Geophysics, geology and geomorphology: controversial data on the provenance of ring-shaped landforms on the East European plain

A number of outstanding ring-shaped geological landforms have been known from the central Mologa-Sheksna Lowland (NW Russia), closely contouring the presumed local margins of the Ostashkov (Late Weichselian) glaciations advance, settled within the lacustrine-alluvial bogged plains. Visual examination of these landforms does not provide clear evidence on their genesis in frames of the accepted regional palaeogeographic context.The “Bor-Timonino” (3.5 km across) and “Yana” (250 m across) sites are most well-preserved ring-shaped landforms in the contemporary relief, exhibit circling slightly elevated sand rims and flat-disk central parts filled with water and peat, respectively.These two landforms were investigated in 2018-2021 using ground penetrating radar (GPR), electric resistivity tomography (ERT), transient electromagnetic method (TEM), magnetic survey (MS) and auger drilling. The results show no clear correspondence of electric and magnetic properties of the constituent Quaternary deposits to the visible surface symmetry of the landforms. Only within the upper 5-10 m of the “Bor-Timonino” site, sandy rim base was traced uniformly around the landform, thus repeating its symmetry. In the deeper part of the section (down to 100 m) no structural patterns corresponding to the present-day relief were observed; it represents the sequence of practically undisturbed layers. A thin (10-30 cm) layer of buried peatsoil has been discovered at depth range of about 1.5 – 4.5 meters below ground surface at several sites within and beyond the ring-shaped landforms, overlain by the sandy deposits of the rims. This peatsoil has Late Glacial age (13.4 – 12.4 cal. ka BP), which indicates that the “Bor-Timonino” landform is presumably older.Supported by the regional geological setting, several apparent ways of the ring-shaped landforms’ origins are suggested, i.e., cryogenic, fluvial, glacial or their combinations. It is possible, though not supported by direct evidence, that late Paleozoic paleokarst occurrences could have caused sinkhole formation at an early stage, which subsequently had been undergoing transformation leading to the ultimate appearance of “rim-disk” landforms. Impact, tectonic or volcanic processes are perceived to be unlikely to form such objects.Nevertheless notable geophysical investigation has been carried out, there is no certainty about genesis of the ring-shaped landforms of the Mologa-Sheksna Lowland. Additional drilling of sand rims and terraces with subsequent lithological, geochronological, geochemical and mineralogical analyses are necessary to progress in this research. Except for their significance for fundamental geology or paleogeography, the ring-shaped landforms also wield great potential as prime sites of regional geoheritage, which is enforced by their position on the territory of Darwin Nature Biosphere Reserve.