A. ELMAS & E. MERİÇ

Department of Geology, Istanbul University, 34850, Avcılar, Istanbul, Turkey

A basin consisting of alluvial fan, fluvial clastics, and lacustrine sediments dating from the late middle Miocene was developed in the southern part of the Thrace Basin, Gelibolu Peninsula, and the northwestern part of the Biga Peninsula. This basin was controlled by a southeastern fault which lies parallel to the northwestern rim of the Biga Peninsula. Shallow-marine sediments which occur widely in the upper parts of the sedimentary sequence in the basin indicate an initial connection between the Sea of Marmara and the Mediterranean during the middle-late Pannonian, extending from southern Thrace to west of Biga Peninsula.

The Saros-Ganos segment, forming the branch of the North Anatolian fault, developed as a positive flower structure in the late Miocene - early Pliocene. This structurally high area obstructed the seaway connection around Saros Bay. Clastics, originating from this positive morphology, filled the Dardanelles Strait area of today.

A late middle Miocene fault which controlled the geometry of the basin from the eastern shoulder in along the Biga Peninsula, was reactivated as a right-lateral strike-slip fault during the late early Pliocene (Ezine-Şarköy fault). Synthetic faults associated with this major tectonic structure caused the opening of the Dardanelles. This tectonism resulted in the reconnection between the Mediterranean and the Sea of Marmara. The ages and foraminifer, ostracod, and nannoplankton assemblages determined from wells in the Gulf of İzmit, indicate intermittent connections instead of a continuous one. The oldest Mediterranean connection was in the late Pliocene-early Pleistocene time. The next connection occurred in the early-middle Pleistocene and the last one occurred during the late Pleistocene-Holocene time.

Key words: Sea of Marmara, Mediterranean, Dardanelles, Neotectonics.

INTRODUCTION

The seaway connection between the Sea of Marmara and the Mediterranean has been investigated extensively. However, when, where and how this connection occured has remained a problem. Most previous investigators (Penck, 1917; Yalçınlar, 1949; Ardel and İnandık, 1957; Erol, 1968; Şentürk and Karaköse, 1987) viewed the Dardanelles as a fluvial valley. Önem (1974) proposed that the Dardanelles is a graben. According to Erol (1992), the Dardanelles is an epigenic fluvial valley developed under the control of Pliocene-early Pleistocene faulting, and the sea invaded the valley during the late Pleistocene-Holocene. Stanley and Blanpied (1980) stated that the connection between the Sea of Marmara and the Mediterranean via the Dardanelles occurred during the last 12,000-9,500 years. The data from the surface sediments of the Gulf of İzmit show that the latest sea connection goes back at least 35,000 years (Çetin et al., 1995).

Herein, the units older than middle Miocene will be treated as basement units. The units developed in the basins which were affected directly by the NAF will be discussed under the heading of middle-upper Miocene / Quaternary units.

BASEMENT UNITS

The oldest unit in the Gelibolu Peninsula is the Upper Cretaceous-Paleocene ophiolitic melange (Okay et al., 1990). It is unconformably overlain by the Ypresian-lower Lutetian (Siyako et al., 1989) regressive sequence (Önal, 1986; Sümengen et al., 1987). An upper Lutetian-upper Eocene transgressive sequence overlies these units. The late Eocene-late Oligocene period is represented by strata displaying a regressive character (Sümengen et al., 1987). The whole sequence forms the basement of the middle-upper Miocene / Quaternary sequence in the Gelibolu Peninsula.

In the Biga Peninsula, three tectonic zones of pre-Tertiary age extend from the northeast to the southwest. These are a Permo-Carbonifereous metasedimentary sequence and an overlying ophiolite (The Ezine Zone), the Upper Cretaceous-Paleocene ophiolitic melange (Çetmi Ophiolitic Melange), and a gneiss-amphibolite-marble complex (Kazdağ Group) overthrusted by a Triassic blocky metavolcano-sedimentary sequence (Karakaya Formation) (Sakarya Zone; Okay et al., 1990). The Jurassic-Upper Cretaceous sequence overlies the Karakaya units. The middle Eocene neritic carbonates and the upper Eocene turbidites with andesitic tuff-lava intercalations form the lowermost parts of the Tertiary sequence. These are overlain by the lower-middle Miocene calcalkaline volcanic and lacustrine deposits. The whole sequence forms the basement of the middle-upper Miocene / Quaternary sequence in the Biga Peninsula.

