Petrography and palynology of Late Oligocene and Middle Miocene coals in the Gelibolu peninsula, NW Turkey

In this study, coal samples from two different coal zones located close to each other were examined. They occur in the Late Oligocene Osmancik and Miocene Gazhanedere formations in the Gelibolu peninsula, in the SW part of the Thrace Basin (NW Turkey). This paper focuses on the organic-petrographic composition and depositional conditions of the two coal zones by using coal petrography, including maceral ratios, and palynology. The maceral composition of the Oligocene coal samples is similar to that of the Miocene coal samples. The coal samples from both locations are characterized by a high content of huminite group macerals, ranging from 46% to 78%. The prevailing maceral from this group is gelinite (31%-65%), which is indicative of a high degree of gelification of organic matter. Relatively low contents of liptinite (not exceeding 9%) and inertinite (not exceeding 8%) were also determined. The tissue preservation of organic matter was poor. The content of mineral matter is inferred from microscopic studies and is variable but generally high, varying from 6% to 46%, similar to that of other Turkish coals; it consists mostly of quartz, calcite, clay minerals, and pyrite. This input of mineral matter is probably related to periodic inundations of swamp. The mean reflectance values of samples ranged from 0.42% to 0.50% Ro, suggesting that the rank of coal is subbituminous according to ASTM classification. Low tissue preservation index (TPI) and high gelification index values suggest that both coal zones were deposited in a limnic environment. The presence of framboidal pyrite and low TPI reflect high pH values (pH 6-7) and reducing conditions during peat deposition. Two different palynological assemblages may be distinguished based on the percentages of palynomorphs. The first assemblage including the stratigraphical marker species Dicolpopollis kockelii (Calamus) suggests the Late Oligocene. The age of the second assemblage is probably the end of Middle Miocene.