Joint nucleation in layered rocks with non-uniform distribution of cavities

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 morpholog y of joints in the dolomite layers of the Judea Group, central Israel, and the role played by spherical cavity-shaped flaws during nucleation and grow th of joints is analyzed. An analysis of cavity geometry and distribution r eveals that joint initiation points are governed by such isolated, relative ly large cavities, preferably located close to the bedding top and by stres s gradients during joint nucleation. In the absence of macroscopic cavities . joint initiation points are located along bedding interfaces. Consequentl y, joints typically show two forms of growth, depending on the abundance an d spatial distribution of cavities within the: layers. In layers with plent iful cavities, joints preferably nucleate at critical cavities, propagate v ertically toward the bedding interfaces and horizontally toward adjacent jo ints, and form elliptical fractures. In layers free of cavities. joints nuc leate at bedding interfaces. commonly propagate downward toward the layer b ase 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 join ts in the Judea Group. (C) 2001 Elsevier Science Ltd. All rights reserved.