Sedimentary organic matter deriving from tubes of sabelliditids, vendotaenids, sapropelic films and kerogens was extracted by acid processing from Upper Vendian siliciclastic successions in the East European Platform. Elemental composition obtained by instrumental neutron activation analysis (INAA) displays the increasing cobalt (Co) concentration from 1 ppm at the bottom of Upper Vendian succession to about 800 ppm in the uppermost part near the Precambrian–Cambrian boundary. This distribution is recorded in all studied successions and is not related to resistant minerals that survived acid treatment. The enrichment in Co and other metals in the sedimentary organic matter is inferred to be caused by the bloom of cyanobacterial microbiota, and bonding of metals in decaying sedimentary organic matter during pre-burial bacterial reworking and post-burial early diagenesis in a low energy, stagnant depositional basin during Kotlin times. The positive Ceanom in probable benthic sabelliditids, and the exceptional preservation of sedimentary organic matter from Kotlinian strata, indicate the anaerobic conditions during their sedimentation. Clear correlations between 87Sr/86Sr, δ13C and Co imply the secondary isotopic signatures of Sr and C in sedimentary organic matter deriving from a restricted epicontinental marine basin.