SEM and thin section observations of mudstones from the Taranaki Basin, New Zealand

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SR_1998-028-pdf
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Larmer, M.P. 1998 SEM and thin section observations of mudstones from the Taranaki Basin, New Zealand. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 98/28; Institute of Geological & Nuclear Sciences science report 1998/28 12, [50] p.


Abstract: Mudstones from various formations of the Taranaki Basin were sampled from outcrops in NW Nelson and the Taranaki Peninsula and from four wells, Kupe South-2, Maui-6, Waihapa-1 and Ariki-1, drilled within the basin. Scanning Electron Microscopy (SEM) and petrographic thin section analyses performed during this study augment previous mineralogical characterisation by x-ray diffractometry (XRD). SEM observations on highly compacted fabrics of mudstones from the older formations of the Taranaki Basin, indicate that these have the lowest porosity and permeability. In contrast, the younger formations from Miocene to Plio- Pleistocene, shao equal zones of strong compaction and relative undercompaction, resulting in microenvironments of higher visible porosity, thus facilitating diagenetic reactions. Diagenetic formation of minerals will also occur in mudstones while they are undercompacted during shallow burial. Diagenetic products recognised in this study include glauconite, calcite, framboidal or discrete pyrite crystals, phosphate minerals, clinoptilolite, syngenite and illite. Except for the clinoptilolite, syngenite and illite, these diagenetic products are commonly seen infilling fossil tests as well as disseminated throughout the matrix. (auth)