THE DESERT EYES PROJECT PART I: POLYGONAL FAULTS IN CRETACEOUS CHALK AND ENIGMATIC FOLD STRUCTURES IN EOCENE LIMESTONES, WESTERN DESERT, EGYPT

The purpose of the Desert Eyes Project is to investigate largely unstudied but pervasively developed bedrock structures that are responsible for large-scale, systematic patterning visible in high resolution satellite imagery of the Western Desert of Egypt. This abstract describes results from a portion of the Desert Eyes Project involving discovery of a set of structures in Cretaceous chalk near Farafra Oasis and a second set of structures exposed over an area of >20,000 km² largely west of Nile in Eocene limestones.

Chalk of the Cretaceous Khoman Formation is well-exposed essentially parallel to bedding over an area of about 700 km² near Farafra Oasis and is partly mantled by aeolian sand over at least 2500 km². The chalk displays a complex polygonal network of thousands of steeply dipping faults that outline polygonal areas 500-1000 m across. Slickenlines, slip fibers, and grooves in both the host chalk and the calcite veins that occur along the faults have rakes of 75-90°, and offsets indicate small amounts of normal slip. Fault geometries indicate that mechanically interacting, multiple fault orientations were active contemporaneously and that the horizontal strain field was essentially isotropic and extensional. All of these features indicate a polygonal fault system formed during burial and diagenesis rather than sets of intersecting faults of different ages and orientations. This is the first extensive exposure of polygonal faults to have been recognized on land.

Eocene limestones west of the Nile display enigmatic fold structures that formed in a narrow time window between Eocene sediment deposition and formation of NNW-SSE faults during Oligocene-Miocene opening of the Red Sea. Fold geometries are unusual. A large portion of the terrain displays narrow (100-300 m wide) branching and merging synclines with two dominant orientations (NNW-SSE and WNW-ESE). In the broad (1-3 km wide) areas between synclines, sedimentary layers are flat-lying; true anticlines and domes are rare. Other areas display a “bubble wrap” terrain of broad, low amplitude domes 100-1000 m across separated by networks of narrow, interconnected synclines. Neither geometry is typical of structures formed in “tectonic” fold and fault terrains and may have formed in response to polygonal faulting and/or mobility in the underlying Esna Shale.