Hypothesized Cambrian medusae from Saint John, New Brunswick, reinterpreted as sedimentary structures
DOI:
https://doi.org/10.4138/atlgeol.2011.002Keywords:
medusae, monroes, sand volcano, gas escape, mud diapir, microbial structure, King Square FormationAbstract
More than a hundred radial and discoidal structures occur on bed tops of shales and very fine sandstones of the Cambrian (Series 3) King Square Formation in New Brunswick, Canada. These structures typically contain a central sediment plug, radial lineations that extend outward from the central plug, concentric rings, and a broad trough surrounding or underlying ring margins. Originally interpreted as fossils of scyphozoan medusae, these structures could represent one of only a half-dozen mass strandings documented from the fossil record. Instead, re-evaluation of their sedimentology and morphology suggests that they are likely sedimentary structures known as Astropolithon. These sand-volcano-like structures formed by subsurface blistering, cracking, and failure of a near-surface or surface bed, triggered by the upward movement of gases or other fluids from underlying beds. RÉSUMÉ Plus d’une centaine de structures radiales et discoïdes apparaissent au sommet des couches de schiste et de grès très fins de la formation King Square du Cambrien (Série 3), du Nouveau-Brunswick, au Canada. Ces structures contiennent généralement un culot sédimentaire central, des linéaments radiaux qui se déploient vers l’extérieur à partir du culot central, des anneaux concentriques, et une vaste cuvette qui ceinture le bord des anneaux ou qui se trouve en dessous. On estimait initialement qu’il s’agissait de méduses scyphozoaires, mais ces structures pourraient correspondre à l’une de la demi-douzaine d’échouages massifs de l’ichnofaune dont les vestiges fossiles ont été retrouvés. La réévaluation de leur contexte de sédimentation et de leur morphologie porte plutôt à croire que ce sont vraisemblablement des structures sédimentaires connues sous le nom d’astropolithon. Ces structures apparentées à un volcan de sable ont été formées par l’apparition d’alvéoles souterraines, le fendillement et la rupture d’une couche près de la surface ou à la surface, et qu’aurait provoquée le déplacement vers la surface de gaz ou d’autres fluides provenant des couches inférieures. [Traduit par la redaction]Published
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