Sea-level fall below the shelf edge, without basin-floor fans

Abstract

A fall of sea level to below the shelf edge (formation of type 1 sequence boundary) increases greatly the likelihood of sand delivery into deep-water basin-floor areas, although an unusually high sediment supply can bring significant volumes of sediment to the shelf edge at any time. An Eocene (Ypresian) data set from seismic- scale, mountainside outcrops of shelf-slope clinoforms illustrates that a fall of sea level below the shelf edge produces significant sand accretion on the shelf margin, but brings no sand out onto the basin floor. Fall and subsequent rise of relative sea level are documented by the style and topographic position of successive shelf edge-to-slope clinoforms, each representing a shelf-edge delta with its greatly extended slope apron of turbidite lobes, sheets, and chutes. A downslope shift of clinothems at the shelf edge and a headward cannibalization of the oldest perched-slope deltas are the criteria for recognition of (1) the time interval of sea-level fall and (2) the position of sea level below the shelf edge. The lowest sea level is recorded by the most basinward clinothem set with a high density of erosive turbidite chutes at its upper surface (the sequence boundary). Wave-eroded terraces at its updip (near shelf edge) end identify the shoreline position at lowest sea level. Aggradation and landward stepping of clinothems reflect subsequent rise of relative sea level. The fall-to-rise configuration and, especially, the occurrence of peak erosion at the time of lowest sea level are compelling evidence for placing the type 1 sequence boundary at the fall-to-rise turnaround and not along the surface onto which the clinothems downlap. The excellent exposure downdip as well as along strike of the clinoform complex demonstrates the absence of sand accumulation beyond the base of slope. The slope turbidites were produced from dilute, nonignitive flows, and caused significant shelf-margin accretion. The data show that such slopes can be extremely sand prone, and that sea-level fall below the shelf edge does not necessarily result in basin-floor sand deposits beyond the base of slope, particularly where canyons have not been eroded into the slope.