الفهرس | Only 14 pages are availabe for public view |
Abstract The present study deals with benthic foraminifera of the Maastrichtain and Early Eocene rocks of the Farafra Oasis, central part of the Western Desert, Egypt. This study was achieved by measuring two stratigraphic sections in the study area; one in Gebel Northren Gunna and the other in Gebel El-Quss Abu Said. The stratigraphic study subdivided the Maastrichtain and Early Eocene succession into five rock units; Khoman Chalk, Dakhla Shale, Tarawan Chalk, Esna Shale and Farafra Limestone. The paleontologic study included the identification of 254 spp. of benthic foraminifera in the recognized rock units. Through the spread and distribution of the benthic foraminifera species, six time markers were recorded and correlated with the standard planktonic biozones. These time markers are: Spiroplectammina knebeli for lower part of Khoman Chalk (Middle Maastrichtian), Bolivinoides draco draco for the upper part of Khoman Chalk (Late Maastrichtian), Lagena hispida – Spiroplectammina dentata for Dakhla Shale (Early Paleocene), Gavelinella rubiginosa - Spiroplecammina dentata for the Tarawan Chalk (Middle Paleocene), Loxostomoides applinae - Vulvulina colei for the lower part of Esna Shale (Late Paleocene) and Marginulina wetherellii zone for the upper part of Esna Shale (Early Eocene). Also, eight larger benthic foraminiferal species were recorded in the Latest Paleocene-Early Eocene interval; Nummulites deserti, N. fraasi. N. solitaries, Alveolina decipiens, A. pasticillata, Operculina libyca, Assilina farafraensis and Discocyclina nudimargo. The paleoecologic study included the calculation and representation of different parameters; the total number of foraminiferal species, the total number of small benthic foraminifera, the total number of planktonic foraminifera and the planktonic / benthic ratio. The interpretation of these parameters revealed an open marine environment with a decrease in the sea water depth in the lower part of Khoman Chalk and an increase in the sea water depth in the middle and upper parts of Khoman Chalk. The obtained results for the Dakhla Shale and Tarawan Chalk reflect the continuing of open marine conditions with a relatively shallow environment. In the lower part of Esna Shale, it was noticed a decrease in the sea water depth due to the flooding occurrence of larger benthic foraminifera; Nummulites deserti, N. fraasi, N. solitarius and Alveolona pasticillata. These larger benthonic foraminifera contributed to a great extent in shallowing sea water during the Latest Paleocene-Early Eocene time. These shallow conditions were followed by an increase in the faunal content indicating to a relatively deeper environment at the end of the lower part of Esna Shale. These open marine conditions decrease gradually in the middle and upper parts of Esna Shale, reflecting a decrease upwardly in the sea water depth till the Farafra Limestone which represents a shallow carbonate deposition. The microfacies analysis revealed that Khoman Chalk is dominated by wackestone facies which graded upward to less dominated packstone facies. The Dakhla Shale consists mainly of highly calcareous shale facies with intercalations of highly argillaceous limestone (packstone). The Tarawan Chalk made up of wackestone/packestone facies which graded upward at El-Quss Abu Said Section to packstone facies. Esna Shale consists mainly of highly ferruginous calacreous shale facies with intercalation at its lowermost part of highly argillaceous wackestone (Northern Gunna section) and grainstone with less dominant packstone (El-Quss Abu Said Section). Meanwhile, the uppermost part of Esna Shale at El-Quss Abu Said Section is intercalated with dolomitic wackestone and packstone. The Farafra Limestone is dominated by the wackestone facies which graded downward to less dominated grainstone facies at Northern Gunna section. At El-Quss Abu Said Section the Farafra Limestone made up of dolomitic packstone which intercalated with wackestone/packstone. The facies zones of the studied microfacies graded from deep sea to deep shelf and deep shelf margin during deposition of Khoman Chalk and Dakhla Shale. These facies zones were repeated during deposition of Tarawan Chalk and Esna Shale. The open marine platform was the facies zone during deposition of Farafra Limestone. |