Introduction

Coal is an important economic resource in many countries globally, due to its decisive position in the world energy mix. In the last 50 years, the share of oil in the world has increased from 46.2 to 33%, natural gas from 16 to 24%, nuclear energy from 0.9 to 4%, renewable energy sources from 1.9%, increased from 5 to 5% (BP 2020). In the same period, the share of coal increased 2.5 times from 24.5 to 27%. According to the estimates made by the National Energy Agency (IEA), based on the assumption that current energy policies will not change much in the future, the world primary energy demand will increase by 45% compared to 2020 and will rise to the level of 20 MTOE (million-ton equivalent oil) in 2040. It is anticipated that there will be no significant differences. Accordingly, in 2040, it is estimated that oil will have 27.5%, coal 27.1%, natural gas 24%, nuclear energy 53%, and other sources 16.1% in the global energy supply (IEA 2020). These probabilistic data show that coal will continue to play a decisive role in the world’s energy resources in the next 20 years and will have a share of approximately 43%.

Together with lignite, asphaltite, and hard coal, the total coal resource in Turkey is approximately 21 billion tons (MTA 2019; TKI 2019). Approximately 92.7% (19.32 billion tons) of this is lignite and asphaltite, while 7.3% (1.52 billion tons, 736 million tons of which is visible) is hard coal resource (TTK 2020).

Paleogene-Neogene aged coal deposits in Turkey are mostly deposited in Miocene and Eocene aged basins (Fig. 1). Although the Miocene basins spreading in the southern regions of Turkey are mostly marine, most of the basins in western and central Anatolia are filled with sediments deposited in terrestrial facies. The vast majority of lignite reserves in Turkey are located within the terrestrial Miocene basins, which can be interconnected (Helvacı et al. 1987; Yağmurlu et al. 1988).

Fig. 1
figure 1

Location of the Mengen Eocene basin in the Central Turkey and geological setting of the other coal reserves in Turkey (modified from Toprak 2009)

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Eocene basins spreading in Turkey mostly contain sedimentary sequences deposited in deep and shallow marine environments (Akarsu 1974; Özen 1975; Gümüşsu 1980; Cerit 1983; Hoş-Çebi and Korkmaz 2013). In the southern regions of Turkey, there are mostly Eocene basins deposited in a deep marine environment. The rock assemblages in these basins are predominantly composed of turbidites, pelagic carbonates, and accompanying volcanic rocks (Ketin 1983; Helvacı et al. 1987; Yağmurlu et al. 1988; Yağmurlu and Helvacı 1995). On the other hand, Eocene marine basins containing coal in Turkey are mostly found in the northern parts of Turkey (Fig. 1). These basins contain mostly reefal carbonates deposited in neritic environment, as well as a sedimentary sequence consisting of bituminous shale, mudstone, sandstone, marl, bituminous marl, and accompanying coal levels (Uysallı 1961; Akarsu 1974; Kaya and Dizer 1981-1982a; Şengüler et al. 1988; Sarı et al. 2004; Sarı and Sonel 1995; Karayiğit et al. 1996).

The Bolu-Mengen Eocene basin has attracted the attention of many researchers in previous years due to the coal deposits it contains and the interesting geological structure it reflects (Blumenthal 1937; Özen 1975; Gümüşsu 1980; Özdemir and Bekmezci 1983; Erdem and Akalın 1983; Karayiğit et al. 1996; Erendil et al. 1991; Sarı 1999a,199b; Sarı and Sonel 1995; Sarı et al. 2004, 2007, 2016; Şengüler et al. 1988; Şener and Şengüler 1998; Tekin and Sarı 1999, 2000; Atalay 2001; Altunsoy et al. 2001, 2003; Koç and Türkmen 2002; Büyükutku et al. 2005; Hoş-Çebi and Korkmaz 2013). In the first studies carried out in the region, the lithostratigraphic features of Paleogene-Neogene aged sediments filling the Bolu and Mengen basins were investigated, and it was determined that these sediments were predominantly composed of deposits with carbonate and clastic lithologies (Blumenthal 1948; Kaya and Dizer 1981-1982b).

