Paleoenvironmental studies of odagbo coal mine sequence, northern Anambra basin, Nigeria: Insight from Palynomorph and geochemical analyses

The palynological study of the northern section of the Anambra basin at Odagbo, southern part of the Benue River was carried out. The coal, silty shale and shaly siltstones were analyzed using standard palynological procedure to enrich the palynological data, date and determine the environment of deposition of the deposits. The heterolithic shales and siltstones yielded some well preserved palynomorphs consisting of 81 angiospermous pollen and 64 pteridophytic spores. Recovered angiosperms include among others Monocolpites, Echitriporites, Psilatricolporites and Psilatricolpites while Laevigatosporites, monolete and trilete spores constitute the spores. The coal seam underlying the sedimentary sequence is barren of sporomorphs. The monocolpates and tricolporates/tricolpates which dominate the taxa constitute 33% and 23% respectively. The association of Zlivisporis blanensis, Longapertites marginatus, Mauritiidites lehmani, Proteacidites spp., Retidiporites magdalenensis and Periretisyncolpites spp. enabled the assignment of Late Maastrichtian age to the deposits. The dominance of palmae flora suggests a tropical to subtropical climate and the recovered assemblage is interpreted as indicative of a warm and humid climate. The geochemical indices such as Th/Cr, Cr/Th, Th/Co and Cr/Ni ratios suggest that these shales were derived from felsic source rocks. The shale units exhibits different degrees of trace-element enrichment, with the approximate order of enrichment relative to an average shale being Y > Zr > Co> Mo > Pb > U>Ba > Cu > Ni > Zn > V > Sr. In addition, based on previously established thresholds, V/Cr, Ni/Co, Cu/Zn, U/Th and V/Sc ratios indicate that these shales were deposited under oxic environment. Besides, the ratio of V (Ni + V) in coal seam and associated shale sequence suggest suboxic to anoxic environment of deposition.