Page 249 - Sedimentology and Stratigraphy

236 Shallow Marine Carbonate and Evaporite Environments

sea, which is itself an important ecosystem and two or three orders of magnitude thicker (Bernoulli &
depositional setting. In open ocean areas coral atolls Jenkyns 1974).
develop on localised areas of shallow water, such as
seamounts, which are the submerged remains of
volcanic islands. In addition to these settings of reef 15.3.3 Carbonate mud mounds
formation, evidence from the stratigraphic record
indicates that there are many examples of patch A carbonate mud mound is a sediment body con-
reefs, localised build-ups in shallow water areas sisting of structureless or crudely bedded fine crystal-
such as epicontinental seas, carbonate platforms and line carbonate. Modern examples of carbonate mud
lagoons. mounds are rare, so much less is known about the
controls on their formation than is the case for reefs.
From studies of mounds of fine-grained carbonate in
Reefs as palaeoenvironmental indicators the rock record (e.g. Monty et al. 1995) it appears
that there are two, possibly three types. Many
Present-day reefs are mainly in tropical seas, occur-
mounds are made of the remains of microbes that
ring up to 35˚ latitude either side of the Equator. It
had calcareous structures and these microbes grew
is therefore tempting to apply this observation to
in place to build up the body of sediment. Others have
the sedimentary record and conclude that if a car-
a large component of detrital material, again mainly
bonate reef body is found it indicates an environ-
the remains of algae and bacteria, which have been
ment of deposition in warm tropical waters. This
piled up into a mound of loose material. It is also
assumption can be made only with certain caveats. possible that some skeletal organisms such as calcar-
First, other reef-builders live in different environ- eous sponges and bryozoans are responsible for build-
ments: in modern seas coralline algae build reefs at ing carbonate mud mounds. They appear to form in
higher latitudes and the environmental tolerances of deeper parts of the shelf than reefs, but within the
extinct taxa are not fully known. Second, even if the photic zone. Cementation of the mud requires circula-
reef is made of corals, then it must be remembered tion of large amounts of water rich in calcium carbo-
that pre-Mesozoic groups, the Rugosa and Tabulata, nate, a process that is not well understood.
may not have had the symbiotic relationship with
algae that is such a distinctive feature of the Mesozoic
to present-day Scleractinia corals, and their distribu- 15.3.4 Outer shelf and ramp carbonates
tion in the seas was more controlled by the availabil-
ity of nutrients.
On the outer parts of shelves carbonate sedimentation
is dominated by fine-grained deposits. These carbo-
nate mudstones are composed of the calcareous
Cessation of reef development
remains of planktonic algae (3.1.3) and other fine-
The growth of coral reefs can normally keep pace with grained biogenic carbonate. This facies is found in
both tectonic subsidence and global sea-level rise. The both modern and ancient outer platform settings and
cessation of reef development is therefore usually due when lithified the fine-grained carbonate sediment is
to changes in environmental conditions, such as an called chalk. Similar facies also occur in deeper
increase in the flux of terrigenous clastic material or a water settings (16.5.1). Chalk deposited in shallower
change in the nutrient supply. When this occurs the water may contain the shelly remains of benthic and
dominant facies formed is fine-grained pelagic mate- planktonic organisms and there is extensive evidence
rial, which is similar in character to deep-sea pelagic of bioturbation in some units (Ekdale & Bromley
carbonates (16.5.1). Pelagic carbonate sedimentation 1991). Chert nodules within the beds are common
is considerably slower than shallow-marine accumu- in places, the result of the redistribution of silica
lation rates resulting in much thinner layers in a from the skeletons of siliceous organisms. Bedding is
given period of time. Successions deposited under picked out by slight variations in the proportions of
these conditions are known as condensed sections clay minerals, which occur in most chalk deposits, or by
and they may have as many millions of years of variations in the degree of cementation. Deposits of this
accumulation in them as a shallow-water deposit type may be found in strata of various ages, particularly