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Introduction

Marine carbonate systems exist in the shallow, warm tropical seas (Figure 1), typically existing down to depths of 200m below sea level. Carbonate systems are teeming with organisms that depend on sunlight, a major reason why these systems have depth restrictions. Carbonate systems yield high biological productivity and biodiversity of shallow marine organisms. Carbonate sediments are produced locally within the environment of deposition (in situ) on the bank top, referred to as the carbonate factory (Belopolsky, 2000). Biochemical grains are one type of sediment produced here, formed from the remains of organisms that precipitate calcite shells. Common biochemical grains are mainly skeletal fragments. Chemical grains refer to inorganic mineral precipitates, such as peloids and ooids. Carbonate sediments are not transported far distances like some clastic sediments but can be transported by currents or storm waves relatively short distances down the carbonate ramp (Brandano et al 2012). Carbonate material weathers to karst topography in terrestrial environments, forming collapses, sinkholes, and conduits (i.e caves). The geomorphology of carbonate systems varies with regional tectonics, sea level, and isostasy.

Geomorphology

Many geomorphic variations of carbonate systems exist in nature, depending on sea level, tidal regime, and exposure to wind, wave, and storm energy (Wright & Burchette, 1998). The two main endmembers are the rimmed platform (Figure 2) and unrimmed platform. Varieties exist between these two endmember forms.

Rimmed Platform/Shelf The rimmed shelf has a slope that levels off into a lagoon, restricted on the seaward side by a built-up deposit called the shelf-edge reef. The platform is flat-topped and is rimmed by shallow reefs (SEPM, 1995). The outer rim along the shelf-edge is subject to high wave energy, creating a steep foreslope (several to 60 degrees) in deeper water towards the basin center (SEPM, 1995). Modern day examples of these systems exist in South Florida and Belize.

Isolated Platform Isolated platforms, also known as atolls, are shallow platforms with very steeply sloped edges, located offshore and separated from the continental shelf by deep water (SEPM, 1995). Isolated carbonate platforms in marine settings overlie volcanic seamounts and are associated with hot spots (SEPM, 1995). The platform itself ranges up to 100s of kilometers across. The platform is commonly characterized by coral reefs and biological fragments. A modern day example is the Bahama Banks.

Epeiric Platform Epeiric platforms are those with a wide (100-10,000 km), shallow platform with a gradual slope leading into the basin. Epeiric platforms occur during periods of relatively high sea level as continent is submerged, creating a shallow sea and thus considered a cratonic interior ramp (Wright & Burchette, 1998). There are no modern day examples of this platform type because present day sea level is not high enough to create expansive continental seas. Epeiric platforms were common during Paleozoic and Mesozoic eras.

Ramp Ramps maintain a fairly consistent low gradient slope that extend 10s to 1000s of kilometers. It is essentially an unrimmed platform with an open shelf. Ramps are commonly divided into inner, middle, outer, and offshore zone (Brandano, 2012) (Figure 3). Shoals or reef complexes can develop in the middle ramp zone. Various ramp types exist. Homoclinal ramps are characterized by relatively consistent, shallow slopes less than 1 degree. Distally-steepened ramps have a broad, deep ramp that separates the top platform from the slope. These ramp types differ by energy levels, which are controlled by tidal regime (Wright & Burchette). Low-energy forms have wide, deep-ramps comprised of mud and high-energy forms only occur in wave-swell dominated coastal margins, where deep-ramps are comprised of lime-sands and slope comprised of muds, breccias, and turbidites. These forms also have wide dune complexes (SEPM, 1995). A modern day example of a carbonate ramp system is the Persian Gulf.

Unrimmed Platform The unrimmed platform has no barriers at its edges, making it an open shelf. Unrimmed platforms are characterized by a gradual slope that persists from 10s to 100s of kilometers, after which the slope breaks into a steep drop. Modern day examples of these systems include the Yucatan Shelf and Campeche Bank.

Depositional Facies & Model

Since carbonates are generally deposited in situ, carbonate facies are typically reflective of the depositional conditions in which they form, such as water depth, chemistry and temperature, biologic activity, wave and wind energy, and currents.

