Geochemical Fingerprints; Implication for Provenance, Tectonic and Depositional Settings of Lower Benue Trough Sequence, Southeastern Nigeria

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).

Keywords: Asu River Group, Cross River Group, Provenance, Geochemistry, Tectonic settings.

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