Beachrock

Figure 6.6. Photomicrographs of common marine cements. A) Whalebone Bay, Bermuda, 3m water depth. B) Beachrock from the Bahamas. C) Subaerial Holocene from the Bahamas. (Photomicrographs courtesy of R. Perkins.)...

The formation of beachrock will be examined as an example of carbonate cement formation, because it has been extensively investigated and because it represents a chance to study carbonate cement emplacement under conditions where the rate of cement precipitation is relatively rapid and the associated solutions can be analyzed directly. It also differs from the cementation process in our model in that carbon dioxide can be degassed to the atmosphere, resulting in major changes in the saturation state of the cementing solution. 

As the name implies, beachrock generally forms in the intertidal to supratidal zone. Magnesian calcite and aragonite are the common cement-forming minerals in... 

Hanor J.S. (1978) Precipitation of beachrock cements Mixing of marine and meteoric waters vs. CO2 degassing. J. Sediment. Petrol. 48,489-501. 

Meyers J.H. (1987) Marine vadose beachrock cementation by cryptocrystalline magnesian calcite—Maui, Hawaii. J. Sediment. Petrol. 57,558-570. 

Schmalz R.F. (1971) Beachrock formation at Eniwetok Atoll, In Carbonate Cements (ed. O.P. Bricker), pp. 17-24. John Hopkins Univ. Press, Baltimore, MD. 

Krumbein, W.E., 1979. Photolithotrophic and chemoorganotrophic activity of bacteria and algae as related to beachrock formation and degradation.(Gulf of Aqaba, Sinai). Geomicrobiol. J., in press. 

Beachrock is a friable to well-cemented sedimentary rock that results from rapid lithification of sand and/or gravel by calcium carbonate cement precipitation in the intertidal zone. It occurs predominantly on tropical ocean coasts, but is also found in temperate realms that extend up to 60° latitude. In contrast to the implications of the name, beachrock precipitation phenomena are not restricted to beaches but also occur on reef ridges, tidal flats and in tidal channels. Intertidal beachrock may be confused with other sediments lithified in the intertidal and subtidal zones, such as hardened crusts or certain reef limestones. 

Beachrock occurs predominantly on tropical to subtropical ocean coasts and islands (e.g. Russell, 1962 Krumbein, 1979 Scoffin and Stoddart, 1983), but it is also found in the Mediterranean (Alexandersson, 1972 El-Sayed, 1988 Strasser et al., 1989), the Black and Caspian Seas (Zenkovitch, 1967, pp. 183-186), South Africa (Siesser, 1974 Cooper, 1991) and as far north as 57° latitude (Knox, 1973 Kneale and Viles, 2000). Beachrock is also reported to form on the coasts of freshwater lakes (Binkley et al., 1980 Jones et al., 1997). Beachrock occurrences from polar regions, i.e. north and south of 60° latitude have not been reported. Collectively, this distributional evidence identifies warm climates with pore waters rich in calcium carbonate as essential cement-precipitation prerequisites for beachrock formation. 

Figure 11.1 Photographs of beachrock outcrops. (A) Seaward dipping layers of beachrock on the northwest coast of Basse Terre, Guadeloupe, West Indies. This is the area that Charles Lyell (1832) mentioned in his Principles of Geolog/. Beachrock consists largely of quartz and volcanic sand cemented by calcium carbonate. (B) Extensive outcrop of beachrock, which also exhibits significant erosion. Windward side of Halfmoon Cay, Lighthouse Reef, Belize. (C) Beachrock ridge on southern shore of Kuramathi island, Rasdu Atoll, Maldives. Note flat and karstified top of outcrop, which resulted from meteoric dissolution.

Figure 11.2 Photographs of beachrock outcrops. (A) Beachrock outcrop with slight inclination of layers towards the sea. Kubbar Island, Kuwait, Arabian—Persian Gulf. (B) Close-up of same location, which shows rectangular fracturing of beachrock. (C) Cemented low beach ridge, which is partly eroded. Tidal flats on...

Figure 11.3 Outcrop and hand specimens of beachrock. (A) Freshly broken piece of beachrock with mostly sand-sized constituent particles. Kuramathi island, Rasdu Atoll, Maldives. Height of picture is 5 cm. (B) Beachrock with mostly cobble-sized fragments of coral. Long Cay, Glovers Reef, Belize. (C) Close-up of fossil beachrock outcrop from Ras Al-Julayah, southern Kuwait. Note how fine-grained layers are better cemented than coarse-grained ones. (For overview of outcrop see Figure 11.9A.)...

Figure 11.4 (A) Diagenetic environments and typical cements (modified from Fliigel, 1978). (B) Formation of beachrock in relation to other cemented coastal deposits (schematic).

Cements from meteoric waters may develop in beachrock during precipitation from percolating calcium-carbonate-saturated rain water during low... 

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