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Beachrock
Beachrock is a friable to well-cemented sedimentary rock that consists of a variable mixture of gravel-, sand-, and silt-sized sediment that is cemented with carbonate minerals and has formed along a shoreline. Depending on location, the sediment that is cemented to form beachrock can consist of a variable mixture of shells, coral fragments, rock fragments of different types, and other materials. It can contain scattered artifacts, pieces of wood, and coconuts. Beachrock typically forms within the intertidal zone within tropical or semitropical regions. However, Quaternary beachrock is also found as far north and south as 60' latitude.〔Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, J.A., eds. (2005) ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ISBN 0-922152-76-4〕〔Scholle, P.A., D.G. Bebout, and C.H. Moore (1983) ''Carbonate Depositional Environments.'' Memoir no. 33. Tulsa, Oklahoma, American Association of Petroleum Geologists. 708 pp. ISBN 978-0-89181-310-1〕
== Overview ==
Beachrock units form under a thin cover of sediment and generally overlie unconsolidated sand. They typically consist of multiple units, representing multiple episodes of cementation and exposure. The mineralogy of beachrocks is mainly high-magnesium calcite or aragonite. The main processes involved in the cementation are : supersaturation with CaCO3 through direct evaporation of seawater,〔Scoffin, T.P. & Stoddart, D.R. 1983. Beachrock and intertidal sediments, Chemical Sediments and Geomorphology. Academic Press, Inc.〕 groundwater CO2 degassing in the vadose zone,〔Hanor, J.S. 1978. Precipitation of Beachrock Cements: Mixing of Marine and Meteoric Waters Vs. Co2-Degassing.〕 mixing of marine and meteoric water fluxes〔SCHMALZ, R.F. 1971. Formation of beachrock at Eniwetok Atoll.〕 and precipitation of micritic calcium carbonate as a byproduct of microbiological activity.〔Neumeier, U. 1998. The role of microbial activity in early cementation of beachrocks (intertidal sediments). PhD Thesis, University of Geneva. Terre et Environment (12).〕
On retreating coasts, outcrops of beachrock may be evident offshore where they may act as a barrier against coastal erosion. Beachrock presence can also induce sediment deficiency in a beach and out-synch its wave regime. Because beachrock is lithified within the intertidal zone and because it commonly forms in a few years, its potential as an indicator of past sea level is important.
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