The Cryogenian was a period of drastic environmental changes across the entire planet, with glacial erosion limiting the preservation of life forms in most of the geological record. Therefore, to explore more of the biogenic evidence and its implications for this period, here we investigate some metavolcano-sedimentary sequences from the Brusque Metamorphic Complex in Brazil, that record a mid-latitude basin so far unexplored from a paleontological point of view. On the one hand, field discoveries reveal the occurrence of Conophyton-type stromatolites preserved in dolomitic marbles that are isotopically incompatible with cap carbonates. On the other hand, 13C-depleted graphitic schists interspersed in these marbles indicate an accumulation from exceptional volumes of organic matter. Raman spectroscopy and transmission electron microscopy also highlight disordered structures, heterogeneous morphologies, and structures similar to ultra-small microorganisms in this graphite, in compatibility with a biogenic origin. Combined with geochronological data, these records reflect formation during the late Cryogenian greenhouse, between the Sturtian and Marinoan Snowball Earth glaciations. From this, we suggest deposition under conditions of warmer and brighter waters without ice cover, favoring the growth of some stromatolitic barriers. Sturtian nutrient-rich meltwater pulses may also have led to exceptional phytoplankton blooms followed by mass mortality, promoting the exceptional deposition of organic-rich horizons during the transition window to a Precambrian algae-dominated world.