The quaternary stratigraphy and environments of Olduvai gorge - Tanzania, based on fossil soils and related dating
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Date
2000
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VRIJE UNIVERSITEIT BRUSSEL
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.
Description
Keywords
Environments, Olduvai gorge, Tanzania,, Fossil soils, Quaternary, Stratigraphy, Quaternary stratigraphy