DO SECULAR VARIATIONS IN SEAWATER CHEMISTRY CONTROL THE MAJOR ION CHEMISTRY OF BASINAL BRINES?

The hypothesis that basinal brines in the deep subsurface of sedimentary basins inherited their chemistries and high salinities from evaporated paleoseawaters was tested by comparing the major ion chemistries of formation waters in Silurian and Jurassic/Cretaceous host rocks, when the world oceans were “CaCl₂ seas”, with those from Permian-Pennsylvanian rocks when the world oceans were “MgSO₄ seas”. Eight of the nine site examples tested have large evaporite deposits of one known age: Silurian (Michigan basin, Illinois basin, Appalachian basin in Ohio), Jurassic/Cretaceous (Central Mississippi Salt Dome basin, Arkansas shelf, and south central Texas), and Permian/Pennsylvanian rocks (Palo Duro basin, Central Basin Platform, and Delaware basin, Texas and New Mexico). Basinal brines examined are assumed to have originally formed from evaporation of the same seawaters that produced major evaporites. Sulfate, Mg, and K in basinal brines are all below the concentrations expected from evaporation of any seawater type, indicating important diagenetic controls on basinal brine chemistry. Basinal brines in Pennsylvanian-Permian rocks are chemically distinct (relatively high Na and low Ca) from basinal brines in Silurian, Jurassic and Cretaceous host rocks, which may reflect original differences in seawater chemistry of these different ages. Basinal brines highly enriched in Ca and depleted in Na relative to evaporated seawater of any type have commonly been interpreted to have formed by albitization of plagioclase feldspar. Another explanation for extreme Ca-enrichment and Na-depletion of basinal brines is mixing of evaporated CaCl₂-type seawater with more dilute water. Some basinal brines are quite close in major ion composition to evaporated seawater of a particular age. Basinal brines, for example, in the Cretaceous Edwards Group carbonates, Texas, only require dolomitization to convert evaporated Mesozoic seawater into Edwards Group brine. Basinal brines in Silurian rocks, Illinois basin, may have obtained their present chemical composition by evaporation of Silurian seawater, dolomitization of limestone and loss of K through precipitation K-feldspar.