Planation surfaces and the long therm geomorphological evolution of Ethiopia

Four major planation surfaces (PS) characterize the Ethiopian geology and landscape. They were modelled near or at sea level before the Ordovician (PS1), before the Late Triassic (PS2), before the Cenomanian (PS3), and before the Oligocene (PS4). These are unconformities in the sedimentary sequence recognizable across the entire country and the surrounding regions that due to uplift can be locally exhumed to generate wide steps in the landscape. At the top of the sequence,thatalsocorrespondstothehigherpartsofthehighlands,theflatdepositionalsurface of the continental flood basalts (CFB) is preserved over large areas. However, a series of deep large palaeovalleys dissecting the CFB have been recognized on the water divides of the Afar, the Main Ethiopian Rift (MER), the Southern Ethiopian Rift (SER), and the Somalian and Sudan lowlands that are the main geomorphological features of the country. The palaeovalleys testify toaradialpatternthat originatedfromthedomecreatedbytheemplacementoftheCFB. They are easily recognizable to the south of Dire Dawa and in the northern part of the country. They were modelled over the pre-volcanic bedrock but in most of the highlands were buried under later volcanic products. The deep erosion of the dome and the later volcanic filling can explain the results of the previous thermochronological investigation. Unfortunately, the thickness of the CFB could have led to the reset of the apatite fission tracks (AFT) during the Oligocene and it is difficult to separate the effects of deep erosion from that of uplift, also considering that they can be closely related. In any case, a progressive incision of the thicker part of the dome is documented soon after the CFB deposition in the Blue Nile Gorge and during the Miocene and the Plio-Pleistocene in many other parts of the country.