DIASTASIS CRACKS - MECHANICALLY GENERATED SYNAERESIS-LIKE CRACKS IN UPPER CAMBRIAN SHALLOW-WATER OOLITE AND RIBBON CARBONATES

Upper Cambrian limestones and dolostones of western Newfoundland, Canada, display conspicuous synsedimentary mud cracks. Cracks occur in carbonate mudstone interbedded with ooid and peloid grainstone (unwashed oolite and ribbon rock lithofacies). The traditional interpretation is that these are desiccation cracks. The weight of evidence supports an alternative explanation: cracks resulted from the differential mechanical behaviour under stress of stiff mud interlayered with loose ooid/peloid sand. The processes envisaged to cause such diastasis cracks may be applicable to a wide variety of both carbonate and terrigenous clastic deposits composed of interlayered sediments of contrasting material properties, and may be a viable alternative to synaeresis. Diastasis cracks are not depth limited, and may form in any subtidal environment from the beach zone to below wave base. If this interpretation is correct, there may not be nearly as many intertidal lithofacies in the rock record as are presently assumed.