Volcanic Stratigraphy and Petrography of the Northwestern Ethiopian Flood Volcanics in Upper Kesem River Section, Central Ethiopia
DOI:
https://doi.org/10.69660/jmpt.v2i2.128Keywords:
Ethiopian volcanic province, lithostratigraphy, Oligo-Miocene, petrography, Upper Kesem riverAbstract
AbstractA petro-stratigraphic study of the volcanic sequence exposed in the Upper Kesem river section is presented. The study area is located in central Ethiopia, North Shewa administrative zone, between Koremash and Shola Gebeya localities, about 80 kilometers north-east of Addis Ababa. The study examined a nearly continuous, ~1080-m thick sequence of stratigraphically preserved basalt flows and intervening silicic pyroclastic deposits. The lower contact of the volcanic sequence is not exposed in this river section. Following a comprehensive field investigation, lithostratigraphic logs for various sections are presented. The sequence is dominated by flood basalts (~78%) with significant proportions of felsic lavas and pyroclastic deposits (~22%) towards the top part of the succession. The very top of the volcanic succession comprises of shield-forming basaltic lava flows and scoraceous deposits. Paleosols and clastic sediments are commonly found as thin layers interbedded within the volcanic sequence, indicating time gaps in eruptive activity. This sequence includes the main phase of flood basalt volcanism and the transition to the termination of flood basalt activity From the constructed composite volcanic stratigraphy , five lithostratigraphic units have been identified, namely 1) Lower Basalt, 2) Lower Rhyolitic Ignimbrite, 3) Upper Basalt, 4) Upper Rhyolitic Ignimbrite, and 5) Shield Basalt. These lithostratigraphic units correlate very well with sequences described in other sections of the Northwestern Ethiopian plateau. A total of 25 representative samples from these stratigraphic units have been studied petrographically. The petrographic studies reveal that only basalts and felsic volcanics have been identified; rocks of intermediate compositions have not been found, indicating a bi-modal petrological distribution for the volcanic sequence. Overall, the basalts exhibit a wide range in their modal mineralogy and phenocryst assemblage. This variation suggests a polybaric plumbing system for the basaltic lavas with several magma chambers developed at various depths within the crust and with each chamber undergoing independent recharge and fractionation processes. Shield volcanism likely represents a decrease in magmatic flux over time. The large-volume silicic pyroclastic deposits further reveal that flood basalt volcanism was accompanied by the development of large and shallow magma chambers, in which basaltic magmas were stored and underwent extensive fractionation and evolution.
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