0.2 - Carbonates

What factors drive carbonate factories?

Carbonate production is largely bio-chemical, favoured by:

• Light

• Nutrients

• Warm water § Elevated salinity

• Protection from terrigenous influx

Production maximal in shallow, (sub) tropical areas: carbonate shelves

How are carbonate depositional environments and lithofacies reflected in stratigraphy?

• Distribution of modern sediments reflects physical environment: water depth, nutrient supply, energy

• Lithofacies: biogenic & abiogenic sediment composition

• Facies analysis (macro- and microfacies): reconstruct depositional environments & processes

What are the tidal zones of carbonate deposition?

The peritidal facies group may encompass all three zones:

• Subtidal

• Intertidal

• Supratidal

What are the features of peritidal facies?

• most peritidal facies develop where tidal range is low (<2m)

• different facies mosaics and diagenesis/hydrology under humid and arid conditions

• topography can influence development of islands and lagoons

• sediment source: subtidal

• channels rework sediment and focus onshore sediment transport during storms

• low faunal diversity

What are the features of peritidal facies supratidal zones?

• Presence of plant debris, rootlets Fenestrae (mangroves), algal mats

• Mostly muddy sediment

• Depositional processes and sedimentary structures:

  • Desiccation cracks

  • Reworking of lithified crusts by storms: mud chips, flat-pebble conglomerates

  • Fenestrae

What are the features of peritidal facies intertidal zones?

• Microbial mats

• Mangrove rootlets

What are the features of peritidal facies microbial mats & stromatolites?

• Presence of rootlets (mangroves)

• Low-diversity, high-abundance skeletal fauna

• Mostly muddy sediment

• Depositional processes and sedimentary structures:

  • Desiccation polygons, fenestrae

  • Crinkly (microbial) lamination

  • Bioturbation in lower intertidal

What are the features of peritidal facies subtidal zones?

• Includes ponds and tidal channels

• High-diversity, high-abundance skeletal fauna

• Depositional processes and sedimentary structures:

  • Bioturbation

  • Sediment transport via tidal channels: fining-upwards successions, levee deposits

What are the features of arid climate peritidal facies supra/intertidal zones?

• Depositional processes and sedimentary structures:

  • Halite crusts, gypsum, anhydrite

  • Dolomite

  • Algal mats

  • Desiccation cracks, teepee structures

  • Wind-blown/storm-deposited sand and mud

• Early diagenetic evaporite precipitation can disrupt/obscure depositional texture

Describe platform exposure

1. Rainwater reacts with soil to release CO 2 and form mild acid

2. Downward percolation may follow pre -existing structural trends

3. In humid setting create dense network of interconnected caves

4. Subtle karst, calcretes in (semi) arid climate

What are the features of shallow subtidal environment lagoons and open shelves?

• Moderate-/low-energy environments

• Muddy sediments, peloids, bioclasts (lime mud/wacke/packstones)

• Optimal conditions for fauna in shallow waters often lead to fossil-rich bioclastic limestones (float/rudstones)

• Fossil content will depend on salinity and substrate nature

• Micritization by cyanobacteria in illuminated shallow water

• Often pervasively burrowed

• Separation based on degree of connection to open ocean:

Open shelf

• No restriction, normal salinity

• Abundant & diverse fauna does not tolerate salinity variation (brachiopods, echinoderms, bryozoa…)

Lagoon

• Restriction, elevated salinity, storm recharge: salinity fluctuations

• Abundant but low-diversity fauna tolerates salinity variation (gastropods, algae, ostracods…)

What are the features of shallow subtidal environment carbonate sand bars/shoals?

• High-energy & high carbonate production rates: shoreface, shelf edge, areas of strong currents (tidal, storms)

• Grainy sediments – important reservoirs

• Analogous to siliciclastic shorelines & sand waves

• Usually very clean, well-sorted carbonate sands

• Lateral and vertical heterogeneous: finer horizons, cemented beds (caliche, beachrock)

What are the features of shallow subtidal environment shelf-margin shoals?

• Stable sand bodies, composed of oolitic and/or skeletal and peloidal carbonate sands

• Geometry and orientation strongly influenced by waves and tidal currents

What are the features of shallow subtidal environment shorefaces?

