CYCLOSTRATIGRAPHY OF MIDDLE PLIOCENE MID SHELF TO UPPER SLOPE STRATA,EASTERN WANGANUI BASIN (NEW-ZEALAND) - CORRELATIONS TO THE DEEP-SEA ISOTOPE

Rangitikei Valley in eastern Wanganui Basin contains a well exposed 75 0-m thick middle Pliocene succession that accumulated during the Gauss Chron. Traditional facies analysis involving laboratory grain size de terminations and benthic foraminiferal census on a suite of closely sp aced (similar to 5 m) samples, show that the Utiku Group (lower 350 m) accumulated predominantly in a mid-shelf environment; the overlying M angaweka Mudstone (400 m thick) accumulated in an outer shelf to upper slope environment. Combined with the identification of sequence strat igraphic 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 outcro p evidence for sequences; seven sedimentary cycles are nevertheless de fined by changes in grain size and by variations in foraminiferal pale obathymetry. The Utiku Group sequences show mostly asymmetric sequence architecture, with comparatively thin TSTs. Rapid deepening across th e Utiku Group-Mangaweka Mudstone boundary at 3.0 Ma, involving an incr ease in water depth of at least 100 m that persisted until 2.55 Ma, or iginated tectonically, probably as a result of plate boundary interact ions. This is the second of two known rapid deepening tectono-sediment ary events, each separated by about 1.5 My. Higher order sea-level cyc licity, involving 11 sedimentary cycles as presently defined, are supe rimposed upon this second tectono-sedimentary cycle. Independent magne tostratigraphies 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 perio dicities of the more major enriched delta(18)O peaks, which range from 41 ka (MG2 to M2), to 210 ka (MG6 to MG2). Thus the duration of cycli cty in the Utiku Group closely corresponds to the duration between sig nificant sea-level lowering events, which are marked by glacial delta( 18)O shifts of > 0.4 parts per thousand on the ODP Site 846 benthic de lta(18)O curve. (C) 1998 Elsevier Science B.V. All rights reserved.