11.2 Carbonate Shelf (Nonreef) Environments
373
form predominantly warm water settings, they can accumulate also in some
co
ol-water, higher latitude environments, as mentioned. In these cool-water envi-
ronments, the carbonate sediment is composed almost entirely of the skeletal re-
mains of organisms. Cool-water assemblages of organic remains have commonly
been referred to as foramol assemblages (Lees and Buller, 1972; Jones and
Desrochers, 1992), named for the dominance of foraminifers and molluscs. They
are composed of benthic (bottom-dwelling) foraminifers, molluscs, barnacles, bry-
ozoans, and calcareous red algae. By contrast, warm-water (>�20°C) assem-
blages of organisms, called chlorozoan assemblages (named from chlorophyta
plus zoantharia corals), are dominated by hermatypic corals (corals that live pri-
marily in the photic zone) and calcareous green algae in addition to foramol com-
ponents. James (1997) suggests that heterozoan association (named for organisms
tt feed through heterotrophic means) is a more appropriate term than foramol
assemblage. He proposes to replace the term chlorozoan assemblage by photozoan
association, to emphasize the light-dependent nature of the major biotic con-
stituents.
In any case, cool-water carbonates make important contributions to the de
posits of some modern shelves (e.g., Farrow, Allen, and Akpan, 1984; Nelson,
1988; James and Clarke, 1997). These cool-water shelves range from those located
in middle to low latitude settings where cool-water currents intrude (Fig. 11.1) to
those located at high latitudes such as Spitsbergen Bank in the Barents Sea. Cool
water carbonate shelf deposits have also been reported in ancient rocks ranging in
age from Tertiary to Paleozoic on several continents, including North America,
Australia, and Europe (e.g., James and Clarke, 1997; Anastas et a!., 1998).
Wa rm-water carbonates may contain, in addition to skeletal remains, sub
stantial amounts of ooids, aggregate grains, peloids, and lime mud. Table 11.1 pro
vides a more complete list of modem warm-water and cool-water organisms and
their ancient coterparts. This table also suggests the manner which these or
ganisms contribute to the makeup of carbonate sediment. Notice the extremely
important roles that organisms play in the formation of carbonate sediment. Mod
em warm-water carbonates, especially reef carbonates, accumulate at a much
faster rate than do cool-water carbonates. the other hand, modern cool-water
carbonates appear to accumulate at about the same rate as did most ancient car
bonates (James, 1997). Reasons for the slow accumulation rates of many ancient
carbonates are poorly understood.
As stated, carbonate sediment accumulates primarily in shallow-water set
tings (Fig. 11.3). The outer shelf is commonly the highest energy environment of
the shelf. It is charactered by the development of lime sand or gravel sheets and
shoals. The middle shelf is a zone of generally low water energy, particularly on
rimmed shelves. Sediments on the middle shelf are typically poorly winnowed,
with a high ratio of micrite to skeletal fragments and other carbonate grains. The
inner shelf in most carbonate environments is also typically a low-energy, tidal
flat environment in which predominantly fine grained, tidal-flat sediments accu
mulate. some ramp platforms, however, a higher-energy nearshore zone may
be present where carbonate beaches or lime shoals develop that are composed of
skeletal fragments, ooids, pellets, and possibly intraclasts. In many cases, carbon
ate beach sands are retransported and reworked by wind to form so-called
eolianites. Numerous examples of Quaternary eolianites have been reported (e.g.,
Abegg, Harris, and Loope, 2001); however, the pre-Quateary record of carbon
ate eolinites is meager.
Some carbonate sediments are deposited in deeper water beyond the shelf
edge. Most carbonate sediment deposited in deeper water results from the fallout
of calcareous plankton (Fig. 11.3)-foraminifers, green algae (coccoliths), and tiny
gastropods. These pelagic calcareous organisms evolved mainly in Jurassic and
post-Jurassic time; therefore, deeper water pelagic carbonates are not important in