Page 127 - Carbonate Sedimentology and Sequence Stratigraphy

118 WOLFGANG SCHLAGER

the highstand input with the possible exception of unusual lower content of metastable minerals are also more prone
breccia bodies (see below). The postulated apron-building to lowstand flushing. Lowstand shedding of Cenozoic cool-
around Pacific atolls during lowstands of sea level has not water carbonates has been described by Driscoll et al. (1991).
withstood close scrutiny (Thiede, 1981; Dolan, 1989). Low- The antagonistic behavior of carbonates and siliciclastics
stand input is compositionally different and can be recog- commonly results in reciprocal sedimentation with high-
nized by petrographic analysis (see below). stand bodies of carbonates and lowstand wedges of silici-
clastics (Delaware Basin – Wilson, 1975; Canning Basin -
The limitations of highstand shedding can be deduced Southgate et al., 1993).
from its causes. The difference between highstand and low- Recently, sea-level studies of platform tops have been
stand production depends on the hypsography of the plat- complemented by compositional analysis of platform-
form. Flat-topped, rimmed platforms with steep slopes derived turbidites on the platform flanks. The depositional
show more pronounced highstand shedding than platforms environment of these calciturbidites is below the range of
with gentle slopes (Fig. 7.17). Besides platform morphology, sea-level fluctuations such that sedimentation is not inter-
the duration of sea-level cycles is important. When sea-level rupted during lowstands. Highstand turbidites differ not
cycles are long (e.g. millions of years) and flanks gentle, the only in abundance but also in composition from their low-
platform can build a lowstand wedge that partly substitutes stand counterparts as they contain more ooids, pellets, and
for the production area lost on the platform top (Fig. 7.17). grapestones - grains that require flooded bank tops for their
Finally, lithification and resistance against lowstand erosion formation. This has been well documented for Pleistocene
vary with latitude and possibly with age. Cool-water car- turbidites in the Bahamas (Fig. 7.16). Reijmer et al. (1991;
bonates are less prone to submarine lithification than their 1994) report on a Triassic example, Everts (1991) on a Ter-
tropical counterparts (Opdyke and Wilkinson, 1990). Lithi- tiary one.
fication upon exposure, too, is reduced because of the low Carbonate petrography reveals not only changes in the
content of metastable aragonite and magnesian calcite. It spectrum of contemporary grains. Lithoclasts derived from
is possible that Paleozoic carbonates with their generally erosion of older, lithified parts of the platform are easily rec-
250 500 250 500 125 500
0 50 100 0 50 100 0 50 100
Fig. 7.16.— Highstand bundling and
compositional signals in calciturbidites.
core depth in m 1 2 4 | 2 4 | Quaternary cores from Tongue of the
1
1
Ocean, a basin surrounded by the Great
dant during interglacial highstands, when
2 Bahama Bank. Turbidites are most abun-
2 |
4 bank tops are flooded and produce sedi-
5
ment; turbidites are thin and scarce dur-
5 ing glacial lowstands. Turbidites also
vary in composition: Highstand layers
6
are rich in pellets and ooids - i.e. grains
4
that form on the shallow banks by the
interaction of tidal currents and winnow-
ing from waves. Lowstand turbidites con-
5
sist mainly of skeletal material, including
6
reef detritus because fringing reefs and
7
6 skeletal sand can migrate downslope
with falling sea level. Glacial-interglacial
stratigraphy is provided by variation in
7
aragonite/calcite ratio, a property that is
closely correlated with the oxygen iso-
6
tope curve. After Haak and Schlager
8
8
(1989).
8
aragonite content 9
nonskeletal/skeletal ratio
(thin line)
(heavy line)
isotopic stages 9
grainsize in microns ungraded turbidite 10
lithology
graded turbidite
periplatform ooze