Tectonic controls of sedimentation within the lower Cambrian Hawker Group, Brachina–Bunyeroo area, west-central Flinders Ranges, South Australia

Abstract

In the Brachina–Bunyeroo area of the Flinders Ranges, South Australia, the lower Cambrian Hawker Group and the overlying Billy Creek Formation, are affected by two clearly defined fault systems. The southern, the Manga Fault System, was initially a synsedimentary fault-flexure. Movement commenced in post-lower Wilkawillina Limestone time and produced a fault-flexure zone separating a depocentre to the south and relative uplift to the north, causing thickness and facies changes in the Wilkawillina Limestone on the down-dropped southern side. Mernmerna Formation deep-water limestones were deposited south of the fault, but not to the north. Oraparinna Shale, up to 400 m thick, was deposited south of the fault, thinning northwards to <200 m across the fault and onlapping the flexure zone north of the fault. The stable archaeocyath reef-tract of the Wilkawillina Limestone north of the fault is covered only by a few metres of Oraparinna Shale. Later movements on the Manga Fault System produced thrust-style displacements, repeating the overlying Billy Creek Formation and Wirrealpa Limestone. In contrast, the Brachina Fault System to the north was only active in the uppermost Hawker Group time and no thickness or facies changes occur across it within the Hawker Group. Uplift of the southern shoulder of the Brachina Fault System, however, allowed erosion of the uppermost part of the Hawker Group, so that Billy Creek Formation disconformably overlies Wilkawillina Limestone; major movement along the Brachina Fault System occurred during Billy Creek Formation deposition. Fault activity had ceased prior to deposition of the Wirrealpa Limestone as this is not displaced by the Brachina Fault System.