Lithology of the Upper Jurassic–Lower Cretaceous deposits of the eastern part of the Myrgovaam and Rauchua depressions, Western Chukotka

Abstract

Upper Jurassic–Lower Cretaceous deposits were formed on the southwestern margin of the Chukotka terrane in an active tectonic environment. Their stratigraphic units are characterized by sedimentary structures and lithological similarities, facies variation and scarcity of reliable fauna findings. Detailed lithological studies are necessary due to the absence of a unified approach to the stratigraphic division of deposits. This paper presents petrographic, geochemical, and isotope-geochemical characteristics of Upper Jurassic–Lower Cretaceous rocks. The stages of changing sedimentation conditions and sources, which determined the differences in sedimentological features and the composition of the studied strata, are reconstructed. The Oxford- Kimmeridgian section is composed of sandy debris flow deposits with an arkosic composition of psammitic differences. Among their sources, ancient granitoids dominated, while siliciclastic rocks, volcanites and metamorphic complexes were secondary. The Volgian-Valanginian interval is characterized by the accumulation of sediments in various parts of the submarine fan. In Volgian sequences fine-, medium- and coarse-grained turbidites with lenses of small-pebble conglomerates are identified. A large amount of simultaneous pyroclastic material in the Volgian deposits indicates synchronous volcanic activity. In the Volgian period, the province was dominated by volcanites, mainly of basaltic and andesitic composition; siliciclastic rocks were present in a smaller amount. The Berriasin section is composed of fine-grained turbidites with single horizons of medium-grained turbidites and gravelitic lenses, as well as slope deposits in the form of rhythmically interbedded sandstones and mudstones with slump structures. Sandstones have greywacke composition and contain an admixture of ash material in the matrix. The main sources for Berriasian deposits were siliciclastic rocks and felsic volcanic complexes. The Valanginian section is represented by fine- and medium-grained turbidites with horizons of amalgamated sandstones. Sandstones are classified as arkoses by the ratio of rock-forming components. The dominant source in the Valanginian time was ancient granitoids, while siliciclastic rocks and volcanites were secondary.