Deltas and Sea-Level Change

Abstract

Abstract Sea-level shift from the innermost shelf out to the shelf edge produces bayhead, inner-shelf, mid-shelf, and shelf-margin deltas. We suggest that these delta types are distinguishable in the ancient record and that such distinction has advantages as compared to the conventional, entirely process-based classification. Bayhead and inner-shelf deltas tend to form thin clinoforms (a few meters to tens of meters amplitude, respectively), and as they aggrade with rising relative sea level they generate a tail of thick paralic deposits. Mid-shelf deltas produce clinoforms as high as the mid-shelf water depth, tend to follow a subhorizontal trajectory, generate little or no paralic tail, and are commonly thinned by transgressive ravinement. Shelf-edge deltas in a stable-to-falling relative sea level usually have no paralic tail, create by far the highest clinoforms, and can have a thick succession of sandy turbidites on the delta fronts. If sea level falls below the shelf margin, the shelf-edge delta becomes incised by its own channels and large volumes of sand can be delivered onto the slope and the basin floor. Many deltas require a strong fluvial drive to attain a shelf transit, though as they approach the outer shelf they commonly become wave dominated. Tidal influence can increase on the outermost shelf if relative sea level is falling, if the shelf-break is poorly developed, and if basinal water depth is shallow. During transgression, the system tends to be tidally and/or wave influenced. Deltas that transit back and forth on the shelf on short time scales (tens of kiloyears to 100 ky) and that are driven largely by sea-level fluctuations are referred to here as accommodation-driven deltas. Deltas that can reach the shelf edge without sea-level fall are termed supply-driven deltas. These highstand deltas deposit thick, sandy, stacked parasequences during their shelf transit, and they tend to have an extensive muddy delta front on reaching the shelf-edge area. Such deltas would not normally be incised at the shelf edge, and they would produce a progradational, shelf-edge attached, sandy slope apron (Exxonian shelf-margin systems tract) rather than basin-floor fans, except in cases of extremely high supply. Sequence boundaries are best developed on accommodation-driven deltas, and are likely to be represented on a variety of time scales (third, fourth, and fifth order). Sequence boundaries in supply-dominated deltas may be identifiable only at lower-order time scales, or they may be non-existent.