Geometry and Microstructures of Differentiated Stylolites in the Lower Vindhyan Limestones, SE Rajasthan, and their Implications for Stylolite Genesis

Stylolites are the product of pressure solution mechanism that plays an important role during diagenesis and Hthification of sedimentary rocks. There are several problems related to geometry, structure and origin of stylolites, some of which have been addressed in this paper.

From a study of the stylolites developed in the limestone of the Nimbahera Formation of the Semri Group of the Lower Vindhyan sequence in Chittorgarh district, SE. Rajasthan, different stages of formation of differentiated stylolites, leading to their variable geometry and microstructures, have been identified. The stylolite geometry changes from an initial planar type through conical type to columnar and wavy types with progressive pressure solution in domains of microtextural instability caused by the non-uniform concentration and random distribution of relatively insoluble opaque and clay mineral grains in layers and clumps, and by an uneven distribution of rock pore-maxima. In the planar type of stylolites the fluid taking the soluble phases (CaCO₃, SiO₂) out of the pressure solution seam, flows transverse to the local maximum compression (a,) direction and parallel to the planar stylolite seam. At a later stage when textural instability that is imparted by and the blockade to fluid-flow that is caused by the concentration of insoluble clay and opaque mineral grains in the stylolite plane lead to a loss of permeability whereby the fluid-flow gets partitioned at the planar stylolite seam. In that situation while the fluid-flow parallel to stylolite seam is reduced, a part of the fluid changes flow polarity, finds new avenues and the flow takes place at a high angle to the planar stylolite seam. The new fluid-flow regime, facilitated by conjugate shear microfractures and microslides at stylolite sides, and enhanced pressure solution due to stress focussing at stylolite head, drives the stylolite head away from the stylolite seam plane. This increases the amplitude of the stylolite cones and columns, causing their displacement parallel to the local maximum compression (a,) direction, given by the stylolite facing direction, away from the stylolite seam plane. This causes the initially planar stylolite seam to differentiate into cones and columns.

At the late-or end-stage of diagenesis when pressure solution diminishes the insoluble residues of opaque and clay minerals forming a mush within the stylolite seam shrink and collapse, giving rise to the secondary compositional layering, and to the kinks and microfaults in the layers. The major part of the saturated fluid film, enveloping the stylolite at that instant, is unable to flow and escape, and progressive crystallisation of quartz first and calcite later takes place at the margins of the stylolite seam. Dimensionally oriented calcite fibres grow in dilation zones from this fluid film under instantaneous extensional regime, caused by volume loss of the seam material and shrinkage-controlled progressive displacement of the stylolite seam-walls, parallel to the stylolite facing direction, to give rise to antitaxiat fibrous bands around the stylolite seam.