Organic vs inorganic contribution to the chemistry of cretaceous black shales in the Mamfe basin, SW Cameroon. Evidence from geochemistry and statistical analysis

Bisse Salomon Betrant; Bokanda Ekoko Eric; Florence Njinto Nkwankam; Ashukem Ethel Nkongho; Nanfa Tefak Fatoumata Maelle; Yugye Jules Alex; Belinga Belinga Cedric; Akono Daniel Florent; Nzesseu Nandjou Valentino; Ekomane Emile

doi:10.1016/j.heliyon.2023.e13748

Organic vs inorganic contribution to the chemistry of cretaceous black shales in the Mamfe basin, SW Cameroon. Evidence from geochemistry and statistical analysis

Heliyon. 2023 Feb 15;9(3):e13748. doi: 10.1016/j.heliyon.2023.e13748. eCollection 2023 Mar.

Affiliations

¹ School of Geology and Mining Engineering, P.O. BOX 115, Meiganaga, Cameroon.
² Department of Geology, University of Buea, Cameroon.
³ National Higher Polytechnic Institute, University of Bamenda, P.O BOX 039, Bambili, Cameroon.
⁴ Department of Earth Sciences, University of Yaoundé I, Cameroon.
⁵ Institute of Mining and Geological Research, P.O. BOX 4110, Yaoundé, Cameroon.
⁶ National Hydrocarbons Society, Yaoundé, Cameroon.

Abstract

The present study focused on evaluating the contribution of both organic and inorganic component to the chemistry of Cretaceous black shales in the Mamfe basin, Sw Cameroon by performing inductively couple plasma spectrometry analyses with the data analysed using multivariate statistical analyses. The shales are classified as calcite enriched (Ca/Mg > 1) and calcite depleted (Ca/Mg < 1). Major elements such as K, Ti, and Al shows significant correlations (>0.71) among themselves and negative correlations with total organic carbon (TOC), total organic nitrogen (TON) and total organic sulphur (TOS) indicating that they were control mainly by inorganic factors. The positive correlation between phosphorus (P) and TOC, TOS, TON maybe due to absorption by organic fraction into the lattice of the shales. Selected both biophilic (Ba, Co, Ni, and Sc) and terrigenous (Zr) trace elements were correlated amongst themselves and with organic components. Zr show negative correlation (-0.36) with organic components and correlates negligibly with Co, and Ni (0.04, 0.16) indicating these elements were derived from organic matter with exception to Ba and Sc which shows positive correlation with Zr (0.77) and negatively correlated with TOS (-0.34, -0.13), carbon (-0.25, -0.17) and TON (-0.17, -0.06). The enrichment of light rare elements over heavy rare earth elements positive europium anomaly on PAAS normalise diagrams and an insignificant to negative correlation with TOC, TOS, and TON indicating their derivation from mainly an inorganic factor. Statistical analyses by hierarchical classification ascending (HCA) and principal component analyses (PCA) confirms solely an inorganic contribution to the chemistry of the studied black shales in the Basin. The positive correlations portray by some elements with organic components maybe due to their absorption by organic fractions into their lattice. Further indirect/direct methods such as sequential extraction and FTIR is required throw light on the origin of the chemistry of black shales in the Mamfe basin.

Keywords: Cretaceous black shales; Geochemistry; Inorganic component; Mamfe basin; Organic components.