Provenance and geochemistry of exotic clasts in conglomerates of the Oligocene Torehina Formation, Coromandel Peninsula, New Zealand

Nelson, Campbell S.; Dix, George R.

Provenance and geochemistry of exotic clasts in conglomerates of the Oligocene Torehina Formation, Coromandel Peninsula, New Zealand

dc.contributor.author	Nelson, Campbell S.
dc.contributor.author	Dix, George R.
dc.date.accessioned	2009-12-16T01:07:13Z
dc.date.available	2009-12-16T01:07:13Z
dc.date.issued	2003
dc.description.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.	en
dc.format.mimetype	application/pdf
dc.identifier.citation	Nelson, C. S., & Dix, G. R. (2003). Provenance and geochemistry of exotic clasts in conglomerates of the Oligocene Torehina Formation, Coromandel Peninsula, New Zealand. New Zealand Journal of Geology and Geophysics, 46, 539-552.	en
dc.identifier.doi	10.1080/00288306.2003.9515028	en_NZ
dc.identifier.uri	https://hdl.handle.net/10289/3499
dc.language.iso	en
dc.publisher	The Royal Society of New Zealand	en_NZ
dc.relation.isPartOf	New Zealand Journal of Geology and Geophysics	en_NZ
dc.relation.uri	http://www.royalsociety.org.nz/Site/publish/Journals/nzjgg/2003/037.aspx	en
dc.rights	This article has been published in the New Zealand Journal of Geology and Geophysics. © The Royal Society of New Zealand 2003.	en
dc.subject	Coromandel Peninsula	en
dc.subject	Torehina Formation	en
dc.subject	Oligocene	en
dc.subject	Conglomerate	en
dc.subject	pebble composition	en
dc.subject	pebble geochemistry	en
dc.subject	provenance	en
dc.subject	Cretaceous	en
dc.subject	Manaia Hill Group	en
dc.subject	Waipapa and Torlesse Terranes	en
dc.title	Provenance and geochemistry of exotic clasts in conglomerates of the Oligocene Torehina Formation, Coromandel Peninsula, New Zealand	en
dc.type	Journal Article	en
pubs.begin-page	539	en_NZ
pubs.elements-id	29591
pubs.end-page	552	en_NZ
pubs.issue	4	en_NZ
pubs.volume	46	en_NZ