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L~te Paleogene rocks in the Fayum Depression of northern Egypt consist of the nearshore marine and fluvial upper Eocene Qasr el Sagha Formation, the conformably overlying, dominantly fluvial Oligocene Jebel Qatrani Formation, and the Widan el Faras Basalt. In its type area, the upper part of the Qasr el Sagha Formation (Dir Abu Lifa Member) consists of 77 meters of nearshore marine sandstones and alluvial lateral accretion channel deposits that record sporadic but gradual late Eocene regression of the Tethys Sea and the western advance of alluvial plain conditions. The Jebel Qatrani Formation is composed of about 340 meters of variegated alluvial rocks: fine to coarse sandstone, granule and pebble conglomerate, sandy mudstone, carbonaceous mudstone, and limestone. Most of these rocks show evidence of profound geochemical and mechanical alterations due to ancient soil (paleosol) formation. The lower part of the Jebel Qatrani Formation is dominated by coarse, multistory, multilateral sand bodies that record deposition by meandering streams under essentially quiescent tectonic conditions. In contrast, the upper part of the formation contains both a greater proportion of fine sandstone and mudstone and more discontinuous sand bodies. These attributes point to deposition in a more active tectonic setting, under the influence of an encroaching marine strandline. Nearshore marine conditions prevailed in some areas of deposition of the upper part of the Jebel Qatrani Formation, but this advance was both minor and short lived and was followed first by a return to alluvial deposition and then by regional erosion. This erosive episode might have been instigated by local uplift or by more regional tilting of the alluvial plain, and erosion was followed by increased tensional stress resulting in normal faulting and basalt extr~sion (Widan el Faras Basalt). In the Jebel Qatrani area, the earhest basalt flows are dated at 31.0± 1.0 million years (late early or early late Oligocene). It is known that early and middle Eocene sedimentation in much of the Western Desert of Egypt consisted of the filling of synclinal basins that formed during the Late Cretaceous. By the end of the middle Eocene, most of these basins on the greater Tethyan shelfhad been filled; however, this research suggests that an inner basin in the Fayum area aligned essentially west-southwest-east-northeast remained active, or at least continued to receive an extraordinary thickness of sediment during the late Eocene and early Oligocene. Because late Eocene-early Oligocene rocks thin markedly to the north and because paleocurrent data indicate deposition by princi~ally west-flowing streams, the Fayum Basin was almost certainly Isolated from the Tethys Sea on the north by highlands. Owing to these topographic constraints, streams during Dir Abu Lifa time and 1University of Colorado, Boulder, Colo. most of Jebel Qatrani time gained access to the Tethys Sea by flowing more or less westward, eventually debouching into coastal mangrove swamps in the vicinity of the El Bahr Depression. Coarser clastics of the Dir Abu Lifa Member of the Qasr el Sagha Formation are dominated by mediumto fine-grained quartz sand, whereas those of even the lowest rocks of the Jebel Qatrani Formation show a major influx of distinctive second-cycle sediment (especially chert granules and pebbles) as well as primary clasts (including relatively unabraded feldspar crystals) derived from felsic igneous rocks. This evidence indicates that sediment sources for these rocks were much closer than the several hundred kilometers away in Upper Egypt that has been suggested by earlier workers. Paleocurrent data show that these source areas must have lain to the east of the Fayum Depression, probably in the vicinity of El Galala el Bahariya in the Eastern Desert. In Oligocene times, the Fayum area was a tropical to subtropical lowland coastal plain characterized by an abundance and variety of vegetation. Mangrove swamps dominated the Oligocene coastline and gave way landward to a forested interior that supported many varieties of trees, large vines, legumes, marshy plants, and aquatic ferns. Many of these floral elements resemble tropical to subtropical species of the present-day Indomalaysian region. Paleosols show evidence of generally damp soil conditions but indicate that rainfall was probably seasonal (monsoonal). Freshwater limestones containing ostracodes and charophytes record the local formation of broad, shallow floodplain ponds. This forested coastal lowland was broken by several meandering streams which debouched westward and northwestward, a short distance away, into the Tethys Sea. The alluvial plain had little relief, and storms or tidal incursions introduced saline water into the streams for several kilometers inland, causing these waters to be cohabited by marine fish, sharks, skates, rays, mollusks, and crabs, as well as by crocodiles, sirenians, and freshwater turtles, fish, and mollusks. The Jebel Qatrani forest supported a large and varied vertebrate fauna dominated by rodents, browsing herbivores, carnivorous mammals, and arboreal quadrupedal higher primates. A large fauna of terrestrial invertebrates is recorded by trace fossils; those of wasps or bees, crayfish, ants, and subterranean termites are especially well represented. In the Fayum area, the tropical to subtropical lowland coastal plain environment typical of the Jebel Qatrani Formation persisted well into the Miocene, and rocks of the dominantly alluvial Kashab Formation closely resemble those of the Jebel Qatrani lithologically. By latest Oligocene and early Miocene time, sedimentation had largely filled the Fayum Basin and streams were at last able to enter the nearby Tethys Sea directly from the south. Periodic and localized marine incursions typified the Miocene depositional r-egime until they finally prevailed in the middle Miocene with deposition of the marine Marmarica Formation in the northern part of the Western Desert.

Geology and paleoenvironment of the Oligocene Jebel Qatrani Formation and adjacent rocks, Fayum Depression, Egypt