Structure of the precambrian sedimentary cover and upper part of the basement in the Central Russian Aulacogen and Orsha Depression (East European Platform)

The Precambrian sedimentary section and upper part of the basement of the Central Russian Aulacogen and Orsha Depression, two largest structures located beneath the Moscow Syneclise are analyzed. It has been established that the Late Riphean Central Russian Aulacogen was initiated on the Proterozoic crust of the Transcratonic belt that separates different-aged geological blocks of the East European Platform basement. The Orsha Depression is superposed both on sedimentary complexes of the aulacogen and rocks constituting structures surrounding the Transcratonic belt. Boundaries of the sedimentary cover and basement are outlined and a new structure (Toropets-Ostashkov Trough) is defined. The Precambrian section recovered by Borehole North Molokovo is proposed to serve as a reference one for the Central Russian Aulacogen. The CMP records demonstrate seismocomplexes, which allow one to trace rock members and sequences defined by drilling. Eight seismocomplexes, combination of which varies in different structures, are defined in the Upper Riphean-Vendian part of the sedimentary section. The section of the Central Russian Aulacogen includes the following sedimentary complexes: dominant gray-colored arkoses (R₃¹), variegated arkoses (R₃²), red-colored arkoses (R₃³), and volcanosedimentary rocks (V₁²). The section of the Orsha Depression consists of dominant red-colored quartz sandstones (R₃⁴), glacial and interglacial (V₁¹), and variegated volcanogenic-terrigenous sediments. The upper seismocomplex (V₂) is composed of terrigenous and terrigenous-carbonate rocks. It represents the basal unit of the Moscow Syneclise, which marks the plate stage in development of the East European Platform. The upper part of the basement corresponds to a seismocomplex (Pr₁) represented by dynamometamorphosed rocks that form a tectonic mélange. Analysis of the lateral and vertical distribution of the defined seismocomplexes made it possible to specify the structure of the Riphean-Vendian part of the sedimentary cover and to revise their formation history in some cases.