Mineralogy and geochemistry of Lobé River sediments, SW Cameroon: Implications for provenance and weathering

Highlights

• The Lobé River sediments consist mainly of very fine and fine sands.

• Post depositional K-enrichment have affected the studied sediments.

• These sediments and their sources underwent a high degree of chemical weathering.

• They derived principally from felsic rocks and subordinate amount of mafic and recycled detritus.

Abstract

The Lobé River (LR) watershed basement rock, Ntem Complex (NC), south Cameroon is covered by a thick lateritic soil. To better constrain the geology of the basement rock, textural, mineralogical and bulk geochemical (major, trace, and rare earth elements) analyses were undertaken on the recent stream sediments from the LR. The aims of this study were to investigate the provenance of sediments and to determine the weathering intensity of the source area. The ternary (Al₂O₃–TiO₂–Zr) and binary (Th/Sc–Zr/Sc) diagrams suggested high sediment recycling and sorting, indicating that the bulk hydraulic energy of the LR controls the sediment texture. Despite the diagenetic processes of K-metasomatism observed on the A-CN-K plot, the (A-K)-C-N plot, Chemical Index of Alteration (CIA), and Plagioclase Index of Alteration (PIA) indicate that the studied sediments and their source areas underwent a high degree of chemical weathering. The heavy mineral suites imply heterogeneous provenance, including igneous, metamorphic and sedimentary source rocks. The ternary diagrams and Co/Th, La/Sc, Sc/Th, Cr/V, and Y/Ni elemental ratios indicate mostly felsic source rocks with lesser contribution from mafic components. The comparison of chondrite normalized REE patterns of LR sediments with potential source rocks reveals that the sediments were derived from the Precambrian basement rocks composed of tonalitic, trondhjemitic, and granodioritic suites, with remnants derived from Archean Greenstone Belt.

Introduction

The geochemistry of sediments from dune (Újvári et al., 2008; Rao et al., 2011; Asadi et al., 2013), lacustrine (Roy et al., 2008; Ekoa Bessa et al., 2018; N'nanga et al., 2019), fluvial (Singh, 2009; Ndjigui et al., 2014, 2015, 2018, 2015; Mbale Ngama et al., 2019), and marine environments (Wang et al., 2014; Zhang and Gao, 2015; Armstrong-Altrin et al., 2017, 2018, 2018; Anthony et al., 2019; Ayala-Pérez et al., 2021) have been analyzed widely to infer the source area weathering, parent rock composition, and depositional processes. The chemical weathering and the pedogenesis cause the degradation of labile feldspar from pristine rocks to secondary clay minerals. This results in the selective leaching of labile cation (Ca²⁺, Na⁺, K⁺, Fe²⁺, and Mg²⁻) relative to hydrolysate constituents such as Al⁺ and Ti⁴⁺ (Nesbitt and Young, 1982). These chemical attributes are finally transferred to sedimentary records and are deemed to be useful indicators of the original composition of parent rocks and subsequent weathering conditions (McLennan, 1993).

River and alluvial terrace sediments are resulting from chronic inundation of adjacent floodplain, which consist predominantly of soil particles and non-biological materials that represent the detrital products of the catchment basins (Arhin and Nude, 2009). These sediments are considered to provide an overview and a more comprehensive understanding of the geochemical signature of source domains and precisely the underlying bedrock, since their chemistry is highly sensitive to compositional variability (Singh, 2009; Kirkwood et al., 2016; Silva et al., 2016).

As they are obtainable and easy to reach, modern river sediments offer opportunities to extract petrographic information from areas which are devoid of extensive outcrop such as the Atlantic equatorial domain in central Africa. In this area, the knowledge of the bedrock is limited by the availability of outcrop. In fact, outcrops are rare, due to the dense vegetation and the thick lateritic cover under the equatorial rainforest. In order to provide a holistic view of the geochemical difference between many sectors of this bedrock, indicating geologically diverse regions, it is worth considering geochemical survey from stream sediments of several or all catchment basins. However, few geochemical studies on alluviums have been reported in the Atlantic equatorial area (Ndjigui et al., 2015, 2018, 2018; Nyobe et al., 2018; Mbale Ngama et al., 2019; Bassanak Ongboye et al., 2019; Mioumnde et al., 2019). Furthermore, these studies have been limited in terms of sampling density and extent, and do not cover most of the river watersheds of the area studied such as the LR watershed. Among the results obtained from previous geochemical studies on alluviums from the Atlantic equatorial area, data on unconsolidated alluvial sands from the Mefou River terrace in Yaoundé area, southern Cameroon, are noticeable, regarding provenance issue (Mbale Ngama et al., 2019). Mbale Ngama et al. (2019) further suggested that the nature of sediments is inherited from the gneiss basement which experienced a high degree of weathering. However, more systematic investigation and quantitative data are required to better constrain the various geology of this basement rock. The lithology of LR watershed and its environs belongs to the Ntem Complex (NC) and consists predominantly of granite, gneisses, migmatite, granitoid plutons of Tonalitic, Trondhjemitic, and Granodioritic (TTG) suites, charnockite (i.e., orthopyroxene bearing granitoid), Banded Iron Formations (BIF) and sillimanite-bearing paragneisses (Clifford and Gass, 1970; Cahen et al., 1976; Bessoles and Trompette, 1980; Nédélec et al., 1990). Although the aforementioned previous studies have been attempted to characterize the features and the genesis of rocks from NC, the structures, depositional settings and relative extents of these rocks remain to be precisely constrained on the restricted geographical area of the LR watershed. In particular, the supracrustal rocks (BIF and sillimanite-bearing paragneisses), representing remnants of greenstone belts, form xenoliths whose geometries are not well mapped (Feybesse et al., 1998; Penaye et al., 2004). Furthermore, the greenstone belts and TTG are characterized by vertical tectonics (vertical foliation, stretching and vertical lineation, and isoclinal folding) which makes it difficult to reconstruct their stratigraphy (Tchameni et al., 2000; Shang et al., 2004, 2007). This thereby results in the uncertainty on the nature and the spatial repartition of the various types of basement rocks existing within the drainage basins. This uncertainty is further enhanced within the LR watershed where outcrops are rare, due to the thick lateritic covers of the underlying basement rocks. Thus, as an alternative approach, the geochemistry of stream sediments from the studied watershed could be used to provide a more comprehensive understanding of the parent rock types, since cartographic ground and field observations are rare (e.g., Singh, 2009; Ndjigui et al., 2014, 2015; 2018; Mbale Ngama et al., 2019). In this regard, the alluvial sediment geochemistry of the LR, which belongs to one of the noticeable coastal river watersheds of the African equatorial region, is not yet documented. The LR is thought to be of importance in the equatorial area, since it has a permanent regime and always carries a non-negligible amount of suspension loads to the Atlantic coast of southwest Cameroon (Olivry, 1986). Hence, it is interesting to provide, for the LR sediments, a geochemical dataset which could help to characterize their source rocks.

In this paper, we report the mineralogical and geochemical features of alluvial sediments from the LR, in order to infer their provenance and the weathering intensity of the source terrane. Since the river draining area is diverse in terms of lithology, it is important to understand to what extent some geological features of the LR sediments are inherited from their predicted parent rocks. This study also help to better establish the nature of the Precambrian bedrock of the Atlantic sector of African equatorial region, since the outcrops of this bedrock are rare and less extensive, resulting in the limitation of the traditional geological survey method.