Detached mud prism origin of highstand systems tracts from mid-Pleistocenesequences, Wanganui Basin, New Zealand

Marine siltstone successions, 1-20 m thick, form the upper part of sequence s in the mid-Pleistocene Castlecliff section (approximate to 0.98-0.35 Ma). The siltstones were deposited within a broad shelf embayment at and about glacioeustatic highstands and are interpreted as highstand systems tracts ( HST). Shell-rich to relatively shell-poor contacts at the base of Castlecli ff HST are interpreted as downlap surfaces, which mark the quenching of tra nsgressive in situ biogenic accumulation (backlap shellbed). Nonetheless, t he basal parts of Castlecliff HST successions are enriched in fossil conten t in the context of the highstand successions as a whole and represent down lap shellbeds. Castlecliff HST are truncated above by sequence-bounding rav inement surfaces, such that complete sandier-upward successions and subaeri al exposure surfaces associated with sequence boundaries sensu stricto are never preserved. Modern highstand sedimentation in the Taranaki Bight offsh ore from Castlecliff is characterized by a mid-shelf mud depocentre and a c oastal shore-connected sand prism, both of which are encroaching upon inter vening shell-rich relict and palimpsest transgressive deposits. The mud dep ocentre is up to 9 m thick, and deposition is influenced by a gyre caused b y bathymetric steering of storm-driven currents along the embayed coastline . Modern highstand deposition in the Taranaki Bight, in which the mud depoc entre is in part detached from the contemporary shore-connected sand prism, is regarded as an analogue for the deposition of the preserved lower parts of Castlecliff HST. The inferred architecture of Castlecliff HST therefore need not refer to the shore-connected, progradational geometry predicted b y traditional sequence models. The model proposed herein may have applicati on to other shelf palaeo-embayments in which mid-shelf focusing of fine-gra ined sediment has resulted from coastal steering of currents.