Carbonates 32, Environments 1

CARBONATE ENVIRONMENTS

Brief Introduction

J.L. Wilson (1975), "Carbonate Facies in Geologic History" Springer-Verlag

Most modern carbonates are accumulating in warm, shallow waters.


Basin margins typically start off as ramps


the ramp slope steepens with continued basin subsidence


Shallow water carbonate sedimentation keeps pace with subsidence around basin margin but can not match the faster rates towards basin center. As water deepens, carbonate sedimentation rates are reduced. Shelf breakof slope results.


The break of slope is a site of high energy from incoming waves. This promotes carbonate precipitation for reef buildingor oolitic and other sand accumulation. Sheltered conditionsdevelop behind the reef or shoal (lagoon or protected shelf).


Rimmed shelf edge or offshore carbonate banks develop in this way, adjacent to deep water areas.

Andros Island and Tongue of the Oceans

oolite shoals

shallow waterplatform carbonates

deep water

Carbonate facies are divided by the shelf margin reef or carbonate sand shoals into back-reef and fore-reef/basin

fondoform clinoform undaform

(1969) GCAGS Transactions, v. 19, p. 11-22


Wilson proposed 9 Standard Facies Belts

Carbonate Environments

• Subaerial exposure– karst

• caves, collapse breccias

– caliche

del Olmo, W.M., and M. Esteban (1983), AAPG Memoir 33, p.93

Esteban, M., and C.F. Klappa, (1983), AAPG Memoir 33, p. 6

Karst, China

Lechugilla Cave, New MexicoHamblin, W.K., and E.H. Christiansen (2001) “Earth’s Dynamic Systems” 9 th Ed., Prentice Hall

Labor Day 2000; 2000/24-22. Mile 490

Collapse breccias above zone of gypsum dissolution, Cretaceous Edwards Group, Texas

Caliche zone in modern soil profile, Texas.

from Moore, C.H., (2001), Carbonate Reservoirs, Porosity Evolution and Diagenesis in a Sequence Stratigraphic Framework. Elsevier. [book and CD]

Caliche glaebules (“pisoliths”), Del Rio, Texas

Caliche glaebules, polished surface of a thin section,reflected light

“Syncline” and “anticline”, Buda/Del Rio contact,1.1 miles east of Comstock, US-90

Lock, B.E., (2000), GCAGS Trans.

Terra Rosa, Queretaro, Mexico


• Subaerial exposure

• Freshwater – lakes and rivers

• freshwater algae (Chara, etc.) and cyanobacteria (stromatolites, oncoliths, etc.)

Algal-precipitated carbonate (LMC) on bottles etc., lake in New York state, USA

Dean, W.E. and T.D. Fouch (1983), AAPG Memoir 33, p. 115


Oncoliths with gastropod nuclei.

Upper: - Recent, Michigan

Lower:- Flagstaff Formation(Paleocene), Utah

Oncoliths, Flagstaff Limestone (Paleocene-Eocene), Utah.Freshwater lake deposits.

Stromatolites, Green River Formation, Eocene, Utah, USA.


SEM views ofChara, ostracods,

small clam, all from freshwater muds,

Lower Cretaceous,Arkansas, USA.

Travertine dams, pools, southern Tamaulipas State,

Mexico

Recent travertine, Seven Rivers, New Mexico

Shore of Great salt Lake, Utah.

inset: ooliths in thin section.

oolith sand



• Freshwater

• Eolian– wind-blown carbonate sands (mainly coastal)

McKee, E. D., and W.C. Ward (1983), AAPG Memoir 33, p. 153

Carbonate eolianite, Castle Rock, Bermuda



• Freshwater

• Eolian

• Coastal– transgressive/regressive?– arid/humid?– wave or tide dominated?– supratidal, intertidal, subtidal zones

CARBONATETIDAL FLAT MODELS

Andros Island, transgressive, humid,

mesotidal

marsh, tidal creeks,intertidal algal flats

beach ridge

Persian Gulf,regressive, arid,

microtidal

sabkha (supratidal),abundant evaporites,intertidal algal mat,

high and low energy lagoon,barrier island

Shinn, E.A. (1983), AAPG Memoir 33, p. 172

Supratidal

• Arid (sabkha)– deflation surfaces– intrastratal evaporites (mainly gypsum or

anhydrite) as lenticular crystals, nodules, contorted beds (enterolithic), massive layers (“cottage cheese”, mosaic or chicken-wire), halite molds