THE MIDDLE-UPPER MIOCENE / QUATERNARY SEQUENCE IN BIGA PENINSULA

The middle-upper Miocene / Pliocene sequence (Dardanelles Group) is represented by disordered blocky conglomerate-sandstone-siltstone at the base (Sarıyar Member; Şentürk and Karaköse, 1987; Gazhanedere Formation; Saltık, 1974). They dip toward basement units on the east (Fig. 1b). Coarse grained poorly sorted material become finer grained and well sorted toward the west. Lateral and vertical transitions of the units are abrupt. The pebbles within the channel fills are imbricated. The unit contains cut-and-fill structures, mud flow, mass flow, sheet flood and stream flood deposits. Thus, this unit whose age is early Pannonian on the basis of stratigraphic position, indicates deposition in an alluvial fan and less commonly, in fluvial environments.

Fluvial deposits (Bayramiç Formation; Siyako et al., 1989) consisting of conglomerate, sandstone and shale rest unconformably on the shallow-marine carbonates. The upper parts of this unit include some lacustrine carbonates (Gülpınar Formation) (Fig. 2). The unit is interfingered with basaltic rocks (Taştepe Basalt) extruded along faults. Radiometric ages from these basalts give 3.8 Ma (Y. Yılmaz 1994, personal communication). The shallow-marine equivalents of the Bayramiç Formation, determined in oil wells in Edremit Bay and outcrops situated in the southern parts of the Çanakkale city center, near the Dardanelles are conformable with the underlying upper Miocene units (Siyako et al., 1989). The age of the unit is accepted as latest Miocene-Pliocene.

THE MIDDLE-UPPER MIOCENE / QUATERNARY SEQUENCE IN GELIBOLU PENINSULA

Middle-upper Miocene fluvial, lacustrine, lagoonal, coastal and off-shore deposits (Çanakkale Formation; Şentürk and Karaköse, 1987) unconformably overlie basement rocks that are composed of Upper Cretaceous-Oligocene sedimentary rocks and an ophiolitic melange in Gelibolu Peninsula and southern Thrace, and volcanic rocks around Enez (Fig. 1b).

Overlying the fluvial deposits (Anafartalar Member) and shallow-marine carbonates (Bayraktepe Member), a unit (Conkbayırı Formation; Kellog, 1973) composed of intercalated mudstone, sandstone, silt-claystone and conglomerate rests unconformably (Şentürk and Karaköse, 1987, Sümengen et al., 1987) (Fig. 2). Channel-fill deposits in the unit were derived from Eocene-Oligocene units croping out west of Gelibolu Peninsula. Mud flow, mass flow, sheet flood, and stream flood deposits are abundant (Şentürk and Karaköse, 1987). The thickness of the unit and the grain size (block-fine sand-silt-clay) of the clastics decreases from west to east. The regressive sequence (Yaltırak, 1995a), forms a syncline whose flank is vertical near the western boundary and dips gently in the east (Şentürk and Karaköse, 1987). The unit including vertebrate and rare fresh-water ostracodes is argued an early Pliocene-early Pleistocene age on the basis of stratigraphic position (Yaltırak, 1995a), and a Turolian age on the basis of micromammals (Şentürk and Karaköse, 1987). Considering the stratigraphic and sedimentologic characteristics, the unit appears to have been deposited in an alluvial fan environment on the slopes of topographically high areas of the western parts of Gelibolu Peninsula (Saner, 1985). In the area between Mürefte and the Gaziköy fault (Fig. 1b), Pliocene coarse clastics (Hoşköy Formation; Bargu, 1989), constitute the equivalent of the Conkbayırı Formation.

A Bacunian-Thyrenian sequence is situated in the upper parts of the middle-late Miocene-Quaternary sequence in Gelibolu Peninsula (Fig. 2). The sequence (Marmara Formation; Yaltırak, 1995b), composed of fluvial deposits at the base and upper parts, shallow-water, warm-marine fauna-bearing carbonates and clastics in the middle parts, overlies unconformably the Conkbayırı Formation. North of Gelibolu Peninsula, along the shore between Gaziköy and Mürefte, the lower-middle parts of the sequence are represented by transgressive deposits, whereas the upper parts are regressive deposits (Gaziköy Formation; Bargu, 1989). The unit is overlain unconformably by a Holocene succession.

