Planktonic Foraminiferal Biostratigraphy of the Late Eocene and Oligocene in the Palmyride Area , Syria

In different geological periods, planktonic foraminifera were deposited over an extensive portion of the world oceans and seas, and are often found in large numbers. Industrial application of planktonic foraminiferal biostratigraphic studies began in the middle of the last century in the Gulf Coast region of the USA and quickly spread through the principal oil-producing parts of the world (STAINFORTH et al., 1975; BOERSMA, 1988). This has resulted in the recognition of their usefulness for both local and regional biostratigraphic zonation and correlation (BOLLI & SAUNDERS, 1985; BERGGREN et al., 1995). Numerous biostratigraphic studies of planktonic foraminifera have originated from the Former Soviet Union – the first zonal scheme for planktonic foraminifPlanktonic Foraminiferal Biostratigraphy of the Late Eocene and Oligocene in the Palmyride Area, Syria


INTRODUCTION
In different geological periods, planktonic foraminifera were deposited over an extensive portion of the world oceans and seas, and are often found in large numbers.Industrial application of planktonic foraminiferal biostratigraphic studies began in the middle of the last century in the Gulf Coast region of the USA and quickly spread through the principal oil-producing parts of the world (STAINFORTH et al., 1975;BOERSMA, 1988).This has resulted in the recognition of their usefulness for both local and regional biostratigraphic zonation and correlation (BOLLI & SAUNDERS, 1985;BERG-GREN et al., 1995).
Numerous biostratigraphic studies of planktonic foraminifera have originated from the Former Soviet Union -the first zonal scheme for planktonic foraminif-
Important biostratigraphical work on the Palaeogene deposits in Syria has been undertaken by KRASH-ENINNIKOV et al. (1996).A previous biostratigraphic investigation in the Palmyride area, on material from deep exploration wells was done by HERNITZ KUČENJAK et al. (2005).
In the present work, a planktonic foraminiferal fauna from three deep exploration wells Jihar-1, Jihar-4 and Jihar-5 drilled in the Palmyride area (Fig. 1) has been examined.A rich, highly to moderately diverse and well preserved planktonic foraminiferal association enabled biostratigraphical zonation of the late Eocene and Oligocene deposits (Fig. 3).On the basis of the composition of the foraminiferal assemblages, diversity, changes of test morphology and wall texture, palaeoecological and palaeoclimatic interpretations have been made.

MATERIAL AND METHODS
The detailed biostratigraphic study was based on material from drill cuttings from three deep exploration wells (Jihar-1, Jihar-4 and Jihar-5) drilled in the Hayan exploration block in the Palmyride area (Fig. 1).Samples for micropalaeontological analyses were taken at every 10 m in Jihar-1 and Jihar-5 wells and at 20 m intervals in the Jihar-4 well.Samples were soaked in water with a small amount of hydrogen peroxide and then washed under running water through 63 µm, 125 µm, 160 µm and 630 µm sieves.These four fractions were dried in a drier and from each fraction foraminifera were picked out onto micropalaeontological slides.Microfossil associations were examined on a stereo microscope, whereas a detailed study of the

Abstract
Three deep exploration wells in the Palmyride area (Syria) have been drilled through Oligocene and Eocene deposits.A detailed micropalaeontological investigation has been performed on the planktonic foraminiferal assemblage from drill cuttings.Standard planktonic foraminiferal zones from P15 to P22 (E15 to O6) have been identified.The test morphology and diversity of the foraminiferal assemblage indicate a general cooling trend during the late Eocene and Oligocene.The specialized Eocene forms (K-mode life strategy) such as turborotaliids, globigerinathekids and hantkeninids become extinct in the uppermost late Eocene.The Oligocene is characterized by the dominance of cooler, opportunistic planktonic foraminifera (r-mode life strategy), such as pseudohastigerinids, chiloguembelinids, globigerinids and tenuitellids.A very low number of benthic foraminifera in late Eocene cuttings implies sedimentation in a deep, open sea environment.A gradual increase of the benthic foraminiferal proportion through the Oligocene indicates shallowing of the sedimentary environment.
planktonic foraminiferal morphology was performed on scanning electron microscope (SEM).
Here, determination of planktonic foraminiferal biozones was based on the last occurrence (LO) of the index taxa to avoid possible mistakes as a result of mixing different foraminiferal associations in drill cuttings.The only exception is the boundary between zones O3 (P20) and O4 (P21a) which is defined by the first occurrence (FO) of Globigerina angulisuturalis.Because of the difficulty in identification of the first occurrence of the zonal marker, this boundary is an approximation.