Eocene basins spreading in Turkey mostly contain sedimentary sequences deposited in deep and shallow marine environments (Akarsu 1974; Özen 1975; Gümüşsu 1980; Cerit 1983; Hoş-Çebi and Korkmaz 2013). In the southern regions of Turkey, there are mostly Eocene basins deposited in a deep marine environment.

In recent years, the discovery of deposits with very large coal reserves in Neogene and Paleogene basins in many regions, especially in Western Anatolia (Eskişehir-Alpu, Afyon-Dinar, Tekirdağ-Malkara, Istanbul-Silivri, etc.), the reconstruction of Neogene and Paleogene basins in Turkey necessitated a detailed examination. In addition, it is seen that the details of the Eocene coals were not revealed in the previous researches related to the Mengen basin. The study area is located around Mengen, 60 km from Bolu (Figs. 1–2).

Fig. 2
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Location map and geological map of the Mengen Eocene basin

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In this study, coal-bearing units and their storage systems in the Eocene Bolu-Mengen basin, which have not been studied in detail before, were investigated. In the Mengen-Gökçesu Eocene basin, in addition to the drilling studies carried out in previous years, with new drilling studies, to reveal the existence of possible new coal seams and to determine the stratigraphic, sedimentological, and tectono-sedimentary characteristics of the coals in the lateral direction and their lithofacies changes in the lateral direction, the coal formed in the lagoon type platform basins. The main purposes of this study are to make paleodeposition conditions of the formations and to make paleogeographic modeling of the basin belonging to the Eocene period.

Materıal and methods

In this study, which mostly covers the middle-eastern part of the Mengen-Gökçesu Eocene basin, detailed 1/10,000-scale geological maps of the basin were taken, and according to the map data, the stratigraphic positions of the rock units in the region and the lateral distribution of the coal outcrops accompanying them were tried to be determined.

The data of the drillings carried out by MTA in previous years (1956, 1957, 1964, 1979, 1983, 1986, and 1987) in the region were re-evaluated and revised, and the locations of the new exploration drillings were determined. Survey drillings and cores of the drillings were evaluated in detail by TKI on the determined locations, and typical sections of each drilling were obtained (M1:508.40 m, M2:671 m, M3:809 m).

By making correlations according to the data obtained from the old and new drillings in the region, the lateral thickness and lithofacies changes in the coal seam/veins and accompanying rock units in the basin were revealed.

Proximate analyzes of the samples taken from the coals cut in the research drillings opened by TKI were made in the MTA coal laboratory. In these analyses, BS ISO 1170–2008 standards (Coal and Coke. Calculation of Analyzes to Different Bases) and following procedure outline by ASTM (1981) were used.

Geological location of the Mengen Eocene basin

The Mengen Eocene basin is located in the southern part of the Pontide belt in main lines. According to Ketin (1983), the Pontides, most of which form the North Anatolian Mountain belt, mainly consists of metamorphic massifs as well as non-metamorphic sedimentary rock assemblages with ages ranging from Ordovician to Pliocene and accompanying volcanic rocks.

The components of the E-W trending North Anatolian Fault Zone (NAFZ), which forms the southern border of the Pontides in most places, pass approximately 40 km south of the Mengen Eocene basin (Fig. 2).

The Mengen Eocene basin, which extends in the NE-SW direction between Mengen-Gökçesu and varies between 2 and 5 km in width, is bounded from the north and south by pre-Eocene basement rocks ranging in age from Mesozoic to Paleozoic.

In the north of the Mengen basin, there are basement rock units consisting of Mesozoic carbonate rocks, as well as Paleozoic-aged metamorphic rocks and accompanying granites belonging to the Sakarya massif. In the south of the basin, pre-Eocene sedimentary rock units consisting of Cretaceous-aged turbiditic sediments are located.

The distribution of the rock units in the Mengen region and the relationships between the fault systems in the region are shown on the geological map given in Fig. 3. As seen on the map, Mesozoic-aged limestones bordering the Mengen basin from the north tectonically overlain the Eocene-aged units filling the basin with a thrust fault.

Fig. 3
figure 3

Revised geological map of the Mengen (Bolu) coal basin (after Gürsoy 1988), and A-B geological cross-section

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Due to this thrust fault, the Eocene-aged units filling the Mengen basin were folded and formed overturned synclines with axial planes inclined towards NW- and NE-axis trending. Besides, the basin is bounded from the south by NE-SW trending normal faults. Therefore, the Mengen basin displays an asymmetrical graben structure.

