FORAMINIFERAL PALEOECOLOGY AND INITIAL SUBSIDENCE OF THE EARLY MIOCENE WAITEMATA BASIN, WAIHEKE-ISLAND, AUCKLAND

Data from 29 fossil foraminiferal faunas from early Miocene strata (Wa itemata Group; Kawau Subgroup) on Waiheke Island are analysed by clust er analysis. The dominant species are grouped into six species associa tions that occur in different inferred environments: Elphidium-beach o r shallow subtidal sand; Buliminella-high energy, inner shelf; Cribror otalia-moderately exposed, inner to mid shelf; Quinqueloculina-inner a nd shallow mid shelf; Nonionella-deep inner to mid-shelf muddy sand; a nd Cassidulina-Outer shelf. The samples are grouped in terms of relati ve species abundances into seven sample associations, and their paleoe nvironments have been determined using the inferred environments of th eir constituent species associations. These assessed paleoenvironments of the seven sample associations are used to reconstruct the changing paleogeography of the Waiheke area during the early Miocene. The Waih eke strata accumulated during a period of subsidence of about 200 m ea rly in the formation of the Waitemata flysch basin. The strata were de posited in three depressions or bays on the sides of the Waiheke greyw acke paleoisland''. The nonmarine and beach conglomerate of Te Rere Po int and Fossil Bay accumulated first in their respective bays. With fu rther subsidence the transgressive, inner to mid-shelf sandstone of Fo ssil Bay was deposited, while nonmarine breccia accumulated to the nor theast in the W (Double-U) Bay - Oneroa depression. Subsidence continu ed, possibly submerging all of the Waiheke ''paleoisland''. Muddy sand stone, which continued to accumulate around the drowned rocky high, on ly began filling the northeastern depression when it had subsided to m id-shelf depths. The highest sediments at W Bay and Oneroa record cont inued deepening to outer shelf depths. These are the richest foraminif eral faunas and most complete sequences that document the initiation o f the Waitemata Basin. The continued subsidence to mid-bathyal depths during a period of sediment starvation is documented by a piecemeal st ratigraphic record in other places.