The quaternary stratigraphy and environments of Olduvai gorge - Tanzania, based on fossil soils and related dating

Abstract

This study reports research results and interpretations based on field geological-stratigraphical- litological-palaeosol sequential studies of Tertiary-Quaternary deposits of Olduvai Gorge (main work), Manonga-Wembere Valley and Holili, (annex work) localities in Tanzania. It is also based on laboratory studies (micromorphology, mineralogy, geochemistry and magnetic susceptibility). Special emphasis is placed on the study of palaeosol levels frequently found in these sedimentary sequences. Geology and stratigraphy In all the three areas, Pliocene-Pleistocene sediments are laid down unconformable on a Precambrian basement complex. A huge unconformity exists between the Precambrian rocks (granite, quartzite and gneiss) and the Pliocene-Pleistocene sediments. The basement is a remnant planation surface, which was formed at the beginning of the Tertiary, often represented by isolated granite kopjes, quartzite and gneiss inselbergs and hills sticking out from the present plain level. In Olduvai Gorge the study revisits the stratigraphy and reveals numerous sediments and palaeosol levels (not earlier reported). These levels and beds are within the earlier recognized general Bed I, II, III, IV, Masek and Ndutu Beds. Bed I (2.2Ma - 1.75Ma) contains at least 43 lithological (clay, sands, gravel, mudstone, limestone and marls) units including palaeosol levels. Bed II (1.75Ma - 1.15Ma) is also a zone of abundant geological depositional environments composed of clay, sand-gravel bed complexes, tuff (ash fall or ash flow), limestone and palaeosol levels. Bed III (1.15Ma - 0.6Ma) is a complex volcano-sedimentary depositional environment marked by lacustrine marly sequences, clay layers, sand-gravel beds and calcarenaceous sediments itercalated by red-brown palaeosol levels. Bed IV and Masek Beds (0.6Ma - 0.4Ma) are not distinguishable in the field and therefore grouped together and names adapted from previous workers. The zone contains about 4 palaeo-Vertisols each developed on a clay layer. Ndutu Bed (0.2Ma and younger) is represented by a series of 7 palaeo-Vertisol levels (similar to the ones in Masek Beds), mudstone, claystone and limestone from bottom to top. Gravel and sand beds across the profile have the highest magnetic susceptibility (MS) values compared to clay, marls, calc-sediments or tuff lithologies. Magnetic susceptibility values of palaeosol levels in sandy units are usually lower than the background values of the sand layers. Palaeosols that developed on clay units have higher MS record than the clay background values. The general MS pattern shows an increasing trend across the stratigraphy from older to younger units. The mineralogical signatures also reflect the general stratigraphical characterization. The lower (Bed 1 and Lower Bed II) stratigraphical unit show higher clay minerals and carbonate concentrations, the middle (Bed II and Bed III) indicate slightly low clay minerals and carbonates abundance and the upper parts of the profile (Masek/Bed IV) show higher concentrations of clay and carbonate minerals. Based on field and some micromorphological studies 57 palaeosol levels are mapped. They are grouped into (a) Red-brown to dark gray palaeosols which are probably palaeo-Alfisols or Ultisols, occur in Upper Bed I, Middle Bed II and Bed III. (b) Olive to gray palaeo-vertisols (Bed I, Masek and Ndutu Beds), (c) Palaeo-Andisols found in Bed I and Bed II are observed to develop on ash fall/flow tuffs and (d) Palaeo-Aridisols are gray to olive palaeosol levels, seem to have developed on sand levels mainly in upper Bed II. Red to brown palaeosols (AlfisoIs/UItisols?) are slightly richer in FeO-Fe2O3 and MnO values than other palaeosol levels. Total soil silica/sesquioxide mole ratios of both palaeo-Alfisols and palaeo-Vertisol resemble modern Alfisols and Vertisols. Micromorphological studies indicate that gray-olive palaeosols (Vertisols, Aridisols and Andisols) contain numerous calcite nodules, calcite coatings/hypocoatings and infillings with rare Fe-Mn hydr(o)xide coatings and nodules. Clay coatings are rare or absent. They commonly show a granostriated or monostriated b-fabric and porphyric c/f related distribution of the basic components. The red-brown and dark gray palaeo-Alfisols contain multiple micromorphological features (red-yellow clay coatings and clay coating fragments, Fe-Mn oxide nodule and coatings with some calcite coatings and in-fillings) that are usually seen, imprinted on one another. Palaeo- Aridsols are regarded to be formed in semi-arid palaeoclimates, while palaeo-Vertisols were formed in alternating wet and dry conditions of the Pleistocene Epoch. The red to brown palaeo- Alfisols are assumed to have developed during wet/humid palaeoclimates in Olduvai Gorge during the Pleistocene. The Manonga-Wembere Valley geology comprises of Pliocene-Pleistocene lacustrine gravel, sands and calcareous-clay deposits and Holocene mbuga clays and alluvial sand. The micromorphology of palaeosol levels from Manonga-Wembere Valley show strong clay illuviation of red to yellow clay coating and clay coating fragments with Fe-Mn (hydr)oxide coatings. The clay coating fragments occur as accumulation of oriented clay-coating fragments in a red to yellow groundmass resembling clay illuviation fronts found in present day warm and humid (Meditterranean) climates. -Manonga-Wembere Valley palaeosol levels are therefore assumed to represent a wet and humid climate and environment during the Pliocene-Pleistocene times. Kaolinite and illite clay minerals together with gibbsite and some zeolites (analcime and stilbite) dominate the mineralogy of these palaeosol levels. The geology and stratigraphy of Holili begins with the Precambrian basement rocks and then covered by lava flows (basalt) of the Kilimanjaro volcanic episode. Then a soil (palaeosol) developed on the basalt. The landscape (palaesol) was finally covered in succession by tuffaceous mudstone and calcareous tuffaceous grit. A hominid tool, fossil leaf impressions of angiosperm dicotyledon flowering plants and animal remains (tooth, horn and canon bone) were recovered in Holili Pleistocene deposits. Palaeoclimatic and palaeoenvironmental variability The palaeoclimatic and palaeoenvironmental variability in Olduvai Gorge, Manonga- Wembere and Holili localities as deduced from facies, magnetic susceptibility, mineralogy, geochemistry, micromorphology and palaeosol occurrences and cyclicities seem to be linked to the global causes of climatic changes. 400ka Gravel beds cyclicity (at about 2.2Ma, 1.76Ma, 1.4Ma, l.OOMa 0.65Ma and 0.2Ma), 40Ka and lOKa palaeosol cyclicity are observed in Olduvai Gorge. The climatic variability of Olduvai Gorge during the Quaternary is found to correlate with that of Greece (Mediterannean). This is evidence that the climate of Olduvai Gorge during the Quaternary also followed global trends. In Olduvai Gorge hominids and other animal fossil remains are frequently found on palaeosols levels or closely associated to palaeosol levels. Long periods of 400Ka marked by gravel bed complexes are generally wet periods. Gravel bed complexes that mark significant short periods of drought in this study are correlated to FAD and LAD of Australopithecus boisei, Homo habilis and Homo erectus hominid species in Olduvai Gorge. Likewise in Manonga-Wembere Valley and Holili deposits animal fossil remains occurrences are associated to palaeosol levels. Future research For future research a complete micromorphological study of all possible palaeosol levels will be helpful in discerning more the climatic variability. Future hominid search would be more successful if palaeosol levels were mapped and followed over long distances. New Ar-Ar dating of the basalt and tuff in Manonga-Wembere Valley and Holili areas would provide better age estimates of the deposits.