Geol 464: Carbonate Geology Semester 062

2007

Lecture 12

Marine carbonates and carbonate platforms

Carbonate platform is a general term widely used for mostly shallow-marine carbonate that develop along passive continental margins. Five major types of carbonate platforms having particular pattern of facies and facies succession are recognized:1. Ramp2. Rimmed shelf3. Epeiric platform4. Isolated platform 5. Drowned platform

Carbonate Platforms/Depositional Systems

• Ramp• Rimmed Shelf• Epieric Platform• Isolated Platform

Rimmed Shelf• Is a shallow-water platform with a distinct break-of-slope into deeper water.• Reefs and carbonate sand bodies occur along the high energy shelf

margin, restricting the circulation in the shelf lagoon behind to a greater or lesser extent.

• Debris from the rimmed-shelf margin is shed onto the adjacent slope and into the basin.

• Example of modern rimmed shelf is Beliz and Queensland (Great Barrier Reef).

Carbonate RampIs a gently sloping surface with a generally high-energy, inner-ramp, affected periodically by storms. Along the shoreline there may be a beach barrier-tidal delta complex, with lagoons and tidal flats behind, or a beach-ridge/strandplain system. Large reefs may occur on the inner ramp, and pinnacle reefs and mud-mounds in deeper water on the outer ramp.

Circulation on a carbonate platform versus carbonate ramp. (A) Platform-edge reef system restricts circulation on the inner platform; evaporation increases salinity. (B) Carbonate ramp lacks a platform-edge reef system resulting in more-open circulation and stenohaline conditions. FWB: fair-weather wave base

The inner ramp lies within fairweather wave base (FFWB), is subject to high-energy agitated conditions, onshore currents push coarse material shoreward and muds into lagoons. The mid ramp lies between FFWB and storm wavebase (SWB), is generally quiet environment disturbed by episodic high-energy storm events and is characterized by decreased organic productivity offshore.The deep ramp lies below SWB, is an area of quiet water suspension deposition of carbonate and clastic sedimentary structures, are typically laminated with some silty rippled beds floored by skeletal lags. Example of modern carbonate ramp is the Trucial Coast of the Arabian Gulf.

Epeiric Platform Epeiric platform is a shallow epicontinental sea covering a large area (100-10 000km across). Along the margin, there may be gentle (ramp-like) or steep (sheet-like) slope into the adjoining basin, but the margin is not a major feature of the platform. Epeiric platforms are dominated by low-energy, shallow subtidal-intertidal sediments.

Isolated Platform Isolated or detached platforms are surrounded by deep water and so are very much affected by prevailing few kilometers to a few hundredkilometers across. Examples of modern isolated platform are the Bahamas and in the Red Sea. Isolated platforms consist of pure carbonate sediment because clasticmaterial, apart from wind-blown dust, is excluded.The margins of these platforms may be shelf or ramp in character.

Drowned platform

Is one that has suffered a relatively rapid sea-level rise so that deeper-water facies are deposited over shallow-water facies. Many pelagic carbonates were deposited in these situation.

The type of carbonate platform developed is determined largely by:1. tectonics2. relative sea-level change. One type commonly evolves into another.

The carbonate sediments of the major depositional environments

Intertidal-supratidalTidal flats areas regularly to rarely covered by water, dominated by weak currents and wave action. They are developed extensively upon epeiric platforms and they occur along the shorelines of low-energy shelves and ramps, typically behind beach-barriers and around lagoons. Tidal-flat carbonates are dominantly lime mudstones, commonly peloidal, although local lenses of coarser sediment (grainstone) may represent tidal-channel fills. The fauna may be restricted in diversity; gastropods in particular may thrive, together with ostracods, foraminifers and bivalves.

Nearshore Inner shelf Outer shelf

Intertidal-supratidal

Thin, coarse layers of subtidal skeletal grains may occur, transported onto the tidal flat by storms. Microbial mats and stromatolites are typical of tidal-flat deposits. Bioturbation is common, and rootlets may occur. Synsedimentary cementation of tidal-flat sediments can produce surface crusts, which may expand to form tepee structures and megapolygons. In arid climate, the evaporite minerals gypsum-anhydrite, and possible halite, will develop in the sediment. Slight uplift and contact with meteoric waters may result in palaeokarstic surfaces, calcretes and black pebbles.