Page 239 - Sedimentology and Stratigraphy

226 Shallow Marine Carbonate and Evaporite Environments

environment, principally in places where reef forma- regions the rainfall, and hence the run-off, is very
tion strongly controls the distribution of energy low, which means that there is little transport of
regimes. Finally, the production of carbonate material sediment to the sea by rivers.
by organisms is rapid in geological terms, and occurs
at rates that can commonly keep pace with changes
Shallow marine waters
in water depth due to tectonic subsidence or eustatic
sea-level rises: this has important consequences for Biogenic carbonate production is inhibited by the
the formation of depositional sequences (Chapter 23). presence of clastic material so the areas of low input
of detritus are potential sites for carbonate deposition.
Under favourable conditions, the amount of biogenic
15.1.1 Controls on carbonate sedimentation carbonate produced in shallow seas is determined by
the productivity within the food chain. Photosyn-
Areas of shallow marine carbonate sedimentation are thetic plants and algae at the bottom of the food
known as carbonate platforms. They can occur in a chain are dependent on the availability of light, and
wide variety of climatic and tectonic settings provided penetration by sunlight is controlled by the water
that two main conditions are met: (a) isolation from depth and the amount of suspended material in the
clastic supply and (b) shallow marine waters. The sea. Relatively shallow waters with low amounts of
types of carbonate grains deposited and the facies suspended terrigenous clastic material are therefore
they form are mainly controlled by climatic condi- most favourable and in bright tropical regions with
tions and they have varied through time with the clear waters this photic zone may extend up to 100 m
evolution of different groups of organisms. The places water depth (Fig. 15.1) (Bosscher & Schlager 1992).
where carbonate platforms occur are determined by Photosynthetic organisms typically flourish in the
tectonic controls on the shape and depth of sedimen- upper 10 to 20 metres of the sea and it is in this
tary basins: tectonic subsidence factors also strongly zone that the greatest abundance of calcareous organ-
influence the stratigraphy of successions on carbonate isms is found. This shallow region of high biogenic
platforms (Bosence 2005). Patterns of depositional productivity is referred to as the carbonate factory
sequences are also affected by sea-level fluctuations (Tucker & Wright 1990). Increased or reduced sali-
(Chapter 23). nity inhibits production and the optimum tempera-
ture is around 20 to 25˚ C. Hermatypic corals
dependent on symbiotic algae are most productive in
Isolation from clastic supply
shallow clear water with strong currents, while most
The primary requirement for the formation of carbon- other benthic marine organisms prefer quieter waters.
ate platforms is an environment where the supply of
terrigenous clastic and volcaniclastic detritus is very
low and where there is a supply of calcium carbonate.

Clastic supply to shallow marine environments can be
limited by both tectonic and climatic factors. Most
terrigenous sediment is supplied to shallow seas by
rivers, and the pathways of fluvial systems are con-
trolled by the distribution of areas of uplift and sub-
sidence on the continents. On most continents the
bulk of the drainage is concentrated into a small
number of very large rivers that funnel sediment to
coastal deltas. Along coastlines distant from these
deltas the clastic supply is generally low, with only

relatively small river systems providing detritus. This
allows for quite extensive stretches of continent to be
areas that receive little or no terrigenous sandy or Fig. 15.1 The relationship between water depth and
muddy sediment. The climate of the continent adja- biogenic carbonate productivity, which is greatest in the
cent to the shelf also has an important effect. In desert photic zone.