As seen in the geological cross-section (Fig. 3), showing the relationships of the Eocene-aged sediments filling the Mengen-Gökçesu basin with the underground structures and the surrounding basement rocks, the Eocene sediments filling the basin and accompanying coal seams, reaching approximately 45–50° depending on the overturned syncline structure. They have gained slopes that can be considered high.

The Eocene-aged sedimentary sequence filling the Mengen basin is divided into two main units. These are, from bottom to top, (1) Sazlar Formation and (2) Tokmaklar Formation (Fig. 4).

Fig. 4
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Stratigraphic column section of the Mengen coal basin

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Sazlar Formation

The unit is predominantly composed of thick-bedded and abundant Nummulites limestones deposited under reefal conditions. The formation name was used for the first time by Sarı et al. (2004). Kaya and Dizer (1981–1982b) defined the carbonate sedimentary levels corresponding to the unit as the Asilik formation. The reefal limestones, which are the dominant components of the Sazlar Formation, are mostly medium to dark grayish, thick, very thick regular bedded, terrestrial massive, and Nummulites fossiliferous. Nummulites fossils, which are common in limestone, locally form biostrom-like organic aggregations (Fig. 5). Intermediate levels, mainly composed of sandy limestone, calcareous mudstone, and clayey limestone, are commonly found within the reefal limestone succession.

Fig. 5
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A view from the limestones of the Nummulitic bank on the Mengen-Pazarköy road (425799/4530433-UTM 6°/WGS84)

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The reefal limestones of the Sazlar Formation rest unconformably on the older units below. Limestones belonging to the Sazlar formation go to the NE of the basin and reach a thickness exceeding 250 m and are completely graded into Nummulites-bearing massive limestones.

Kaya ve Dizer (1981–1982b), Nummulites burdigalensis (de la Harpe), Assilina placentula (Deshayes), Nummulites uronensis Heim, Nummulites preaturicus Schaub, Nummulites obtusus (Sowerby), N. uronensis Heim, N. uronensis var, pauncigranulata Doncieux, Nummulites gallensis (Heim), Assilina spira (de Roissy), Assilina exponens Sowerby ve Assilina amillata d’Archiac, based on the presence of foraminifera, determined the age of the unit as Ypresian and Lutetian.

The texture and compositional properties of the limestones, which are the dominant components of the Sazlar formation, indicate that they may be carbonate rocks deposited in reef conditions, mostly offshore. Biostrome-like organic deposits formed by Nummulites fossils within the limestone succession are important in terms of showing the presence of barrier reefs and accompanying patch reefs on the carbonate plain.

Tokmaklar Formation

The Tokmaklar Formation, which sits on the Sazlar Formation with a harmonious and graded contact, is predominantly composed of shale, bituminous shale, coal, bituminous clayey limestone, Nummulites limestone, sandstone, mudstone, and marl levels from bottom to top (Fig. 6). The name of the formation was first used by Kaya and Dizer (1981–1982b) for units containing coal in the Gökçesu region.

Fig. 6
figure 6

Bituminous shale-limestone levels in the Tokmaklar Formation (402,318/4523688-UTM 6°/WGS84)

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Bituminous shales form the dominant component of the Tokmaklar Formation; it is medium-to-dark grayish, locally yellowish-brown, thin-medium layered, and regularly laminated. Intermediate levels consisting of claystone and mudstone are normally found in the bituminous shale level. Sandstones, which constitute the other dominant component of the Tokmaklar Formation, are mostly greenish-gray in color, medium to thick-bedded, fine- to coarse-grained, and medium to poorly sorted. Intermediate levels, consisting mainly of mudstone and conglomerate, are normally found in the sandstone section. Sedimentary structures such as planar and grooved cross-bedding, grain orientation, and load patterns are commonly observed sedimentary structures in sandstones and conglomerates. The clastic grain components that make up the sandstone and conglomerate are predominantly derived from the pre-Eocene metamorphic and sedimentary basement rocks and the Nummulites-bearing limestones of the Sazlar Formation. For this reason, fossiliferous intercalations consisting of Nummulites remains are normally observed in sandstones.

