Foraminifera of the Eptahorion Formation (early Oligocene) of the Mesohellenic Basin, northern Greece

Planktonic and benthic foraminifera from the type area of the early Oligocene Eptahorion Formation (Mesohellenic Basin, northern Greece) have been ranged and partly described and figured. Altogether, about 130 taxa were identified. According to the current age assignments, the formation is attributable to Blow’s Zones P20 and P21. The record of benthic palaeoecologic indices has thrown light on bathymetric history of the formation. The deposition of the whole sequence, after the initial transgressive phase, occured in a marine, relatively deep environment. Palaeontologic evidence indicates few changes, ranging from the epibathyal to the outer neritic bathymetric domain.


INTRODUCTION
The Oligocene-Miocene succession of the Mesohellenic Basin has received littleattention in terms of detailed biostratigraphic investigations. As recently described and named by Ori et a1.(1990) during their regional study of the area, the Oligocene and early Miocene rocks of the northern Mesohellenic Basin are wholly represented by four main units, from the base to the top they are the Eptahorion Formation, theTaliaros Formation, the Voion Formation and the Pendalofos Formation. A detailed examination of the biostratigraphy and palaeoecology of the first unit by means of foraminifera is the subject of this study. Brunn (1956) described in detail the formation ("Marnes d'Eptakhorion") in its type area, around the village of Eptahorion. He referred the unit to the Stampian on the basis of the recovery of stratigraphically significant molluscan fauna. Desprairies (1979) described the stratigraphy and sedimentology of the formation ("Formation d'Eptakhorion") accepting the age of the unit according to the biostratigraphic data of Brunn. He again correlated the formation with the "Marnes d e 1'Arakhtos" and the "Gres d'Anemorakhi", of the Internal Ionian Zone (western tectonic zone of the Hellenids), without defining their chronostratigraphic limits or stratigraphic rank. In the Ionian Zone, Bizon & Bizon (in IGRS-IFP, 1966) described poorly fossiliferous, flysch deposits lithologically and chronologically comparable to those of the Mesohellenic Basin. In recent years, papers dealing with the biostratigraphic evolution of the Mesohellenic Oligocene have been published by Zygojannis (1979a andb, 1980). The basic material of their study were some sections cropping out in the southern area of the Oligocene basin (Meteora region). Data from the north (Eptahorion area) are limited to some planktonic and benthic foraminiferal lists obtained from unknown stratigraphic locations. Listed planktonic species are Glohi~yr~rinnnrnyli~pert iira,G. nrzgirlisirfurdis, G.ciperociisis,G.

PREVIOUS STRATIGRAPHIC WORK
selliiand Globorofah opiina. Marginal marine facies (neritic and littoral) referred to the Eptahorion Formation are described from the Grevena Basin and the northern part of the Thessalia Basin, both located southeast of the type area and chronologically assigned to the late Oligocene (Steininger et al. (1985). More recently, Ori et a1.(1990) discussed in detail the Oligocene-early Miocene stratigraphy of the northern Mesohellenic Basin. The biostratigraphic and palaeoenvironmental framework was based on foraminiferal data and on relationships between lithofacies and biofacies.

AREA OF INVESTIGATION, LITHOLOGY AND DEFINITION
The Eptahorion Formation extends over a large geographic area in north-western Greece. It typically outcrops in the area around the village of Eptahorion, western Macedonia, in a series of well exposed hillsides. The extent of the formation in the study area is shown in Fig. 1.
The formation unconformably overlies the ophiolites of the Subpelagonian Zone. The contact of the formation with the overlying Taliaros Formation (late Oligocene), mainly sandstones and arenaceous shales, is marked by an unconformity. In its type area the unit consists of a generalized fining upward sequence, more than 1100 meters thick (Ori et al., 1990), with mainly arenaceous conglomerates at the base totally devoid of macro and microfossils. The formation grades upward into sandstones interbedded with silty mark and shales with relatively rich foraminiferal contents. The fine-grained interbeds in the lowest part of the formation contain a few mollusc and ostracod-rich horizons.

