Paleoenvironmental Reconstruction of the Adigrat Sandstone Formation Around Dire Dawa Area, Southeast Ethiopia
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Date
2014-06-06
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Addis Ababa University
Abstract
The Adigrat Sandstone Formation of southeastern Ethiopia was not previously studied independently from the other Mesozoic sedimentary units. Its environment of deposition has been inferred as braided stream from regional studies with limited data. The present research work has tried to study this unit in more detail to reconstruct paleoenvironment (depositional processes and environments) that led for the formation of this sandstone unit. The data used in this study are mainly field data such as lithology, color, sedimentary structures, textures, composition and trace fossils together with petrographic analysis of thin-sections. Using these field data and laboratory results thirteen facies types have been identified and interpreted. These facies types are: poorly sorted massive pebbly sandstone facies, poorly sorted medium grained massive sandstone, trough cross-bedded fine sandstone facies, greenish color massive mudstone facies, brownish yellow shale-dolomite-gypsum unit, herringbone cross-stratified fine sandstone facies, hummocky cross-stratified fine sandstone, slightly bioturbated normal graded sandstone, intensively bioturbated fine-medium grained sandstone, horizontally laminated fine grained sandstone, massive and well sorted quartz rich sandstone, calcareous sandstone and sandy limestone. From these, related facies types have been grouped together by which four facies associations are established and interpreted in terms of depositional environments. These are: meandering stream, shore zone clastic, lagoonal-tidal flat and shallow marine mixed carbonate & clastic facies associations from bottom to top respectively. The depositional environment and the subsequent sediment accumulation for the Adigrat Sandstone Unit of southeastern Ethiopia could have a close relationship with the Late Triassic-Early Jurassic paleogeography of Eastern Africa and to the sea level fluctuation related to the Gondwana breakup. Based on datings on the Adigrat Sandstone in more northern locations of Ethiopia and by considering the age of the top part of the presently studied unit, the sea level fluctuation responsible could be that of Norian-Callovian time. Accordingly, southeast facing and gradually subsiding marginal environment is the probable depositional site for this unit.
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Paleoenvironmental, Reconstruction, Adigrat Sandstone, Formation Around, Dire Dawa Area, Southeast Ethiopia