Varieties of glauconite in late Cretaceous and early Tertiary rocks of the South Island of New Zealand, and new proposals for classification

Abstract

Glauconites from 142 samples of Upper Cretaceous and lowermost Tertiary rocks in the South Island of New Zealand, and Chatham and Campbell Islands, were analysed comprehensively to provide data for paleoenvironmental, paleogeographic, and stratigraphic studies. All morphological varieties and all internal textures of glauconite that have been previously distinguished by Triplehorn and Konta were found in the suite of samples. Two new morphological classes are defined: (1) fragmentary glauconite, comprising angular (Subclass A) or rounded (Subclass B) fragments of once larger grains; (2) spongy glauconite, comprising grains with a porous surface morphology which is subdividable with scanning electron microscopy into “cauliflower” and “serrulate” varieties. Fragmentary grains reflect transportation and abrasion; spongy grains probably reflect diagenetic alteration. Neither glauconite type is suitable for radiometric determination of the time of sediment accumulation. One new class of internal texture is proposed— patch-oriented microcrystalline.

Crystallographic classification of glauconites examined by X-ray diffraction is modified. Degraded illite occurs amongst the expandable layers of some glauconites; its presence should be reported in future analyses. Because degraded illite layers probably result from leaching, glauconites with a significant quantity of them should be avoided in radiometric age determinations. Critical values of a new parameter, the Disorder Coefficient (DC), are proposed as one basis for crystallographic classification of glauconite. The DC takes into account all causes of disorder and establishes objective limits between classes; its use will avoid some existing problems of worker bias.

The combination of morphology, internal texture, crystallography, and geochemical characteristics of glauconite are shown to have great potential in stratigraphic analysis, discrimination of paleoenvi-ronments, and determination of geological history.

Keywords:

• glauconite
• Upper Cretaceous
• Dan-nevirke Series
• South Island
• Chatham Islands
• Campbell Island
• geochemistry
• petrography
• stratigraphy
• glauconitic composition
• sedimentary rocks