The Bobs Cove Beds and their relationship to the Moonlight Fault Zone

Abstract

Abstract The Bobs Cove Beds, a sequence of Oligocene marine sediments in the Lake Wakatipu district, are described and discussed. The sediments include breccia‐conglomerates, siltstones, sandstones, and bioclastic limestones and are mainly preserved as thin fault‐involved slivers along the Moonlight Fault Zone. At the type locality at Bobs Cove, the basal part of the 450 m thick sequence represents shallow marine (“shelf”) environments of deposition. The remainder of the sequence, from the limestones upwards, comprises redeposited sediment, deposited largely from mass‐transport on a submarine slope leading to a deep marine flysch basin. All other known occurrences of the Bobs Cove Beds are briefly described. An occurrence of Bobs Cove Beds south of Lake Wakatipu has been relocated, and is shown to be preserved along a southern continuation of the Moonlight Fault Zone that can be traced for at least 40 km to the south. It is shown that the Moonlight Fault Zone has undergone at least three phases of movement: (1) before Cainozoic sedimentation at Bobs Cove; (2) synchronous with, and resulting in, the deposition of the Bobs Cove Beds in an extensional marine trough; and (3) in the Upper Tertiary when the regional stress pattern changed and the Moonlight Fault Zone became a high‐angle compressive feature. While redeposited marine sediments accumulated in the Bobs Cove basin during the second phase of faulting, Manuherikia Group lacustrine sedimentation continued unchanged on the schists to the east; a most important palaeogeographic and tectonic boundary is thus inferred to run just east of the Moonlight Fault Zone. It is demonstrated that the upper Cainozoic movement on the fault zone scissors across Lake Wakatipu, and this fact explains both the unique mode of preservation of the Bobs Cove Beds and several other features of the regional geology. The zone of deformation in part represented by the Moonlight Fault is a regional feature that can be traced as far south as the coast at Te Waewae Bay and probably north to the Alpine Fault. Late Quaternary fault traces and seismic activity along this Moonlight Tectonic Zone demonstrate that deformation still continues.