ABSTRACT

The Otway Shelf is covered by cool ocean waters and veneered by bryozoan-dominated carbonate sediments. Radiocarbon dating and stratigraphy of shelf vibracores and slope gravity cores document late Pleistocene/Holocene deposition. Shelf sediments of the late Pleistocene highstand (60-26 ky) are rare, either never having been deposited or having been removed during the following sea-level fall (26-17 ky). During the subsequent lowstand the shelf was exposed, facies shifted basinward, and beach/dune complexes were constructed near the shelf edge. The deep shelf was characterized by nondeposition and hardground formation, and the shelf margin became locally erosional. Upper-slope bryozoan/sponge assemblages continued to grow actively, and lower-slope foraminifera and nannofossil ooze was increasingly enriched in hemipelagic terrigenous mud swept off the wide shelf. Coarse shelf debris and lowstand dune sands were erosively reworked and transported onto the upper slope and redistributed to deep-slope aprons during early transgression (17-10 ky). This event locally terminated shelf-edge bryozoan and sponge growth. Continued erosive shoreface retreat formed a bored and cemented ravinement surface, mantled by bivalves and lithoclasts, across the open shelf. Base-level rise reduced the amount of terrigenous material reaching the continental slope. Slower transgressive flooding from 10 to 6.5 ky allowed infilling of shallow embayments on the shelf, and outer-shelf sediment accumulation began by 7 ky. Sea level stabilized near its present level by 6.5 ky and the area is now par itioned into three zones by an energetic wave and swell regimen: (1) inboard, protected embayments and/or beach/dune complexes with high sediment accumulation rates (about 100 cm/ky), (2) an open shelf where, although sediment production is active, accumulation is patchy (3-5 cm/ky) to nonexistent within the zone of wave abrasion, and minimal (about 23 cm/ky) between maximum abrasion depth and swell wave base, and (3) a deep shelf edge and upper slope where sediment is accumulating at rates of 2-50 cm/ky. Interpreted deep-water (> 100 m) bryozoan/sponge mounds are growing at rates of 105 cm/ky. Accumulation decreases progressively down slope with deep-slope, pelagic carbonate sedimentation at rates of 5 cm/ky.

The late Quaternary shelf record resembles that of flat-topped, warm-water platforms with Holocene sediment overlying Pleistocene/Tertiary limestone, but for different reasons. Much of this high-energy shelf is still within "lag depth", more than 10 ky after flooding. The "steady-state" deep-water, shelf-margin bryozoan factory has been largely unaffected by 50-100 m eustatic changes in sea level, and has contributed to slow but continuous platform progradation. The slow growth potential, uniform profile of sediment production and distribution, and inability of constituent organisms to construct rigid frameworks favor maintenance of a shallow ramp profile and makes the cool-water carbonate system an excellent modern analog for interpretation of many ancient ramp successions.