POLYGONAL PATTERNS AND DESERT EYES: RECONNAISSANCE SATELLITE IMAGE STUDY OF FOLD AND FAULT STRUCTURES IN LATE CRETACEOUS AND EARLY TERTIARY LIMESTONES OF THE WESTERN DESERT, EGYPT
TEWKSBURY, Barbara1, KATTENHORN, Simon A.2, SAYLER, Frances1, TEWKSBURY-CHRISTLE, Carolyn3 and SAINT-JACQUES, David4, (1)Dept. of Geosciences, Hamilton College, 198 College Hill Rd, Clinton, NY 13323, (2)ConocoPhillips Company, 600 N. Dairy Ashford, Houston, TX 77079, (3)United States Air Force, Dept. of Earth and Planetary Sciences, University of Tennessee, 1412 Circle Dr, Knoxville, TN 37996-1410, (4)Canadian Space Agency, 6767 Route de l'Aeroport, Saint-Hubert, QC J3Y 8Y9, Canada, btewksbu@hamilton.edu
High resolution satellite imagery reveals previously unstudied structures in Cretaceous chalk of the Khoman Formation where it is exposed in the broad regional Farafra anticline. The oldest units of the Khoman chalk display a complex network of low, narrow (1-2 m) ridges that outline roughly polygonal areas 500-1000 m across. Offsets of bedding indicate that these are fault structures, and their positive relief suggests the presence of either deformation bands or mineralization. These polygonal networks occur in continuous surface exposure over an area of about 700 km2 and extend over at least 2500 km2 partly mantled by aeolian sand. Overlying this fault network across the entire Farafra region is a terrain of isolated, nearly circular basins. The basins range from ~50-200 m in diameter and have layering with very shallow inward dips and very little topographic relief. In the oldest rocks of this terrain, the polygonal ridge network cuts and locally offsets the basins, and basins are spatially associated with the ridge networks. Ridges are rare in the upper portions of this terrain. The top part of the Khoman Formation displays basins of similar size, but here the basins stand in relief, forming a polka-dot network of weakly aligned, eye-shaped mesas locally associated with narrow ridges aligned approximately EW. Curiously, the immediately overlying Paleocene Tarawan Formation has low amplitude dome structures of similar size.
About 120 km to the east, Eocene carbonates display thousands of low-amplitude domes up to 1000 m across separated from one another by a polygonal network of narrow interconnected synclines, forming structures with the geometry of bubble wrap. In the north and west parts of this terrain, structural trends are weak and faults are rare. In the south and east portions, a NNW-SSE structural alignment of narrow synclines dominates, and NNW-SSE faults are common.
The structures near Farafra Oasis bear a striking resemblance in both scale and geometry to polygonal fault systems and fluid escape structures reported from young marine sediment sequences in the North Sea, Lower Congo Basin, and offshore Namibia. Structures in the Eocene carbonates are strikingly similar in both scale and geometry to broad hummocks and narrow synclines in sediments above polygonal fault networks in the North Sea and elsewhere.
