Geology of Mangapanian (late pliocene) strata, Wanganui Basin: lithostratigraphy, paleontology and sequence stratigraphy

Abstract

Fourteen Mangapanian and early Nukumaruan (late Pliocene) cyclothems in Wanganui Basin have been studied, stratigraphically logged and geologically mapped in thirteen sections from west to east across the basin. The Mangapanian succession contains two subgroups. The Paparangi Subgroup contains sparsely fossiliferous siltstone and sandstone, and extends from the base of the Mangapani Shell Conglomerate to the base of the Wilkies Shellbed. Constituent formations include the Atene Formation, Mangaweka Mudstone, Pitangi Formation and Moukuku Formation. The Okiwa Subgroup is cyclothemic, fossiliferous, and includes all strata between the base of the Wilkies Shellbed and the Hautawa Shellbed. Constituent formations include the Whauteihi Formation, Whakaihuwaka Formation and Parikino Formation. Formational boundaries are mainly located at the base of shellbeds. Individual sandstone, siltstone and shellbed units within formations are assigned member status.

New collections of fossil molluscs allow refinement and minor emendations to the NZ Geological Timescale for bioevents occurring at the Waipipian-Mangapanian (Wp-Wm) and Mangapanian-Nukumaruan (Wm-Wn) Stage boundaries. One significant discovery is that the FO of dextrally coiled Globorotalia crassaformis does not mark the Wp-Wm Stage boundary. Comparison between molluscan and formainiferal bioevents is possible in the Wanganui River section. Two rhyolitic tephra (Eagle Hill and Otere) have been numerically dated. These tephra have U-Pb SHRIMP ages of 2.85 ± 0.20 Ma and 2.71 ± 0.25 Ma, respectively. Single crystal U-Th/He dating of zircon from the Otere Tephra yields an age of 2.57 ± 0.04 Ma. These numerical ages constrain new and revised magnetostratigraphic data for several sections. The Gauss / Matuyama paleomagnetic transition and “X” event cryptochron both occur within the Mangapanian succession. Shellbeds in the succession have been correlated with δ¹⁸O Stages G10-88 (2.79 Ma - 2.28 Ma), providing a numerical age range for each cyclothem within the succession.

The Mangapani Shell Conglomerate (2.79 Ma), Wilkies Shellbed (2.50 Ma) and Hautawa Shellbed (2.28 Ma) have been selected for detailed investigation, and each displays an inner-shelf to upper bathyal depositional profile. Molluscan water-depth estimates for each of these units indicate that they are transgressive deposits, and together with their overlying siltstone and sandstone units comprise a 41 k.y. sequence. Similarities in the character of these three sequences provide constraints upon which a model cyclothem is based. Comparison of other 41 k.y. sequences within the Mangapanian succession with the model cyclothem reveal subtle patterns in sequence architecture, from which predictions of paleoshelf position can be made for particular sections / logs. While most sequences exhibit a consistent westward shallowing trend, many sequences appear to exhibit expression of their shallowest parts in the central parts of the basin, with increases in water depth both eastward and westward of this point. The point of inferred closest proximity to a paleoshoreline for outcrop sections is named Point “S”. Tracing the locality of Point “S” for successive sequences reveals that it occurs in the central parts of the basin during the late Mangapanian. This pattern is correlated with the transfer of strain across the deep subduction interface of the Indo-Australian and Pacific plates. Paleogeographic maps depicting the shape of Wanganui Basin at both the beginning and end of the Mangapanian show the southward migration of the basin depocentre, and that a western basin margin probably separated the Wanganui Basin from the Tasman Sea.