Abstract
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 sediments shows that fluviatile environment in flood plain stage is suggested for the deposition of the lower Shumaysi Formation. While the middle 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 between the eruptive centres as indicated by the enrichment with volcanic tuffs and precipitated silica. Finding of planktonic foraminiferal (Globigernoides) fossils at the bottom and mollusca fauna with plant remains (Oogonia) at the top gives an Oligocene-Early Miocene age for the upper Shumaysi Formation.
The heavy mineral suite is dominated by ultrastable constituents; zircon and rutile. The gross mineralogy of the heavy mineral assemblage remains unchanged throughout the succession. It reflects deeply weathered igneous rocks with dominant flat relief conditions in the Red Sea area during that time.
Shumaysi Formation appears to preserve a much earlier rifting history where Late Oligocene-Early Miocene sedimentary rocks are the earliest rr:cord of Red Sea rifting.