Morphodynamics and Morphotectonics of the Varzuga River Estuary Area in the Late Glacial and Holocene (Terskiy Coast of the White Sea).

The late-glacial and post-glacial history of the development of the White Sea coastal zone in the area of the Varzuga River estuary is considered as a result of the interaction of endogenous and exogenous factors of coastal morpholithogenesis. Based on the geomorphological investigations, study of Holocene deposits by lithostratigraphic, diatom, and radiocarbon analyses, as well as collection and analysis of published data, new results on the development of relief of the area for ~13 cal ka have been obtained. The features of the regional hierarchical morphostructure and local post-glacial tectonics of the territory—the spatial relationships of blocks and the rate of vertical movements—were determined. The superimposed linear Nizhnevarzugskaya Depression, which determined the configuration of the Varzuga River estuary in the late-glacial and post-glacial periods, was identified for the first time. The influence of the spatial ratio of blocks and differentiated post-glacial uplift on the coastal morpholithogenesis was established. The course of changes in the relative sea level (RSL), development conditions, and morphodynamics of the open coast and the estuary of the Varzuga River were reconstructed, and new data on the rhythms of coastal geomorphologic processes (coastal, estuarine, and eolian) were obtained. Three stages of development of the coastal zone were identified, which corresponded to regional rhythms of changes in the relative sea level and climate. They are (I) the Late Glacial transgression and Early Holocene regression (~12 to 9.8 cal ka BP), (II) the Middle Holocene Tapes transgression (7.8 to 4.9 cal ka BP), and (III) the Late Holocene regression (after 4.9 cal ka BP). The upper marine boundary of the Late Glacial transgression was traced at heights of ~54 or 55 m to the west of the Nizhnevarzugskaya depression, 39 or 40 m to its east, and 22 to 25 m a.s.l in the depression. The shores of lower morphostructural blocks up to ~10.2–9.8 cal ka BP were probably blocked by dead ice. During the Tapes transgression, the RSL reached a maximum (~20 m a.s.l.) of ~7.8 to 7.6 cal ka BP and slowly decreased to ~15 m a.s.l. in the interval of ~7.6 to 4.9 cal ka BP. The prevailing directions of sediment fluxes and the approaches of winds and waves became similar to those of today and have not changed significantly since that time. The main source of the coastal sediment supply was the erosion of glaciofluvial sediments and the input of sands from the seabed. In the interval of ~4.9 to 1.7 cal ka BP, the RSL decreased to ~5 m a.s.l. and then slowly approached the modern one. Activation of coastal (~5–1 cal ka BP) and channel (~4.9–4.7 to 3.6–3.4 cal ka BP) processes contributed to rapid filling of the estuary and formation of accumulative sand terraces on exposed banks. The sediment runoff of the Varzuga River became the main source of coastal sediment supply. After ~2.3 cal ka BP, several stages in the intensification of eolian processes were revealed. [ABSTRACT FROM AUTHOR]