CARBONATE SHELF DEPOSITS

 

Under-water view of calcareous algae Halimeda (above) and Penicillus (below) in Florida Bay

Carbonate shelves are typically illuminable shallow water environments adjacent to a continental landmass where calcium carbonate (CaCO3) and/or magnesium carbonate is biochemically or chemically mediated. Although the majority of these deposits form in-situ within the basin of deposition, there may be some delivery of terriginous carbonate sediment from continental weathering into the system which may then be incorporated into and preserved within the physical and chemical matrix. While hydrological perturbation and circulation may scatter these autochthonous sediments into tidal flats or slope deposits, the vast majority remains proximal to the continental shelf environment.

Carbonate Sediments

Siliciclastic Sediments

Most sediments occur in shallow, tropical environments

Climate is not a constraint; sediments occur worldwide and at all depths

Most sediments form in marine environments.

Sediments form in either marine or terrestrial environments.

Sediment grain size depends on the size of organisms' skeletons and calcified hard parts.

Sediment grain size depends on the hydraulic energy of the environment.

Lime mud indicates presence and growth of organisms whose calcified parts are mud-sized crystallites.

Mud indicates settling out of suspension.

Shallow water lime sand bodies result from localized chemical reactions (i.e., physiochemical reactions or biological fixation of carbonate).

Shallow water sand bodies result from ocean current and wave action.

Sedimentary environments may change without a change in hydraulic regime. (Local build-ups of sediments can alter properties of surrounding sediments)

Changes in the sedimentary environments correspond to changes in the hydraulic regime .

Sediments are cemented on the sea floor.

Sediments remain unconsolidated in the environment of deposition and on the sea floor.

Periodic exposure of sediments during deposition will cause diagenesis (i.e., intensive cementation and recrystallization).

Periodic exposure of sediments during deposition does not affect the deposits.

Signs of sedimentary facies are destroyed during low-grade metamorphism.

Sedimentary facies discernible after low-grade metamorphism.

Reproduced from University of British Columbia.

Globally, carbonate deposits currently cover few continental shelves. Modern examples can be found off the coasts of southern Florida, Honduras, the Yucatan, and Australia. The force behind this geographical limitation is the sensitivity of carbonate-secreting organisms to environmental parameters including water depth, siliclastic influx, and ambient temperature and salinity. Geologically, although carbonate platforms may develop in a range of geotectonic settings, the majority can be found along passive continental margins and back–arc basins to foreland basins

Carbonate platforms generally take on one of two categories: 1) open shelves, those in which the outer shelf to slope break is in tens of meters of water (Figure 1); and 2) rimmed shelves, which have a continuous or nearly continuous barrier along the shelf margin (Figure 2) (Ginsburg and James, 1974).

Figure 1. An unrimmed (shoal) platform. Present day occurrence is on the leeward side of tropical banks and cool water environments. Unrimmed platforms may have a constant gentle seaward slope (ramp) or an open shelf morphology. Ramps tend to pass gradually into deeper water and then basinal sediments while shelves distally tend to rapidly grade into basinal sediments with associated gravity flows. The lack of any physical barriers on the platform itself results in high wave energy which in turn creates complex nearshore facies with high rates of sediment transport. Although barrier reefs are absent, pinnacle reefs and reef mounds may be found in an unrimmed environment. Modern shoal-rimmed platforms can be found on the Trucial Coast of the Arabian Gulf, Shark Bay in western Australia, and along the eastern Yucatan. From James (1983).

Figure 2. A reef-rimmed platform contains both the features of a wave-agitated margin and a calm, landward lagoonal environment. Along the high-energy barrier reefs and/or oolitic sand shoals and cements can be found. Organic productivity is high here, especially if upwelling is occurring. The lagoon is usually an environment of lower energy, although the extent of which depends on how well the marginal reefs and sand shoals dissipate the wave energy. If wave energy is negligible within the lagoon, the environment will be one of little circulation (aside from large storm events) and possible hypersalinities during drier seasons. Conversely, if the lagoon is more open to turbulence tidal effects and minor storm events will effect the sedimentary environment. Modern rimmed platforms may be found in the Caribbean and coastal Belize as well as Australia's Great Barrier Reef. From James (1983).

Index to carbonate shelf sediments below or to link back to USC website on sequence stratigraphy click here

Evolution Lag time Antecedent Topography
Biology Climatic Zone Siliclastic Influx
Temperature & Salinity Sea Level Clastic Input
Tectonism Platform Morphology Stratigraphic Succession
Water Depth & Turbidity Subsidence Lithofacies
Circulation Carbonate Growth Potential Eustatic Response

Proceed on to the carbonate platform question set.