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Significance of early-diagenetic water-rock interactions in a modern marine siliciclastic/evaporite environment: Salina Ometepec, Baja California

¹ Department of Geology, Amherst College, Amherst, Massachusetts 01002-5000, USA
² Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063, USA
³ Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA
⁴ Department of Earth and Environmental Sciences, Rutgers State University, Newark, New Jersey 07102, USA
⁵ Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063, USA

Although marine brines are a significant component of pore waters in sedimentary basins, there are few geochemical studies of modern analogues of such systems, especially in siliciclastic settings. For these reasons, we chose the evaporite-associated siliciclastic sediments deposited in the salt flats of the Salina Ometepec, Baja California, for an integrated investigation of sediment, pore-water, and overlying brine geochemistry. Here, the detrital components include quartz, K-feldspar, plagioclase, chlorite, biotite, and smectite, and authigenic minerals are dominated by halite, gypsum, and K-rich magnesium smectite.

Thermal and saline stresses on the sediments of the Salina Ometepec keep both organic and inorganic carbon concentrations in the sediments unusually low relative to other coastal marine environments. Sediment pore waters exhibit little microbial sulfate reduction, and dissolved inorganic C contents are also very low. As a result, we did not observe carbonate and sulfide mineral authigenesis in the Salina Ometepec sediments. Instead, pore-water geochemical evolution is largely controlled by evaporative concentration of seawater, evaporite-mineral dissolution and recrystallization, and diagenetic alteration of detrital aluminosilicates.

Evaporite-mineral recycling affects the compositional evolution of surficial brines even before they infiltrate the sediment. Specifically, Na⁺ and Cl^– concentrations are increased owing to halite dissolution. We see significant Br^– enrichment relative to expected seawater evaporation trends in near-surface pore water, secondary to dissolution of K- and Mg salts. Because bacterial sulfate reduction is inhibited in the Salina Ometepec sediments, sulfate concentrations are more accurate indicators of the degree of evaporation than Br^–, a usually conservative element during geochemical reactions.

Pore waters exhibit down-core increases in dissolved Mg²⁺, K⁺, and H₄SiO₄⁰ over the upper 1 m. Authigenic K-rich Mg-smectite formation is promoted by the concurrent processes of brine concentration, selective dissolution of K- and Mg-bearing salts, and dissolution of detrital aluminosilicates. Pore waters at a depth of 1 m have ⁸⁷Sr/⁸⁶Sr ratios that require input of Sr that is less radiogenic than that of Gulf of California seawater. This Sr is likely derived from weathering of detrital aluminosilicates from nearby volcaniclastic sources. These results show that significant chemical interactions among marine brines, evaporite minerals, and detrital aluminosilicates can occur relatively soon after sedimentation.

Key Words: diagenesis • evaporites • hydrogeochemistry • pore waters • salina