Transgressive systems tracts and onlap shellbeds from mid-pleistocene sequences, Wanganui Basin, New Zealand

Sequences in the Castlecliff section (mid-Pleistocene, Wanganui Basin, New Zealand) are composed of transgressive and highstand systems tracts, and were deposited in response to fifth-order (c. 100 ky) and sixth-order (c. 40 ky) glacio-eustatic fluctuation on the paleo-Taranaki Shelf, Castlecliff transgressive systems tracts (TST), the topic of this paper, are ideally represented by fining-upward facies successions up to 22 m thick, Above a basal sequence-hounding ravinement surface, these successions usually comprise a subtidal cross-bedded shell-rich gravel (onlap shellbed) which is overlain by a fining-upward succession of inner shelf siltstones and silty fine sandstones, TST successions are capped by mid-cycle condensed shellbeds (discussed in detail elsewhere) which represent terrigenous sediment starvation at the seaward feather edge of TST deposition. Castlecliff TST facies architecture differs substantially from other examples of transgressive stratigraphy. First, Castlediff TST are relatively thick in comparison to overall cycle thickness, Second, sequence-bounding ravinement surfaces at the base of Castlecliff TST everywhere rest directly on shelf siltstone constituting the truncated highstand systems tract of the underlying sequence; this indicates erosion of (1) coeval paralic TST landward of the ravinement surface, (2) the lowstand coastal-plain comprising the sequence boundary sensu stricto as well as any incised fluvial channels, and (3) the presumably coarsening-upward upper part of the underlying sequence. Such differences can be reconciled by consideration of local factors relating to the active tectonic setting of Wanganui Basin, Deposition occurred in a storm-influenced shelf embayment adjacent to a rising hinterland, In this setting, high-energy transgressive shorefaces scoured 10-20 m, At the same time, the embayed coastline provided a trap for the abundant sediment supply.