Environmental conditions in the Late Cretaceous African Tethys: conclusions from a microscopic-microchemical study of ooidal ironstones from Egypt, Sudan and Nigeria

Abstract

Two types of ooidal ironstone deposits, all of Late Cretaceous age, were recognised in an area trending from Nigeria in a northeasterly direction via Sudan to Egypt. The two types of ironstones are of the kaolinite-type deposits (Agbaja area, Bida area, Sokoto and Potiskum, Nigeria; Shendi and Wadi Haifa, Sudan; and Kalabscha, Egypt) and of the chamosite-type deposits (Aswan, Egypt and Okigwe, Nigeria). Post-diagenetic ferruginisation of these deposits is reflected in only small variations in the chemical composition of the two types. Obvious differences include the varying MgO concentrations, which are considerably higher in the chamosite-type (in the range from ∼0.5–6.75 wt%). In the kaolinite-type, MgO varies from 0.0 to ∼0.4 wt%. One of the principal characteristics of the chamosite-type deposits is the occurrence of fossils, especially of brecciated shell material and bioturbation. These features are unknown in the kaolinite-type. In both types, pyrite and siderite are common constituents. These minerals are of diagenetic origin and were formed under reducing conditions in the presence of either chamositic clay minerals or kaolinite. The protoliths of both the kaolinite and the chamositic types were lateritic weathered rocks of the hinterland, transported via fluvial drainage systems and deposited in marine basins. The differences of the two types have been explained by the attendant environmental conditions. The environment of the chamosite-type is compatible with fully marine conditions and normal salinity, resulting in the availability of Mg leading to the formation of chamositic clay during diagenesis. The environment of the kaolinite-type is thought to be a marginal basin with high river discharge and thus lower salinity with zero or negligibly low Mg concentration. In this environment, a diagenetic transformation of the sedimentary kaolinite precorsur into chamositic clay failed to occur. The model is supported by the distribution patterns of the two ironstone types. Within the study area, the chamositic-types are located at the extreme positions and are thus closest to the open sea where fully marine conditions are most likely to have occurred.