The initiation of the Blue Nile River and sediment sources for the Nile Delta: New insights from the Northwestern Highlands of Ethiopia

The Nile River feeds a delta that hosts a major producing hydrocarbon system in the Eastern Mediterranean. The river has three main tributaries: the White Nile, the Blue Nile, and the Atbara-Tekeze rivers. The Blue Nile (53-61%) and the Atbara-Tekeze (30-42%) rivers dominate the sediment supplied to the main Nile River in Egypt. This is important as these two tributaries are sourced in the Northwestern Highlands where they drain the Oligocene-Miocene Ethiopian Flood Basalt Province (EFBP). Therefore, the generation and arrival of volcanic debris to the delta and their impact on reservoir and/or sealing properties is important for hydrocarbon exploration. Understanding the incision history of the Northwestern Highlands is crucial in unravelling the potential contribution of volcanic debris to the Nile Delta. Although the northern part of the Ethiopian Flood Basalt Province, sourced by the Atbara-Tekeze River, has been isotopically dated (e.g. 31-29 Ma; Early Oligocene in the Limalimo area), the age of the province incised by the Blue Nile River to the south is less certain due to the paucity of stratigraphically constrained ages. This study addresses this uncertainty by combining field logging and isotopically dating seven basalt lava flows at key stratigraphic intervals exposed in the Jemma River, a major tributary of the Blue Nile River in Ethiopia. These new ages have been used to calculate incision rates that radically changes our understanding of the development of the Blue Nile River during the last 30 Myr.

The uplift and subsequent incision by the Blue Nile River and its tributaries has exposed a >1.5 km thick Mesozoic sedimentary succession belonging to the Blue Nile Basin. Lithostratigraphic logs collected across the basin detail the unconformable transition from marine limestones and fluvial sandstones to the subaerial lava flows of the Ethiopian Flood Basalt Province. Field observations suggest that valleys were incised into the Mesozoic succession that were subsequently buried by the Flood Basalts. Newly acquired Ar-Ar ages for the Ethiopian Flood Basalt Province in the Jemma River shows that volcanism commenced in the Late Oligocene, lasted for at least 4 Myr and finished in the Early Miocene. The eruption of the Ethiopian Flood Basalt Province started at least 3 Myr after the Flood Basalts were emplaced in the Limalimo area (31-29 Ma; Early Oligocene) and therefore, suggests that the Ethiopian Flood Basalt Province is not coeval between the two areas. This is important as previous incision rate models have commonly used 29 Ma for the initiation of the Blue Nile River following the emplacement of the Ethiopian Flood Basalt Province, but the new ages would imply this is incorrect by at least 7 Myr. The presence of the sub-volcanic valleys, however, does imply that uplift had begun prior to the emplacement of the lava flows. The incision of the underlying siliciclastic sandstones is also supported by a clean sandstone found interbedded between some of the earliest lava flows within the Blue Nile Basin.

In addition to the Ethiopian Flood Basalt Province, two lava flow sequences are found filling the lower reaches of the Jemma River valley. Two basalt lava flow flows have been dated from each of these sequences by Ar-Ar geochronology. The lava flows are Quaternary in age and the youngest of the two lava flows is found downriver, suggesting the Jemma River has been inundated at least twice since the emplacement of the Ethiopian Flood Basalt Province. Using the new Ar-Ar data, incision rates for the Jemma River have been estimated between the volcanic sequences and the present-day river level. Modelling suggests incision rates in the Jemma River have increased significantly during the last ~10 Myr. This coincides with the onset of rifting in the northern and central sections of the Main Ethiopian Rift and therefore, has important regional implications for the uplift history of Ethiopia. For example, it would have taken between 9 and 14 Myr for the Jemma River to incise through the 375-680 m thick Oligocene-Miocene Ethiopian Flood Basalt Province and into the underlying Mesozoic sedimentary strata. Consequently, the Jemma River would have been dominated by volcanic debris from the Early to Late Miocene when it likely became increasingly diluted with non-volcanic siliciclastic and carbonate debris. This compositional change could be reflected in fluvial deposits downriver and potentially, further afield, in the Nile Delta. The periodic damming of river valleys by lava flows could also influence the compositions of fluvial deposits downriver, which in turn could affect reservoir properties. Important aspects for hydrocarbon exploration, but also CO₂ sequestration.