Carbonate depositional environments

Carbonate Platforms

A carbonate platform is defined as:

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

Most platforms:


Nomenclature (Dunham)

Mudstone

Carbonate mud in warm, shallow environments is derived from:

Wackestone and packstone

Bafflestone and bindstone

Grainstone and rudstone

Storm deposits


Facies models

Unrimmed platforms

Modern unrimmed shelves are characterised by:

Eg Trucial coast in the Persian Gulf - lithofacies include:

Rimmed Shelves


Allocyclic Controls

Three end member states: exposure, flooding and drowning .


Reefs

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:


Facies distribution

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

reef front facies

fore reef facies

reef crest facies

reef flat facies

back reef facies

Nutrient/sediment zonation


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

Depositional slopes

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

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


Facies Models

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

Carbonate aprons

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