POSSIBLE MASS TRANSPORT DEPOSITS AND FLUID ESCAPE PIPES IN EOCENE LIMESTONES OF THE DUNGUL FORMATION, SOUTHEAST WESTERN DESERT, EGYPT

The southernmost portion of the Western Desert Limestone Plateau in Egypt is capped by quasi-horizontal limestones of the Eocene Dungul Formation. Differential erosion has produced extensive bedding surfaces 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 bands organized in patterns and confined almost entirely to one stratigraphic horizon, and 2) thousands of pits 10-100 meters in diameter developed in several stratigraphic horizons.

The bands consist of lighter-colored stripes a few meters to 10s of meters wide separated by topographically lower, darker-colored stripes 1-5 m wide. Sets of bands display parallel, sinuous, lobate, plumose, and arcuate patterns. Patterns extend beneath the next younger limestone layer, and immediately overlying and underlying layers are unpatterned, indicating that the patterning is part of the limestone layer in the stratigraphic sequence. Individual bedding plane exposures of the patterning range up to ~4.5 km² in area, with pattern-parallel lengths ranging up to ~2.5 km. Pits occur on bedding surfaces as small, quasi-circular, low-relief depressions that commonly display a concentric pattern of more and less resistant layers. 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 occur as aligned sets.

The layer-confined patterning is strikingly similar to patterns typical of marine mass transport deposits, and we propose that these patterns resulted from mobilization on a shallowly sloping carbonate shelf, although the triggering mechanism is unclear. The southeast Western Desert is cut by a major set of E-W faults that have been repeatedly active since the late Mesozoic, so seismicity may have played a role. Because the pits resemble fluid escape pipes, 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.