MATERIAL AND METHODS
One continuous section, almost totally exposed within thearea of investigation, with minor portions covered by vegetation, was measured (Fig. 2). Planktonic and benthic foraminifera were studied from 38 samples collected in the sediments northern Mesohellenic Basin, and location of exposed along the road in the hillsides that run westward from the village of Eptahorion (Fig. 1). Samples were collected at intervals of about 50-100 metres within the lower portion of the section, and at intervals of 5-20 metres in the middle and upper parts.
Samples were crushed and disaggregated by hydrogen peroxide solution (30%) or dimethylbenzilammonium chloride solution (lo%), from at least one hour to 24 hours, and washed with distilled water through a 200 mesh sieve.

AGE AND ZONE ASSIGNMENTS
Zones are assigned using both the classic zonal scheme established by Blow (1969) and that more recently proposed by Bolli & Saunders (1985). The last two authors mainly used the Oligocene zonal definitions as originally defined by Bolli (1957).
Zones recognized are defined as follows: Globorotalia opima opima Zone represents a taxon-range zone and it is therefore defined by the range of the zonal marker (Bolli and Saunders, 1985). Zone P21 has its base defined by the first occurrence of Globigerina angulisuturalis (Blow, 1969). Thus, according to the original definition, the lower part of the G.opima opima Zone, which corresponds to the interval immediately below the first occurrence of G. angulisuturalis, belongs to Zone P20.

PLANKTONIC FORAMINIFERA
A total of 33 planktonic taxa were identified, some of them having stratigraphic significance. Their distribution and abundance is shown in Fig. 3. Samples collected from the lower part of the section, a total of about 300 metres, are totally devoid of planktonic species. Almost all the fossiliferous samples investigated contain deformed or crushed specimens, occasionally pyritized. Within some intervals planktonic specimens are rare and poorly preserved ( e. g. from samples 31 to34). But the interval including the boundary between Zones P20 and P21 (from samples 8 to 18) yielded relatively diverse and well preserved assemblages. The four main genera representingmost of therecorded taxa are Catapsydrax, Globigerina, Globoquadrina and Paragloborotalia.
The generalized morphologic affinity within the planktonic foraminifera1 population is due to a peculiar characteristic of the Oligocene assemblages, namely a worldwide dominance by small pentacamerate globigerinids and small to large tetracamerate globigerinids (discussed in detail by Stainforth, 1975 andLamb, 1981). The relatively low diversity between temperate and tropical areas in the Oligocene oceanic plankton assemblages (Kennett, 1978;Sancetta, 1979) is well known According to the stratigraphic ranges of the planktonic foraminifera provided by Baumann (1970), Blow (1969), Bolli (19661, Bolli & Saunders (19851, Stainforth et a/ (1975 and Stainforth & Lamb (1981), the assemblages of the Eptahorion Formation range from the upper part of Zone P20 to Zone P21 of Blow's zonal scheme. With respect to this relatively short stratigraphic interval, most of the recorded taxa are long ranging.
The first occurrence of Globigerina angulisuturalis provides an excellent worldwide datum used by a number of authors to define the base of their Globorotalia opima Zone (e. g. Baumann, 1970;Stainforth & Lamb, 1981). The boundary between Zone P20 and P21 is placed in the present section with sample 10, on the basis of the appearance of G.angulisuturalis. The first record of Paragloborotalia opiim occurs in the same sample and is believed to be non evolutionary. There is no trace, indeed, of any overlapping range between Globigerina ampliapertiira and P.opima, which would correspond to the lowest interval of Globorotalia opima opirna taxon-range Zone. The concurrence of the above two taxa is widely reported in the literature, both in low latitude (Blow, 1969;Stainforth et nl. (1975) and in the Tethys area (Baumann, 1970;Molina, 1979). Nevertheless, this point is still puzzling and several authors reject the assumed overlap (Beckmann ~t nl. 1981;Bolli and Saunders, 1985).
Late Oligocene-Miocene taxa, such as "Globorofalin" sinkelisis and Globoqmdrina deliisceiis praedelrisceiis, whose appearance is usually placed within Zones P21/P22, are also present. These two species generally exhibit a close first occurrence, as confirmed by the Eptahorionassemblages, with partial overlap of the P. opima range. Finally, the P.opima appearance is here believed to be a "local lateappearance", presumably due to sedimentary factors rather than to ecologic ones.
G. arnpliapertura represents an excellent marker to identify in the broadest term the lower part of the Oligocene. In the studied material no specimens of this widespread taxon were No main discrepancies within the planktonic foraminiferal distribution are noted, with the remarkable exception of Globoquadrina tapuriensis. This rare taxon has been recorded in just two samples from Zone P21. It is seldom given because of the difficulty in keeping the subspecies separate from the closely related G. tripartita tripnrtitn. Nevertheless, its distribution exhibits an extinction level which usually does not exceed Zone P20. Such shifting upward of the range of G. tapuriensis has already been described in the Hungarian Oligocene (Sztrakos, 1979). The range of the other significant species is similar, with minor differences, to those usually reported in the literature.
Despite the lack of foraminiferal records in the lower part of the section, one should be aware that there is no evidence for these beds in a zone older than P20. On the contrary, the early occurrence of taxa that typify Zones P20/P21, close to the G. angulisuturalis datum, provides some confidence in the attribution of the whole lower section to Zone P20.
There is no evidence, within the planktonic record, of any remarkable change in temperature. During the time interval of the deposition of formation planktonic foraminifera maintained their warm water characteristics. According to Keller (1983) a major cooling episode, related to an eustatic sea level drop (Haq et a1.1987) occurred at the P20/P21 boundary.

