Biostratigraphic and Paleoecologic studies on the Maastrichtian–Early Eocene Benthonic Foraminifera of Central Western Desert, Egypt

Abstract

The present work deals with the biostratigraphy, paleoecology and microfacies analysis of the Maastrichtian-Early Eocene sequence exposed in Gebel Northern Gunna and El-Quss Abu Said of Farafra Oasis, Western Desert, Egypt. The Farafra Oasis lies in the central part of Western Desert, Egypt. The Maastrichtian-Lower Eocene rock units are widely distributed in and around the Farafra Oasis. The measured sections of the study area are located between Longitudes 28º 03׳ E and 27º 54׳, E and Latitudes 27º 11 ׳ N and 27º 04׳ N. The studied Maastrichtian-Lower Eocene rock units are represented by from base to top as; Khoman Chalk, Dakhla Shale, Tarawan Chalk, Esna Shale and Farafra Limestone. One hundred and twelve samples were collected from the studied rock units of Gebel Northern Gunna section (G1- G112). On the other side, ninety samples were collected from the rock units of El-Quss Abu Said section (S1-S90). All samples were treated by standard micropaleontological techniques. The identified fossil species were picked, photographed using the Scanning Electron Microscope (SEM) and illustrated in 18 plates. Representative forty five samples were selected for microfacies analysis. The collected rock samples were investigated megascopically and their thin sections were examined microscopically. The detailed textural characteristics of the studied carbonates were used to classify these rocks according to the classifications adopted by DUNHAM (1962). Also, the litho and biofacies characteristics of the different rock units were correlated with the standard microfacies types (SMF) of FLǕGEL(2004) and the facies zones (FZ) of the rimmed carbonate platform (WILSON, 1975) are determined. Lithostratigraphically, the Khoman Chalk is made up of a lower part (25m) of snow-white chalk and chalky limestone with some hard siliceous limestone and an upper part of argillaceous limestone (30m). 211 Also, this formation contains numerous species of small benthonic foraminifera, in addition to some macrofossils such as Pecten farafrensis and Ventriculites poculum. The Dakhla Formation overlies the Khoman Formation and underlies the Tarawan Chalk and consists of light green to dark gray shales and marl with siltstone, interbeds. The Tarawan Chalk overlies the Dakhla Shale and underlies the Esna Shale. It consists of whitish chalky limestone containing bands of marl. It is represented by samples (G65-G68) and its thickness attains 10m, in Northren Gunna sction, while in El-Quss Abu Said section consists of argillaceous and chalky limestone. It is represented by samples (S1-S7) and its thickness attains 16m. The Esna Shale overlies the Tarawan Chalk and underlies the Farafra Limestone. It is made up of green shale and marl enclosing carbonate intercalations and becomes more calcareous toward the top. It is represented by samples (G69-G109) and covering about 105m. thick in Northren Gunna section, while in El-Quss Abu Said section, it consists of dark green shales with gypsiferous bands and shale-limestone intercalations and becomes more calcareous towards top. It is represented by samples (S8-S86) that covering about 182m. thick. The Farafra Limestone is made up of very hard dolomitic limestone. It is represented by samples (G110-G112) and covering about 8m. thick in Northren Gunna sction. In El-Quss Abu Said section, it consists of hard limestone, and represents the samples (S87-S90) which reaching about 12m. thick.This formation is barren of planktonic foraminifera and contains a few smaller benthonic. The study of planktonic foraminifera was devoted for biostratigraphy and revealed ten biozones. The Maastrichtian was subdivided into two biozones; Globotruncana aegyptiaca and Gansserina gansseri, with missing for the Abathomphalus mayaroensis Zone and considering Globotruncanella havanensis Zone, to occupy the basal part of the Globotruncana aegyptiaca Zone. The Paleocene-Early Eocene interval was subdivided into eight biozones are ; Morozovella trinidadensis, Morozovella uncinata, Morozovella angulata, Planorotalites pusilla 211 pusilla, Planorotalites pseudomenardii, Morozovella velascoensis, Morozovella subbotinae, Morozovella formosa formosa. On the other hand, the study of benthonic foraminifera included the identification of two hundred and fifty four species 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: 1) Spiroplectammina knebeli for the lower part of Khoman Chalk (Middle Maastrichtian), 2) Bolivinoides draco draco for the upper part of Khoman Chalk (Late Maastrichtian), 3) Lagena hispida – Spiroplectammina dentata for Dakhla Shale (Early Paleocene), 4) Gavelinella rubiginosa - Spiroplecammina dentata for the Tarawan Chalk (Middle Paleocene), 5) Loxostomoides applinae - Vulvulina colei for the lower part of Esna Shale (Late Paleocene) and 6) Marginulina wetherellii Zone for the upper part of Esna Shale (Early Eocene).