Beachrock formation

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 (1978) also discussed the mechanisms that may be responsible for degassing of groundwaters in the supratidal zone. His ranking of processes in order of probable importance was 1) tidally-induced oscillations of the groundwater table, 2) tidal pumping at the sediment-atmosphere interface, 3) a decrease in the average depth of the water table, and 4) a seaward increase in porosity. 

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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 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. 

Cooper, J.A.G. (1991) Beachrock formation in low latitudes implications for coastal evolutionary models. Marine Geology 98, 145-154. 

Davies, P.J. Kinsey, D.W. (1973) Organic and inorganic factors in recent beachrock formation, Heron Island, Great Barrier Reef. Journal of Sedimentary Petrology 43, 59-81. 

Important reviews on beachrock formation are included in Ginsburg (1953), Nesteroff (1954), Russell (1960), Stoddart and Cann (1965), Friedman (1968), Scoffin (1970), Friedman and Gavish (1971), Schmalz (1971), Davies and Kinsey (1973), Siesser (1974), Friedman (1975), Gavish (1975), Hanor (1978), Krumbein (1979), Scoffin and Stoddart (1983), Gavish et al. (1985), Hopley and Davies (1986), Meyers (1987), El-Sayed (1988),... 

Beachrock formation is a diachronic and wide-ranging sedimentary process, and since lithifica-tion takes place at the coastline, beachrocks have been used as indicators in Quaternary sea-level and neotectonic studies (e.g., Kelletat 2006). 

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).

In order to understand the formation of beachrock, some fundamentals of the marine carbonate system (C02-H20-CaC03) and calcium carbonate precipitation and dissolution have to be considered. Only limited aspects of the subject can be discussed here as the topic is complex and treated at length in various textbooks (e.g. Lippmann, 1973 Berner, 1980 Morse and Mackenzie, 1990). 

Beachrock forms due to cementation of beach sediments under a thin cover of sediment in the intertidal zone. The sediment cover is important because in order to precipitate cement, a stable substrate (i.e. stable pore space) is required. Erosion of the cover of unlithified sediment leads to beachrock exposure. The rapid formation of beachrock in the intertidal zone was well-known to 19th century writers (e.g. Moresby, 1835 Gardiner, 1898). For example, inhabitants of Indo-Pacific islands were known to harvest beachrock for building stone where new occurrences formed on the same beach within a few years. Further reports confirming rapid formation... 

Frankel, E. (1968) Rate of formation of beachrock. Earth and Planetary Science Letters 4, 439-440. 

Diagenetic environments and typical cements, and the formation of beachrock in relation to other cemented... 

Nesteroff W. (1954) Sur la formation des gres de plage on beachrock en Mer Rouge. Comptes Rendus des Seances de I Academie des Sciences 238, 2547—2548. 

The coastal geomorphological features that are used as sea-level indicators are the result of either erosional or depositional processes. Ero-sional features can only be preserved on hard, solid rocks, and in some cases they crmstitute indicators of sea-level change. Such indicators are marine notches, potholes, abrasion platforms, etc. Among the depositional formations, marine terraces and beachrocks stand out as the most important sea-level indicators. 

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