Core study of the Oldest Halite (Na1) deposits from wells documenting the Mechelinki rock salt deposit (Werner, 1975) and from leaching wells of the Underground Gas Storage Kosakowo (Czapowski & Tomaszczyk, 2012) enabled construction of the preliminary 3D model of deposit structure (Fig. 1). Preliminary structural and genetic conclusions are as follow: 1) the Oldest Halite (Na1) seam is composed of two salt facies (Czapowski, 1987; Czapowski et al., 1993): the thicker deposits of open salt basin (OBS; mainly monomorphic to polymorphic and giant transparent halite types, rhythmically laminated with anhydrite laminae) in the lower part of succession and overlain by the deposits of salt lagoon (LS; dominant polymorphic halite with anhydrite concentrations). Such profile resulted from successive infill by chlorides the bathymetrically differentiated salt basin, with an inadequate bottom subsidence caused the consequent basin shallowing; 2) in the lower part of salt seam in the whole deposit occur two correlable layers of intrasalt anhydrite (A1s1 and A1s2), several-several tens cm thick, at the almost constant depth. They registered the longer periods of brine dilutions and become the significant obstacle during caver leaching; 3) in the top of underlain deposits of Lower Anhydrite (A1d), north-eastward from wells K-1, K-2, K-5 and Me4, is visible a linear depression, 4) such intraseam salt shifting (Wilkosz et al., 2012) was confirmed by occurrence of 2-3 zones with increased dip of salt and sulfate laminae and layers, located in the bottom and the upper part of salt seam. Similar disturbances were registered in the seam profiles (Me-1, Me-2 and Me-5 wells), placed on the uplifted top of Lower Anhydrite unit
