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 .
Highlights
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We demonstrate that some of the enigmatic sedimentary structures commonly observed within late Ediacaran successions - and interpreted as fossils - should be re-interpreted in terms of mat-driven processes
We propose that the very small-scale fluid injection features result from the influence of the microbial mats
Summary
This document is made available in accordance with publisher policies. Full terms of use are available: http://www.bristol.ac.uk/red/research-policy/pure/user-guides/ebr-terms/. The dynamic influence of microbial mats on sediments: fluid escape and pseudofossil formation in the Ediacaran Longmyndian Supergroup, UK. 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 fluid injection can explain the formation of several types of enigmatic discoidal impressions, common in marginal marine facies of this age, which have previously been attributed to the Ediacaran macrobiota. Our observations emphasize the non-actualistic nature of shallow-marine
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