Journal of African Earth Sciences 47 (2007) 1–8 www.elsevier.com/locate/jafrearsci Palynology of some Cretaceous mudstones from southeast Aswan, Egypt: significance to regional stratigraphy Magdy S. Mahmoud *, Mahmoud A. Essa Geology Department, Faculty of Science, Assiut University, Assiut 71516, Egypt Received 2 February 2006; received in revised form 5 October 2006; accepted 18 October 2006 Available online 30 November 2006 Abstract The basal mudstones from the El-Nom borehole in the Gebel Abraq area in southern Egypt have yielded a diverse and relatively well preserved terrestrial palynoflora that includes Balmeisporites holodictyus, Crybelosporites pannuceus, Foveotricolpites gigantoreticulatus, Nyssapollenites albertensis, Retimonocolpites variplicatus and Rousea delicipollis. These suggest an Albian–Cenomanian age and deposi- tion in a fluvio-deltaic environment; no marine phytoplankton is reported. The fern-dominated palynoflora and the overwhelming pres- ence of kaolinitic clays suggest a warm, humid palaeoclimate. According to available knowledge, the mudstones in the Gebel Abraq area, equivalents of the so-called ‘‘Timsah Formation’’, might be correlated with an older rock unit, the Maghrabi Formation, based on the new palynological age assessment. This new definition of local stratigraphy implies that the Bernice sheet of geological map of Egypt [Klitzsch, E., List, F., Po¨hlmann, G., 1987. Geological map of Egypt, sheet NF 36 NE Bernice, 1: 500000. Conoco and the Egyptian General Petroleum Corporation, Cairo] ought to be reconsidered. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Terrestrial palynology; Stratigraphy; Cretaceous; Egypt 1. Introduction and geological setting The siliciclastics of the ‘‘Nubian Sandstone’’ rocks are of predominantly continental origin. They are widely exposed This study was conducted as part of a sustainable devel- in central and southern Egypt, and are mostly Cretaceous in opment project in southern Egypt in a trial to improve the age. These rocks are overlain by marine upper Cretaceous quality of living of the Ababda inhabitants in the Eastern rocks of the Duwi (phosphate) and Dakhla formations. A Desert of Egypt. One of the main objectives of this project formal subdivision of the ‘‘Nubian Sandstone’’ in central is to explore underground water for drinking and irrigation. and southern Egypt (Fig. 2) into six formations has been The El-Nom borehole was drilled in the Gebel Abraq area established (see Hermina et al., 1989 and references therein). (Fig. 1) for this purpose and offered the opportunity to study These are, from base to top: the Six Hills (Basal Clastics), palynomorphs from fresh unweathered samples. The target Abu Ballas (Lingula Shale), Sabaya (Desert Rose Beds, of this study is threefold. Firstly, to elucidate the age of the Desert Rose Unit), Maghrabi (Plant Beds, Plant Bed Unit), palynomorph-productive rocks of the study area, for which Taref and Quseir (Variegated Shales, Mut) formations. biostratigraphic information is so far lacking; secondly, to Names in parentheses refer to commonly used informal syn- apply this biostratigraphic information in regional correla- onyms. The lithostratigraphy adopted in Aswan, including tion, and thirdly to use palynological and clay mineralogical the Gebel Abraq area as appearing in the Bernice map sheet data to determine depositional environments. NF 36 NE of Klitzsch et al. (1987), records Nubian units that are lateral time equivalents of the Taref–Quseir forma- tions. These are the sand-dominated Abu Aggag Formation * Corresponding author. E-mail addresses: [email protected] (M.S. Mahmoud), maessa that rests unconformably over an eroded basement surface, [email protected] (M.A. Essa). the clay-dominated Timsah Formation and the overlying 1464-343X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2006.10.002 2 M.S. Mahmoud, M.A. Essa / Journal of African Earth Sciences 47 (2007) 1–8 Fig. 1. Geological map of Gebel Abraq area, SE Aswan (after Klitzsch et al., 1987; the Geological Map of Egypt 1: 500000). sand-dominated Umm Barmil Formation. However, rocks age, and may be diachronous, becoming younger from equivalent to the Maghrabi Formation have not previously the west (Dakhla) to the east (Kharga), with a shift in been recognized around Aswan. depocenters. Marine palynomorphs support a marginal Palynology has contributed significantly to the stratigra- marine environment for the Maghrabi Formation (Schrank phy and palaeoecology of the above-mentioned rock units and Mahmoud, 2000). In the Kharga Oasis area, well-pre- (e.g. Schrank, 1984, 1990, 1992, 2001; Schrank and Mah- served pollen and spores indicate an early Campanian age moud, 1998, 2000, 2002; Mahmoud, 2003; Mahmoud and for the base of the Quseir Formation (Mahmoud, 2003). Schrank, 2003) that were for a long time considered unfos- In the east Aswan area, in a location near to Gebel siliferous. Among contributions on the Dakhla area, pal- Abraq, a palynological association with Droseridites seno- ynofloras from the basal Six