The mineralogy, geochemistry and origin of Lower Tertiary smectite-mudstones, East Coast deformed belt, New Zealand.

Abstract

Marine smectite-mudstones of Lower Tertiary age (Teurian to Runangan) occur throughout the East Coast Deformed Belt of New Zealand. In Marlborough, Marl lithofacies of the Amuri Limestone comprise calcareous, siliceous smectite-mudstone alternating with biomicrite. In Wairarapa, the Kandahar Formation consists of calcareous smectite-mudstone, micritic limestone beds and mass-flow greensand beds. Calcareous smectite-mudstone is also a minor interbedded lithology in the Mungaroa Limestone of Wairarapa. The Wanstead Formation in Hawkes Bay comprises uncemented smectite-mudstone with interbedded mass-flow greensands. Lower Tertiary sequences throughout the East Coast Deformed Belt are typically disrupted by thrust faults and associated shear/mélange zones which have developed in the weak smectite-mudstone lithology.

Insoluble clay fractions of the smectite-mudstones are composed of well crystallised smectite + illite ± quartz (chert). Both the smectite and illite clays are discrete phases with no interstratification suggestive of post-sedimentary transformation of smectite to illite. From detailed phase analysis, the smectite clay overall is a montmorillonitic species, but with varying interstratification of other dioctahedral smectite species and varying layer charge. No distinct stratigraphic trends in clay fraction mineralogy or smectite mineralogy are apparent. Sand fractions of the mudstones are dominated by authigenic or non-volcanic detrital minerals.

Average smectite + illite structural formulas calculated from chemical analyses are commonly non-ideal, with deficiencies in aluminium particularly apparent. The dominant exchangeable cations are calcium in Marlborough mudstones and sodium in Hawkes Bay mudstones. Trace element geochemistry of the smectite-mudstones is similar to that of typical shale and carbonate rocks. Variations in trace element abundances·reflect the lithological character of the mudstones and do not appear to be a useful tool for regional stratigraphic correlation.

Combined sedimentological, mineralogical and geochemical features of the smectite-mudstones indicate a non-volcanic origin. They did not form by in-situ alteration of ash-falls and are unlikely to have formed from transported/reworked ash. Previous use of the term 'bentonite' for the smectite-mudstones implies such a mode of genesis and should be discontinued. Hemipelagic sedimentation and/or mass-flow redeposition of detrital or neoformed clay in an open oceanic, relatively deep water environment is proposed as the origin of the smectite-mudstones.