The Kyzyl Kum area is underlain by three main tectonic units (Fig. 1). The oldest (basement) consists of metamorphosed and folded Lower Palaeozoic carbonaceous and sulphidic clastic rock referred to as the Besopan Formation. The Besopan Formation was metamorphosed and deformed during the Lower Palaeozoic Caledonian orogeny. After erosion and exhumation it became basement to an unconformable cover sequence of Devonian to Early Triassic carbonate and clastic sediments and volcanic rocks referred to herein as Cover Sequence 1.
Cover Sequence 1 has been divided into a lower carbonate-dominant section reflecting passive margin sedimentation. The upper part is marked by a change to terrestrial evaporitic facies in the Karatau Range (Cook e t a l ., 1997; Lapointe et al., 1997) and extrusion of alkalic andesites in the so-called "Valerianovsky volcanic belt". These andesites have been intersected in petroleum wells and are inferred at depth from aeromagnetic data (see Drew et al., 1996). Palynological data from overlying sediments suggest that these volcanic rocks are pre-Triassic in age consistent with Carboniferous K-Ar dates (Burshtein, 1998). Alkalic chemistry suggests deposition in a rift environment ( Y. Savchuk, pers. comm., 1998).
Cover Sequence 1 was intruded by granites during the Late Carboniferous Hercynian orogeny. Some authors infer a major Carboniferous collisional suture beneath the sedimentary rocks of Cover Sequence 2 (Fig. 1). There is, however, little evidence of active subduction here during this time. Cover Sequence 2 (Syr Daria or Kyzyl Kum Basin) comprises over 2 km of mainly terrestrial sedimentary rocks which unconformably overlie Cover Sequence 1 and Hercynian intrusions. Sedimentation during the Jurassic was confined to narrow NW- and NNE-trending basins. During the Cretaceous and Tertiary, however, sedimentation extended beyond these narrow basins and covered rocks of the basement and Cover Sequence 1 throughout the Kyzyl Kum region (Fig. 1).