Abstract:
This thesis maps a 70 km2 region in the Wanganui River Valley, Wanganui Basin, consisting of late-Pliocene to early Pleistocene (2.3-1.15 Ma) dominantly shallow marine strata, and compiles a 540 m thick stratigraphic section from which a facies and cyclostratigraphic analysis is undertaken.
The Wanganui Basin is an ovoid Plio-Pleistocene basin, which developed in the backarc setting of the Hikurangi subduction zone. Subsidence occurred in the late Pliocene through to the early Pleistocene allowing 4000 m of mostly shallow marine sediments to accumulate. Regional tilting caused progressive submergence from the north and migration of the basin depocenter to the south. Consequently, sediments have been uplifted with very little deformation, and dissected by rivers offering good exposure of strata.
Sediments analysed in this study include, in ascending order; 85 m of upper-Okiwa Group inner-shelf to shoreface sandy muds, shellbeds and sands; 240 m of Nukumaru Group middle-shoreface to sub-tidal sands that alternate with shellbeds and coquina limestones; 170-200 m of Maxwell Group heterolithic sands, carbonaceous muds and fluvio-deltaic sands; and 25 m of lower-Okehu silty sands, and volcaniclastic sands. In addition, the stratigraphy includes tephra beds, small paleosols, a 5 m thick volcaniclastic conglomerate, and contains a significant unconformity in the top of the succession (≤ 350 ka c.). Sub-formation lithological units were mapped at 1:15 000 to establish the lateral extent of units and the significance of bounding surfaces.
A facies analysis divided sediments into 10 facies (including 10 sub-facies) with interpreted environments of deposition ranging from marine inner-shelf to lowland fluvial. Six facies successions are recognised representing the depositional response to relative sea-level changes across both wave-graded and protected shelf-shorelines, and marginal marine to terrestrial environments. The vertical stacking of facies, their In general cyclic sediment packages are bound by unconformities or their correlative conformities, and commence with a basal transgressive lag or terrigenous starved carbonate, which is overlain by a shallowing up sand-dominant succession. Sediment cycles have been deposited in response to transgression/regression cycles across the shelf-shoreline. Analysis of facies architecture within each cycle revealed a trend from cycles dominated by marine facies deposited on a normal wave-graded shelf-shoreline in the Okiwa Group, through to cycles dominated by non-marine facies deposited at the feather-edge of the paleo-basin depocenter. Shifts in the dominant facies architecture through time reflect a larger scale shallowing and south-southeast migration of the basin depocenter.
Correlations of key lithological units and tephra beds into the Rangitikei Valley Section, where the chronology is well-constrained, has provided the chronological controls for establishing the periodicity of cyclic sediment packages in the Wanganui Valley field area. Average cycle periods are of the 6th order, which approximate the Milankovitch orbital cycle of obliquity (41 ka). Both the frequency of cyclicity and the amplitude of relative sea-level change displayed in the facies architecture are consistent with Plio-Pleistocene glacio-eustasy.
