Facies and Microfossil Analysis of Jurassic Carbonate Unit In Lemi Area (Jema Section) Northern Shewa; Central Ethiopia.
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Date
2018-05-30
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Addis Ababa University
Abstract
A thick Jurassic carbonate succession of about 157m thickness was studied in Jema section, the Blue Nile basin; of the Northwestern Ethiopian Plateau. This study was mainly aimed on exploring the stratigraphy, paleontology, facies analysis, biostratigraphy and determination of the depositional environments of carbonate unit of the Jema section. Based on the petrographic study of 18 representative carbonate rock samples and 8 marl samples; the compositional and textural analysis of rocks were studied and grouped into six (6) microfacies associations and about 22 foraminiferal and 7 ostracodals species are identified; which characterizes for intertidal zone, shoal restricted marine and shelf facies to open circulation environments. The presences of micritic, bioclastic wackestone and peloidal grainstone microfacies in the Jema section indicate shoal to shelf facies deposits. The Pelsparite and algal stromatolite mudstone are indicators of carbonate intertidal to shoal deposits of shallow marine setting.
Based on the presence of index fossils such as Kurnubia cf. morrisi, Alveosepta jaccardi and Kurnubia palastiniensis with their associations of Conicokurnubia orbitoliniformis, Valvulina lugeoni, and Nautiloculina oolithica; the age of carbonate succession was determined to an age from Callovian to Oxfordian. Paleontological study of the section in Abay basin shows the carbonate unit is rich in micro fossils and less in macro fossils that supply additional data for stratigraphic and Paleoenvironmental interpretations; particularly of foraminifers and ostracods explored. Among the macrofossils few groups of bivalves, gastropods brachiopods and echinoids are also identified. The carbonate deposits of the logged section are correlated with chronostratigrphically equivalent and lithostratigrphically similar basins in North western and Southeastern plateau of the country regionally and with the previously logged sections in the Bilu Nile Basin locally.
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Depositional Environment, Microfacies, Facies, Microfossils, Paleoenvironment, Biostratigraphy, Carbonate Unit