Isotopic-Geochemical Features of Rocks of the Middle-Upper Cambrian Verkhnyaya Lena Formation in the Siberian Craton

Isotopic compositions of carbon and oxygen in carbonates and sulfur in sulfates of the Verkhnyaya Lena Formation (E-2-E-3), which terminates the Cambrian section of the Irkutsk Amphitheater of the Siberian Craton, are studied. Sulfates of the Verkhnyaya Lena Formation are marked by unusually low delta S-34 values (4.6-12.0 parts per thousand) relative to sulfates of the underlying Angara Formation. This is likely caused by variations in the facies-paleogeographic sedimentation at the transition of the Angara and Verkhnyaya Lena formations, as well as associated variations in the water and salt alimentation budget in sedimentation basins, due to their isolation from open sea and intensification of the continental and underground discharge. The delta O-18((PDB)) value in carbonates decreases from -4.4 parts per thousand at bottom to -10.4 parts per thousand at top, reflecting variation in postsedimentary transformations and probable continuous freshening of sedimentation basin. Isotopic composition of carbon in most samples shows normal marine delta C-13 values (0 + 1 parts per thousand). Only in some samples, does the delta O-18 value increase up to -3.8 and 2.2 parts per thousand due to specific features of postsedimentary processes. The Rb-Sr systems of the clayey component of marls from the 500-m-thick section of the Angara Formation and bottom of the Verkhnyaya Lena Formation record an age of 512 +/- 10 Ma, which is close to the assumed stratigraphic age of the Verkhnyaya Lena Formation. The Sr-87/Sr-86 initial ratio is 0.7082 +/- 0.0004.