New Zealand Journal of Geology and Geophysics abstracts

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Sedimentology and petrography of early diagenetic chert and dolomite in the Late Cretaceous-early Tertiary Amuri Limestone Group, eastern Marlborough, New Zealand

MARK J. F. LAWRENCE

Geology Department
University of Canterbury
Christchurch New Zealand*

*Present address: Department of Geology and Petroleum Geology, Meston Building, King's College, University of Aberdeen, Aberdeen AB9 2UE, Scotland.

Abstract The dominantly micritic Amuri Limestone Group of eastern Marlborough, New Zealand, consists of six regionally extensive units: the Mead Hill Formation, Teredo Limestone Formation, Lower Limestone Formation, Middle Limestone Formation, and the Lower and Upper Marl Formations, which were deposited in a northwest-trending trough. Six separate diagenetic zones (I-VI) are recognised, based on visually estimated proportions of chert, dolomite, or limestone. Zones I-V occur in the Mead Hill to Lower Limestone Formations, and Zone VI occurs in the Middle Limestone Formation. There is no chert or dolomite in the two Marl formations. The bulk of the chert and dolomite occurs in the basin centre (mainly in the Mead Hill Formation) where the limestone is thickest, but decreases in abundance stratigraphically upwards and towards the basin margins. Zone I consists almost entirely of chert which is either detrital rich or detrital poor. Zones II and IV consist of chert and dolomite in approximately equal proportions. Zones III and VI contain varying proportions of chert and limestone. Zone V is a thin, chertified, fine sandstone-mudstone of limited extent. Sulphide nodules, usually <10 mm in diameter, are common in all zones. Chert usually forms discontinuous nodular beds or nodules. Some elongated chert nodules which may be in excess of 2 m (chert bands) are found near the top of the chert sequence. Dolomite occurs as <1 mm diameter rhombohedra in discontinuous chert-cemented beds and lenses. The common pinch and swell morphology displayed throughout most zones results from differential compaction of relatively chert-free limestone, or dolomite, around chert nodules. Stylolites are generally absent, but where present they are deflected around chert nodules, indicating chert formation prior to stylolitisation.
In thin section, Zone II dolomites contain inclusions arranged in cloudy-centre clear-rim patterns. Zone IV dolomite has inclusions arranged in diagonal planes. Both dolomite types are associated with abundant disseminated pyrite. All cherts consist of microcrystalline or crypto-crystalline quartz or chalcedony, with common disseminated pyrite. Petrographic evidence and δ¹⁸O data indicate that the dolomite formed at temperatures <60°C, probably in the upper sediment column. The presence of pyrite suggests that dolomite formed in association with sulphate reduction. The δ¹⁸O composition of the chert is very similar to that of the dolomite, implying that chert formation may have occurred at similar depths and times in the sediment as the dolomite. The association of pyrite with chert suggests that sulphate reduction may also have promoted silica nucleation. Therefore, the sedimentological and petrographic evidence is consistent with the early nucleation of both chert and dolomite.

Keywords Amuri Limestone Group; diagenesis; micritic limestone; chert; dolomite; pyrite; quartz; chalcedony; opal-CT; sulphate reduction; petrography; sedimentology; trace fossils

Received 20 February 1992; published 13 April 1993
New Zealand Journal of Geology & Geophysics, 1991, Vol. 36: 9—25
0028Ð8306/06/3601—0009 ©The Royal Society of New Zealand 1991

PDF file of entire paper: medium quality (5296K); (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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