DEFINITION OF CARBONATE

A sediment or sedimentary rock containing the

carbonate ion (CO3

-2). It may be formed by the organic or

inorganic precipitation from aqueous solutions in warm

waters. Typical carbonate rock types include limestone and

dolostone, most of which are deposited on shallow marine

platforms, mounds, or in reefs, where there is a lack of siliciclastic

sediment input.

Carbonate platforms form most commonly along subsiding

passive margins, where sedimentation or reef growth

keeps up with the rate of subsidence, maintaining shallow

water depths. The platforms grow laterally by shedding carbonate

sediment into the adjacent basin until the water

depths are shallow enough for the platform or reef to grow

outward over the slope deposits. At times of high eustatic sea

level, carbonate muds may cover vast areas of the continents,

forming shallow inland seas that can accumulate tens or even

hundreds of meters of carbonate sediments. Carbonate reefs

may also form fringing complexes around volcanic atolls.

Several types of carbonate platforms are distinguished.

Carbonate ramps are gently seaward-sloping surfaces with no

marked break between shallow and deep water. These typically

have carbonate sands and oolitic deposits formed near

the continent and fringing reefs, grading seaward into skeletal

sands, muddy sands, and mud. Rimmed shelves have sharp

breaks between shallow and deep water, with a high energy

facies near the slope/shelf break. The shelves are typically

covered by carbonate sands, muddy sands, and muds, with

isolated patch reefs. The shelf edge may develop extensive

reef complexes (such as the Great Barrier Reef) that are fed

by nutrient-rich upwelling water and shed reef debris into the

adjacent basin during storms.

Reefs are framework supported carbonate mounds built

by carbonate-secreting organisms, whereas carbonate mounds

are general positive features on the seafloor built by biological

processes. Reefs contain a plethora of organisms that

together build a wave-resistant structure and provide shelter

for fish and other organisms. The spaces between the framework

are typically filled by skeletal debris, which together

with the framework becomes cemented together to form a

wave-resistant feature that shelters the shelf from high-energy

waves. Reef organisms (presently consisting mainly of

zooxanthellae) can only survive in the photic zone, so reef

growth is restricted to the upper 328 feet (100 m) of the seawater.

Carbonate mounds differ from reefs in that they do

not have a rigid framework but are formed by sedimenttrapping

processes such as those of sea grass, algae, and

other organisms.

Carbonates, carbonate platforms, and reefs are represented

in all geological eras from the Archean to the present,

although the organisms that have constructed these similar

morphological features have changed through time. Carbonates

are the primary source rocks for the world’s hydrocarbon

deposits.

See also ATOLL; CARBONATE MINERALS; CARBON CYCLE;

DOLOMITE; LIMESTONE; PASSIVE MARGIN; REEF; SEDIMENTARY

ROCKS.

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