Abstract

Modern marine carbonate environments do not correspond precisely to the ancient ones, qualitatively or quantitatively. General classification of marine sediments should be redesigned to include Pelagic, Terrigenous, Volcanic and Biohermal (where "biohermal" is taken to mean all the sediments associated with organic reef structures, massive, clastic or chemical). Both pelagic and biohermal suites may be largely carbonate, and the others may contain smaller quantities. In origin the bulk of modern carbonates are organic or organogenic, being partly neritic, partly bathyal (pelagic). In view of the organic origin of petroleum, a basic genetic relationship exists with carbonate environments.

In the past the bulk of limestones were laid down in neritic environments. Their formation would be favored by thalassocratic sea-level conditions. In these warm shallow regions large amounts of magnesium carbonate were concentrated within organic calcite crystal lattices. This is the only important source of marine magnesium carbonate, but the resultant solid solutions are unstable. This leads to mobilization of small amounts of Ca and Mg ions which, in an alkaline medium (under reducing conditions and a high pH) and an unlimited supply of additional Mg ions (from sea or connate water), provides a potential source of dolomite. Under thermodynamic control, dolomitization may proceed at any appropriate stage of diagenesis, from soon after sedimentation to the post-orogenic period.