Antitaxial Fibrous Bands in Differentiated Stylolites

Syngenetic and congruent fibre bands (FB) of variable thickness occur between the differentiated stylolite seams and the host-rock blocks within the limestone of the Nimbahera Formation, Lower Vindhyan Supergroup, Rajasthan. The FBs contain three microstructural zones: selvage zone of quartz and diagenetic chlorite at FB/seam-wall interface, fibrous calcite zone in the middle and epitaxial growth zone of calcite at FB/rock-wall interface. The selvage and the epitaxial zones show syntaxial 'crack-seal' type growth, but the fibrous zone developed antitaxially, probably under Taber-growth conditions.

The fibre growth kinetics is asymmetric and the growth geometry has been controlled by the selvage-wall asperity, the latter acting as grain boundary attractors (GBA). The preferred dimensional orientation of the fibres parallel to the stylolite axial plane trace or to σ₁, irrespective of the seam orientation, track the FB opening trajectory. This indicates displacement-controlled fibre growth. A few curved fibres track transtension related dilation caused by strain resolution and local non-coaxial strain at the inclined seam margins.

A model for the origin of the FBs has been proposed. The stylolite formation being a pressure-solution phenomenon, the stylolite seams initially contain supersaturated fluid charged with insoluble residues of clay and opaque minerals. When stress relaxation takes place at the end-stage of stylolite formation, fluid pressure drops, the seam walls collapse due to volume loss and the dilation zones are created at the seam margins. These low pressure dilation zones receive the solute by either advective fluid-flow or diffusive mass-transfer, the fluid having been expelled from the shrinking seam mush. This causes the antitaxial fibre growth. The FBs give useful information on pressure-solution mechanism and fibre growth kinetics in relation to stylolite genesis.