RELIEF INVERSION BY LONG-TERM CHEMICAL DEGRADATION IN SAUDI ARABIA

Most raised channels around the globe, where relief inversion is recognized, are interpreted to be the result of incision and/or deflation. In northern Saudi Arabia, however, inverted topography has been created by long-term chemical degradation. Scores of channels in well-defined, parallel drainage basins are found on the Umm er Radhuma Formation (UeR) in Saudi Arabia, which is known as the principal aquifer in eastern third of the country and host of many cave systems. The UeR is a Paleocene–early Eocene limestone formed in shallow marine waters, that has undergone extensive diagenesis since mid Tertiary time and is now primarily a dolomite with minor limestone and gypsum content. A combination of chemical and wind erosion has created a rough bedrock landscape of over 15,000 km² that is dominated by wind-shaped hills and flat dolines that are filled with aeolian fines and clayey chalky marl. Chemical degradation occurs primarily by weathering of interbedded chalky dolostones to soft marl and by concentrated dissolution in Arabian dolines called khabras. Khabras initially form on interstream divides as dolines and over time deepen, enlarge in diameter, and merge with adjacent khabras. Roadcuts in the region expose the internal structure of khabras as sinkholes filled with chaotic infills of surface debris, aeolian sediment, and dissolution breccia. Tertiary channels were likely cemented by duricrust during drier climates in mid-late Pliocene time. Duricrust in channels is significantly more resistant to chemical and wind erosion than the adjacent dolomite, which has resulted in dendritic patterns of raised channels 6-to-14 m above former interstream divides. The shapes of old, well–developed khabras are now defined by the patterns of relic Tertiary channels. Two to three former khabra surfaces are commonly found adjacent to active khabras, which likely reflects the effects of Quaternary wet-dry cycles. U-series ages of fracture filling, laminated carbonate from surface duricrust on bedrock indicate that dolomite dissolution occurred during Marine Isotope Stages 7 and 9. Dissolution in the khabras, enhanced by runoff from the slopes below the raised channels, appears to be active, albeit slow, under present arid conditions with as little as 105 mm annual precipitation.