Geomorphology of the Adwa District

Abstract

This chapter describes the geologic and geomorphic characteristics of the Adwa district and provides a reconstruction of the major phases of the long-term landscape evolution from the Late Proterozoic to the Quaternary. Two landscape features of the Adwa district exemplify the control of tectonic uplift and erosion, often in a feedback loop: the plateau landscape and river incision. The plateau landscape developed upon metamorphic, sedimentary and volcanic materials, with major contacts marked by planation surfaces correlative with major regional- and sometimes continental-scale unconformities. In the southern Adwa district, the most prominent landscape feature is a regionally extensive erosion surface bevelled across the Precambrian basement and later buried by Palaeozoic and Mesozoic sedimentary rocks. In the northern district, the flat-top horizontal structural plateaux are formed by the Mid-Cenozoic upper basaltic Trap sequence, intruded by trachyte and phonolite plugs originated during the Miocene and Pliocene volcanic activity, and a lower plateau developed over the laterised Adigrat Sandstone Formation. The post-Pliocene landscape evolution has led to exhumation of the former erosion surfaces and dissection of the drainage network. These stream channels have cut large amphitheatre headwaters and flow southwards to join the regional base level of the Weri River, a contributor to the Nile basin. The Quaternary sedimentary record in the Adwa area is composed by consolidated carbonate Late Pleistocene rocks (travertines), unconsolidated Pleistocene/Holocene alluvial fan deposits, Holocene fluvial deposits (river terraces and valley fill deposits) and historical to present-day colluvial deposits.