Sedimentogenesis and volcanosedimentary formations in the Mesozoic-Cenozoic continental rift depressions of the Baikal region and Southern Mongolia

Abstract

Study of the Mesozoic and Cenozoic infill of rift depressions in the Baikal region (Transbaikal and Baikal regions) and Gobi Altai in southern Mongolia allowed us for the first time to consider regularities of sedimentogenesis in these structures and to establish peculiarities of their typical volcanosedimentary formations1. It is shown that continental rift depressions include plain fan and orogenic molasse formations, which were previously united into the orogenic molasse group. Abundance of the coarse-clastic material, which served as a basis for assigning the fan formations to the molasse group, was related to the destruction of near-wall benches (hereafter, escarps) of the grabens in ancient hillocky plains. The lithofacies and sedimentary environments of rift fan and molasse formations in the arid and humid zones are described. Specific features, which make it possible to discriminate the paragenetic associations, are presented. It was established that the riftogenic fan formations of the relief peneplanation epoch have no analogues beyond rift zones. In contrast, similar (in structure, composition, and sedimentary depositional environments) molasses were formed in various tectonic settings during the orogenic epochs.