Cleavage development in dolomite of the Elbrook Formation, southwest Virginia

Well-developed cleavage in argillaceous Cambrian Elbrook Formation dolomite has been analyzed petrographically and chemically to evaluate the deformation mechanisms responsible for cleavage formation and the lithologic controls on these mechanisms. Two main types of cleavage occur: moderately spaced cleavage defined by coarse, continuous, wavy domains, and narrowly to closely spaced cleavage defined by fine, discontinuous, anastomosing domains.

The two most important deformation mechanisms during cleavage formation were pressure solution and mechanical rotation. Evidence for pressure solution includes off-set sedimentary structures, grain dissolution, syn-cleavage fibrous veins, and accumulations of quartz, phyllosilicates, and pyrite in cleavage domains. Pressure solution was enhanced by the presence of clay minerals.

A model for cleavage development in dolomite is presented in which cleavage initiates by the dissolution of dolomite grains at dolomite-phyllosilicate grain boundaries. Rotation of phyllosilicate grains concomitant with dolomite removal resulted in a strong preferred orientation of layer silicates within cleavage domains. Finer cleavage domains coalesced to form coarser domains as the intervening lithon dolomite was dissolved. This model is comparable to those previously proposed for cleavage development in argillaceous limestones and siltstones.