Pb-Pb age of Jatulian carbonate rocks: The Tulomozero Formation of southeast Karelia

Several deep drillholes were made in the Onega palaeobasin by the Fennoscandian Arctic Russia-Drilling Early Earth Project (FAR-DEEP) of the International Continental ScientificDrilling Program (ICDP). These provided fresh core material documenting the LomagundiJatuli Isotopic Event (LJIE), its termination, and the start of the Shunga Event (SHE) of an enhanced accumulation of organic matter. The cored section represents the most complete known record of the end of the LJIE.The late phase of the LJIE, recorded in FAR-DEEP Core 11A, is represented by the upper part of the Tulomozero Formation (TF), a 300-m-thick unit composed of variegated dolostones, siltstones and shales. Accumulated under oxic conditions on a shallow-water carbonate platform, the TF hosts the earliest known thick formations of halite and massive anhydrite. Low-temperature greenschist facies metamorphism caused partial recrystallisation of the rocks; however, a primary carbonate phase, the dolomite, exhibits a negligible degree of post-depositional alteration of the carbon isotope system. The dataset of 46 bulk-carbonate analyses of carbon and oxygen isotopes yielded  13 C carb ranging between +6.8 and +11.8‰, and revealed a positive excursion from +8 to +11.8‰ followed by a decline to +8‰ towards the top of the 300-m-thick succession.The cored part of the overlying Zaonega Formation (ZF) recovered in holes 12AB and 13A is an 800-m-thick unit composed of mixed siliciclastic-carbonate facies with numerous mafic lava flows and tuff beds intruded by gabbro sills. The formation represents an unprecedented episode of enhanced accumulation of organic-rich rocks and preserves the earliest known supergiant, petrified oilfield, all typifying the SHE. The sedimentological, petrographic, geochemical and C-and O-isotopic study of over 800 m of core throughout the ZF suggests that the initial carbonate deposition occurred in a lacustrine environment, then rapidly proceeding through a lagoonal setting to a rifted deepwater shelf, and finally to a carbonate platform. Both the deepwater shelf and the platformal settings are marked by organic-rich environments. The primary dolomite was potentially exposed to syn-and post-depositional hydrothermal/metasomatic alterations, organic matter diagenesis, metamorphic mineral reactions, CO 2 degassing, and finally a low-temperature, post-metamorphic calcitisation resulting in overall large fluctuations of  13 C carb between -22 and +8‰. The dataset of over 413 bulk-carbonate analyses of carbon and oxygen isotopes, screened petrographically and A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT4 geochemically against post-depositional alteration, revealed 28 least-altered samples within three stratigraphic intervals, suggesting multiple positive and negative  13 C carb excursions throughout over 1000 m of stratigraphy. A shift from +5 to +9‰ followed by a drop to near zero values marks the lower part of the ZF. A  13 C carb decline from +8‰ to below zero in the middle and upper part of the ZF defines the end of t...

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“…3). The only currently published age is 2090 ± 70 Ma obtained by a Pb-Pb technique on dolomite (Ovchinnikova et al, 2007).…”

Section: Depositional Age and Lithostratigraphymentioning

confidence: 99%

Carbonate deposition in the Palaeoproterozoic Onega basin from Fennoscandia: a spotlight on the transition from the Lomagundi-Jatuli to Shunga events

Melezhik

Fallick

Brasier

et al. 2015

Earth-Science Reviews

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“…Here they were abundant both quantitatively and taxonomically (Melezhik et al 1997). In Russian Karelia (eastern part of the shield) stromatolites are occurring in terrigenous-carbonate sequence of the Upper Jatulian Onegian Formation which has a Pb-Pb age 2090 ± 70 Ma (whole rock determination) (Ovchinnikova et al 2007). The 800 m thick succession of Onegian Formation mainly consists of reddish 13 C-enriched dolostones with mud-cracked siltstone and ripple-marked quartzite interbeds.…”

Section: Early Precambrian Stromatolite Assemblages Of the Baltic Shieldmentioning

confidence: 96%

Geobiology of Stromatolites

Reitner¹,

Quéric²,

Reich³

2008

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“…The minimum age of the Zaonega Formation is constrained at c. 1.98 Ga by several whole-rock and mineral Sm-Nd, Re-Os and Pb-Pb isochrons obtained from a differentiated mafic intrusion in the overlying volcanic succession of the Suisari Formation (Puchtel et al 1992(Puchtel et al , 1998(Puchtel et al , 1999. The maximum age is constrained at 2.090 AE 0.07 Ga, which is the age obtained by Ovchinnikova et al (2007) for the underlying Tulomozero dolostones using the Pb-Pb technique. Preliminary Re-Os data on C org -rich rocks from the Zaonega Formation suggest an age of~2.05 Ga (Hannah et al 2008).…”

Section: Source Rocksmentioning

confidence: 99%