PALEONTOLOGIC DATA

In the Gulf of İzmit, samples comprising a unit 118.45 m in total thickness was obtained by drilling nine boreholes (Figs 1a, 3) between Hersek and Kaba promontories. As a result of analysis, the arrival periods of the Mediterranean waters to the Gulf of İzmit was determined as late Pliocene, Pleistocene, and Holocene.

Nannoplankton assemblage

At the base of the 118.45 m section, between 112.45-112.00 m, the Discoaster brouweri Zone was identified (Toker and Şengüler, 1995). Thus part of the sequence was deposited during the latest Pliocene. On the other hand, the existence of genera and species belonging to the Pleistocene Pseudoemiliana lacunosa Zone between 106.45-106.00 m and 70.45-70.00 m depth indicates that the Gulf of İzmit was influenced by Mediterranean waters during this period.

Foraminiferal assemblage

The deepest well between the Hersek and Kaba promontories is KS-2 borehole (Fig. 3). Between 118.45-118.00, 112.45-112.00, and 76.00-75.55 meters of this drill hole, foraminifers of Mediterranean origin (Meriç and Sakınç, 1990; Cimerman and Langer, 1991; Sgarella and Montcharmont-Zei, 1993) were found together with late Pliocene nannoplankton in the younger sedimentary rocks (Table 1). This indicates that Mediterranean waters reached the Gulf of İzmit during this period. The age deduced from mollusc shells in the KS-2 well between 55.20-54.75 m yielded 817.000 ± 105.000 yrs by the ESR (electron spin resonance) method. Therefore, the age of these parts of the sequence is eopleistocene (Koreneva and Kartashova, 1978; Yanko, 1990).

The existence of early-middle Pleistocene sediments in the upper parts (KS-2, S-3, S-2, S- 4 wells) of the investigated sequence (Fig. 3) was determined by the ESR method (Çetin et al., 1995). They obtained ages of 693,000 ± 126,000 yrs (S-4, 71.70-71.25 m), 664,000 ± 94,000 yrs (KS-2, 42.10-41.60 m), 320,000 ± 37,000 yrs (S-3, 70.45-70.00 m), 306,000 ± 39,000 yrs (S-2, 79.25-78.80 m), 254,000 ± 34,000 yrs (S-7, 77.00-76.55 m), 199,000 ± 22,000 yrs (S-8, 74.20-73.75 m), 198,000 ± 23,000, 195,000 ± 20,000 yrs (S-4, 55.45-55.00 and 52.45-52.00 m) and 186,000 ± 20,000 yrs (S-8, 64.45-64.00 m). In the KS-2 (42.10-41.60 m), S-3 (70.45-70.00 m) and S-2 (79.25-78.80 m) wells, characteristic Mediterranean foraminifers (Meriç and Sakınç, 1990; Cimerman and Langer, 1991; Sgarella and Montcharmont-Zei, 1993) are found (Table 1). These data show that the Gulf of İzmit again received Mediterranean waters in the early-middle Pleistocene.

Finally, the region was influenced by Mediterranean waters from 35,200 ± 8,100 yrs. The obtained age data are: 35,200 ± 8,100 yrs (50.05 m), 33,700 ± 7,200 yrs (40.45-40.00 m), 31,700 ± 5,400 yrs (38.45-38.00 m) in the S-5 drill hole; 24,800 ± 3,700 yrs (44.45-44.00 m) in the S-3 drill hole; 15,400 ± 3,700 yrs (26.60-26.15 m) in the S-5 drill hole; 11,600 ± 2,800 yrs (5.20-4.75 m), 6,600 ± 700 yrs (3.20-2.75 m) in the KS-2 drill hole; 5,000 ± 900 yrs (8.20-8.00 m) in the S-5 drill hole; and 500 ± 200 yrs (37.45-37.00 m) in the S-3 drill hole (Çetin et al., 1995).

The foraminiferal assemblage observed in the upper levels of the wells were evaluated collectivelly. Results show the abundance of foraminifers of Mediterranean origin in the Gulf of İzmit during 12,000 ± 1,900 and 500 ± 200 yrs (Table 1).

