Key words: Jardas, planktic foraminifera, Al Jabal al Akhdar, Qasr Al Abid, Libya.

Transcription

1 Cenomanian Microfossils from the Qasr al Abid Formation, Al Jabal al Akhdar, NE Libya. Ahmed M. Muftah 1 and Ali A. El-Mehaghag 2 ABSTRACT Calcareous Nannofossils and foraminifers have been identified from the Cenomanian Qasr al Abid Formation in Al Jabal al Akhdar, NE Libya. Based on six outcrop samples, 18 calcareous nannofossils species, and 14 benthic foraminifers accompanied with planktic hedbergellids are documented herein for the first time. The Qasr al Abid Formation is assigned to the UC1- UC3 (a-d) of Burnett (1998), based on Corollithion kennedyi and Lithraphidites acutus. This provides a Cenomanian age for the succession. This is supported by the presence of the index agglutinated foraminifera Thomasinella punica. These sediments were deposited on shelf to upper slope as indicated by the deeper habitat hedbergellids and lenticulinids as well as the calcareous nannosfossils assemblage changed upwards to shallow inner shelf environment as indicated by the dominance of the shallower Thomasinella punica and Thalassinoides burrows mixed with planktic foraminifers via deep water currents. Key words: Jardas, planktic foraminifera, Al Jabal al Akhdar, Qasr Al Abid, Libya. 1 University of Benghazi, Faculty of Science, Department of earth Sciences, P.O.Box 9480, Benghazi, Libya; a_muftah@yahoo.com 2 Arabian Gulf Oil Company, Exploration Division, P.O.Box 263, Benghazi, Libya.

2 INTRODUCTION The Al Jabal al Akhdar anticlinorium is a major geological feature in the northeastern part of Libya on the Mediterranean foreland of the African Shield. The samples used herein are collected from one of the common cretaceous inliers in the region, represented by a small excavation near the Jardas al Abid village, about 15 miles south of the city of El-Merj, in the northeastern part of Al Jabal Al Akhdar (Figs. 1 and 2). Fig. 1: Location map of the studied Qasr Al Abid section in Al Jabal al Akhdar, NE Libya. Fig. 2: View of the uppermost part of the studied Qasr al Abid outcrop. 2

3 The upper Cretaceous carbonates of Qasr Al Abid samples have been examined for their nannofossils and foraminiferal contents. The stratigraphical relationship between the exposed Qasr Al Abid, Al Baniyah and Al Hilal formations are shown in figure 3, Qasr Al Abid Formation was believed to be equivalent to the lower part of the exposed Al Hilal Formation at Marsa al Hilal type locality Barr (1972). Recently, El Mehaghag and Muftah (1996) dated Marsa al Hilal type section as Santonian, accordingly the lower part of the exposed Al Hilal Formation at coastal region of Al Jabal al Akhdar is not equivalent to Qasr Al Abid Formation, however probably likely in subsurface. Fig. 3: Surface stratigraphic relationship of Qasr al Abid, Al Hilal, Athrun, and Jarads Formation in Al Jabal al Akhdar, NE Libya (Modified after Barr and Weegar (1971). The purpose of this study is to record the calcareous nannofossils and the foraminifera of Qasr Al Abid Formation, in order to define the assigned biozones, and to determine the prevailing paleoenvironment during the deposition of this Formation. GEOLOGICAL SETTING AND PREVIOUS WORK Kliensmeide and van den Berg (1968) proposed four members of the Jardas Formation in northeast Libya: the Qasr Al Abid Marl Member, the Al Benia 3

4 Limestone Member, the Got Sas Marl Member, and the Al Feitah Limestone Member (Fig. 3). More recently, Elwerfalli et al., (2000) defined the Qasr Al Abid Formation as the oldest formation exposed in northern-cyrenaica. This formation is present as two small inliers in the core of the Jardas Al Abid dome. Lithologically, the Qasr Al Abid consists of thin, green to yellow beds of ocherous marl interbedded with partly glauconitic and pyritic marly limestone Röhlich, (1974). Petrographicaly, it is echinodermal packstones containing foraminifers, echinoderm fragments, pelecypods (particularly oysters, some Inoceramus prisms) partly replaced by authigenic quartz, and sponge spicules. Glauconite is present in minor amounts. The lower boundary is not exposed in the study area. The upper boundary is sharp and conformable with Al Baniyah Formation El Hawat and Shelmani (1993) (Fig. 3). This Formation is equivalent to the lower part of Al Hilal Formation at coastal sections (El Mehaghag and Muftah, 1996). Banerjee (1980) documented a diverse foraminiferal assemblage from the Qasr Al Abid Formation, based on examination of samples from several subsurface wells. The assemblage includes Globigerellinoides aspera, G. ehrenbergi, Rotalipora cushmani, R. reicheli, Praeglobotruncana stephani, Heterohelix moremani, Hedbergella ex. gr. delrioensis, H. planispira, H. ex. gr. amabilis, Thomasinella punica, and Lenticulina div. sp. The planktonic Rotalipora cushmani Biozone has been recognized by Muftah, (2014) from the lower successions ( ) of an exploration oil well D1-NC152 in the vicinity of Jardas al Abid area as suggested by the presence of the diagnostic planktic foraminifers Rotalipora cushmani, in association with R. greenhornensis, Praeglobotruncana stephani and P. gibba, although, this biozone is not reported in the Jardas al Abid schools outcrop. 4

