CHOCOLATE CLAYS OF THE NORTHERN CASPIAN SEA REGION (DISTRIBUTION, POSITION AND STRUCTURE)

Abstract

Chocolate clays are one of the most common facies of Lower Khvalynian sediments of Northern Caspian and Lower and Middle Volga region. After more than a hundred years of research, there are still a number of unresolved issues related to the conditions of its bedding, structure, facial structure and colority.

Chocolate clays are confined solely to pre-Khvalynian relief depressions: terraces, paleo down cuttings and junctions of large tributaries at river valleys, and varied in form and origin paleoravines, estuaries, saline-dome depressions (Elton Baskunchak) and ancient deflationary depressions (Kalmykia) at watersheds. Chocolate clay roofing according to the general slope of the surface of the Lower Volga and Northern Caspian gradually decreases from north to south. The highest level of occurrence of clays in the Caspian depression is usually noted at levels +20-25 m. To the north, within the valley of the Volga, clay roof rises to a level35 mand above.

There are two types of chocolate clay by the pattern of its distribution: continuous and intermittent (mosaic). Continuous distribution prevails in the Volga River valley and is observed in ancient estuaries and junctions of Volga River’s major tributaries: the Large and Little Irgiz, Large and Little Karaman, Torgun etc.

Depending on structure features facies (macrofacies) of chocolate clay are divided into subfacies: monoargillaceous, stratified, sand-clay and silt-clay. The most common monoargillaceous subfacies have total clay composition. It’s massive unstratified or hidden-stratified dark chocolate brown clay with a large block jointing, sometimes interbedded with lighter silt varieties.