SEDIMENTOLOGIC AND STRUCTURAL FEATURES OF AN EXHUMED INTRACONTINENTAL SAG BASIN; THE MEKELLE SEDIMENTARY BASIN IN ETHIOPIA

The Mekelle Sedimentary Basin in Northern Ethiopia, contains ~2 km thick mixed carbonate-siliciclastic sediments thought to be deposited in an interior rift developed in response to continental fragmentation and dispersal during late Paleozoic-Mesozoic of Gondwana. This study summarizes recent findings of a previously unrecognized structural and sedimentologic features of the Mekelle Sedimentary Basin. We used an integrated fieldwork, remote sensing and geophysical data to investigate structural and sedimentologic evolution of the Mekelle Sedimentary Basin. Our results reveal, unlike what is expected for most rift basins, that the Mekelle Basin is characterized by the following: (1) circular to elliptical shape in map view: both Landsat Thematic Mapper (TM) images and geologic maps of the basin show the circular appearance of the basin as opposed to the more elongate geometry characteristic of rift basin; (2) saucer shaped geometry in cross section view; (3) absence of a significant structural control of sedimentation and, unlike rift basins, the major faults in the area do not bound the basin, cross cut the sedimentary rocks, and are post-depositional; (4) terrestrial and shallow marine sedimentary strata with simple layer-cake stratigraphy, bounded by coral and stromatoporoid reefs, and (5) concentric gravity minima at the center of the basin. These are characteristic features of an IntraCONtinental Sag (ICONS) basins, groups of basins that show no to very little extensional tectonic features and generally form above a stable lithosphere, away from plate boundaries. Accordingly, we interpret the Mekelle Sedimentary Basin as an ICONS. Due to its location at the western flank of the Afar Depression, the Mekelle ICONS is exhumed. Other ICONS have remained buried and most of our knowledge about their sedimentologic and structural evolution comes from geophysical and numerical modelling. We believe, the Mekelle ICONS present an unparalleled opportunity in which sedimentary formations and geological structures associated with an ICONS are completely exposed on the surface allowing for the construction of an accurate three-dimensional (3D) of its geometry.