Petrostratigraphic Study of Volcanic Rocks in Sela Dingay Area North Shewa Central Ethiopia

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Addis Ababa University


A petrostratigraphic study of volcanic rocks in the parts of NW Ethiopian Plateau is presented in this paper. The study area is located in the North Shewa district, specifically around Sela Dingay, in central Ethiopia, about 200 kilometers north of Addis Ababa. The studied stratigraphic successions are cut by Mofar River starting from its tributaries in the highly elevated Termaber section (~3315m) downward into the lowly elevated Gawna section (1540m) in a way from Sela Dingay to Sasit in the central part of the study area. The main objective of the study was to provide a complete lithostratigraphy of the research area based on a thorough field investigation (mapping and description), stratigraphic sampling, and petrographic (thin section) analysis of representative samples. Furthermore, the study attempts to integrate these data to relate them with regional studies on geochemical, geochronological, and magma emplacement behavior data about the whole volcanic provinces of Ethiopia. From the composite stratigraphic section constructed in this study, five volcanic lithostratigraphic units (Lower Basalt, Sela Dingay Rhyolitic Lava flow, Upper Basalt, Sela Dingay Rhyolite-Ignimbrite, and Termaber Shield Basalt) which are a part of Cenozoic continental flood basalt, are identified. The volcanic rocks are underlain by Mesozoic (Cretaceous) Sandstone formation. The basaltic lava flows (fissural flows) are the dominant product with interlayered rhyolitic-pyroclastic deposits with the felsic products dominating the top part and the most top part of the succession is represented by central type shield basalt. These stratigraphic units are separated by a considerable time gap marked by alteration/erosional surface, paleosol, or sediments, which are all related to a decrease in magma flux in the system. Exposed flow fields display three-part features: flow top, flow core, and flow bottom structures. Flow tops are often made up of glassy to very fine-grained vesicular basalt, or scoriaceous to vesicular basalt fragments (brecciated flow top) that lie above a zone of non-fragmented, infrequently vesicular to columnar jointed basaltic core. The flow bottom is characterized by a limited zone of sparsely vesicular, glassy to very fine-grained basalt. A tabular to compound braided flow facies is confirmed by field observations of lava flow fields. A total of 24 petrographic samples from these stratigraphic units are studied by using a petrographic microscope. The majority of the samples are from Basaltic units and have a variety of textures ranging from aphyric to mildly phyric to porphyritic. Plagioclase and Olivine are the major phenocryst phase with a proportion of pyroxene and Fe-Ti-oxides. In some samples, the groundmass is made up of microlites of the same phase with a glassy texture. Cumulophyric, ophitic, and poikilitic to sieve textured phenocryst phases are embedded in a felty or trachytic groundmass in the porphyritic samples. In most basaltic samples, these features combine to generate porphyritic, holocrystalline to hypocrystalline, and Vitrophyric textures. Alkali feldspar, quartz, plagioclase, and lithic fragments with glassy groundmass predominate in rhyolitic and ignimbrite rock samples. The petrographic data from analyzed samples characterizes a part of HT1 flood basalt in geochemically zoned Ethiopian flood basalt. The predominance of plagioclase mineral as a phenocryst and groundmass phase and the occurrence of significant felsic-pyroclastic products coincides with the regional studies relating these character for the late-stage (termination phase) of flood basalt volcanism.



Petrostratigraphic Study, Volcanic Rocks, Sela Dingay Area, North Shewa, Central Ethiopia