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



Marine siltstone successions, 1–20 m thick, form the upper part of sequences in the mid-Pleistocene Castlecliff section (≈ 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 Castlecliff HST are interpreted as downlap surfaces, which mark the quenching of transgressive in situ biogenic accumulation (backlap shellbed). Nonetheless, the basal parts of Castlecliff HST successions are enriched in fossil content in the context of the highstand successions as a whole and represent downlap shellbeds. Castlecliff HST are truncated above by sequence-bounding ravinement surfaces, such that complete sandier-upward successions and subaerial exposure surfaces associated with sequence boundaries sensu stricto are never preserved. Modern highstand sedimentation in the Taranaki Bight offshore from Castlecliff is characterized by a mid-shelf mud depocentre and a coastal shore-connected sand prism, both of which are encroaching upon intervening shell-rich relict and palimpsest transgressive deposits. The mud depocentre is up to 9 m thick, and deposition is influenced by a gyre caused by bathymetric steering of storm-driven currents along the embayed coastline. Modern highstand deposition in the Taranaki Bight, in which the mud depocentre 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 by traditional sequence models. The model proposed herein may have application to other shelf palaeo-embayments in which mid-shelf focusing of fine-grained sediment has resulted from coastal steering of currents.