carbonate plateform

8/2/2019 Carbonate Plateform

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SUMMARYVariations in carbonate depositional settings through time produce correspondingchanges in their lateral and vertical facies geometry. High frequency changes in faciesare induced by sea level, and climate, whereas long-term low frequency changes arecommonly related to paleo-geography and plate configurations or the evolutionary

changes in the carbonate producing organisms. The sections below provide a generaland classical introduction to the eclectic responses of carbonates to changes in theirsetting.Click here to link to a more detailed examination of the details of the sequencestratigraphy of carbonates.

CONTROLS ON CARBONATE DEPOSITIONThe geometry and facies relationships of carbonate accumulations are closely tied tothe paleogeographic setting and changes in that setting through time, particularly waterdepth variation. Paleolatitude and clastic influx are major influences on the distribution

of the carbonates, with facies largely controlled by the proximity of the depositionalbasin to open marine circulation and the position of the depositional setting within thebasin. Carbonate sediments and their associated buildups are largely the by products oforganisms whose evolution has a significant influence on carbonate deposition. Wilson(1975) reviews in detail some of the similarities and differences that occur in carbonatesof different ages.As outlined in an earlier section, the position of the depositional setting controls thegeometry and continuity of deposits. For instance, deep-water basinal deposits arecommonly widespread, thin beds of fine-grained carbonate formed by a combination ofpelagic fauna and suspended shelf muds. At the edges of the basin, contributions from

the shallow platform are greater. These basin-edge deposits range from turbidite fansand debris flows to reef talus. Porosities may be higher in these deposits than in basinalmuds, but continuity may vary greatly. In contrast the margins of the carbonate platformor shelf may consist of linear to mound-like reef or shoal buildups that may have localporosity. To the lee of the margin the "back-reef" lagoon sediments occur in widespreadbeds of even thickness with discontinuous and mounded patch reefs scattered withintheir more seaward portions. Toward the updip limit of the platform, the lateral continuityof platform carbonate sands decreases, and supratidal carbonates associated withevaporites and elastics are common.
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An example of water depth changes in the Cretaceous.

Changing water depth primarily causes changes in the depositional setting. Worldwidechanges in relative sea level have occurred repeatedly and cyclically through geologictime. The rate of relative sea level rise has an obvious effect on the sediment type andnature of deposition, whereas the rate and extent of relative sea level fall has a markedeffect on the diagenesis and erosion of carbonate sequences. As indicated elsewhere inthis site eustatic sea level changes are believed to have dual origins: either they areglacially induced and have a high frequency or there is a change in the shape of the


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oceans due to tectonics and low frequencies occur. Despite their small size, the rapidityof high frequency eustatic events makes them the driving mechanism behind much ofthe cyclic nature of sediments. These sea level changes may be sinusoidal but thecorresponding relative changes and movements of the coastline, whether over a narrowor wide shelf, are asymmetric (figure below).

Relative sea level changes may be the products of eustatic fluctuations, but may alsobe a response to subsidence or uplift of the depositional setting. These later effects maybe related to faulting, thermal regime, salt diapirism, or isostacy (See figure below).Local differences occur where the underlying sediment compacts at different rates.Other relative changes are initiated when basins become isolated by the developmentof sedimentary or tectonic sills in conjunction with eustatic drops in sea level. When thisisolation occurs at low latitudes, evaporative drawdown often produces a correspondingdrop in sea leve


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RESPONSES TO RELATIVE SEA LEVEL CHANGES

The response of the carbonate depositional surface to relative sea level changes

include drowning of the surface, catching up with sea level rise, keeping up with therise, or build up to exposure. Drowning of a reef or platform is caused by the failure ofcarbonate production to keep pace with a relative rise of sea level so that as the waterdeepens carbonate accumulation slows and is outpaced by clastic deposition.


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The sedimentsurfaceleaves therealm ofshallow-water

carbonatesedimentationaltogetherand becomessubmergedbelow theeuphotic zone(right figure).The onset ofdrowning isexpressed by

a changefrom shallow-water faunasto deeper-watercommunitiesin reefs andon lagoonalfloors.Buildups trulyabandoned

by a risingsea arecommonlycapped by asubmarinehardgroundandenveloped bya shale capor deepwaterlimestone. An

example of adrownedrampreservoir isthe DevonianOnadaga ofNew York.


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Reservoirs in drowned buildups on rimmed margins include the Devonian Swan Hills,Leduc and Rainbow reefs of Western Canada.The Middle to Upper Triassic of RagusaField in Sicily is an example of a drowned isolated platformThe survival

of the rim ofthe platformbut not itsinterior is acomplex, butcommon,responseintermediatebetweencompletefailure and

completesuccess of aplatform'sability tosurvive. Therate ofrelative sealevel rise issuch that onlysedimentationon the

platform rim(normally areef) and/orisolated patchreefs on theplatforminterior keepspace whilethe remainderof theplatform is

drowned,becoming adeep lagoonor shelf sea(right &figuresbelow).


