LITHOLOGY AND SEDIMENTOGENESIS OF THE EARLY OLIGOCENE LOWER PLANORBELLIAN DEPOSITS IN THE NORTHWESTERN PART OF THE INDOLO-KUBAN DEPRESSION

Abstract

On the basis of lithological, lithmological, mineralogical-petrographic, and granulometric studies of the Lower Planorbellian deposits of the northwestern side of the Indolo-Kuban depression, in combination with the data of facies interpretation of logging results (radioactive methods), the environments of Early Planorbellian sedimentation were reconstructed. It has been established that the lithological structure of the Lower Planorbellian sequence of the Maikop of the northwestern side of the Indolo-Kuban depression is characterized by an almost uniform distribution of mudstones (34–71%, average 49%) and siltstones (27–57%, average 41%) with a subordinate development of sandstones (3–27%, average 10%). According to the features of the spatial distribution of these components within the studied region, four lithological complexes of three series were distinguished: psammite-siltstone-argillite (sandstones 0–25%, siltstones 50–75%, mudstones 25–50%) complex of the siltstone series; predominantly mudstonesiltstone (sandstones 0–25%, siltstones 25–50%, mudstones 25–50%) and mixed terrigenous (sandstones 25–50%, siltstones 25–50%, mudstones 25–50%) complexes of mixed psammitic-siltstone-argillite series and argillite with interlayers of sandstones and siltstones (sandstones 0–25%, siltstones 0–25%, mudstones 50–75%) complex of the argillite series. The genetic interpretation of the results of granulometric analyzes of terrigenous rocks showed the dominance within the region of formations of shelf basins with a directed system of watercourses. A cyclic (transgressive-regressive) character of the Early Planorbellian sedimentation was revealed, which manifested itself in the upsection replacement of sandy formations of the initial stages of the development of the basin by siltstoneargillaceous silts in the middle and again by the accumulation of clastogenic deposits at the final stage. On the whole, the sedimentation basin was a sublittoral shoal with local directed watercourses, within which formations of estuarine and alongshore bars, alluvial fans, and shelf plains are distinguished.