Petrostratigraphic framework of a bimodal volcanic sequence at Degolo and Meragna sections, Central sector of the Ethiopian volcanic province

Abstract

Detailed stratigraphic sections have been generated in the central sector of Ethiopian volcanicprovince; approximately 310 km north of Addis Ababa, near Degolo town in the South Wollo zone andMeragna town in the North Shewa zone. Complete volcanic stratigraphic sequences presented from thebasal flow contacts with the Mesozoic sandstone unit (Debre Libanos sandstone). The compositestratigraphy of Degolo-Meragna area exposes 1100m thick bimodal volcanic sequence. The bimodalsequences at Degolo and Meragna sections show variations in flow thickness, phenocryst assemblages, anddegree of welding in ignimbrite units, lithic fragment sizes in ignimbrites. The basalt flows in the Degolosection are 908 meters thick divided into Lower Basalt, Middle Basalt, and Upper Basalt units.Petrographically, these basalts exhibit a range of textures from aphyric to porphyritic; classified withphenocryst assemblages as Plag-Cpx phyric, Cpx-phyric, Cpx-Opx phyric and Cpx-Plag phyric basalts.The basalt flows at Meragna section are 880 meters divided into Lower Basalt and Upper Basalt unitswhose textures range from aphyric-microcrystalline to porphyritic. Petrographically basalts classified asCpx-phyric-ophitic to subophitic, Plag-Cpx-glomerophyric, Cpx-phyric-subporphyritic-microcrystalline,and aphyric-microcrystalline. The individual ignimbrite flows have thicknesses that vary from 18 to 90meters; and the ignimbrites have total thickness of 226 meter at Degolo and 254 meters at Meragna section.The variations in magma flux, depth of stalled magma and magma recharge constrained from diversity inbasalt textures, mineral modal assemblages, mineral abundances, lava flow morphologies, and volcanichiatus markers (paleosols, sediments, weathering horizons). The recurrence and cyclicity of basalticrhyolitic suites and mineral modal compositions logged through stratigraphic sequences are indicatives ofcomplex magmatic process. The presence of inter-lava sediments (carbonates and siliceoussediment/chert), paleosols, and weathering horizons (lateritic horizons) indicate significant volcanicquiescence and pulsed bimodal volcanic eruptions in Late Oligocene-early Miocene periods