Sedimentology and diagenetic evolution of the Neogene ‘Intermediate Sandstone Unit’ in the halite deposits of the Granada Basin (SE Spain): the turning point in the change from marine to continental sedimentation
Résumé
The Granada Basin is a small (50 × 50 km) Neogene intramontane basin located in the central part of the Betic Cordillera (Spain). In the latest Tortonian, the Granada Basin desiccated and a thick salt succession formed, encompassing three halite-bearing units: the ‘Lower Halite Unit’, the ‘Intermediate Sandstone Unit’ (ISU), and the ‘Upper Halite Unit’ (UHU). ISU deposits record the onset of marine to non-marine conditions in the Granada Basin. The main purpose of this paper is to study the environment of formation and the diagenetic evolution of the ISU salt-bearing unit, in order to assess the events leading to and resulting from the continentalization of this basin in the late Miocene. This study includes visual core descriptions, petrographic (conventional petrography and scanning electron microscopy), and geochemical (δ34S and δ18O and 87Sr/86Sr) analyses. ISU deposition took place in a coastal lake, isolated from the open sea by a sand barrier. Lake evolution was from a very shallow, hypersaline lacustrine environment to a deeper, perennial lake undergoing frequent storm-induced marine flooding and, finally, to a shallower, perennial saline lake. Isotope analyses point to a mixture of different inflow waters, including marine- and underground (hydrothermal)-water inputs for the origin of the brines. Halite dissolution occurred after flooding events and clear-halite cement was precipitated inside primary-halite dissolution cavities. Early diagenesis involves halite re-crystallisation during repetitive, dissolution–precipitation cycles, gypsum replacement by halite, halite replacement by nodular anhydrite, and framboid pyrite formation. Intermediate- to late-diagenetic processes are silica (megaquartz, chalcedony, and lutecite) replacement of halite and anhydrite, and celestine replacement. Megaquartz formation relates to sulfate-depleted, UHU percolating brines. Chalcedony and lutecite crystallization took place sometime later from sulfate-rich percolating brines, during deposition of the gypsum sequence occurring on top of the salt. Celestine, replacing lutecite, resulted from the interaction with Sr-rich underground waters (via dissolution of previously formed celestine).