A genetic explanation for the anhydrite–halite cyclic layers in the Middle Jurassic Louann Salt, U.S. Gulf Coastal Region

Abstract

ABSTRACT Successions of bipartite layers (couplets) form the bedding fabric of salt stocks of the U.S. Gulf Coastal Region, south-central U.S.A. The cyclic couplets typically have a thickness range of 3–20 cm and a mean of about 10 cm. Each couplet consists of a white band of halite interstratified with a thinner, gray band of halite mixed with anhydrite (generally 3–8 wt % of each couplet). Successions of couplets, which lack siliciclastic beds and significant hiatuses, make up the thick (∼ 1.5 km) Middle Jurassic Louann Salt. Deeply buried (by up to ∼ 18 km), the Louann couplets moved laterally by ductile flow into the base of salt stocks and rose (or appeared to). The Dead Sea is a unique, modern analog of the Louann Salt. The huge lake is a model of how surface evaporation, brine temperature, and double-diffusive salt fingering combined to form the Louann couplets. Halite varves (couplets) presently precipitate from the Dead Sea’s deep (∼ 300 m), marine-like brine. If these modern varves are to be a valid genetic model for the Louann couplets, they too must be annual layers. An explicit similarity of the Louann couplets with two modern and four ancient varve-established evaporitic couplets demonstrates that the Louann anhydrite–halite couplets are truly varves. The Louann varves accumulated at such a rapid rate (∼ 1 m every 10 years) that a deep-water depositional basin must have pre-existed. In the basin, an abrupt destabilized thermocline existed in summer between a surface zone (tens of meters thick) of warm, high-salinity brine and a deep zone (hundreds of meters thick) of cooler, lesser-salinity brine. This disposition resulted in a vertical flux from the thermocline of alternating rising and sinking salt fingers (millimeter wide, centimeter long, and millions per km2). When heat diffused from the warm surface zone into the cooler fingers, their density decreased, they rose, and blended with the ambient brine; and when heat diffused from the warm fingers into the cooler deep zone, their density increased, they sank, and blended with the ambient brine. Salinity also diffused into and from fingers but at a rate ∼ 100 × slower than heat. In summer, the Louann surface zone was NaCl undersaturated because: 1) solar heating caused enhanced NaCl solubility, 2) ascending fingers transferred low-salinity brine into the higher-salinity surface zone, and 3) descending fingers transferred high-salinity brine into the lower salinity deep zone. The Louann surface zone, however, was CaSO4·2H2O supersaturated because of: 1) gypsum’s retrograde solubility and 2) gypsum concentration by evaporation in the surface zone. Gypsum cumulates rained from the surface zone onto the seafloor. As fingers descended from the thermocline into the Louann deep zone, their NaCl salinity state changed from saturated to supersaturated. Cumulate and bottom-growth halite precipitated, and the halite and gypsum accumulated together on the seafloor. With subsequent dehydration of the gypsum, the precipitates formed the thin, gray, anhydrite(halite) summer band of the Louann varves. In the fall, brine above the thermocline began to cool. By mid-winter, energized by wind and convection, the brine column overturned, became significantly cooler, and intensely NaCl-supersaturated. Halite cumulates crystallized throughout and rained onto the seafloor to form the thicker, white, halite winter band of the Louann varves.