The Girvanella oncoids from Upper Ordovician deposits in northern Estonia - evidence of the beginning of microbial carbonate factory prior to the Hirnantian glaciation in Baltica.

Abstract Oncoids are widespread components of Phanerozoic carbonate systems, however, their occurrence in the Ordovician of Baltoscandia has remained poorly documented. This study presents the first detailed description of oncoids from the Upper Ordovician (Katian–Hirnantian) of Estonia, based on a unique historical collection assembled over nearly three decades by the Estonian scientist L. Põlma that has not previously been available for detailed scientific analysis. The material originates from northern and central Estonia. Macroscopically, the oncoids occur within lenticular interlayers of oncolitic grainstones and are predominantly spheroidal to ellipsoidal in shape, with typical diameters of 1–2 cm and rare specimens reaching up to 10 cm. Petrographic analysis indicates that the oncoids belong to the porostromatic type. Well-preserved filamentous and tubular microstructures attributable to the calcareous cyanobacteria Girvanella were identified. Oncoid nuclei consist mainly of strongly micritized fragments of skeletal fauna. The extreme rarity of oncoids in the Katian–Hirnantian successions of Estonia is interpreted not as an environmental limitation, but rather as a consequence of preservation bias related to platform morphology and sediment bypass on Ordovician shoals. The association of Girvanella oncoids with peloids and cortoids indicates an intense microbial activity and suggests the operation of a microbial carbonate factory on shallow, high-energy platform highs prior to the Hirnantian glaciation. These findings provide new insights into Late Ordovician carbonate production on the Baltoscandian platform and highlight the importance of unique legacy collections for reconstructing underrepresented depositional environments.