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.