Abstract:

Eastern China holds many Cenozoic basins where saline giants developed episodically throughout geologic time. Yet, in part, because modern analogous for ancient halitic depositional environments are lacking, in terms of both size and mineralogy, the stratigraphy of evaporite deposition in eastern China, especially whether the depositional environments were “deep basin-deep water” or “deep basin-shallow water”, remains controversial after more than 40 years of research. In order to better understand the factors controlling the formations of ancient evaporite deposits, previous researchers have investigated the hydrology and hydrochemistry of many modern salt lakes with different mineral assemblages, but the forming conditions and indicators for deep-water evaporite deposits are still unclear. In this paper, we compared the salt-forming characteristics of modern salt lakes that possess deep-water evaporite deposits, such as the Dead Sea and Freefight Lake, with that of the Dongpu Sag in eastern China, and discussed the salt-forming conditions, including salinity, water depth, lake level fluctuation and brine stratification. In addition, we analyzed the fabrics in and distributions of the deep-water evaporite sediments and summarized the indicators for deep-water salt deposition. Studies have shown that multistage cyclicity in halite sequence core and log data can be used for salt rock identification. The thickness of the brine profile obviously controls the formation and distribution of evaporites, as thick single layered homogenous salt deposit with lateral stability is more likely to form in the inland “deep basin-deep water” type of salt lakes, such as the Dead Sea and Dongpu Sag. The deep-water evaporite deposits around the depocenter coexist with the sallow-water salt deposits along the lakeshore, and they are all formed by brine concentration as lake level recedes. The rock salt texture is closely related to the sedimentary facies. The coarse grained evaporite minerals mainly precipitate and aggregate at the brine-lakefloor sediment interface in the deep-water region, and are often interbedded with black shales rich in organic matter and pyrite. The shallow-water salt deposits along the lakeshore usually contain more finer grained terrigenous clastics. The evaporite deposits in both modern salt lakes, such as the Dead Sea, and ancient evaporite basins, like the Dongpu Sag, show that declining lake level and thicker brine profile promoted the “deep basin-deep water” type halite deposition. This study revised the current understanding of the mechanisms of salt deposition in inland lake, especially the brine stratification phenomenon in modern salt lakes. It has many implications for other evaporitic events in the geologic record. The indicators for “deep-water evaporite deposition” on the basis of modern salt lakes can provide constraints in building the depositional model of ancient rock salt.

Key words: deep-basin, deep-water evaporite deposition, salt deposited in the depression center, brine stratification, forming conditions, Dongpu Sag, Dead Sea