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Carbonate buildups
Cathy Hollis
Definition of carbonate buildups Carbonate buildups are laterallyrestricted structures, which have usually undergone organicallymediated growth. They can be grossly divided into:
Reef (mud) mounds (automicrites) are inorganically and/or
biogenically constructed but lack a rigid skeletal framework and unable to withstand high energy wind/wave action
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
However, reef nomenclature remains an area of significant discussion
Reef types Framebuilt Reef mound Mud mound
Coral Stromatoporoid Red algae Stromatolites
Bryozoan Phylloid algae Sponges
Codiacean algae Sea grass Crinoids
Microbial mats
Frame builders
Binders
Bafflers
Sediment contributors
Precipitators
Modified from Tucker and Wright, 1990
Distribution of modern day reefs
http://en.wikipedia.org/wiki/Coral_reef
Controls on reef morphology Biological controls
Modern day coral growth controlled by temperature, salinity, light, wave energy, water turbidity, nutrient
concentration
W.M.Fenner/S
EPM; Q
uinana Roo, M
exico
Reef at 1520m water depth
Controls on reef morphology Topographic controls
• Often, reefs tend to preferentially develop on topographic highs, including – older reefs – karst – erosional terraces – siliciclastic or volcanic features
• May also be a (salt) tectonic control
Bosence, 2005
Controls on reef morphology Sea level • Since most reefs grow in
shallow water they are highly susceptible to sea level fluctuations
• During rising sea level, if rate of rise > rate of growth, reef will backstep and/or drown (‘give up’)
• If reef growth can keep pace with sea level rise (‘keep up’) an accretionary geometry will be developed
• If reef growth exceeds the rate of sea level rise, then flanks will prograde
• Progradation may also occur during sea level fall, which can also result in expsosure
Growth rate
Rate of sea level rise
Modified from Tucker and Wright, 1990
Drowining
Vertical accretion
Backstepping
Retreat Progradation
Textural classification of reefs Allochthonous Autochthonous
Original components not organically bound during deposition
Original components organically bound during deposition
>10% grains >2mm
Matrix supported
Supported by >2mm component
By organisms that act as baffles
By organisms which encrust and bind
By organisms which build a rigid
framework Floatstone Rudstone Bafflestone Bindstone Framestone
Modified from Embry and Klovan (1971) and James (1984)
Reef dynamics 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 reefderived detritus
Organic (skeletal) reefs Framebuilding organisms Autotrophic producers Selfnourishing, primarily by photosynthesis
• Cyanobacteria • Green algae • Red algae
Autotrophic producers via symbionts
• Large foraminfera • Hermatypic coral (rely on algae for nutrition)
• Some bivalves (?rudists)
Heterotrophic producers Nourished from an external supply of organic matter
• Foraminfera • Sponges • Ahermatypic corals (rely on planktonics for nutrition)
• Gastropods • Cephalopods • Arthropods • Brachiopods • Bryozoan • Echinoderms
Organic (skeletal) reefs Types of carbonate buildup
after Tucker and Wright, 1990
Fringing reef
Patch reef
Barrier reef
Faro reef Atoll Pinnacle
Fringing reef: attached to coastline
Barrier reef: separated from coastline by a lagoon Patch reef: isolated reef
Faro reef: atoll within a lagoon Atoll: ringlike structure with central lagoon in deep water Pinnacle: flattopped, no lagoon, deep water
Growth form of reefbuilding metazoans Growth form Wave energy Sedimentation
Modified from Pomar et al, 1985; Tucker and Wright, 1990
Delicate, branching
Thin, delicate, plate like
Globular, bulbous, columnar
Robust, dendroid, branching
Hemispherical, domal, irregular, massive
Encrusting
Tabular
Low
Low
Moderate
Moderate high
Moderate high
Intense
Moderate
High
Low
High
Moderate
Low
Low
Low
Reef facies Reef front and crest
Colum
bus Cay, B
elize, Central A
merica, E. G
. Purdy Pu
rdy et.
