Evidence of rain shadow in the geologic record: repeated evaporite accumulation at extensional and compressional plate margins

Arid climates have been common and effected water resources throughout earth history. This climatic history provide a key to understanding current causes for desertification and a means to devise realistic strategies for coping with its effects. Desert climates are often indicated in the geologic record by thick sections of evaporites (anhydrite, gypsum and halite) that have accumulated in both lacustrian and marine settings either adjacent to margins of recently pulled apart continental plates, in compressional terrains of colliding margins, or in areas of local tectonic uplift or sediment accumulation that have isolated standing bodies of water from the sea. These linear belts of evaporitic rocks can be directly related to rain shadow caused by: 1) The aerial extent of adjacent enveloping continental plates 2) The occurrence of uplifted crust marginal to linear belts of depressed crust 3) The occurrence of linear belts of depressed crust, with surfaces that are often below sea level 4) The occurrence of internal drainage and/or limited access to open ocean waters 5) The location within a climatic belt already characterized by low rainfall Examples of evaporite generation in depressed extensional basins belong to the Mesozoic sedimentary section of the North and South Atlantic margins: the Mesozoic of the northern Gulf of Mexico; the Mesozoic of the Yemen rift belt; the Mesozoic and Tertiary of Eritrea; the East African Rift; the Dead Sea, and so on. In contrast the current Arabian Gulf and its underlying Mesozoic to Tertiary rock section is a prime example of a linear intercontinental compressional zone that has a history punctuated by limited access to the sea and repeated desert climates. Other comparable examples include sections of the Silurian of the Michigan Basin and western New York State; the Devonian of western Canada and the Northwest USA; the Pennsylvanian of the Paradox Basin; the Permian of New Mexico and west Texas; the Permian of the Zechstein Basin; the Jurassic of the Neuquen Basin of Argentina; the Tertiary of the Mediterranean; and the Mesozoic and Tertiary of the final phases of the Tethys Sea (e.g., the Caspian and Aral Seas, etc.). Examples of evaporite accumulation behind barriers developed by structure and sediment buildup include the Permian Khuff Formation and the upper Tuwaiq Mountain Group, both of which accumulated on the eastern margin of the Arabian Shield and were isolated from the Tethys Ocean. The recognition of the strong tie between plate setting and climate can be used to predict the evolution of the climatic conditions within present day desert settings. The water resources in these areas of rain shadow and their proximity to the continental margins of lakes and narrow marine bodies match those of the past. These resources are often finite and need to be husbanded. Though some effects of deserts associated with rain shadow can be circumvented through river diversion and creation of artificially dammed water reservoirs, reverse osmosis etc., many other desert areas are subject to depletion of fossil water resources no matter the care taken to avoid this effect! The geologic record of the earth has a strong message for us all, particularly hydrologists, suggesting that despite human intervention, the effects of desertification are difficult to contend with and often almost impossible to avoid. The overwhelming signal from Nature suggests that the solution to water resource problems is often a mix of better engineering of the current resources and thoughtful political decisions.

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