ORIGIN OF AN EXTENSIVE NETWORK OF ENIGMATIC SYNCLINES IN EOCENE LIMESTONE OF THE WESTERN DESERT, EGYPT

Early Eocene limestones of the Thebes Group in the Western Desert of Egypt contain an enigmatic, extensively developed network of thousands of synclines. Both scale and geometry differ from typical tectonic fold structures. Synclines are 100-400 m across, with no parasitic folds and no larger structures, and commonly occur as isolated structures 1-3 km apart in otherwise flat-lying limestone. Although two orientations dominate (NNW-SSE and WNW-ESE), synclines branch, merge, and curve into one another, forming a network. Although synclines are not equally well-developed everywhere, reconnaissance mapping using high resolution satellite imagery reveals that synclines occur in the Thebes Group over an area of nearly 100,000 km² across the Limestone Plateau of the Western Desert and in smaller areas of the Eastern Desert near the Nile. Synclines are cut by faults related to Red Sea rifting. Lack of correlation with modern wadi hydrology indicates that the synclines are relict features, and erosion has removed a minimum of 30 m of limestone since syncline formation.

This syncline network is best explained by non-tectonic sag of limestone layers. The mechanism, however, remains unclear. Continuity of layering in syncline cores in roadcut and wadi exposures to depths of over 100 m suggests a deep volume reduction process. Recently collected, but as yet unanalysed, geophysical data should allow us to constrain this. Dissolution of evaporites at depth is precluded by lack of evaporites in the stratigraphy underlying the Thebes. Consistent syncline orientation over huge areas suggests that subsurface mobilization of shale is an unlikely explanation. Collapse of paleokarst is unlikely, given the lack of evidence of long-term subaerial exposure in older limestone-bearing sequences. Syncline geometry, plus a strong correlation between syncline, joint, and fault orientations in the Eocene limestone, suggests that hypogene speleogenesis might be a reasonable model. Upward movement of aggressive fluids along joints and faults in the Esna Shale and into overlying Thebes Group limestones may have caused dissolution lower in the Thebes and sag at shallower levels. Mafic igneous activity, which was widespread in Egypt after the early Eocene, may have played a critical role in creating warm, rising, CO₂-charged fluids.