New Zealand Journal of Geology and Geophysics abstracts

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Relation of heavy mineral populations to stratigraphy of Cretaceous formations in Marlborough, New Zealand

DAVID SMALE

Institute of Geological and Nuclear Sciences
P.O. Box 30 368
Lower Hutt, Wellington

M. G. LAIRD

Institute of Geological and Nuclear Sciences
c/- Geology Department
University of Canterbury
Private Bag 4800
Christchurch, New Zealand

Abstract  Cretaceous lithostratigraphic units in Marlborough contain two main heavy mineral populations, epidote rich and epidote poor, clearly differentiated by cluster analysis. The epidote-rich population is typical of the Torlesse Supergroup (which has been the dominant provenance throughout) and overlying units up to base of the Wallow Group (Ngaterian). The epidote-poor population has resulted from burial leaching (in sediments that have allowed pore-fluid circulation) of the Wallow and overlying groups. The stratigraphic position of the base of the Wallow Group is in some places ambiguous from field relationships, and where it is obscure the change in heavy mineral population (although resulting from leaching, and commonly itself complex in the same places) provides independent ground for discrimination. Three smaller populations result from variations in provenance and leaching. Garnet is conspicuous in alternating sandstones and mudstones (late Motuan-Ngaterian) of the Awatere Valley. It has a similar compositional range to garnet in Cretaceous sandstones of the North Island East Coast, and has probably been recycled from garnet-rich Torlesse. Pyroxene in mudstone underlying the Lookout Volcanics near Mt Lookout suggests that igneous activity had started slightly earlier than indicated by the volcanics themselves. Biotite in the Coverham/Dubious Stream area occurs in correlative rocks, and may have been formed in part by contact metamorphism from this early igneous activity on surrounding Torlesse.

Keywords  Marlborough; Cretaceous; sequence boundary; leaching; heavy minerals; unconformity; stratigraphy; provenance

New Zealand Journal of Geology and Geophysics, 1995, Vol. 38: 211-222

0028-8306/95/3802-0211 $2.50/0 (c) The Royal Society of New Zealand 1995

PDF file of entire paper: medium quality (2035K); (scanned from paper original: notes about this process)

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