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. 2024 Jun 18;10(12):e33256.
doi: 10.1016/j.heliyon.2024.e33256. eCollection 2024 Jun 30.

Lithotectonic setting, paleoweathering, maturity, sediment recycling and paleoredox conditions of quaternary alluvium from Mayo Sina (Tchad). A geochemical approach

Amaya Adama  1 , Bisse Salomon Bertrant  1 , Bokanda Ekoko Eric  2   3 , Akono Daniel Florent  4 , Belinga Belinga Cédric  4
Affiliations

Lithotectonic setting, paleoweathering, maturity, sediment recycling and paleoredox conditions of quaternary alluvium from Mayo Sina (Tchad). A geochemical approach

Amaya Adama et al. Heliyon. .

Abstract

The alluvial material in the two PITs of the Mayo Sina in the southern part of Chad, were subjected to a geochemical analysis in order to determine the source rock lithological composition, tectonic context, degree of weathering, type of climate at the period of weathering, and cycles of sedimentation and maturity. Plotting source lithology charts with values of La/Sc, Th/Sc, La/Co, and Cr/Th indicated that the sediments originated from a felsic source rock composition. The Enrichment of LREE/HREE and a negative chondrite Eu anomaly confirm the felsic source rock composition. A passive tectonic context for the source rock of the sediments is revealed by binary charts (SiO2 versus K2O/Na2O) in conjunction with innovative discrimination plots (DF 1 and DF 2 (Arc-Rift-Col)M1, DF (A-P)M, and DF (A-P)MT). The exceptional degree of plagioclase removal and weathering is demonstrated by the results of the Plagioclase Index of Alteration (PIX) and Chemical Index of Alteration (CIX). The proportion of SiO2/Al2O3 and the new index of compositional variation (ICVnew), which exclude iron and calcium oxides from the formula, indicate that the sediments are immature to mature. Schematic representations of K2O/Na2O ratio indicate that the sediments have undergone both primary to secondary phases of sedimentation.

Keywords: Alluvial materials; Intensity; Maturity; Mayo Sina; Source lithology chemistry; Tectonic scenery.

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Conflict of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Fig. 1
Fig. 1
(a) Map of Chad showing the location of the study area (b) shematic facies map of the study area (adapted from Isseini, 2011; Toteu et al., 2006).
Fig. 2
Fig. 2
Diagram of alluvial pits showing PIT01 and PIT02 with description of the different layers.
Fig. 3
Fig. 3
X-ray diffraction spectra characteristic of clays fraction mineralogy: (a) selected samples for alluvial pit 01 (PIT01); (b) selected samples for alluvial pit 02 (PIT02).
Fig. 4
Fig. 4
Spectral and bar chart diagrams showing major elements normalised to PAAS and their proportion variation with respect to weathering indices (a) PIT01 samples, (b) PIT02 samples, (c,d) PIT01, PIT02, PAAS and UCC.
Fig. 5
Fig. 5
Spectral and bar chart diagrams showing trace elements normalised to PAAS and their proportion variation with respect to LILE, HFSE & TTE (a) PIT01 samples, (b) PIT02 samples, (c,d) PIT01, PIT02, PAAS and UCC.
Fig. 6
Fig. 6
PAAS and Chondrite normalised diagram for the studied samples (a,c) PIT01 samples, (b,d) PIT02 samples.
Fig. 7
Fig. 7
Depositional environment plots for the studied PIT samples (a–c) Binary diagram of Th/U, Y/Ho and Sr/Ba ratio by Bokanda et al. (2018a;2022a), (d–f) depositional condition plots of U/Th, Ni/Co and V/Cr ratio by Bokanda et al. (2018a).
Fig. 8
Fig. 8
Binary diagrams for Past weathering and maturity of the studied PITs samples (a,b) for CIX and PIX respectively, (c) for Sr/Cu (c–d) for ICVnew and oxide of Si/Al respectively and (e) ratio of oxide of K/Na.
Fig. 9
Fig. 9
Source lithology illustrations for the studied sand (a-d) La/Sc*100, Th/Sc*100, Cr/Th, La/Co*100 correspondingly after Bokanda et al. (2021).
Fig. 10
Fig. 10
Tectonic discrimination plots for the studied sand samples (a–b) K2O/Na2O vs SiO2, (b–c) DF1 (arc-rift-col)m1 Vs DF2 (arc-rift-col)m1 from Verma et al. (2013), (d,e) DF (A-P)M & DF (A-P)MT from Verma and Armstrong-Altrin (2016).
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