Palynology of Lower Oligocene brown coal and lowermost Middle Miocene sand deposits from the Łukowa-4 borehole (Carpathian Foredeep, SE Poland) – implications for palaeogeographical reconstructions

Abstract

Brown coal and overlying sand strata from the Łukowa-4 borehole, located in the northeastern sector of the Carpathian Foredeep in Poland, were studied for palynology. These strata are underlain by Upper Eocene sands, and covered by Middle Miocene rocks. Coal beds yielded infrequent sporomorphs and freshwater algae Botryococcus. The presence of the latter indicates that these deposits accumulated in a freshwater environment whereas sporomorph assemblages point to the presence of mixed forests with a significant portion of thermophilous taxa. Age of the coal beds has been established based both on the presence of species that appear for the last time in the Early Oligocene, and on the similarity with sporomorph spectra from the Lower Oligocene of the Polish Lowlands. Overlying sands yielded marine dinoflagellate cysts, which point to a marine sedimentary setting, and frequent sporomorphs. The latter indicate the presence of mixed mesophytic forests, bush swamps, swamp forests, and riparian forests in the vicinity of lacustrine environments. Stratigraphic analysis of dinoflagellate cyst and sporomorph assemblages suggests that the sands accumulated in early stages of Miocene transgression in the Carpathian Foredeep in the latest Early–early Middle Miocene. Our climatic interpretation of the sporomorph spectra suggests that the climate during deposition of the strata was relatively warm, although less frequent thermophilous taxa recorded in the Miocene sands suggest a slightly cooler climate than that deducted from the spectra yielded by the underlying Lower Oligocene coal beds. Correlation of Lower Oligocene coal beds with neighbouring coeval marine sands suggests diverse morphological conditions in the Carpathian foreland at that time, partly covered by a sea, and partly emerged. A similar, morphologically diverse basement in the Carpathian foreland favoured accumulation of Lower Miocene phytogenic deposits. A similar stratigraphic position of both Lower Oligocene and Lower Miocene coal beds in the Carpathian Foredeep may result in a false correlation of these strata devoid of fossils, which are commonly regarded as Miocene.