al. (1975)/S
EPM
Spur and groove
Reef crest
Reef flat algal coated reef rubble
Reef apron carbonate
sand
• Reef front extends from highest point on reef to seaward depth where little/no framebuilding (up to 100m)
• Reef crest is the highest, most exposed part of reef and therefore very high energy
Reef front
Reef facies Reef crest: composition • Reef crest 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 are high. May be undergo periodic subaerial exposure
• Recognised in ancient carbonates as bindstones/framestones with laminar encrusting organisms
Acropora Palmata
Millepora (fire coral)
Porites
Ed Purdy/SEPM
Reef facies Reef front
few m’s to ~20m
BarryGuimbellot, Grand Cayman
BarryGuimbellot, Grand Cayman
W.Mayhew (1988)/Quintana Roo, M
exico
<10m
~10m
1015m
Acropora Palmata
Monastrea annularis
SEPM
• Coral growth is extensive seaward of the reef crest and forms the ‘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 (probably constructional and wave influenced)
• Biota evolves with depth as light penetration and energy decreases
• Low preservation potential due to bioerosion and early diagenesis
W.Mayhew (1988)/Quintana Roo, M
exico
Gorgonians (soft coral)
Halimeda
Sponge
Reef facies Reef front
SEPM
H.Roberts/Cayman Brac
Monastrea, Diploria, Agaricia
BarryGuimbellot, Grand Cayman
Agelas clathrodes (elephant ear sponge)
>15m (to ~200m)
26m
1520m
Spatial distribution of growth forms
Pomar , 1991
Reef facies Reef front spur and groove
Lighthouse reef, Belize, Central America Lowaltitude oblique aerial photograph showing linear zonation of the Belize Barrier Reef
Cliff Jordan/SEPM
spurandgroove zone
surf zone
grainstone belt
muddy carbonate sands
coral growth
Reef facies Forereef slope
~115m
~30m
Rock wall
Forereef talus
Noel P
. Jam
es/SEPM
/ Belize Barrier
The coarse forereef sediment rich in Halimeda plates from a water depth of 1025 meters. Scale bar is 2 mm long
• Forereef slope is positioned seaward of the reef front, and provides the transition into the basin
• Sedimentation is dominated by gravity flow mechanisms and deposition of pelagic sediments
• Depositional/accretionary reef margins slope continuously into the basin • Bypass margins have a steep escarpment seperating the reef from reef talus
Carbonate platforms Rimmed carbonate shelf margins
Tucker and Wright, 1990; Moore, 2001
Reef facies Reef flat: pavement
Landward edge of reef flat with large gastropods (Strombus gigas), soft corals, and Halimeda on gravelly substratum. L.A. = 1.7 m Sal J. Mazzullo/SEPM
• Reef flat located behind the reef crest and is partially protected by it
• Reef pavement forms directly shoreward of the crest, few metres of water depth and possibly exposed. Some coral growth, abundant rubble
• Zone of intense bioerosion. Boulders may be algal encrusted
• Up to +100m wide (Belize)
Reef facies Reef flat: sand apron
• Develops behind reef pavement, water depths up to 10m. May be up to 160km long, parallel to reef and 100200m wide
• Comprise reworked reef debris and carbonate sand, with local colonisation by sea grass and algal mats
• Gradational contact with backreef lagoon
Reef crest and front
Rum point Sand apron
Back reef lagoon
Harry Roberts/SEP
M/ G
rand Cayman
Reef facies Backreef lagoon • Lagoons are areas of low energy sedimentation, protected from the open ocean by the reef rim
• Width varies from 100’s metres to km wide. Water depth may be <10m, up to >70m in some Pacific atolls
• Usually areas of relatively low energy sedimentation: finer grained sediment than towards reef margin.
