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.