The coals in the lower part of the Tokmaklar Formation are mostly found in alternation with bituminous shale and bituminous clayey limestones. Layer thicknesses in coals vary between 10 and 200 cm. The total thickness of the coals in the coal horizon, which consists of many layers, can reach up to 15 m.

The Tokmaklar Formation overlies the sediments of the underlying Sazlar Formation with a harmonious and transitional contact. Pliocene-aged coarse-grained sediments belonging to the Mengen Formation overlie the Tokmaklar Formation with an eroded unconformity contact.

Kaya and Dizer (1981-1982b), Nummulites millecaput (Boubée), Nummulites uroniensis var. paucigranulata (Doncieux), Assilina exponens (Sowerby), Assilina mamillaîa (d'Archiac), Discocyclina scalaris (Schlumberger) ve Discocyclina sella (d'Archiac), Nummulites crassus (Boubée), Nummulites uroniensis (Heim), Assilina praespira (Douvillé), Discocylina scalaris (Schlumberger) ve Discocylina sella (d'Archiac), determined the age of the unit as Middle Lutetian based on the presence of foraminifera.

The organic matter-rich shale, bituminous shale, and accompanying bituminous clayey limestones and coal layers that make up the Tokmaklar Formation mostly reflect the presence of a deposit developed in a lagoon environment.

It is possible to evaluate the well-sorted sandstones and cross-layered conglomerate channel fillings, which are common in the lower parts of the Tokmaklar Formation, as clastic components of tidal channels developed in a lagoon environment. On the other hand, the lenticular carbonate intercalations of nummulitic limestone found at different levels of the Tokmaklar Formation are important in that they reflect the presence of patch reefs or biostromes that developed in the lagoon environment.

Mengen Eocene coals

The coal deposits in the Mengen basin are mostly located in the Gökçesu region in the western part of the basin. Coal in this region is still processed for underground mining today. Located in the western part of the Mengen basin, the coalfield extends for approximately 15 km in the NE-SW direction between Mengen and Gökçesu. The Eocene basin containing coal is limited by reverse and normal faults from the north and south in this region. Overturned syncline structures have developed in the coal basin, especially due to the reverse faults limiting the basin from the north. In this syncline, which is axially trending mainly in NE direction, the axial plane is inclined to northwest between 50 and 55° (Fig. 3). When the road cuts between Mengen-Gökçesu are examined carefully, it is observed that the sandstone layers within the Tokmaklar Formation are completely inverted, reflecting the southward-turned syncline structure (Fig. 7).

Fig. 7
figure 7

The appearance of the layers within the overturned syncline structure of the Tokmaklar Formation

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It is possible to see the coal seams intercalated with bituminous shale and bituminous limestones in the lower part of the Tokmaklar Formation and the accompanying bituminous shales in the road and stream cuts in the Gökçesu region (Fig. 8).

Fig. 8
figure 8

Organic matter-rich levels observed in the Tokmaklar Formation containing coal and bituminous shale

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Drilling logs

In the section of Bolu province, between Merkeşler and Salıpazarı, approximately 60 drillings were carried out by MTA at different times between 1956 and 1983 and approximately 40 million tons of reserves (probable + apparent) were found (Erdem and Akalın 1983).

In the region between Mengen and Gökçesu (west of the study area), 14 drillings were carried out by MTA in 1986 and 1987 (79/2A, 82/5–7-9–11-17, 86/1–2-3- 5, 87/1–2-4–5), and according to the coal seams cut from these drillings, it was revealed that there are approximately 3.9 million tons of reserves (probable + probable) (Gürsoy 1988) (Fig. 9).

Fig. 9
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Positions and vertical sections of the coal-cutting drillings in the region between Mengen and Gökçesu

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In this study, exploration drillings were carried out at three different points in the SW (M1), NE (M2), and NW (M6) of Mengen. A continental clastic sedimentary succession consisting of fluvial and fluvio-lacustrine sediments belonging to the Neogene Mengen Formation was cut in the M1 drilling.

The fluvial sediments cut in this drilling are predominantly composed of conglomerate, sandstone, and local siltstone-mudstone components. Fluvio-lacustrine sediments, on the other hand, are mostly composed of mudstone and claystone components, and intermediate levels consisting of sandstone and conglomerate. The area where the M1 drilling is located in between the NE-SW trending Yörükler fault and the Mengen fault (Fig. 10).

