Page 159 - Carbonate Sedimentology and Sequence Stratigraphy

150 WOLFGANG SCHLAGER

Sediment reworking is fast and effective since cementa- New-Zealand shelf. These authors found that sediment ex-
tion is slow in both the terrestrial and the marine envi- port from the shelf was high during the last glacial lowstand
ronment. As in siliciclastic sequences, significant portions and has virtually ceased in the Holocene highstand. On the
of the highstand systems tract may be shaved off during Eucla shelf of southern Australia, Saxena and Betzler (2003)
sea-level fall such that the highstand tract has to be recon- found that lowstand sedimentation had a distinct maximum
structed from the preserved parts of the prograding clino- on the upper slope whereas sedimentation of transgressive
forms( Feary and James, 1998; Saxena and Betzler, 2003; for and highstand tracts was more uniform; they also showed
siliciclastic analogues see Anderson et al., 2004). that lowstand erosion largely removed the shallow (most
One of the best examples of sequence stratigraphy in characteristic) portion of the transgressive tract and high-
C carbonates is the Neogene – Quaternary Eucla shelf off stand tract. The cumulative result of several cycles is a thick
southern Australia (Fig. 8.5; Feary and James, 1995; 1998; shelf-margin wedge that thins both downslope and shore-
James, 1997). Unconformity-bounded sequences are well ward (Fig. 8.6).
defined in the high-sedimentation area of the outer shelf and
Sequence stratigraphy of the C factory in deep-water
upper slope. Bounding surfaces converge and partly merge environments
on the middle shelf where reworking and non-deposition
are most intensive. A second high-sedimentation belt is in One of the most profound differences between the T fac-
the coastal zone. tory on the one hand and the C and M factory on the other
is the depth window of production. The window of the T
Highstand shedding or lowstand shedding? factory is extremely narrow and restricted to the sunlit part
of the ocean. The other two factories can produce in a much
The standard model of sequence stratigraphy postulates wider depth range. The window of the C factory certainly
that the influx of sediment to the slope and basin is high extends to abyssal depths: production is virtually indepen-
during lowstands of sea level when shelf accommodation dent of water depth and temperature. It may commence
is small and rivers discharge their load on the outer shelf. wherever currents sweep the sea bed, remove the fines and
We have seen that the sequences of the T factory approx- provide sufficient nutrients for benthic growth.
imately swing in antiphase to this model (p. 116f). Trop- The depth-indepence of production has profound impli-
ical platforms export most sediment during highstands of cations for sequence stratigraphy. The siliciclastic standard
sea level when the production area is large, and waves and model rightly proceeds on the premise that any sediment ac-
tides can move the sediment efficiently to the margin. Ex- cumulations in the deep sea must somehow be connected to
port during lowstands is small because the production area a terrestrial source, and that the succession of environments
is reduced and rapid cementation of exposed sediment pre- from shore to abyss represent one system. This premise also
cludes lowstand flushing, i.e. extensive mechanical erosion holds for the T factory, albeit with the modification that the
of sediment from the preceding highstand. source is in the shallowest part of the sea.
The effects that cause highstand shedding in the T factory For the C factory, the concept of a connected system link-
are small or absent in the C factory as the following analysis ing the shore with the deep sea need not be valid at all.
shows. There are linked environments, to be sure. Canyon-fan sys-
➤ The depth window of production is 5 – 10 times wider tems exist even though well-documented examples are rare.
than that of the T factory. Consequently, the width One example is the Tertiary of the Gippsland Basin where
of the production window exceeds the amplitude of major canyons were filled with redeposited C-carbonates
nearly all eustatic fluctuations in the sequence-stratigra- from the shelf (Wallace et al., 2002). However, there are other
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phically relevant time domain of 10 –10 years. areas where the C factory produces large volumes of carbon-
➤ The production area directly affected by sea-level cycles ate material at bathyal and abyssal depths without any con-
is a steadily seaward-dipping shelf with a round shelf nection to shoal-water. One example is the northern North
break. Thus, the difference in production area between Atlantic (De Mol et al., 2002). Part of this deep production
highstands and lowstand is much smaller. (It is nil on a accumulates in reefs or mound-shaped buildups, another
perfectly planar epeiric ramp). part is transported by deep-sea currents and remodeled into
➤ Cementation in the meteoric and the marine environ- sediment drifts, often mixed with terrigeneous and pelagic
ment is slow because seawater is less supersaturated sediment.
with calcium carbonate and there is less metastable Stratigraphic sequences can be recognised in these envi-
aragonite to dissolve. ronments if one defines them as conformably layered pack-
Based on these considerations one expects the C factory ages bounded by unconformities as proposed by Vail et al.
to differ from the T factory and closely obey the standard (1977, p. 55). The link of sediment anatomy with sea level
model. is more subdued than on the shelf but the stratigraphic con-
The question of highstand or lowstand shedding in cool- trol may be excellent if sufficient plankton is preserved. In
water carbonates has not been studied as extensively as the Quaternary, for instance, pelagic sediments intercalated
in tropical carbonates. Nelson et al. (1982) confirmed in cool-water carbonates may allow one to correlate them
lowstand-shedding for the most recent glacial cycle on a with eustatic highstands and lowstands (e.g. Passlow, 1997).