Abstract:
The Rangitikei River valley, between Utiku and Mangaweka in eastern Wanganui Basin, contains a mid-Pliocene (c. 3.5-2.5 Ma), c. 750 m thick, southward dipping (6-12 degrees ), cyclothemic marine succession. Eleven sedimentary cycles occur within the succession. The lower four sand-dominated cycles are assigned to the Utiku Group (new), which includes three newly defined formations: Tarare, Kawhatau, and Manui. Each formation contains one or more unconformity-bounded cyclothems, which comprise a lower silty sandstone unit and an overlying sandstone unit. We correlate the cyclothems with oxygen isotope stages MG8 to 103 defined for ODP Site 846. Discrete sandstone, silty sandstone, and siltstone units are assigned member status and correspond to systems tracts in terms of sequence stratigraphic nomenclature. The distribution of the three newly defined formations is presented on a new geological map of the region. The upper seven siltstone-dominated cycles are assigned to the Paparangi Group, which in eastern Wanganui Basin comprises Mangaweka Mudstone (emended). High-resolution textural and microfaunal analyses through the Mangaweka Mudstone indicate regular changes in water depth from outer neritic/upper bathyal to mid/outer neritic environments. The 80-100 ka duration of the cyclothems is consistent with a glacio-eustatic origin and corresponds to an interval of progressive climatic deterioration before the initiation at c. 2.54 Ma of major continental Northern Hemisphere glaciations. The Utiku-Mangaweka region has simple postdepositional structure. Upsection shallowing in the dip (12-8 degrees ) of strata is attributed to synsedimentary tilting and subsidence of the shelf concomitant with migration of the tectonic hinge southward into the basin. The distribution of outcrop of the Utiku Group and Mangaweka Mudstone are influenced by this regional tilt and also by a major northeast-southwest striking, high-angle reverse fault (Rauoterangi Fault), which deforms strata in eastern parts of the mapped area. The sedimentation rate decreased upwards through the Utiku Group from c. 0.8 m/ka to c. 0.5 m/ka. This is consistent with accumulation of the shallowest water deposits in the uppermost part of the Utiku Group, when basin infilling was in its most progradational state and the rate of sediment supply exceeded basin subsidence. Thereafter, the sedimentation rate rapidly increased to c. 0.9 m/ka. This coincided with a marked deepening of the paleobathymetry from inner neritic to outer neritic/upper bathyal depths, and a persistent shift from sandstone to mudstone (Mangaweka Mudstone) deposition.