Cyclostratigraphy of middle Pliocene mid shelf to upper slope strata, eastern Wanganui Basin (New Zealand): correlations to the deep sea isotope record

Abstract

Rangitikei Valley in eastern Wanganui Basin contains a well exposed 750-m thick middle Pliocene succession that accumulated during the Gauss Chron. Traditional facies analysis involving laboratory grain size determinations and benthic foraminiferal census on a suite of closely spaced (∼5 m) samples, show that the Utiku Group (lower 350 m) accumulated predominantly in a mid-shelf environment; the overlying Mangaweka Mudstone (400 m thick) accumulated in an outer shelf to upper slope environment. Combined with the identification of sequence stratigraphic surfaces (sequence boundary, transgressive surface of erosion and downlap surface) four sequences have been identified in the Utiku Group. In the outwardly massive Mangaweka Mudstone there is no outcrop evidence for sequences; seven sedimentary cycles are nevertheless defined by changes in grain size and by variations in foraminiferal paleobathymetry. The Utiku Group sequences show mostly asymmetric sequence architecture, with comparatively thin TSTs. Rapid deepening across the Utiku Group-Mangaweka Mudstone boundary at 3.0 Ma, involving an increase in water depth of at least 100 m that persisted until 2.55 Ma, orignated tectonically, probably as a result of plate boundary interactions. This is the second of two known rapid deepening tectono-sedimentary events, each separated by about 1.5 My. Higher order sea-level cyclicity, involving 11 sedimentary cycles as presently defined, are superimposed upon this second tectono-sedimentary cycle. Independent magnetostratigraphies allow the sedimentary cycles to be correlated to the astronomically tuned oxygen isotope stratigraphy for ODP Site 846. For the interval 3.55-3.0 Ma the sedimentary cyclicity reflects the periodicities of the more major enriched δ 18O peaks, which range from 41 ka (MG2 to M2), to 210 ka (MG6 to MG2). Thus the duration of cyclicty in the Utiku Group closely corresponds to the duration between significant sea-level lowering events, which are marked by glacial δ 18O shifts of > 0.4 ‰ on the ODP Site 846 benthic δ 18O curve.