Fig. 10
figure 10

Vertical section of drillings M1 and M2

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During the M2 drilling carried out at the NE end of the Gökçesu-Mengen Eocene basin, a clastic succession (Tokmaklar Formation) consisting of Nummulitic reefal limestone (Sazlar Formation) and overlying lagoonal sediments conformably was cut from bottom to top. As a result of the gradation of the dominant lagoonal conditions in the Gökçesu section of the basin to reef conditions in the NE direction, organic sedimentary deposits consisting of only clayey coal, with a thickness of 30 to 50 cm, were deposited at 181 and 192 m in the lower part of the Lagunar succession (Fig. 10). A clastic succession consisting of lagoonal sediments of the Neogene aged Tokmaklar Formation was cut in the M6 drilling as well. The coal-bearing zone was entered at 727.60 m of the succession, which mainly consists of bituminous shale, mudstone, marl, sandstone, and limestones with Nummulites, and this zone continued up to 809.00 m. The thickness of the coal cut into two veins in the upper coal horizon (12.30 m) reached 2.90 m, and its thickness in the lower coal horizon (16.70 m) reached 5.20 m (Fig. 11).

Fig. 11
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Core views of the coals cut in M6 drilling in Mengen region

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Correlation

The thickness of the coal horizon cut in the lower part of the Tokmaklar Formation reaches a maximum of 150 m during the drillings made by TKI and MTA in the Çukurca-Ağacalar and Gökçesu regions to the west of Mengen. While the coal seams in this horizon, which are mostly alternated with bituminous shales, show thickness changes between 10 and 200 cm, the total thickness of the coal seams within this horizon can reach up to 10–15 m (Fig. 9).

The coals spreading in the western part of the Mengen basin are predominantly blackish, semi-glossy and shiny, locally banded, hard, and brittle. Secondary minerals cannot be observed in the coal texture (Fig. 11). When evaluated macroscopically, Mengen-Gökçesu coals dominate; it is observed that it is made of clarite and to a lesser extent vitrine and durite. According to the chemical analysis results of the coal samples taken from the cores of the drillings in the Mengen-Gökçesu region, it is possible to define the Eocene aged coals in the region as bituminous-C with low volatility (ASTM 1991) (Table 1).

Table 1 Chemical analysis results of coal samples taken from drillings in Mengen region
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In the Mengen-Çukurca and Gökçesu region, the lateral correlation of the drillings made by MTA and TKİ in the previous years and the sections and coal seams of the exploration drillings opened in this study shows that the thickness of the coal horizon changes significantly in the lateral direction depending on the facies changes in the basin. The coal horizon, which reaches a thickness of 150 m in the Gökçesu region, decreases to a thickness of 100 m at the location where the M6 drilling is located towards the east of the basin. In addition, the coal horizon, approximately 100 m thick, cut in the upper part of the M6 drilling, thins rapidly due to the facies changes towards the east and ends by thinning in the region where the M2 drilling opened in the east of Mengen.

The M2 drilling is located in the NE end of the Mengen basin where Nummulites reef limestones are common. For this reason, thick reefal limestones were entered after the thin coal levels in the lower part of the Tokmaklar Formation were cut in the area where the M2 drilling is located (Fig. 12). However, 3,920,315 tons of reserves (probable + apparent) were calculated from the coal seams cut from the drillings (79/2A, 82/5–7-9–11-17, 86/1–2-3–5, 87/1–2-4–5) with the deepest 418.9 m in the region between Mengen-Gökçesu in the study area (Gürsoy 1988).

Fig. 12
figure 12

Lateral correlations of the sections and coal seams of the drillings in the Mengen-Gökçesu basin

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In the exploration drillings opened within the scope of this study, depths of 671 m (M2 drilling) and 809 m (M6 drilling) were reached, and in the calculation made according to the coal seams cut in the drillings, a reserve of 135,200 tons for the upper seam and 1,108,640 tons for the lower seam (probable) was found. In particular, the coal seam in the lower coal horizon cut in the M6 drilling; it is important in terms of its character and potential that can significantly affect the coal reserve of the region since it could not be cut in the drilling works carried out in previous years.

