Epochs of weathering crust formation in the Cenozoic of the Voronezh Anteclise

Abstract

Introduction: Cenozoic formations of the Voronezh Anteclise, with a depth of up to 100 m, are accessible for research in the outcrops on the slopes of river valleys, gulches, ravines, and the holes of shallow wells. They are represented by sandy clay rocks of various genesis. The Paleogene is characterized by marine deposits,while the Neogene is characterized by alluvial and lacustrine deposits. Quaternary formations, which are the most representative based on the genetic types of rocks, are composed of alluvial, river terrace, glacial deposits, and cover loams. In the history of the Cenozoic of the Voronezh Anteclise, three epochs of crust formation are distinguished: the Paleocene, Oligocene-Miocene, and Late Quaternary. The deposits of each epoch are associated with certain minerals. The most valuable siliceous-zeolite raw materials are confined to the last epoch. The article discusses the relationship of climate, surface topography, type of rocks with the mineral composition of weathering crust. A genetic interpretation of the formation of minerals in hypergenic processes is presented. Methodology: The study of the mineral composition of the rocks, which constitute both the weathering crust (WC) and the host sediments, was carried out by X-ray diffraction and electron microscopic analysis methods. Results and discussion: The Paleocene era is represented by a weathering crust of a linear-areal type, deposited on the carstified surface of the Upper Cretaceous deposits. Its formation occurred under warm and humid subtropical climate (it can’t be excluded that the climate was tropical, based on the presence of gibbsite in some sections). The Oligocene-Miocene era was detected by “mature” correlate rocks in the upper Oligocene and lower Miocene, represented by the continental Shapkinskaya sequence. The kaolinite composition of its lower part and montmorillonite composition of the upper part, represent an inverted section of the weathering crust. Its formation took place under a climate less favourable for weathering due to lower temperatures and humidity, which resulted in a reduction of the thickness of the eluvium. In the Late Quaternary era, a modern relief was formed on the studied territory under temperate climate. The peculiar siliceous WC, widely developed under the Quaternary deposits in the western and central parts of anteclise, was formed on easily decomposed chalk-marl rocks. Due to the slow infiltration of acidic surface waters, calcium carbonate was replaced by silica. At the same time, the crystal structures of minerals were destroyed as they are unstable in an acidic environment. Conclusions: According to stratigraphic nonconformities and mineral associations, three epochs of weathering crust formation that had been previous identified were confirmed. Various minerals are associated with the considered WC. These minerals include trace minerals of rocks the Khopersky horizon, ceramic clays, and monomineral quartz sands of the Oligocene-Miocene age. The most valuable is zeolite-siliceous raw materials, which are used in many sectors of the national economy.