Determination Of The Sedimentological And Geochemical Characteristics Of The Sediment In The Niger Delta Basin

Project and Seminar Material for Geology

Determination Of The Sedimentological And Geochemical Characteristics Of The Sediment In The Niger Delta Basin


Chapter One


Introduction

The mineralogical and chemical composition of clastic sedimentary rocks are controlled by various factors, including (1) the composition of their source rocks, (2) environmental parameters influencing the weathering of source rocks (e.g., atmosphericchemistry, temperature, rainfall and topography), (3) duration of weathering (4)transportation mechanisms of clastic material from source region to depocenters, (5)depositional environment (e.g., marine versus fresh water), and (6) post-depositional processes (e.g., diagenesis, metamorphism) (Hayashi et al., 1997). Numerousinvestigations are substantiating the above aspects pertaining to genesis of both ancient andmodern siliciclastic sediments (e.g., Dickenson et al., 1983; Nesbitt and Young, 1982, 1984; Bhatia, 1983; Roser and Korsch, 1988; McCann, 1991; Condie et al., 1992; Condie, 1993; McLennan et al., 1993; Nesbitt et al., 1996; Cullers, 2000; Hessler and Lowe 2006; Nagarajan et al., 2007; Spalletti et al., 2008). Several studies have also been focused on the identification of palaeotectonic settings of provenances based on geochemical signatures of siliciclastic rocks (e.g., Dickinson and Suczek, 1979; Bhatia, 1983; Bhatia and Crook, 1986; Roser and Korsch 1986; McLennan and Taylor, 1991).

Among the terrigenous sedimentary rocks, shales are considered to represent the average crustal composition of the provenance much better than any other siliclastic rocks (e.g., McCulloch and Wasserburg, 1978). Shales retain most of the mineral constituents of the source and their bulk chemistry preserves the near-original signature of the provenance and more faithfully reveal palaeoweathering conditions (e.g., Pettijohn, 1975; Graver and Scott, 1995).

The present note examines the geochemistry of sediment from part of the subsurface Niger Delta Basin province, attempts to constrain there paleo redox and tectonic setting and provenance. Owing to limitations of analytical facilities, the present work is based on chemical analyses data of major and select trace elements of the investigated sediment of the study area.

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Chapter Five


Conclusion

Provenance of the Sediments. Based on major oxidesmost of the sample plots in the fields were felsic igneous provenances suggesting high content of silica from an acid rock most probably granite or gneiss or dacite or any acidic (felsic) igneous rock.

The provenance and prevalent conditions of deposition from various elemental ratios indicate that the Th/U ratiohas an average of 4.1 which is very close to that of upper continental crust of 3.8. The high ratios of Th/Sc and Zr/Sc indicate a slight input of felsic materials from recycled sed-imentary provenance. Higher abundances of incompatible elements like Th indicate felsic rather than mafic sources. Elevated values of thorium with respect to uranium may imply a felsic source. It will be observed that most values for the Al2O3/TiO2 ratio fall between 15 and 70 (the range for igneous rock) which is an indication that the source rock is felsic or acidic igneous rock such as granite, granodiorite, rhyolite, dacite, or aplite. Th/Sc values for the analyzed samples were in the range of 0.83–2.83, implying a felsic igneous provenance. The same applies for the Th/Co ratio as most of the values are above 0.27 and less than 19.5 (Th/Sc and Th/Co values for felsic rocks are 0.84–20.05 and 0.27– 19.5, resp.). Thus the source of the rock weathered to give the sediment is a felsic or acidic igneous rock, probably granite. Th/Co versus La/Sc logarithmic plot shows that the samples are sourced from felsic or acidic silicic rocks, and very few of the samples tend towards intermediate provenance.

Provenance from REE and negative EU anomaly points to the fact that average REE pattern of the sediments is interpreted to reflect the average upper continental crust. Coupled with a negative Eu anomaly, conclusions can be drawn that shallow, intercrustal differentiation involving pla-gioclase differentiation (through either melting or fractional differentiation) must be a fundamental process in removal of feldspar from a felsic melt. The LREE enrichment as well as relatively flat HREE pattern also confirms felsic source rock. T he relative REE patterns and Eu anomaly size have also been utilized to deduce sources of sedimentary rocks [20, 37]. Mafic rocks contain low LREE/HREE ratios and tend not to contain Eu anomalies, whereas more felsic rocks usually contain higher LREE/HREE ratios and negative Eu anomalies [38]. A negative Eu anomaly is a confirmation of the sediment’s provenance from felsic sources. Thus from the enrichment LREE or higher LREE/HREE, we can conclude that the provenance of the sediments is felsic rock.

Tectonic Settings. From major oxides it can be concludedthat the tectonic setting of the Niger delta is active continental margin and this confirms the cretaceous rift systems of West and Central Africa. The rift system extends for over 4000 km from Nigeria northwards into Niger and Libya and eastwards to Sudan and Kenya. This cretaceous rift system forms a trough in which those sediments are deposited. The trace elements confirmed the tectonic settings of the sediments as active continental margins. The trivariate plots of La-Th-Sc, Th-Sc-Zr/10, and Th-Co-Zr/10 all register the provenance of the sediments to be active continental margin. The Th/Sc versus Zr/Sc diagram after McLennan et al. [14], confirms the zone of sediment recycling in upper crust input.

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