Depositional characteristics of the Mengen Eocene basin

In the sedimentary succession filling the Mengen-Gökçesu Eocene basin, two facies were distinguished from bottom to top, (1) reefal and (2) lagoon.

The sedimentary rock group constituting the reefal facies is predominantly composed of Nummulites limestone and accompanying sandy limestone, calcareous mudstone, and clayey limestone components. Nummulites fossils, which are widely found in reefal facies sediments, locally form biostrom-like organic aggregations.

Neritic foraminifers found within the biostromal limestones, mainly composed of Nummulites and Discocylina are found in a matrix consisting mostly of microspar calcite, as terrestrially alternating. In Nummulites, grain long axes are mostly observed in parallel and semi-parallel shapes. The texture and compositional properties of reefal limestones indicate that they can be defined as packstones and boundstones rather than Dunham's (1962) classification.

Sedimentary facies in the Mengen-Gökçesu Eocene basin and their lateral lithofacies changes indicate that the carbonate rocks forming the reefal facies grade into fine- and coarse-grained clastic sediments forming a lagoon environment towards the west of the basin. The presence of patch reefs and/or biostromes is common, especially in the reef core facing the lagoon (Fig. 13).

Fig. 13
figure 13

The showing block diagram of the depositional model of the Mengen Eoceane coal

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The carbonaceous shale, bituminous shale, and accompanying bituminous clayey limestones and coal layers, which constitute the dominant component of the Tokmaklar formation, mostly reflect the presence of a deposit developed in a lagoon environment. Varv-like lamination with charred vegetal remains is commonly observed in lagoonal sediments. In the lower part of the Tokmaklar Formation, well-sorted sandstones and cross-bedded conglomerate channel fills and accompanying mudstones are common. It is possible to evaluate these coarse clastic sediments as sedimentary components belonging to tidal channels that probably developed in the lagoon environment. Upward grain thinning, cross-bedding, grain long-axis orientation, and sedimentary structures consisting of overlapping pebbles are commonly observed in clastic sediments of tidal channel fills. The clastic sediments belonging to the tidal channel fillings are graded within short distances into the shale and mudstones rich in organic matter belonging to the tidal plain.

Coal levels in the Mengen-Gökçesu basin are mostly observed in the lower part of the Tokmaklar Formation, alternating with bituminous shale, dark grayish mudstone, and bituminous clayey limestones deposited in lagoon facies. Coal beds in the region were deposited in the shallow parts of the lagoon environment close to the land. However, the subsidence developed under the control of the syn-sedimentary faults affecting the basin must have led to the development of many coal layers and accompanying lagoonal sedimentary succession in the lagoon environment in the region.

In the examined coal samples, as macroscopic structural elements, clarite (30–40%) and durite (30–35%) are dominant in microlithotype components, whereas vitrinite is present at a small rate (10–20%) and carbonaceous shales are 5–10% in some samples. It has been found that there are volumetric distributions at the level of coals analyzed according to the results of coal petrography analysis, chemical analysis data and microlithotype distributions show sub-bituminous coal characteristics (Table 1, Hacquebard et al. 1967). The distribution of coal-bearing sediments in the coal-bearing Eocene basins in Turkey is quite limited, and accordingly, the data on the Eocene pollen records are also quite limited. Palynological data obtained so far in these basins show that the Eocene coal-bearing sediments generally include mangrove plants such as Nypa, Pelliciera, and Avicennia, which indicate deposition in the marine coastal environment (Akkiraz et al. 2017).

Although there are limited pollen data in the coaly lower levels of the Tokmaklar Formation of the Mengen-Gökçesu basin, species belonging to the genus Leiotriletes, dinoflagellates, and Monocolpopollenites crassiexinus, which are characteristic for the Eocene, are abundant. Fine clastic sediments in the upper levels are richer in marine dinoflagellates, conifers, and spores. The formation is also diverse in terms of ostracod and foraminifera content. There are also forms Spinizonocolpites sp. and Longapertites retipiliatus that indicate a low rate of mangrove and back-mangrove environments (Durak, 2020).

