Desert sedimentary environments, present and past—A summary

Abstract

Tropical deserts develop where the potential rate of evaporation exceeds that of precipitation, where rainfall is too low or spasmodic to support vegetation. These usually occur in the trade-wind belts north and south of the equator and in the rain shadow of mountain ranges. About half the area of deserts comprise outcrop subject to erosion and deflation. The rest consists of sediments of fluvial (wadi), lacustrine (salina), paralic (coastal sabkha) and aeolian origin, and of evaporites. Ephemeral streams deposit sands and gravels over basin-margin alluvial fans, and over desert plains where sub-surface cementation commonly takes place. Evaporation of basin-centre temporary lakes results in a crust of halite (inland sabkha). Fluvial sands are an important source of dune sand; the finer fractions are removed from the desert in suspension to become loess and the coarser grain remain as a deflation lag. The axes of sand dunes are transverse to moderate winds and parallel to strong winds. Giant forms of the latter dune type were prevalent during the last ice age, and in coastal areas carbonate-cemented dune sands extend below present sea level; this suggests a causal relationship between polar glaciations and strong desert winds. Desert fluvial sediments rarely reach the sea, so the warm and clear coastal waters are ideal for the manufacture of organic carbonates. Longshore currents sweep them into offshore bars behind which evaporitic lagoonal conditions develop. The lagoons become sabkhas as they are infilled with algally bound marine and wind-blown sediment. Although there are desert sequences as old as about 2000 Ma, they seem to have occurred only sporadically during the earth's history. It seems likely that their occurrence was controlled by the past size of continents, their location relative to the equator, by the freedom of circulation of global oceanic water and by the presence of ice caps. The Permo-Triassic was a time of extensive evaporite deposition and dune activity. By implication, other known occurrences of evaporites may be marginal to wadi and dune sequences that have yet to be identified.