Early Diagenetic Concretions Associated with Intrastratal Shrinkage Cracks in an Upper Proterozoic Dolomite, Tasmania, Australia

Abstract

The Upper Proterozoic Irby Siltstone contains an unnamed 127-m-thick member, predominantly composed of fine-grained dolomite and dolomitic mudstone deposited below fair-weather wave-base in a quiet marine shelf environment. Storm-deposited layers, consisting of planar- to ripple-laminated dolosiltite beds with erosional bases constitute a minor, more proximal facies. The quiet-water sediments contain an abundance of intrastratal shrinkage cracks and small (5-50 mm) early diagenetic nodules that in places have coalesced to form essentially continuous diagenetic layering Shrinkage cracks, appearing as narrow, spar-filled veinlets subperpendicular to bedding, are most abundant in silt-free lutites. Conversely, concretion development was favored and shrinkage cracks somewhat inhibited where minor primary silty laminae are present. Most of the shrinkage cracks appear considerably deformed by sedimentary compaction, but many, particularly those in concretion-rich beds, were surrounded by zones of early cementation that developed into tabular, bedding-subperpendicular concretions that resisted compaction. Transport of ions to concretion sites was evidently facilitated along the cracks. Rare small clastic dykes may be sediment-filled synaeresis cracks which probably formed by synaeresis in very early diagenesis. A phase of tectonism imposed a cleavage steeply inclined to bedding, a bulk plane strain and a preferred orientation of the diagenetic structures in the cleavage direction.