Quantitative Characterization of Architecture Elements and Their Response To Base-Level Change In A Sandy Braided Fluvial System At A Mountain Front

Abstract

Abstract: The Rockcave Member of the Yungang Formation (Middle Jurassic), in the Datong Basin, China, is a multistory sandstone that is interpreted as a sandy braided fluvial deposit based on grain size, lithofacies, and architectural elements. The depositional setting was the mountain front of the Datong Basin, which was tens of kilometers from the shoreline of a lacustrine basin during the Middle Jurassic. The concept of base level is used to analyze the architectural elements of the braided system obtained from photographic mosaics and high-resolution measurement of six outcrop sections from Yungang Rock Cave. Grain size, lithology, and bounding surfaces together with scale and dimension parameters of the sedimentary structures in various architectural elements were used to quantitatively characterize each group of architectural elements and stratigraphic units. Width/thickness values of trough crossbeds were found to be a fundamental component of the hydrodynamic regimes. During base-level rise the braided fluvial system developed large-scale channel units, bar units, and overbank fills. The relatively high proportion of bar units and overbank fills reflect processes operating during positive accommodation. However, during base-level fall there are a high proportion of small-scale channel units that reflect incision-dominated processes such as channels with low W/T values for trough crossbeds and less common overbank fines.