GEOLOGICAL SETTING
The foraminiferal fauna has been studied from the Hayan exploration block (Fig. 1), which is situated in the Palmyride area (Central part of Syria).The Palmyrides are an inverted Mesozoic rift basin (LUČIĆ & FORŠEK, 2000;BREW, 2001;BREW et al., 2001).Inversion began in the Late Cretaceous, but the majority of the inversion took place in the Miocene (BREW, 2001).
Syria can be divided into four major tectonic zones: the Palmyride area, the Abd el Aziz-Sinjar area, the Euphrates Fault System, and the Dead Sea Fault System (BREW, 2001;BREW et al., 2001).The Palmyrides are an intracontinental mountain fold belt which extend In the geological evolution of Syria during the Palaeogene, five main steps have been observed by KRASHENINNIKOV et al. (1996), which correspond to the following lithostratigraphic formations: cal- In the Palmyrides, the Oligocene marine sedimentation stopped at the Neogene boundary, and the marine Palaeogene is overlain by continental sediments or by unconformable and transgressive marine Lower Miocene deposits.

LITHOLOGY
In the studied exploration area, the Middle Eocene argillaceous limestones underlie the Upper Eocene deposits.These argillaceous limestones are greyish white to grey-greenish wackestone/packstones.They occasionally grade into calcareous marl, grey to brownish in colour.In places there are a few occurrences of grey quartz arenites.The Upper Eocene sedimentary rocks (biozones E14 to E16) are represented by greyish-white to grey-greenish argillaceous limestones of wackestone to wackestone/packstone types.Occasional occurrences of marls have been found, but only in the Jihar-4 well.The Lower Oligocene sediments are composed of light grey to grey marls, and white to grey-greenish argillaceous limestones of wackestone/packstone types.Deposits of the Upper Oligocene consist of grey, sporadically sandy-clayey marls, with occurrences of very light grey fine-to medium-grained quartz arenites.The Upper Oligocene deposits are overlain by the light grey sandstones of quartz wacke to quartz arenite type, and grey to brownish grey marls, possibly Early Miocene in age.
According to the lithological characteristics of deposits and composition of the foraminiferal association (plankton/benthos ratio approx.70:30), the Upper Eocene rocks were probably deposited in the open shelf environment, whereas the Oligocene deposits (plankton/benthos ratio approx.40:60) were accumulated in the outer to middle shelf environment.

Zone E14 Globigerinatheka semiinvoluta
In the material studied, a very well preserved planktonic foraminiferal association has been presented.The lower boundary of this Zone is defined by the extinction of all morozovellids and large acarininids and corresponds very well with previous investigations in the Mediterranean and Atlantic areas (PREMOLI SILVA & BOERSMA, 1988;PREMEC-FUĆEK et al., 1998;WADE, 2004).
Due to difficulties in identifying the first appearance datum of the index species in the material from drill cuttings, the top of the zone is established approximately by the first occurrence of Globigerina angulisuturalis (BERGGREN et al., 1995;BERGGREN & PEAR-SON, 2005).First occurrence of G. angulisuturalis in the investigated Syrian wells has been recognized in the following samples: Jihar-1 -200-210 m, Jihar-4 -495-515 m and Jihar-5 -320-330 m.KRASHENINNIKOV et al. (1996) reported the regressive nature of the Oligocene deposits.In the Palmyrids the Oligocene marine sedimentary regime terminated at the Neogene boundary, and the marine Palaeogene is unconformably and transgressively overlain by marine Lower Miocene deposits.

