The Mahurangi Limestone from Puketotara Peninsula, Northland, New Zealand

Abstract

The Mahurangi Limestone (Oligocene) on Puketotara Peninsula, is a highly deformed, whitish grey, fine-grained rock that typically lacks distinct bedding and identifiable macrofossils. It is faulted against the Waitemata Group (Miocene) and is probably overlain by the Onerahi Chaos-breccia (Upper Tertiary — Quaternary?). The basal contact was not seen. Petrographically the rock is an argillaceous biomicrite containing both terrigenous and organic grains “floating” in an argillaceous, micritic matrix that is seen to be composed mainly of coccoliths under an electron microscope. The limestone is believed to have accumulated as an organic ooze in a sub-tropical, marine environment the depth of which is unknown. The terrigenous grains appear to have been derived from an igneous and sedimentary terrain. In the Lower Miocene, volcanic-rich sediments of the Waitemata Group were deposited on top of the limestone which was later sheared and faulted, this deformation possibly being related to the Upper Tertiary Kaikoura Orogeny.