ABSTRACT

Inorganic precipitation of calcium carbonate from modern seawater is currently taking place inside pore spaces of some recently weathered and dedolomitized ferroan dolostones of Carboniferous age exposed in the littoral zone on the North Sea coast of northeast England. The precipitating carbonates fill partly or completely microcavities, solution pores and microfractures and occur in four texturally different forms, namely: 1) brown finely crystalline fringes; 2) clear bladed-equant crusts; 3) granular mosaic; and 4) blocky and massive cements. The only precipitating carbonate phase is magnesian calcite whilst aragonite is entirely absent.

The ferroan dolostones are currently weathered by modern seawater and are gradually dedolomitized and altered into rusty-looking yellowish brown rocks containing iron hydroxides, dedolomite calcite with or without some dolomite relics. During the process of dedolomitization and alteration, apparently, some of the ferroan dolomites undergo dissolution releasing some Ca²⁺, Mg²⁺ and HCO₃^- into seawater remaining within pore spaces during low tides. Aided by evaporation, pore water may become sufficiently supersaturated with respect to CaCO₃ to allow the precipitation of intergranular calcite cement to take place. Presumably, this level of supersaturation as well as the temperature of the waters are never high enough to permit the pre ipitation of aragonite.

The MgCO₃ contents of the intergranular calcite cements are below 10 mole percent. This is lower than expected in calcites precipitating from seawater in equilibrium with the atmosphere. Temperature and dissolution of altered ferroan dolomites appear to be the critical factors which determine the MgCO₃ content of these cements.