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