Geology Reference
In-Depth Information
Studies of mid- and high-latitudinal non-tropical car-
bonates started rather late in the 1970s, but 'temperate'
and 'cold-water' carbonates are becoming continuously
better documented (Nelson 1988; Henrich and Freiwald
1995; Rao 1996; James and Clarke 1997) and have be-
come one of the most exciting tools in carbonate re-
search. The importance of non-tropical coral reefs was
underlined by Teichert (1958). Chave (1967) pointed
out that shelf carbonates could form at all latitudes, re-
gardless of water temperature. Based on a thorough
analysis of modern shelf sediments, Lees and Buller
(1972) recognized the fundamental differences between
tropical and temperate carbonates, and the importance
of water temperature and salinity as major controls on
the composition of shelf carbonates. The authors em-
phasized the significance of 'grain association patterns'
for distinguishing between tropical and non-tropical car-
bonates. This concept provides a high potential for
evaluating the climatic and oceanographic controls on
ancient carbonates using microfacies data (Sect. 12.2).
The last twenty years have produced a wealth of pa-
pers dealing with non-tropical coastal and shelf car-
bonates in the Northern and Southern Hemisphere and
comprising a latitudinal range of 30 to 80° N and 30 to
70° S. Temperate and cold-water shelf carbonates were
studied in the northern Atlantic (northwestern France,
England, Ireland, Scotland, Norway), in the Greenland
and Barents Sea, on the Canadian Scott Shelf, from
various parts of the Mediterranean Sea (Tunisia, Sic-
ily, Adriatic Sea, Eastern Mediterranean) as well as from
Pacific regions (Western Canada, India). In the South-
ern Hemisphere extensive temperate carbonates are
forming today off Australia, Tasmania and New Zealand
on tidal flats, gulfs, shelves, platforms and ramps. The
largest modern temperate-water carbonate shelf pro-
duction occurs on the outer shelf, shelf edge and upper
slope of the southern and southeastern margin of Aus-
tralia (Boreen and James 1993). The eastern Tasma-
nian shelf is an excellent example for the deposition
and diagenesis of cool-water carbonates. These carbon-
ates have been extensively studied with regard to com-
positional, mineralogical and geochemical criteria and
are, therefore, a reference standard for recognizing and
interpreting ancient non-tropical carbonates.
Informative reviews and collections of case studies
are available (Nelson 1988; Henrich and Freiwald 1995;
James and Clarke 1997). Rao (1997) gives an excel-
lent overview on the present state of knowledge and
includes a comprehensive bibliography.
rigenous sources, (b) appropriate sea-water tempera-
tures (which may be relatively 'warm' - the Mediterra-
nean type of parts of southern Australia, or 'cool tem-
perate' - e.g. Otway shelf, southeastern Australia; Tas-
mania), (c) the relative abundance of calcareous biota,
(d) the availability of nutrients, (e) sufficient levels of
saturation of CaCO 3 in seawater (allowing cementation
and reducing dissolution), (f) high water energy for ac-
cumulating calcareous sands, and (g) the rate of accu-
mulation (approximately an order of magnitude lower
in temperate shelves than in tropical shelves). Many
non-tropical carbonates form at low CO 2 levels in re-
gions of upwelling waters, which maintain CaCO 3 satu-
ration and provide nutrients for luxuriant growth of
fauna. Lowering of sea level results in wave-base ero-
sion and redistribution of sediments from shallow to
deeper water and below the wave base.
Temperate and polar carbonates: Bioclastic carbon-
ates of shelf platforms:
These sediments are represented by moderately to
well-sorted accumulations of coarse and fine bioclas-
tic sands (e.g. bryozoan-, mollusc-bryozoan or coral-
line algal sands; Pl. 4/2, 3, 5) with minor amounts of
fines and gravels (e.g. molluscan gravel). The sands
consist mainly of bioclasts, some micritic peloids and
angular, rounded and bored intraclasts (reworked sub-
marine hardgrounds or semilithified carbonate). Ooids
are absent. Sedimentary particles are permanently re-
worked and swept basinward along the slopes, where
the grains mix with autochthonous or older particles.
Many sediments are a mixture of modern calcareous
sands and Pleistocene relict carbonates.
Australian case studies demonstrate the occurrence
of two major facies zones: Above the zone of winnow-
ing the sediment consists of coarse skeletal harsh and
some relict particles. Below, the deposits are muds, rich
in planktonic material and some benthics (Nelson et al.
1988, James and von der Borch 1991). Carbonate mud
sedimentation occurs on the temperate shelf of south-
eastern Australia (Blom and Alsop 1988). The calcitic
muds are produced by the accumulation and disinte-
gration of nannoplankton, as well as from the biodeg-
radation of skeletal grains accumulating on the sea floor.
Many high-latitude carbonate muds are bioerosional
and result from maceration of shells (Farrow and Fyfe
1988). Bioerosion down to about 25 m is intensive in
the photic zone. Shelly substrate is affected by e.g.
clionid sponges, endolithic algae, bivalves, regular echi-
noids, crabs, fish. Bioerosional products are angular
silt- and gravel-sized chips, or mud-sized disintegra-
tion products of fecal pellets.
Controls on temperate carbonates:
Major controls on the origin and composition of tem-
perate carbonates are (a) the rate of dilution from ter-
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