The dynamic influence of microbial mats on sediments: fluid escape and pseudofossil formation in the Ediacaran Longmyndian Supergroup, UK

Abstract

Microbial mats are thought to have been widespread in marine settings before the advent of bioturbation, and the range of their influence on sediments is gradually becoming recognized. We propose that mat sealing can dynamically affect porewater conditions, and allow the build-up of overpressure that can drive dewatering and degassing to produce a suite of atypical fluid-escape features. Finely bedded silty and sandy laminae from the c . 560 Ma Burway Formation of the Longmyndian Supergroup, Shropshire, England, reveal evidence for sediment injection, including disrupted bedding, clastic injections, sill-like features and sediment volcanoes at sub-millimetre scale. These features are associated with crinkly laminae diagnostic of microbial matgrounds. Matground-associated sediment injection can explain the formation of several types of enigmatic discoidal impressions, common in rocks of this age, which have previously been attributed to the Ediacaran macrobiota. Serial grinding of Longmyndian forms previously described as Medusinites aff. asteroides and Beltanelliformis demonstrates that such discoidal features can be fully explained by fluid escape and associated load structures. Our observations emphasize the non-actualistic nature of shallow-marine Ediacaran sediments. Matground-associated sediment injection features provide a new insight into the interpretation of Proterozoic rocks and the biogenicity of their enigmatic discoidal markings. Supplementary materials: A document containing further images of fluid escape and loading features observed in the upper Burway Formation at Ashes Hollow, together with an annotated diagram of features appearing in one typical vertical cross-section, is available at www.geolsoc.org.uk/SUP18870 .

Cryptic Disc Structures Resembling Ediacaran Discoidal Fossils from the Lower Silurian Hellefjord Schist, Arctic Norway

“…Partly or wholly biological processes may produce discoidal structures, including scratch circles [70], bacterial colonies [71] and water or gas escape from microbial mats [72,73]. The surface expression of vertical burrows may also produce discoidal structures.…”

A mixed Ediacaran-metazoan assemblage from the Zaris Sub-basin, Namibia

“…The structures exhibit several features that negate an abiotic origin or interpretation as water-or gas-escape structures (see e.g., Menon et al, 2016). First, the conical/chevron infill visible in some examples is oriented concave-upwards, which is inconsistent with gas escape (Frey et al, 2008).…”

“…Second, in some examples thin mud drapes can be seen extending laterally from the matrix down into the base of infill chevrons; this illustrates that laminae are not always cross-cut (as is typical for both gas-and water-escape structures), but instead sediment was able to accumulate alongside the organism, and settled into the core of the burrow as the organism adjusted its position upwards. Almost all of these features are at odds with the Medusinites and Beltanelliformis fossils reinterpreted by Menon et al (2016) as fluid escape structures, which typically do not extend deeper than 1 mm (the examples described here penetrate >5 mm), apparently rely on the presence of microbial mats for formation (which are not evident in our examples), and do not exhibit the combination of upwards-deflected laminae and chevron cone-in-cone infill that is characteristic of biological behavior (Abad et al, 2006). Moreover, we do not see any horizontal sill-like 'injections' of sediment, which would suggest an abiotic origin as a sand volcano (Menon et al, 2016).…”

“…Almost all of these features are at odds with the Medusinites and Beltanelliformis fossils reinterpreted by Menon et al (2016) as fluid escape structures, which typically do not extend deeper than 1 mm (the examples described here penetrate >5 mm), apparently rely on the presence of microbial mats for formation (which are not evident in our examples), and do not exhibit the combination of upwards-deflected laminae and chevron cone-in-cone infill that is characteristic of biological behavior (Abad et al, 2006). Moreover, we do not see any horizontal sill-like 'injections' of sediment, which would suggest an abiotic origin as a sand volcano (Menon et al, 2016).…”

Pistas fósiles y dubiofósiles del Ediacárico y Cámbrico del Anticlinal de Alcudia, España

“…5E) very similar to some pseudofossils of the Longmyndian Group (England). Menon et al (2015) demonstrated these Beltanelliformislike concavities to be pseudofossils originated by the small-scale sinking of a sand cone after an upwards fluid injection obstructed by a microbial mat. Other structures suggesting substantial microbial development in the same kind of bed tops consist on "bubbles" (Fig.…”