GEOCHEMISTRY OF EKENKPON AND NKPORO SHALES, CALABAR FLANK, SE NIGERIA: IMPLICATIONS FOR PROVENANCE, TRANSPORTATION HISTORY AND DEPOSITIONAL ENVIRONMENT

Matthew E. Nton, Oto S. Akpan, Olajide J. Adamolekun

Abstract


The Cretaceous Nkporo and Ekenkpon Shales within the Calabar Flank were investigated  using  Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) and Atomic Emission Spectroscopy (ICP-AES) to deduce the provenance, transportation history and depositional environment. The results show the dominance of SiO2 (45.27 – 46.45%; 44.50 – 54.83%), Al2O3 (22.27 – 23.57%; 19.20 – 20.20%) and Fe2O3 (8.30 – 9.04%; 5.64 – 7.30%) constituting the bulk major oxides of Nkporo and Ekenkpon Shales respectively. The Index of Chemical Variation (ICV) ranges from 0.42 – 0.56 and 0.57– 0.68 for Nkporo and Ekenkpon Shales respectively, suggesting moderate and high degree of weathering. The enrichment of Sr (124 – 350ppm; Nkporo Shale and 176 – 856ppm; Ekenkpon Shale) compared to Post Australian Archean Shale (PAAS; 14.6ppm) could be attributed to feldspars in the source area, and is easily incorporated in the clay minerals of the shales.  Also the values of  Th and U which are 15.00 – 17.20ppm; 14.6-23.2ppm and 3.5-4.2ppm; 1.9-6.2ppm  respectively in Nkporo and Ekenkpon sediments  show enrichment compared to PAAS of 14.6ppm and 3.1ppm for respective values of Th and U. The Eu anomaly of 0.04 to 0.07 suggests felsic source rocks for both formations while plots of TiO2 vs. Al2O3 indicate an intermediate to dominantly felsic granodiorite source for both shales, with little input from felsic volcanic provenance plus traces of quartzose sedimentary rocks. The mobility of Na, Ca, and K due to progressive weathering of the shales is evident in the bivariate plots of Na2O wt% vs. PIA, CaOwt% vs. PIA and K2O vs. PIA. Values of Th/U ratios range from 3.37 – 4.91(Nkporo) and 3.10 – 7.68 (Ekenkpon), indicating moderate to high weathering and reworking of sediments. It is envisaged that the area is associated with  passive to active continental margin tectonics, where sediments were mainly sourced from felsic rocks of the adjoining terrain  and deposited in oxic, continental to transitional marine environment based on Al2O3–K2O +CaO+MgO-Fe2O3+MgO; AKF plot.


Keywords


Cretaceous, Calabar Flank, provenance, transportation history, depositional environment.

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