BENTHIC FORAMINIFERA
The marly and shaly samples collected in the Eptahorion sectioncontainagenerally diversebenthic foraminiferalfauna. These assemblages usually outnumber the planktonics, by 50% to more than 95%. The section may be divided into four palaeobathymetric zones where 92 taxa have been recorded and ranged (figure 4). A number of long ranging, frequently occuring species, such as Guttulina communis, most of the nodosariids and lenticulinids, have not been listed. Although benthic foraminifera are generally more preservable and resistant to mechanical breakage than planktonics (Douglas, 1973), the benthic assemblages constantly contain crushed and deformed specimens. The assemblages, as well as any single taxon, have limited biostratigraphic value, most of the common taxa exhibiting wide stratigraphic range.
Species reported worldwide from the Oligocene strata include A few species, generally the rare ones, can be useful in dating sediments. The occurrence of Boliuina semistriata in the lower part of Zone P21 should be emphasized. This taxon seems to reach its stratigraphic top within the Rupelian (early Oligocene) in the northern Alps (Lindenberg, 1965;Hofmann, 1967). could exhibit significant stratigrafic value, Schnitker (1979) and Miller (1983) gave the range this species up to the late Oligocene in the Bay of Biscay (Zones P21 /P22).

PALAEOBATHYMETRY
The Epthhorionsection has been divided into four zones based on foriminiferal abundance and palaeobathymetric inference (Fig. 4).
Samples from 1 to 5 identify the zone A. The interval is almost totally barren, a few unidentifiable foraminiferal specimens, crushed and spathized, were collected from sample 1. Molluscan and ostracod shells and fragments were collected from the same level of sample 5 (Colalongo, personal communication, recognized an oligotipic, brackish ostracod fauna). Abundant glauconite and pyrite are mineral accessories constituents. Despite the extremely poor fossil record, sedimentological evidence suggests a proximaldeltaic, shallow water environment. (Desprairies, 1979;Ori et a/, 1990). Zone B ranges from samples 6 to 28 and includes the boundary between Blow's Zones P20 and P21. Benthics are particularly abundant and diverse. The lower boundary of the zone B is based on the first, sudden foraminiferal recovery. The dominant taxa are Spiroplectammina carinata, Dorothia beloides, uvigerinids, Clobocassi~irlinnglobosa /subglobosagroirp, Melonis affinis, Bulimina palmerae, Alinnena hieroglyphica, gyroidinoids. In most of these samples a significant number of costateuvigerinids is recorded, they include Uuigerina cocoaeizsis group, U.gallozuayi and U . eocaenn. This group, well known as an excellent bathymetric marker, is generally assigned to the upper epibathyal environment (Pflum & Frerichs, 1976;Pujos-Lamy, 1984;Boersma, 1986). Direct relationship between ornamentation and depth was suggested by Sztrakos (1983) on the basis of the Hungarian costate uvigerinids. The Eptahorion material contains comparable results: predominance of costate specimens within zone B and, by contrast, weakly costate specimens of shallower conditions, in the overlying zone C. It is probable that Uuigeriiia cocoaensis sirbspecies (cocoaensis and jacksonensis, undivided in this study) represent a cline (ecophenotipic variants). Together with the listed taxa, environmentally diagnostic species include Valvuliizeria palmarealensis, Sigmoiliizita teiiiiis and Birliinirm ros t rn to.

