Ordovician contourites in the Lachlan Fold Belt, eastern Australia

Abstract

The Ordovician sequence in the northeastern Lachlan Fold Belt consists of three lithologic units representing an overall fining-upward succession from turbidites to black shale. The lowest unit, dominated by turbidite beds, represents a small segment of an extensive Ordovician submarine fan sequence that covered much of southeastern Australia. The overlying unit consists predominantly of ripple-bedded sandstone with beds 3–5 cm thick. These beds are texturally mature, containing heavy minerals, mica and silt concentrated on the cross-laminae. The ripple-bedded unit has a mean palaeocurrent trend to 321° which is at 60° to sole marks and almost normal to the trend of ripple cross-bedding in the underlying and associated turbidite beds. This unit is here interpreted as a sandy contourite sequence that can be attributed to contour currents moving northwards along the eastern margin of Gondwana from areas of subpolar downwelling off southern Africa and America. The two major fining-upward successions in the Ordovician sequence in southeastern Australia may reflect eustatic sea level rises during the Middle and Late Ordovician and thus provide evidence of interglacial periods in the Darrawilian and late Gisbornian to Bolindian. The contourites would have accumulated in the latter stages of the glacial periods after sea level had risen sufficiently to cut off major sediment supply to the submarine fan but before the temperature had risen to the extent of preventing deep thermohaline circulation.