Definition of the Mississippian–Pennsylvanian Boundary in the Lviv–Volyn Coal Basin (Western Ukraine), Based on Palynological Data

Abstract

The Lviv–Volyn basin (LVB) is situated in the Volyn–Podillya margin of the East European platform, and is composed of Carboniferous (Mississippian and Pennsylvanian) coal-bearing deposits. The main problem of the regional stratigraphy is the definition of the Mississippian–Pennsylvanian boundary in local sequences of the LVB. This definition is possible only with a detailed study of the major coal-bearing sediments of the Bug Formation in the LVB. A palynological facies study of the Carboniferous of the LVB demonstrates that the boundary between the Mississippian and Pennsylvanian and dating of the Bug Formation can be clearly resolved using palynological data. In the Bug Formation, two palynozones are established, namely Raistrikia nigra–Bellispores nitidus (NN) and Neoraistrikia splendidus–Raistrikia fulva (SF). The lower NN zone, distinguished in the interval between the limestone N3 and coal bed n7, is an assemblage zone. It is defined by two index species, namely Raistrikia nigra and Bellispores nitidus. In this palynozone, 65 taxa (43 widespread, 32 typical) were identified. Nine species disappear at the top of the zone and one appears close to its base. Based on the miospore composition, the NN zone corresponds to the Serpukhovian (Mississippian). The SF zone is recognized in the upper part of the Bug Formation, in the interval between coal bed n7 and limestone B1. It is a concurrent-range zone that is defined by two index species, namely Neoraistrikia splendidus, and Raistrikia fulva. In this palynozone, 60 taxa are recorded: 31 widespread and 29 typical, five of which appear at the base of the zone (Florinites similis, Radiizonates aligerens, Alatisporites pustulatus, Cirratriradites saturni, and Raistrikia fulva). These species are typical for the Pennsylvanian of surrounding regions. Deposits of the SF zone and, accordingly, the upper part of the Bug Formation of the LVB, are included in the Bashkirian of the Pennsylvanian. Thus, according to palynological data, the Bug Formation corresponds to the following ages: the lower part of the formation is Serpukhovian (Mississippian) and the upper part is Bashkirian (Pennsylvanian). A considerable turnover of spores and pollen assemblages is observed at the level of coal bed n7. This is the boundary between the NN and SF palynozones. This level corresponds to the boundary between the Mississippian and Pennsylvanian in the LVB.