As a result, the seaway connection between Mediterranean and the Sea of Marmara was established in the late Pliocene and Quaternary.

Ostracod assemblage

Samples from the wells between Hersek and Kaba promontories contain a rich ostracod fauna. Especially the ostracod assemblage (Gülen et al., 1995) (Table 2) from the KS-2 well (118.45-118.00 m, 94.45-94.00 m and 55.20-54.75 m) (817,00 ± 105,000 yrs; Çetin et al., 1995) confirmed the arrival of ostracodes of Mediterranean, Aegean, and Adriatic origin to the Sea of Marmara during late Pliocene (Toker and Şengüler, 1995) - early Pleistocene time.

In the upper parts of the young sediments, Mediterranean, Adriatic, and Aegean ostracodes (Table 2), were obtained (Gülen et al., 1995) yielding ages of 320,000 ± 37,000 yrs (S-3, 70.45-70.00 m), 306,000 ± 29,000 yrs (S-2, 79.25-78.80 m) (Çetin et al., 1995). These results indicate that they lived in the Sea of Marmara during the early-middle Pleistocene. The ostracodes reported by Gülen et al., (1995) from the upper Pleistocene-Holocene sediments are also of Mediterranean, Aegean, and Adriatic origin (Puri et al., 1969; Barbeito-Gonzalez, 1971; Bonaduce et al., 1975; Ruggieri, 1976; Yassini, 1979) (Table 2).

An eastern Mediterranean form, Urocythereis britannica (Atthersuch) is present in Pliocene sediments of the Aegean region (Sissingh, 1972), in Cyprus (Athersuch, 1977) and in some areas in the Aegean Sea such as Edremit Bay and west of Bozcaada (C. Kubanç 1994, personal communication). Thus, the species reached the Mediterranean and Aegean Sea in the Pliocene, and to the Sea of Marmara in the early-middle Pleistocene, from the Atlantic through a seaway connection.

DISCUSSION AND EVOLUTION

At the lower parts of the middle-upper Miocene / Quaternary sequence (Dardanelles Group and Çanakkale Formation) in the region, the observed east-to-west facies changes (Figs 2, 4) indicate the existence of a topographic high to the east of the Lapseki-Ezine line in Biga Peninsula during late Aragonian - middle Pannonian time. An alluvial fan (Sarıyar Member; Şentürk and Karaköse, 1987) formed toward the west from the topographic high produced by a fault (Ezine-Şarköy fault). Due to this topographys streams flowed from the east to the west (Anafartalar Member; Şentürk and Karaköse, 1987), generating down-stream lacustrine environments (Göksu Member; Şentürk and Karaköse, 1987) (Fig. 4b). On the other hand, the lacustrine basins (Çamrakdere Member; Şentürk and Karaköse, 1987), which developed since the late Vallasian, formed in the central parts of the area produced by river deposits. The sites of these basins coincide with the modern-day Dardanelles (Fig. 4b).

The middle parts of the Dardanelles Group are represented by a transgressive sequence (Alçıtepe Formation, Önem,1974; Bayraktepe Member, Şentürk and Karaköse, 1987) begins with clastics and passes upward to carbonates. The middle-upper Pannonian shallow marine carbonates which forming the upper parts of the sequence, overlie also basement rocks in the Biga Peninsula (Fig. 4c). The unit forms the unique parts of the Dardanelles Group in which marine fauna existed. The shallow marine carbonates crop out almost everywhere in the region, except for tectonically uplifted areas such as north of Ganos fault, west of Gelibolu Peninsula and near-east of the Dardanelles (Fig. 1b). All the characteristics of the unit indicate the following deductions: a) Following the middle Pannonian, the areas of lacustrine environment became a shallow sea, b) This shallow sea covered a wide area, c) In the middle (?) - late Pannonian (9-10 Ma), the Sea of Marmara and the Mediterranean were connected by a wide seaway around the modern-day Dardanelles, Gelibolu Peninsula and Saros Bay, and d) However, lagoonal deposits are exist in the western parts of the Gelibolu Peninsula (Yaltırak, 1995b). In addition to this, the fluvial deposits progresivelly filled the lacustrine basins (Çamrakdere Member) in the northwest part of the Biga Peninsula and the west of the Gelibolu Peninsula in the Pannonian. Thus, land areas developed in the center of the wide seaway. During the middle (?) - late Pannonian, the connection between the Sea of Marmara and the Mediterranean occurred essentially via the modern-day Dardanelles and Saros Bay (Fig. 4c). The western part of the Gelibolu Peninsula was in the position of small islands within the wide seaway in the middle (?) - late Pannonian.

