Td. Boreen et Np. James, HOLOCENE SEDIMENT DYNAMICS ON A COOL-WATER CARBONATE SHELF - OTWAY, SOUTHEASTERN AUSTRALIA, Journal of sedimentary petrology, 63(4), 1993, pp. 574-588
The Otway Shelf is covered by cool ocean waters and veneered by bryozo
an-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 high-stan
d (60-26 ky) are rare, either never having been deposited or having be
en removed during the following sea-level fall (26-17 kv). During the
subsequent lowstand the shelf was exposed, facies shifted basinward, a
nd beach/dune complexes were constructed near the shelf edge. The deep
shelf was characterized by nondeposition and hardground formation, an
d the shelf margin became locally erosional. Upper-slope bryozoan/spon
ge assemblages continued to grow actively, and lower-slope foraminifer
a and nannofossil ooze was increasingly enriched in hemipelagic terrig
enous mud swept off the wide shelf. Coarse shelf debris and lowstand d
une sands were erosively reworked and transported onto the upper slope
and redistributed to deep-slope aprons during early transgression (17
-10 kv). This event locally terminated shelf-edge bryozoan and sponge
growth. Continued erosive shoreface retreat formed a bored and cemente
d 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 o
uter-shelf sediment accumulation began by 7 ky. Sea level stabilized n
ear its present level by 6.5 ky and the area is now partitioned into t
hree zones by an energetic wave and swell regimen: (1) inboard, protec
ted embayments and/or beach/dune complexes with high sediment accumula
tion rates (about 100 cm/ky), (2) an open shelf where, although sedime
nt production is active, accumulation is patchy (3-5 cm/ky) to nonexis
tent within the zone of wave abrasion, and minimal (about 23 cm/ky) be
tween 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 gro
wing at rates of 105 cm/ky. Accumulation decreases progressively down
slope with deep-slope, pelagic carbonate sedimentation at rates of 5 c
m/ky. The late Quaternary shelf record resembles that of flat-topped,
warm-water platforms with Holocene sediment overlying Pleistocene/Tert
iary limestone, but for different reasons. Much of this high-energy sh
elf is still within ''lag depth'', more than 10 ky after flooding. The
''steady-state'' deep-water, shelf-margin bryozoan factory has been l
argely unaffected by 50-100 m eustatic changes in sea level, and has c
ontributed to slow but continuous platform progradation. The slow grow
th 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 c
arbonate system an excellent modern analog for interpretation of many
ancient ramp successions.