DELTA VARIABILITY AND SEA-LEVEL CHANGE

Sea-level shift from innermost shelf out to shelf edge produces a family of delta types, including bayhead, inner-shelf, mid-shelf and shelf-margin deltas. We suggest that these delta types are distinguishable in the ancient record by their own sets of characteristics. Bayhead and inner-shelf deltas tend to form relatively thin clinoforms (few to 10's of m amplitude respectively), and as they aggrade with rising relative sea level 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, and generate little or no paralic tail. Mid-shelf deltas can be thinned by subsequent wave-generated transgressive erosion. Shelf-edge deltas usually have no paralic tail because it is easier for them to reach the edge of wide shelves if sea level is still falling. Shelf-edge deltas create by far the highest clinoforms because they build into the deepest water setting. Their delta fronts have a thick succession (many 10's of m) of sandy turbidites, because of superposition on a pre-existing shelf-margin slope. If sea level falls significantly 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 basin-floor. Deltas that transit back and forth on the shelf on short time scales (10s to 100 ky) and driven largely by sea-level fluctuations are referred to here as accommodation-driven deltas. Deltas fed by large rivers can reach the shelf edge without sea-level fall, and can remain at the outer shelf (albeit with lateral shifting) for longer periods. Such deltas, most common where rates of sea-level rise are modest and shelf width narrow to moderate, are termed supply-driven deltas. These highstand deltas deposit broad, thick sandy lithosomes with stacked parasequences during their shelf transit, and usually have an extensive muddy delta front on reaching the shelf edge area. Deltas can undergo regime change and sediment budget partitioning as they transit across the shelf.