AN ENIGMATIC SYSTEM OF FOLDS AND RELATED FAULTS IN STABLE PLATFORM LIMESTONES OF THE WESTERN DESERT, EGYPT

Eocene carbonates of the Egyptian Stable Platform display pervasively developed bedrock structures that are responsible for large-scale, systematic patterning visible in high resolution satellite imagery over an area of at least 20,000 km² west of the Nile. Structures vary with both geographic location and stratigraphic level. In the north, a “bubble wrap” terrain of broad, low amplitude domes 100 m - 1 km across separated by networks of narrow, interconnected synclines characterizes the Samalut Formation and the upper and western Minia Formation. Farther south, a terrain dominated by branching and merging narrow synclines with two dominant orientations (NNW-SSE and WNW-ESE) characterizes most of the Drunka Formation and produces a conspicuous sinuous pattern in the satellite imagery. In the broad areas between synclines in this terrain, sedimentary layers are flat-lying, and true anticlines and domes are rare. Between these two terrains, a hybrid terrain of long narrow synclines separated by clusters of bubble wrap-style domes characterizes the lower and eastern Minia Formation and the upper Drunka Formation. In the southern-most terrain, the El Rufuf and lower Drunka display both narrow synclines and NNW, N, and NNE-striking faults spaced 10-15 km apart. Domes and basins occur in association with these faults. Structures in all four terrains pre-date well-developed NNW-SSE graben and related structures that have been recognized on both sides of the Nile by other workers and attributed to reactivation of basement faults during Oligocene-Miocene opening of the Red Sea.

These structures formed in a narrow time window between Eocene sediment deposition and intrusion of basalts associated with uplift of Gebel Gebeil at 23 Ma. Although a Late Eocene tectonic event affected the Syrian Arc to the north, geometries of the structures described above are difficult to fit into that regional picture. Furthermore, the bubble wrap structures are strikingly similar to hummocky terrain overlying polygonal faults imaged with 3D seismic beneath the North Sea. Gradation between the bubble wrap terrain in the north and the unusual syncline terrain in the south suggests a common mechanism, at least in part, and perhaps involving diagenesis, with stronger control by basement faults in the older and more southerly sequences.