Stratigraphy and sedimentology of the Kapuni formation, Taranaki, New Zealand

Abstract

The stratigraphy and sedimentology of the Focene-Oligocene Kapuni Formation have been studied from data obtained from eight drillholes in onshore Taranaki. These holes are all sited in the Taranaki Graben, which is an infilled Cerozoic basin extending from the Cape Egmont Fault Zone eastward to the Taranaki Fault. The type section of the Kapuni Formation is taken as the interval between 3,245m and 3,976m below derrick floor in the Kapuni-1 well. The formation is a quartzose sandstone-coal measure sequence which is here divided into four members: (1) Upper Sandstone Member; (2) Middle Sandstone Member; (3) Coal Member; and (4) Lower Sandstone Member. Sandstone, shale and coal-bearing sediment lithofacies can be recognised in these members. Trace fossils are varied and often abundant. Deep burrows indicate a hiqh energy environment, probably shallow tidal or subtidal. Small vertical burrows and horizontal burrows indicate deeper, quieter conditions. Occasional escape structures indicate periods of very rapid deposition. The probable provenance for the sandstones was predominantly low-grade metamorphic, although some sediments from plutonic and reworked sedimentary sources are likely. The three sources proposed are Triassic and Jurassic greywacke-argillite metasediment, Pakawau Group sediments and Tasman Intrusives. The Kapuni Formation. was deposited in a generally marine environment which varied from middle or outer shelf through shallower water to lagoonal or terrestrial conditions. The facies and members recognised permit speculative correlations to be made across the basin, and suggest a general regressive-transgressive sequence. The Lower Sandstone and Coal Members were deposited close to a fluctuating but gradually westward-moving shoreline. The Middle and Upper Sandstone Members were deposited in deeper water, and the transgression reached a maximum during deposition of the overlying Oligocene-Lower Miocene Mahoenui Formation. The sediments have been modified, often considerably, since deposition by diagenetic processes that include: (1) quartz cementation and solution; (2) formation of stylolites; (3) feldspar alteration; (4) clay precipitation; (5) carbonate cementation; (6) pyrite precipitation; and (7) coalification of organic material.