Mineralogical characterization and transport pathways of dune sand using Landsat TM data, Wahiba Sand Sea, Sultanate of Oman
Introduction
The purpose of this study is to characterize the spatial patterns of the mineralogies of dune sand within the Wahiba Sand Sea and to use those patterns to suggest the source and the transport pathways of the sand. Large-scale characterization of sand mineralogy in a dune field is spatially restricted by the limitations of field-based, point-specific sampling methods. Whereas in situ field sampling is necessary for detailed analysis of sand composition, logistical constraints often limit sampling of large areas to a relatively coarse grid. Our study limits this problem by using Landsat Thematic Mapper (TM) data in conjunction with field sampling. A classification of these data were used to determine the distribution of sand mineralogy and assess the potential sources of the sediment in the Wahiba Sand Sea.
The Wahiba Sand Sea receives sediment from multiple sources, resulting in several mineralogical populations. A number of possible local sediment sources exist, including (a) ultramafic minerals from ophiolite sequences, (b) marine sedimentary sandstone, limestone, and chert, (c) coastal sediments and (d) reworked aeolianite sediments. Despite the complex sources, three basic mineral groups (quartz, carbonate, and mafic) make up most of the sediment in the sand sea. Such distinct mineral groups permit discrimination of sand populations with Landsat TM data (e.g., White et al., 1997, Blount and Lancaster, 1990). Mineralogic data from in situ sample sites linked to the specific representative region in a TM scene, create a method for determining the spectral responses of individual, mineralogical, sand populations. The determination of spectral signatures for those regions of sand that are mineralogically similar allows the characterization of mineralogical regions to be extended beyond the site of field sample where the composition of the sediment is known.
Section snippets
Study area
The Wahiba Sand Sea is located on the eastern coast of the Sultanate of Oman (Fig. 1) and covers over 12,000 km2 (Goudie et al., 1987). The southeastern edge of the Wahiba Sand Sea is bordered by the coastline and intermittent sabkhas. To the north, the sand sea is truncated by the eastern Hajar Mountains, and on the west by a fan/wadi complex and localized sabkhas (Fig. 1). The sand sea is composed of four basic physiographic units. The upper Wahiba region consists of a compound/complex
Climate
The Wahiba Sand Sea is located in a hyper-arid desert region. The primary climatic patterns for the region are monsoon systems from the southwest during the summer and from the northeast during the winter Miller and Keshavamurthy, 1968, Pedgley, 1970, Findlater, 1971, Olsen, 1990. The Northeast Monsoon is weaker than the Southwest Monsoon, therefore, the most pronounced sediment transport in the Wahiba Sand Sea is thought to occur during the summer months (Warren, 1988b). The Shamal, a more
Data
Sand samples were collected from the Wahiba Sand Sea and the surrounding environments (e.g., wadis, sabkhas, and beaches). In the dune field, samples were collected approximately every 10 km along transects accessible by four wheel drive vehicle. Bulk samples of about 100 g were taken from the top 1–3 cm of the crests of dunes. Most sites were active and so bulk samples were thought to be representative of the mixed surface layer. Sites with excessive lag deposits were not used in this study
Band ratios
Band ratios, the ratio between two spectral responses, are useful for geologic purposes because they reduce the influence of differential slope and shadowing caused by topography (Drury, 1987). Band ratios were used as a first-order estimation of the location and extent of individual mineral populations in the Sand Sea. Spectral signatures for most common mineral types in the range of sand size are known from laboratory and field studies Hunt and Salisbury, 1970, Hunt and Salisbury, 1971, Hunt
Sediment sources and transport pathways
The general patterns of mineral concentrations in the Landsat TM images are consistent with patterns suggested by samples collected in the field. The following interpretations of sand sources use both sets of data. Previous interpretations of the sources of sediment in the Wahiba Sand Sea have suggested that the upper Wahiba mega-ridge complex is considerably older, and as such, mineralogically distinct from the lower Wahiba region Juyal et al., 1997, Gardner, 1988, Warren, 1988a. This analysis
Conclusions
Interpretation of mineral data beyond point sample sources in an aeolian sand sea was improved by Landsat TM data. These data provided a view of the spatial pattern of mineral distribution across the entire dune field. The discrimination of individual sand populations based on varying mineral assemblages was successful and enabled better assessment of the mineral variability and sand source regions than was possible using field data alone. Because these patterns are based on spectral data, they
Acknowledgements
Funding for this work was provided by the National Science Foundation (OCE-9311263) and the National Aeronautics and Space Administration (NAGW-3652 and NAG5-3753). This paper is contribution number 439 to the US Joint Global Ocean Flux Study (U.S. JGOFS) as part of the Arabian Sea Process Study. We are grateful to Philip Townsend for reviewing an early draft of this manuscript. We also thank His Majesty Sultan Qaboos bin Said Al Said and the authorities and people of the Sultanate of Oman for
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