Relict drainages, conical hills, and the eolian veneer in southwest Egypt—Applications to Mars

Abstract

Viking images indicate that no single process is sufficient to explain the origin and evolution of Martian landforms. Landforms common to both earth and Mars result from a complex interplay of geologic agents, including water, mass wasting, wind, and possibly ice. The relict wadis of the Gilf Kebir Plateau in southwest Egypt strongly resemble fluvial channels on Mars, particularly those of the fretted terrain. Like the fretted channels on Mars, the Egyptian ‘wadis’ are dry, flat floored, and deeply incised in a plateau surface; they have stubby, theatre‐headed, wide, and deep tributaries that commonly join the trunk valleys at odd angles; they apparently lack drainage basins and are separated by areas that appear, on most of the available images, to be undissected. The wadis grade into adjacent low‐lying, smooth plains mantled by eolian debris and dotted with myriads of conical and flat‐topped inselbergs that have obviously undergone mass wasting. The unusual geomorphic characteristics of the Gilf region are herein interpreted as the results of an extreme eolian and mass‐wasting overprint on a landscape originally carved by running water. The relict networks of southwest Egypt were initiated by streams that drained formerly extensive Gilf highlands. Those highlands are inferred to have been a major Tertiary watershed in North Africa. As Quaternary dessication set in, these rivers vanished and were replaced by ephemeral gullies. Cliff sapping, wind abrasion, and deflation have widened the Gilf valleys and reduced many interfluves to clusters of flat‐topped and conical hills. These now rise as inselbergs on the adjacent desert plains, on which little evidence of former fluvial activity remains. Wind has deflated the surface and redistributed the finer particles from fluvial sediments and the debris of mass wasting into eolian sand sheets, drifts, and dunes. Dunes are migrating southward through the region along eolian thoroughfares that follow the apparent paths of ancient master streams toward distant basins of sand accumulation in the Central Sahara. The fluvial and mass wasting origin of the particles in the eolian deposits, the segregation of these materials according to grain size, and the migration of those particles capable of saltation to areas of accumulation in lowland basins of the Sahara are suggested as analogs for the formation and accumulation of ‘sand’ sheets and dunes on the northern plains and in the polar erg on Mars. Outliers of the Martian plateau in the fretted terrain appear to have been dissected, at least initially, by channels whose upstream portions are incised in the uplands. The Martian ‘wadis’ have many geomorphic peculiarities similar to those of the Gilf wadis, and like the Egyptian features, they have been attributed to mass wasting. Although basal sapping and removal of debris by wind have almost certainly modified the Martian features, their deep incision in the plateau and their inferred northward extensions on the northern plains seem to require not only initial downcutting by fairly energetic streams but also prolonged and long‐distance flow of water. On Mars, as in southwest Egypt, running water is now nonexistent. As suggested by Milton (1973), the northern plains may be ‘the most extensive of the landforms produced during a fluvial stage of Martian history.’ Inferred integrated stream networks in the fretted terrain on Mars, as in the Gilf Plateau; may be relics of earlier, less arid climatic intervals when runoff collected and flowed with sufficient power to carve valleys in bedrock. In this interpretation, the uplands might once have stood as a watershed on a much less arid Mars.