TRANSGRESSIVE SYSTEMS TRACTS AND ONLAP SHELLBEDS FROM MIDPLEISTOCENE SEQUENCES, WANGANUI BASIN, NEW-ZEALAND

Sequences in the Castlecliff section (mid-Pleistocene, Wanganui Basin, New Zealand) are composed of transgressive and highstand systems trac ts, and were deposited in response to fifth-order (c. 100 ky) and sixt h-order (c. 40 ky) glacio-eustatic fluctuation on the paleo-Taranaki S helf, Castlecliff transgressive systems tracts (TST), the topic of thi s paper, are ideally represented by fining-upward facies successions u p to 22 m thick, Above a basal sequence-hounding ravinement surface, t hese successions usually comprise a subtidal cross-bedded shell-rich g ravel (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 elsew here) which represent terrigenous sediment starvation at the seaward f eather edge of TST deposition. Castlecliff TST facies architecture dif fers substantially from other examples of transgressive stratigraphy. First, Castlediff TST are relatively thick in comparison to overall cy cle thickness, Second, sequence-bounding ravinement surfaces at the ba se of Castlecliff TST everywhere rest directly on shelf siltstone cons tituting the truncated highstand systems tract of the underlying seque nce; this indicates erosion of (1) coeval paralic TST landward of the ravinement surface, (2) the lowstand coastal-plain comprising the sequ ence boundary sensu stricto as well as any incised fluvial channels, a nd (3) the presumably coarsening-upward upper part of the underlying s equence. Such differences can be reconciled by consideration of local factors relating to the active tectonic setting of Wanganui Basin, Dep osition occurred in a storm-influenced shelf embayment adjacent to a r ising hinterland, In this setting, high-energy transgressive shoreface s scoured 10-20 m, At the same time, the embayed coastline provided a trap for the abundant sediment supply.