ABSTRACT This paper investigates two types of macroscopic surficial structures that are characterised by hemispherical depressions. Both structure types are reported from Ediacaran outcrops of Conception Bay, Newfoundland. We document the petrographic, geochemical and morphological aspects of structures previously described as bubble trains and their morphological variation to investigate their mode of formation. Bubble trains preferentially occupy the troughs of ripples/wavy bedding and were cast by a carbonate‐cemented sandstone after degradation of the associated microbial mat. In contrast, the EM Coombs Surface exhibits numerous widely distributed hemispherical depressions that do not aggregate but share a similar casting process to bubble trains, referred to as ‘bubble fields’. Petrographic analyses demonstrate that bubble trains in Crocker's Cove contain carbonate cements, some of which fill pre‐existing conduits in the silty mudstones below bubble trains and are interpreted as fractures formed by gas bubble growth and migration. Carbon isotope ratios of the carbonate cements demonstrate an isotopically light δ 13 C carb value of −13.21‰ on average. The analysed carbonate cement contents indicate early diagenesis within the manganese reduction zone. These data suggest that rising gas bubbles accumulated beneath an impermeable microbial mat, creating hemispherical depressions by displacing unlithified sediment beneath the mat. These depressions and conduits formed by this migration process were later cemented, with anaerobic oxidation of methane and organic carbon as potential carbon sources for carbonates involved. The case of bubble trains is unusual in that the only impermeable layers are the linear and patchy microbial mats in ripple troughs. In contrast, the extensive mat of the EM Coombs Surface allowed gas accumulation over a wide area rather than in strips. Findings of this research imply that microbial mats can be invoked in the creation of both ‘bubble trains’ and ‘bubble fields’, thereby allowing their classification as forms of microbially induced sedimentary structures.
Chida et al. (Mon,) studied this question.