Karastic features associated with unconformity surfaces, a case study fromthe United Arab Emirates

The regional unconformity that marks the Cretaceous/Tertiary boundary and t he Oligocene-Miocene contact in the United Arab Emirates exhibits an irregu lar palaeokarstic surface. Polyphases karat development related to dissolut ion-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 j oints with precipitates and endogenetic or exogenetic sediments are observe d. Also present are celestite, silcretes, gypcretes, coarse columnar calcit e crystals and ferruginous material as related products to karastification processes. The heterogenetic nature of these void-filling deposits is relat ed to changes in climatic conditions through time. Petrographic study point s to alternation between semi-arid or humid climate to arid-semi-arid condi tions during their precipitation. Preservation of fibrous gypsum could indi cate extreme aridity and limited effect of leaching of water-soluble salts near the ground surface. (C) 2000 Academic Press.