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Spyridon M. Bellas, Dimitris Frydas:
Oligocene/Miocene boundary calcareous nannofossil events stratigraphy from siliciclastic successions 'Flysch' of the External Ionian Zone in Epirus and Corfu, NW Greece

During the latest Eocene, Oligocene and Early Miocene in the External Hellenides orogenic belt, the Ionian palaeobasin was largely a clastic sedimentation area, bordered to the east and to the west by the Gavrovo and the Apulian carbonate platforms, respectively. Tectonic deformation of the primary basin caused development of many smaller, asymmetrical subbasins, usually oriented parallel or subparallel to a NNW-SSE direction, running in turn parallel to the Aegean Island Arc and Trench System. The position and extension of this region is largely in question. There is increased economic interest in the study of the Ionian Zone, since it is considered to be the main hydrocarbon-producing region in Albania to the north (together with the Durres Basin). Thrust tectonics, in combination with the role of giant evaporites controlling structural style and producing hydrocarbon seals, were recently studied by Velaj et al. (1999). Recoverable oil reserves are found in both Tertiary clastic and carbonate reservoirs.

Diachronism at the Eocene/Oligocene boundary has long been proved for the basal deposits of the siliciclastics, or 'Flysch', in the Ionian Zone of Epirus (co. IGRS/IFP, 1966). This fact was initially suggested by planktonic foraminifera (Bizon, 1967; de Mulder, 1975), and it was later supported by detailed biostratigraphical studies on dinoflagellates (Wilpshaar, 1995) and processed results of calcareous nannofossils (Bellas, 1997). Therefore, microfossils proved very useful for studying the onset of Flysch sedimentation, which presents a younging sequence from east to west. This in turn has been considered important evidence for palaeobasin evolution during the Cenozoic, the thrusting migration in Greece, and the present complex geotectonic situation of northwestern continental Greece, including the Ionian Islands.

In the present work, we investigate the Oligocene/Miocene boundary in terms of calcareous nannofossil data and make a biostratigraphical synthesis of important bioevents for the external part of the Ionian Zone based on the four sections, below: a) the Anacharavi section located to the north of the island of Corfu (western part of the external Ionian Zone); and b) the Argyrotopos (upper part), National Road and Monos outcrops, all three located in the Plataria Syncline which is situated in the eastern part of the External Ionian Zone.

Before considering the observed bioevents, it is useful to have in mind, firstly, that the studied calcareous nannofossils are coming from a mid-latitude zone located in the eastern Mediterranean Sea and, secondly, that they are recovered from outcrops (onshore sections). Therefore, they represent a mixed assemblage comprising low- and high-latitude taxa, recorded in various frequencies relative to the palaeoenvironmental conditions which prevailed each time. As a consequence, a new biostratigraphic scheme was developed, which accounts for mid-latitude sectors, lasting from latest Eocene to Early Miocene time. The Oligocene/Miocene boundary, especially, is placed at the new Reticulofenestra scissura Interval Zone/Triquetrorhabdulus spp. Partial Range Subzone boundary. This definition is biostratigraphically equivalent to the already-known MNP25b/MNN1a Subzone boundary s.s. for the Italian sections (sensu Fornaciari & Rio, 1996), and it is correlatable s.l. with NP25/NN1 or CP19b/CN1a sensu Martini & Müller (1986) and Okada & Bukry (1980), respectively.

On the basis of the above proposed biozonation scheme, the Oligocene/Miocene boundary was placed at the last common occurrence (LCO) level of R. scissura. Individuals of this species continued to be present above the boundary but they are very rare and are considered to be redeposited. Parallel to the main definition, nine subordinate bioevents were also recorded in respect to the LCO level of R. scissura, four of them below, and the next five above, it. A short abundance interval of Helicosphaera recta (1), as well as an interval with Sphenolithus conicus (2), were found to precede the LCO of R. scissura. An abundance acme of Sphenolithus delphix (3) at a level below the O/M boundary is correlatable to the one reported from the boundary stratotype section of Lemme-Carrosio in Italy (co. Steininger et al., 1994). Moreover, within the same stratigraphic interval, the first occurrence (FO) of Triquetrorhabdulus carinatus (4) was recorded. Above the O/M boundary we have firstly the last occurrence (LO) of Zygrhablithus bijugatus (5), which is followed by the acme-end interval of Cyclicargolithus abisectus (6). Within the latter interval, the FO of Discoaster druggii (7) is identified, followed upwards by the LO of Ilselithina fusa (8) and an occurrence interval of Sphenolithus capricornutus (9).

All the above bioevents of calcareous nannofossils may help in the better definition of the Oligocene/Miocene boundary, and represent a refinement of the present knowledge in terms of micropalaeontology and concerning Palaeogene/Neogene biostratigraphy, at least in the area of the eastern Mediterranean Sea and at the zone-sector of mid-latitudes.

References

Bellas, S.M. 1997. Calcareous nannofossils of the Tertiary Flysch (post-Eocene to Early Miocene) of the Ionian Zone in Epirus, NW Greece: Taxonomy and Biostratigraphical correlations. Berliner Geowiss. Abh., E22: 1-173.

Bizon, G. 1967. Contribution à la connaissance des foraminifères planktoniques d'Epire et des îles ioniennes (Grèce occidentale) depuis le Paléogène supérieur jusque'au Pliocène. Publ. Inst. Francais Pétrole: 142pp.

Fornaciari, E. & Rio, D. 1996. Latest Oligocene to early Middle Miocene quantitative calcareous nannofossil biostratigraphy in the Mediterranean region. Micropaleontology, 42(1): 1-36.

IGRS/IFP Institute for Geology and Subsurface Research (Athens)/Institut Francais du Petrole 1966. Etude géologique de l'Epire (Grèce nord-occidentale): 306pp.

Martini, E. & Müller, C. 1986. Current Tertiary and Quaternary calcareous nannoplankton stratigraphy and correlations. Newsl. Stratigr., 16: 99-112.

Mulder, E.F.J. de 1975. Microfauna and sedimentary-tectonic history of the Oligo-Miocene of the Ionian islands and western Epirus (Greece). Utrecht Micropal. Bull., 13: 139pp.

Okada, H. & Bukry, D. 1980. Supplementary modification and introduction of code numbers to the low-latitude coccolith biostratigraphic zonation (Bukry, 1973; 1975). Mar. Micropaleontol., 5: 321-325.

Velaj, T., Davison, I., Serjani, A. & Alsop, I. 1999. Thrust tectonics and the role of evaporites in the Ionian Zone of the Albanides. AAPG Bull., 83(9): 1408-1425.

Wilpshaar, M. 1995. Applicability of dinoflagellate stratigraphy to the analysis of passive and active tectonic settings. Geologica Ultraiectina, 134: 1-132.

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