Secular variation in seawater chemistry and the origin of calcium chloride basinal brines

CaCl₂ basinal brines, which are present in most Phanerozoic sedimentary basins, inherited their chemistries and salinities from evaporated paleoseawaters when the world oceans were Ca rich and SO₄ poor (CaCl₂ seas). CaCl₂ seas coincided with periods of rapid seafloor spreading, high influxes of mid-ocean-ridge brines rich in CaCl₂, and elevated sea levels, conditions that favored accumulation of marine CaCl₂ brines in marginal and interior continental basins. Typical basinal brines in Silurian-Devonian formations of the interior Illinois basin, United States, show the same compositional trends as those of progressively evaporated CaCl₂-rich Silurian seawater. Chemical deviations can be accounted for quantitatively by brine-rock reactions during burial (dolomitization, dolomite and K-feldspar cement . This explanation for the origin of CaCl₂ basinal brines contrasts with others that assume constancy of seawater chemistry and involve more complex brinerock interactions.