The sedimentary environment and models of mixed rocks in saline lake basins: a case study on Upper Shahejie Formation ( 4th Member) of Luojia area in Zhanhua depression, Bohai Bay Basin, China

Mixed rocks refer to the rocks formed by the mixture of terrigenous clasts and carbonate sediments. In recent years, the development characteristics and sedimentary patterns of mixed rocks in saline lake basin in Upper Shahejie Formation (4^th Member) of Luojia area in Zhanhua depression Bohai Bay Basin have yet to be solved, which brings certain limitation to the exploration of oil and gas in this area and some other places with the same environment. This study used detailed core description, sequence stratigraphy analysis, sedimentary analysis, and thin section observation to study the mixed rocks characteristics and sedimentary patterns of Upper Shahejie Formation (4th Member) in Luojia area, systematically. The research shows that the 4^th Member of Upper Shahejie Formation in Luojia area is divided into a whole three-level sequence, including lowstand systems tract (LST), transgressive system tracts (TST), highstand systems tract (HST). Mixed rocks mainly develop in highstand systems tract, with a small amount developing in lacustrine transgressive system tracts. The lacustrine transgressive system tract has strong provenance supply, resulting in water dilution and a narrowly mixed phase zone. At highstand systems tract, the water in the lacustrine basin shrinks, and water salinization occurs. The decrease of water leads to the increase of terrigenous clast supply, forming mixed rocks and mixed strata with carbonate rocks.

During the highstand systems tract, mixed rocks and mixed strata mainly distribute in the front of fan delta and the near-fan shallow lake area. The mixed rocks in the front of fan delta are mainly affected by provenance supply. The migration of fan delta lobes caused the enhancement or weakening of provenance supply, mainly forming mixed rocks composed of rich terrigenous clasts. Lacustrine wave transformation carried the terrigenous debris from the fan delta into the shallow lake area far away from the fan delta. Frequent lake level fluctuations control the occurrence of mixing, resulting in the superposition of multiple mixing tidal flats, mainly forming mixed rocks with terrigenous clasts and carbonate rocks.