10 - Carbonate platforms

What are the parameters and producers for carbonate precipitation?

Carbonate supersaturation (Variable degrees of biologically influenced/induced, Abiotic)

• High temperature, high salinity, oceanic restriction

  • Non-skeletal grains, mud, microbes (ooids, peloids, stromatolites, algal mats, whitings, cements)

Light (Variable degrees of biologically influenced/induced, Biologically controlled)

• High temperature, shallow depth, water transparency

  • Photozoan s.s. (hermatypic corals, green algae)

  • Symbiotic foraminifers

• Water transparency, moderate hydrodynamics

  • Red algae

Organic matter (Variable degrees of biologically influenced/induced, Biologically controlled)

• Food supply Plankton & pelagic OM Seagrass, macroalgae

  • Heterozoan s.s. (w/o red algae)

What are the production modes of the carbonate factory?

M, Mud mound micrite

• Biotically-induced/influenced, Heterotrophic

C, Cool-water-controlled precipitates

• Heterotrophic, Autotrophic

T, Tropical topmost water

• Bioticallyinduced/influenced, Heterotrophic, Autotrophic

How are carbonate platforms classified?

Shallowest areas with waves & currents & photic energy -> main carbonate production; best locations for emersion/karstification:

• Ramp

• Rimmed Shelf

• Epeiric Platform

• Isolated Platform

Describe the features of rimmed shelf carbonate platforms

• Rimmed shelves are characterized by a shelf margin break, defined by reef growth or carbonate sandbodies

• The shelf margin is ocean-facing, high energy and turbulent

• Up-welling of ocean currents makes the shelf break a zone of high organic productivity

• Landward of the shelf margin is typically protected, characterised by lower energy/quieter water

• Sediment is transported offshore of the shelf break by sediment and gravitydriven processes

Describe the features of ramp carbonate platforms

• Ramps are shallow-dipping carbonate platforms (often <1o)

• The ramp is subdivided on the basis of the influence of waves/ storms on sedimentation

• Sand shoals may develop in the high energy/wave agitated inner ramp, offering some landward protection from wave/storm energy

• Sedimentation in the mid ramp is frequently storm influenced, whilst the outer ramp is only infrequently storm-reworked

• The basin is very low energy with negligible storm influence

Describe the features of epeiric platforms carbonate platforms

• Epeiric platforms were deposited in epi-continental seas

• Platforms < few thousands of km wide with negligible topography

• No modern analogues as present day sea level is relatively low

• Dominated by shallow water, storminfluenced, shallow subtidal-intertidal sedimentation

• Wide facies belts Examples: Permian – Cretaceous Middle East

What are the depositional environments and lithofacies of high-energy platform margin: reefs?

Belt along windward margin and small patches in lagoon

Depositional processes:

• In-situ growth of carbonate build-ups

• Reworking by storms, currents: rubble

• Export to basin by gravity processes

What are the depositional environments and lithofacies of oolite carbonate sand shoals?

High-energy locations

Depositional processes:

• In-situ precipitation of ooids

• Reworking by tidal currents, storms, waves

What are the depositional environments and lithofacies of bank interior/lagoon?

Low-energy areas of platform

Depositional processes:

• Accumulation of faecal pellets (worms, gastropods)

• Disintegration of algae: mud

• Chemical precipitation: whitings

What are the depositional environments and lithofacies of tidal flats?

Land-attached areas

Depositional processes:

• Reworking by tidal currents and waves

• Storms move subtidal sediment onto tidal flat and into lagoon