MICROFACIES ZONATION AND SPATIAL EVOLUTION OF A CARBONATE RAMP - MARGINAL MOROCCAN PHOSPHATE SEA DURING THE PALEOGENE

The paleoenvironment of the southwestern portion of the Paleogene Moro ccan phosphate sea has been reconstructed in detail by microfacies stu dies for a complete transgressive-regressive cycle. In front of the An ti-Atlas, forming the southern border of the Atlantic gulf, a bioclast ic and phosphatic carbonate ramp extended to the north. The ramp was a homoclinal, very gentle dipping surface. Four stages of ramp evolutio n can be distinguished. They are (1) the marginal marine initial stage , (2) the marine transgressive stage, (3) the marine regressive stage and (4) the final silting up stage. The ramp is characterized by a dis tinct facies zonation. A sabkha plain, documented by mudstones and dol ocretes, was situated between the arid hinterland and the ramp. Nearsh ore, open lagoons existed, populated by chlorophycean algae and miliol id foraminifera. These environments were protected from the open sea b y a chain of oyster banks. A belt of echinoid enrichments probably mar ked the wave base. Sea areas with deeper and only temporarily, highly- agitated water are characterized by a benthic faunal assemblage of var ied molluscs, with bryozoans becoming more abundant to the east. The c arbonates are intensively bioturbated and interfinger with phosphorite sands, which dominated in the deepest part of the ramp. All phosphori tes studied are clastics, poor in matrix and are not associated with p hosphate source rocks. A process of extensive reworking, transport and redeposition, which is related to transgressions and regressions, has to be assumed.