carbonate platforms

8/13/2019 Carbonate Platforms

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Carbonate Platforms


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I. Intro

1. Limestones are biochemicalrocks

2. Composed of CaCO31. Calcite, Aragonite

3. Calcium carbonatesediments depend on:1. Temperature2. Pressure3. Agitation

4. Abundance of calcareous-shelled organisms

1. abundant light2. constant so/oo3. clear, warm water

1. Organisms like shallow

water, away from river


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II) Platform and shelf carbonates

A. Carbonate depositon mostly onplatforms

B. Carbonate platform is defined as:1."a large edifice formed by the

accumulation of sediment in anarea of subsidence"C. Examples: rimmed platform,

isolated platform, ramp
http://woodshole.er.usgs.gov/project-pages/caribbean/tsunami.html


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III. Platform Characteristics A. Most platforms:

1. have a flat top

2. steep sides3. can be several

kilometres thick4. extend over many

hundreds of squarekilometres5. can be rimmed or

unrimmed


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Platform Characteristics

ContinuedB. Platform seds also in epeiric seas

1. distinguishing betweenshelf/epeiric sea carbonatesdifficult

2. distinction based onpaleogeography

C. Most carbonate productionseaward

1. upwelling/nutrientsi. e.g. reefs on platform marginii. if no reef, ooid shoals--oosparite

2. landward- subtidal carbonatei. poor preservation - stormsii. seds transported

offshore/onshore (tidal flats,channels)

3. nearshore protected areasi. micrite/biomicrite/pelmicrite

4. above high tide (supratidal)--

dolomite


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IV. Carbonate deposits

A. Micrite mud mounds1. 100 meter thick, 1km diameter

i. deep & shallow water originii. lack internal structureiii. Mostly Paleozoic some Mesozoic

iii. pelloid deposit?1. skeletal component low

iv. plant stabilizationv. sed trapping organism--bryzoans


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Devonian Saharan Micrite Mud

Mounds

Asymmetrical mud mounds at AzelMatti (Ahnet Basin, Algeria) Mud mounds (25-30 m high) at Azel Matti (Ahnet Basin, Algeria)


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V. Reefs --Defined

A. a reef - structure constructed of largeelements (usually > 5 cm) capable ofthriving in energetic environments


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VI. Reef Structure & Rocks

A. Low latitude, shallow water env.1. framework organisms vary

with timei. archaeocyathids, sponges,

corals, algae, bryzoa, etc.B. Reef core = massiveC. Forereef = talusD. Reef limestone types

1. framestone in situ fossilsform supporting framework

2. bindstone--encrusting &

binding organism3. bafflestone-- stalk-shapedfossils--reduce rate of waterflow

E. Reef core bounded by forereeftalus

1. forereef-allocthonus seds

2. c.gr--floatstone, rudstone


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VII. Reef Facies A. Reefs generally comprise three facies:

1. Core facies - massive unbedded carbonate withor without skeletons

2. Flank or forereef facies - bedded carbonatesand and conglomerate of in place and/or corederived material, dipping and thinning away fromthe core

3. Interreef or open platform facies - subtidallimestone to terrigenous clastic sediment,unrelated to reef growth.


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Reef Facies

Miocene Llucmajor ReefComplex Components , MallorcaSpain strata.geol.sc.edu/.../049-Reef

Components.jpg

Miocene Reef Facies, Mallorca
http://strata.geol.sc.edu/Miocene-Mallorca/images/049-Reef-Components.jpghttp://strata.geol.sc.edu/Miocene-Mallorca/images/049-Reef-Components.jpghttp://strata.geol.sc.edu/Miocene-Mallorca/pages/018-Reef-Simulation-Mallorca.htmlhttp://strata.geol.sc.edu/Miocene-Mallorca/pages/018-Reef-Simulation-Mallorca.htmlhttp://strata.geol.sc.edu/Miocene-Mallorca/images/049-Reef-Components.jpghttp://strata.geol.sc.edu/Miocene-Mallorca/images/049-Reef-Components.jpg


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VIII. Deep sea carbonates

A. Deposition seaward of

terrigenous seds1. max depth about 5 km2. seawater colder w/depth

i. more dissolved carbon-dioxide than warm water

ii. increase H 2CO3= increasedissolution

iii. Temp more importantthan P


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B. CCD = depth belowwhich no carbonateseds accumulate1. deeper in equatorial

wateri. more carbonate

production2. shallow near margin

i. More organics3. CCD changes w/ time

i. depends on CO2content &carbonateproduction


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C. Deep sea redclay orsiliceous sedbelow CCD


