NORMAL FAULTS-RELATED DEFORMATION BANDS IN THE PALEOZOIC-MESOZOIC SANDSTONES OF THE MEKELE SEDIMENTARY BASIN IN NORTHERN ETHIOPIA

We report, for the first time, the presence of extensive deformation bands associated with normal faults in the Mekele Sedimentary Basin in northern Ethiopia. This Paleozoic-Mesozoic, ~100 km wide, semi-circular basin has been interpreted as an intra-continental sag basin that was exhumed during the Cenozoic time because of its location on the western uplifted flank of the Afar Depression. The basin is rimmed by the Paleozoic Edaga Arbi Glacials and the Enticho sandstone and the Mesozoic Adigrat sandstone, both are separated from the Precambrian crystalline rocks of the Arabian-Nubian Shield by an angular unconformity. The center of the basin is dominated by the Mesozoic Antalo limestone and the Agula shale. The basin is dissected by a number of WNW-tending normal fault zones as evidenced by normal-slip displacement of marker horizons combined with the presence of numerous fault planes that contain down-dip slickenlines. Our examination of the deformation bands associated with the northern fault zone, which is referred to as the Wukro fault belt, shows the following: (1) The deformation bands seems to occur within the entire length of the fault belt, especially where the Enticho sandstone is faults against the Precambrian crystalline rocks and the Antalo limestone is faults against the Adigrat sandstone. (2) Almost all of the deformation bands were developed within the medium-grained, granular Enticho and Adigrat sandstones. However, less-developed deformation bands are observed in few places within the Antalo limestone where an oolitic texture is present. (3) The deformation bands are characterized by reduction in grain size as well as reduction in porosity. This often makes the deformation bands more resistant to weathering; hence they are preserved as sharp mesoscopic ridges. (4) The change in grain size and porosity between the deformation bands and the undeformed sandstones is very sharp and it does not show any gradation. (5) Most of the deformation bands are less than one centimeter in width and are a few centimeters long and these form networks that do not show any preferred orientation relative to the trends of the slip planes associated with the fault belt. However, in places some of the individual deformation bands are aligned within up to ~30 cm wide zones that extend for several meters parallel to the slip planes.