Lithologic mapping in arid regions with Landsat thematic mapper data: Meatiq dome, Egypt

Abstract

Digital Landsat thematic mapper (TM) data were evaluated for lithologic mapping capabilities over the Meatiq dome area in the hyper-arid Eastern Desert of Egypt. Bi-directional spectral reflectance data (0.4–2.5 µm) for powders of the major rock types exposed in the dome and published spectral reflectance data were used as guides in selecting TM band reflectance ratios that maximize discrimination of individual rock types on the basis of their respective mineralogies. Comparison of TM data with field and petrographic observations shows (1) increasing amounts of magnetite and other opaque minerals, with low, flat spectral reflectances, decrease the ratio of TM band 5 (1.55 to 1.75) to band 1 (0.45 to 0.52 µm); (2) increasing amounts of hydroxyl-bearing minerals, with hydroxyl ion vibrational absorptions in TM band-7 wavelength region (2.08 to 2.35 µm), increase the ratio of TM band 5 to band 7; (3) increasing amounts of Fe-bearing aluminosilicates that absorb in the band-4 wavelength region (0.76 to 0.9 µm) increase the product of the following two TM ratios: band 5 to band 4 and band 3 (0.63 to 0.69 µm) to band 4; and (4) thin (≤5 µm), desert varnish that covers many outcrops modulates, but does not obscure, the spectral reflectance signatures of the Meatiq rocks. The varnish consists of amorphous to poorly crystalline dioctahedral smectite, iron oxides, and/or oxyhydroxides. Serpentinite, mafic mylonite, massive amphibolite, quartzofeldspathic mylonite, biotite schist, and quartz phyllonite were mapped on the basis of their unique values in one or more of the three ratio images, whereas coarse- and fine-grained granites, granite gneiss, and tonalite, with similar mineralogies and TM band ratios, were mapped as a group. Finer subdivisions were made where field traverses provided local verification. Results demonstrate that appropriate processing and presentation of Landsat TM data can significantly augment field observations for lithologic mapping of large areas in arid regions.