Trench-Slope Channels from the New Zealand Jurassic: The Otekura Formation, Sandy Bay, South Otago

  1. Dorrik A. V. Stow
  1. Robert M. Carter

Published Online: 29 APR 2009

DOI: 10.1002/9781444304473.ch29

Deep-Water Turbidite Systems

Deep-Water Turbidite Systems

How to Cite

Carter, R. M. (1991) Trench-Slope Channels from the New Zealand Jurassic: The Otekura Formation, Sandy Bay, South Otago, in Deep-Water Turbidite Systems (ed D. A. V. Stow), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304473.ch29

Editor Information

  1. Department of Geology, University of Southampton, UK

Author Information

  1. Department of Geology, Otago University, Dunedin, New Zealand

Publication History

  1. Published Online: 29 APR 2009
  2. Published Print: 11 NOV 1991

ISBN Information

Print ISBN: 9780632032624

Online ISBN: 9781444304473

SEARCH

Keywords:

  • Hokonui facies belt of the Rangitata Geosyncline;
  • medium-thick bedded sandy flysch;
  • directional indicators;
  • Omaru formation;
  • shelf-slope break deposit

Summary

The Otekura Formation (Early Jurassic, Pseudaucella zone) at Sandy Bay comprises part of a 10+ km thick, regressive, forearc shelf and slope sequence, the Hokonui facies belt of the Rangitata Geosyncline. The Otekura Formation is dominantly fine grained, being mostly mudstone, silty mudstone and siltstone. The sediments are volcanogenic throughout. The upper 150 m of the formation contains two 20 m thick, channelized bodies of medium-thick bedded sandy flysch, each associated with thin bedded muddy flysch interpreted as overbank turbidites. Directional indicators within the channel sequence indicate emplacement from the south-southwest. In contrast, rare turbidites that occur below the channel sequence, within the background mudstone sediment, were emplaced from the east, i.e. at right angles to the channelized flows. The immediately overlying Omaru Formation contains more abundant macrofossils, intraclastic conglomerates, and appreciable amounts of traction-emplaced cross-bedded sand. Bioturbated calcareous siltstones with an in situ molluscan fauna follow (Boat-landing Formation), and are of shelf origin. The Omaru Formation is therefore interpreted as a shelf-slope break deposit, and the Otekura Formation as an upper slope facies.

Reconnaissance studies indicate that the Otekura Formation is underlain by several kilometres of dominantly fine grained, deep water slope sediments, containing occasional sand and conglomerate filled channels similar to those here described in detail from the Otekura Formation. Such channels are inferred to form when a mass-transported sand, derived from failure higher on the slope, ploughs erosively into the sea floor. After their incision, the channels served for a short time as conduits for downslope transport of sediment, the redeposited deposits of which are found filling each channel. Both channel fills at Sandy Bay are capped by thin-bedded turbidites inferred to have overspilled from similar channels nearby on the slope.