Palynology and stratigraphy of the Upper Miocene Chad Formation, Bornu Basin, northeastern Nigeria

摘要

Abstract Both lithological and palynological analyses were employed to decipher sedimentary stacking patterns and to date the sediments within the interval 50–325?m in Kemar-1 well, Bornu Basin, northeastern Nigeria. Eight different lithofacies units deposited in various environments were recognized. They are: (1) the grayish sandy claystone, rich in organic matter and associated with lignite, deposited in a continental to lacustrine environment; (2) the micaceous claystone, rich in muscovite flakes and organic matter, deposited in a lacustrine environment; (3) the lithified claystone which intercalates the dark gray shale, and deposited in a marine setting; (4) the shale; (5) the micaceous sandstone, which is characterized by presence of muscovite, silty to granular grain sizes, well sorted and deposited in a meandering setting; (6) the poorly-sorted sandstone. It is poorly sorted, poorly-graded, and coarse-grained, with erosive surfaces to underlying facies, and deposited in a fluvial environment; (7) the heterolithic clayey sandstone, poorly graded, grayish, with a degree of organic richness, suggesting an anoxic lacustrine environment; and (8) the grayish claystone which is non-lithic and rarely ferruginized marking the Upper Miocene/Eocene boundary. One main palynological zone, i.e., the Echitricolporites spinosus assemblage zone, has been established. The base of the studied section at 385?m is characterized by the last up-hole occurrence of Grimsdalea magnaclavata suggestive of Eocene boundary while the overlying sediments are characterized by Late Miocene marker species such as E. spinosus, Elaeis guineensis, Anthocerus sp., Nymphaea lotus, and Retistephanocolpites gracilis. The Chad Formation is dated as Late Miocene age unconformably overlying the Eocene Kerri-Kerri Formation which indicated that the Chad Formation was probably deposited during an alternation of lacustrine and continental settings, due to climatic change.