Lithological mapping in the Eastern Desert of ... - Semantic Scholar

Journal of African Earth Sciences 44 (2006) 196–202 www.elsevier.com/locate/jafrearsci

Lithological mapping in the Eastern Desert of Egypt, the Barramiya area, using Landsat thematic mapper (TM) Sabreen Gad *, Timothy Kusky Department of Earth and Atmospheric Sciences, Saint Louis University, 3507 Laclede Ave., Saint Louis, MO 63103, United States Received 1 October 2004; received in revised form 29 June 2005; accepted 7 October 2005 Available online 3 January 2006

Abstract The Barramiya area, Central Eastern Desert, Egypt, comprises a variety of Late Proterozoic rocks including serpentinites, island-arc assemblage of metasediments and metavolcanics, and syntectonic granitoids. Lithological mapping in the Barramiya area is carried out by using Landsat thematic mapper (TM) image enhancement techniques, including RGB band ratio and supervised classification images. The significant spectral characteristics of serpentinites have been used previously by other authors to map these rocks using different band ratio images. In the current study, band ratio images (5/3, 5/1, 7/5), and (7/5, 5/4, 3/1) together with supervised classification techniques are used and proved effective for lithological mapping of the serpentinite rocks. Integrating previous knowledge lithological mapping studies of the study area and our visual interpretations of the different enhanced Landsat images, we mapped, in detail, the distribution of the Pan-African serpentinites in the Barramiya area. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Landsat TM; Band ratio images; Supervised classification; Egypt; Late Proterozoic

1. Introduction As part of the Pan-African Arabian-Nubian Shield, the Eastern Desert of Egypt is occupied by igneous and metamorphic rocks that were formed in the East African Orogen during the collision between East and West Gondwana and the closure of the Mozambique ocean 600 Ma (Stern, 1994; Kusky et al., 2003) and are well exposed all over the Red Sea Hills in Egypt, Sudan, Ethiopia, and Saudi Arabia. The Central Eastern Desert, Egypt, where the study area is located, is almost exclusively built up of ophiolitic me´lange and associated rocks, together with subordinate molasse-type sediments and late-tectonic volcanics and granitoid intrusives (El Ramly et al., 1993). The Barramiya area, Central Eastern Desert (Fig. 1),1 is

*

Corresponding author. Tel.: +1 314 677 3131; fax: +1 314 977 3117. E-mail address: [email protected] (S. Gad). 1 For interpretation of color in Figs. 1, 4–8, the reader is referred to the web version of this article. 1464-343X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2005.10.014

one of the historic places where the ancient Egyptians mined their gold, and is occupied mainly by serpentinites and the associated rock types (ophiolitic me´langes ‘‘metavolcanics and metasediments’’ and granitic rocks). Lithological mapping using Landsat TM band ratioing techniques has been used increasingly by several authors (e.g. Abrams et al., 1983; Kaufmann, 1988; Abdelsalam and Stern, 1999; Sultan et al., 1986; Kusky and Ramadan, 2002; Gad, 2002; Frei and Jutz, 1989; Sabins, 1999). In the current study we employed techniques, e.g. band ratio images, developed to map ophiolitic rocks using Landsat TM imagery in NE Africa (Sultan et al., 1986, 1987). Prior to interpretation, we applied image enhancement techniques (i.e. band ratios and supervised classifications) in order to map the serpentinite rocks in the Barramiya area, and the results are compared to the RGB band ratio images used by Sultan et al. (1986) and Sabins (1999). The distinctive mineral assemblages of serpentinites impact their spectral characteristics (Fig. 2) and enhance their discrimination using the Landsat TM band ratio images (Sultan et al., 1986; Kusky and Ramadan, 2002;

S. Gad, T. Kusky / Journal of African Earth Sciences 44 (2006) 196–202

197

Fig. 1. Landsat enhanced thematic mapper (ETM+) image (7, 4, 2) in RGB for southeastern Desert of Egypt showing the location of the study area.

