Miocene benthonic foraminiferal morphogroups in an oxygen minimum zone, offshore Cabinda

This paper is part of the special publication No.153, The oil and gas habitats of the South Atlantic. (eds: N.R. Cameron, R.H. Bate and V.S. Clure). Two hundred and seventy-eight benthonic foraminiferal species were recorded (126 agglutinating and 152 calcareous) from CABGOB 128-3, offshore Angola. This assemblage is comprised of a cosmopolitan fauna named the Agua Salada Fauna, known from the Caribbean, in the Vienna Basin, West Africa, Libya and Borneo. The fluctuating relative abundance of benthonic foraminiferal morphogroups in combination with 'total organic carbon' (TOC) data is suggested to reflect variation in the intensity of the oxygen minimum zone as it steadily increases upwards. An infaunal agglutinated 'opportunistic' (r-selected) group moves from antiphase into phase with TOC upwards in CABGOC 128-3. A morphologically comparative calcareous group displays the opposite trend, moving from phase into antiphase with TOC data. Planispiral agglutinated foraminifera emulate the steady increase in TOC upwards in CABGOC 128-3. This group includes Cyclammina cyclops, previously only recorded in the Arctic. Elongate, flattened calcareous morphologies undergo a sudden expansion towards the top of the well and are apparently unaffected by TOC. This overall trend is supported by concurrent poor preservation amongst distinct calcareous groups susceptible to dissolution, as expected in an area of dysaerobic conditions. The disappearance of all agglutinated faunas in the late Miocene is considered to be a reflection of oceanogrphic change.