Fluvio-deltaic environment and in situ pedogenesis of Middle Miocene kaolinitic sandstones of the Pugu coastal area of Tanzania

The Middle Miocene Pugu Sandstone of Central Tanzania is built of (1) micaceous, (2) hard and (3) soft kaolinitic sandstones, the hard kaolinitic variety consisting of poorly crystallized ragged anhedral kaolinite crystals and the soft type consisting of well-crystallized hexagonal crystals of kaolinte. Sedimentary structures in the sandstone suggest that they were deposited in a fluvially dominated flood plain without a significant marine influence. On the basis of heavy-mineral analyses, palaeocurrent data and the existence of strongly undulatory polycrystalline quartz, the provenance of the sandstones is surmised to be the Precambrian metamorphic rocks of the Usagaran System and sediments of Permo-Carboniferous (Karroo) to Cretaceous age. Three palaeo-soil horizons suggest that the sedimentary sequence was interrupted by at least three phases of subaerial weathering.