Calcrete on
Pleistocene Coastal Ridges, West Alexandria, Egypt:
Sedimentary Nature and Applications
Mohamed A. Rashed
Dept. of Geology, Fac. Sciences, Alexandria Univ., Egypt |
The Pleistocene limestones (calcarenite) of
the NW coastal area, Egypt, are covered with an indurated calcrete composed dominantly of
low-Mg calcite micrite may and/or may not incorporate some of the underlying limestones.
The calcrete was formed in a vadose diagenetic environment. Some of the CaCO3
was derived from the limestone bedrock, soil cover, and some from wind blown salt spray.
The limestone bedrock was altered by dissolution, brecciation, micritization,
sparmicritization and biogenic processes (algae, fungi and root hairs). The calcite
precipitation was passive (cement), displacive or replacive. Displacive introduction of
low-Mg calcite took place from highly super-saturated solutions due to rapid evaporation
(with CO2 loss) of vadose waters with increase of Mg content in solution (in a
closed system). This was combined with Mg release during high-Mg to low-Mg calcite
transformation, inducing precipitation of authigenic Palygorskite and minor dolomite.
The studied calcrete have carbon-isotope composition markedly lighter (lower C13
/C12 ) than those of the limestones bedrock. The 3 most likely sources as
revealed by isotopic composition are rain water, soil and plants, and grains of the
Pleistocene limestone bedrock. The present information suggests that organic sources of
carbonated carbon are important to the formation of the calcrete crust in the NW coastal
area.
For their abundance, physical, chemical and mechanical characterstics, these calcretes are
found to be suitable to be used as a coarse aggregate for concrete making and as a
railroad ballast.
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