Globigerina cipcroenrir Globigcrina cryptornphola C o t o p t y drax u nicavur "Globiporina" ruopertura
The prevailing palaeobathymetric interpretation of the benthic assemblages,suggests epibathyal conditions, with water depth not exceeding 500-700 meters. The very rare Fig. 4 . Stratigraphic ranges of benthic foraminifera from the Eptahorion Formation.
recovery of a number of taxa commonly widespread in the Oligocene bathyal strata, such as Glomospira spp., Cyclammina spp., Melonis spp. and Cibicidoides hauanensis, confirm this interpretation. Assemblages comparable with those described fromzone B are reported by Sztrakos (1979) from theepibathyal sediments of the Hungarian Oligocene. Zone C extends from sample 28 to 37. It is assigned to the upper epibathyal toward outer neritic environment. This interval is characterized by a sharp decrease in the species number. Dominant taxa of the underlying zoneare still present, but reduced in number. New taxa rarely occur and include several Pararotalia species, Elphidium carpaticum, E . ortenburgensis, Cancris auriculus primitiuus, Uvigerina tenuistriata. It is believed that most of these species represent displaced specimens from shallower water. Other relatively significant taxa, already listed in the previous zone, are Uvigerina cocoaensis group, .Almaena hieroglyphica, Spiroplectammina carinata. The recovery of U.tenuistriata has some interest. Sztrakos (1983)  reported by authors as a lower epibathyal taxon, has its upper depth limit around 600 meters (Wright, 1978;van Morkhoven et al., 1986). The moderate deepening and shallowing evidenced by the benthonic foraminifera during the environmental evolution of the Eptahorion Formation, isusually linked with associated lithologic changes. Particulary during the major, dramatic change, from zones A and B, variation in the sedimenation occurred. Although the evidence of relation between sediment and benthic foraminiferal abundanceas pointed out, for instance, by Winkler (1983) in the turbidite depositsthe changes in paleodepth appear to be the major factor controlling faunal change mechanisms in the Eptahorion sediments.
Further faunal considerations can be summarized in the following points : a) the plankton/benthos ratio generally gives to the benthonics a strong predominance. On the contrary, recent deep-sea sediments of the Mediterranean contain generally planktonic foraminifera in excess of 95% (Cita & Zocchi, 1978). b) high number of foraminiferal shells, mainly planktonics, broken or deformed, are constantly present. c) all the investigated samples contain very small amount of species displaced from shallow water.
The joint occurrence of points a) and b) should be simply due to the fact that benthic are more resistant to mechanical breakage than planktonic specimens during post mortem transport and accumulation. Moreover, the lack of species clearly reworked excludes extraformational contaminations.

CONCLUSIONS
The detailed observations of the foraminifera in the Eptahorion Formation lead to assign the sequence within the total range of Paragloborotalia opima, except for the lower portion, in which age-diagnostic foraminifera are virtually missing. An attribution to Zone P20 is, therefore, indirectly derived from thezonal attributionof theoverlying strata. A finer delineation of the zonation is not possible from the data at hand.