JOINT NUCLEATION IN LAYERED ROCKS WITH NON-UNIFORM DISTRIBUTION OF CAVITIES

Abstract

Characterization of 'critical flaws' that initiate into systematic joints is essential for understanding the process of fracturing in sedimentary rocks. In this study, advantage is taken of the well-developed surface morphology of joints in the dolomite layers of the Judea Group, central Israel, and the role played by spherical cavity-shaped flaws during nucleation and growth of joints is analyzed. An analysis of cavity geometry and distribution reveals that joint initiation points are governed by such isolated, relatively large cavities, preferably located close to the bedding top and by stress gradients during joint nucleation. In the absence of macroscopic cavities, joint initiation points are located along bedding interfaces. Consequently, joints typically show two forms of growth, depending on the abundance and spatial distribution of cavities within the layers. In layers with plentiful cavities, joints preferably nucleate at critical cavities, propagate vertically toward the bedding interfaces and horizontally toward adjacent joints, and form elliptical fractures. In layers free of cavities, joints nucleate at bedding interfaces, commonly propagate downward toward the layer base and adjacent joints, and form semi-elliptical fractures. In both cases, bedding interfaces between the dolomite layers impedes joint propagation. Fractographic methods based on joint surface morphology yield satisfactory, first-order estimates of the driving paleostress required to initiate joints in the Judea Group.