FAULTS, FRACTURES, A NETWORK OF NON-TECTONIC SYNCLINES, AND LANDSCAPE EVOLUTION IN THE WESTERN DESERT OF EGYPT, PLUS A CAUTIONARY TALE ABOUT MAPPING KATKUT GRAVELS USING SATELLITE IMAGERY

Our previous work has established that Eocene limestones across an area of ~100,000 km² in Egypt display a variably developed network of thousands of narrow synclines that pre-date Red Sea Rift-related grabens and Oligocene(?)/Miocene gravels of the Katkut Formation. We have established that the synclines are non-tectonic sag synclines produced by volume reduction at depth and sag of overlying limestone layers.

New mapping north of Kharga Valley reveals two dominant orientations of synclines, WNW-ESE synclines in otherwise flat-lying limestone and NS synclines parallel to faults. As elsewhere, synclines from both trends branch and merge. Riedel geometries suggest left lateral strike slip along the NS faults. As the faults die out northward, dominant orientations of synclines change to WNW-ESE and NNW-SSE, with the latter parallel to fractures and faults associated with Red Sea rifting. Closer to the Nile, the syncline network is polygonal. By the end of the Oligocene, rocks in the region contained multiple sets of faults and regional fractures of different orientations and patterns produced 1) during a late Eocene contractional event in the Syrian Arc, 2) during early phases of Red Sea rifting, and 3) perhaps by polygonal faulting of shales underlying the limestones. We propose that fluids rose from depth along this complex fault/fracture system, causing volume reduction at depth (hypogene karst, silica diagenesis, or dolomitization), sag in overlying limestone layers, and mineralization along fractures. Syncline orientations reflect local patterns of fluid conduits in the latest Oligocene. Later phases of Red Sea rifting offset the syncline network. Landscape and patterning in satellite imagery reflect differential erosion of layers in synclines, yardangs in flat-lying, jointed limestone between synclines, and resistant fracture networks.

Our work has also revealed that Katkut Formation gravels have been inaccurately mapped by previous workers using ASTER and Landsat imagery. Although areas of true Katkut are dark gray-brown in satellite imagery, our field work and detailed mapping on high resolution satellite imagery shows that large areas with similar color are not Katkut gravels at all, but, rather, a thin veneer of angular, desert varnished chert fragments weathered from the underlying limestone.