Accompanying the development of the late middle Miocene-late Miocene units (Dardanelles Group and Çanakkale Formation), Marmara trough also started to subside. The cause of this is the development of several pull-apart structures along the northern branch of the NAF in the region (Şengör et al, 1985; Barka and Kadinsky-Cade, 1988; Barka, 1992). According to Şentürk and Karaköse (1987), the development of the pull-apart basins started in the pre-late Miocene, whereas Barka (1992) suggests a Pliocene age. The ages of non-marine and lacustrine deposits on the basement units around Dardanelles, Saros Bay and Sea of Marmara, indicate that the western prolongation of NAF, which controls the development of these basins, has existed since the late middle Miocene. According to Şengör et al, (1985), the westward movement of the Anatolian plate caused by the NAF, was started in the middle-late Miocene (late Serravalian-Tortonian). In addition to this, the early-middle Miocene basins in the neighbourhood of NAF are unrelated to the fault but the late Miocene-Pliocene basins controlled by releasing bends of the fault zone (Barka, 1992).

During the late Miocene, the development of a basin (Enez Graben) between Semadirek and Hisarlıdağ highs took place (Fig. 4a). The seismic sections indicate that Dardanelles Group / Çanakkale Formation is the initial unit covering the Oligocene upper age basement units in the Enez and Saros grabens, as in the land areas (Saner, 1985). The lower parts of Mio-Pliocene deposits (they are the equivalents of the Dardanelles Group / Çanakkale Formation), that determined in the seismic sections, are cut by the syn-sedimentary faults. Afterwards, they transgressively overlie the Semadirek and Hisarlıdağ highs (Saner, 1985). Thus, the subsidence in the Enez Graben began since the development of Dardanelles Group / Çanakkale Formation (in the late middle Miocene) and ended in the late Miocene. The horizontal position and the stability of the Quaternary deposits in the region imply absence of an active faulting recently (Saner, 1985).

Anafartalar fault (Yaltırak, 1995a), that formed as a positive flower structure (Christie-Blick and Biddle, 1985), raised the western parts of the Gelibolu Peninsula. The seaway connection obstructed by this reason around the Saros Bay since the latest Miocene. The uplifted area between the Anafartalar fault and the fault bounding the Saros graben to the south, caused the development of the alluvial fan deposits (Conkbayırı Formation; Kellog, 1973) at the eastern areas (Saner, 1985; Yaltırak, 1995a). The high area extends from Mursallı to the southwest edge of the Gelibolu Peninsula. Conkbayırı Formation is situated to the east of the uplift (Figs 1b, 5b). The compression and uplift in the western parts of the Gelibolu Peninsula have not been resulted in faulting everywhere in the region (Fig. 5). Şentürk and Karaköse (1987) reported the existence of occasionally observable reverse faults. In the middle parts of the Gelibolu Peninsula, the units forming the basement of the middle-upper Miocene deposits are overturned towards the southeast (Saner, 1985). Erkal (1991) reported, that the presence of the strike-slip faults branching from the NAF at the north of Şarköy. The northern blocks of these faults overthrusted towards the southeast (Sümengen et al., 1987). Thus, the uplift and occasional breakup of the western parts of the Gelibolu Peninsula are restricted with the initial stage of the development of Conkbayırı Formation whose age is thought to be late upper Miocene (?) - lower Pliocene.