5 Duronio et al. (1991) reported Early Cenomanian forams such as Rotalipora, Thomasinella, and Planomalina from the subsurface well A1-NC120. METHODOLOGY Calcareous Nannofossils Preparation Technique Smear slides were prepared following the Pipette Strew Slide method, Bown and Young (1998). The surface of each sample was trimmed with a blade until a fresh surface was obtained. The sample was crushed in a paper towel to avoid contamination and the powder was then placed in a beaker and immersed with distilled water. The solution was stirred and left for some time to allow the sediments to disintegrate. A small amount of the suspension was decanted into a test tube and diluted with distilled water. A labeled glass slide was flooded with the suspension and dried on a hot plate. A cover slip was mounted using optical mounting media. All slides were examined with an Axio Plan2 universal Zeiss microscope at 1000x & 2500x magnification. Each slide was examined under cross-polarized and normal Light. Species were photographed using a Sony video camera CCD-IRIS/RGB and Mitsubishi Vidoprinter CP.700 E (D). Some of the observed species are illustrated in Plate 1. Estimation of the relative abundance of species present on the field of view was based on three traverses. The slides are stored in the Geological laboratory of Arabian Gulf Oil Company, Benghazi Libya. Foraminiferal Preparation Techniques Samples were disaggregated by immersion in a mixture of boiled water and hydrogen peroxide. They were then washed over a 63-µm sieve in order to remove the 5

6 silt and clay size-fractions. The samples were dried in a convection oven and microfossils picked. a) Calcareous nannofossils biostratigraphy The six samples from the Qasr Al Abid outcrop yielded a few specimens. Only eighteen species of Cenomenian nannos were recorded including Axopodorhabdus albianus, Zeugrhabdotus scutula, Tranolithus orionatus, Tranolithus orionatus, Corollithion kennedyi, Tegumentum stradneri, Eiffellithus turriseiffelii, Retecapsa crenulata, Orastrum perspicuum, Owenia cf. O. hillii, Biscutum ellipticom, Watznaueria manivitiae, Watznaueria biporata, Watznaueria barnesae, Zeugrhabdotus diplogrammus, Zeugrhabdotus? noeliae and Thoracosphaera sp.. The stratigraphic distribution and interpreted biostratigraphy is shown in figure 4. For more detail all the reported taxa are listed, fully-authored and referenced in Bown (1998). The biozonation schemes of Burnett (1998) were applied herein, in which the investigated surface samples are assigned to the Late Cenomanian UC3a-d based on the occurrences of Lithraphidites acutus and Corollithion kennedyi with completely absence of G. segmentatum which is reported in the nearby subsurface water well (Figs. 4 and 5). The established biozone of this study is corresponding to M. decorates zone (CC10) of Sissing, (1977) and Perch Nielsen, (1985). 6

7 Fig. 4: Distribution chart of the reported calcareous nannofossils from the studied Jardas school section. (R: 1; F: 2-3; C: >3) 7

8 Fig. 5: Calcareous nannofossils from Jardas al Abid Formation close to Jardas School exposure. (All photos are under cross nicols and magnifications at X2500). (Note: Sample no. is at bottom left corner) 8

9 b) Benthic and Planktic foraminifera biostratigraphy The foraminiferal zonation of the upper Cretaceous successions in northeast Libya is established based on planktic foraminifera by Barr (1971, 1972); The Cretaceous micropalaeontological zonation scheme applied is based on the worldwide standard Cretaceous (Albian-Maastrichtian) planktic foraminiferal zonation scheme of Caron (1985). Together with nannofossils data, this section is interpreted to present an integrated age-dating analysis. The planktic and benthic foraminifers present are abundant and moderately well preserved. The benthic foraminifera is dominated by T. punica, Spiroplectanella sp., Textularia sp., Gavelinella sp,?rienholdella sp., Discorbis sp., with few of Conorotalites cf. C. aptiensis, Saracenaria sp., Bigenerina sp., Tolypammina sp. and Osangularia sp. However, planktic foraminifers dominate the faunal assemblage of finer fraction, with very common hedbergellids, including Hedbergella planispira, H. delrioensis and H. spp. Accordingly, this part of Qasr al Abid Formation is assigned to the Cenomanian age (Fig. 4). Thomasinella punica This adherent monospecific genus has been identified from broken specimens, although the spiral initial part is not observed, some specimens exhibit bifurcation growth (Fig. 6). The agglutinated grains used in wall construction are mainly medium to coarse-grained and of carbonate type, although glauconite and other clay minerals are also incorporated into the wall. The wall appears to exhibit alveolar structure, with common calcareous cement when examined under the transmitted light microscope. Stratigraphic range: Cenomanian, abundance. However, this species proved to be an excellent guide fossil for the Cenomanian strata of the Tethyan region, Gohrbandt (1966) reported from another exposure in Jefren Marl Member of Ain Tobi Formation northwestern part of Libya. 9

