Palaeoenvironmental significance of the clay mineral composition of Olduvai basin deposits, northern Tanzania

Quaternary deposits in the southeastern part of the Olduvai basin, northern Tanzania, consist of lake margin deposits, followed by a series of fluvial sediments. The clay mineral fraction of the lake margin deposits (Bed I and lower part of Bed II) is composed of smectite and subordinate illite. All smectite is largely dioctahedral and shows indications for a limited degree of irregular interstratification by illite. In the overlying fluvial deposits (Beds II–IV), illite is the most abundant clay mineral. Smectite only occurs in lower parts of the fluvial deposits (up to the middle of Bed III), where it generally shows a high degree of irregular interstratification. Differences in clay mineral composition between the lake margin deposits and the fluvial deposits record differences in sediment source area and degree of alteration. Dioctahedral smectite in the lake margin deposits and the oldest fluvial deposits is derived from a region with volcanic material extending to the east and south of the basin, which also supplies a certain amount of illite. Illite in the fluvial deposits of Bed IV originates from an area with a metamorphic bedrock to the west and north. Alteration of detrital clay minerals resulted in Mg-enrichment of dioctahedral smectite in part of the lake margin deposits and partial illitization of smectite in the older fluvial deposits. Formation of clay minerals during diagenesis or soil development is not documented by analysis of the total clay fraction.