The fault bounding the Saros graben to the south, and the Anafartalar fault are also responsible for the termination of the Ezine-Şarköy fault, that activated in the late middle Miocene-early late Miocene period, until the early Pliocene (Fig. 5). The late upper Miocene-lower Pliocene stream / lacustrine deposits (Bayramiç and Gürpınar formations; Siyako et al., 1989) in the Biga Peninsula are the time equivalents of the Conkbayırı Formation which controlled by the Anafartalar fault in the Gelibolu Peninsula (Fig. 2). The lithological characteristics of the lower and middle parts of these deposits indicate that the fault (Ezine-Şarköy fault) terminated its activity in their development period. The shallow marine equivalents of the Bayramiç and Gülpınar formations found in the well in the Edremit Bay and their outcrops near to Dardanelles in the south of Çanakkale, are conformable with the underlying upper Miocene units (Siyako et al., 1989). Therefore, the shallow marine environment continuing since the late Miocene around Edremit Bay. The shallow seaway (occupying the Dardanelles also), which existed between the Sea of Marmara and the Mediterranean in the late Miocene, was obstructed by the erosion of the area raised due to the Anafartalar fault and the filling up of the trough situated in the east, during the latest Miocene-early Pliocene (Fig. 5b). But, in the farther east areas (the western part of Biga Peninsula), where the filling material could not have been reached, the deposition has continued in a fluvio-lacustrine environment since the late Miocene (Fig. 5b). In spite of the shallowing up, the continuity of the deposition should have been caused by the westward escape of the southern block of the fault bounding the Saros graben to the south. This escape subsequently, will have caused the reactivation of the fault (Ezine-Şarköy fault) in the northwest of Biga Peninsula during the late early Pliocene time.

In Biga-Bayramiç-Çanakkale area, the nature of volcanic activity are calc-alkaline in the Miocene (Ercan 1981), alkaline basaltic at the end of Miocene (Borsi et al., 1972) and alkaline in the late Pliocene (Ercan, 1981). The lavas (Taştepe basalt; Siyako et al., 1989) extruded along faults in the Biga Peninsula are intercalated with the upper parts of the Bayramiç Formation and rest on it. Radiometric ages from these basalts, which extend through the line between the north of Ezine and the south of Lapseki (Fig. 1), give 3.8 Ma (Y. Yılmaz 1994, personal communication). The fault (Ezine-Şarköy fault), which extruded the basic lavas, forms the boundary between the basement units in the east and the middle-upper Miocene / Pliocene units in the west. Thus, this fault, that affecting previously the development of the late middle Miocene-early upper Miocene alluvial fan / stream deposits (Sarıyar Member and Gazhanedere Formation) in the Biga Peninsula, reactivated since the late lower Pliocene (Fig. 6). The subsidence on the western part of the fault caused the development of a trough. The Ezine-Şarköy fault is the prolongation of a dextral strike slip fault (Ambraseys, 1970; Allen, 1975; Sümengen et al., 1987) causing to the development of a compressional trough (Şengör et al., 1985) in the off-shore region of Şarköy and Kumbağ (Figs 7, 8). Furthermore, the situation of this fault is compatible with the strike slip faults of the pull-apart basins (Şengör et al., 1985; Barka and Kadinsky-Cade, 1988; Barka, 1992) beneath the Sea of Marmara (Fig. 7). The opening of the Dardanelles should have been controlled by the structural entities related to these strike-slip fault zones. These structural features are simple synthetic faults, that constitute one of the fracture sets making 60-70° with each other, and that make 10-30° and has the same sense of slip with the main fault (Wilcox et al., 1973) (Fig. 6). The structures should have been caused the seaway connection between the Sea of Marmara and the Mediterranean. Because the drill hole data from the Gulf of İzmit indicate the connection of the Sea of Marmara and Mediterranean since the late Pliocene. Although these structures can be observed in the land, and their larger scaled traces should be hidden in the Dardanelles. Moreover, westward escape of the southern block of the fault bounding the Saros graben to the south accelerated the extension and the subsidence in the region.

The formations of the extensional areas and troughs in the region, should have been caused by the strike slip faults owing to vertical component. The faults bounding the Saros graben have to the horizontal component together with the vertical component (McKenzie, 1978). In addition to that, the distribution of M > 2.5 earthquakes, which recorded by the Kandilli Observatory between the 1976-1990 in the Marmara region, indicates that the pull-apart basins along the northern branch of the NAF have continuous activity (Üçer et al., 1985). However, the connecting strike-slip faults and the branches of the NAF in the Biga Peninsula have little or no activity (Barka, 1992). In this case, the strike slip components of the faults, which cause to the opening of the Dardanelles, are dominating recently.

The fault system, which gave rise to the opening of the Dardanelles, follows the boundary between the two tectonic units welded in the early Tertiary. One of these is the Upper Cretaceous-Paleocene ophiolitic melange (Çetmi Ophiolitic Melange / Gelibolu Zone; Okay, et al., 1990) which forms the basement in the Gelibolu Peninsula. The other one is the Permo-Carbonifereous metasedimentary rocks and the ophiolite nappe thrust onto it (Ezine Zone; Okay et al., 1990) in the basement of the northwestern parts of the Biga Peninsula (Fig. 9).

