Late Miocene to early Pliocene stratigraphic record in northern Taranaki Basin: condensed sedimentation ahead of Northern Graben extension and progradation of the modern continental margin

The middle Pliocene-Pleistocene progradation of the Giant Foresets Formation in Taranaki Basin built up the modem continental margin offshore from western North Island. The late Miocene to early Pliocene interval preceding this progradation was characterised in northern Taranaki Basin by the accumulation of hemipelagic mudstone (Manganui Formation), volcaniclastic sediments (Mohakatino Formation), and marl (Ariki Formation), all at bathyal depths. The Manganui Formation has generally featureless wireline log signatures and moderate to low amplitude seismic reflection characteristics. Mohakatino Formation is characterised by a sharp decrease in the GR log value at its base, a blocky GR log motif reflecting sandstone packets, and erratic resistivity logs. Seismic profiles show bold laterally continuous reflectors. The Ariki Formation has a distinctive barrel-shaped to blocky GR log motif. This signature is mirrored by the SP log and often by an increase in resistivity values through this interval. The Ariki Formation comprises (calcareous) marl made up of abundant planktic foraminifera, is 109 m thick in Ariki-1, and accumulated over parts of the Western Stable Platform and beneath the fill of the Northern Graben. It indicates condensed sedimentation reflecting the distance of the northern region from the contemporary continental margin to the south. The latest Miocene (Kapitean) age of Ariki Formation beneath the Northern Graben gives a maximum age for the start of crustal extension in the basin; a minimum early Pliocene (Opoitian) age is provided by the age of the base of the Mangaa Formation, which infills part of the graben. Uplift on the margins of the graben allowed condensed Ariki Formation to continue to accumulate into the Waipipian. The Mangaa Formation has blocky to tabular profiles on SP and resistivity logs, particularly in Mangaa-1. The formation comprises several sandstone-dominated submarine fan deposits that thicken into the master graben fault. Accumulation of the Mangaa Formation within the graben continued into the Waipipian when it was supplanted by the bottom-sets of the Giant Foresets Formation. It was not until the latest Pliocene (Nukumaruan) that sedimentation rates exceeded subsidence rates in the graben, leading to progradation of the Giant Foresets Formation over the Western Stable Platform in northern Taranaki Basin.