New Zealand Journal of Geology and Geophysics abstracts

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Brunner Coal Measures at Golden Bay, Nelson: an Eocene fluvial-estuarine deposit

WILLIAM L. LEASK

Department of Geology
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand*

*Present address: Ian R. Brown Associates Ltd, P.O. Box 9043, Wellington, New Zealand.

Abstract Brunner Coal Measures (previously named Motupipi Coal Measures) of Bortonian to ?latest Runangan (Eocene) age, are preserved as three contrasting units in Golden Bay, Nelson, New Zealand.
The lower Takaka valley contains up to 350 m of cross-bedded medium to coarse grained sandstone (mainly of granitic provenance), interlaminated fine sandstone and siltstone, carbonaceous mudstone, and thin coal seams. These have been grouped into five facies associations: (1) a combination of the above lithologies with a dominance of mudstone and interlaminated sandstone-siltstone, interpreted as meandering river floodplain deposits; (2) mainly bio-turbated muddy sandstone, interpreted as tidal flat deposits; (3) cross-bedded sandstone, interpreted as braided river or flood tidal delta deposits; (4) thickly bedded sandstone and conglomerate, interpreted as fluvial channel deposits; and (5) alternating sandstone, conglomerate, and mudstone with a distinctive metamorphic provenance, interpreted as aggrada-tional sheetflood deposits.
The Washbourn Limonite Member (new name) consists of blocky iron-ore which forms irregular masses up to 50 m thick on ridge crests west of Onekaka. It is interpreted as a bog iron deposit developed over a marble karst basement.
The Quartz Wash Member (new name) consists of coarse quartzose conglomerate and sandstone with basal carbonaceous mudstone, as scattered remnants up to 40 m thick on the southeast flank of the Aorere and Parapara valleys. This is interpreted as a braided river deposit.
In the Takaka valley, a NNW-oriented paleobasin is inferred from isopachs, north-trending paleocurrents, and the largely granitic source of the sediments. Sedimentation was controlled by subsidence on a northeast- striking normal fault, which is inferred to be the precursor of the reverse Pikikiruna Fault active during Miocene time. With increasing distance from the fault, and probably also towards the top of the formation, eustatic sea-level changes exerted greater influence. Most of the formation in the Takaka area reflects fluvial deposition succeeded by estuarine deposition. At the top of the formation, coarse sandstone and conglomerate (associations 3, 4, 5) indicate fluvial and tidal delta deposits that prograded during a highstand of sea level. The Washbourn Limonite and Quartz Wash Members are inferred to have been deposited during one or more transgressions and highstands.
A paraconformity between the Brunner Coal Measures and the overlying Takaka Limestone indicates nondeposition during the entire Whaingaroan Stage. The Golden Bay area appears to have been a stable platform during Oligocene time, in contrast to rapidly subsiding areas to the south and east.

Keywords Nelson; Golden Bay; Brunner Coal Measures; Quartz Wash Member; Washbourn Limonite Member; Eocene; sedimentology; facies associations; paleocurrents; fluvial environments; estuarine environments; new strati-graphic names

Received 16 July 1992; published 13 April 1993
New Zealand Journal of Geology & Geophysics, 1991, Vol. 36: 37—50
0028Ð8306/06/3601—0037 ©The Royal Society of New Zealand 1991

PDF file of entire paper: medium quality (2822K); (scanned from paper original: notes about this process). Digitisation of this article from the printed journal was kindly facilitated by the Geological Society of New Zealand

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