The Zonguldak basin of northwestern Turkey is the only basin in Turkey with mineable bituminous coal deposits. It has been mined for coal at least since 1848. The basin takes its name after Zonguldak, Turkey, and lies at approximately 41° N. It is roughly elliptical in shape with its long axis oriented roughly southwest to northeast, and is adjacent to the Black Sea. Three main regions have been recognized in the Zonguldak basin: from west to east, Armutcuk, Zonguldak, and Amasra.Sinayuç, C., and Gümrah, F. (2009) Modeling of ECBM recovery from Amasra coalbed in Zonguldak Basin, Turkey, International Journal of Coal Geology, 77, 162-174
Depositional history The Zonguldak basin has undergone two major periods of deposition. The first period began in the Paleozoic, and the second began in the Cretaceous. Isolated areas of deposition in the basin occurred during the Late Permian through the Triassic as well as the Latest Jurassic.
Paleozoic deposition The Zonguldak basin first experienced deposition in the Ordovician.Yalçin, M.N. and Yilmaz, I. (2010). Devonian in Turkey – A review. Geologica Carpathica, 61 (3), 235-253 Deposition begins with the lower Ordovician Soğuksu Formation. The Soḡuksu Formation is 700–1100 meters thick. At its base it consists of green shale and sandstone and coarsens upwards to arkosic conglomerates. The lower Ordovician Aydos Formation conformably overlies the Soḡuksu. It is a conglomerate of quartzitic sandstone and is 50–200 meters thick. The Findikli Formation was deposited during the upper Ordovician, Silurian, and lowermost Devonian in the Zonguldak basin. It is 300–450 meters thick. Its facies are indicative of a mixed siliciclastic-carbonate shelf environment that is shallowing through time.
The red, cross-bedded sandstones of the Ferizli Formation overlie the marly deposits of the Fendikli Formation. The oolitic sandstones contain iron and an iron ore. The formation, like the Findikli Formation, shows a shift toward shallower depositional environments and a shift to higher energy areas of deposition. The younger sediments of the Ferizli Formation become progressively more enriched in calcium carbonate and eventually give way to the Yilanli Formation.
The Yilanli Formation is Visean in age and is the beginning of the coal-related sequences in the Zonguldak basin.Görür, N., Monod, O., Okay, A.I., Sengör, A.M.C., Tüysüz. O., Yiğitbaş, E., Sakinç M., and Akkök, R. (1997) Paleogeographic and tectonic position of the Carboniferous rocks of the western Pontides (Turkey) in the frame of the Variscan belt. Bull. Soc. Géol. France, 168 (2), 195-205Hoşgörmez, H. (2007). Origin and secondary alteration of coal bed and adjacent rock gasses in the Zonguldak Basin, western Black Sea, Turkey. Geochemical Journal, 41, 201-211 The Yilanli is a dolomitic limestone unit with calcareous black and gray shales. It was deposited in a shallow marine passive margin setting. It is conformably overlain by the Alacaagzi Formation, and has accumulated over 1000 m of sediment. The Alacaagzi Formation contains economic deposits of coal. It is composed predominantly of crossbedded black shales and silts in the lower sections of the unit and progressively becomes composed of sands, shale bearing coals, and conglomerates towards the top of the formation. Facies analysis in the Alacaagzi Formation is suggestive of coastal environments including lacustrine, fluviatial, and fan deposits.
Conformably overlaying the Alacaagzi Formation is the Kozlu Formation. The Kozlu contains 19 coal seams totalling 30–32 m. The Kozlu is composed of successional conglomerate, sand, silt, mud, and coal deposits. The Karadon Formation conformably overlies the Kozlu. The Karadon is lithologically similar to the Kozlu Formation, but contains less coal seams. An angular unconformity spans the top of the Karadon formation and ranges from 46.5 Ma–194 Ma in duration.Yalçin, M.N., Schenk, H.J., and Schaefer, R.G. (1994). Modelling of gas generation in coals of the Zonguldak basin (northwestern Turkey). International Journal of Coal Geology, 25, 195-212
Localized deposition Deposition resumes in the western portion of the Zonguldak basin with the deposition of the Çakraz Formation. The Çakraz spans the upper Permian through the Lower Jurassic. Unconformably overlying the Çakraz is the Inalti Formation. The Inalti was deposited during the upper Jurassic and is truncated by an unconformity. The carbonates of the Inalti are representative of a shallow passive margin setting.Channel, J.E.T., Tüysüz, O., Bektas, O., and Sengör, A.M.C. (1996). Jurassic-Cretaceous paleomagnetism and paleography of the Pontides (Turkey). Turkey, 15 (1), 201-212
Cretaceous deposition During the late Cretaceous, the Zonguldak basin was roughly 25° N, and was experiencing subsidence due to the formation of a back arc, the Black Sea. As a result, the Zonguldak experienced deposition from the early Cretaceous through the Eocene. Different authors present differing stratigraphic columns of the Zonguldak basin and this analysis will preferentially report more current research. The lithologies deposited during this period of deposition include limestones, mudstones, siltstones, and dolomites. The Albian, 105–100 Ma, Zonguldak Formation is predominantly limestone with areas of dolomitization.Hoşgörmez, H., Yalçin, M.N., Cramer, B., Gerling, P., Faber, E., Schaefer, R.G., and Mann, U. (2002). Isotopic and molecular composition of coal-bed gas in the Amasra region (Zonguldak basin—western Black Sea). Organic Geochemistry,33, 1429-1439 It is conformably overlain by the Albian, 105–112 Ma, Kilimli Formation. The Kilimli is composed of sandstone and carbonaceous sandstone.
