Sedimentary Geology And Geochemical Studies Of Upper Cretaceous Sandstones Northern Bida Basin

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The sediments in the northern Bida Basin, northcentral Nigeria consisting of conglomerate, sandstone, siltstone and claystone lithofacies were studied using facies analyses (facies characterization, architectural elements, geometry, textural and compositional characteristics), X-Ray Fluorescence (XRF), Inductive Coupled Plasma Mass Spectrometry (ICPMS), Scanning electron microscope (SEM) equipped with backscattered electron microscope (BSE) and cathodoluminescence (CL) with the intention of determining the provenance, paleotectonics, paleoenvironment reconstruction and diagenetic history.rnThe facies analysis reveal five (5) outcrop Facies Association comprising of matrix-supported, massive conglomerate (Gmm), clast-supported, massive conglomerate (Gcm), clast-supported, crudely bedded conglomerate (Gh), crudely stratified to massive sandstone (Se), planar cross-bedded sandstone (Sp), rippled laminated sandstone (Sr), horizontally stratified sandstone (Sh), massive or slightly laminated sandstone (Sm), finely laminated sandstone, siltstone and claystone (Fl) and massive siltstone and claystone (Fsm). They were interpreted as alluvial intraformational conglomerate, braided fluvial channels, meandering channels, tidal channels and fluvial overbank/floodplain. Six identified architectural elements within the study area; channels (CH), gravel bars and bed forms (GB), lateral accretion (LA), laminated sand sheet (LS), sandy bedforms (SB) and overbank fines (OF) showed a river system ranging from floodplains to more high energy, braided fluvial systems of low sinuosity. Sheet and blanket sandbody geometry with typical vertical and lateral connectivity of sand bodies are common in the central and northern part while wedge and lobate types are common in the southern and eastern parts.rnSedimentary structures with paleocurrent importance include; planar lamination, planar and trough cross-bedding, compound cosets of strata with low-angle inclined bounding surfacesrnand imbricated pebbles were mapped and measured. Dominant southwestern with minor south-eastern paleocurrent transport directions were obtained indicating a continuous influence of southwestern sediment source areas across the basin axis. Regionally, the proportion of overbank sediments increases to the central and western parts, with thick alluvial units occurring toward the south and southeast part of the basin.rnThe ternary plots based on petrographic data suggest that the sandstones petrofacies are mainly quartzarenite with minor sublitharenite while geochemical ternary plots classified the sandstones as mainly litharenite, sublitharenite and subarkose indicating multiple provenance of craton interior (in rifted continental margin basin setting) to orogenic sourced sediments. Petrographic characteristics and other variables with geochemical proxies indicate felsic igneous rocks as the dominant parent rock with minor contribution from metamorphic that has been intensely weathered physically, in tropical humid climatic conditions and low relief of the source area.rnApplication of statistical tools (Correlation matrix, Factor Analysis and Principal Component Analysis) show the main elemental distributions in the studied sediments and the geological processes which include source area composition, weathering process and diagenesis.rnThe geochemical proxies; Al2O3/TiO2, SiO2/Al2O3, K2O/Al2O3, La/Th, La/Co, Th/Co, La/Sc, Cr/Th, Zr/Nb and Zr/Th revealed sediments‟ derivation mainly from felsic source rocks. Moreover, chondrite-normalized REE patterns, light REE enrichment, heavy REE flat pattern and negative Eu anomalies with discriminant diagrams also support felsic rock source derived from craton interior to orogenic recycled sources and deposition in a passive continental margin during the Cretaceous. Discrimination plots of log(K2O/Na2O) versus SiO2, TiO2 versus (Fe2O3+MgO), Al2O3/SiO2 versus (Fe2O3+MgO) and SiO2/20-(K2O+Na2O)-(TiO2+Fe2O3+MgO), La/Y versus Sc/Cr, La-Th-Sc, Th-Sc-Zr/10 indicate passive continental margin paleotectonic settings. A higher Zr/Sc and Zr/Hf ratio with Th/Sc and Zr/Sc binary plot reflects considerable zircon enrichment in the source area.rnThe studied sandstones show relatively high chemical index of alteration (CIA= 71.73-95.4.68), plagioclase index of alteration (PIA= 92.33-95.4.76), chemical index of weathering (CIW= 93.90-95.4.76), chemical proxy for alteration (CPA= 97.74-95.4.90) with low weathering index of Parker (WIP= 0.16-27.34) which when compared with the UCC and PASC values imply that the source area was affected by intensive chemical weathering and therefore chemically mature. Similarity in the mineralogical constituents may be due to the heterogeneous resistance of minerals to the transport processes and represent two distinct tectonostratigraphic formations in the northern Bida Basin.rnDiagenetic imprints identified in these sandstones include early cementation by silica and iron-oxide cement, compaction by development of quartz overgrowths, quartz fracture and replacement and dissolution of detrital feldspar. Generally, these diagenetic imprints have modified these sandstone through both porosity reduction and porosity enhancement.rnIn conclusion, this research shows that northern Bida Basin sediments were deposited by fluvial conditions during warm humid paleoclimatic settings. The sediments were sourced from felsic igneous rocks from the Basement Complex of southwestern Nigeria. Evidences of various degree of diagenetic phases including compaction, cementation, quartz overgrowth, feldspar and rock fragment dissolution leading to the formation of secondary (sp) and microporosity (mp) were revealed.

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Sedimentary Geology And Geochemical Studies Of Upper Cretaceous Sandstones Northern Bida Basin

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