Palaeoproterozoic, rift-related, ¹³C-rich, lacustrine carbonates, NW Russia. Part I: Sedimentology and major element geochemistry

Abstract

The 150-m-thick Kuetsjärvi Sedimentary Formation (KSF) from the Pechenga Greenstone Belt, NW Russia, is one of the key formations in the study of a positive δ13Ccarb excursion occurring globally in the Palaeoproterozoic. The KSF formed in an intracratonic rift setting and is sandwiched between two, 2-km-thick subaerially erupted volcanic units. The KSF was previously interpreted as shallow marine, but new data reported here indicate that it is a non-marine unit deposited on a deeply subaerially weathered surface mantling the underlying volcanic rocks. The lowermost part of the KSF represents an alluvial–fluvial plain, followed by a laterally and vertically variable succession of variegated to mottled fine-grained siliciclastic rocks and ‘red beds‘, dolostones containing stromatolite sheets, hydrothermal travertine deposits and abundant desiccation features (e.g.tepees, surfical silicified crusts and dissolution cavities), including probable pseudomorphed evaporites. Measured S and Corg concentrations for the carbonate and siliciclastic rocks are low. Combined, these features indicate that the carbonate rocks of the KSF accumulated in a shallow lacustrine setting. Major types of carbonate facies were formed by: (1) biologically-induced precipitation; (2) evaporitic removal of CO2 in a closed lake environment; and (3) chemical precipitation from thermal springs. Apparently, none of these carbonate facies was in full isotopic equilibrium with atmospheric CO2. This interpretation shows the importance of taking into account the interplay between global and local depositional factors when interpreting the isotopic signature of the KSF dolostones and its implication for the Palaeoproterozoic carbon isotope excursion

Palaeoproterozoic, rift-related, 13C-rich, lacustrine carbonates, NW Russia. Part II: Global isotope signal recorded in the lacustrine dolostones

“…The Rb and Sr concentrations were determined by isotope dilution. For a detailed geological map of the Pechenga Greenstone Belt, see Melezhik & Fallick (2005). Strontium isotope analyses were performed in static mode on a Finnigan MAT-261 mass spectrometer.…”

“…2). The third 13 C offset of w1‰, at the base of Excursion 4, is defined by a rapid switch in the depositional setting from a carbonate shoreline to a rift-bound playa-lake (Melezhik & Fallick 2005). However, the upward decrease in 13 C from +8 to +7‰ coincides with the transition from interdistributary areas on the coastal plain to a shallow-water lacustrine facies.…”

“…They are rich in clastic quartz and contain 0·5-53·5 wt.% SiO 2 (20 wt.% on average) (Melezhik & Fallick 2005). They are rich in clastic quartz and contain 0·5-53·5 wt.% SiO 2 (20 wt.% on average) (Melezhik & Fallick 2005).…”

“…However, net bioproduction approximates to zero, and many carbonate lithologies are highly oxidised rocks (Melezhik & Fallick 2005). However, net bioproduction approximates to zero, and many carbonate lithologies are highly oxidised rocks (Melezhik & Fallick 2005).…”

“…Evaporitic conditions and a negative water balance seem to have appeared regularly through the lake history (Melezhik & Fallick 2005) and may have led to further modification of the carbon isotopic composition of ambient lake water. Evaporitic conditions and a negative water balance seem to have appeared regularly through the lake history (Melezhik & Fallick 2005) and may have led to further modification of the carbon isotopic composition of ambient lake water.…”