Carbonate Platforms

A carbonate platform is defined as:

"a large edifice formed by the accumulation of sediment in an area of subsidence"

Most platforms:

• have a flat top
• steep sides
• can be several kilometres thick
• extend over many hundreds of square kilometres
• can be rimmed or unrimmed

Nomenclature (Dunham)

Mudstone

Carbonate mud in warm, shallow environments is derived from:

• breakdown of calcareous algae
• precipitation from seawater
• breakdown of larger skeletal particles

• mostly accumulates in quiet water environments (below wave base, lagoons)
• can also form in higher energy environments where it is held in place or baffled by sea grasses or similar

Wackestone and packstone

• on platforms - current activity but insufficient to remove all the mud
• many are formed of faecal pellets

Bafflestone and bindstone

• form where plants act as baffles to fine sediment or bind the sediment

Grainstone and rudstone

• occur in high energy environments- wave action can remove the fine muddy material
• rudstones - around patch reefs or behind reefs and form largely due to storm activity

Storm deposits

• similar to those in terrigenous environments
  

Facies models

Unrimmed platforms

Modern unrimmed shelves are characterised by:

• a seafloor, 10m to 300 km wide, gently sloping offshore from a continental area
• facies belts of variable width that closely parallel bathymetric contours
• gradual transition of facies belts from inner, shallow shelf to outer, deep shelf to basin
• high-energy, carbonate sands in the wave and/or tide agitated inner shelf
• skeletal muddy sands to muds in quiet deeper outer shelf that are only periodically affected by storms
• no continuous reef trends
• localised patch reefs and sand shoals.

Eg Trucial coast in the Persian Gulf - lithofacies include:

• a back ramp with microbial intertidal flatsthat pass landward into an evaporitic basin and skeletal-pelleted sands to pelleted lime muds in protected lagoons
• a shallow ramp with high energy skeletal/oolitic sand shoals, beach barrier systems and coral reefs
• a deep ramp that is transitional from aggregate/skeletal sands dominated by molluscs and foraminifera to skeletal muddy sands dominated by mollusc debris
• a gradual transition into bivalve rich marls of deeper water
  

Rimmed Shelves

• shallow water shelves - grass covered sands and muds on their inner parts and skeletal sands and patch reefs on the outer parts
• deep water shelves - lagoons with water depths up to 30m, floored by mud; outer reefs and patch reefs surrounded by reef talus; skeletal sands in nearshore areas.
  

Allocyclic Controls

Three end member states: exposure, flooding and drowning .

• sea level falls below the edge of the platform
  • carbonate factory is shut down
  • karst topography can develop
  • terrigenous sediment can build out over the carbonate platform
• rapid rise in sea level
  • carbonate organisms below the photic zone
  • carbonates may resume growing when sediment accumulation brings the platform back into the photic zone or
  • carbonate production may keep up with the rapid sea level rise and result in a thick reef succession and a deep lagoon
• flooding
  • carbonate will continue to build up with a high accumulation rate
    

Reefs

• a reef - structure constructed large elements (usually > 5 cm) capable of thriving in energetic environments
• a mound - structures built by smaller commonly more delicate organisms in tranquil settings

Reefs generally comprise three facies:

Core facies - massive unbedded carbonate with or without skeletons

Flank or forereef facies - bedded carbonate sand and conglomerate of in place and/or core derived material, dipping and thinning away from the core

Interreef or open platform facies - subtidal limestone to terrigenous clastic sediment, unrelated to reef growth.

Sedimentary processes

Any living reef is a balance between 4 factors:

• upward growth of in-place calcareous elements
• continual destruction by a host of raspers, borers and grazers
• prolific sediment production by rapidly growing, short-lived, attached calcareous benthics
• concurrent inorganic or organically induced cementation.

• The modern reef growth window
• Continuing the carbonate nomenclature of Dunham to reefs

Facies distribution

Reefs show distinct lateral as well as vertical facies zonations - a response to variations in water depth and energy.

reef front facies

• lies between about 10-100 m
• diverse reef builders varying in shape from hemispherical to branching to columnar to dendroid to sheet-like (dependent on species that exist at time of reef formation)
• accessory organisms and niche dwellers common
• below 40 m light and wave intensity is low and corals are platy

fore reef facies

• gravel and sand composed of skeletal debris, reef limestone blocks, reef builder skeletons
• grade basinwards into muds

reef crest facies

• down to max 15 m
• receives most wind and wave energy
• organisms range from encrusting to short and stubby branching types depending on wave and wind energy

reef flat facies

• in areas of intense waves - pavement of cemented, large skeletal clasts with scattered rubble and coraline algal nodules
• moderate wave energy - shoals of well washed lime sand
• most material swept in from reef crest

back reef facies

• where much of the mud formed on the reef comes out of suspension
• prolific growth of sand and mud-producing bottom fauna (eg algae)
• corals are stubby and dendroid or large and globular

Nutrient/sediment zonation

• there is commonly a cross shelf zonation due to decreasing nutrient and sediment supply as you move outward from the coast
  

Response to sea level rise

Keep-up reefs - maintain their crests at or near sea level

Catch-up reefs - either began as shallow reefs which became deeper when growth could not keep up with the rise in sea level but then later grew quickly to catch up OR started on a deep substrate then quickly grew upward

Prograding deposits - once built up to sea level, reefs can only grow through prograding

Give-up reefs - initially grew as the other s did but then stopped growing due to changes in environmental conditions eg dropping below the photic zone

Carbonate Slopes

• extend from the shallow water environment of the reef and fore reef down into deep water of ocean basins
• processes are similar to those of terrigenous slopes and deep water
• tend to be cut by a number of parallel gullies
• facies belts are parallel to the platform margin
• down slope carbonate deposition controlled by the carbonate compensation depth

Depositional slopes

• either smooth slopes that extend from shallow to deep water with the thickness of accumulated sediment decreasing seaward
• or bypass slopes where the upper slope is largely bypassed with sedimentation on the lower slope

Erosional slopes

net removal of material due to a number of mechanisms including slumping and carbonate dissolution

Sediment types and facies

Slope sediments have several origins:

Pelagic sediments

• accumulation of the shells of microscopic to very small marine organisms of the open ocean

Platform carbonate

material that is derived from the shallow water platform

mud to boulder-sized fragments

can be transported as much as 120 km from the platform

Hemipelagic sediment

fine-grained terrigenous clastic material

Autochthonous carbonate

faecal pellets, skeletons of organisms of the slope, carbonate cements

Processes operating on the slope

• material settled out of suspension
• sediment gravity flows such as turbidites, slumping, debris flows and creep
• reworking by bottom currents

• laminated mudrocks to megabreccias that indicate large scale collapse of the platform margin
  

Facies Models

Carbonate fan models - similar to siliciclastic slopes but in reality they appear to be very rare

Carbonate aprons

• wedge-shaped to lenticular bodies that generally thicken toward the platform margin
• several types depending on slope type
• facies distribution also depends on whether the carbonate platform is developing in an ocean, an open seaway between platforms or a closed seaway

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