Upper Paleocene-Lower Eocene Calcareous Nannofossil Boistratigraphy and Paleoecology of Gebel Matulla Section, Southwestern Sinai, Egypt
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THE FIFTH INTERNATIONAL CONFERENCE ON THE GEOLOGY OF AFRICA Vol. (1), P-P.VII-33 – VII-51 (OCT. 2007) ASSIUT-EGYPT UPPER PALEOCENE-LOWER EOCENE CALCAREOUS NANNOFOSSIL BOISTRATIGRAPHY AND PALEOECOLOGY OF GEBEL MATULLA SECTION, SOUTHWESTERN SINAI, EGYPT Abu Shama, A. M.1, Faris, M.2 and Al-Wosabi, Kh. A.3 [1] Biological and Geological Sciences Department, Faculty of Education at Kafr El Sheikh University, Egypt, E-mail: [email protected] [2] Geology Department, Faculty of Science, Tanta University, Egypt, E-mail: [email protected] [3] Taiz University, Yemen, e-mail: [email protected] ABSTRACT The calcareous nannofossil biozones NP7/8 through NP12 were identified from the Tarawan, Esna and base of the Thebes formations at Gebel Matulla, west central Sinai. The Subzone NP9a spans the Tarawan/Esna contact indicating a conformable relationship between the two formations, which in further emphasized by a gradational lithologic characters in the field. The Zone NP10 is easily differentiated into 4 subzones (a-d). The NP9/NP10 zonal boundary can be identified at the level of increase frequency of Neochiastozygus junctus. Zone NP11 occupies the topmost part of the Esna Formation and the base of the Thebes Formation indicating conformable relation between them. The Paleocene/Eocene boundary is traced between the NP9a/NP9b subzonal boundary which is marked by the first appearances of Rhomboaster spp. It is located at the basal part of the Esna Formation and conforms lithologic change from blackish calcareous shale to brown calcarenite. Kaolinite peak and abrupt increase in calcium carbonate content is also observed. On the basis of nannofossils, the Paleocene Eocene Thermal maximum (PETM) interval can be divided into two parts (lower and upper). The major assemblage shift across the P/E boundary suggests a change from colder, more productive surface waters to warmer, more oligotrophic conditions. Above the PETM interval, the niche of Fasciculithus spp. was filled by Neochiastozygus juncts and Rhabdolithus solus, the cooler and more eutrophic taxa. INTRODUCTION The late Paleocene/early Eocene interval is characterized by major global climatic and environmental changes that reflect various paleontological, sedimentological, mineralogical and isotopic variations. In Egypt, the Paleogene rocks are widely distributed in the Eastern Desert, Western Desert, Nile Valley and Sinai. These rocks offer a good chance to studying the P/E boundary events. In the central Nile Valley, the Paleocene-Eocene succession reaches up to 2km thick and attracted the attention of many geologists since many decades. A Global Standard Stratotype- section and Point (GSSP) for the Paleocene/Eocene has been proposed recently in the lowermost part of the Esna Shale at the Dababiya section in central Egypt (Aubry et al. 2002). In Sinai, the Paleocene-Eocene succession is greatly reduced in thickness. Calcareous nannofossils of some Paleocene-Eocene rocks in Sinai were studied before and after the determination of the late Paleocene/early Eocene GSSP such as that of El-Dawoody, 1992; Bolle et al., 2000; Faris et al., 2000; Faris and Zahran, 2002; Abu Shama, 2002; Ayaad et al., 2003 and Faris and Abu Shama, 2007). The previous studies on the calcareous nannofossils of the Paleocene-Eocene rocks should be correlated with the Dababiya section. The main purpose of this work is: 1) to present a study of the calcareous nannofossil biozonation for the Tarawan, Esna and the base of the Thebes formations at G. Matulla section. VII-34 Abu Shama, A. M., Faris, M. and Al-Wosabi, Kh. A 2) definition of the Paleocene-Eocene boundary according to the last reports of the International Subcommission on Paleogene Stratigraphy (ISPS). 3) to investigate the calcareous nannofossil events related to paleoecologic conditions at this boundary. MATERIAL AND METHODS At Gebel Matulla, Abu Zenima area, west central Sinai (Fig. 1), 55 samples are collected from the upper Paleocene-lower Eocene rocks at intervals between 30-70 cm. Smear slides were prepared from well-mixed and unsettled sediment of 1gm suspension. No ultrasonic cleaning or centrifuge concentration was applied in order to maintain the original nannofossil composition. Nannofossils were observed in the light microscope at 1250X magnification and identified following the taxonomic schemes of Perch-Nielsen (1985). For statistical analyses across the Paleocene/Eocene boundary, 50 views of smear slides were counted and analyzed (Table 1). Carbonate content and clay mineralogy of the carbonate free fraction are studied around the P/E boundary. Fig. 1: Location and stratigraphy of Gebel Matulla. LITHOSTRATIGRAPHY In Central Egypt, the Paleocene-Eocene sequence is generally subdivided into Dakhla, Tarawan, Esna and Thebes formations respectively from base to top. In west central Sinai, this sequence is recorded in many exposures but not as thick as in central Nile Valley. In Sinai, because of this many authors dealt with Dakhla Formation, Tarawan