EXTENSIVE SYNCLINE NETWORK IN EOCENE LIMESTONE OF THE WESTERN DESERT, EGYPT: REGIONAL FOLDING? COLLAPSED PALEOKARST? MOBILIZATION OF UNDERLYING SHALE?

Eocene limestones east and west of the Nile in central Egypt display a network of hundreds of long, narrow synclines that produce a strong regional patterning in high resolution satellite images over an area of >20,000 km². Synclines are narrow (100-200 m across), with shallow limb dips, porpoising hinges with shallow plunges, and multiple basin closures along their lengths. Synclines are similar in scale across the region, with no parasitic folds and no larger structures. Synclines are typically separated by 1-3 km and form isolated downwarps in otherwise flat-lying limestone. Where synclines are locally more closely spaced, inter-syncline areas are broadly anticlinal. Although two dominant orientations are common (NNW-SSE and WNW-ESE), synclines from the two trends branch, merge, and curve into one another, forming a network of interconnected synclines. Where synclines are dissected immediately east and west of the Nile, high resolution satellite imagery indicates that syncline limbs are commonly cut by faults striking parallel to bedding but dipping more steeply than bedding toward syncline cores. The geometry suggests a genetic relationship, with faults cutting upward through synclines formed earlier. The syncline network developed in a narrow time window between Early Eocene deposition of the limestones and formation of cross-cutting faults associated with Red Sea rifting.

The syncline network geometry is not typical of tectonic fold and fault terrains. The structures share similarities with sag structures developed above collapsed paleokarst systems, although the stratigraphic record does not indicate long-term erosional unconformities in underlying karst-susceptible sequences. Sag structures could also have been produced by other localized volume reduction processes associated with alteration or diagenesis. Alternatively, a network of sag structures in the Eocene carbonates could have formed during mobilization of the underlying Esna Shale. A Late Eocene tectonic event affecting the Syrian Arc uplifted portions of northern Egypt and may have triggered translation of the Esna Shale to the south or southwest. The presence of broad domes 10-50 km across in the southern part of the Sinn El Kaddab Plateau may reflect subsurface thickening of the Esna associated with south to southwest mobilization.