On the other hand, the sedimentological features reflected by the sediments of the Tokmaklar Formation accompanying the coals and the distribution of microlithotype components; it shows that the examined coals mostly occurred in the limno-telmatic swamp zone formed in the coastal part of the lagoon environment close to the land, and then occurred under gradually deepening conditions, tropical and rainy climatic conditions (Hacquebard et al. 1967; Stach et al. 1982). In addition, the locally observed charred root zone and the lateral continuity of the coal layers and the presence of fossiliferous levels in the lower part of the coals in the Gökçesu section of the Mengen basin indicate that these coals were formed by in situ bedding (autochthonous origin).

Conclusions

The Mengen Eocene basin is located in the north of the North Anatolian fault system along the main lines. Mesozoic-aged carbonate rocks and Cretaceous-aged turbidic sediments, as well as Paleozoic-aged metamorphic rocks and accompanying granites belonging to the Sakarya massif located in the north of the basin, constitute the main pre-Eocene basement rock units in the region.

The Mengen-Gökçesu basin, which extends mainly in the NE-SW direction, is bounded from the north by NE-trending thrust faults and from the south by NE-SW trending normal fault systems. Therefore, the Mengen basin displays an asymmetrical graben structure.

The Eocene sedimentary sequence filling the Mengen basin has been divided into two different rock units, taking into account the dominant lithological features. They consist of (1) Sazlar Formation and (2) Tokmaklar Formation from bottom to top. Sazlar Formation is mostly composed of thick-bedded and abundant Nummulites-bearing limestones deposited under reefal conditions and accompanying biostrom-like organic aggregates. The Tokmaklar Formation also consists predominantly of shale, bituminous shale, coal, bituminous clayey limestone, Nummulites limestone, sandstone, mudstone, and marl levels deposited in lagoon conditions.

The sediments belonging to the Tokmaklar Formation overlie the carbonate sediments of the Sazlar Formation below with a conformable and graded contact. Although the reefal sediments of the Sazlar Formation are common in the eastern part of the Mengen basin, the coal-bearing lagoonal sediments of the Tokmaklar Formation are more common in the Gökçesu region in the western part of the basin.

Coal deposits that can be considered economical in the Mengen basin are mostly located in the lagoonal sediments in the lower part of the Tokmaklar Formation, which spreads in the Gökçesu region in the west of the basin. Chemical analyzes of samples taken from the upper horizon of the Mengen-Gökçesu coals show that these coals are in the bituminous-C stage with low volatility.

In the study area between Mengen and Gökçesu, approximately 3,920,000 tons of reserve (probable + apparent) has been calculated from the coal seams cut in the drillings with a depth of 418.9 m in previous years. In the exploration drillings opened within the scope of this study, depths of 671 m (M2 drilling) and 809 m (M6 drilling) were reached, and in the calculation made according to the coal seams cut in the drillings, a reserve of approximately 135,000 tons for the upper seam and 1,109,000 tons for the lower seam (probable) has been revealed. The coal seam in the lower coal horizon cut in the M6 drilling; it could not be cut at all during the drilling works carried out in previous years. Therefore, assuming that the coal seam uncovered in the M6 drilling was also excavated in previous years, the coal potential in the region should be expected to be 5 times higher than the current situation. However, the results of chemical analysis of the coals cut in the lower coal horizon (727–809 m) in the M6 drilling revealed that the coals in this section are in the high volatility bituminous-C stage. This situation reflects that the coals are made of hard lignites (sub-bituminous coal) with a blackish shine in terms of quality.

In addition, according to the microlithotype components uncovered, it is possible to state that the examined coals were formed in the limno-telmatic swamp zone, which is mostly formed in the near-land part of the lagoon environment, discharged by sulfate-rich river systems, and a calcium-rich suboxic lake or lake-swamp environment. The lithofacies characteristics of coal and accompanying lagoonal sediments are important in terms of showing that non-arid humid climatic conditions are dominant in the region during the coalification period.

In Northern Anatolia, there is an Eocene basin containing many coals of various sizes, similar to the Mengen basin. For example, Sinop-Durağan, Çorum-Karakaya, and Amasya-Suluova basins are Eocene basins containing this type of coal. It would be beneficial to carry out systematic and detailed studies to reveal new coal deposits that have not yet surfaced in Eocene basins with similar characteristics in the northern part of the Anatolian peninsula.