MINERALOGY AND STRATIGRAPHY OF 3 DEEP LATERITIC PROFILES OF THE JOS PLATEAU (CENTRAL NIGERIA)

The Fluviovolcanic Series (= FVS) of the Jos plateau, Central Nigeria are an upper Eocene to middle Miocene time-equivalent to the Continental Terminal of the surrounding basins. It can be divided into a lower part with mafic to intermediate and even felsic volcanic rocks (according to the Zr/TiO2), fluvial or limnic sediments and an upper part of mafic volcanic rocks with intercalated transported pisoids and quartz grains. Palynomorphs of a limnic clay reflect a subtropical to montane tropical vegetation rich in plant species. The lower part of the FVS often shows distinct tilting as a result of neotectonic movement along a reactivated conservative plate boundary in continuation of the transatlantic Romanche fracture zone. The various rocks have been completely converted into deep (often > 80 m) polygenetic soils which consist of repetitive sequences of saprolite, mottled horizon and ferricrete. The ferricrete represents a paleosurface and protects the profiles against erosion. The mineralogy of 3 profiles consists essentially of kaolinite with various amounts of goethite and hematite; gibbsite appears in one confined horizon, whereas anatase and residual (weathered) ilmenite and quartz occur as accessories. The presence or absence of ilmenite, quartz and anatase in the various sections of the profiles were taken as an indicator for different parent rocks. The crystallinity of kaolinite (Hinckley index), the crystal size and Al substitution of goethite and hematite varied sectionwise indicating different weathering environments. For example, the goethite in laminar-massive ferricretes appearing at the base of all 3 profiles had lower Al substitution and somewhat larger crystals than that of the pisolitic-vermiform ferricretes and was associated with redoximorphic conditions. On the basis of mineralogical, paleontological, sedimentological and absolute K/Ar age of basalts, the profiles were placed into the transition from Paleogene to Neogene and stratigraphic correlation was suggested.