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Carbonate ramps

The bathymetric profile of a carbonate ramp and the physical processes within the sea and on the sea floor

are very similar to an open shelf with clastic deposition. The term ramp may give the impression of asignificant slope but in fact the slope is a gentle one of less than a degree in most instances (Wright &

Burchette 1996), in contrast to slope environments associated with rimmed shelves, which are much

steeper. Modern ramps are in places where reefs are not developed, such as regions of cooler waters,increased salinity or relatively high input of terrigenous clastic material. However, in the past carbonate

ramps formed in a wider range of climatic and environmental settings, especially during periods when

reef development was not so widespread. In macro- to mesotidal regimes tidal currents distribute

carbonate sediment and strongly influence the coastal facies. Wave and storm processes are dominant in

microtidal shelves and seas. The effects of tides, waves and storms are all depth-dependent and ramps can

be divided into three depth-related zones: inner, mid- and outer ramp.

Distribution of facies on a carbonate rampThe inner ramp is the shallow zone that is most affected by wave and/or tidal action. Coastal facies along

tidally influenced shorelines are characterized by deposition of coarser material in channels and carbonate

muds on tidal flats. Wave-dominated shorelines may have a beach ridge that confines a lagoon or a linear

strand plain attached to the coastal plain. Ramps with mesotidal regimes will show a mixture of beachbarrier, tidal inlet, lagoon and tidal-flat deposition. Agitation of carbonate sediment in shallow nearshore

water results in a shoreface facies of carbonate sand bodies. Skeletal debris and ooids formed in the

shallow water form bioclastic and oolitic carbonate sand shoals. Benthic foraminifers are the principal

components of some Tertiary carbonate ramp successions. The mid-ramp area lies below fair-weatherwave base and the extent of reworking by shallow-marine processes is reduced. Storm processes transport

bioclastic debris out on to the shelf to form deposits of wackestone and packstone, which may include

hummocky and swaley cross-stratification (14.2.1). In deeper water below storm wave base the outer

ramp deposits are principally redeposited carbonate mudstone and wackestone, often with the

characteristics of turbidites. Redeposition of carbonate sediments is common in situations where the outer

edge of the ramp merges into a steeper slope at a continental margin as a distally steepened ramp.

Homoclinal ramps have a consistent gentle slope on which little reworking of material by mass-flow

processes occurs. In contrast to rimmed shelves reefal build-ups are relatively rare in ramp settings.Isolated patch reefs may occur in the more proximal parts of a ramp and mud mounds are known from

Palaeozoic ramp environments.

Non-rimmed carbonate shelves

Non-rimmed carbonate shelves are flat-topped shallow marine platforms that are more-or-less horizontal,

in contrast to the gently dipping morphology of a carbonate ramp. They lack any barrier at the outer

margin of the shelf (cf. rimmed shelves) and as a consequence the shallow waters are exposed to the full

force of oceanic conditions. These are therefore high-energy environments where carbonate sediments are

repeatedly reworked by wave action in the inner part of the shelf and where redeposition by storms affects

the outer shelf area. They therefore resemble storm-dominated clastic shelves, but the deposits are

predominantly carbonate grains. Extensive reworking in shallow waters may result in grainstones andpackstones, whereas wackestones and mudstones are likely to occur in the outer shelf area. Coastal facies

are typically low energy tidal-flat deposits but a beach barrier may develop if the wave energy is high

enough.

Rimmed carbonate shelves

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A rimmed carbonate shelf is a flat-topped platform that has a rim of reefs or carbonate sand shoals along

the seaward margin. The reef or shoal forms a barrier that absorbs most of the wave energy from the open

ocean. Modern examples of rimmed shelves all have a coral reef barrier because of the relative abundance

of hermatypic scleractinian corals in the modern oceans. Landward of the barrier lies a low-energy

shallow platform or shelf lagoon that is sheltered from the open ocean and may be from a few kilometres

to hundreds of kilometres wide and vary in depth from a few metres to several tens of metres deep.

Distribution of facies on a carbonate rimmed shelfIn cases where the barrier is a reef, the edge of the shelf is made up of an association of reef-core, fore-

reef and back-reef facies (15.3.2): the reef itself forms a bioherm hundreds of metres to kilometres across.

Sand shoals may be of similar extent where they form the shelf-rim barrier. Progradation of a barrier

results in steepening of the slope at the edge of the shelf and the slope facies are dominated by

redeposited material in the form of debris flows in the upper part and turbidites on the lower part of the

slope. These pass laterally into pelagic deposits of the deep basin. The back-reef facies near to the barrier

may experience relatively high wave energy resulting in the formation of grainstones of carbonate sand

and skeletal debris reworked from the reef. Further inshore the energy is lower and the deposits are

mainly wackestones and mudstones. However, ooidal and peloidal complexes may also occur in the shelf

lagoon and patch reefs can also form. In inner shelf areas with very limited circulation and under

conditions of raised salinities the fauna tends to be very restricted. In arid regions evaporite precipitationmay become prominent in the shelf lagoon if the barrier provides an effective restriction to the circulation

of seawater.