, Ron, Donatella Mellere, Piret Plink, Jeff Crabaugh, Jack Deibert, Maija Schellpeper, Tore Loeseth, and M. Schellpeper: The WOLF-consortium at the Dept. of Geology & Geophysics, Univ. of Wyoming, Laramie, WY

Abstract

A study of Cretaceous and Eocene shelf-edge successions, as well as shelf-slope-basin floor clinoforms, are spectacularly exposed along mountainsides in the Central Basin of Spitsbergen. 'Walk-out' correlation along seismic-scale outcrops shows that:

1. Thickest accumulations of deep water turbidite sands on the basin floor correspond with episodes of river development on the time-equivalent shelf. The shelf edge is particularly prone to collapse at this time, producing small growth faults, slumps, mudflows, block rotation and foundered slides of course-grained, fluvial and other shelf-edge strata into the slope muds.
2. Thick accumulations of turbidite sands on the slope, usually wedge-shaped and tapering downslope, correspond with episodes of delta development on the shelf and at the shelf edge. These episodes produce little sand on the basin floor.
3. Where both of the above scenarios are present in close association, there is a time development from (1) to (2) above, corresponding with falling and lowstand of relative sea level, leading to delta development during early rise of sea level within the lowstand wedge.

The slope sand wedges, when ocurring without any association to (1) above, are commonly 70 m thick just below the shelf edge and thin significantly onto the lower slope. They have an internal architecture consisting of a series of downw ard-displaced and younging, perched-slope deltas. Unconformities, decapitating the upper levels of the deltas signify the position of sea-level or subaerial erosion below the shelf edge, and demonstrate that relative sea level fall (and not only sediment supply increase) was necessary to bring the deltas across the shelf and onto the slope.

The slope sand wedges, dominated by thin-bedded and downslope-thinning turbidites generated on the slope-delta fronts, appear to have been emplaced by hyperpycnal flows that rarely were able to ignite on the slope. The shelf-edge rivers, on the other hand, were able to produce slumps and flows that eroded on the slope, and developed into thick basin-floor fans with thick-bedded turbidites.