PALEONENVIRONMENTAL INTERPRETATION OF SYNRIFT AND PRERIFT SEDIMENTS OF THE EASTERN FLANK OF THE CENTRAL RED-SEA, JEDDAH REGION, SAUDI-ARABIA

Field examination, sedimentological and paleontological investigations have been carried out on the elastic Shumaysi Formation exposed east of Jeddah region. Paleoenvironmental interpretation of prerfit basin s ediments shows that fluviatile environment in flood plain stage is sug gested for the deposition of the lower Shumaysi Formation. While the m iddle Shumaysi Formation was deposited in fresh water lake environment . The upper Shumaysi Formation (synrift sediments) represents an early transgressive marine environment with final fresh water lake in betwe en the eruptive centres as indicated by the enrichment with volcanic t uffs and precipitated silica. Finding of planktonic foraminiferal (Glo bigernoides) fossils at the bottom and mollusca fauna with plant remai ns (Oogonia) at the top gives an Oligocene-Early Miocene age for the u pper Shumaysi Formation. The heavy mineral suite is dominated by ultra stable constituents; zircon and rutile. The gross mineralogy of the he avy mineral assemblage remains unchanged throughout the succession. It reflects deeply weathered igneous rocks with dominant flat relief con ditions in the Red Sea area during that time. Shumaysi Formation appea rs to preserve a much earlier rifting history where Late Oligocene-Ear ly Miocene sedimentary rocks are the earliest rr:cord of Red Sea rifti ng.