Paleodepositional Environment and Sequence Stratigraphy of Outcropping Sediments in Parts of Southern Middle Niger Basin, Nigeria

摘要

The determination of paleodepositional environment and sequence stratigraphic framework within which geologic factors interplayed to create the sedimentary facies present in outcropping sediments in parts of the Southern Middle Niger Basin, in order to establish sedimentary architectures and chronostratigraphically significant surfaces; erecting a stratigraphic framework through quantitative palynological and sedimentological analysis of sediments by detailed field mapping and laboratory techniques, forms the focus of this study. The Lokoja Formation was mapped on outcrops exposed in Robinson Street, Mount Patti, Banda and Filele in Lokoja area, while the Patti Formation was mapped in Ahoko, Gegu Beki, Gegu Egba, Adabor, Orehi and Akpasi along the Lokoja-Abuja express way. The study area lies within Latitude N 070 81’ and N 080 30’; Longitude E 0060 73’ and E 0060 86’. Three main lithofacies, sand, shale and silt were defined with seven subfacies as silty sand, fine-grained sand, medium-grained sand, coarse-grained sand, clayey siltstone, silty clay, and sandy silt. Sorting ranged from 0.72 – 1.29, indicating poor – moderate sorting for the Lokoja Formation. Depositional environments defined by values of binary plots of skewness versus standard deviation and mean versus standard deviation for samples obtained from the Lokoja area, confirmed by binary plot of skewness versus median for same samples indicate a continental fluvial and shallow marine to transitional paleodepositional environment for the Lokoja Formation and the Patti Formation respectively. Three systems tracts, LST, TST and HST, were established in the study with the Lokoja Formation as an LST, the lower section of the Patti as a TST and the upper section as an HST. An SB and a candidate MFS of probably Late Cretaceous age was established at the base of the Lokoja Formation and in the Patti Formation respectively. Age-significant Cretaceous palynomorph species recovered from the Patti Formation which includes Cyathidites sp., Monocolpopollenites spheroidites, Proteacidites dehanni, P. longispinosus, Cingulatosporites ornatus, Echitriporites triangulatus, Baculatisporites sp., Longapetites veneendenburgi, and Monocolpites marginatus indicate and confirm a Late Cretaceous age for the Patti Formation. Keywords: Sequence stratigraphy, Middle Niger Basin, Patti Formation, Lokoja Formation, Depositional Environment.