100 90 80

Reflectance

70 60 Granite

50 40

Serpentinite

30

Amphibolite Andesit

20 10 0

500

1000

1500 Wavelength (Nanometer)

2000

2500

Fig. 2. Spectral reflectance of the serpentinites, granites and metavolcanics (Andesite and amphibolite) for the Eastern Desert, Egypt (Frei and Jutz, 1989).

198


Frei and Jutz, 1989). This situation has also been illustrated by the case study of mapping Oman Ophiolites (Abrams et al., 1988). 2. Geologic setting The terrane protoliths at central Eastern Desert of Egypt are represented by rootless ophiolite nappes, the largest of which crop out in the Gerf and the Barramiya areas (Johnson and Woldehaimonti, 2003). Serpentinites in the Eastern Desert, like those exposed in the Barramiya area, form a lithotectonic terrain dominated by rocks of oceanic affinity (Stern, 1981) and they form elongated fault-bounded sheets that extend for several kilometers trending NE–SW (Fig. 3). The outcrop pattern and the elongation of serpentinite bodies is controlled by regional folds which trend mainly northwest–southeast and northeast–southwest respectively (Ries et al., 1983; El Gaby et al., 1984; El Ramly et al., 1984). In Barramiya area a variety of Late Proterozoic rocks are exposed including serpentinites, island-arc assemblage of metasediments and metavolcanics, and syntectonic granitoids. The different rock units of the study area (El Ramly, 1972) are also shown in Fig. 3. In some cases, the distribution of linear belts of mafic rock units and serpentinised ultramafic sequences can be used to constrain the approximate locations of suture zones (Abdelsalam and Stern, 1996; Sultan et al., 1987).

3. Methods used Two important success factors are assessed when carrying out lithological mapping using remote sensing techniques: (1) the localization of increased concentration of minerals relative to the background and (2) the characterization of the mineral assemblages (Frei and Jutz, 1989). Image enhancement techniques are applied in the current study by using RGB band ratio images and supervised classifications. Band ratio images are used to suppress the topographic variation and the brightness difference related to grain size variation (Adam and Felic, 1967; Sultan et al., 1987). Band ratios have been used successfully in lithological mappings for the Arabian Nubian shield and for other areas worldwide. Band selection for the different ratio images used is based on the spectral signature of these rocks. When ratioing techniques are applied, all the reasonable grouping of minerals are best discriminated by a combination of ratios that include short wavelength bands (i.e. 3/1, 4/1 or 4/2), the ratio of the long wavelength bands (5/ 7) and a ratio of one band each from short and long wavelength band groups (e.g. 5/4 or 5/3) (Crippen, 1989). The distinctive spectral reflectance of serpentinites (Fig. 2) is caused by the abundance of antigorite, lizardite, clinopyroxenite and magnetite in the mineral composition. Comparison with the spectral reflectance of other rock units, e.g. granites and metavolcanics, is also given in Fig. 2. In the current study we applied band ratio image enhancement techniques, (e.g. Sultan et al., 1986; Sultan et al., 1987; Rothery, 1987; Frei and Jutz, 1989) besides the supervised classification technique (Sabins, 1997) to map the serpentinites in the Barramiya area, central Eastern Desert. 4. Results and discussion

Fig. 3. Geologic map of the Study area (El Ramly, 1972).

RGB band ratio images (5/3, 5/1, 7/5) and (7/5, 5/4, 3/1) (Figs. 4 and 5) are used in the current study and proved to be very effective in the lithological discrimination of the serpentinites and the associated rock units. In the RGB band ratio image (5/3, 5/1, 7/5, Fig. 4), the serpentinite appears in a dark brownish green color while the metavolcanics have a pinkish yellow color. Whereas in the band ratio image (7/5, 5/4, 3/1, Fig. 5) the serpentinites appear in dark brownish color while the associated metavolcanics rocks appear yellowish green. Comparison with the previously used RGB band ratio images (5/7, 5/1, 5/4 * 3/4) (Sultan et al., 1986) and (3/5, 3/1, 5/7) (Sabins, 1999) (Figs. 6 and 7 respectively) is also applied for the study area. In Sultan’s ratio the serpentinites appear bright in color (which is assigned to red) owing to the band 7 absorption by MgO– and OH– bearing minerals, while in Sabins ratio the serpentinites appears in violet color. Crippen (1989) listed a group of band ratio combinations that can be used for lithological mapping in arid areas. In the same sense, Sultan et al. (1986) proved that band ratio image (5/7, 5/1, 5/4 * 3/4) effectively map


Fig. 4. Landsat TM RGB band ratio color image (5/3, 5/1, 7/5) for the study area. Serp: serpentinites and MV: metavolcanics.

