Biostratigraphy and sequence stratigraphy of the Lower Cretaceous in the NW part of the Mid-Polish Trough

Abstract

By comparison with the Lower Cretaceous of central and SE Poland, that of NW Poland (the Pomeranian, Szczecin, and Mogilno-Łódź troughs) has scarce biostratigraphic data. But, despite the lack of Lower Cretaceous exposure in the NW Polish Lowlands, borehole data, including borehole-cores and geophysical logs, allow analysis of complete successions. We refine the stratigraphic units using parallel studies of ammonites, microfauna and calcareous nannoplankton collected from the same intervals, and by correlating age-defined intervals with geophysical logs. Ostracod zones F to A are documented by the presence of ostracod assemblages representing the interval between the Upper Tithonian (ostracod zone F) and the lower part of the Upper Berriasian (ostracod zones E to A). The fragmentary and poorly preserved ammonites allowed only for distinguishing the uppermost Middle and Upper Berriasian (Ryazanian), while the informal subdivisions from the central part of the basin could not be identified unequivocally. Nannoplankton recognized in the succession analysed was very rare due to shallow marine facies of the strata. Only one nannoplankton zone was recognized in the lower part of the succession studied: the CC2 Stradneria crenulata Zone (uppermost Middle and Upper Berriasian and lowermost Valanginian). An additional study only on nannoplankton enabled recognition of certain boreal taxa typical of the BC2 zone of the Uppermost Riazanian. Valaginian ammonites occur in core material located closer to the central part of the trough. Some planktonic foraminiferal species indicate the Lower Aptian. Some Upper Cretaceous nannoplankton zones were also recognized: the CC9 Eiffellithus turriseiffeli (Uppermost Albian to Lower Cenomanian) and UC0, UC1-2 and UC3 zones which correspond to the Upper Albian and Lower as well as Middle Cenomanian. The sequence stratigraphic interpretation was based on geophysical logs with the application of gamma-ray, neutron-gamma, spontaneous potential and resistivity logging, as well as caliper logging. These studies allowed recognition and correlation of sedimentary sequences within the part of the sedimentary basin analysed, characterized by a similar cyclic pattern of geological phenomena described using depositional sequences as in the central and SE part of the Polish Basin. Third-order depositional sequences with maximum flooding surfaces were distinguished. Effective correlation of depositional cycles with biostratigraphy and with the global sea level curve was demonstrated for several boundaries, confirming the applicability of this method for the Polish part of the the Central-European Basin. Other boundaries recognized that are not correlatable and shifted relative to Haq’s curve may reflect autogenous factors (e.g., local tectonics) overlapping with the global changes controlled by allogenic processes