Geology Reference
In-Depth Information
temperate, and in polar cold-water environments (Fig.
2.9). The warm-temperate zone running roughly paral-
lel to the equator at latitudes of approximately 30° S
and 30° N forms the link between the tropical zone and
the cool-temperate zone. Basic differences between
tropical and non-tropical shelf carbonates relate to biota
and diagenetic patterns as well as to the composition
of grain associations.
warm-temperate and cold-temperate carbonates (Brook-
field 1988). The predominantly siliceous polar hyalo-
sponge sediments lie beyond the 5 °C isotherm, often
in areas with semipermanent or permanent ice cover
(Henrich et al. 1992).
Photozoan and Heterozoan Associations
James (1997) accentuating the light dependence/in-
dependence of benthic carbonate-producing organisms
introduced two major association categories:
Grain association analysis
Grain associations (sometimes also called grain as-
semblages) are characterized by the dominating skel-
etal and non-skeletal grains. Ooids, aggregate grains
and peloids are abundant constituents of warm-water
carbonates, but are absent in cool-water carbonates ex-
cept for the peloids. The abundance and association of
skeletal grains are controlled by the dependence of
benthic organisms on environmental factors discussed
in the preceding sections. The composition of skeletal
grain associations is influenced by many factors includ-
ing light, water temperature, salinity, and the rate of car-
bonate production versus the rate of siliciclastic input.
Lees and Buller (1972) and Lees (1975) tied the
composition of modern low- and high-latitude shelf
carbonates to temperature and salinity. The authors in-
troduced the term
chlorozoan
skeletal association for
tropical carbonates composed of calcareous green al-
gae (
Chloro
phyta) and hermatypic corals (
Zoan
tharia).
The name
chloralgal
refers to tropical carbonates domi-
nated by
chloro
phycean
alg
ae. The term
foramol
skel-
etal association was introduced for non-tropical car-
bonates consisting predominantly of
fora
minifera and
mol
lusks. The term has proven to be less useful be-
cause of the wide compositional spectrum of non-tropi-
cal cool-water sediments. As a result, other expressions
have come in use, partly based on the investigation of
recent sediments (e.g. bryomol; Nelson 1988), partly
on studies of Tertiary (Hayton et al. 1995), Cretaceous
(Carannante et al. 1988) and Late Paleozoic (Beauchamp
1994; Beauchamp and Desrochers 1997) shelf lime-
stones.
The original concept favoring water temperature
and, to a lesser content, salinity as the main controls on
the global distribution of chlorozoan (chloralgal) and
foramol associations draws the boundary between both
associations at the 15 °C to 18 °C
or more generally at
the 20 °C annual isotherm, which parallels the tropical
zone in most parts of the world, except in areas affected
by Coriolis-driven cold-water currents. In these areas,
upwelling of nutrient-rich cold water favors the domi-
nance of autotrophs (e.g. calcareous red algae) over
heterotrophs. A significant impoverishment in the
foramol association at the 10 °C and 5 °C isotherms
leads to differentiation of temperate carbonates into
• The
Photozoan Association
is characterized by an
association of benthic carbonate grains including
(1) skeletons of light-dependent organisms (algae; in-
vertebrates, e.g. scleractinian corals and foraminifera,
containing photosymbionts), and/or (2) non-skeletal
grains (ooids, peloids and others), and (3) skeletons
from the Heterozoan Association. The term replaces
the term chlorozoan association and covers the chlor-
algal as well as many association types based on an-
cient limestones (see Box 12.9). It reflects the exist-
ence of shallow, warm-water, benthic calcareous com-
munities and their resulting sediments, today mostly
confined to tropical and subtropical settings.
• The
Heterozoan Association
consists of benthic
carbonate particles produced by (1) organisms that are
light-independent, and plus or minus (2) calcareous red
algae. The Heterozoan Association comprises many
grain associations established for Cenozoic shelf car-
bonates (Box 12.9). It occurs from the poles to the equa-
tor and from the intertidal zone to the deep ocean.
Cool-water carbonates are always heterozoan. But
the Heterozoan Association does not necessarily indi-
cate cool-water sediments! Heterozoan associations
predominate where the supply of other sediments
(photozoan, pelagic or siliciclastic) is limited by envi-
ronmental constraints.
In warm water the Heterozoan Association charac-
terizes environments below the photic zone. But hetero-
zoan grain associations can also be produced in the
photic zone as demonstrated by the Mediterranean Sea,
where heterozoan-type organisms living on sea grass
form carbonates in warm-temperate settings (Betzler
et al. 1997).
The photozoan and heterozoan concept is useful in
tracing major and prolonged paleoclimate trends and/
or paleolatitudinal shifts. A rigorous use of the con-
cept, however, can mask differences better indicated
by the individual grain associations listed in Box 12.9.
Grain association types
Box 12.9 summarizes the criteria of grain associa-
tion types of modern and ancient shelf carbonates. All
