Structure and sedimentology of the Curdimurka subgroup, northern Adelaide Fold Belt, South Australia

The Curdimurka Subgroup is the upper unit of the Callanna Group, the basal group of a thick Neoproterozoic metasedimentary dominated succession that is preserved as the Adelaide Fold Belt. Throughout the fold belt, the Curdimurka Subgroup typically crops out as megaclasts within breccias, the majority of which formed as salt diapirs or other salt-tectonic related bodies. However, within the Willouran Trough, a depocentre that formed early in the evolution of the northern Adelaide Fold Belt, the package is less fragmentary, albeit bounded by tectonic and brecciated contacts. In this type area of the Curdimurka Subgroup, about 6,000 m of semi-coherent strata are preserved. By examining the stratigraphy and structure of the Curdimurka Subgroup, this study intends to develop an understanding of the early development of the northern Adelaide Fold Belt. Deposition of the Curdimurka Subgroup was mainly in low energy, shallow water to emergent conditions, with periods of sub-wave base deposition. Evidence from evaporite mineralogy and stable isotope geochemistry suggest that it was deposited in marine conditions, but a lacustrine setting cannot be ruled out. Curdimurka Subgroup depocentres evolved within a rift environment, subsequent to an initial phase of syn-rift, mantle-plume related volcanism. The outcropping part of the succession is lacking in coarse-grained rudaceous facies, a feature interpreted to record sedimentation distal to basin-marginal sediment input points. Detrital zircon age populations demonstrate a clear upsection trend from proximalto distal-source area contribution, a trend which is consistent with conceptual models of broadening drainage patterns with increasing rift basin maturity. Field relationships show that the area was subject to three deformation events. D1 produced a diverse array of macroscopic structures, including listric normal faults, inclined to recumbent folds and low angle reverse faults. Deformation is interpreted to have accommodated bulk extension associated with gravity spreading above a decollement positioned at the base of the Curdimurka Subgroup. Localised compressional structures formed in response to translation above perturbations along both the decollement surface, and additional layer-subparallel fault zones positioned higher in the Curdimurka Subgroup succession. Withdrawal and local diapirism of a parent salt layer positioned in older Arkaroola Subgroup strata provided the gravitation instability to drive D1. The timing of this event is constrained by associated high level accommodation development, involving complex sub-basin growth, during Cryogenian Umberatana Group sedimentation, D2 produced upright, northwest – southeast trending folds with a penetrative axial plane cleavage, and northeast dipping reverse faults. D3 produced open, southwest trending folds, local refolding of F2, and a spaced cleavage oriented at a high angle to bedding. Both D2 and D3 occurred during a Cambrian basin inversion event, the Delamerian Orogeny. Monazite geochronology shows that peak metamorphism occurred during the Delamerian Orogen, at 509 ± 5 Ma. A second thermal event occurred at 429 ± 9 Ma, approximately contemporaneous with the intrusion of the British Empire Granite in the Mt Painter Inlier, 130 km east of the study area.