FORAMINIFERAL PALEOBATHYMETRY AND MID-CYCLE ARCHITECTURE OF MIDPLEISTOCENE DEPOSITIONAL SEQUENCES, WANGANUI BASIN, NEW-ZEALAND

Mid-Pleistocene strata in the Castlecliff section New Zealand show a c yclic alternation of shoreline to shelf lithofacies. A typical cycle i s bounded by erosional disconformities, and shows a threefold subdivis ion. The lower unit is characterized by a basal cross-bedded shell-ric h gravel, which is overlain by shoreline-nearshore siltstones, and/or sandstones. The middle unit is disconformity bounded, and is character ized by condensed shellbed facies deposited in terrigenous, sediment s tarved, inner-shelf settings. The upper unit comprises relatively thic k and monotonous successions of shelf siltstone. Castlecliff cycles ar e interpreted as depositional sequences, driven by glacio-eustatic sea -level fluctuation. The component facies successions represent transgr essive systems tracts (TST), mid-cycle condensed shellbeds (MCS), and highstand systems tracts (HST) respectively. Six Castlecliffan foramin iferal associations have been defined, and their paleodepth significan ce determined by uniformitarian comparison with modern New Zealand fau nas. The shallowest (shoreface-innermost shelf) faunas are confined to TST successions, while the deepest faunas (outer inner-shelf) are con fined to HST successions. The stratigraphic position of maximum paleod epth, and therefore the position of the conceptual maximum flooding su rface (MFS), varies. Sequences in which the maximum paleodepth occurs within the mid-cycle condensed shellbed closely correspond to ideal se quence models, in which conditions of maximum sediment starvation coin cide with maximum paleodepth. Sequences showing maximum paleodepths in HST shelf siltstones indicate that the MFS may not necessarily be coi ncident with mid-cycle condensed facies or their associated disconform ities. Sequences with maximum foraminiferal paleodepths spanning some or all of MCS-HST successions indicate that the MFS may be represented as an interval rather than a specific disconformity or stratal surfac e. These results suggest that the MFS concept, as a means to separate TST from HST strata, may be of little use in shoreline inner-shelf pal eogeographic settings.