Carbonate Depositional Environments - Microfacies of Carbonate Rocks

Geology Reference

In-Depth Information

Box 2.3. Criteria of peritidal and subtidal zones.

Supratidal zone (supralittoral): Shore zone above normal high tide, flooded only during storms and semi-monthly

high spring tides. The zone includes marshes, mangrove forests and diverse coastal ponds of different salinity. May

become evaporitic in arid or semiarid climates. Evaporitic supratidal flats are called sabkhas. The width of this zone

depends on the relief and dip of the coasts.

Intertidal zone (littoral, eulittoral) : Near-coast zone between normal high- and low-tide levels which is alternatively

flooded and exposed on a diurnal or semi-diurnal basis. Transitional area from marine to terrestrial conditions, dotted

with brackish and saline ponds and dissected by subtidal creeks. This zone exhibits a variety of biological, sedimentary

and diagenetic criteria. Because many of these criteria are preserved in the geological record, ancient intertidal

carbonates are of major importance as key markers in facies studies.

Subtidal zone (sublittoral, circalittoral) : Marine zone below low-tide level, seaward of the tidal flats, extending down

to the edge of the continental shelf and underlying the neritic zone. Water depths between a few decades of meters

and 100 to 200 m. Includes low- and high-energy environments. The shallowest parts can be influenced by tidal

currents and may be briefly exposed by semi-monthly neap tides. The zone is illuminated and includes shallow and

deep subtidal subenvironments. Subdivisions are predominantly based on the distribution and composition of benthic

biota.

ditions resulting in an extreme rich variety of sedimen-

tary, early diagenetic and paleontological features re-

corded in carbonate rocks.

(5) Owing to the large number of combinations of

circulation, salinity, tides and climate, care must be

taken, to use just a single 'model' in describing ancient

tidal deposits.

(6) Tidal flats are wave-protected and, therefore pref-

erential sites for the accumulation of mud. Offshore

mud is transported shoreward onto tidal flat currents

and storms, and mud can also be produced on supratidal

flats.

(7) Carbonate tidal flats nucleate on a continental

mainland (Persian Gulf), isolated islands with a lagoon

(Florida Bay), and platform-margin islands (Great Ba-

hama Banks).

(8) Warm and clear waters of shallow tropical envi-

ronments are conducive to the profilific growth of al-

gal and microbial mats which influence carbonate depo-

sition by binding, trapping and precipitation in supra-

and intertidal zones.

(9) Early cementation is crucial in the preservation

of primary structures and textures.

(10) Carbonate tidal-flat facies typically occur as

stacked cycles, each ideally composed internally of an

upward succession of subtidal to intertidal to supratidal

units.

Basics: Modern transitional environments

Ginsburg, R.N. (ed., 1975): Tidal deposits, a case topic of

Recent examples and fossil counterparts. - 428 pp., Ber-

lin (Springer)

Hardie, L.A. (ed. 1977): Sedimentation on the modern tidal

flats of Northwest Andros Island. - John Hopkins Univer-

sity Studies in Geology, 22 , 202 pp.

Hardie, L.A., Shinn, E.A. (1986): Carbonate depositional en-

vironments, modern and ancient. Part 3. Tidal flats. - Colo-

rado School of Mines Quarterly, 81 , 1-74

Inden, R.F., Moore, C.H. (1983): Beach environment. - In:

Scholle, P.A., Bebout, D.G., Moore, C.H. (eds.): Carbon-

ate depositional environments. - American Asociation of

Petroleum Geolists Memoirs, 33 , 211-265

Pratt, B.R., James, N.P., Cowan, C.A. (1992): Peritidal car-

bonates. - In: Walker, R.G., James, N.P. (eds.): Facies mod-

els. Response to sea level change. - 303-322, Ottawa (Geo-

logical Association Canada)

Shinn, E.A. (1983): Tidal flat environments. - In: Scholle,

P.A., Bebout, D.G., Moore, C.H. (eds.): Carbonate depo-

sitional environments. - American Association of Petro-

leum Geologists Memoir, 33 , 171-210

Shinn, E.A. (1986): Modern carbonate tidal flats: their di-

agnostic features. - Colorado School of Mines Quart. 81 , 7-

35

Neumeier, U. (1999): Experimental modelling of beachrock

cementation under microbial influence. - Sedimentary Geol-

ogy, 126 , 35-46

Wright, V.P. (1984): Peritidal carbonate facies models: a re-

view. - Geological Journal, 19 , 309-325

Wright, V.P.: (1990): Peritidal carbonates. - In: Tucker, M.E.,

Wright, V.P.: Carbonate sedimentology. - 137-164, Ox-

ford (Blackwell)