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Geometries of carbonate platforms

Several factors influence the geometry of a carbonate platform, including inherited topography, synsedimentary tectonics, exposition to currents and trade winds. Two main types of carbonate platforms are distinguished on the base of their geographic setting: isolated (as Maldives atolls) or epicontinental (as the Belize reefs or the Florida Keys). However, the one most important factor is perhaps the type of carbonate factory. Depending on the dominant carbonate factory, we can distinguish three types of carbonate platforms: T-type carbonate platforms (produced by "tropical factories"), C-type carbonate platforms (produced by "cool-water factories"), M-type carbonate platforms ("produced by mud-mound factories"). Each of them has its own typical geometry.

T-type carbonate platforms

The depositional profile of a T-type carbonate platforms can be subdivided into several sedimentary environments.

The carbonate hinterland is the most landward environment, composed by weathered carbonate rocks. The evaporitic tidal flat is a typical low-energy environment.

The internal lagoon, as the name suggests, is the part of platform behind the reef. It is characterised by shallow and calm waters, and so it is a low-energy sedimentary environment. Sediments are composed by reef fragments, hard parts of organisms and, if the reef is epicontinental, also by a terrigenous contribution. In some lagoons (e.g., the Florida Bay), green algae produce great volumes of carbonate mud. Rocks here are mudstones to grainstones, depending on the energy of the environment.

The reef is the rigid structure of carbonate platforms and is located between the internal lagoon and the slope, in the platform margin, in which the framework produced by large-sized skeletons, as those of corals, and by encrusting organisms will resist wave action and form a rigid build up that may develop up to sea-level. Survival of the platform depends on the existence of the reef, because only this part of the platform can build a rigid, wave-resistant structure. The reef is created by essentially in-place, sessile organisms. Today’s reefs are mostly built by hermatypic corals. Geologically speaking, reef rocks can be classified as massive boundstones.

The slope is the outer part of the platform, connecting the reef with the basin. This area acts as sink for excess carbonate sediment: most of the sediment produced in the lagoon and reef is transported by various processes and accumulates in the slope, with an inclination depending on the grain size of sediments, and that could attain the settlement angle of gravel (30-34°) at most. The slope contains coarser sediments than the reef and lagoon. These rocks are generally rudstones or grainstones.

The periplatform basin is the outermost part of the t-type carbonate platform, and it is dominated by density-cascating processes.

The presence of a rim produces restrict circulation in the back reef area and a lagoon may develop in which carbonate mud is often produced. When reef accretion reaches the point that the foot of the reef is below wave base, a slope develops: the sediments of the slope derive from the erosion of the margin by waves, storms and gravitational collapses. This process accumulates coral debris in clinoforms. Clinoforms are beds that have a sigmoidal or tabular shape, but are always deposited with a primary inclination.

The total dimension of the platform can be tens of kilometers.

C-type carbonate platforms

C-type carbonate platforms are characterized by the absence of early cementation and lithification, and so the sediment distribution is only driven by waves and, in particular, it occurs above the wave base. They show two types of geometry or depositional profile, i.e., the homoclinal ramp or the distally-steepened ramp. In both geometries there are three parts. In the inner ramp, above the fair weather wave base, the carbonate production is slow enough that all sediments may be transported offshore by waves, currents and storms. As a consequence, the shoreline may be retreating, and so in the inner ramp there may be a cliff caused by erosional processes. In the middle ramp, between the fair weather wave base and the storm wave base, carbonate sediment remains in place and can be reworking only by the storm waves. In the outer ramp, below the storm wave base, fine sediments may accumulate. In distally steepened ramps, a distal step is formed between the middle and outer ramp, by the in situ accumulation of gravel-sized carbonate grains (e.g., rhodoliths) only episodically moved by currents. After the outer ramp there’s the basin, but here there are no carbonate sediments and it isn’t part of the carbonate platform.

Carbonate production along the full depositional profile in this type of carbonate platforms, with an extra production in the outer part of the middle ramp, but less than in the T-type carbonate platforms.

M-type carbonate platforms

M-type carbonate platforms are characterized by an inner platform, an outer platform, an upper slope made by microbial boundstone, and a lower slope often made by breccia. The slope may be steeper than the angle of repose of gravels, with an inclination that may attain 50°.

In the M-type carbonate platforms the, carbonate production mostly occurs on the upper slope and in the outer part of the inner platform.