Foraminifera‐based estimates of paleobathymetry using Modern Analogue Technique, and the subsidence history of the early Miocene Waitemata Basin

Abstract

Cluster and Canonical Correspondence Analyses were used to group 52 early Miocene (Waitemata Group) benthic foraminiferal faunas into eight associations and relate them to proxies for paleobathymetry and bottom‐water energy. Modern Analogue Technique (MAT) was used to estimate the paleodepth of each fossil fauna by comparing their generic composition with 371 modern New Zealand faunas. MAT estimates are mostly consistent with, but no more precise than, those inferred by conventional subjective means. MAT estimates could be improved by broadening the modern database. The less precise paleodepth estimates for outer shelf and bathyal faunas were improved using the known upper depth limits of several key bathyal taxa. The combined use of MAT and conventional methods is advocated to provide the most robust paleoenvironmental interpretations. The paleobathymetry estimates are consistent with previously inferred regional subsidence during formation of the Waitemata Basin. The faunas document the submergence of a land with up to 100 m of rolling relief, with initial creation of both sheltered bays (Elphidium and Melonis associations) and exposed coasts (Amphistegina and Cribrorotalia, Gaudryina, Cibicides associations). Dysoxic faunas (Nonionella dominated) provide evidence for the accumulation of fine sediment on the quiet floor of several sheltered, deeper water inlets (20–40 m). As subsidence continued, the former ridge crests became islands and finally disappeared, coincident with cessation of coarse terrigenous sediment supply. Paleobathymetry estimates imply that several coarse, shallow‐water gravel or shell hash units slid down the submarine slopes of these small islands to be interbedded with mid‐outer shelf sediment (with Bolivina and Cibicides dominated faunas). Aperiod of sediment starvation (hiatus or thin mudstone) ensued: in the south, it spanned an interval during which the basin subsided from c. 150 m down to >400–500 m; in the north, it lasted until the basin floor reached lower bathyal depths (c. >1700 m, Oridorsalis‐Nodosaria association), before sand turbidites flowed in from the northwest.