FLUVIAL GRAVELS OF THE KATKUT FORMATION, OLIGOCENE/MIOCENE LANDSCAPE, AND CONSTRAINTS ON TIMING OF FORMATION OF NON-TECTONIC SYNCLINES IN EOCENE LIMESTONES OF THE WESTERN DESERT, EGYPT

Eocene limestones in the Western Desert of Egypt display an extensive network of narrow synclines. Our previous work, which combined analysis of high resolution (1-2 m/pixel) satellite images, audio-magnetotelluric surveys, and field work, has established that these synclines are non-tectonic sag synclines. The synclines are relict features that predate Pleistocene/Holocene hydrologic systems, and the syncline network is cut and offset by faults associated with opening of the Red Sea Rift. Other workers have placed this latter event at the Oligocene/Miocene boundary, although the actual age of faulting in the Western Desert has not been determined directly. Our current work aims to better constrain the timing of syncline formation.

Previous workers mapped two large arcuate patches of fluvial gravels of the Katkut Formation that lie unconformably on limestones west of the Nile. These gravels lie 180-240 m above the Nile floodplain and pre-date incision of a through-going Nile in the late Miocene. Satellite image analysis combined with field work and analysis of SRTM DEM data reveals a third large arcuate patch of Katkut gravels that was missed by previous workers due to masking by light-colored aeolian sand. An inverted channel deposit extends across the Limestone Plateau, ending at the Katkut. Where the Katkut has been dissected by erosion, high resolution imagery also reveals many small sinuous interfluves that we interpret as inverted channel deposits. We see no evidence that the Katkut was deposited in a graben complex, as proposed by others. Although normal faults are ubiquitous across the Limestone Plateau, faults do not define the arcuate margins of the Katkut, which are more consistent with erosion in a fluvial system.

Field work combined with analysis of high resolution imagery indicates that the unconformity is not deeper in syncline cores, which argues against sag during or after deposition of the Katkut. The long, inverted channel deposit feeding into the Katkut crosses multiple synclines and faults at a high angle, and neither the faults nor the synclines have any influence on the channel deposits. Both indicate that fluvial gravels of the Katkut were deposited on a landscape developed on limestone with an existing, eroded syncline network, which supports an Oligocene/early Miocene age for the sag synclines.