beachrock Beachrock is a lithified sediment which forms as a result of cementation, usually by calcium carbonate, in the beach zone of mainly tropical and subtropical coasts. Similar cementation also occurs within some marine intertidal flat sediments and storm ridges which lie above the normal high water limit. Examples of lake-shore, estuarine, and lagoon beachrock are also known. In the majority of beachrocks the detrital sediment grains remain in contact with one another; that is, the sediment maintains a clast-supported fabric, while between 10 and 100 per cent of the intergranular porosity is filled by precipitated calcium carbonate.
The size distribution of the detrital sediments varies widely, ranging from relatively clean, well-sorted sands to poorly sorted conglomerates and muddy sands. Volcaniclastic grains are common on volcanic islands (for example the Canary Islands); quartz, feldspars, and lithic grains dominate on many continental mainland shores (such as Egypt, Israel, and Crete); while carbonate grains, including ooids, coral fragments, shell debris and algae, are predominant on beaches with relatively low rates of clastic sediment supply (for example the Bahamas and the Florida Keys). Aragonite (CaCO
3) and magnesium calcite (containing 10–14mol per cent MgCO
3) are the two most important cementing minerals, particularly in tropical and subtropical beachrocks. Occasionally low-magnesium calcite, dolomite, siderite, and other minerals also occur as major components of the cement. These minerals tend be more common in lacustrine, estuarine, and lagoonal settings.
Most marine beachrocks form below the surface of the beach, in the zone affected by fluctuating groundwater. As long ago as (1851), J. D. Dana was one of the first scientists to suggest that tropical marine beachrock forms as a result of evaporation of sea water at low tide, a view endorsed by many subsequent authors. An alternative mechanism involves lithification by magnesium calcite, aragonite, or dolomite brought about by the subsurface mixing of sea water with fresh (or brackish) waters draining from the land. A few beachrocks, composed mainly of low-magnesium calcite, have also apparently formed entirely as a result of evaporation of land-derived fresh waters rich in calcium carbonate.
Although beachrock formation can be an entirely inorganic process, microbial activity and degradation of organic matter, including upward movement of microbial degradation products (such as carbon dioxide and methane) from greater depths, often play a part. Analysis of the stable carbon and oxygen isotope composition of the cements, together with the nature of trace and rare-earth elements, is often useful in identifying the relative importance of different carbonate sources.
A wide variety of cement textures has been described from beachrocks. Textures developed in the relatively free-draining vadose zone above the lowest level of the beach groundwater table are often dominated by
meniscus cements, formed by precipitation during evaporation of water films held at points of grain contact, and
pendant cements, formed by evaporation of moisture held by gravity on the underside of grains. Cements formed by precipitation from supersaturated fluids below the lowest level of the beach groundwater table often form
rim cements which encircle the grains, or
blocky cements which completely fill the pore spaces. In practice, many beachrocks show more than one type of cement fabric, because of spatial and temporal fluctuations in groundwater levels. Partial or complete replacement of detrital grains, composed of both carbonate and siliciclastic material, is common.
The cementation of beachrock can be very rapid by geological standards. Beachrocks on Pacific atolls which contain military artefacts have reportedly formed within less than seven years. Reports have also indicated that on some Indian Ocean islands it is possible to make an annual harvest of beachrock for use as a building stone. In other instances, however, beachrocks have been shown by uranium-series and radiocarbon dating to be several thousands to hundreds of thousands of years old. Fossil beachrock has been used widely as a palaeo-sea-level indicator, and lateral variations in the elevation of beach rocks can provide useful information about recent tectonic deformation associated with earthquakes and volcanic activity. Accurate radiometric dating of beachrocks can be difficult, however, owing to diagenetic changes which involve exchange of radioelements between the cements and ambient fluids, possibly leading to under or overestimation of the true ages.
Beachrock is often exposed at the surface on shores where the overlying sediment has been eroded, or where the beachrock has been exposed subaerially by a fall in relative sea level. In such instances the upper surface of the beachrock may exhibit a variety of karstic weathering features, including miniature dolines (sink-holes) and pinnacles. Exposure to rainwater, sea spray, and endolithic organisms may also bring about changes in the rock texture and mineralogical composition.
K. Pye
Bibliography
Scoffin, T. P. and and Stoddart, D. R. (1983) Beachrock and intertidal cements. In Goudie, A. S. and Pye, K., (eds) Chemical sediments and geomorphology, pp. 401–25. Academic Press, London.
