Chapter 23 Glacial deposits of the Bokson Group, East Sayan Mountains, Buryatian Republic, Russian Federation

Abstract

The Bokson Group (Gr.) forms the platform cover in northern part of the Tuva-Mongolian Massif, south of the East Sayan Mountains. Widespread diamictites occur in the lower part of the Bokson Gr., in the Zabit Formation (Fm.). These contain erratic, faceted and striated stones along with dropstones. They have disconformable or gradual lower boundaries and conformable or transitional upper boundaries with overlying deposits. The isotopic d 13 C crb curve records some maximum and minimum values as a background of insignificantly varying values (from -3 to 2‰). Maximum values (up to 5.5‰), are recorded in deposits underlying diamictites, whereas the overlying beds of the Zabit Formation (Fm.) yielded three peaks or minima down to 24‰.

Neoproterozoic glacial origin of the Great Unconformity

“…Maintaining low preserved sediment abundance over the 92 My of the Ediacaran (and particularly the 39 My from the Gaskiers to the base of the Cambrian) through erosional means would be trivial compared with the kilometer-scale erosion we propose for the Cryogenian. While a late Ediacaran glaciation has been suggested (68, 69), precise geochronological constraints are lacking and key observations [e.g., Cloudina in the matrix of an Ediacaran diamictite (70)] have not been replicated. ii Nondeposition resulting from sediment starvation may be expected if glacial peneplanation (71) sufficiently reduces the available topography; reduced sediment supply could persist on tectonic timescales until orogenesis provides a renewed clastic input. However, there are Ediacaran basins which are not sediment starved. iii Chronological bias may result in an underestimation in the volume and extent of Ediacaran sediments if ambiguous units are mistakenly assigned to the Cambrian.…”

“…Maintaining low preserved sediment abundance over the 92 My of the Ediacaran (and particularly the 39 My from the Gaskiers to the base of the Cambrian) through erosional means would be trivial compared with the kilometer-scale erosion we propose for the Cryogenian. While a late Ediacaran glaciation has been suggested (68, 69), precise geochronological constraints are lacking and key observations [e.g., Cloudina in the matrix of an Ediacaran diamictite (70)] have not been replicated.…”