OUTCROP ANALOG FOR INTRACONTINENTAL SAGS (ICONS): A CASE OF THE MEKELE BASIN IN NORTHERN ETHIOPIA

The Mekele Sedimentary Basin (MSB) in Ethiopia, is a Paleozoic-Mesozoic IntraCONtinental Sag (ICONS) that is completely exhumed due to its location along the western escarpment of the Afar Depression. ICONS represent a group of sedimentary basins formed within the stable part a lithosphere, though the underlying subsidence mechanisms are not entirely understood. Since most ICONS have remained buried following their formation, testing the processes through which ICONS are formed will require expensive subsurface data. The use of outcrop analogs will present a cheap and valuable data for deciphering their development. This work presents an integrated outcrop based study of the MSB using field and remote sensing data.

The MSB represents a case in which the 3D stratigraphic and structural architecture of an ICONS is exposed, providing a detailed insight into their subsurface geometry. It contains ~2 km thick mixed carbonate-siliciclastic sediments deposited in continental-shallow marine environments, overlying a Neoproterozoic basement of metamorphosed sedimentary and volcanic rocks, and granitoids. The basement belongs to the Arabian Nubian Shield, an accretionary crust formed due to consolidation of Gondwana. Basin formation, therefore, follows a scenario in which, accommodation space is formed due to thickening and cooling of the lithosphere.

In this study, the sedimentary fill of the Mekele is subdivided into three phases (Pres-sag, Sag and Post-sag). The Pre-sag phase represents the time just after the final accretion where processes associated with stabilization of the Neoproterozoic basement dominate through erosion and peneplanation. Sag phase include all sedimentary formations from Ordovician to Late Jurassic. These includes ~350 m thick Paleozoic, a predominantly glacial deposits (Enticho Sandstone and Edaga Arbi Glacial). Paleozoic rocks are overlain by ~650 m thick Triassic-Jurassic Adigrat Sandstone deposited in continental and marginal marine setting. This is followed by marine transgression during Middle-Late Jurassic culminated in the deposition of Antalo Limestone (~700 m thick). The Sag phase ended following deposition of the Agula Shale (~250 m). Post-sag phase represents the deposition of the middle Cretaceous Amba Aradom Formation and Cenozoic sparsely distributed basalts.