Paper No. 338-12
Discoidal impressions are common in rocks of Ediacaran age, and their occurrence in these and older rocks is often used as evidence for the presence of an assemblage of Ediacaran biota, representing complex life. Fluid escape structures are recognized as a potential cause of discoidal markings on bed tops. Positive relief impressions on bed soles have however generally been regarded as more likely to be genuine body fossils. Several discoidal forms found on bed soles of c.
560-Ma upper Burway Formation rocks of the Long Mynd, Shropshire, England, and previously interpreted as fossils (McIlroy et al., 2005), are here shown to be pseudofossils resulting from sediment injection driven by microbial mat sealing (Menon et al., 2016). We provide a new interpretation of the remaining discoidal impression in these rocks, Longmyndian Intrites
(McIlroy et al., 2005), which occurs as mounds with a central depression, or pairs of arcs, of diameter typically 2-4 mm, on both bed soles and tops. Examination in cross-section reveals a torus-like structure, frequently with pale internal laminae, and bounded by microbial mat layers. Longmyndian Intrites
are here interpreted as resulting from microbially mediated build-up of sediment layers on the raised topography of small sediment volcanoes produced by fluid effusions through microbial mats. These “Intrites”
can thus be regarded as a type of torus-shaped microstromatolite, a new form of microbially influenced sedimentary structure (MISS). This study widens the range of MISS, and shows how combinations of physical and microbial processes may produce discoidal impressions on bed soles, urging reassessment of similar discs in non-actualistic, mat-dominated successions.
McIlroy, D., Crimes, T.P., and Pauley, J.C., 2005. Fossils and matgrounds from the Neoproterozoic Longmyndian Supergroup, Shropshire, UK. Geological Magazine, 142, 441-455.
Menon, L.R., Liu, A.G., McIlroy, D., and Brasier, M.D., 2016. The dynamic influence of microbial mats on sediments: fluid escape and pseudofossil formation in the Ediacaran Longmyndian Supergroup, UK. Journal of the Geological Society London, in press.