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Discussion papers
https://doi.org/10.5194/se-2019-34
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2019-34
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 04 Mar 2019

Research article | 04 Mar 2019

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Solid Earth (SE).

Precipitation of dolomite from seawater on a Carnian coastal plain (Dolomites, northern Italy): evidence from carbonate petrography and Sr-isotopes

Maximilian Rieder1, Wencke Wegner2, Monika Horschinegg2, Stephanie Klackl1, Nereo Preto3, Anna Breda3, Susanne Gier1, Urs Klötzli2, Stefano M. Bernasconi4, Gernot Arp5, and Patrick Meister1 Maximilian Rieder et al.
  • 1Department of Geodynamics and Sedimentology, University of Vienna, Althanstr. 14, 1090 Vienna, Austria
  • 2Department of Lithospheric Research, University of Vienna, Althanstr. 14, 1090 Vienna, Austria
  • 3Department of Geosciences, University of Padova, Via Gradenigo 6, 35131 Padova, Italy
  • 4Geological Institute, ETH Zürich, Sonneggstr. 5, 8092 Zürich, Switzerland
  • 5Geoscience Centre, University of Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany

Abstract. The geochemical conditions conducive to dolomite formation in shallow evaporitic environments along the Triassic Tethyan margin are still poorly understood. Most of the Triassic dolomites in the Austroalpine and the South Alpine realm are affected by late diagenetic or hydrothermal overprinting, but recent studies from the Carnian Travenanzes Formation (South Alpine) provide evidence of primary dolomite. Here a petrographic and geochemical study of the dolomites intercalated in a 100-m-thick Carnian sequence of distal alluvial plain deposits is presented to gain better insight into the conditions and processes of dolomite formation. The dolomites occur as 10- to 50-cm-thick homogenous beds, mm-scale laminated beds and nodules associated with palaeosols. The dolomite is nearly stoichiometric with slightly attenuated c-reflections. Sedimentary structures indicate that the initial primary dolomite or precursor phase consisted largely of unlithified mud. Strontium isotope ratios (87Sr / 86Sr) of homogeneous and laminated dolomites reflect Triassic seawater, suggesting precipitation in evaporating seawater in a coastal ephemeral lake or sabkha system. However, the setting differed from modern sabkha or coastal ephemeral lake systems by seasonally wet conditions with a significant siliciclatic input and inhibition of significant lateral groundwater flow through impermeable clay deposits, thus representing a non-actualistic system in which dolomite formed along the ancient Tethyan margin.

Maximilian Rieder et al.
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Maximilian Rieder et al.
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Short summary
The formation of dolomite (Ca,Mg(CO3), an abundant mineral in Earth's geological record, is still incompletely understood. We studied dolomites embedded in a 100-m-thick succession of coastal alluvial clays of Triassic age in the Southern Alps. Observation by light microscopy and Sr-isotopes suggest that dolomites may have formed spontaneously from concentrated evaporating seawater, in coastal ephemeral lakes or tidal flats along the western margin of the Triassic Tethys sea.
The formation of dolomite (Ca,Mg(CO3), an abundant mineral in Earth's geological record, is...
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