Paleoenvironmental analysis of quaternary strata in the Levin area : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Quaternary Science at Massey University

Abstract

Marine transgression during the Last Interglacial resulted in widespread inundation of the southern Manawatu area. The Otaki Formation constitutes the relatively thick blanket of predominantly marine sand deposited at the height of the transgression and is now exposed in a partially dissected marine terrace abutting the Tararua Range. Sedimentation was controlled by basement block faulting related to a regional strike-slip tectonic regime on the south­ eastern margin of the South Wanganui Basin. Wave-induced longshore currents from the north-west supplied abundant sediment to the coast. North-east of Levin the Kairanga Trough, occupying a north-east-trending structural depression between uplifted basement blocks, formed the centre of an embayment during the transgression. Tide-dominated depositional processes predominated around the margins of the embayment. In the Forest Lakes area, the absence of seaward barriers resulted in an open wave-dominated coastline. Between Ohau and Shannon mixed wave/tide processes predominated. Stabilisation of sea level resulted in shoreline progradation which was especially marked south of Levin where a dune belt formed, mantling the coastal cliff and later migrating inland. Retreat of the sea was followed by differential uplift and dissection of the newly exposed marine terrace. Two later marine transgressions cut treads in the earlier marine terrace, their strandlines being controlled by the previously established drainage pattern.·Ameliorating climate associated with the major sea level regression of the Last Glacial was accompanied by several phases of loess and minor dune sand accumulation on the exposed marine terraces. At the same time large areas of the terrace coverbeds were removed due to river aggradation. Final truncation of the Last Interglacial marine terraces occurred during the Holocene transgression. Tectonic warping of the marine terraces is continuing along pre-existing basement faults.