Geochemical Fingerprints; Implication for Provenance, Tectonic and Depositional Settings of Lower Benue Trough Sequence, Southeastern Nigeria
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Date
2013
Journal Title
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Volume Title
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The International Institute for Science, Technology and Education (IISTE)
Abstract
The study areas, Asu River Group (ARG) and Cross River Groups (CRG) belong to Lower Benue Trough. The
Trough is thought to have been deposited by marine transgression and regression. ARG covers Awi, Abakaliki
and Mfamosing Formations while Ekenkpon, Eze-Aku, New Netim, Awgu and Agbani Formations fall within
CRG. Sampling was done to cover both the Abakaliki Anticlinorium and Calabar Flank. The study aimed at
using geochemical approach to deduce weathering, provenance, tectonic setting as well as depositional
environment in a holistic manner which hitherto has not been used by any worker.
A total of 56 fresh outcrop samples were obtained from the study area. The samples were subjected to detailed
lithologic description by visual examination. Geochemical analysis was done using Inductively Coupled Plasma
Mass Spectroscopy and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-MS/AES) to
determine major, trace and rare-earth elements using lithium metaborate/tetraborate fusion method.
Weathering Indices of Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), Chemical
Index of Weathering (CIW) and Ruxton Ratio (RR) of ARG has a range of (0.18-86.1, 0.13-99.3, 0.18-99.5 and
1.80-25.2) with median of (76.7, 92.6, 93.5 and 5.76) respectively while CRG has a range of (2.93-97.7, 2.78-
99.7, 2.94-99.7 and 1.83-46.4) and median of (76.1, 85.5, 87.7 and 10.9) respectively, indicating moderate to
high weathering at the source. The Al2O3-(K2O+CaO+MgO)-(Fe2O3+MgO), (AKF) ternary plots reveals
sediments of ARG and CRG deposited in Continental, Transition and Marine zone and dominated by
argillaceous, carbonaceous argillite, carbonaceous and a ferruginous argillites confirming a chemically altered
sediments deposited in oxidizing and shallow marine environment. The plots of Log (Fe2O3/K2O) vs Log
(SiO2/Al2O3) reveals sediments deposited in the Fe Shale, Shale, Wacke, Subarkose and Quartz arenite field. The
discriminant function plots of Herron characterized the sediments as been derived from Quartzose sedimentary
provenance, Intermediate igneous and Felsic provenance. Trace elements ternary plots of La-Th-Sc, Th-Sc-Zr/10
and Th-Co-Zr/10 reveal deposition within Continental Island Arc, Passive Margin and Oceanic Island Arc
settings. This confirmed the tectonic discriminant plots of K2O/Na2O vs SiO2. This discriminant function
diagram proposed by Roser and Korsch (1986) distinguish the sources of the sediments into four provenance
zones, mafic, intermediate, felsic, igneous and quartzose sedimentary using ratio plots and raw oxides. Most of
the sediments of ARG and CRG fall in, quartzose sedimentary provenance using raw oxide plots, and exceptions
are the limestones that fall in intermediate igneous provenance this also corroborates with the ratio plots.
Conclusively, the study shows that the Cretaceous clastic sediments of ARG and CRG have multiple
provenances subjected to moderate to high weathering conditions and were deposited within an oxidizing and
shallow marine setting and derived from Upper Continental Crust (UCC).
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Citation
Adeigbe and Jimoh, 2013