Petroleum/Tectonics Groups of the Geological Society, London SALT TECfONICS 14-15 September 1994 Early initiation of salt pillows in evaporite deposits of two continental Neogene basins in Central Spain.

Classical halokinetic models consider tIlat a thick overburden is necessary before a salt pillow begins lo develop (Trusheim, 1960). However,!he initiation of diapirs is one of the least understood aspects of salt tectonics, and recent studies high1ight the fact that salt might begin to move soon after deposition (Jackson & Vendeville, 1994). Two cases are described in Central Spain, where ear1y initiation of salt pillows occurs: the Madrid and lbe Calatayud basins (ME & CB; Fig. 1). In these basins, the endorreic Neogene sedímentary sequence is characterized by Ibree units of continental derived deposits: 1) a "Lower Unit " comprising up to 500 m Aginian/Aragonian sediments wilb a centripetal facies distribution Le. detrital deposits in the borders passing lo evaporites in the center; and 2) up lo 150 m of detritic and carbonatic sediments constituting the "Intermediate and Upper Units " of the Miocene eyc1e in Central Spain. Several arguments suggest the early bu1ging of the evaporites forming a paleorelief in Ibe central sectors of lbese basins, before and during the deposition of the "Intermediate Unít": 1) Paleokarsts with exokarstic depressions filled with "in situ " sediments develop in the roof of Ibe evaporitic unit; 2) The initial sediments of the "Intermedíate Unit " correspon