PALAEOECOLOGY AND CLIMATIC CHANGES
Planktonic foraminifera have great potential in palaeoecological studies (MURRAY, 1991a).These tiny monocellular organisms with calcite shells are distributed throughout the world seas and oceans, from the polar water masses to the equator, and have different depth preferences of habitat in a stratified water column (HEMLEBEN et al., 1989;MURRAY, 1991a;PEARSON, 1998).Biotic and abiotic factors affect their diversity, morphology, and distribution.Abiotic factors which may influence the distribution of planktonic foraminifera are temperature, salinity, water density, nutrients, light, turbidity, and currents (HAL-LOCK, 1987;HEMLEBEN et al., 1989;MURRAY, 1991a).Biotic factors include food supply, symbiotic relationships, productivity and interfaunal relationships (BÉ, 1982;MURRAY, 1991a;HALLOCK et al., 1991).Planktonic foraminifera are, therefore, very useful in palaeooceanological and palaeoclimatological studies.Much of the world oceans in tropical and subtropical areas are well stratified and oligotrophic.The ecology of the various species in these environments is closely related to their preferred niche in the water column (HEMLEBEN et al., 1989;MURRAY, 1991a;PEAR-SON, 1998;COXALL et al., 2000;WADE, 2004;SEX-TON et al., 2006).In general, planktonic foraminiferal species live in two preferred depth habitats: (1) Shallow-dwelling planktonic foraminifera tend to have thinner or more porous, less dense shells, more globular chambers and they are often spinose (PEAR-SON, 1998).The function of spines is to carry external cytoplasm with symbionts and to prevent escape of prey (HEMLEBEN et al., 1989;PEARSON, 1998).Shallow-dwelling species have their primary aperture usually in the central position at the umbilical side of the test (PEARSON, 1998).Some of them have multiple supplementary apertures.Isotope analyses of the planktonic foraminiferal calcite shells provide one of the most important insights into oceanographic changes during Earth history.Stable isotope studies of the surface dwelling planktonic foraminifera usually indicate the lightest δ 18 O and heaviest δ 13 C values in modern as well as in fossil planktonic foraminiferal shells.
(2) The majority of modern deeper-dwelling forms and some ancient examples tend to have more angular and flattened chambers.The position of a primary aperture is more extraumbilical and there are no supplementary apertures (PEARSON, 1998).Detailed isotope analyses from two species Turborotalia pomeroli and T. cerroazulensis from the Adriatic Sea indicate a thermocline habitat for the Turborotalia cerroazulensis group (PREMEC FUĆEK & PEARSON, 2006).Species within the genera Subbotina and Catapsydrax are interpreted as having occupied a relatively deep thermocline habitat (PEARSON et al., 1993(PEARSON et al., , 2001;;SEXTON et al., 2006).

Late Eocene
The Upper Eocene succession of the Syrian wells contains a rich and diverse planktonic foraminiferal association.Muricate forms including Acarinina medizzai and A. collactea have only been represented in zone E14.Previous isotope investigations of acarininids show the lightest δ 18 O values suggesting a habitat in the uppermost part of the water column.Most positive δ 13 C values indicate a relationship with algal simbionts (SHACKLETON et al., 1985;PEARSON et al., 1993;D'HONDT et al., 1994;WADE, 2004;SEXTON et al., 2006).In the analysed samples, few species of Globigerinatheka in Zone E14 and Zone E15/E16 have been found.The Eocene genus Globigerinatheka includes spinose forms (PREMEC FUĆEK et al., 1998), as in many modern species in tropical and subtropical regions (HEMLEBEN et al., 1989), possessing algal symbionts.They are limited to the shallow, mixed layer of the oceans and seas because of the photosynthetic requirements of their algae (BÉ, 1982;MUR-RAY, 1991a).These aforementioned species indicate a subtropical to warm climatic bioprovince for the east Mediterranean area during the late Eocene.Representatives of the non-spinose genus Turborotalia (T.pomeroli, T. cerroazulensis and T. cocoaensis) occupy an upper thermocline habitat (PEARSON et al., 2001;WADE, 2004;PREMEC FUĆEK & PEARSON, 2006).Stable isotope records of the turborotalid tests show relatively light δ 13 C signals suggesting that they have no association with algal symbionts (PREMEC FUĆEK & PEAR- SON, 2006).During the late Eocene, a few extinctions occurred.Acarinina medizzai and Acarinina collactea became extinct at the end of zone E14.Due to the absence of the zonal marker Globigerinatheka semiinvoluta, the last appearance of these two species has been used to approximately define the E14/E15 boundary (see distribution range of A. medizzai in PEARSON et al., 2006).Globigerinathekids decline in abundance and diversity and at the lower part of the E15/E16 (P16/P17) zone become extinct.In BERGGREN & PEARSON (2005), the LO of Globigerinatheka mark the top of the zone E15.The most dramatic extinction event happened close to the Eocene/Oligocene boundary when the last specimens of Turborotalia cerroazulensis (HOWE & WALLACE), T. pomeroli (TOUMARKINE & BOLLI) and T. cocoaensis (CUSHMAN) disappeared.Also Cribrohantkenina inflata (HOWE) disappeared almost simultaneously, while the species Turborotalia increbescens (BANDY) and Turborotalia ampliapertura (BOLLI) persisted in the early Oligocene.In the same horizons, typical Palaeogene opportunistic foraminifera including biserial heterohelicids, low spired subbotinids, and tenuitellids (HALLOCK et al., 1991) became more and more frequent in assemblages.

