SEDIMENTOLOGY, DEPOSITIONAL ENVIRONMENTS AND BASIN EVOLUTION OF COAL AND OIL SHALE-BEARING SEDIMENTS IN THE DELBI-MOYE BASIN, SOUTHWESTERN ETHIOPIA

Abstract

The Inter-Trappean coal and oil shale-bearing sedimentation in the Delbi-Moye Basin took place in tectonically controlled grabens and half-grabens formed by extensional fault systems and accompanied by passive subsidence. The sedimentation history of the basin is related to the tectonic events that affected East Africa. The Delbi-Moye Basin consists of two sedimentary units (upper and lower sedimentary units). The upper and lower sedimentary units attain maximum thicknesses of 40 and 278 meters respectively. The upper sedimentary unit is characterized by one sedimentary cycle. The lower sedimentary unit is characterized by three sedimentary cycles, and defined by five sedimentary facies (coal-bearing sediments, upper oil shale facies, upper mudstone facies, lower oil shale facies and lower mudstone facies). The coarser clastic sediments were deposited at the basin margins, whereas the concentration of fine-grained clastic sediments and organogenic deposits (oil shale and sapropelic coal) was high at the depocenter of the basin. The sedimentation is defined by alluvial fan, fluviatile, off shore lacustrine, shoreline lacustrine and paludal and/or swampy depositonal environments.