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| Titel |
Factors controlling growth and morphology of early Miocene stromatolites from Montaigu-le-Blin, France |
| VerfasserIn |
Tomaso R. R. Bontognali, Federica Barilaro, Judith A. McKenzie  , Crisogono Vasconcelos |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250113185
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| Publikation (Nr.) |
 EGU/EGU2015-13380.pdf |
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| Zusammenfassung |
| A large variety of stromatolite morphotypes are present in the early Miocene sedimentary
sequence that outcrops in the region of Montaigu-le-Blin, France. Thanks to its good
degree of preservation, this formation is an ideal place to evaluate the key factors
controlling the occurrence and morphology of the stromatolites, as well as to study the
diagenetic processes that transform a soft, slimy lithifying microbialite into a solid,
laminated sedimentary rock. Facies analysis suggests that the Montaigu-le-Blin
stromatolites formed in shallow waters, in a restricted, laterally heterogeneous basin.
Various evidence suggests a biological origin for the stromatolites, whose formation
cannot be explained through abiotic background precipitation driven by evaporation,
temperature changes, or migration of metamorphic fluids. Microscopic observations
revealed that the Montaigu-le-Blin stromatolites are mainly comprised of authigenic
calcite that precipitated directly from the basin waters (no evidence for trapping and
binding) in close association with algae, microbial cells, and exopolymeric substances.
Well-preserved microfossils of photosynthetic organisms are widespread within the
thin sections. The calcite constituting the stromatolites is characterized by positive
δ13C values (1.7 to 4.4 o), suggesting that microbial mediation of calcite was
mainly related to the metabolic activity of photosynthetic organisms or anoxygenic
phototrophs, and not to metabolisms in which organic material is degraded (e.g. sulfate
reduction). The laminated habit of the stromatolites results from alternating layers of
porous, digit-like sparite and more compacted organic-rich micrite. No correlation
was identified, through isotopic analysis, between these two types of laminae and
external environmental factors (i.e.Âtemperature, salinity, eutrophication state).
Rather, we hypothesize that periodic fluctuations in the dominant microbial/algal
species within the mat influenced the morphology and the porosity of the different
laminae. The results of this study provide insights for interpreting stromatolites that are
morphologically and petrographically similar to that of Montaigu-le-Blin, but are included
in older and less well-preserved rocks, which makes it difficult to evaluate their
biogenicity and differentiate primary features from late stage metamorphic textures. |
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