Precambrian and Cambrian Regional Stratigraphy of Mongolia

Abstract

In this paper, we demonstrate the Mongolian Precambrian and Cambrian stratigraphic classification of sedimentary, sedimentary–volcanogenic, volcanogenic, and metamorphic rock sediments spread over the territory of Mongolia according to the new stratigraphic classification approved by the International Stratigraphic Commission. The Mongolian republic is situated in the central part of the Asiatic continent. Geological studies in different parts of Mongolia began in the middle of the twentieth century, but regular and extensive examination started in the 1970s and 1980s along with stratigraphic analyses of the oldest deposits within the sedimentary basins. The Joint Russian–Mongolian Scientific Research Geological and Palaeontological Expeditions have played a leading role in the study of the region. These studies commenced more than 40 years ago and are still in progress to some extent. Precambrian rock units, exposed mainly in northern Mongolian, have been divided into three regional metacomplexes (Baidrag, Buteel, and Bumbuger) and four sedimentary formations (Muren, Khug, Darkhad or Zavkhan, and Khubsugul or Tsagaanolom). Isotopic age determinations on rocks and minerals from Archean granulite–gneiss blocks demonstrate an evolution from ~3.1 to 2.6 Ga. The Palaeoproterozoic sedimentary complex is well exposed in northern and central Mongolia and is distinguished by marbles, various calciphyres, amphibolites, and iron quartzites. The age of the complex is 1600–2050 Ma. Detailed studies of the middle Neoproterozoic and early Cambrian faunal (comeosiliceous polyactinal sponges, ichnofossils, soft-bodied fauna, medusa, chiolites, anabarites, archaeocyathids, trilobites) and floral (stromatolites, microphytolite assemblages, cyanobacterial mats, microfossils, calcareous algae) fossils provide the opportunity to construct the first regional stratigraphic subdivision of different facies sediments. Glaciogenic deposits are widely distributed in the Neoproterozoic successions around the world, but only a few of them contain unequivocal macroscopic fossils. Precambrian glacial marine deposits (Ediacaran Maikhanuul Formation) were discovered in the Zavkhan region of western Mongolia in the 1990s; they discordantly overlie the volcanic rocks of the Zavkhan Formation (732–777 Ma) and are conformably overlain by limestones of the Tsagaanolom Formation. Recent studies suggest that the discoidal macroscopic remains from the intertillite beds of the Maikhanuul Formation, as well the assemblage of microfossils from the Tsagaanolom Formation, are comparable to those from the Doushantuo Formation of South China (600–550 Ma).