Benthonic Foraminiferal Paleoecology Indicates an Oxygen Minimum Zone and an Allochthonous, Inner Neritic Assemblage in the Brasso Formation (Middle Miocene) at St. Fabien Quarry, Trinidad, West Indies

Abstract

Six samples were collected at 1m intervals from the Middle Miocene age (Globorotalia fohsi fohsi Zone) Brasso Formation at St. Fabien Quarry, west central Trinidad. They yielded 123 species of benthonic foraminifera, of which only 18 comprised >5% of the recovery from any one sample. The fauna comprised a mixture of (1) common outer-neritic to upper-bathyal species that confirm the paleoenvironment indicated by abundant planktonic foraminifera, and (2) rarer foraminifera indicative of shallow-water, reefal, oligotrophic conditions. There exists a strong correlation between the percentage of the fauna as planktonics and the diversity of the deeper-water benthonic association, both of which increase upwards through the section, indicating that the sequence comprises part of a transgressive systems tract. The top of the sequence, with common Bolivina jiattongi and Globocassidulina subglobosa, culminates within the upper part of an oxygen minimum zone. The relatively high abundance (19.8%) in the middle of the section of the reefal assemblage, which includes the epiphytal species Amphistegina gibbosa, Planorbulinella trinitatensis, and Quinqueloculina lamarckiana, is postulated to derive from seaward transport of epiphytal specimens on seagrass debris during a one or more storms. The sequence stratigraphic and directional drilling possibilities of these findings are emphasised.