Depositional and Eruptive Mechanisms of Density Current Deposits from a Submarine Vent at the Otago Peninsula, New Zealand
- William McCaffrey,
- Ben Kneller and
- Jeff Peakall
Published Online: 17 MAR 2009
Copyright © 2001 The International Association of Sedimentologists
Particulate Gravity Currents
How to Cite
Martin, U. and White, J. D. L. (2009) Depositional and Eruptive Mechanisms of Density Current Deposits from a Submarine Vent at the Otago Peninsula, New Zealand, in Particulate Gravity Currents (eds W. McCaffrey, B. Kneller and J. Peakall), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304275.ch18
School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, West Yorkshire, UK
- Published Online: 17 MAR 2009
- Published Print: 24 APR 2001
Print ISBN: 9780632059218
Online ISBN: 9781444304275
- depositional and eruptive mechanisms of density current deposits from submarine vent at the Otago Peninsula, New Zealand;
- small-volume explosive volcanic eruptions forming cones, rings;
- Allans beach volcanic stratigraphy;
- phonolitic pumice deposits (unit c);
- density current deposits from submarine vent;
- deposits invaded by number of dykes - in form of irregular lobes;
- gas thrust to aqueous flows
The remnant of a small Miocene volcano, consisting largely of phonolitic pumice, is preserved on the Otago Peninsula at Allans Beach. Its 80-m thick deposits record a variety of particulate gravity flows from a subaqueous eruption. Lack of wave-generated structures suggests deposition below wave base, and for the eruptive centre a depth of about 200 m is estimated, based on the present day wave climate along the Otago Peninsula and an overlying pillow/hyaloclastite unit.
A thick unit of phonolitic pumice represents the main phase of the Allans Beach eruption. A lower, crudely bedded zone at the base is characterized by alignment bedding defined by elongated pumice fragments. The overlying part of the unit is well bedded, crudely doubly graded and locally shows weak cross-stratification. A thin-bedded upper zone is fine grained and lacks clear current indicators. A 20-m thick pillow/hyaloclastite unit overlies the pumice deposits.
This phonolitic pumice sequence indicates deposition from dense suspension out of a series of closely-spaced high-particle concentration granular flows, followed by high- and low-concentration turbidity currents, shed directly from a subsiding subaqueous eruption column during the waning of an initially higher flux eruption. Development of the eruption column was a result of the high gas content of the magma. The column formed and collapsed entirely beneath seawater. Pumice shed from the column chilled rapidly and the grains mixed with water to form aqueous density currents from which deposition occurred.