干旱区生态环境知识资源中心

The regional unconformity that marks the Cretaceous/Tertiary boundary and the Oligocene-Miocene contact in the United Arab Emirates exhibits an irregular palaeokarstic surface. Polyphases karat development related to dissolution-precipitation processes commonly results in the collapse of the breccia of the limestone with planar-walled fissures filled with red mudstone and clasts of host limestone. Moreover, solution pipes, caves, sinkholes, and joints with precipitates and endogenetic or exogenetic sediments are observed. Also present are celestite, silcretes, gypcretes, coarse columnar calcite crystals and ferruginous material as related products to karastification processes. The heterogenetic nature of these void-filling deposits is related to changes in climatic conditions through time. Petrographic study points to alternation between semi-arid or humid climate to arid-semi-arid conditions during their precipitation. Preservation of fibrous gypsum could indicate extreme aridity and limited effect of leaching of water-soluble salts near the ground surface. (C) 2000 Academic Press.