The Kilimli is unconformably overlain by the sandy limestone of the Cemaller Formation. The unconformity lasts at least 6.5 Ma and the Zonguldak basin experiences continuous deposition from the Turonian – Campanian. The Cemaller is overlain by the siltstone and limestone of the Baþköy Formation. The Dinlence Formation overlies the Cemaller and consists of andesites and andesitic tuffites. It is possible that the Dinlence is the Yemislicay Formation since it too contains andesites and andesitic tuffites. The Dinlence is overlain by the marls, and limestones of the Alapý Formation.
Tectonic history
Paleozoic During the middle Paleozoic, the Zonguldak basin was part of the south-facing passive margin of the Laurasian plate.Goncuoglu, M.C. and Kozlu, H. (2000). Early Paleozoic evolution of the NW Gondwanaland: Data from southern Turkey and surrounding regions. Gondwana Research, 3, 315-324 During the Carboniferous, the temperature of the sediment water interface was near 25 °C and heat flow into the Zonguldak basin was about 1.3 heat flow units (HFU). Backstripping analysis of the Alacaagzi Formation, the lowest coal-bearing formation, reaches a maximum temperature and depth of 100 °C and 2.4 km, respectively, in the Zonguldak area during the Carboniferous. Similarly, the base of the Kozlu Formation reaches maximum temperatures of 85, 85, and 100 °C in the Armutcuk, the Zonguldak, and the Amasra regions, respectively.Yalçin M.N., İnan, S., Gülbin, G., Mann, U., and Schaefer, R.G (2002). Carboniferous coals of the Zonguldak basin (northwest Turkey): Implications for coalbed methane potential. American Association of Petroleum Geologists bulletin, 86 (7), 1305-1328
The Zonguldak basin was tectonically active during the late Paleozoic and this strongly influenced its structural and burial history due to the Hercynian orogeny. The Hercynian orogeny was the result of the continent–continent collision between Laurasia and Gondwana. This collision created many E-NE/W-SW striking faults, folds, and tilted the Paleozoic sediments. This uplift of the basin, near the end of the Westphalian, halted deposition and created the angular unconformity at the top of the Karadon Formation. Heat flow remained constant in the Zonguldak basin during the Hercynian Orogeny, while the uplift decreased the temperatures the sediments experienced. For example, at the end of the Permian the top of the Alacaagzi was roughly 70 °C and 1.6 km below the sediment surface in the Zonguldak area.
Mesozoic During the Cretaceous, the Zonguldak basin experienced general subsidence, rifting, and faulting. This led to another period of deposition in the region and faulted the coal seams. This faulting provided a pathway for meteoric water to enter the coal seams.
During the Aptian, the Intrapontide Ocean, the ocean separating the Western Pontides tectonic region of Turkey form the Sakarya Continent, underwent subduction. This led to the formation of a back arc basin, the Black Sea. The start of this subduction is responsible for the unconformity between the Kilimi and Cemaller formations and marks the beginning of the Alpide orogeny in the region. The andesitic volcaniclastic sediments of the Yemislicay support the subduction of oceanic crust in the region during this time. The Zonguldak basin was able to sustain deposition after the start of the Alpide orogeny due to rifting in the Black Sea basin. The rifting in the Black Sea also increased heat flow into the Zonguldak basin during the Cretaceous. Heat flows were as high as 1.5–1.75 HFU, and the temperature of the sediment water interface was about 25 °C.
Cenozoic The Intra-Pontide Ocean stopped subducting with the collision between the Western Pontides terrain and the Eastern Pontides terrain. The Aplide orogeny stopped deposition and uplifted the Zonguldak basin during the Eocene epoch beginning at 42 Ma. The coal bearing formations experienced the highest temperatures at the onset of the Alpide orogeny. For example, the base of the Kozlu Fm experienced temperatures of 125, 175, and 140 °C in the Armutcuk, the Zonguldak, and the Amasra regions, respectively.
The Alpide tectonic provinces in Anatolia, from north to south, are the Pontides, Anatolides, Tarides, and Border Folds. These provinces have roughly an east–west strike. The Zonguldak basin is currently being uplifted by the Alpide orogeny. Progressively older sediments outcrop toward the north of the basin.
Source rock
Coals The coals of the Alacaagzi, Kozlu, and Karadon formations are of bituminous rank.Yürüm, Y., Bozkurt, D., and Yalçin, M.N. (2001). Change of the structure of coals from the Kozlu K20 G borehole of Zonguldak Basin with Burial Depth: 1. chemical structure. Energy Sources, 23, 511-520 The Alacaagzi, Kozlu, and Karadon formations contain greater than 70%, 81%, and 81% total organic carbon (TOC), respectively.
Structure The coals of the Zonguldak basin follow the mean evolution of type III kerogens. The coals of the Zonguldak basin show vitrinite reflectances (Ro) of 0.45–1.70%. Hoşgörmez et al. (2002) determined that the coals of the Kozlu formation exhibit Ro of 1.0–1.2%, which gives them a coal rank of high volatile A bituminous.Strąpoć, D., Mastalerz, M., Dawson, K., Macalady, J., Callaghan, A.V., Wawrik, B., Turich, C. and Ashby, M. (2011). Biogeochemistry of microbial coal-bed methane. Annual Review of Earth and Planetary Sciences, 39, 617-656 Additionally, coalification increased with depth, and the coals became more aromatic with depth.Karayigit, A.I., Gayer, R.A., and Demirel, I.H. (1998). Coal rank and petrography of Upper Carboniferous coal seams (Amasra, Turkey). International Journal of Coal Geology, 38, 277-294 The calorification of the coals also increased with depth.
Methane