The Antalo Group – Amba Aradam Fm. transition in Central Ethiopia: a Late Jurassic-Cretaceous regression cycle within the Gondwana break up

The Paleozoic-Mesozoic sedimentary cover of Ethiopia rests unconformably onto a Precambrian basement. It crops out in a very wide area across the whole Horn of Africa, southwestern Arabia peninsula, southeastern Africa and western India. The succession is thought to yield the records of crucial climatic and geodynamic events, such as an Ordovician major glaciation connected with the late Paleozoic global icehouse settings, or the Mesozoic Gondwana break up. The stratigraphic relationships among depositional units of the Paleozoic-Mesozoic sedimentary cover of Ethiopia is rather debated, and particular attention is dedicated to the occurrence and stratigraphic position of the main unconformities. In a general view, the whole succession is composed of (from bottom up): Enticho Sandstones and Edaga Arbi Glacials formations (Ordovician-early Silurian), Adigrat Sandstones (?Paleozoic-Early Jurassic), Antalo Limestones and Agula Shales (Jurassic), and Amba Aradam Fm. (Early Cretaceous). The present study shows preliminary results of an ongoing field research project, focused on the boundary between the Antalo Limestones and the Amba Aradam Fm. cropping out in the Muger Valley, northwest of Addis Ababa. The Antalo Limestones s.s. pass gradually upwards to a ten metres-thick succession of gypsum-bearing mudstones that in turn fade upwards into mudstones and limestones, scattered sandstones and polychromous shales (Agula Shales), capped by a thick sandstone unit, which can be correlated with the Amba Aradam Fm. The coarsening-upwards sandstone unit is covered by very thick (some hundred metres) Oligocene trap basalts. Sandstones are subarkose to arkose in composition, whereas to the north, the formation consists mainly of conglomerates and breccias. The sandstone unit, about 50 metres thick, is mainly organized into large-scale, gently inclined beds, commonly erosively-based, cross-stratified or massive, with interlaid bioturbated red shales. In terms of paleoenvironments, the Antalo Limestones have been referred to a shallow marine setting, whereas the overlying Agula Shales could be referred to an evaporitic setting typical of lagoonal to coastal environments, though episodes of marine ingression cannot be excluded. The polychromous shales were deposited on alluvial or inner coastal plain, passing to a fluvial dominated environment as witnessed by the sandstones of the Amba Aradam Fm. For the Muger Valley succession, our preliminary results point out to a regressive trend with gradual transition from a shallow marine/lagoonal realm (Antalo Limestones and Agula Shales) to a continental environment (Amba Aradam Fm.). This contrasts with the known stratigraphic framework for northern Ethiopia, where marked surfaces of angular unconformity and Cretaceous planation have been reported, associated to a sealed normal fault system. These considerations suggest that the Antalo Group–Amba Aradam Fm. transition could be expressed by a composite surface with different stratigraphic significances: mostly unconformable in northern Ethiopia and likely conformable in central-western Ethiopia. If this hypothesis will be confirmed, the Jurassic - Cretaceous transition in the Horn of Africa area must be accounted for by the development of progressive unconformities. Therefore, the derived paleogeographic scenario would indicate a non-erosional regressive pattern as the most convincing for the southern basinward areas of Ethiopia.