Diagenetic minerals in Cenozoic sandstones of the Te Anau Basin, Western Southland

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Watters, W.A.; Turnbull, I.M. 1997 Diagenetic minerals in Cenozoic sandstones of the Te Anau Basin, Western Southland. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 97/33 23 p.

Abstract: A sequence of terrestrial and marine Cenozoic quartzofeldspathic sandstones up to 4800 m thick in the Te Anau Basin, western Southland, contains widespread though usually minor diagenetic minerals. The main secondary mineral is calcite, but the zeolites analcime, laumontite, stilbite and heulandite / clinoptilolite; clay minerals; minor quartz and rare prehnite are also present. Several of the minerals (particularly calcite, but also laumontite, stilbite, quartz and prehnite) are found as lensoidal inclusions within detrital biotite. Stabilisation of these minerals within biotite was probably connected with increase in pH of adjacent pore water, resulting from changes in electrical charge distribution along exposed biotite surfaces. The only diagenetic clay mineral so far identified is a mixed-layer chlorite / smectite. The sequence is overlain by at least 3000 m of Late Miocene to Quaternary gravels, and with a suggested thermal gradient between 20 degrees and 25 degrees C/km, the maximum temperature at the base of the sequence was between 140 degrees and 200 degrees C. The secondary minerals show little relationship between their distribution and depth of burial, and the petrographic evidence indicates that at least some of the zeolitisation took place at shallow burial depths. Contemporary volcanic rocks are absent and hydrothermal activity was unlikely; at least some of the secondary alteration took place at temperatures below 90 degrees C and probably lower than this in places. We suggest that minor solution of silicate minerals by acids, produced by alteration of organic polymers in sparse carbonaceous beds in the sequence, was important in the production of the secondary silicates. Much of the pore-filling calcite resulted from re-distribution of carbonate from shell material, although some calcite may have formed from combination of carbon dioxide, produced during oxidation of carbonaceous matter, with calcium ions present in pore water. (auth)