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IX. Unrimmed Shelves A. Characteristics of unrimmedshelves:

1) a 10m -300 km wide seafloorgently slopes offshore from acontinental area

4) lithofacies are generally grainy5) high-energy, carbonate sands in

the wave and/or tide agitatedinner shelf

6) skeletal muddy sands to muds inquiet deeper outer shelfi. periodically affected by storms

7) localized patch reefs and sandshoals,8) no protection from onshore

waves


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XI. Rimmed Platforms A. Shallow platforms 0-30 m deepbounded at outer edges by high-energyfacies (typically reefs) and a pronouncedbreak in slopeB. Reduced connection between open-ocean and shelf due to protection byreefs, sand shoals, or islands, thusdampening wave energyC. Lithofacies are generally muddy

D. Shelf depth determines facies type anddistribution1. Shallow water shelves have grassy

covered sands and muds on their innerparts andskeletal sands and patch reefs on theouter parts while

2. deep water shelves (lagoons withwater depths < 30m) are floored bymuds.

E. Behind the reef are muddy carbonatesediments that contain lots of marineorganisms

http://wrgis.wr.usgs.gov/open-file/of01-448/images/fig18.jpg


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Growth of Rim


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Belize

belizemodernfacies.com/?page_id=19
http://belizemodernfacies.com/?page_id=19http://belizemodernfacies.com/?page_id=19


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Shallow Rimmed Shelf-Florida, Deeper Rimmed Shelf--Belize

strata.geol.sc.edu/.../Sea_Level_Changes.htm
http://strata.geol.sc.edu/MARINESEDIMENTS/CarbonateShelf/Sea_Level_Changes.htmhttp://strata.geol.sc.edu/MARINESEDIMENTS/CarbonateShelf/Sea_Level_Changes.htm


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XII. Isolated Platform

A. E.g. BahamasB. Interior = skeletal l.s.,

peloid sands & mud

C. Platform margins =shoals of ooidgrainstone

D. Talus slope & slump &gravity flows

E. Platform evolution:1. Develop on horst2. graben = deeper

water


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Isolated Platform

http://www.geologie.uni-stuttgart.de/online_kurse/virtfoss/CPR%20ordner/Figuren/Bilder%20Lec3/Fig.3-3.gif


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X. Ramps

A. gentle slope (


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Trucial Coast Ramp

strata.geol.sc.edu/.../Sea_Level_Changes.htm
http://strata.geol.sc.edu/MARINESEDIMENTS/CarbonateShelf/Sea_Level_Changes.htmhttp://strata.geol.sc.edu/MARINESEDIMENTS/CarbonateShelf/Sea_Level_Changes.htmhttp://www.geochem.com/rg-CarbonateModels.htm


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Generalized model of a Carboniferous carbonateplatform (from Richards, 1989a).

http://www.ags.gov.ab.ca/publications/wcsb_atlas/A_CH14/FG14_18.html


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XI. Intrashelf Basins on Rimmed Shelves andRamps

A. Rimmed shelf w/inshore basinB. Pass landward into coastal siliciclastics

C. Seaward-basin pass to rim of skeletal orooidsD. Basin depth few 10's of m

1. below fairweather wave base

2. seds = shale, quartz sand3. If below wave base, euxinic, dysaerobic l.s. &

shale


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Intrashelf Basin- I couldntfind one for carbonate setting

Carbonateswould

occur atouter edge


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Stop At This Slide! Succeeding

Slides are extraneous.


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u - o ac es nc u e:

a back ramp with microbial intertidal flatsthat passlandward into an evaporitic basin and skeletal-pelleted sands to pelleted lime muds in protectedlagoonsa shallow ramp with high energy skeletal/oolitic sandshoals, beach barrier systems and coral reefsa deep ramp that is transitional fromaggregate/skeletal sands dominated by molluscs andforaminifera to skeletal muddy sands dominated bymollusc debrisa gradual transition into bivalve rich marls of deeperwater


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Sedimentary processes

Any living reef is a balance between 4factors:upward growth of in-place calcareouselements

continual destruction by a host of raspers,borers and grazersprolific sediment production by rapidlygrowing, short-lived, attached calcareousbenthicsconcurrent inorganic or organically inducedcementation.The modern reef growth windowContinuing the carbonate nomenclature ofDunham to reefs


