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

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.

Fig. 1

(a) Map of Cameroon in the Africa continent; (b) The Cameroon volcanic line, Mamfe basin and Benue trough in the Cameroon Map; (c) modified geologic map from Ref. [14]of the Mamfe Basin showing the study locations.

Fig. 2

Field photos of Cretaceous black shales from the Mamfe basin. (a) Millimetric to centimetric laminated black shales, (b) massive weathered shales, (c) laminated shales showing nodules of siderite (Sn) interbedding with limestone (lst), (d) centimetric laminated shales.

Fig. 3

(a–m). Photomicrographs of shales in the Mamfe Basin. (a, h, f, m) scattered minerals mixed with organic matter, (b–d, g) Shales with millimetric to centimetric lamellae composing or organic matter mixed with minerals (i, j) Dolomite-rich algal mat facies; with filamentous calciphytes and phyllodes, (k, l) shales with organic matter occupying an entire domain. Qtz = quartz, Ca = calcite, OM = organic matter, Mus = muscovite.

Fig. 4

(a) Rare earth element + Y PAAS pattern of whole rock samples (b, c) PAAS normalized cross plot diagram of Ce/Ce* versus Pr/Pr* used as a proxy for the Ce and La anomaly modified from Ref. [12]. Notes: Field I: neither Ce nor La anomaly; Field IIa: positive La anomaly, no Ce anomaly; Field IIb: negative La anomaly, no Ce anomaly; Field IIIa: positive Ce anomaly, negative La anomaly; Field IIIb: negative Ce anomaly.

Fig. 5

Statistical analyses diagram (a) PCA for major elements oxides, (b) PCA for trace elements, (c) PCA for rare earth elements (REEs), (d) HCA for major elements oxides, (b) HCA for trace elements, (c) CCA for rare earth elements (REEs).

Fig. 6

Xrd spectral lines for the studied black shales in the Mamfe basin. (a, c, d) dolomitic shales, (b) carbonated shales.