Foraminiferal biofacies in a silica-rich neritic sediment, Late Eocene, South Australia

Abstract

The highly siliceous Blanche Point Formation accumulated during the Late Eocene (Zones P15–P16) in the St. Vincent Basin, when the climate was warm. It differs from shelly, neritic carbonates above it and below it by higher but fluctuating numbers of benthonic foraminifera of the Family Uvigerinidae, and by high dominance in different horizons of infaunal crustacean traces and of infaunal gastropods. Within the Blanche Point Formation, changes from a banded siliceous (opal-CT) sediment to a homogeneous spongolite are paralleled by biofacies changes. High numbers of Uvigerinidae, and in part of the Formation high numbers of Robertinacea, Bolivinidae, Sphaeroidina, Cassidulina, are more consistent with generally lowered levels of oxygen at the sediment surface than with other forms of environmental stress. The study is consistent with a recent conclusion, based on mineralogy, that the abundant silica came from volcanic ash. It suggests too that restricted circulation permitted biogenic and other mechanisms of fix the silica in the St. Vincent Basin whilst silica was being flushed away from more open environments facing the nascent Southern Ocean.