GEOCHEMICAL SIGNATURES OF PALEODEPOSITIONAL AND DIAGENETIC ENVIRONMENTS: A STEM/AEM STUDY OF AUTHIGENIC CLAY MINERALS FROM AN ARID RIFT BASIN, OLDUVAI GORGE, TANZANIA

Olduvai Gorge, Tanzania (East African Rift) exposes a 100-m thick Plio-Pleistocene sequence of volcaniclastic sediments deposited in a 40-km wide closed basin containing a playa lake. Source rocks on the western margin are Precambrian quartzose-feldspathic rocks, whereas source rocks on the eastern margin are trachytic volcanics (Ngorongoro complex). Previous investigations have shown that the lake size varied in response to regional fluctuations in aridity. A STEM and AEM study of authigenic clay minerals in sediments from representative depositional environments in the basin (fluvial, lake, lake margin, and soils) was undertaken to determine whether clay compositions could provide unique geochemical fingerprints characteristic of source area and paleoenvironmental conditions.

The tuffaceous pyroclastic fan deposits (eastern margin) are dominated by a highly disordered Al-rich dioctahedral smectite derived from the alteration of volcanic glass. A Mg-rich (saponitic) component is also present resulting in octahedral Al:Fe:Mg mole proportions (%) of ~50:20:30. Soils developed on the distal pyroclastic fan contain smectite that is Fe-rich and Mg-poor (Al:Fe:Mg ~45:35:20) relative to the pyroclastic material. The silty claystones of the fluvial plain (western margin) contain smectite that is more Al-rich, and Mg- and Fe-poor relative to the eastern-margin pyroclastic material (Al:Fe:Mg ~60:20:20). The Al-rich chemistry is consistent with weathering of Al-rich quartzose-feldspathic source material. Lake sediment contains Mg-rich smectite (saponitic: Al:Fe:Mg ~25:15:60) and Mg-rich illite (celadonitic; Al:Fe:Mg ~10:20:70). These lake-clays occur as pseudohexagonal euhedral crystals (~50-100 nm) consistent with neoformation.

Our study shows that clay minerals formed in the various depositional environments retain unique compositional signatures inherited from their parent source rocks, as well as acquiring a paleoenvironmental fingerprint during neoformation and diagenetic/pedogenic alteration. Determination of clay mineral compositions at a basin-wide scale provides a useful tool for constraining the distribution of depositional and diagenetic environments, which are in turn records of paleoclimatic conditions.