Eocene/Oligocene boundary
The significant changes of planktonic foraminiferal assemblages indicate a general cooling trend and eutrophication of the euphotic zone during late Eocene and Oligocene times (KELLER, 1983;BOERSMA & PREMOLI SILVA, 1991).A highly diversified, subtropical to warm fauna, where a K-mode life strategy predominated, was gradually replaced by less diverse assemblages dominated by r-strategy taxa indicating cooler waters.The Eocene/Oligocene boundary is marked by the extinction of Hantkenina.The climate event (Oi-1) occurs 200 kyr after the boundary (WADE & PÄLIKE, 2004) and is the result of the ocean temperature drop, the formation of Antarctic ice sheets and the development of heavy cold water circulation locally on the bottom of the ocean (CAVELIER et al., 1981;ZACHOS et al., 1996ZACHOS et al., , 2001;;MACKENSEN, 2004;WADE & PÄLIKE, 2004).The results of cooling have been a shallowing of thermocline depth and reduction in the abundance of shallow-and warm-water species (SPEZZAFERRI et al., 2002).The faunal overturn is indicated by the extinction of specialised Eocene forms (K-strategists), such as turborotalids, globigerinathekids, hantkeninids, and their replacement by generalised globigerinid form species.In addition, the decrease in the size of specimens is remarkable, as well as increased abundances of the opportunistic taxa and cool water indices, increased numbers of the non spinose, deeper dwelling forms, and reduction in diversity (PREMOLI SILVA & BOERSMA, 1988;BOERSMA & PREMOLI SILVA, 1991).

Oligocene
The investigated pseudohastigerinids, chiloguembelinids and tenuitelids within the planktonic foraminiferal association suggest cooler water conditions in the early Oligocene (SPEZZAFERRI & PREMOLI SILVA, 1991).Pseudohastigerinids did not persist into Zone O2.
The extinction of Chiloguembelina cubensis marks the top of Zone O4.Species of the Globigerina ciperoensis group (G.ciperoensis, G. angulisuturalis) were best adapted to the Late Oligocene environmental conditions.They possess a small lowered trochospiral test with a cancellate and spinose wall texture.These species are abundant at low latitudes outside upwelling areas, and they seem to prefer higher salinity water conditions (SPEZZAFERRI & PREMOLI SILVA, 1991;SPEZZAFERRI, 1994).In Zone O6, very tiny microperforate and the rare species Streptochilus pristinum (Pl.6, Fig. 10) have been found.Furthermore, Globigerinoides primordius has its first occurrence in the planktonic foraminiferal assemblage of the late Oligocene.This species with a smooth and spinose wall texture indicates a shift to warmer climatic conditions in the Late Oligocene.

CONCLUSION
The Palmyride area in Syria contains a succession of Upper Eocene to Upper Oligocene deposits.On the basis of a rich planktonic foraminiferal association, biozones from E14 (P15) to O6 (P22) have been identified.Biostratigraphic zonal boundaries are determined on the last occurrence of the index taxa.Significant changes marked by the extinction of muricate forms, except for small acarinids, have occurred at the beginning of Zone E14.During the Late Eocene, the number of warm shallow dwelling species decreases, thus indicating the onset of global cooling, which culminated in the earliest Oligocene.This significant climatic change is well documented by the planktonic foraminiferal assemblage collected from the studied area.A faunal overturn is indicated by the extinction of specialized Eocene forms (K-mode life strategy), such as turborotalids, globigerinathekids, hantkeninids and their replacement by cooler, opportunistic globigerinid forms.The early Oligocene, as a period of glacial episodes, is characterised by dominance of the r-strategist planktonic foraminiferal species, such as pseudohastigerinids, chiloguembelinids, globigerinids, and tenuitelids.The planktonic foraminiferal association of the Late Oligocene shows a trend towards warming, due to the presence of specimens from the Globigerina ciperoensis group and Globigerinoides primordius.
A very low number of benthic foraminifera in the association imply sedimentation in the deep, open-sea environment during the Late Eocene.Through the Oligocene, the proportion of benthic foraminifera in the assemblage gradually increased, thus indicating the shallowing of the sedimentary environment, connected with the global sea level decrease and regional tectonic events.

Fig. 1
Fig. 1 Map of Syria showing the location of the main tectonic zones (modified from BREW, 2001).A circle represents the approximate location of the Hayan exploration block.