Geochemical features of the Vendian-Cambrian carbonate deposits of Olenek uplift

(Arctic, Russia)

Pisareva N. I.

V.S. Sobolev Institute of Geology and Mineralogy, SB RAS, Novosibirsk, Russia

[e-mail]

In the study most of the Precambrian sedimentary complexes rather acute question of the age of their accumulation. This is due to the absence of biogenic residues and a small amount of geochronological data. An effective method for solving this problem is the method of isotope chemostartigraphy. Sr-isotopic characteristics are used for regional correlations and for the correct establishing the position of sedimentary complexes in the stratigraphic sequence. We have studied carbonate sediments of the Khorbusuonka Group of the Olenek Uplift (north-eastern part of Western Siberia) with a total thickness of about 380 m, which is subdivided into the Maastakh Fm about 50 m of conglomerate and dolostone, the Khatyspyt Fm up to 180 m of black, bituminous limestone and the Turkut Fm 80-300 m of buff dolostone [5]. Silicified carbonates of the Mastakh Formation contain poorly preserved sheaths of mat-forming cyanobacteria, also stromatolites Boxonia grumulosa and Stratifera irregularis occur in the Mastakh Fm. The upper Turkut Fm contains Colleniella singularis, Paniscollenia emergens, Boxonia (?) grumulosa, and Stratifera irregularis. The Khatyspyt Fm contains fifteen taxa of Ediacara-type soft-bodied fossils and Vendotaenids are locally abundant in the carbonaceous limestones.[3]

The upper limit of sedimentation in this series is determined 543.9 ± 0.3 Ma by dating sinsedimentory volcanic breccia at the base of the Kessyuasa Fm overlying Khorbusuonka Group [1]. To determine the lower age limit of the Khorbusuonka Group and use in the future for the global correlation of Late Precambrian sedimentary complexes we have brought the method of Sr-isotope chemostratigraphy.

The samples were investigated by the standard technique of decomposition of carbonates for the atomic-absorption study and the technique of selective dissolution with removal of the secondary structures. By atomic absorption analysis were identified content of Ca, Mg, Fe, Mn and Sr. As a result of geochemical studies have shown, that the carbonate rocks of the Maastakh Fm and of the Turkut Fm consist mainly of dolostone (Mg/Ca ≥ 0.42), while the Khatyspyt Fm is composed of limestone (Mg / Ca ≤ 0.005). The Maastakh Fm is characterized by middle Mn/Sr (3-27), high Fe/Sr (38-500) rations and low content Sr (30-70 ppm); for Khatyspyt Fm is characterized by low Mn/Sr (0.01-7) and Fe/Sr (0.48-7.5) rations and high content Sr (290-2160 ppm); for Turkut Fm is characterized by middle Mn/Sr (0.83-6.25) and high Fe/Sr (9-90) rations and low content Sr (20-60 ppm). Geochemical criteria for these deposits [4] indicate insignificant postsedimentary changes. Geochemical criteria for these deposits (for dolostone: Mg/Ca ≥ 0.608, Mn/Sr ≤ 1.2, Fe/Sr ≤ 3.0; for limestone: Mg/Ca ≤ 0.024, Mn/Sr ≤ 0.2, Fe/Sr ≤ 5.0) indicate the suitability of their use in studying the initial isotopic composition of Sr.

Fig. Comparison of the variations of the Sr-isotopic composition in the Late Precambrian waters in the paleoocean and in the carbonate sediments of the Khorbusuonka Group.

Symbols: the curves of variation of the Sr-isotopic composition in the Vendian-Cambrian by 1 - [6,4,7], 2 - [2] 3 - ⁸⁷Sr/⁸⁶Sr ratio of the least altered samples of carbonate rocks: ht - Khatyspyt Fm, ms - Maastakh Fm and tr - Turkut Fm.

The isotopic composition of Sr was measured on a mass-spectrometer Finnigan MAT-262 (Baikal Analytical CCU SB RAS, Irkutsk). The content of Sr and Rb was measured by mass spectrometer MI 1201T (Analytical Center IGM SB RAS, Novosibirsk).The value of Sr isotopic composition of dolomite of the Maastakh Fm is 0.70821, which proves the Vendian age of these carbonates, that for a long time did not have the exact stratigraphic position. Limestones of the Khatyspyt Fm have relatively small variations in the ⁸⁷Sr/⁸⁶Sr value, that lying in the range 0.70783-0.70806.

A comparison with the curve of variation of the isotopic composition of Sr in the water of late Precambrian (fig.) shows that these deposits were formed 590-580 Ma. Investigation of Sr isotope composition of dolomite Turkut Fm determined, that the ratio of ⁸⁷Sr/⁸⁶Sr can be lower or close to 0.70854, that is typical for middle Vendian and for the Early Cambrian segment of the curve [2].

References:

1. Bowring S.A., Grotzinger J.P., Isachsen C.E. et al. Calibrating rates of Early Cambrian evolution // Science, 1993. V. 261. p. 1293–1298.

2. Halverson G.P., Wadeb B.P., Hurtgenc M.T et al. Neoproterozoic chemostratigraphy // Precambrian Research, 2010. V.182. p. 337-350

3. Knoll A.H., Grotzinger J.P., Kaufman A.J. et al. Integrated approaches to terminal Proterozoic stratigraphy: an example from the Olenek Uplift, northeastern Siberia // Precambrian Research, 1995. V.73. p. 251-270

4. Kuznetsov A.B., Semikhatov M.A., Gorokhov I.M. et al. The isotopic composition of Sr in the carbonate rocks of the Southern Urals Karatavskoy group and standard curve of variation of the ratio ⁸⁷Sr/⁸⁶Sr in the Late Riphean Ocean // Stratigraphy. Geol. correlation. 2003, T. 11, №5. p. 3-39.

5. Pelechaty S.M., Kaufman A.J., Grotzinger J.P. Evaluation of δ¹³C chemostratigraphy for intrabasinal correlation: Vendian strata of northeast Siberia //Geological Society of America Bulletin, 1996.V.108. p.992-1003

6. Kaufman A.J., Jacobsen S.B., Knoll A.H. The Vendian record of Sr- and C-isotopic variations in seawater: implications for tectonics and paleoclimate // J. Earth and Planet. Sci. Lett. 1993. V. 120. № 4. P. 409-430

7. Semikhatov M.A., Petrov P.Yu., Obchinnikova G.V., Gorokhov I.M., Kuznetsov A.B., Kaurova O.K. Pb-Pb isochron age and Sr-isotopic signature of the upper Yudoma carbonate sediments (Vendian of the Yudoma-Maya trough, Eastern Siberia) // Doklady Earth Sciences. V 393. № 8. pp. 1093-1097

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