# A Review of Evaporite Beds Potential for Storage Caverns: Uncovering New Opportunities
## 2 Evaporites and Bedded Evaporite Deposits
### Origin of Engorites  
Several theories have been developed to explain the origin and formation of evaporites from various perspectives. The first theory, known as the Bar Theory [53], was created to describe the formation of thick saline deposits [54]. Proposed initially by [PERSON] (1888), based on ideas from [PERSON] (1864), Bar Theory suggests that evaporites are deposited through the precipitation of dissolved salts in a restricted arm of seawater, isolated by a barrier that limits inflow [55]. Bar Theory outlines the deposition process in five stages (Figure 2) [56] as follows:  
Figure 1: Solar heating and brine concentration in the formation of evaporites [51]. 1. Isolation of an arm of the sea by a barrier with periodic inflow of fresh seawater;
2. Increase in mineral concentration within the arm;
3. The continued flow of seawater into the arm further raises mineral concentration and brine density;
4. Deposition of various evaporites leads to a reduction in basin depth;
5. Separation of the basin from the sea due to geological factors, like uplift. While the Bar Theory explains the marine origin of evaporites, other theories have emerged to account for non-marine evaporite origins. For example, the Desert-Basin Theory, proposed by [PERSON] (1894), addresses the absence of fossils in saline deposits. [PERSON] (1913) used the Desert-Basin Theory to explain the origin of the Silurian Salina Formation in the U.S. by connecting evaporite deposits to desert redbeds and suggesting that connate water in pre-existing sediments provides a sufficient salt source for evaporites [54]. [PERSON] (1915) introduced the Ring Theory (Modified Bar Theory), proposing that different evaporites are deposited in separate basins. This theory explains the occurrence of monomineralic evaporites by hypothesizing separate basins for deposition: the outer basin is where gypsum is deposited, while the inner basin is reserved for halite [53; 54; 56]. Figure 3 illustrates the concept of evaporite deposition according to the Ring Theory [56]. Figure 2: Evaporite formation based on the Bar Theory [56]. Another modified Bar Theory, known as the Reflux Theory, was introduced by [PERSON] (1947) to explain the formation of thick, extensive gypsum deposits. This theory posits that when seawater enters an evaporation basin through a channel in the barrier, two layers of brine with different densities develop within the basin. The denser brine remains at the bottom, and a process called backflow, or reflux, allows this denser brine to maintain a constant density and composition, facilitating the deposition of thick gypsum layers without halite formation [53; 54; 57]. [PERSON] (1953) further expanded on the Reflux Theory by suggesting a lateral salinity gradient in basins where evaporites precipitate (Figure 4) [53; 54; 55]. Further theories have been developed to explain both deep- and shallow-water evaporite depositions. The Mediterranean evaporite serves as an example used in [54] to illustrate three distinct theories: the \"Deep-water Deep-basin Model\" by [PERSON] (1969), the \"Shallow-water Shallow-basin Model\" by [PERSON] (1957), and the \"Desiccating Deep-basin Model\" by [PERSON] (1972). Figure 5 provides a schematic representation of these three models of evaporite deposition [58]. Figure 4: Lateral salinity gradient postulated by [PERSON] (1953) [53]. Figure 3: Evaporite deposition concept according to the Ring Theory [56]. The Fractional Sedimentation Theory, proposed by [PERSON] (1965), [PERSON] (1971), and [PERSON] (1983), describes the cyclic nature of evaporite deposition. This theory attributes cyclic evaporite deposition of substantial thickness to marine transgressions and regressions (Figure 6), resulting in various sequences of evaporite layers due to the precipitation of brines with differing mineral concentrations [56]. The theories reviewed in this section address diverse aspects of evaporite formation.