Abstract:

Through the observation and measurement on three braided river sedimentary outcrops of Yanqing Silicified Wood National Geopark Section in Beijing, three braided river microfacies of braided channel, floodplain and splay are been identified, and their sedimentary characteristics are analyzed. Two full medium-term cycles and some short cycles that constitute the medium-cycles are divided using the base-level cycles change principle of high-resolution sequence stratigraphy, and the stratigraphic framework is established. The cycle conversion surface identification of the braided river outcrop is summed up as the flooding surface and the sequence boundaries. Flooding surfaces include the wet floodplain shale development segments with a certain thickness and distribution, the splay stacked locations with a larger thickness and the isolated channel layers. Sequence boundaries include the locations  with little or no development of flood plain mudstone, the locations of stacked river channel on the erosion surface and the positions of the river channel that can be correlated and has a big scale in a certain range. The braided channel sand bodies produced in low A/S ratio, are superimposed into a film horizontally and overlaid each other vertically, with good connectivity, big thickness, little mud, good heterogeneity and strong contractility. While the braided channel sand bodies that are produced in high A/S ratio, are isolated and less overlaid, with small thickness, limited range in horizon, high clay contain and poor connectivity. As a result, the high-resolution sequence stratigraphy of braided river facies is initially formed, which can provide the basis for the establishment of high accuracy correlation framework and the recognition and prediction of reservoir in outcrop braided river depositional system.

Key words: braided river, outcrop, high-resolution sequence stratigraphy, base-level cycle, facies, Yanqing