Fig. 5. Landsat TM RGB band ratio image (7/5, 5/4, 3/1) for the study area.

199

200


Fig. 6. Landsat TM RGB band ratio image (5/7, 5/1, 5/4 * 4/3, Sultan et al., 1986) for the study area. The serpentinite rocks (red color) are distinguished from metavolcanic rocks (pale green). For interpretation of color, the reader is referred to the web version of this figure.

Fig. 7. Landsat TM RGB band ratio image (3/5, 3/1, 5/7, Sabins, 1999).


201

Fig. 8. Supervised classified image for the study area.

the serpentinites in the Eastern Desert of Egypt. With referring to those studies (Sultan et al., 1986; Crippen, 1989), and from the results we obtained in the current study we add to the list the two RGB band ratio images (5/3, 5/1, 7/5) and (7/5, 5/4, 3/1) (Figs. 4 and 5, respectively) that successfully map the serpentinites and the associated rocks in the study area and can be used for lithological mapping in other arid areas. Beside the RGB band ratio images, Landsat TM supervised classified image is also used in the current study (Fig. 8). Five classes are assigned to five colors each corresponds to a certain rock unit; serpentinites (green), metavolcanics (red), granodiorites (dark blue), younger granites (yellow) and undifferentiated sediments (black). The results obtained from the band ratio and supervised classification images are also compared with the detailed geologic maps for the study area by El Ramly (1972) (Fig. 3) and the geologic map of Wadi Al Barramiyah (Geological Survey of Egypt, 1992). 5. Conclusions The enhancement techniques applied in the current study integrated with previous lithological mapping studies of the Barramiya area, allow the discrimination of serpen-

tinites and the associated lithologies. Sultan et al. (1986) and other authors applied several ratio images to map these rocks in the Eastern Desert of Egypt. Herein we use the same techniques, i.e. ratioing, but with different band ratio images in addition to supervised classification to show how different ratios can selectively highlight different features. It is advisable to integrate several discriminating band ratio images for a successful detailed lithological mapping of serpentinites or other rocks. Discrepancies among ratio maps may help recognizing strength/weakness of using certain ratio for certain mapping assignment. Calibration and evaluation of resolving power of any band ratio are to be achieved utilizing other proven ground truth data. Acknowledgement The authors would like to thank Dr. Michaela Frei, Prof. Dr. D. Klemm, Dr. M. Beyth and Dr. O. Crouvi for the useful and the constructive comments about the paper. References Abdelsalam, M.G., Stern, R.J., 1996. Sutures and shear zones in the Arabian Nubian Shields. Journal of African Earth Sciences 23 (3), 289–310.

