Page 158 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 8: SEQUENCE STRATIGRAPHY OF C AND M FACTORIES 149

summarizes work by Reeckmann and Gill (1981) on Qua- bonate. This zone of Holocene erosion and reworking sep-
ternary calcarenites in southern Australia. After 10 ky, only arates a coastal zone of deposition from an outer-shelf-and-
few percent of calcite cement had formed and the sediment slope zone of deposition. James (1997) notes furthermore
was well cemented after 60 ky of exposure. In southern Aus- that at least part of this shaved shelf was also bare during
tralia, too, James and Bone (1989) showed that calcarenites the Pleistocene sea-level falls.
virtually devoid of primary aragonite are still loose or friable
after 10 My of exposure, whereas an overlying unit with 20-
Geometry of systems tracts
30% of primary aragonite content is now well cemented and
the aragonitic shells have been replaced by molds.
C-factory sequences resemble siliciclastic sequences with
A transgressive surface or transgressive interval is fre- rounded shelf breaks and gentle slopes. There are two im-
quently well developed on modern cool-water shelves. portant exceptions to this general similarity: C-factory sys-
Beautiful examples of this phenomenon occur in southern tems tracts lack point sources of sediment input from rivers
Australia (Fig. 8.4; James, 1997): The shelves are hundreds of and they have the ability to build seismically recognizable
kilometers wide and have shelf breaks far below 100 m. The reefs, albeit not at sea level but at greater depth. Morpho-
large swell generated in the roaring fourties of the south- logically, the C-factory reefs have convex tops because they
ern ocean moves sand at depths of 100 m and more. Car- are not planed off by wave action. This difference to the flat,
bonate production extends from the shoreline to the upper wave-breaking reefs of the T factory can be resolved seismi-
slope but the middle part of this C factory is what James et cally. Unfortunately, tropical systems that slowly drown and
al. (2001) called a "shaved shelf" - a zone covered with relict subside below the zone of wave action also develop convex
Pleistocene material but virtually devoid of Holocene car- tops (Figs 7.21, 7.23).

A)
LOWSTAND Fig. 8.4.— Transgression and
regression on Australian shelves
of the C factory. Slow marine
lithification leads to intensive re-
working of sediment, particularly
zone of wave abrasion
storm on ocean-facing shelves where
wave base erosion, waves that move sand at 100 m
abrasion
& cementation depth are common. The result is a
broad zone of non-deposition that
autochthonous +
allochthonous is being reworked and abraded
sedimentation during both transgressions and
regressions. After James (1997),
B) modified.
TRANSGRESSION & REGRESSION

zone of wave abrasion
storm erosion,
wave base abrasion
autochthonous +
allochthonous & cementation
sedimentation

C)
HIGHSTAND
zone of wave abrasion
storm
wave base autochthonous +
progradational allochthonous
wedge sedimentation