GPR INVESTIGATION OF MODERN AND ANCIENT AEOLIAN DUNE STRATIGRAPHY

Ground penetrating radar (GPR)is non-invasive and produces high resolution images of the shallow subsurface which has transformed the study of recent sediments. In the past, we had to infer the correlation between boreholes, dig shallow trenches to investigate lateral continuity and variability. Nowadays, with suitable ground conditions, we can collect GPR profiles that show the subsurface stratigraphy. In addition, 3-D GPR can resolve subsurface stratigraphy with high resolution allowing much greater appreciation of sandbody geometry and architecture. However, GPR is not a universal panacea, it works well in ground with a high resistivity, especially sands and gravels. Soils with a low resitivity, including most clay rich soils and areas with saline groundwaters, cause rapid attenuation and poor signal penetration.

This paper includes examples of GPR data from a variety of modern and ancient aeolian dunes. The high resistivity of aeolian dune sands gives excellent depth of penetration while the large size of aeolian dune cross-stratification results in good resolution of sedimentary structures. In aeolian sand and sand dune deposits GPR reflections can be related to bedding, sets of cross-stratification, palaeosols and bounding surfaces. Radar stratigraphy and radar facies analysis can be used to interpret GPR profiles. Horizontal and dipping reflections and cross-cutting relationships such as toplap, downlap, offlap and truncation of reflections, are used to establish a relative chronology. A combination of 2-D and 3-D profiles shows sets of cross-stratification and bounding surfaces within dune sands allowing extraction of bedding characteristics and sandbody geometry. In addition, the relative chronology derived from radar profiles can be used to develop a sampling strategy for geochronological dating. The combination of shallow geophysical profiles and geochronological dating methods has great potential for palaeoenvironmental studies and improving Holocene and Quaternary stratigraphy. A combination of GPR and optical dating has been used to investigate the timing of dune migration in the Namib Sand Sea and coastal dunes in North Wales and Maputaland in South Africa. The Jurassic Navajo Sandstone at Zion Canyon National Park, USA provides a case study of an ancient aeolian sandstone.