Disturbed Stratification in Late Pleistocene Sediments of the Khibiny Pluton (Kola Peninsula)

Abstract

Abstract —The paper presents a description of three Late Pleistocene sections within the Khibiny pluton, outcropping on the proximal slope of the Kukisvum morainic ridge, a fluvioglacial terrain in the valley of the Kukisiok River, and a glacial complex in the valley of the Vuonnemiok River. Various small disturbances of the primary stratification were found in the first two sections. These structures do not form horizons sustained along the strike and are associated with the top of sand–silt sediments with horizontal or wavy bedding of shallow lacustrine genesis. They form small wavy bends, ‘tongues”, ovoids, and flexural microfolds that could not be preserved at the bottom of the basin and, consequently, formed after overlapping deposits accumulation. Analysis of the cross bedding of the coarse clastic deposits of the fluvioglacial terrace has allowed us to relate its formation to north-to-south glacial water discharge along the trough valley of the Kukisiok River. In the glaciolacustrine varve clays of the third section (the Koashva open pit), extended horizons of plicative disturbances 0.5–2 m in thickness and 300 m in length were studied. The soft-sediment deformations are covered with an unstructured horizon composed of coarse grains dispersed in sand–silt matrix, which means that the deformation was caused by either a glacial flood or a mudflow. The study has revealed no signs of seismic liquefaction during the formation of the folded structures. The relevance of the performed study is determined by the fact that it has become possible not only to clarify the formation conditions of the Quaternary Khibiny deposits but also to develop the objective criteria for determination of the soft-sediment deformation structures associated with thixotropic effects in weakly consolidated sediments. Similar structures are sometimes regarded as seismic convolutions, which can lead to unjustified overestimation of the regional seismic hazard level.