The Response of Architectural Elements to Sequence Stratigraphy in Braided Fluvial System

Abstract

The Yungang Formation of Middle Jurassic in Datong Basin, north China, is a set of typical sandy braided fluvial deposits. Photographic mosaics and high resolution measurement of six outcrop sections nearby Yungang Rock Cave, northwest Datong, allowed the identification of twelve lithofacies and nine architectural elements in braided fluvial deposits. With developing architectural analysis and identifying different hierarchy of bounding surfaces, this study reconstructed stacking patterns of fluvial deposits, and revealed the relationship between architectural elements and base level change in braided fluvial system. At the high flow stages as the base level rise, large scale complete bar and channel elements could be preserved at depth resulting from downstream and laterally accretion in channel complexes. Overbank elements commonly occur at the upper parts of these channel fills, corresponding to the maximum accommodation. In contrast, small scale and incised channel elements (e.g. chutes) with quantities appear to be the main architecture elements in channel-belt at the low discharges, which is according with a decrease of accommodation as the base level fall. Migration of those incised channels modified the upper portions of the high flow braid bar or channel deposits.