Oligocene storm-induced lacustrine deposits in the Yaxi Area of the Jiuxi Basin, northeastern margin of the Tibetan Plateau

Storm-induced deposits in the sedimentary record can now be easily identified on account of a better understanding of processes of unidirectional and oscillatory flow. Thin-bedded sandstone, siltstone, and interlayered mudstone deposits of the Oligocene Jianquanzi Member in the Yaxi Area of the Jiuxi Basin along the northeastern margin of the Tibetan Plateau were formed by shallow lacustrine storm-generated flows. Petrologic analysis reveals that the sandstones exhibit moderate to high compositional maturity but are texturally sub mature to immature. Based on investigation of drill cores, six storm-deposit lithofacies are identified, including massive or graded sandstone, parallel and trough cross-bedded sandstones, hummocky cross-stratified sandstone, wave-rippled sandstone and siltstone, massive or horizontal bedded mudstone, and structurally deformed sandstone and mudstone. In a vertical succession, these lithofacies occur as three fining-upward lithofacies associations, each of which represents different architectural elements (and depositional processes) of proximal to distal storm deposits. The relatively coarse proximal architectural element is formed by strong density flows and unidirectional-dominated combined flows; the transitional architectural element is dominated by unidirectional- and oscillatory-dominated combined flows to pure oscillatory flows, resembling Tb-e divisions of the classic Bouma sequence; the relatively fine distal architectural element is dominated by unidirectional-dominated combined flows with a very weak oscillatory flow component. The origin of storms responsible for these deposits is attributed to Asian winter monsoons associated with the paleoclimatic and paleogeographic setting.