• Fairweather wave base to low tide

• Sand spits (longshore drift) and mobile sand shoals

• Beach-barrier island complex

  • Tidal channels

  • Stable, low-gradient coast

• Strandplain systems

  • Low/moderate topography, often rocky coastlines

• Depositional processes and sedimentary structures:

  • Wave-controlled sedimentation

  • High rates of carbonate sediment supply

  • Low-angle cross-stratification in shoreface oolitic/skeletal grainstones

  • Early lithification: beachrock

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

How does Walther’s Law impact carbonate dynamics?

Walther’s Law: “Facies occurring in a conformable vertical sequence were formed in laterally adjacent environments.” -

• Lateral migration of facies belts due to extrinsic or intrinsic forcing leads to vertical superposition of facies with time

What factors control eustasy in relation to sea level change?

Eustatic sea level change is controlled at long time scales >106 years) by:

• Volume of water in oceans – especially driven by glacial ice volumes

• Volume of ocean basins – related to spreading rates/plate tectonics

What are the features of peritidal carbonates shallowing-up successions?

• Most modern and ancient carbonates have stacked vertical successions of increasingly shallow water facies

• Evidence of seaward facies migration

• Controls may be related to sedimentary processes and/or eustatic sea level fluctuations

What are the affects of transgressions on ocean chemistry

Transgressions:

• Advect deeper-water nutrients to shelf: increased productivity

• Cause retention of nutrients in shallow seas: increased productivity

• Promote stratification and dys/anoxia: increased preservation

• Cause retention of clastics nearshore: less dilution

What are the affects of clastic input carbonate production?

• Input of clastic material inhibits carbonate production

• Suspended mud increases turbidity and decreases light levels (‘clastic poisoning’)

• Mixed carbonate-clastic deposition can occur as a result of delta lobe ‘switching’

How did reef ecology impact carbonate production during the Oxfordian?

• Callovian-Early Oxfordian: Wetter climate, increased siliciclastic input, transgressive anoxia do not favor carbonate productivity

• Middle-Late Oxfordian: Drier climate, low siliciclastic input favor carbonate productivity and build up development

Name some of the controls on carbonate sedimentation?

• Tectonics

• Topography

• Sea level

• Water depth

• Clastic sediment input

• Turbidity

• Light

• Climate

• Temperature

• Salinity

• Water circulation

What is the influence of evolution on carbonates through time?

Changes in biota through time mean that uniformitarianism does not strictly apply

What is the influence of plate tectonics on carbonates through time?

• Rates of sea floor spreading influences the abundance of atmospheric CO2 (and thus precipitation/dissolution)

• Regional and local tectonism may control platform geometry

• Plate tectonics controls the distribution of clastic sediment: influencing hinterland topography and river drainage

What is the influence of ocean chemistry on carbonates through time?

• Stratigraphic variation in ooid mineralogy (Sandberg, 1983)

• Recognition of ”aragonite seas” and “calcite seas” denoting dominant primary marine carbonate mineralogy

• Link to global sea level and greenhouse/icehouse cyclicity

• Precipitation of low magnesium calcite is favoured by high pCO2 and low Mg/Ca ratios

• Precipitation of aragonite and high magnesium calcite is favoured by low pCO2 and high Mg/Ca ratios (>5)

What is the influence of a greenhouse world on carbonates through time?

• High spreading rates

• Dispersed continents

• Relatively high sea level - large continental areas flooded

• Low latitudinal temperature gradient- warm at the poles

• No continental ice

• Increased pCO2 by production of CO2 from processes at subduction zones and MOR

• Lower Mg/Ca ratio of seawater via hydrothermal alteration of basalts at MOR

• Dominantly calcite seas

What is the influence of an icehouse world on carbonates through time?

• Low spreading rates

• Assembled continents

• Relatively low sea level - little continental areas flooded

• High latitudinal temperature gradient - cold at the poles

• Continental ice sheets at the poles

• Decreased pCO2 through increased photosynthesis

• Increased Mg/Ca ratio: increased Mg- supply via continental weathering, removal of Ca via evaporite precipitation

• Dominantly aragonitic seas

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