ABSTRACT
ABSTRACT: Hydrochemical and stable isotope profiles of 64 saline groundwaters samples from 20 different locations in the Benue Trough, Nigeria are presented and discussed in relation to the source of primary salinity with respect to the inland extension of the Gulf of Guinea during the Cretaceous period. Average electrical conductivity (EC) values of the saline groundwaters in the lower and middle region of the trough are 56,300µS/cm and 22,200µS/cm respectively. The saline groundwaters are Na Cl type enriched in Ca and Sr and depleted in Mg and SO4 relative to a seawater evaporation trend and have δ18O values of −5.99‰ and δ2H values of up to −40‰. The interpretations of the hydrochemical data strongly support salt/halite dissolution as the primary source of salinity and rule out evapo-concentrated and hydrothermal sources. However, the lower δ18O and δ2H values for most of the analyzed samples also suggest that the dissolution of halite/disseminated salts are related to influx of seawater during the inland transgressive extension of the Gulf of Guinea and subsequent remobilization of fossil/connate formation water rather than mineralization of the infiltrated meteoric water. The observed hydrochemical characters indicate that the original marine chemistry has been modified by water-rock interactions, involving dolomitization and enrichment of Ca, through cation exchange process.
