Abstract

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-bounding 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, Castlecliff 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.

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