What is Controlling the Gorge of the Nile in Ethiopia? An Integrated Approach from ASTER, RADARSAT, DEMs, and Field Studies

D. Nahid Sultana
The University of Texas at Dallas, Richardson, TX

Digital Elevation Models (DEMs) extracted from Shuttle Radar Topography Mission (SRTM) with 90 m x-y resolution and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) with 15 m x-y resolution are used in conjunction with ASTER and RADARSAT images and field studies to understand the geological controls on the 1.6 km deep Gorge of the Nile in Ethiopia that was formed by the Blue Nile on the NW Ethiopian Plateau. The Blue Nile forms a near-circular loop around Tertiary to Quaternary Age shield volcanoes and exposes a geological section ranging from Precambrian crystalline rocks to Mesozoic sedimentary rocks and topped with Tertiary-Quaternary volcanic rocks. Our work shows that: (1) 3D perspective views generated by image draping are effective in mapping sub-horizontal lithological units and lithologically-defined structures. (2) The side-looking geometry of the Standard Beam RADARSAT data effectively maps morphologically-defined structures due to radar shadow-illumination effect. (3) Color Normalization Transformation (CNT) fusion technique enhances mapping ability as the fused image preserves spectral information of ASTER and incorporates morphology from RADARSAT. (4) ASTER band-ratio (4/5 – 3/1 – 3/4) image which reduces the topographic effect between multi-spectral bands, better resolves lithological units and lithologically-defined structures. (5) SRTM DEMs are effective for characterizing 3D spatial relationships between the river's course and regional geomorphological features. The possible geologic controls of the Gorge of the Nile might be related to base-level adjustment due to regional uplift, obstruction and diversion due to bedrock structures, and differential incision due to varying lithology.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005