Hills were proved to be nicus dated the mudstone unit known as the ‘‘Timsah For- exclusively of early Cretaceous age (Schrank and Mah- mation’’ as Coniacian (Sultan, 1985). However, in Upper moud, 1998), as opposed to the previous late Jurassic Egypt the formation has been given a Coniacian to Santo- assumptions for the basal part of the formation. It is nian age based on the occurrence of the bivalves Inoce- now known that the Sabaya Formation may range down ramus balli and I. cycloides (Klitzsch, 1986), few floral into the Aptian and that the overlying Maghrabi Forma- remains such as angiosperm leaves and fruits, and some tion is of Albian-early Cenomanian to late Cenomanian ichnofossils, in highly bioturbated shallow-marine sand- M.S. Mahmoud, M.A. Essa / Journal of African Earth Sciences 47 (2007) 1–8 3 Fig. 2. Correlation of Cretaceous formations in central and southern localities of Egypt. See above listed references for more details. stones (Lejal-Nicol, 1987; Germann et al., 1987). The 2.1. Palynomorphs and palynofacies underlying Abu Aggag Formation contains poorly-pre- served leaf remains and ichnofossils in the area of Bir Eighteen mudstone samples, between depths 30 and Sibrit, and is dated as Turonian (see Klitzsch and Hermina, 118 m, were productive and yielded palynofacies. Eight 1989). The Umm Barmil Formation, at the top, contains samples were productive of palynomorphs but only three small lamellibranches and Ostrea rouvillei and is dated in were found to be rich in palynomorphs. Seventy other sur- the areas southeast of Aswan as Santonian to early Camp- face samples were collected and processed, but proved to anian (Klitzsch, 1986; Hendriks et al., 1987). be palynologically barren. The samples were prepared To date, there is little palynostratigraphic information according to standard palynological preparation tech- concerning the strata of southeast Aswan in general and niques. Carbonates were first removed by reaction with the area of Gebel Abraq in particular, except for a few pal- HCl (35%) for approximately 24 h. Silicates were then ynofloras which were documented from a surface mud- removed from neutralized residues using HF (40%) for stone horizon in the Gebel Abraq area, as reported by approximately 24 h. No oxidation or ultrasonic treatments Mahmoud et al. (1995). Although the palynomorphs are were carried out. The digested residue was sieved using not well preserved, an angiosperm-dominated assemblage 10 lm polyamide nylon sieves. For each sample five perma- from mudstones equivalent to those studied here (previ- nent slides were prepared by using glycerine jelly as a ously named as the Abraq Formation, Mahmoud et al., mounting medium. From each productive sample at least 1995), with tricolporates and polyporates (Cretacaeiporites 200 grains were counted for semi-quantitative abundance polygonalis), is dated as late Albian to Cenomanian? The estimates. Additional slides were prepared from certain overlying sandstone-dominated unit, although also originally samples for qualitative studies and documentation of sig- misidentified as the Umm Sidida Formation (Mahmoud nificant or rare palynomorphs. et al., 1995), was allocated to the Cenomanian–Turonian? interval on the basis of regional correlation. As noted in the introduction, the main aim of this study 2.2. Clay mineralogy beside routine palynological investigations is to provide evi- dence for the age of the mudstone rocks encountered in the The clay fraction (<2 lm) was separated from seven El-Nom borehole and, consequently, to comment on its mudstone samples from the borehole taken at depths of lithostratigraphic setting with respect to other units shown 30, 54, 66, 86, 92, 100 and 114 m (see Fig. 3a). The samples on the Bernice map sheet NF 36 NE of Klitzsch et al. were disaggregated using ultrasonic treatment and washed (1987). Some palaeoenvironmental conclusions have also in distilled water until all soluble salts were removed, been drawn. before being dispersed with Calgon. Separation of the clay fraction was performed using the pipette method (Gale- 2. Materials and methods house, 1971). Oriented samples were mounted on glass slides and two X-ray diffractograms were made per sample The study is based on the description and analysis of 24 (one untreated and one heated at 550 °C for 3 h). The sam- cuttings samples collected from the El-Nom borehole, ples were run using a Philips X-ray diffractometer appara- drilled in the Gebel Abraq area of SE Aswan (Fig. 3a). tus with CuKa radiation, 45 KV and 35 mA and scanning 4 M.S. Mahmoud, M.A. Essa / Journal of African Earth Sciences 47 (2007) 1–8 Fig. 3. (a) The El-Nom borehole section in Gebel Abraq area, with sample positions and inferred lithostratigraphic units according to Klitzsch et al. (1987). (b) Percentage frequency pie diagrams of major palynomorph groups encountered in the Albian–Cenomanian interval of the El-Nom borehole section (1 – at sample depth 104 m, 2 – at sample depth 108 m, 3 – at sample depth 114 m). between 2 and 40 2h at the rate of 1.2 2h/min.