Loss of geoheritage objects in the Main Devonian Field

Objects of geoheritage include natural (rock outcrops, landforms) and artificial (some quarries and artificial caves) objects that are valuable from a geological point of view; a special group among geoheritage is the key stratigraphic sections that clearly reflect the stages of geological development of the area. The authors make a revision of the state of stratotypes and a number of key sections of the Lower and Middle Frasnian of the eastern Main Devonian Field (MDF), many of which are the objects of geoheritage, having the status of both approved and unapproved geological monuments. Among the objects of regional geoheritage with the environmental status, there are stratigraphic (stratotypes of some subregional stratigraphic units), geomorphological, hydrogeological and complex geological monuments. For the second half of the 20th century and to this day, there has been a significant reduction in the number of objects of regional geoheritage, clearly visible as the loss of many key and stratotype sections of the Lower and Middle Frasnian deposits. In addition to anthropogenic factors (mineral mining, agricultural and land reclamation works, deforestation and timber rafting), which represent the main danger to geoheritage, a key role is played by the lithological and facies features of the Lower and Middle Frasnian rocks, which determine the nature and dynamics of the development of hazardous geological processes. For the east of MDF, two structural-facies zones (SFZ) are distinguished—the Baltic–Ladoga and Izbor–Ilmen zones. The first zone in the Early and Middle Frasnian corresponded to the area of the internal depression of the marine basin (clay–carbonate nature of the section) and the marginal part of the sea (essentially carbonate nature of the section). In the Baltic–Ladoga SFZ in the Early and Middle Frasnian, there were conditions of coastal–marine and coastal–continental sedimentation (carbonate–clayey–terrigenous sections). The maximum loss of objects of geoheritage occurs in the areas of distribution of clayey–carbonate (the Shelon basin) and terrigenous–clayey–carbonate (the Syas basin) Frasnian rocks.