Substratum sedimentology and topography of two riparian peat bogs in the Bieszczady Mountains (Carpathians)

The valley of the upper course of the San River in the Bieszczady Wysokie mountain range is, after the Orawa-Nowy Targ Basin, the area with the largest number of peat bogs in the Polish Carpathians. This study combined a levelling survey with data from dense networks of boreholes on two riparian bogs (Łokieć and Dźwiniacz) in the Bieszczady Mountains (East Carpathians) to characterise the morphology of the two peatlands, create a digital elevation model (DEM) for Łokieć, and describe the underlying sediments. Previous radiocarbon dating of basal peat samples has indicated that both peat bogs began to grow during the younger part of the Holocene, but not at the same time. The results of the coring undertaken in our study demonstrated that both bogs fill 0.5–1.5 m deep troughs cut into a fluvial terrace situated 4–6 m above the level of the San River. The bog domes are asymmetrical, more than 2 m high, and their summits lie directly above the deepest parts of the troughs. The troughs are lined by sediments of mainly fluvial origin (silt, sand, fine gravel), and in their deepest parts the peat lies directly on clays originating from weathering of shales in the terrace base. On this basis, the troughs are interpreted as palaeochannels (infilled oxbows) representing fragments of the ancient San River channel, cut off by erosion and situated at the former level of the valley bottom. Raised bogs seldom occur in mountain valleys, where reworking of floodplain deposits proceeds at a higher rate and peatlands are frequently eroded by fluvial action. The two bogs that we studied in the Bieszczady Mountains are exceptional in that they are set in preserved palaeochannels.