In the eastern coast of the Gelibolu Peninsula, Baccunian-Tyrhenian sequence (Marmara Formation; Yaltırak, 1995b) unconformably rests on the upper Miocene and lower Pliocene sedimentary rocks. The sequence, which is transgressive in the lower-middle parts and regressive in the upper parts (Bargu, 1989), includes shallow marine levels. The sequence is an evidence on the land areas for the connection between the Sea of Marmara and the Mediterranean via the Dardanelles during the late Pleistocene. In addition, the Tyrhenian marine units (Erinç, 1956) of the graben in the Gulf of İzmit, were developed by the influence of the meso-scaled strike slip and other type faults (Şengör et al., 1982). The equivalents of the Tyrhenian deposits (Altınova Formation, Sakınç and Bargu, 1989) in the İzmit-Karamürsel region can be observable in the southern area of the Gaziköy fault. The upper Pleistocene deposits on the southern block of the Gaziköy fault was uplifted as much as 80 m (Bargu, 1989).

CONCLUSIONS

The stratigraphic and paleontologic data which obtained from the land areas, indicate an initial seaway connection between the Sea of Marmara and the Mediterranean during the middle-late Pannonian, extending from southern Thrace to west of Biga Peninsula. This seaway connection is not directly related to the tectonism. The Saros- Ganos segment, forming the branch of the North Anatolian fault, developed as a positive flower structure in the late Miocene - early Pliocene. This structurally high area obstructed the seaway connection around Saros Bay. Furthermore, clastics originating from this positive morphology, filled the Dardanelles Strait area of today. In addition, the foraminifer, ostracod and nannoplankton assemblages of Mediterranean origin are found in the late Pliocene-Holocene deposits of the Gulf of İzmit. These data suggest that the Gulf of İzmit and the Sea of Marmara has been connected to the Mediterranean. But this connection related directly to the tectonism and occurred since the late Pliocene via Dardanelles. The data and the foraminiferal assemblage of sediments in İzmit Bay indicate intermittent connections instead of a continuous one. The oldest Mediterranean connection occurred in the late Pliocene-early Pleistocene time. The next connection was in the early-middle Pleistocene and the last one occurred during the late Pleistocene-Holocene time.

Acknowledgements. The authors are thankful to Dr Tuncay Taymaz for his help.

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FIGURE CAPTIONS

Figure 1. (a) The localities of the drilling wells in the İzmit Bay. (b) Simplified geological map of the southern Thace, Gelibolu and Biga Peninsula (Modified from Sümengen et al., 1987; Şentürk and Karaköse, 1987; Siyako, et al., 1989; Yaltırak, 1995 a-b; Erkal, 1991)

Figure 2. Columnar section showing late middle Miocene - Holocene units in the Gelibolu and Biga Peninsula.

Figure 3. The distribution of the Mediterranean foraminifers in the İzmit Bay for the late Pliocene and Quaternary time.

Figure 4. Structural map (a) and block diagrams of the study area for the late Aragonian - early Pannonian (b), middle / late Pannonian (c) time.

Figure 5. Structural map (a) and block diagram (b) of the study area for the late upper Miocene - early Pliocene.

Figure 6. Structural map (a) and block diagram (b) of the study area for the late Lower Pliocene - late Pliocene.

Figure 7. Enhanced Landsat 5 TM image of the neighbourhood areas of Dardanelles USGS (White arrow points to trace of the Ezine - Şarköy fault).

Figure 8. Neotectonic faults of the Marmara region (Adapted from Saner, 1985; Barka and Kadinsky-Cade, 1988; Siyako, et al., 1989; Smith et al., 1995).

Figure 9. Geotectonic map of the region around the Marmara Sea (from Okay, et al., 1990).

Table 1. Foraminifera assemblage (from Meriç, 1995) for the late Pliocene - Qaternary period of the İzmit Bay. Numeric values from Yanko (1990).

Table 2. Ostracod assemblage (from Meriç et al., 1995) for the late Pliocene - Quaternary period of the İzmit Bay. Numeric values from Yanko (1990).