Detrital Limestone on Bermuda
January 21, 2006
Limestone is a 'chemical sedimentary rock' composed primarily of calcite (CaCO3) precipitated from water. It may form inorganically by precipitation of calcite from saturated water, biochemically in the formation of animal hard-parts, or as a combination of the two processes. Examples of inorganically formed limestones are stalactites and stalagmites, found in caves. Calcium carbonate dissolved in the water that drips in caves may precipitate out as icicle like formations. Examples of biochemically produced limestones are coral reefs and chalk. Coral reefs are formed of the skeletons of coral invertebrate animals. Chalk is a soft rock made up of the skeletal parts of microscopic marine organisms that settle to the bottom of warm shallow seas.
A combination of inorganic and biochemical processes forms coquina. Coquina is the name given to limestone in the form of loosely compressed and cemented shell fragments. This is sometimes called detrital limestone if it contains large shell fragments or complete shells. Oftentimes, the limestone is softer or less acid-resistant than the shell fragments. This results in the fragments presenting on the surface of the limestone, as the surrounding material dissolves and/or erodes away. This phenomenon is analogous, in miniature, to the formation of volcanic chimneys (such as Devil's Tower) or intrusive dikes (such as the Palisades).
The photograph above shows an example is of coquina, or detrital limestone -- it was taken on the south coast of Bermuda in March of 2005. An embedded spiral-shelled gastropod is seen presenting, as the softer surrounding limestone is dissolved into seawater and eroded by wind and sand.Related Links: