Polysphaeridium subtile, Davey and Williams, 1966

Kumar, A, 2016, New palynological evidence for the age of the Beda Formation, Sirte Basin, Libya, Palaeontologia Electronica 4 (3), pp. 1-14 : 5-11

Polysphaeridium subtile

Williams and Bujak, 1985: Early - Middle Eocene

Tityrosphaeridium cantharellus

Williams and Bujak, 1985: Middle Eocene - Early Miocene

Powell, 1992: Base is at Middle Eocene

Williams et al., 1993: Middle Eocene (Lutetian) -

Early Miocene (Aquitanian)

Stratigraphic ranges of the significant dinoflagellate cysts indicate an Early-Middle Eocene age for the studied samples of the Beda Formation.

Some age diagnostic forms like Adnatosphaeridium multispinosum , Apteodinium australiense , Batiacasphaera compta , Cordosphaeridium cantharellus , Fibrocysta cf. bipolaris , Homotryblium floripes , and Polysphaeridium subtile co-occur in the Middle -Late Eocene well sections of the Nile Delta of Egypt (El-Beiley, 1990), Nigeria (Jan du Chene and Adedrian, 1985) and Salt Range, Pakistan ( Edwards, 2007). On the basis of palynological data from several wells from northeast Libya, Stead (2007) provided age ranges for the following dinoflagellate cyst species that occur in the Beda Formation; they are Adnatosphaeridium multispinosum (Subzone - LPT12a: Middle to Early Eocene), Cordosphaeridium cantharellus ( Tityrosphaeridium cantharellus ) (Middle Eocene (Lutetian) - Early Miocene (Burdigalian), Homotryblium floripes (Middle Eocene (Lutetian) - Early Miocene (Burdigalian), Polysphaeridium subtile (Early Eocene (Ypresian) - Eerly Pliocene (Zanclian). Palaeocene-Eocene Thermal Maxima (PETM) event occurs at the Palaeocene-Eocene boundary in the marginal marine environments is known to be marked by the Apectodinium acme in the Tethyan realm ( Crouch et al., 2003), but Apectodinium was not observed in the present study. On the basis of above observations it is suggested that the present dinoflagellate cyst assemblage represents an Early-Middle Eocene (late Ypresian - Lutetian) age for the studied samples of the Beda Formation.

Pollen grains. The pollen grains and spores recovered from the Beda Formation are commonly known to occur in the Paleogene sediments from different parts of the world ( Kumar and Takahashi, 1991; Kumar, 1994). A comprehensive catalogue of Tertiary fossil spores and pollen from India ( Saxena and Trivedi, 2006; Saxena and Tripathi, 2012) provides stratigraphic occurrences of all the known fossil pollen and spores from India. Several ele-

A. Dinoflagellate Cysts 1. Adnatosphaeridium multispinosum Williams and Downie, 1966

2. Apteodinium australiense ( Deflandre and Cookson, 1955) Williams, 1978

3. Apteodinium sp.

4. Batiacasphaera compta Drugg, 1970

5. Batiacasphaera cf. sphaerica Stover, 1977

6. Batiacasphaera sp.

7. Canningia sp.

8. Cordosphaeridium sp.

9. Cyclonephelium sp.

10. Fibrocysta cf. bipolaris ( Cookson and Eisenack, 1965) Stover and Evitt, 1978

11. Homotryblium floripes ( Deflandre and Cookson, 1955) Stover, 1975

12. Hystrichosphaeropsis sp.

13. Operculodinium centrocarpum ( Deflandre and Cookson, 1955) Matsuoka et al. 1997

14. Polysphaeridium subtile Davey and Williams, 1966

15. Spiniferites ramosus group (Ehrenberg, 1838) Mantell, 1854

16. Tityrosphaeridium cantharellus ( Brosius, 1963) Sarjeant, 1981 B. Acritarchs

1. Leiosphaeridia spp.

2. Micrhystridium spp.

3. Pterospermella spp. C . Pollen Grains

1. Arecipites indicus Venkatachala and Rawat, 1972

2. Brevitricolpites sp.

3. Neocouperipollis sp.

4. Palmaepollenites spp.

5. Palmidites sp.

6. Proxapertites operculatus van der Hammen, 1956

7. Proxapertites cursus van Hoeken-Klinkenberg 1966

8. Retibrevitricolpites sp.

9. Retitricolpites sp.

10. Spinizonocolpites sp.

11. Tricolpites microreticulatus Venkatachala, 1973

12. Tricolpites spp.

13. Tricolporopollenites sp.

14. Trisyncoloporopollenites sp. D. Spores

1. Laevigatosporites sp.

2. Polypodiaceoisporites sp. E . Miscellaneous palynomorphs

1. Foraminieral linings

2. Fungal spores

3. Fungal remains ments of the pollen and spore assemblage the Beda Formation, for example, Arecipites indicus , Proxapertites operculatus , and Tricolpites microreticulatus co-occur in the Indian basins, and their time ranges indicate a Paleogene age. El Beialy (1998) reported several taxa of angiosperm pollen from the Eocene of Oman that also occurs in the Beda Formation.

Thus the palynological results, especially based on the dinoflagellate cysts of the four samples from the core # 1 of well X43 of Sirte Basin indicate an Early-Middle Eocene age for the Beda Formation. Since this result does not conform with the currently accepted age of the Beda Formation, which is considered to be Middle Paleocene (Montian), it is important to discuss further reasons for this discordant result and find a scientific and logical solution to this problem. A literature survey of the published and unpublished micropaleontological data of the Paleocene of the Sirte Basin was carried out to understand why age of the Beda Formation was assigned to the Montian Stage .

Unpublished reports of the Robertson Group plc of the subsurface well sections (wells B1-12 and D1-32) from the Sirte Basin show presence of thick Paleocene-Early to Middle Eocene Limestones with subordinate calcareous claystones. The age of these rocks is primarily based on the poor microfaunal records of benthic as well as planktic foraminafers ( Edwards et al., 1991a, 1991b).

There are few references on the foraminiferal studies covering Paleocene section of the Sirte Basin ( Berggren, 1969, 1974; Tmalla, 1992, 1996; Lehmann, 1964). Additionally, there are several unpublished reports on the biostratigraphic studies of the wells in the Sirte Basin by Robertson Group Plc, where they have defined Paleocene sections in several wells ( Edwards et al., 1991a, 1991b). However, I have not been able to find any information or reference on integration of the Beda Formation with Middle Paleocene (Montian) foraminiferal biostratigraphic zones.

There is a definite presence of the Paleocene marine sedimentary rocks in the Sirte Basin as demonstrated by both planktic foraminiferal evidence ( Berggren, 1969) as well as benthic foraminiferal evidence ( Berggren, 1974). Berggren (1969) presented a biostratigraphic subdivision of the Tertiary of northern and central part of the Sirte Basin in which he defined presence of the Montian stage. This information is not applicable to the age of the Beda Formation because this formation was proposed much later by Barr and Weegar in 1972 for a well section in the western part of the basin. Although this certainly does not mean that there is no Montian section in the western Sirte Basin, but the point of contention is that there is no data to prove the presence of Montian stage in the western Sirte Basin that could conclusively prove that Beda Formation is of Montian age. Berggren’s later paper ( Berggren, 1974) on the Paleocene biostratigraphy of the Sirte Basin does not mention presence of the Beda Formation; probably at the time of writing of this paper he was not aware of the publication of Barr and Weegar (1972). He ( Berggren, 1974) divides Paleocene in the central part of the Sirte basin into Early (Danian) and Late (Thenetian) and there is no mention of Montian stage in this paper.

Since Barr and Weegar (1972) consider that Beda Formation overlies Hagfa Shale Formation (Lower Paleocene) and underlies Dahra Formation (Paleocene) thus it could be of Middle Paleocene (Montian) age (also according to them supported by the foraminiferal data), but according to Berggren (1974, figure 2) the age of Hagfa Shale Member (not formation as mentioned by Barr and Weeger, 1972) ranges from Late Cretaceous (Maestrichtian) to Late Paleocene (lower part of Thanetian). Thus the age the Beda Formation becomes latest Paleocene or Thanetian (not Middle Paleocene as suggested by Barr and Weeger, 1972) or possibly even Early-Middle Eocene. In the south and west of the Sirte Basin, Lower and Middle Paleocene are composed of three units: the lower shale, the middle limestone, and the upper shale. The lower shale passes laterally into coral-algal reef limestone locally on structural highs. A rich benthic and planktic foraminiferal fauna occurs in these shales and indicates deposition in depths ranging from <30 m to somewhat <200 m ( Berggren, 1974). It is quite evident from the above discussion that there is no definite foraminiferal evidence to say that the Beda Formation belongs to the Middle Paleocene (Montian) age. Thus the age of the Beda Formation is still debatable.

Marine Eocene carbonate rocks are present in the subsurface throughout the Sirte Basin, forming major hydrocarbon reservoirs. In the Sirte Basin the dominant lithologies of the Eocene rocks are shallow-water carbonates, dolomites, and evaporites ( Megerisi and Mamgain, 1980).

The palynological data from the four samples of the core #1 of the well X43 indicates an Early-Middle Eocene age thus suggesting a discordant view that this core belongs to the Beda Formation of the Montian age. This result indicates that either the age of the Beda Formation ranges from Middle Paleocene (Montian) to Early-Middle Eocene or this core is not from the Beda Formation and instead belongs to the Eocene carbonate reservoirs of the Sirte Basin. An extensive study of large numbers of subsurface sections (cores from wells) is required to generate new micropaleontological (foraminiferal and other microfossil groups) and palynological data that will help conclusively to determine the age of the Beda Formation.