Extensively developed network of non-tectonic synclines in Eocene limestone of the Western Desert, Egypt: an example of hypogene speleogenesis?
Barbara Tewksbury, Hamilton College
Elhamy Tarabees, Damanhour University
Charlotte Mehrtens, University of Vermont
High resolution satellite imagery of the Western Desert of Egypt reveals an extensive network of long narrow synclines developed in Early Eocene limestone. These structures are unusual and have characteristics not typical of tectonic fold structures. The terrain is dominated by long narrow synclines (100-400 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. Two dominant orientations are common (NNW-SSE and WNW-ESE), parallel to two prominent joint sets in the limestones. Synclines from the two trends branch, merge, and curve into one another, forming a network. Over large parts of the area, narrow synclines are the only fold structures present and form isolated downwarps 1-3 km apart in otherwise flat-lying limestone. Where synclines are locally more closely spaced, inter-syncline areas are broadly anticlinal, but geometries suggest that they are "accidental anticlines" formed by the proximity of two nearby syncline limbs rather than by an active anticline-forming process. Syncline limbs are commonly cut by faults striking parallel to bedding but dipping more steeply than bedding toward syncline cores. The syncline network and related faults 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 characteristics of the syncline network are consistent with sag of limestone layers, which can be caused by a variety of subsurface volume reduction mechanisms or by mobilization of underlying shale sequences. All the likely suspect mechanisms have problems, however. No evaporites have been reported in the Western Desert stratigraphic column. We see no evidence of epigenic karst processes at a scale that could produce the synclines. Long-term subaerial exposure of units deeper in the section occurred in shale units, rather than limestones, which argues against collapse of paleokarst. Mobilization of underlying shales and accompanying sag of overlying limestone layers sounded plausible until we did a country-wide survey of synclines, which revealed remarkably consistent orientations over huge distances. Silica diagenesis remains a possibility, although consistency of orientations is also a problem for this mechanism. We are currently exploring hypogene speleogenesis, with upward migration of fluids along joints and faults and dissolution of the limestones at depth. Fluid aggressiveness might have come from CO2 (perhaps supercritical) associated with widespread igneous activity during the Eocene and Oligocene in Egypt.