| Popis: |
The results of a geochemical study and paleogeographic, paleoclimatic interpretation for a cyclically constructed section of upper Cretaceous deposits near Volsk city, Saratov region, are presented. Elementary formation cyclites and cyclic variations of a number of certain parameters were associated with the Milankovich astronomical-climatic cycles. The curves of changes of paleotemperature, humidity, paleobathymetry were compiled. The results obtained give an idea of the migration of the arid belt boundaries in the upper Cretaceous and the overall climatic zonation, which is important for regional and global paleoclimatic reconstructions, as well as the history of the development of shelf seas that covered the Russian plate (especially Ulyanovsk-Saratov trough). Paleotemperatures of the land surface in the denudation areas are obtained from the chemical index of alteration (CIA). In the Turonian-Campanian interval selected climatic cyclicity, including period of relative cooling (Turonian–Coniacian) with paleotemperature about 20 °С, the period of relative warming in the mid-late Campanian (20–24 °С), the cooling time at the end of the late Campanian (19–21 °С) and the period of warming at the turn of the Campanian and Maastrichtian and in the early Maastrichtian time. In Maastrichtian age, there are two climatic cycles, beginning with a time of relative cooling (about 19 °С) and ending with a time of relative warming (about 20 С, at the end of Maastrichtian to 25 °С). The cycles of climate humidity change are also determined: two cycles in Campanian time, three cycles in early Maastrichtian, and one cycle in late Maastrichtian. The boundary of the early and late Maastrichtian corresponds to the change of arid conditions to humid ones. The paleobatimetry curves show transgressive-regressive cycles: one in the late Turonian-Coniacian time, two in the late Campanian time, five in the early Maastrichtian time, and one in the late Maastrichtian time. Depth variations were estimated: in the Turonian-Coniacian time in the range of 70–80 m, in the Campanian-Maastrichtian time, the paleobatimetry consistently increased and changed from 100 to 200 m (on average about 150 m). The results obtaired give and idea of the migration of the boundaries of the arid belt in the Late Creataceous and main features of the climatic zonation, which is important for regional and global paleoclimatic reconstructions, as well as for the history of the development the Russian plate in the Ulyanovsk-Saratov region. |
