Sedimentary processes controlling halite deposition, Permian Basin, Texas
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Evaporites preserve considerable information about their depositional and diagenetic history. The facies and geochemistry of two exceptionally well-preserved evaporite sections within the Permian Basin the Guadalupian to Ochoan shelf environments of the Palo Duro Basin and the Ochoan section of the rapidly infilled Delaware Basin are examined as case studies in evaporite sedimentology. Key depositional processes documented by evaporite fabrics are 1) synsedimentary evaporite recycling and 2) development of stratification because of density contrasts in saline brines. Shallow-water evaporite deposition is characterized by efficient circulation of surface water to the floor of the brine pool. Bottom-growth predominates during evaporation; halite dissolution during flooding creates insoluble residues beneath anhydrite or carbonate. Diagenesis of shallow water evaporites is driven by brine evolution in the depositional environment. Deeper water deposition is characterized by isolation of surface water from basin floor sediments. Isolation is apparent during evaporite precipitation, when cumulate deposition dominates; during sea level rise, when sulfates or carbonates precipitate from surface water and are deposited on halite without dissolution; and during sea level fall, when higher salinity brine superseded lower salinity brines but cause minimal cementation and diagenesis in previously-deposited sediments, indicating minimal interaction between sediment and brine. The composition of evaporite brines documented by analysis of evaporites deposited across a facies tract depends on 1) sources of water, 2) sources of solutes, and 3) modification by diagenetic reaction of brines within the evaporite environment. Marine-dominated evaporites are characterized by marine ⁸⁷Sr/⁸⁶Sr and [partial derivative symbol] ³⁴S in anhydrite. Bromide concentrations in halite reflect lateral evolution of brine in response to evaporation and halite precipitation. Oxygen and hydrogen isotopic composition of brine inclusions in halite indicate a dominant contribution of evaporated marine brine. Southward expansion of evaporite environments during the Permian progressively decreased marine input, increased meteoric contribution, and increased synsedimentary recycling and rock-water interaction. ⁸⁷Sr/⁸⁶Sr and [partial derivative symbol] ³⁴S signatures in anhydrite vary from marine to highly modified; Br concentrations in halite decrease to less than 30 ppm, and brines in fluid inclusions have meteoric oxygen and hydrogen isotopic compositions.