Structure, Age, and Settings of Formation of Ordovician Complexes of the Northwestern Frame of the Kokchetav Massif, Northern Kazakhstan

This work presents the data on the structure, geochronology, and formation settings of the Ordovician sedimentary and volcanogenic-sedimentary complexes of the Sterlitamak, Mariev, and Imanburluk structural and formational zones located in the western and northwestern frames of the Kokchetav massif (Northern Kazakhstan). In addition, the results of detailed stratigraphic, geochemical, and geochronological studies of the reference section of the Ordovician deposits of the Mariev Zone are given. The studied section is composed of carbonate, terrigenous, and less commonly volcanogenic-sedimentary deposits, confined to a wide stratigraphic interval from Tremadocian Stage of the Lower Ordovician to the lower Sandbian Stage of the Upper Ordovician. For the first time, the study of conodont assemblages made it possible to establish the Early to Middle Ordovician age of the most ancient limestone–dolomite sequence, which was previously conventionally attributed to the Cambrian. The above-lying tuffaceous–terrigenous Kupriyanovka Formation is now attributed to the Middle Ordovician. On the basis of compositional features of the lithoclastic tuffs composing the middle part of the formation, we assume that it was formed within the island arc zone. Limestones from the base of the youngest terrigenous–carbonate Kreshchenovka Formation are attributed to the lower part of the Sandbian Stage of the Upper Ordovician. The study of the geochronology of detrital zircons from terrigenous rocks of the limestone–dolomite sequence has shown that the Early Neoproterozoic quartzite–schist sequences of the Kokchetav massif were the most probable provenance area during its deposition. It was established that there was the change of sedimentation environments from closed lagoons to a relatively deep sea basin with normal salinity and intense circulation of water masses in the northwestern frame of the Kokchetav massif during the Ordovician. During this period of time, there was a sufficiently high level of erosion of provenance areas that resulted in the deposition of thick strata of terrigenous material. A general tendency of the deepening of sedimentation environments from the Early to Late Ordovician was interrupted by sea level rises in the Dapingian and early Darriwilian ages.