Depositional architecture and sequence stratigraphic framework of the fluvio-lacustrine Ash Shumaysi Formation, Jeddah-Makkah Region, Saudi Arabia: Implications for climatic and tectonic changes in a local-scale sub-basin

This study aims to interpret and document the depositional architecture styles and sequence stratigraphic framework of the Ash Shumaysi Formation in the Jeddah-Makkah region, the west-central part of Saudi Arabia, and presents an example of rarely discussed, local-scale sub-basins (half grabens). It also shows the relationships between synchronous sedimentary processes and pre, syn and post-rift conditions. The described lithofacies and their facies associations indicate the presence of seven architectural depositional styles: proximal-distal braided fluvial, meandering fluvial (point bar), crevasse splay, floodplain, estuarine and lacustrine. A proposed depositional model for the Ash Shumaysi Formation is drawn. The Ash Shumaysi Formation forms a second order depositional sequence, which is organised into two third order depositional sequences (sequences I and II) bounded by three sequence boundaries. Each third-order sequence encloses the low accommodation systems tract and high accommodation systems tract. The low accommodation systems tract represents the coarse-grained, braided-distal fluvial facies developed during low accommodation space associated with high sediment supply (high discharge). The high accommodation systems tract encloses the fine-grained deposits of point bar, estuarine and lacustrine facies that reflect the creation of significant accommodation space and low sediment supply (low discharge). Vertical and lateral variations of the inferred depositional architectural styles, sequences and systems tracts reflect that tectonic forces and climate are the main controlling factors during deposition of the Ash Shumaysi Formation, although base-level changes in response to sea-level changes cannot be ruled out.