Palynostratigraphy and palaeoenvironmental shifts in Oligocene and Miocene strata from offshore Angola, west-central Africa

Abstract

A palynological investigation of 15 ditch cutting samples from Borehole 8, located off the Angolan coast, west-central Africa, revealed Late Oligocene (Chattian) to latest Middle to earliest Early Miocene (Serravallian/earliest Tortonian?) marine dinoflagellate cysts, freshwater colonial algae and terrestrial palynomorphs. Various early Miocene pollen characterising the Verrutricolporites rotundiporus Zone of Legoux (1978) confirm the location of the Oligocene–Miocene boundary in relation to a new short-ranging early Miocene dinoflagellate cyst taxon Cristadinium headii sp. nov. The Oligocene to Miocene dinoflagellate cyst assemblages reflect three periods, A–C, with high palaeoproductivity, corresponding to periods in the latest Oligocene (late Chattian), Early Miocene (late Aquitanian–early Burdigalian?) and the base of the Middle Miocene (Langhian). Early to middle Miocene acme intervals of Cleistosphaeridium placacanthum and Cribroperidinium tenuitabulatum are considered to reflect two regional oceanographic events due to intense upwelling along the West African coast. A distinct Early Miocene episode of brackish-water outflow from the nearby Angolan mainland is also reflected by the palynological assemblages, perhaps linked to the global Mi-1 event. Changes in relative abundance of grass pollen indicate a gradual change towards a drier and perhaps also warmer Burdigalian–Langhian climate during which the Angolan savanna developed, followed by cooler and perhaps more humid conditions following the Miocene Climatic Optimum.