Ramp Systems Distribution of facies along the ramp may vary based on the systems exposure to wind, wave, and storm energy (Wright & Burchette, 1998). Bedload transport of sediments is most common. The slope in most ramp systems is generally a low enough gradient where gravity transport does not occur (Wright & Burchette, 1998). Below is a generalized depositional model of a carbonate ramp subject to sea level rise and corresponding facies descriptions. Figure 4 illustrates the differentiation between ramp subsettings, based on biological components, sedimentary structures, and corresponding flow regimes. Inner: Shallow, nearshore facies include ooid-peloid shoals or skeletal banks (SEPM, 1995). Lithofacies dominated by euphotic organisms, such as larger foraminifera species and genticulated red algae (Brandano, 2012). Highest flow regime, therefore lacking sedimentary structures. Autochthonous sediments dominate this area. Middle: Spans across through the mesophotic and oligophotic light zones, with large benthic forams that decrease in abundance toward distal end. Echinoids, bivalves and bryozoans present in situ are transported downslope. Parachthonous material from inner ramp includes shallow water forams, supported by abrasion features on shells. Proximal area of middle ramp consists largely of reworked inner ramp deposits and storm deposits (Wright & Burchette, 1998; Brandano, 2012). Planar to angled cross-bedding reflective of subaqueous dune structures; suggest upper flow regime. Energy decreases and cross-bedding becomes sigmoidal in distal parts of middle ramp. Outer: calmer, low flow regime allowing deposition of horizontal mud beds, with internal bioturbated tabular beds. Dominant facies is comprised of aphotic organisms.

Platform Systems

Tidal-flat: Upward-shallowing cycles of subtidal-intertidal burrowed limestone in humid conditions, which are dolomitized in arid conditions. Lagoons: lithofacies vary from peloid limestone to cherty/lime mudstone, to burrowed skeletal packstone or mudstone. Metazoans form the main biological constituents. Shoal-water complex: Includes reefs, banks, and ooid/pellet shoals. Take the form of either skeletal banks or skeletal reef rimmed shelves. Banks mound-shaped deposits composed of biostromes and skeletal fragments. Deeper Shelf: Skeletal packstone to wackestone characterized by diverse, open marine fauna, many of which are preserved as whole body fossils. Contains fining-upward storm beds. Slope and Basin: These areas are characterized by breccias and turbidities. These deposits are thinly interbedded layers of periplatform lime and terrigenous mud with few gravity-transport deposits. Basin deposits are commonly shale. The shale becomes increasingly anoxic, reflected by shale laminations and lack of bioturbation.

Controls on Depositional System Evolution

Isostasy

Given that shallow, warm water is most conducive to carbonate production rates, carbonate systems are highly sensitive to slight changes in subsidence and uplift (Wright & Burchette, 1998). Such processes control the geomorphology and water depth of the carbonate ramp or platform. Accommodation space is created or destroyed through subsidence or uplift, which are functions of carbonate sediment supply. A high rate of carbonate production and low rate of erosion creates more accommodation space by causing the depositional surface to sink under the mass of accumulated sediment. When that sediment is removed by erosional forces, this causes the surface to rebound and uplift. Sea Level

Accommodation space can also be created or destroyed by changes in sea level (Belpolsky, 2000). Changes in sea level cause an upslope or downslope shift in lateral facies associations (Wright & Burchette, 1998). If rate of carbonate production exceeds that of accommodation space creation, aggradation and progradation of the platform occur (Belpolsky, 2000). Such settings are associated with periods of marine regression. If sea level declines enough where the platform is subaerially exposed, carbonate production will either cease or the platform will migrate downslope (Belpolsky, 2000). During marine transgression, the rate of accommodation space creation exceeds that of carbonate sediment production, resulting in a landward shift of the platform. The amount of accommodation space and rate of sedimentation ultimately influences the geometry of the carbonate platform in regard to aggradation and progradation (Belpolsky, 2000) (Figure 5) Tidal regime is another variable that shapes the geometry of architectural elements and corresponding stratigraphy (Wright & Burchette, 1998).

References

Baniak, G.M., Amskold, L., Konhauser, K.O., Muehlenbachs, K., Pemberton, S.G., Gingras, M.K., 2014, Sabkha and Burrow-Mediated Dolomitization in the Mississippian Debolt Formation, Northwestern Alberta, Canada; Ichnos, v. 21, p. 158-174. Research Gate.

Belopolsky, A.V., April 2000, Tectonic and Eustatic Controls on the Evolution of the Maldive Carbonate Platform; PhD Thesis, Rice University, Houston, Texas

Brandano, M., Lipparini, L., Campagnoni, V., Tomassetti, L., 2012, Downslope-migrating large dunes in the Chattian carbonate ramp of the Majella Mountains (Central Apennines, Italy): Sedimentary Geology, v. 255-256, p. 29-41.

Society for Sedimentary Geology (SEPM), 1995, Milankovitch Sea-Level Changes, Cycles, and Reservoirs on Carbonate Platforms in Greenhouse and Icehouse Worlds (SC35), Ch. 2: Basin Types of Carbonate Platforms.

Wright, V. P. & Burchette, Z. P. 1998. Carbonate ramps: an introduction. In: WRIGHT, V. P. t~; BURCHETTE, Z. P. (eds) Carbonate Ramps. Geological Society, London, Special Publications, 149, 1-5.