– dolomite

Enterolithic (contorted) anhydrite, deflation surface, Abu Dhabi sabkha


cores of sabkha sediments, Abu Dhabi

marine subtidal sediment with gypsum crystals algal mat cottage cheese anhydrite nodules

Sabkha lithofacies, Arab Formation (Jurassic), Abu DhabiGrotsch, et al. (2003), GeoArabia v. 8, p. 47-86

Sabkha lithofacies, Arab Formation (Jurassic), Abu Dhabi

Grotsch, et al. (2003),GeoArabia v. 8, p. 47-86

Inner ramp cross-bedded ooliticgrainstones

Mid ramp and outerramp bioturbated

wackestones

gyps nodule molds

molds from dissolution of gypsum nodules, Cretaceous, Texas

Evaporites may dissolve and leave collapse breccias (Texas Cretaceous)

supratidal (sabkha)dolomite bed,

Cretaceous, Texas

parasequence boundary

dolomite (dark rock)

Supratidal

• Arid (sabkha)

• Humid– marshes (peat/coal), roots, cerithid

gastropods


Andros Island

Intertidal

• Tidal creeks– high to moderate energy; alternating– grainstones, flat-pebble conglomerates– wavy, lenticular and flaser bedding,

bidirectional currents– fining-up

Shinn, E.A. (1983), AAPG Memoir 33, p. 192Tidal creek, based on Andros Island

Flat-pebble conglomerate, tidal flats. Kindblade Formation, Ordovician, Oklahoma.

Shoe, for scale

dolomitized tidal bedding, Cretaceous, Texas

“herring-bone” cross bedding (tidal, bi-directional), Cretaceous, Texas.

Intertidal

• Tidal creeks

• Flats– fine grained sediments– scoured surfaces common– burrowed– algal laminations, stromatolites, birdseye

structures– surface crusts (dolomite)

Birdseye (= fenestral) structure, Mississippian, Kentucky, USA.

scoured surface

Algal mat, Baffin Bay, south Texas coast

“crinkly” algal mat lamination, Permian, New Mexico

Stromatolites, tidal flats. Kindblade Formation, Ordovician, Oklahoma.

5 cm

Arid tidal flats, Sonoran coast, Mexico (siliciclastic sands): note tidal creeks feedsediment on to the flats. Dark areas are halophyte plants.

Dolomitic crust (white) beneath erosional surface, Cretaceous tidal flats, El Abra.

Modern tidal flats, Sonora. Dolomitic crusts eroded by tidal creek.

Subtidal/Lagoon

• High or low energy, depending on lagoon size, etc.– carbonate mudstones to grainstones

• Salinity may fluctuate (fresh water flooding, high salinities from evaporation)– high stress: flora and fauna may not be normal marine

• miliolid forams etc., serpulids, ostracods, no echinoderms.

• Shoaling-up cycles

Peritidal cycles(supratidal/intertidal/subtidal),

Cretaceous, Texas

Peters et al., GeoArabia v. 8 (2), 2003


Surface-exposedsalt domes, Oman.

Blocks of interbedded

shallow waterlimestone are

scattered over the ground.

] saltlimestone reservoirs

Limestones were interbeddedwith the original salt (latestPrecambrian) and contain

stromatolites, crinkly laminationand other shallow water features. Microbial mat

limestones are dark and fetid,believed to be source rocks in deeper parts of the basin. Red

clastics are interbedded withthe carbonates.


Beach and/or Barrier Island

• High energy, coarsening-up sequence

• Seaward-dipping lamination, low-angle cross-bedding (summer-winter variation in beach foreshore slope), some higher angle landward-dipping cross-beds (offshore bars).

• Festoon cross-bedding in tidal channels (coarse-based units)

• Beach-rock (vadose and phreatic cement textures, marine cement mineralogies)

Inden, R.F., and C.H. Moore (1983), AAPG Memoir 33, p. 213

Festoon cross-bedding, tidal inlet channel

Beach rock, Bahia Adair, Sonora.

Thin-section photomicrograph, beach rock from Bahia Adair. Note aragonite needle cements

Carbonates 32, Environments 1

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