New Zealand Journal of Geology and Geophysics abstracts
Provenance and geochemistry of exotic clasts in conglomerates of the
Oligocene Torehina Formation, Coromandel Peninsula, New Zealand
George R. Dix
Ottawa-Carleton Geoscience Centre
Department of Earth Sciences
Carleton University
1125 Colonel By Drive
Ottawa, Ontario, Canada K1S 5B6
email: gdix@ccs.carleton.ca
Campbell S. Nelson
Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand
Abstract Non-marine pebble to cobble conglomerates of
the lower Torehina Formation (Oligocene) crop out along western Coromandel
Peninsula and overlie, with strong angular discordance, continental-margin
metasedimentary rocks (Manaia Hill Group) of Mesozoic (Late Jurassic to ?Early
Cretaceous) age. The conglomerates contain provenance information that identifies
a pre-Oligocene depositional history obscured by the unconformable juxtaposition
of these Tertiary and Mesozoic strata. Most clasts in the lower Torehina
Formation are visually similar to local bedrock lithologies, including metamorphosed
sandstones and argillites, but are kaolinitic and contain more detrital and
authigenic chert, quartz, and potash feldspar. Local derivation of these
clasts seems unlikely. By comparing geochemical ratios with those defined
for continental margin sandstones, and well characterised New Zealand tectonic
terranes, we interpret the majority of clasts in the lower Torehina Formation
to have been derived from a dissected orogen, with mixtures of felsic and
volcanogenic-derived sediment. The most likely sources are the Waipapa and
Torlesse Terranes. The remaining 20–30% of the clasts in the lower Torehina
Formation were originally friable, are coarse grained, and appear to be lithologically
exotic relative to known metamorphosed sandstones in basement terrane sources
on North Island. Some clasts contain coal laminae and particles, and all
contain detrital kaolinite as lithic fragments and matrix. Such characteristics
imply a non-marine to marginal-marine source containing sediment derived
from strongly weathered granite or granodiorite. Mechanical fragility implies
a likely proximal, easily erodible source. We propose that this group of
clasts was derived from an Upper Cretaceous sedimentary cover, either part
of a locally developed basin fill or part of a once regionally extensive
cover on North Island. Either case defines a more widely distributed Cretaceous
source than found today
Keywords Coromandel Peninsula; Torehina Formation; Oligocene;
conglomerate; pebble composition; pebble geochemistry; provenance; Cretaceous;
Manaia Hill Group; Waipapa and Torlesse Terranes
G02021; Received 15 April 2002; accepted 19 May 2003; online publication
date 13 November 2003
New Zealand Journal of Geology & Geophysics, 2003, Vol. 46: 539–552
0028–8306/03/4604–0539 $7.00/0 © The Royal Society of New Zealand
2003
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