Foraminiferal paleoecology and initial subsidence of the early Miocene Waitemata Basin, Waiheke Island, Auckland

Abstract

Data from 29 fossil foraminiferal faunas from early Miocene strata (Waitemata Group; Kawau Subgroup) on Waiheke Island are analysed by cluster analysis. The dominant species are grouped into six species associations that occur in different inferred environments: Elphidium— beach or shallow subtidal sand; Buliminella—high energy, inner shelf; Cribrorotalia—moderately exposed, inner to mid shelf; Quinqueloculina—inner and shallow mid shelf; Nonionella—deep inner to mid‐shelf muddy sand; and Cassidulina—outer shelf. The samples are grouped in terms of relative species abundances into seven sample associations, and their paleoenvironments have been determined using the inferred environments of their 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 Waiheke strata accumulated during a period of subsidence of about 200 m early in the formation of the Waitemata flysch basin. The strata were deposited in three depressions or bays on the sides of the Waiheke grey wacke “paleoisland”. The nonmarine and beach conglomerate of Te Rere Point and Fossil Bay accumulated first in their respective bays. With further subsidence the transgressive, inner to mid‐shelf sandstone of Fossil Bay was deposited, while nonmarine breccia accumulated to the northeast in the W (Double‐U) Bay ‐ Oneroa depression. Subsidence continued, possibly submerging all of the Waiheke “paleoisland”. Muddy sandstone, which continued to accumulate around the drowned rocky high, only began filling the northeastern depression when it had subsided to mid‐shelf depths. The highest sediments at W Bay and Oneroa record continued deepening to outer shelf depths.

These are the richest foraminiferal faunas and most complete sequences that document the initiation of the Waitemata Basin. The continued subsidence to mid‐bathyal depths during a period of sediment starvation is documented by a piecemeal stratigraphic record in other places.

Keywords:

• Waiheke Island
• Waitemata Basin
• subsidence
• Kawau Subgroup
• Waitemata Group
• foraminifera
• paleoecology
• paleoenvironment
• Miocene
• Otaian
• cluster analysis