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This response, which probably occurs when the rising sea flooded the platforms topsafter a period of exposure, is probably the result of small but rapid eustatic rises. Thepattern tends to develop in stages: initially the rate of rise exceeds the growth rate ofboth rim and interior, and the depositional setting shifts to deeper and more open-marine conditions. Carbonate accumulation slows and widespread submarine

hardgrounds develop.

The lag phase is followed by catch-up. During the catch-up phase the reef rim andnewly established patch reefs in the interior accumulate faster and build to sea level.


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A third phasemay followwhen theinterior lagoon

fills up and aflat platformtop is re-established.Reservoirexamples ofsuch rimmedmargins arethe LowerCretaceousSligo and

Stuart Citytrends of theU. S. GulfCoast.Pennsylvanianproduction inAneth Field inthe ParadoxBasin is froma catch-upramp.

Examples ofan interiorshelf that hascaught up andkept up arethe JurassicArab "D" andCretaceousNatihFormations ofthe Middle

East and thePermianGrayburgFormation ofthe MidlandBasin.


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In the keep-up response,growthpotential ofrim and

interiormatches orexceeds therate ofrelative rise(figureright). Theplatforminterior fills tosea level,and in most

cases theplatform rimprogradesseaward,building onexcesssedimentdumped onthe flanks.These rimsmay consist

of reefs orstackedcarbonatesand shoalsthat showlittleapparentchange inwater depthduringdeposition.

The depositional environment over the platform interior varies from supratidal to very-shallow subtidal. During sea level rises, shelf width usually increases and elastics areconfined to the landward side of the shelf.

Shoaling upward carbonate sequences usually represent sedimentation, particularlytoward the seaward margin of the shelf. Occasionally, during rises, isolated depressionsland-ward of the shelf margins produced by wind deflation during a sea level low or


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during growth of a rimmed margin become evaporative lagoons. Individual shoalingcycles tend to be widespread and where clastic supply is low, are frequently terminatedby supratidal evaporite sequences. On narrow shelves with low clastic supply,carbonates may dominate the seaward margin of a clastic-dominated shelf. Whereclastic supply is high, carbonates and elastics interfinger rhythmically. When a relative

sea level rise slows a carbonate shelf system can be expected to fill up to the supratidalwith the excess sediment causing the coast to prograde seaward. Examples ofproducing shelf margins where carbonate production has kept up with sea level rise arethe Permian reservoirs in the Delaware and Midland Basins.Shoaling upward cycles in carbonates are common in stable platform and shelves (figure below).

The supratidalevaporites associatedwith these shoaling

upward carbonatesare formed onlyduring sea level risesand should notmistakenly beinterpreted as formingduring sea level falls.The shelf interior hasfew complete shoalingupward cyclesbecause hiatuses are

common and not allsea level rises extendall the way across theshelf interior. Incontrast, the shelfmargin and basincenters may lackshallow watersediments becausethe subsidence is sorapid that evidence of

the progradationcycles is obscured.

Thus, where subsidence is extremely fast, as on a basin margin immediately followingcontinental breakup, the effects of rapid subsidence may hide cycles. Instead of theasymmetric shoaling upward cycles common to stable shelves, symmetrical shoalingand deepening cycles might be predicted.


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While carbonate platforms and reefs have the potential to keep pace with all but thefastest rises in relative sea level, they are very poorly equipped to shift the loci ofcarbonate production and deposition when there is a relative drop in sea level. Theflanks of platforms are usually so steep that reefs or other carbonate fades belts areunable to gradually migrate down slope following the retreating sea. Beach erosion and

subsequent terrestrial weathering, quickly remove what little sediment is depositedduring this retreat. Consequently, the most common record of sea level drops oncarbonate platforms is a subaerial hiatus associated with karst development, clifferosion, leaching and possibly dolomitization.


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Duringstillstands inthe retreat ofthe sea, theconnections

of the basinto the opensea may beclosed byfringingreefs orstructuralhighs. Thistendencytowardisolation of

the basinand the lackof clasticinflux makescarbonatebasinsparticularlyprone toevaporitedepositionwhen sea

level drops(figureright).Typically asea leveldropterminatescarbonatedeposition;the exposedshelf is

cementedso littledetritus isshed anderosion ofthe elasticstrapped onthe shelf is


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minimal.

However, some basins at sea level lows are dominated by shelf derived elasticsbecause their access to the open sea makes them non-evaporative (figure above).

Reservoir examples of clastic offlap and smothering of downramp buildups are thePermian Scurry and Jameson Formations of the Midland Basin.

carbonate plateform

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