THE SHAPE AND STRUCTURE OF PHOSPHORITE-BEARING PART OF THE VENDIAN-CAMBRIAN FORMATION IN THE HUBSUGUL BASIN, MONGOLIA: AN ATTEMPT TO ELABORATE SEARCHING STANDARDS

Types of sequences and features of the phosphorite-bearing part of the thick carbonate Hubsugul series (Vendian-Cambrian) of the Hubsugul basin (Western Mongolia and East Sayan) are described. In cyclic structure of the series productive beds occur in the most contrasting cycle, and sediments reveal the maximum shallowness. The productive cyclical dolomite part of the series overlaps non-phosphatic noncyclical pelagic melanocratic limestones. The productive epoch climaxed in drying and erosion in some places of this water paleoarea. In the beginning of a new phase of transgression the material redeposited from weathering crusts on the adjoining land. The southern (Dzabhan Subbasin) and northern (Oka Subbasin) parts of the Hubsugul basin were more shallow and less productive than the central one which possessed optimal depths of water. Types of sediments and their mutual positions in the Hubsugul phosphorite-bearing formation have considerable resemblance with these features of phosphoritic interval in the coeval sequence of the Karatau basin (Kazakhstan). The main difference between these two basins is that the deposits in Hubsugul are primary phosphatous phytolites, while in Karatau they are phosphatic sands, the reworked product of these organomorphous sediments.