GEODYNAMIC CONTEXT AND DEPOSITIONAL SEDIM ENTARY ENVIRONMENT OF THE MIOCENE STEVENSITE DEPOSIT (RHASSOUL) OF MOROCCO - LACUSTRINE OR EVAPORITIC ENVIRONMENT

The Miocene clay minerals called <<rhassoul>> and exploited in Morocco have been previously considered to be an authigenic stevensite formed contemporaneously in a closed evaporitic, gypsum-rich basin. A new in vestigation of the geodynamic context of the Missour basin, and the re lationship between the rhassoul clay and other depositional facies of the formation, demonstrates that the rhassoul did not form in an evapo ritic, gypsum-rich environment but rather in a fresh or brackish-water lacustrine environment. The basin deposits show evidence for distinct sedimentary stages: a lacustrine marry-carbonate stage containing the rhassoul, which follows a marly-gypsiferous evaporitic stage without rhassoul. The gypsum of the lower formation penetrated secondarily int o the rhassoul formation, had thus imparted a misleading evaporitic as pect. Laterally, toward the margin of the basin, these marry-carbonate and marly-gypsiferous facies pass rapidly into detrital fan-deltaic d eposits. This interpretation of the depositional environment of the rh assoul deposit as fresh or brackish water lake, rather than an evapori tic gypsum-rich environment, should allow a new interpretation for the genesis of stevensite.