MULTIPLE EPISODES OF YARDANG EVOLUTION IN SAUDI ARABIAN DOLOMITES

Yardangs are wind–eroded landforms predominantly found on desert floors around the globe. Commonly, yardangs are carved into soft earth materials such as lakebeds. Bedrock yardangs, however, are relatively rare. A large region of yardangs carved into dolomite has been recently located in northern Saudi Arabia. The Saudi yardangs are unique, because they have formed on 3 to 4 sequentially higher bedrock surfaces above the modern drainage floor. Yardang evolution has taken place in response to the following proposed geologic history. Multiple parallel dendritic drainages formed in the Umm er Radhuma Formation in response to mid Miocene uplift and tilt of the Arabian Shield. Surface lowering by karstic dissolution on interstream divides took place during the wet-to-semiarid climates of the Neogene. Duricrust cementation in scores of channels occurred during a dry period(s) in mid-late Pliocene. Dissolution continued in sediment-filled dolines, Arabian khabras, during wetter climates; however, duricrust-filled channels were not affected by chemical weathering. Growth of the An Nafud and Ad Dhana sand seas began in mid Pleistocene time and created a zone of wind-blown sand and dust parallel to the sand deserts that supplied the sediment necessary for wind abrasion. Local relief increased by continued dissolution in khabras during wet, interglacial conditions, and yardangs were eroded by sand abrasion during dry, and very windy, glacial conditions. Return of wet conditions in the next climate cycle renewed chemical degradation. Another generation of yardangs was carved during the subsequent glacial episode in bedrock ledges created during the most recent episode of chemical erosion in the khabras. A minimum of three, possibly four, khabra-yardang cycles has taken place during the Quaternary. Yardangs are subject to degradation by slow weathering of thin units of interbedded chalky dolostones. The oldest yardangs are preserved as rounded domes, while the youngest are low, flat and sharp landforms. Beyond the zone of blowing sand, the dolomite landscape is characterized primarily by exposed bedrock with karstic landforms and duricrust surfaces.