ABSTRACT: "Diastasis Cracks": Mechanically Generated "Synaeresis-Like" Cracks in Ancient Shallow Water Oolitic Carbonates

COWAN, CLINTON A., Queen's University, Kingston, Ontario, Canada

Oolitic lithofacies in Upper Cambrian limestones and dolostones of western Newfoundland, Canada, display conspicuous synsedimentary/shallow substratal cracks that have been previously interpreted as desiccation cracks. Reexamination of these features, however, reveals that they are mechanical in origin. Such diastasis cracks, a term proposed here, resulted from mixed shear/tensile failure of stiff dololutite layers resting on, or within, a loose and shifting oolite. The resulting cracks are locally indistinguishable from what many workers have erroneously called synaeresis cracks.

Associated with diastasis cracks in this oolitic lithofacies are other syn/meta-depositional deformation features, such as cuspate/lobate sedimentary interfacies, and ragged in situ disintegration of dololutite layers. These related deformation structures, along with the cracks themselves, point to the critical role that competence contrasts

between the oolite and dololutite played in the ultimate forming of this spectrum of fabrics. The occurrence of diastasis cracks in interlayered grainy and muddy sediments reflects an underlying, causative mechanical relationship rather than a contingency on intertidal exposure or chemical change in the sediment or water column, and thus diastasis may have broad applicability to the generally abiotic (oolitic/peloidal) and unburrowed lithofacies that characterize many Proterozoic and lower Paleozoic successions.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)