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 brine-rock interactions.