Origin of the Qattara Depression, Egypt
As a working hypothesis, we propose that the Qattara Depression originated as a stream valley that was subsequently dismembered by karstic processes during the late Miocene epoch and afterward was deepened by deflation and otherwise modified by mass wasting and fluviatile processes. The Depression is excavated in sedimentary rocks of Tertiary age that have a regional dip of a few degrees toward the Mediterranean Sea. The precipitous northern wall is the scarp slope of a cuesta capped by carbonate rocks of middle Miocene age. Its cliffy slopes descend to pediments graded southward to an axial valley. That valley, now extensively covered by saline crusts, sabakha deposits, and salt marshes, slopes westward into the deeper quadrangular part of the basin where elevations as much as 134 m below sea level have been measured. Nearby sinkholes and cavernous subsurficial limestone attest to past karstic activity. Presumably karstification reached a peak during the recession of the Mediterranean Sea late in the Miocene epoch when the north-dipping aquifers discharged into the then almost empty basin. As the Mediterranean Sea receded, what is now the Western Desert of Egypt and its off-shore continuations were subjected to extensive erosion by streams. A major stream issuing from the Gilf Kebir Highlands flowed northward to near the present site of Siwa Oasis, its alluvium now mostly covered by aeolian deposits of the Great Sand Sea. If it reached the Mediterranean Basin, then it may have done so through the Sahabi Channel System of Libya, or by a much shorter route through the Qattara area to an exit near the head of the Ras Alam er Rum submarine canyon offshore near Alexandria. In either case, the stream responsible for the Qattara cuesta, whether a tributary of the "Gilf River" or a part of that river's descending course into the Mediterranean Basin, apparently became inactive after its waters were diverted into underground courses through sinks and caverns. With the refilling of the Mediterranean and the accompanying diminution of subsurface aqueous activities and with the advent of more and climatic regimes, surface processes associated with deserts have completed the shaping of the Depression to give it its present configuration and character.