Brine-issuing vents have been observed at 1920 m water depth on top of Green Knoll, an isolated salt diapir rising seaward of the Sigsbee Escarpment in the northern Gulf of Mexico. The brines are seven times saltier than ambient sea water. Their venting, both by lateral flow and by diffusion upward through the underlying sediments, gives rise to drainage patterns similar to braided streams. Brine transport pathways are floored by unindurated red-orange deposits similar to the iron oxide deposits of the Red Sea brine pools. The relations between major and minor constituents (Na, K, Cl, SO4, Br, and Li) and the δ18O compositions indicate that brines are formed by congruent sea-water dissolution of the underlying Louann salt containing a mixture of halite, sylvite, and anhydrite. Net removal of Mg corresponding to a proportional increase in Ca suggests that dolomitization is active in the subsurface. Ferrous iron is extracted from the salt diapir and precipitated as ferric hydroxide on floors of drainage networks during the venting and mixing of the brines with the oxygen-rich ambient water. The Green Knoll brines, whose chemistry is uncompromised by severe diagenetic overprints, could serve as an end-member analogue for formation waters that have acquired complex chemical histories during their long migration in sedimentary basins.