Abstract

The geometry, microtextures, and c-axis fabrics of calcite “beef” veins in the Lower Jurassic black shales (Wessex Basin, UK) were characterized to investigate the mechanism responsible for widening following fracture propagation. Isolated beef veins exhibit planar tapering tips, whereas closely spaced veins are characterized by blunt tips. Vein surfaces are generally smooth and flat; however, circular ridges appear on vein surfaces that protrude into the host clays where there are solid inclusions below or above the ridges. Fossils with well-preserved morphologies, which are separated by subvertical calcite fibers, are observed on both the lower and upper surfaces of single veins. The shale laminations around beef veins are folded and parallel to vein margins. The beef veins commonly contain blocky zones of small, equant calcite crystals, pyrite, and organic matter. The fibers exhibit a preferred subvertical c-axis orientation, whilst crystals in the median zones and blocky zones have random c-axis orientations. The different crystal sizes, morphologies, and c-axis orientations of the fibers from the blocky crystals suggest that the fibers grew without competition with each other under a nonhydrostatic stress field. The displacive widening of calcite beef veins, which is evident from vein interactions and deformation of individual fossil skeletons, demonstrates that fibers grew incrementally because the crystallization pressure of calcite exceeded the overburden load. The force of crystallization is suggested to be responsible for the c-axis orientations of calcite fibers, whereby crystals with free surfaces normal to c-axis orientations grew preferentially. The present study suggests that the fibrous widening of calcite veins in shale postdates their initiation and may result from displacive crystallization rather than fluid overpressure.