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Palynology of the Paleogene Oshosun Formation in the Dahomey Basin, southwestern Nigeria

2007; Volume: 39; Issue: 1 Linguagem: Inglês

0556-655X

S. I. Bankole, Eckart Schrank, Bernd‐D. Erdtmann,

Aquatic Ecosystems and Phytoplankton Dynamics

espanolSe ha estudiado, desde el punto de vista palinologico, una serie paleogena de la Formacion Oshosun a partir de dos sondeos (IB10 e IL3) y de una seccion en superficie (cantera Sagamu) en la Cuenca de Dahomey, suroeste de Nigeria. Entre los palinomorfos encontrados, los mas abundantes fueron los quistes de dinoflagelados, si bien tambien se registro polen de angiospermas tipicas de areas tropicales paleogenas, como Bombacacidites africanus, Proxapertites cursus, Retistephanocolpites williamsii, Spinizonocolpites baculatus y S. echinatus. La presencia de quistes de dinoflagelados diagnosticos de edad (como por ejemplo, Apectodinium homomorphum, A. quinquelatum, A. paniculatum, Kallosphaeridium cf. brevibarbatum, Hafniasphaera septata, Ifecysta pachyderma) indican una edad Paleoceno superior a Eoceno inferior para la Formacion Oshosun. La reltiva abundancia de especies del genero termofilo Apectodinium sugiere que la Formacion Oshosun se deposito durante el maximo global termico del Paleoceno-Eoceno. Los cambios en los porcentajes de los grupos de palinomorfos mas abundantes indica la existencia de variaciones en las condiciones paleoambientales de la Formacion Oshosun. La asociacion Apectodinium-Homotryblium-Kallosphaeridium apunta a la presencia de un medio costero y de salinidad reducida o fluctuante, asociado a polen y esporas relativamente comunes procedentes de medios terrestres. Esta influencia terrestre es mas acusada en las dos localidades occidentales (sondeos IB10 e IL3). La existencia de unas condiciones mas neriticas viene indicada por la relativa abundancia de Spiniferites y/o Cleistosphaeridium y por la poca frecuencia de palinomorfos terrestres (IB10 superior y seccion Sagamu). La escasa presencia de las algas verdes Debarya y Ovoidites en dos de las tres secciones investigadas (IB10 y cantera Sagamu) indica un influjo ocasional de medios de agua dulce. Se propone Triorites takahashii Schrank, nom. nov. para reemplazar a Triorites scabratus Takahashi & Jux, 1989, un homonimo anterior de Triorites scabratus Couper, 1954. EnglishA palynological investigation has been conducted on the Paleogene mudstone sequence of the Oshosun Formation encountered in two boreholes (IB10 and IL3) and one surface section (Sagamu Quarry) in the Dahomey Basin, southwestern Nigeria. Among the palynomorphs found during this investigation dinoflagellate cysts are most abundant, but angiosperm pollen typical of Paleogene tropical areas such as Bombacacidites africanus, Proxapertites cursus, Retistephanocolpites williamsii, Spinizonocolpites baculatus and S. echinatus are also present. Occurrences of age-diagnostic dinoflagellate cysts (e.g. Apectodinium homomorphum, A. quinquelatum, A. paniculatum, Kallosphaeridium cf. brevibarbatum, Hafniasphaera septata, Ifecysta pachyderma) support a late Paleocene to early Eocene age for the Oshosun Formation. The relative abundance of species of the thermophilic genus Apectodinium indicate deposition of this formation during the global Paleocene-Eocene thermal maximum. Changes in the percentages of major palynomorph groups suggest variations in the palaeoenvironmental conditions of the Oshosun Formation. Near-shore and reduced or fluctuating salinity conditions are indicated by an Apectodinium-Homotryblium-Kallosphaeridium assemblage which is associated with relatively common land-derived pollen and spores. A relatively strong contribution from a terrestrial source and hence proximity to a land area is noted for the two western localities (boreholes IB10 and IL3). Slightly more open marine (neritic) conditions are indicated by relative abundance of Spiniferites and/or Cleistosphaeridium and scarcity of terrestrial palynomorphs (upper IB10 and Sagamu section). The rare presence of the green algae Debarya and Ovoidites in two of the three investigated sections (IB10 and Sagamu Quarry) indicates an occasional influx from freshwater environments. The new name Triorites takahashii Schrank, nom. nov. is introduced as a replacement of Triorites scabratus Takahashi and Jux 1989, a junior homonym of Triorites scabratus Couper 1954.