Sedimentary Facies and Palaeoenvironment Analyses on Yixian FormationBottom Layers of Yixian Basin,in Western Liaoning Province

Key words: 
• sedimentary facies /
• sedimentary paleoenvironment /
• Mashenmiao sedimentary deposit /
• Laogonggou sedimentary deposit /
• Yixian Formation /
• Yixian basin

Abstract: Mashenmiao and Laogonggou sedimentary strata develop as the bottom of early Cretaceous Yixian Formation in the Yixian basin, western Liaoning province. Mashenmiao sedimentary stratum, which forms under arid palaeoclimate, could be divided into 6 sublayers from the lower to the upper portion, the lower conglomerate sublayers come into being proluvial fan bottom facies and fan surficial streamflood filled facies; the middle part of Mashenmiao layers is ployenous conglomerate containing tuffaceous interlayer; mesotypicbasic vulcanite overlay Mashenmiao sedimentary deposit. The Laogonggou bottom is politic siltstone distal facies of alluvial fan and gritstonepebbly gritstone alluvial facies of proluvial fan to basin centre part, the latter is formed by ephermeral stream's fluviation. Bentonitic siltstone and bentonitecarbonate rock sedimentary deposit are aggraded as the top of Laogonggou stratum; it is a shallow lacustrine sedimentary set. In the initial stage of basin evolution, regional climate is arid and basin margin has high hypsographic feature. There are proluvial fans around basin margin, the flood stream sedimentary process occur on fan surface. In the transition part of fan to lacustrine, there are ephemeral streams; the top facies of Laogonggou indicates a palaeogeographic and palaeoclimatic characteristics of regional aridity, intensive volcanic eruption and lacustrine waterdepth fluctuation. The lower sedimentary of Yixian Formation show form basin margin to centre three facies coexistence paleogeography characteristics of planar, linear and relatively quiescence water mass depositions.