Subsurface stratigraphy, paleoenvironments, palynology, and depositional history of the late Neogene Tauranga Group at Ohinewai, Lower Waikato Lowland, South Auckland, New Zealand

Abstract

At Ohinewai, South Auckland, New Zealand most of the 200 m of overburden on Paleogene coal measures comprises terrestrial sediments of the late Neogene Tauranga Group. A detailed study of 15 continuous cores through these deposits located 16 stratigraphically unconfined lithofacies which, on the basis of preferred associations, have been grouped into 5 major lithofacies assemblages. These assemblages are lithologically correlatable with formations previously established for the group, including the Whangamarino, Puketoka, and Karapiro Formations. Five pollen zones are recognised within the Tauranga Group, ranging in age from Late Miocene to Holocene. The constituent formations at Ohinewai are older and span a longer time period than previously determined. Key spore and pollen types are used to date the sequence, but several anomalies exist suggesting that northern North Island may have formed a late Neogene refuge for many plants that became extinct earlier elsewhere in New Zealand.

The late Neogene depositional history of the Tauranga Group at Ohinewai is described in terms of six major events, comprising meandering river, muddy floodplain, and back swamps (Late Miocene); meandering river and floodplain lakes (Late Miocene-Late Pliocene); low-energy braidplain and lakes (Late Pliocene-early Pleistocene); high-energy braidplain (early-middle Pleistocene); erosion, weathering, and ashfalls (middle-late Pleistocene); floodplain lakes and peat bogs (latest Pleistocene and Holocene). These events were strongly influenced by the tempo and nature of silicic volcanism in the hinterland, by ongoing modification to the landscape and drainage patterns as a consequence of tectonic movements, and by glacio-eustatic fluctuations of sea level and associated climatic and vegetation changes. The sediments are mainly sands, silts, muds, and peats deposited on a coastal lowland. Their terrigenous fraction is dominated by silicic volcanic materials derived from Coromandel Peninsula and, latterly, central North Island sources, with persistent and locally abundant mineral assemblages from weathered or fresh “greywacke” or coal measure local sources. At least five distal ignimbrites occur in the Ohinewai cores.

Keywords:

• Tauranga Group
• Neogene
• stratigraphy
• cores
• Ohinewai
• South Auckland
• palynology
• correlation
• historical geology
• fluvial sedimentation
• lacustrine sedimentation
• peat
• ignimbrite