The Marine Middle Jurassic Karajigu Formation in Northern Karakorum Mountains, Western China, and its Tectonic Implications

Abstract

The Karajigu Formation is proposed to be for the marine Middle Jurassic strata in the Karajigu area on the northern Karakorum Mountains, western China. The formation, with a total thickness of about 2, 200 m, comprises three lithologic members ; the clastic rock member, the massive limestone member and the thin-bedded limestone member, in the ascending order. The lithology corresponds to a shallow neritic facies and the Middle Jurassic age is determined by the contained bivalve, brachiopod and ammonite fossils. Its stratigraphic relationships with the underlying Lower Permian and Upper Triassic strata and the overlying Upper Cretaceous strata in Karajigu, together with regional stratigraphies and geologic structures throughout the northern Karakorum Mountains to the northern-central Tibetan Plateau, elucidate a major profile of the East Tethys evolution during the late Paleozoic through Mesozoic. The Karajigu Formation was formed on the northern continental margin of the East Tethys in Middle Jurassic age, subsequent to the closure of the Late Triassic Karakorum-Hoh Xil Ocean (newly proposed) through the Indosinian movement. Its unconformable relation to the overlying Upper Cretaceous red beds demonstrates the influences of end-Jurassic Yanshanian movement to the pre-Cretaceous construction of principal parts of the northern Karakorum Mountains. The Tertiary Himalayan movement was the final marking in the structural history of the Karajigu area.