Morphodynamics and morpotectonics of the varzuga river mouth area (terskiy coast of the white sea) in the late glacial and holocene

Abstract

The Late- and post-glacial history of the development of the White Sea coastal zone in the area of the Varzuga River mouth is considered as a result of the interaction of endogenous and exogenous factors of coastal morpholithogenesis. Based on 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 area’s relief development for ~13 cal ka BP 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 speed of vertical movements – were determined. The superimposed linear Nizhnevarzugskaya depression, which determined the configuration of the Varzuga River estuary in the late and postglacial time, was identified for the first time. The influence of the spatial ratio of blocks and differentiated postglacial 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 morpholithogenesis processes (coastal, estuarine, and aeolian) obtained. Three stages of the coastal zone development were identified, corresponding to regional rhythms of changes in the relative sea level and climate: (I) Late Glacial transgression and Early Holocene regression (~12–9.8 cal ka BP), (II) Middle Holocene Tapes transgression (7.8–4.9 cal ka BP), (III) Late Holocene regression (after 4.9 cal ka BP). The upper marine boundary of the Late Glacial transgression is traced at the elevation of ~54–55 m a. s. l. to the west of the Nizhnevaruzgskaya depression, — ~39–40 m a. s. l. to the east of it, and — 22–25 m a. s. l. in the depression. The shores of lower morphostructural blocks were probably blocked by dead ice up until ~10.2–9.8 cal ka BP. During the Tapes transgression, the RSL reached a maximum (~7.8–7.6 cal ka BP; ~20 m a. s. l.), and by 4.9 cal ka BP fall to ~15 m a. s. l. The prevailing directions of sediment fluxes, winds and wave approach became similar to those of today. However, the main source of the coastal zone sedimentary supply was the erosion of glaciofluvial sediments and the input of sands from the seabed. In the interval of ~4.9–1.7 cal ka BP, the RSL decreased to ~5 m a. s. l. The sediment runoff of the Varzuga River became the main source of feeding the coastal zone.