An interpretation of oncoid mass-occurrence during the Late Silurian Lau Event, Gotland, Sweden

Abstract: The geological significance of the microbial-formed oncoid is not as well understood as the recognized, analogous stromatolite but can, as well be used as an environmental indicator. In association with the larger Phanerozoic mass extinctions, a resurgence of microbialites as a disaster fauna is at present an accepted view. The same model, however, is under debate concerning whether or not it can be applied to minor extinction events such as the Lau Event, a coupled extinction-isotopic event recorded from Late Silurian. The Lau Event displays one of the largest positive carbon isotope excursions (CIE) during the entire Phanerozoic with detrimental effects on various marine fauna. The last appearance datum (LAD) of the conodont species P. siluricus in the uppermost Hemse Group on Gotland marks the commencement of the event and the superimposing Eke Formation is extremely rich in oncoids. To analyze the specific controlling determinants and the potential for oncoids as environmental indicators, I measured size and shape for over one hundred oncoids deriving from Gotland and Bjärsjölagård, Sweden. Thin sections and cross-sections of oncoids were produced and Rothpletzella was by far the most abundant calcimicrobe in oncoid cortex, often associated with Wetheredella. Results herein strengthen the theory of the mass abundant oncoids constituting an anachronistic facies. Furthermore, oncoid- shape and morphology are concluded herein to, with caution, be used as indicators of water energy which sets the deposition of the Eke Formation under a phase of transgression.