LATE PERMIAN EARLY TRIASSIC SEDIMENTARY EVOLUTION AND TECTONIC SETTING OF THE LINXI REGION, INNER MONGOLIA

Early Triassic strata in the southern Da Hinggan Mountains has been evidenced by discovery of fossils of ostracods, conchostracans and bivalves in the Linxi region, Inner Mongolia. The Lower Triassic is of fluvial lacustrine facies with red bed features and notably different from its underlying Upper Permian Linxi Formation. Before this work the sedimentary facies of the Linxi Formation was considered as Late Permian non marine deposits. Sedimentological, paleontological and oxygen carbon isotope studies suggest that the mid lower sequences of the Linxi Formation consist mainly of marine deposits with turbidity current deposits, which reflects the existence of a remnant sea basin in the southern Da Hinggan Mountains till the middle Late Permian. This should be the main cause why the Late Permian plants on both sides of the Xar Moron River still maintain the features of different floras. An analysis of Late Permian Early Triassic sedimentary evolution and tectonic setting in the Linxi region shows that the Late Paleozoic plate tectonic movement in the southern Da Hinggan Mountains continued till the Indosinian tectonic cycle.