Palaeoclimatic Control on the Composition of Palaeozoic Shales from Southern Ghana, West Africa

Abstract

The Late Ordovician to Early Cretaceous Sekondian Group of southern Ghana is made up of seven major sedimentary formations. One important feature of the Sekondian Group is the abundance of fairly fresh plagioclase in the lowest two formations, i.e., the Ajua Shale and Elmina Sandstone, and at the uppermost formation (i.e., Essikado Sandstone); but a virtual absence throughout the rest of the Group. The basal part of the Sekondian Group has been interpreted as of glacial origin whereas the upper formations are thought to have formed in a warmer stage. The abundant plagioclase in the lowest two formations could, therefore, be due to less pervasive chemical weathering rather than erosion of a distinct source. In this paper, geochemical data for fine-grained sedimentary rocks of the Ajua Shale and the Takoradi Shale (that overlie the Elmina Sandstone) formations are used to investigate the influence of palaeoweathering on the composition of the Sekondian Group. The concentrations of the transition trace metals Cr, Ni, V, and Cu, and Cr/V-, Zr/Y-, and Ba/Co-ratios are similar in the samples from both formations and suggest that they were largely derived from the granitoids that intrude the Birimian supracrastal rocks. However, the Takoradi Shale samples are enriched in Rb, Cs, Th, U, and Nb relative to the Ajua Shale samples, and this may be related to differences in the degree of weathering in the source rocks. The Takoradi Shale samples are characterized by high chemical index of alteration (CIA) values and Rb/Sr ratios (CIA > 75; Rb/Sr > 1) indicating high degrees of weathering of their source rocks. In contrast, the Ajua Shale samples have low CIA values and Rb/Sr ratios (CIA