Geology Reference
In-Depth Information
Fig. 16.24. Reef thickness. Mean thickness values of reefs were calculated from the ratio of the sum of thickness values for
all reefs of a stage divided by the number of reef sites. The interval values were translated into metric values to produce
expressive means: Each point in the graph corresponds to a stage in the figure. Mean thickness was moderate in the Early and
Middle Cambrian and in the Early to early Middle Ordovician reefs and rose sharply in the Ashgillian. The trend towards
thicker reefs up in the Late Devonian is interrupted by several setbacks. Mean thickness increased in the Givetian and
reached a maximum in the Frasnian, followed by a decline during the Carboniferous with a significant drop in the Early
Pennsylvanian. A pronounced and continuous increase took place until the Artinskian, followed by a decrease during the
Middle and Late Permian and a significant reduction in the Scythian microbial reefs. A prominent peak is shown by the
coralline sponge-microbe reefs of the Ladinian. Subsequent Triassic stages are marked by descreasing mean thickness val-
ues. After a sharp decline in the Hettangian, moderately thick reefs developed during the Early Jurassic and Aalenian.
Thickness rose sharply from the Callovian to the Kimmeridgian and declined from the Tithonian to the Hauterivian. In the
Cretaceous moderate thickness values were achieved only from the Barremian to the Cenomanian, whereas later Cretaceous
stages are characterized by low thickness. Tertiary reefs exhibit fluctuating mean thickness values and a distinct trend to-
wards thicker reefs in the Neogene. Modified from Kiessling (2002).
shows that there is no overall trend in reef thickness
during the Phanerozoic, but that stages with great mean
reef thickness are commonly followed by stages with
minor reef thickness. Maximum reef thickness values
in the Frasnian and Artinskian are due to stacked reef
mounds and reefs and may be biased by the high per-
centage of subsurface reefs included in the calculation.
Subsurface reef studies tend to overestimate reef thick-
ness because of the inclusion of non-reefal carbonates
in the reef volume, or because the 'reefs' are more likely
to seismic reefs.
peak occurs in the Late Oligocene to Neogene inter-
val. Smaller highs in reef debris production occur in
the late Early to Late Silurian, the Middle Permian to
Late Triassic, and the mid-Cretaceous.
16.7.1.3 Pelagic Carbonates
Pelagic carbonate production today is greater in terms
of mass than shallow-water carbonate production
(Milliman and Droxler 1986), but the sedimentary
record of pelagic carbonates is poor in pre-Jurassic rocks
(Kuznetsov 2000). Paleozoic pelagic carbonates depos-
ited in rather shallow water depths are represented by
cephalopod and tentaculitid limestone (Sect. 15.8.2.1).
The primary locus of global limestone deposition
has gradually shifted from shallow cratonic to deep
oceanic settings since the Jurassic. This change is gen-
erally coincident with the appearance of planktonic
foraminifera and the diversification of coccolitho-
phorids (Boss and Wilkinson 1991).
The Jurassic planktonic foraminifera (Pl. 69/12)
show a dramatic evolutionary radiation throughout the
Cretaceous and Cenozoic (Pl. 73) and have been im-
portant components of the oceanic plankton ever since.
Reef debris. Many ancient reef complexes consist
predominantly of debris, whereas the proportion of in-
situ organic framework can be very low. Because reefal
debris may be porous and can contain significant hy-
drocarbon accumulation, knowledge of secular varia-
tions in the amount of reef debris is important. The
calculation of debris production in Phanerozoic reefs,
taking in consideration reef abundance, reef sizes and
debris potential demonstrates extreme variations be-
tween times of high and low debris production
(Kiessling 2002). The two largest peaks are evident in
the Givetian to the Famennian time interval and the
latest Middle Jurassic to Late Jurassic interval. A third
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