202


Abdelsalam, M.G., Stern, R.J., 1999. Mineral exploration with satellite remote sensing imagery: examples from Neoproterozoic Arabian shield. Journal of African Earth Sciences 28, 4a. Abrams, M.J., Brown, D., Lepley, L., Sadowski, R., 1983. Remote sensing for porphyry copper deposits in southern Arizona. Economic Geology 78, 591–604. Abrams, M.J., Rothery, D.A., Pontual, A., 1988. Mapping in the Oman Ophiolite using enhanced Landsat Thematic Mapper images. Tectonophysics 151, 387–401. Adam, J.B., Felic, A.L., 1967. Spectral reflectance 0.4–2.0 micron of silicate rock powders. Journal of Geophysical Research 72, 5705–5715. Crippen, R.E., 1989. Selection of Landsat TM band and band-ratio combinations to maximize lithologic information in color composite displays. In: Proceedings of the Seventh Thematic Conference on Remote Sensing for Exploration Geology II, pp. 912–921. El Gaby, S., El Nady, O., Khudeir, A., 1984. Tectonic evolution of the basement complex in the Central Eastern Desert of Egypt. Geologische Rundschau 73, 1019–1036. El Ramly, M.F., 1972. A new geologic map of the Eastern and SouthWestern Deserts of Egypt. Scale 1:1000.000. Annals Geological Survey of Egypt 12, 1–18. El Ramly, M.F., Greiling, R.O., Rashwan, A.A., Rasmy, A.H., 1993. Geologic map of Wadi Hafifit area. Scale 1:100.000. Egyptian Geological Survey, 68. El Ramly, M.F., Greiling, R., Kro¨ner, A., Rashwan, A.A., 1984. On the tectonic evolution of the Wadi Hafafit area and environs, Eastern Desert of Egypt. Bulletin of King Abdelaziz University 6, 113–126. Frei, M., Jutz, S.L., 1989. Use of Thematic Mapper data for the detection of gold bearing formations in the eastern Desert of Egypt. In: Proceedings of the 7th thematic conference on remote sensing for ore exploration geology II, pp. 1157–1172. Gad, S., 2002. Exploration for mineralized granites in Central Eastern Desert, Egypt, M.Sc. Thesis, Faculty of Science, Aswan, South Valley University, Egypt, 118pp. Geological Survey of Egypt, 1992. Geological map of Wadi Al Barramiyah Quadrangle, Egypt, Scale 1:250.000. Johnson, P., Woldehaimonti, B., 2003. Development of the Arabian shield: perspective on accretion and deformation in the northern East

African Orogen and the assembly of Gondwana. In: Yoshida, M., Windely, B.F., Dasgupta, S. (Eds.), Proterozoic East Gondwana, supercontinent assembly and breakup, 206. Geological Soc., London, pp. 289–325 (Special publication). Kaufmann, H., 1988. Mineral exploration along the Aqaba—Levent structure by use of Landsat TM data; concepts, processing, and results. International Journal of Remote Sensing 9, 1639–1658. Kusky, T.M., Ramadan, T.M, 2002. Structural controls in the Neoproterozoic Allaqi suture: an integrated field, Landsat TM, and radar C/X SIR SAR images. Journal of African Earth Sciences 35, 107–121. Kusky, T.M., Abdelsalam, M.G., Tucker, R.D., Stern, R.J., 2003. Evolution of the East African and related Orogens, and the assembly of Gondwana. Precambrian Research 123 (2–4), 81–337. Ries, A.C., Shackleton, R.M., Graham, R.H., Fitches, W.R., 1983. PanAfrican structures, ophiolites and me´langes in the Eastern Desert of Egypt: a traverse at 26°N. Geological Society of London journal 140, 75–95. Rothery, D.A., 1987. Improved discrimination of rock units using Landsat Thematic mapper imagery of the Oman Ophiolite. Journal of the Geological Society, London 144, 587–597. Sabins, F., 1997. Remote Sensing: Principles and Interpretation, third ed., 494pp. Sabins, F.F., 1999. Remote sensing for mineral exploration. Ore Geology Reviews 14, 157–183. Stern, R.J., 1981. Petrogenesis and tectonic setting of Late Precambrian ensimatic volcanic rocks, Eastern Desert of Egypt. Precambrian Research 16, 195–230. Stern, R.J., 1994. Arc Assembly and continental collision in the Neoproterozoic East African Orogen: implications for consolidation of Gondwanaland. Annual Review Earth Planetary Science 22, 319– 351. Sultan, M., Arvidson, R.E., Sturchio, N.C., 1986. Mapping of serpentinites in the Eastern Desert of Egypt using Landsat Thematic Mapper data. Geology 14, 995–999. Sultan, M., Arvidson, R.E., Sturchio, N.C., Guinnes, E.A., 1987. Lithologic mapping in arid regions with Landsat TM data: Meatiq dome, Egypt. Geological Society of America Bulletin 99, 748– 762.