Chalk and Esna Shale as one unit termed Esna Shale (Formation). However, in Abu Zenima area, Anan (1992) dealt with these units as recognizable formations. In the present work, the base of Gebel Matulla is composed of Sudr Formation that overlain by Dakhla, Tarawan, Esna and Thebes formations, respectively (Fig. 1). The Tarawan Formation (Awad and Ghobrial, 1965) At Gebel Matulla, the total thickness of the Tarawan Formation is about 4.1 m. It is composed of 2.5m of cliff forming yellowish limestone that grades upwards and downwards into marly limestone. The cliff forming yellowish limestone is dissected by two thin (about 3- Upper Paleocene-Lower Eocene Calcareous Nannofossil Boistratigraphy ,…. VII-35 5cm) disconnected chert bands in its middle part. The carbonate contents range from about 70 to 84%. In the present study, the contact between the Tarawan Formation and the overlain Esna Formation is arbitrary put below sample no. 74, where the carbonate content decreases to about 60%. However, the lithologic boundary between the Tarawan and the Esna formations at the Dababiya section was arbitrary put by Dupuis et al. (2003) at level where the carbonate content falls bellow 50%. Table 1 : Changes in the abundance of species across the Paleocene/Eocene boundary, G. Matulla section, Egypt. VII-36 Abu Shama, A. M., Faris, M. and Al-Wosabi, Kh. A The Esna Formation (Beadnell, 1905) At Gebel Matulla, the Esna Formation consists of about 20m of gray to greenish calcareous shale and marls. It conformably overlays the Tarawan Formation and underlies the Thebes Formation. It is disconnected by brown calcarenite bed (at ~2.1m above the base) and three beds of very hard marlstone to limestone all form four conspicuous ledges (stringers) due to differential weathering (Fig.1). The lower part of the Esna Formation is mostly composed of calcareous shale where the upper part consists of nodular marls. At the Dababyia section, the Esna Shale is divided by Dupuis el al. (2003) into three informal rock units: a lower unit (Unit Esna1); a middle unit (Unit Esna 2); and an upper unit (Unit Esna 3). The Thebes Formation (Said, 1960) The basal part of this Formation at G. Matulla consists of chalky limestone. A gradational contact between the Esna and Thebes formations is present in studied section. The lithologic boundary between them is arbitrary put at the limestone stringer which lies about 2m below the cliff forming chalky limestone. Calcareous nannofossil Biozones: The standard zonation scheme of Martini (1971), complemented by Aubry (1996) and Aubry et al. (1999) was followed in this work (Fig. 2). At Gebel Matulla, the calcareous nannofossils are generally abundant and well to moderately preserved. The stratigraphic distribution of the calcareous nannofossil species and biozones are presented in Figures 3. Some of calcareous nannofossils are illustrated (Plates 1, 2). Fig. 2: Proposed latest Paleocene-earliest Eocene nannofossil magneto-biochronology correlated with the present work. Upper Paleocene-Lower Eocene Calcareous Nannofossil Boistratigraphy ,…. VII-37 1-Discoaster mohleri Zone (=NP7/8): Hay (1964) emend. Romein (1979). Zones NP7 and NP8 are difficult to be differentiated in case of the absence of the marker species Heliolithus riedelii at the top of Zone NP7 (Romein, 1979). The base of this interval is defined at the lowest occurrence (LO) of Discoaster mohleri where its top is defined at the LO of Discoaster multiradiatus. It occupies the Tarawan Formation and is represented by Samples 67-72 with total thickness of about 3.6m. Samples 68-71 are composed of cliff forming yellowish cherty chalky limestone (~ 2.6m thick.) whereas Samples 67 and 72 (~ 1.0m) compose of white gray marly limestone (Fig. 1). Fig. 3: Stratigraphic distribution of the calcareous nannofossil species at Gebel Matulla section, west central Sinai, Egypt. VII-38 Abu Shama, A. M., Faris, M. and Al-Wosabi, Kh. A Fig. 3: Continued In east central Sinai, Zone NP7/8 was recorded by Faris and Abu Shama (2007) from the hard yellowish brown limestone stringer (1m) intercalating the Esna Formation at Gebel Mishiti. According to these authors, this limestone stringer is equivalent to the Tarawan Formation in the present study. The calcareous nannofossil assemblages are well to moderately preserved and they are abundant. Discoaster okadi, Ericsonia robusta and Discoaster nobilis are recorded from the top of this zone. The upper part of this zone displays common abundance of genera Fasciculithus; Sphenolithus and Ericsonia (Fig. 3). These taxa reflect relatively warm and, probably oligotrophic surface water. 2- Discoaster multiradiatus Zone (=NP9): Bramlette & Sullivan (1961), emend. Martini (1971) and Bukry & Bramlette (1970). The base of this zone is well defined by the LO of Discoaster multiradiatus where its top is marked by the LO of Tribrachiatus bramlettei. In the absence of the primary index species for NP10, the top of this zone is provisionally placed at the highest occurrence (HO) of Fasciculithus involutus following Perch-Nielsen (1985). However, in the present study few reworked specimens of F. involutus and other Fasciculithus spp.