Lithological-mineralogical characteristics of the Lower Khvalynian chocolate clays of the Lower Volga region (a case study of Raygorod and Srednyaya Akhtuba sections)

Chocolate clays are polyfacial marine sediments of disputable genesis, widely distributed in the Lower Volga region. We present the results of detailed analysis of microstructure features, granulometric (42 samples) and mineralogical (24 samples) compositions of different layers in the thickness of the Lower Khvalynian chocolate clays of the Srednyaya Akhtuba and Raygorod sections. The sections are located on opposite sides of the Volga-Akhtuba floodplain. Based on the data obtained with a laser diffractometer, lithological coefficients were calculated. This allowed us to distinguish seven lithotypes common to the two sections, each of which also has specific mineralogical and micromorphological characteristics. The mineralogical structural features of minerals in the <1 μm fraction in each lithotype and the quantitative ratios of clay minerals were determined by X-ray diffractometry. It was revealed that the most clayey lithotypes are characterized by the predominance of trioctahedral illite over smectite. The opposite situation was observed in the silty lithotypes, where smectite predominated over dioctahedral illite. The swelling components were represented by highly charged beidellite in the Srednyaya Akhtuba section and highly charged montmorillonite in the Raygorod section. Such differences in the ratio and structure of clay minerals may be caused by different sources of drift or by their different synsedimentation or diagenetic transformations. Micromorphological data allowed us to establish biogenic textures in the clay lithotypes of the Lower Khvalynian Chocolate clays. In silty lithotypes, oblique layering and carbonaceous organic matter were determined, which suggests the proximity of their sedimentation zone to the coastline. The obtained materials made it possible to show the complex composition of individual layers of the chocolate clay sequence and determine new directions in the study of their genetic features. Studies of individual lithotypes of the Lower Khvalynian chocolate clays with a detailed reference to the depths were carried out for the first time.

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