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Facies distribution

reef front facieslies between about 10-100 mdiverse reef builders varying in shape fromhemispherical to branching to columnar todendroid to sheet-like (dependent on speciesthat exist at time of reef formation)accessory organisms and niche dwellerscommonbelow 40 m light and wave intensity is lowand corals are platy


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fore reef facies gravel and sand composed of skeletaldebris, reef limestone blocks, reefbuilder skeletonsgrade basinwards into muds


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reef crest facies down to max 15 mreceives most wind and wave energyorganisms range from encrusting toshort and stubby branching typesdepending on wave and wind energy


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reef flat facies in areas of intense waves - pavement ofcemented, large skeletal clasts withscattered rubble and coraline algalnodules

moderate wave energy - shoals of wellwashed lime sandmost material swept in from reef crest


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back reef facies where much of the mud formed on thereef comes out of suspensionprolific growth of sand and mud-producing bottom fauna (eg algae)corals are stubby and dendroid or largeand globular


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Carbonate Slopes

extend from the shallow water environment of thereef and fore reef down into deep water of oceanbasinsprocesses are similar to those of terrigenous slopesand deep water

tend to be cut by a number of parallel gulliesfacies belts are parallel to the platform margindown slope carbonate deposition controlled by thecarbonate compensation depth


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Depositional slopes either smooth slopes that extend fromshallow to deep water with thethickness of accumulated sedimentdecreasing seaward

or bypass slopes where the upper slopeis largely bypassed with sedimentationon the lower slope


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Erosional slopes net removal of material due to anumber of mechanisms includingslumping and carbonate dissolution


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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 oceanPlatform carbonate material that is derived from the shallow water platformmud to boulder-sized fragmentscan 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


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Processes operating on the slope material settled out of suspension

sediment gravity flows such asturbidites, slumping, debris flows andcreep

reworking by bottom currentslaminated mudrocks to megabrecciasthat indicate large scale collapse of theplatform margin


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http://www.gly.fsu.edu/~salters/GLY1000/11Seds_sedrocks/Slide35.jpg


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USGS
http://woodshole.er.usgs.gov/project-pages/caribbean/tsunami.htmlhttp://woodshole.er.usgs.gov/project-pages/caribbean/tsunami.html


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Cambrian PaleogeographyI) Intro

A. Most of N. America alongpaleoequator

B. Deposition of carbonate & qtzsand, shaleC. Early Cambrian seaencroachmentD. L. Cambrian sea coveredlarge areaE. Much carbonate depositionF. E.g.. Sauk Sequence

1. erosion and weathering of xlnbasement2. encroachment of marinewater3. deposition of clean sand4. sandy deposition replace bycarbonatesi. algae flourished5. sands pass westward intofiner clastics & carbonates6. E.g. Grand Canyoni. Cambrian Tapeats ssii. passes to Bright Angle Shaleiii. followed by Mauv L.S.iv. together form transgressivesequence

http://jan.ucc.nau.edu/~rcb7/Ear_Camb.jpg


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Devonian Saharan Micrite MudMounds

Asymmetrical mud mounds at AzelMatti (Ahnet Basin, Algeria)

Mud mounds (25-30m high) at Azel Matti(Ahnet Basin, Algeria)

Generally, the mounds were established in a deeper subtidal environment as

suggested by the lack of indications for ground-moving waves. Non-photic conditionsare indicated by the absence of cyanobacterial activity (e.g. micritic envelopes) andcalcareous algae. The mounds are reefs in the biological sense which means that

they are autochthonous structures with a significant relief build by organisms.However, the typical Devonian reef builders, stromatoporoids and colonial rugose

corals, are lacking. No rigid framework of organisms is present as that formed by the

hermatypic scleractinian corals in modern reefs. The dominating faunal elements arecrinoids, tabulate corals, brachiopods, trilobites, sponges, ostracods and bryozoans:

thus mostly filter feeding organisms which preferred the exposed mound positionbecause of better oxygenation and better food supply. But all these organisms are all

mound dwellers, not mound builders. More than 80% of the mound volume isconstituted of a fine-grained carbonate (micrite) whose origin is one of the mainmiracles of mud mounds. High accumulation rates (0.2-0.8 m/1000 a), purity of

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carbonate platforms

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