"Barren bodies" are meter-size lens-like features in sylvite beds where sylvite is absent or depleted. They are commonly found when mining potash deposits. Most barren bodies are believed to be formed by selective dissolution of sylvite, due to the circulation of diagenetic or later fluids. Contrary to this commonly favored postdepositional interpretation, barren bodies of the Subiza mine, Navarra, Spain are thought to be the result of synsedimentary processes. This potash deposit contains a 100 m thick Upper Eocene succession of alternating claystone and evaporites (sulfate, halite, and sylvite). The evaporites accumulated in an elongated basin forming one of the depocenters of the 250 km long South Pyrenean foreland basin. Along the margin of the basin, slope instability, promoted probably by tectonism, created mass wasting, forming mounds 0.5-2 m high and tens of meters in extension. As evaporation progressed, two stratified brines formed. Halite precipitated at the air-brine interface and sank to the bottom of the basin with terrigenous clays. Sylvite, however, precipitated from the lower brine. The mounds extended into the upper brine, thus sylvite did not precipitate over these upper zones. With progressive deposition the lower brine covered the mounds, the sylvite beds overlapping the mounds. The mineralogical and petrographical features of the barren bodies and their surroundings are continuous. Analyses of primary fluid inclusions from the halites of the barren bodies show a Mg-K-Cl rich composition. This corresponds to a primary brine and is unlike the Na-Cl rich brines expected from replacement processes. The thickness of the sylvite-forming brine (<1 m) inferred from the mounds is inconsistent with the thickness required to form centimeter-thick beds of sylvite in a closed basin. Alternatively, the existence of a more restricted subbasin, open to a halite-forming basin similar to that observed in saline ponds, is proposed. Numerical simulations of the evaporation processes under these conditions give results that match the mineral associations observed and the solute content of the fluid inclusions.