Abstract

Patterns of joints in flat-lying Permian rocks have been compared with those of folded Mathinna beds, of Siluro-Devonian age, across an exposed unconformity, and with those of a nearby late Devonian granite. The patterns of the Permian rocks and the granite are similar, and each joint set appears to be related to parallel-trending post-Permian faults in the region. However, there is no representation of the joint sets of the Permian rocks in the Mathinna beds, considered to be due to shear failure either along bedding up to 18 degrees from the expected position, or along pre-existing open joints up to 65 degrees away. Lack of apparent order in the deflection and termination of joints against one another in the Mathinna beds indicates contemporaneity of formation of the open continuous fractures of differing attitudes, suggesting that the initial form of joints is as minor discontinuous cracks, which later develop into the open joints examined in surface outcrops.

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