10 Fig. 6: Thomasinella punica from the exposed Qasr al Abid Formation close to Jardas School exposure (Muftah, 2014). PALEOECOLOGY The agglutinated foraminifera Thomasinella punica is considered to be an indicator of a warm shallow-marine environment warm water Weidich and Al-Harithi (1990). However, the coexistence of the shallower setting assemblages dominated by T. punica with the small sized open marine hedbergellids at the lowermost sample is believed to be deposited on shelf to upper slope and the open marine globular shaped hedbergellids are justified as current derived taxa due to the environmental variability, differential settling velocities and lateral transport of empty globular tests (Fig. 7). 10

11 Fig. 7: Schematic depositional environment shows the shelf current faunal mixing hypothesis (modified after Leckie, 1987) Based on the following evidence, we interpret this measured section to be representative of shallow water shelf to upper slope conditions. 1) The foraminiferal assemblage is characterized by low diversity and relatively rich in both benthic and planktic foraminifers. The benthic foraminifers are mainly represented by T. punica and others such as Lenticulina, while the planktic suite is represented by extremely small-sized hedbergellids (oppurtunistics) with Rare ostracodes such as Bairdia sp., and Cytheropteron sp. is present as well. 2) Sparse calcareous nannofossils. 3) Associated macrofossils such as gastropods, oysters and echinoids, as well as Thalassinoides bioturbation are also reported in the same sediments. 4) Finally the presence of glauconite grains. 11

12 CONCLUSIONS 1- Eighteen species of calcareous nannofossils are documented from the Qasr al Abid Formation near Qasr al Abid section in northeastern Libya. 2- These sediments are assigned to the M. decoratus Zone (CC10) of Sissingh (1977) and Perch-Nielsen (1985). Therefore, the Qasr al Abid Formation is Late Cenomanian in age. 3- Paleoecologically, the agglutinated foraminifera Thomasinella punica, together with abundant planktic hedbergillids, suggest that deposition occurred on shelf to upper slope and the open marine globular shaped hedbergellids drifted by either bottom current or near surface wave. ACKNOWLEDGEMENTS The authors are gratefully indebted to Dr. H. Daniels and Mr. K. Ashahomi for their comments on the early draft of this manuscript. Also, our thanks to Mr. S. Atia, N. Gargoum and N. Ben Saoud for foraminiferal slides and smear slides preparations. REFERENCES Banerjee, S Stratigraphic Lexicon of Libya, Industrial Research Center, Tripoli, Bull. 13, Barr, F. T Cretaceous biostratigraphy and planktonic foraminifera of Libya. Micropaleontology, 18 (1), Barr, F. T Biostratigraphy and planktonic zonation of the Upper Cretaceous Atrun Limestone and Hilal Shale, northeastern Libya. In: Proc. 2 nd Int. Conf. Plank. Microfossils. A. Farinacci, Ed. Rome, pp

13 Barr, F. T. and Weegar, A. A Stratigraphic Nomenclature of the Sirte Basin, Libya. Petrol. Explor. Soc. Libya, Tripoli. Caron, M Creatceous planktic foraminifera. In: (ed. A. Farinacci), Proceedings II Planktonic Conference, Roma, 1, Bown, P.R., (Ed.) Calcareous Nannofossils Biostratigraphy. Kluwer Academic, London, 315p. Bown, P.R. and Young, J.R Techniques. In: P.R. Bown (Ed.) Calcareous nannofossil Biostratigrphy. Chapman & Hall/Kluwer Academic Publishers: Burnett, J.A Upper Cretaceous. In: Bown, P.R., Ed., Calcareous Nannofossil Biostratigraphy. Chapman and Hall, Cambridge; Duronio, P, Dakshe, A., and Bellini, E Stratigraphy of the offshore Cyrenaica (Libya), in The Geology of Libya, M.J.Salem, O.S.Hammuda and B.A.Eliagoubi Eds. Elsevier. El Hawat, A. S. and Shelmani, M. A Short notes and guidebook on the geology of Al Jabal al Akhdar, Cyrenaica, NE Libya. Inprinted Limited Malta, 70p. El-Mehaghag, A. A., and Muftah, A. M Calcareous nannofossils and foraminiferal biostratigraphy of Al Hilal Formation, North-East Libya. In: The 3 th International conference on the Geology of the Arab World, Cairo University, Elwerfalli, A, Muftah, A., El Hawat A. and Shelmani, M A guidebook on the geology of Al Jabal al Akhdar, Cyrenaica, NE Libya. Sedimentary basins of Libya 2 nd Symposium on Geology of Northwest Libya,

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