• Bioturbation may be pervasive • Abundant calcareous algae • Peritidal environments may develop Turks and Caicos
Open ocean
Reef crest and front
Back reef lagoon (with patch reefs)
Reef facies Backreef lagoon:patch reefs • Isolated reefs, commonly in backreef lagoons
• Growth initiated as coral knobs which coalesce and grows upwards to reach wave base; may show bio zonation
• Switch to lateral accretion as reef reaches sea level
• In lower energy environments, sediment trapping and binding by algae and sea grasses may lead to mud mound development
E.Shinn/SEPM; Belize
Patch reef
~200m
Pleistocene, Turks and Caicos
Nonframe constructed reefs Reef mound/‘Cluster reefs’/bioherm
• Buildups in which insitu skeletons are not in contact
• Stability provided by matrix, limited cementation
• Organic components include bivalves (rudists), stromatoporoids, corals, sponges. Can grade into framebuilt reefs
• Low topographic relief do not offer significant wave resistance, but can trap sediment
Riding, 2001
Nonframe constructed reefs Microbial buildups
• Organic reef, formed from microbially trapped sediment (Riding, 2001), with some early cementation
• Modern examples in Shark Bay, Australia, and Bahamas • Low topographic relief
http://www.aussieactionadventures.com/images/Warwickst4250L.jpg
Riding, 2001
Nonframe constructed reefs Mud mounds
• Fine grained, mud (micrite)dominated buildups that display topographic relief and few or no stromatolites or inplace skeletons’ (Riding, 1990)
• Occur mostly in Palaeozoic, either during periods where frame building metazoans did not flourish or in deep water settings
• Automicrite (ie. nonmicrobially derived micrite) is main constituent
• May be colonised by metazoans
Devonian mud mounds, NW Sahara; Wendt et al., 1993; Wendt et al, 1997
http://www.unigraz.at/bernd.kaufmann/mudmounds.htm
Nonframe constructed reefs Muleshoe Mound, Sacremento Mountains
Kirkby and Hunt, 1996
Exhumed mudmound (500m long, 100m high), traditionally interpreted as deep water, but with evidence for growth in a relatively high energy, currentinfluenced setting. Five facies associations, each seperated by a hiatal surface I lime mudstone core II lime mudstone core and crinoidal packstone/grainstone flank III bryozoan cementstone and grainstone core, crinoidal pst/gst flank IV bryozoan cementstone and gst core, breccia and megabreccia on flanks V lime mudstone core and flank
Stratigraphic distribution of reefs
Flügel 1997
corals & algae
rudists & corals
sponges
& corals
algae &
bryozoans &
microbes
sponges
& corals
sponges
microbes
microbes
extinction events Proterozoic
C
O
S
D
C
P
T
J
C
T
Palaeozoic
Mesozoic
Caino zoic
100
200
300
400
500
600
reefs
mud mounds
increasing abundance • The dominance of
reefs and mud mounds has varied through time in response to changes in ocean basin morphology, climate and ocean chemistry
• The composition of framebuilt reefs has changed through time in response to evolution
InfraCambrian Namibia
• Deposition predates Cambrian explosion • Microbialites dominate: stromatolites and thrombolites • Mostly form sheetlike bioherms, but can develop into patch reefs and pinnacle reefs
Grotzinger et al., 2005
InfraCambrian Ara Formation, Oman
• Unique oil and gas reservoirs in South Oman
• Saltencased ‘rafts’ of carbonate, with inner ramp succession comprising microbial buildups and carbonate sand
Schroeder et al., 2005
Devonian Canning Basin
• Dominated by stromatoporoidsponge communities. Cavities may be filled by cyanobacteria (Renalcis [R]) and radiaxial calcite [C]
• Principal reservoir in West Canada Basin, Canada, where pervasively dolomitised
Domal stromatoporoid
laminar stromatoporoid
sponge
Wood, 1998
Carboniferous Sierra del Cuella, Asturias, Spain
Kenter et al., 2005
• Laminated microbial crusts with cement filled voids and minor skeletal debris (crinoids, bryozoa, brachiopods, bivalves and massive, clotted microbialites
• Elsewhere (eg. UK) see fringing coral reefs and deep water mud mounds
Carboniferous Tengiz Field, Kazakhstan
Kenter et al., 2005
Permian Capitan Reef • Wave resistant, porous structure forming platform margin
• Dominantly constructed by sponges and bryozoa
• Capitan reef and associated facies form major reservoirs in Permian Basin of West Texas
Wood et al., 1994
S= sponge, B= bryozoan, M =microbial micrite Scale bar = 10cm
Mesozoic Reef mounds
• Skeletal organisms capable of framebuilding not abundant; dominated by microbialbound (rudist and coral) buildups
• Platform margin development locally in Lower Cretaceous, but more common as isolated buildups in midCretaceous
• Major hydrocarbon reservoirs in the Middle East
Hillgartner et al., 2003 Length= 4mm
Miocene, Mallorca
Pomar, 1991, 2003
• Shallow water, euphotic (autotrophic) coral algal reef assemblages forming steeply dipping platform margins
Miocene, Pinnacle reefs Luconia Province, Malaysia
Epting, 1989
Summary • Carbonate buildups are laterallyrestricted structures, which have usually undergone organicallymediated growth
• The growth and morphology of carbonate build ups is strongly influenced by relative sea level and the type of reefbuilding organisms. These organisms have changed through time as a function of evolution
• The morphology of the reef and the associated reef facies exert a strong control on reservoir properties