Foraminiferal paleobathymetry and mid-cycle architecture of mid-Pleistocene depositional sequences, Wanganui basin, New Zealand

Mid-Pleistocene strata in the Castlecliff section New Zealand show a cyclic alternation of shoreline to shelf lithofacies. A typical cycle is bounded by erosional disconformities, and shows a threefold subdivision. The lower unit is characterized by a basal cross-bedded shell-rich gravel, which is overlain by shoreline-nearshore siltstones, and/or sandstones. The middle unit is disconformity bounded, and is characterized by condensed shellbed facies deposited in terrigenous, sediment starved, inner-shelf settings. The upper unit comprises relatively thick and monotonous successions of shelf siltstone. Castlecliff cycles are interpreted as depositional sequences, driven by glacio-eustatic sea-level fluctuation. The component facies successions represent transgressive systems tracts (TST), mid-cycle condensed shellbeds (MCS), and highstand systems tracts (HST) respectively. Six Castlecliffan foraminiferal associations have been defined, and their paleodepth significance determined by uniformitarian comparison with modern New Zealand faunas. The shallowest (shoreface-innermost shelf) faunas are confined to TST successions, while the deepest faunas (outer inner-shelf) are confined to HST successions. The stratigraphic position of maximum paleodepth, and therefore the position of the conceptual maximum flooding surface (MFS), varies. Sequences in which the maximum paleodepth occurs within the mid-cycle condensed shellbed closely correspond to ideal sequence models, in which conditions of maximum sediment starvation coincide with maximum paleodepth. Sequences showing maximum paleodepths in HST shelf siltstones indicate that the MFS may not necessarily be coincident with mid-cycle condensed facies or their associated disconformities. 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 surface. 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 paleogeographic settings.