4 - Carbonate Platform Margin Depositional Environments

What are carbonate reefs and build-ups?

Carbonate build-ups are laterally-restricted structures, which have usually undergone organically-mediated growth. They can be grossly divided into:

• Organic (skeletal) reefs, built by organisms with a rigid calcareous frame, may be matrix or skeleton supported and deposited in warm or cold water and able to withstand high energy wind/wave action.

  • Microbes M Factory, Photozoa T Factory

• Reef (mud) mounds are inorganically and/or biogenically constructed but lack a rigid skeletal framework and unable to withstand high energy wind/wave action

  • Microbes M Factory, Heterozoa C Factory

What are the features of reef and mud mounds?

• Fine grained, mud (micrite)-dominated build-ups

• Stability provided by matrix, limited cementation

• Organic components include bivalves, corals, sponges, bryozoa, microbes, stromatoporoids

• Heterotrophic and biologically influenced/induced carbonate precipitation

• Low topographic relief - do not offer significant wave resistance, but can trap sediment

• When skeletal framebuilders were absent or in deeper-water setting (common Paleozoic)

What are the processes in reef dynamics and ecology?

• Constructive processes: Biological processes through direct growth, baffling or binding

• Destructive processes Wave damage and biological destruction

• Cementation Early cementation from seawater

• Sedimentation Accumulation of biogenic matter and reef-derived detritus

What are the organism roles in reef dynamics and ecology?

• Frame builders

• Binders

• Bafflers

• Sediment contributors

• Precipitators

What are is the biological succession in reef dynamics and ecology?

Oxfordian reef ecological succession (Morocco):

1. Pioneer stage

2. Colonisation stage

3. Diversification stage

4. Diversification stage /domination stage

What are the growth forms of frame-building organisms in reef dynamics and ecology?

Growth form - Wave energy - Sedimentation

Delicate, branching - Low - High

Thin, delicate, plate-like - Low - Low

Globular, bulbous, columnar - Moderate - High

Robust, dendroid, branching - Moderate/High - Moderate

Hemispherical, domal, irregular, massive - Moderate/High - Low

Encrusting - Intense - Low

Tabular - Moderate - Low

What are the features of reef facies reef flat: sand aprons?

• Behind reef pavement, water depths up to 10m. May be up to 160km long and 100- 200m wide

• Reworked reef debris, carbonate sand, local colonisation by sea grass and algal mats

• Some coral growth, intense bioerosion, algal encrustation of boulders

• Gradational contact with back-reef lagoon

What are the features of reef facies reef crest: compositions?

• Dominated by encrusting organisms, especially red algae, usually coating dead coral/coral debris. May be encrusted by forams, gastropods etc

• Low energy crests may be composed of Millepora or Acropora Palmata

• Skeletal breakage, abrasion, bioerosion high

• Periodic subaerial exposure possible

• Bindstones/framestones in ancient carbonates, with laminar encrusting organisms

What are the features of reef facies reef fronts?

• Extensive coral growth seaward of reef crest: ‘reef core’ preserved in ancient reef limestones

• Close to the crest, in the high energy zone, spur and groove structures form oblique to the shoreline

• Biota evolves with depth as light penetration and energy decrease

• Low preservation potential due to bioerosion and early diagenesis

What are the features of reef facies forereef slopes?

• Forereef slope is positioned seaward of the reef front, transition to basin

• Sedimentation dominated by gravity flow mechanisms and deposition of pelagic sediments

• Depositional/accretionary reef margins slope continuously into the basin

• By-pass margins have a steep escarpment seperating the reef from reef talus

What are the features of ancient reef Devonian reef complexes?

• Small reef mounds to barrier reefs

• Framebuilders: stromatoporoids, corals, plus sponges, bryozoa, algae, echinoderms

What are the features of ancient reef Cretaceous rudist bioherms?

• High-energy platform margin to low-energy lagoon environments

• Biostromes and patch-reefs

• Reduced role of framebuilding corals: unfavorable environmental conditions

What factors have affected the biological evolution of reefs through time?

The composition of the skeletal components of carbonates has varied through time in response to:

• Evolution

• Extinction events

• Changes in ocean chemistry

Changes in continent configuration Reefs as organic build-ups are good mirrors of these changes

What are the depositional processes in slope and basin depositional environments?

On ramps and other gentle slopes:

• Below fair weather wave base - current-/storm-dominated sedimentation

• Below storm wavebase - finegrained limestones, siliciclastic shales

Slopes and basins adjacent to rimmed shelves:

• Remobilisation

  • Debris/gravity/fluidised flows

  • Calciturbidites

  • Rock fall, slumps

Deposition from bottom currents

in situ pelagic fall out

What are the features of slope deposition?

• Turbidites: varying degrees of completeness of Bouma sequence

• Grain flows: well-sorted carbonate sands with reverse grading

• Debris flows: little grading/sorting, rudstones and floatstones

What are the features of basin deposition?

• Reduced influence of platform margin in deep -water environments

• Settling -out of suspended biogenic material (plankton): pelagic sediments

• Foraminifera, coccolithophorids, diatoms, radiolaria, pteropods…..

• Distribution controlled by productivity and ocean currents

How have pelagic sediments changed over time in basin deposition?

Nature of plankton changed over time

• Planktonic foraminifera and coccolithophorids only important since Cretaceous

• Triassic-early Jurassic: pelagic bivalves, cephalopods

• Paleozoic: calcareous plankton ~absent

• Radiolaria present since Precambrian

What are planktonic organisms are important to basin deposition?

• Plants with calcite skeletons: Coccolithophorids

• Plants with silica skeletons: eg Diatoms

• Animals with calcite skeletons: planktonic Foraminifera

• Animals with aragonite skeletons: Pteropods (pelagic gastropods)

• Animals with silica skeletons: Radiolaria