Enigmatic structures in Eocene limestones of the Dungul Formation, southeast Western Desert, Egypt: Mass transport deposits and fluid escape pipes?

Eocene limestones of the Dungul Formation cap the remote southern part of the Limestone Plateau in the Western Desert of Egypt. Bedding is quasi-horizontal, and limestone layers with different erosional resistances are interlayered at the scale of meters, resulting in extensive bedding surfaces on resistant layers that step from one to the next across low scarps underlain by less-resistant limestone. Using high resolution satellite imagery, we have recently discovered two sets of enigmatic and previously unrecognized features on these bedding surfaces: 1) light and dark banding organized in patterns and confined almost entirely to one stratigraphic horizon, and 2) thousands of 10-100 m pits developed in several stratigraphic horizons. The banding consists of lighter stripes a few meters to 10s of meters wide separated by topographically lower, darker bands 1-5 m wide. Bands are organized into sets with parallel, sinuous, lobate, plumose, and arcuate patterns. Patterns extend beneath outliers of the next younger layer, and the immediately overlying and underlying layers are unpatterned, indicating that the patterning is part of the limestone layer in the stratigraphic sequence. The largest individual bedding plane exposures of the patterning are 4.5 km2 in area, with pattern-parallel lengths up to 2.5 km. Pits occur as small, quasi-circular, low-relief depressions in bedding surfaces. Pits commonly display a subtle concentric structure of more and less resistant layers, and some pits show clear inward dips in bedding. Prominent regional WNW-ESE joints cut the limestone inside the pits, indicating that pits are not young sinkholes. Most pits are randomly distributed, although some are aligned. We propose that the layer-confined patterning was generated by mobilization of a layer of carbonate sediment, because the patterning is strikingly like the longitudinal shears, secondary flow fabrics, transverse pressure ridges, and fold-thrust systems in marine mass transport deposits. The triggering mechanism is unclear. The pits resemble fluid escape pipes, so episodic fluid escape may have been a trigger. The patterned layer is also co-located with several large, low amplitude domes in bedding across an area of ~4000 km2, and diapirism in underlying shales may have been a factor. Seismicity may also have played a role.