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Der Es Safya Graben im Nordsudan - Geologie und Tektonik
Fiedler-Volmer, Rainer
The Es Safya Graben (ESG), a so far unknown structural feature of the North Kordofan Block (NKB), has been revealed by applying multidisciplinary methods such as satellite image interpretation and field-geological and geophysical investigations. The ESG is 65 kilometres long and some 25 kilometres wide, containing a sedimentary section of 200 to 500 metres. The highly complex structure is formed by three half-graben-type segments which are linked by a transfer zone and a basement uplift. The low- to high-angle normal boundary faults show various amounts of displacement. The ESG is bounded to the north by a series of faults comprising the Sodiri Shear Zone (SSZ). To the south the graben is bounded by the Abu Harr Shear Zone (AHSZ), a structure which could be revealed by this study. Fault orientations and the direction of relative movements provide evidence that left-lateral wrenching along and in between the Sodiri and Abu Harr Shear Zones transtensionally formed the ESG using pre-existing, antithetic Riedel faults, thus suggesting similarities to a pull-apart structure like development. Sedimentological and structural evidence indicate a post-sedimentary normal faulting. Up to now Triassic lateral displacement along the SSZ was assumed to be the latest deformation of pre-existing Late Proterozoic lines of lithospheric weakness (structural grain) of the NKB. In the light of the structural and statigraphic setting of the ESG it can now be proved that during Late Cretaceous and Tertiary times a further tectonic development took place, which now can be added as D5 to the known deformational phases D1-D4 (Late Proterozoic to Triassic). Crystalline rocks comprising the graben shoulders consist of Pan-African basement and anorogenic magmatic complexes. The plutonites have a calc-alkalic composition being typical for subduction-related magmatism. They indicate the existence of fossil plate margins and give for the Es Safaya area a scenario of a complex geodynamic pattern of terranes during the Pan-African orogeny, similar to the known picture of the Gebel Rahib area in the N of the NKB. Alkalic magmatism took place in the Es Safaya region, like in the entire of Sudan, during several episodes in between Ordovician and Jurassic times. K-Ar age data indicate that some of the anorogenic magmatites are approximately 290 Ma old, thus providing ages which sofar have only little been known for the NKB. The distribution of alkalic complexes in the study area is controlled by the SSZ and AHSZ. Both shear-systems are interpreted to be branching elements of the Central African Fault Zone (CAFZ). The graben sediments have been subdivided into lower and upper Hamrat el Wuz Formation. The lower sequence is composed out of deltaic to lacustrine interbedded sandstone and mudstone yielding Albian to Turonian palynomorphs. The upper sequence is composed out of heterogeneous shaly to conglomeratic sediments of mainly fluvial origin whose age coud be assigned to the Campanian/Maastrichtian because of the palaeoflora Salvinia. Shallow-marine ingressions are indicated by the occurrence of Diplocraterion and Thalassionoides, thus proving a far more southerly advance of Campanian/Maastrichtian transgressions than previously supposed. Knowledge of graben-fill and structural geometries allows in connection with data on climate and water consumption a gross evaluation of the groundwater situation of this arid to semi-arid area. Dated findings of medieval ironworks prove a progressive climatic deteriation and are of great archaeological interest.
