Sedimentary Geochemistry of the Riphean Stratotype (Meso/Neoproterozoic) in the South Urals, Russia

E. Z. Gareev Institute of Geology, Ufa Branch, Russian Academy of Sciences, K,Marx Str. 6, Ufa 450000, Russia

One of the best Upper Precambrian sections in the world is known in the South Urals. This is a stratotype of the Riphean and the Asha series of the Vendian (Meso/ Neoproterozoic). The deposits with unessential epigenetic transformations aged 1650 to 570 Ga make up a cyclic section 12 to 15 km thick in total.There are four large megacycles (series): Burzyanian (Lower Riphean), Yurmatinian (Middle Riphean) - Mesoproterozoic; Karatavian (Upper Riphean) and Asha series (Vendian) - Neoproterozoic, Each series begins with fragmental terrigenous rocks and ends with carbonates.

Clay and fragmental terrigenous rocks of mainly quartz, feldspar-quartz and less frequently polymictic composition have moderate Na2O/K2O, TiO2/Al2O3, Fe2O3+FeO+MnO/ TiO2+Al2O3 values, simple REE composition, and low Ti, V, Cr, Ni and Mn contents. All these features correspond to terrigenous deposits formed at the expense of acid rocks in the base of the Russian plate and products of their destruction.

Arid climatic conditions prevailing in the area of supply and the sedimentation itself going on at a high rate have resulted in the accumulation of low-differentiated sand
sediments and clay deposits of mainly hydromicaceous composition with chlorite admixed. Glauconite as an
indicator of humid environments is found at some horizons of the Karatavian and Asha series only. The arid climatic conditions with an alkali environment have served to the
K accumulation in the Precambrian crusts of weathering. There are three levels of the abnormal K accumulation in the section (K2O higher than 5 pc, K2O/Al2O3 higher than 0.31).

The deposits of the section have been accvumulated mainly in the near-shore shallow zones of the epicontinental marine basin with a normal salinity. However, both the Burzyanian and Yurmatinian are known for some levels of accumulation of high-magnesian and high-iron carbonates similar to evaporites.

During the Riphean/Vendian the tectonic environments vary from moderately active (riftogenic intracratonic) to passive ones at the intermediate and final stages of each cycle.

Variations of the rock geochemical characteristics in the section are governed by a sedimentological cyclicity. The Na2O/K2O ratio increases in the basal horizons of the series where volcanogenic structures and products of their destruction occur. However, the range of petrochemical moduli is more complicated. Their unidirectional change takes place in the Burzyanian, that can be interpreted by a successive washout of one and the same source of supply and weakening in the tectonic processes. In the Yurmatinian, Karatavian and Asha series the moduli are of a saw-tooth pattern suggesting that some additional sources of supply might appear in the middle of the cycles and/or the climate might change. At the same time one can see the FeO/Fe2O3 and MgO/CaO decrease in the terringenous and carbonate rocks respectively. The FeO/Fe2O3 ratio exceeds 1 in the Burzyanian and Yurmatinian that corresponds to black carbon-bearing sedimentary rocks developed widely in the section. Upwards, redbeds begin to predominate in the Karatavian and Vendian. It suggests a redox potential growing in the course of time. The MgO/CaO ratio falling in the carbonates from bottom to top may be a consequence of climatic changes.

The geochemical investigations have led to a number of practical results. Thus, several geochemical horizons have been determined in the Riphean/Vendian section rich in one or another chemical element (Fe, Mg, Ba, Sr, F).