U–Th–Pb Isotopic Dating of Detrital Zircons and Garnet and Tourmaline Indicators from Molassic Deposits of the Belorechenskaya Formation (Western Cis-Caucasia): Geological Implications

In order to determine the provenance areas of Cenozoic sedimentary basins in the Western Cis-Caucasia and to reconstruct the paleotectonic and paleogeographic environments of orogenic molasse formation in these basins, the sands and sandstones from Quaternary sequences corresponding to two stratigraphic levels were studied: (1) the lower part of the Belorechenskaya Fm section (Gelasian) (sample MK-29) and (2) sequences forming a series of adjacent terraces (Upper Neopleistocene) on the southwestern outskirts of the Maikop town (sample MK-30). U‒Th‒Pb isotope dating and morphological study of detrital zircons (dZr), as well as mineralogical analysis of garnet and tourmaline grains extracted from the heavy fraction of the samples have been performed. Density probability curves (DPC) characterizing the distribution of dZr ages from the studied samples were obtained. Together with previously published similar data on the Eopleistocene sands from the middle part of the Belorechenskaya Fm (sample K23-073) and the Holocene sands of the modern alluvium of the Belaya River (sample K22-032), this made it possible to obtain a summary characteristics for four successive stratigraphic levels of Quaternary deposits, demonstrating a temporal change in the provenance signal in the orogenic molasse of the Western Cis-Caucasia. The studied complex of clastic rocks participating in the Quaternary orogenic molasse contains well-defined signs of the “southern” (Caucasian) provenance signal presented in the sets of U‒Pb isotope ages of dZr grains. In the case under consideration, the “southern” signal is represented by peculiar components associated with the erosion of: (1) Lower and Middle Jurassic volcanogenic-sedimentary, volcanogenic, subvolcanic, and intrusive rocks of the Cimmerian structural level (Cimmerian provenance signal); (2) Early Paleozoic and Late Neoproterozoic igneous and metamorphic formations of the Cadomian complex (Cadomian provenance signal) as a part of the Hercynian basement. A tendency of a gradual decrease in the intensity of the Cimmerian and increase in the intensity of the Hercynian provenance signals in the stratigraphic units of the Quaternary sequence from the lower to the upper levels of the section has been established. This tendency characterizes a successive deepening of the erosion level of the Greater Caucasus orogen. In the beginning, at the early stages of the formation of the western segment of the orogen in Gelasian, erosion spanned rock complexes mainly of the Cimmerian structural stage. Later, starting from the Eopleistocene and up to the present time, the main source of detrital material became the exposed complexes of the Hercynian basement.