Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 1710
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528 MINERALS/Carbonates
subsequently modified with the introduction of terms
such as boundstone, bafflestone, and bindstone,
which are typically applied to reefal limestones. In
some cases, prefixes (e.g., skeletal grainstone) are
used to signify the dominant allochem.
Marine Carbonate Deposits
Distribution and Composition
Depending on local conditions, many different minerals can be precipitated from marine waters. Seawater contains many salts, but under conditions of
increasing temperature, CaCO3 is one of the first
compounds to be precipitated, either as calcite or
aragonite. In modern oceans, carbonate sediments
have the potential of forming anywhere and accumulating wherever the water depth is shallower than the
carbonate compensation depth (CCD) and where
there is little or no influx of siliciclastic sediments. In
areas where large volumes of siliciclastic sediments
are being introduced (e.g., where the Mississippi
River empties into the Gulf of Mexico), the carbonate
sediments, which form at a much slower rate, will be
masked to the extent that they will appear to be
non-existent. Extensive areas of carbonate sediment
accumulation are found on shallow-water shelves
(typically <30 m deep) that receive little or no influx
of siliciclastic sediment, or on isolated banks that are
surrounded by deep oceanic waters. In modern
oceans, the mineralogy of the carbonate sediment is
largely a function of the water temperature and pressure. Thus, aragonite dominates sediments forming in
shallow tropical seas whereas calcite dominates temperate waters and the deeper, cooler waters in the
tropics.
Depositional Systems
Marine carbonate sediments typically accumulate on
a platform that develops as sediments accumulate in
an area of subsidence. These depositional systems,
which commonly cover hundreds of square kilometres, are referred to as a carbonate ‘shelf’ or a
‘bank’. A carbonate shelf has a continental landmass
on one side and deep oceanic waters on the other side,
whereas a bank is completely surrounded by deep
oceanic water (Figure 6). Carbonate shelves are classified as a ramp, an unrimmed shelf, or a rimmed
shelf according to the slope of their depositional surface and the nature of their margins (Figure 6).
A ramp slopes gradually (typically <1 ) from the
shoreline to the deep ocean with no appreciable
change in slope. In contrast, rimmed and unrimmed
shelves have a distinct break in slope on their oceanic
margin (Figure 6). A rimmed shelf has islands, sand
shoals, or actively growing reefs along their oceanic
margins whereas an unrimmed shelf is devoid of such
structures (Figure 6).
The margins of many modern tropical platforms
are commonly characterized by large, robust coral
reefs, especially on the windward coasts (Figure 1A).
Spur and groove structures develop on the oceanward
side of the reef crest. Unrimmed shelves or ramps are
usually found on the leeward side of tropical islands
and in temperate zones. The presence of islands,
sand shoals, and/or reefs along the margin of a
rimmed shelf significantly dampens the strength of
any waves that encroach from the open ocean. As a
result, low-energy environments and little current
movement generally characterize the shelf. Only
major storms and hurricanes will interfere with
these tranquil settings, by generating high waves and
strong currents for short periods of time. In stark
contrast, ramps and unrimmed shelves have open
communication with the ocean and therefore receive
the full impact of onshore waves and currents.
The fact that ocean waves move freely across the
shelves with little or no impediment means that
ramps and open shelves are characterized by highenergy conditions, complex arrays of nearshore
facies, and physical transportation of sediment across
the shelf.
The ‘carbonate factory’, where most carbonate
sediment is generated, is typically located on the shallow, illuminated parts of the shelves. Conditions are
ideal for the precipitation of carbonate sediment directly from seawater and for the growth of animals
and plants that are the sources of aragonite and/or
calcite skeletons (Figure 2F, G, and H). Most of
the sediment stays in or close to the place where it
formed. Nevertheless, storm-generated waves and
currents can move sediment shoreward onto the
Figure 6 Schematic diagram showing differences between rimmed and unrimmed shelves, a ramp, and a bank.
