Title: Spheroidal sandstone concretions. Their formation and significance in an Eocene seep system (Bulgaria)
Authors: De Boever, Eva ×
Birgel, D.
Muchez, Philippe
Peckmann, J.
Dimitrov, Lyubomir
Swennen, Rudy #
Issue Date: Jun-2009
Publisher: Pergamon-Elsevier Science Ltd.
Host Document: Geochimica et Cosmochimica Acta vol:73 issue:13 pages:A271-A271
Conference: Annual VM Goldschmidt Conference "Challenges to Our Volatile Planet" edition:19 location:Davos, Switzerland date:21-26 June 2009
Abstract: Spheroidal sandstone concretions (< 0.1 – 15 cm diameter)
are confined to Early Eocene shallow marine sands of the
Dikilitash Formation near Varna (NE Bulgaria). Low calcite
δ13C values down to −45‰ VPDB demonstrated the link
between their formation and methane-bearing fluids. Based on
integrated field, petrographic, geochemical and lipid
biomarker results, this study aims at clarifying the process of
spheroidal concretion formation and the significance of these
concretions in a methane seep system.
Field observations showed that the onset of concretion
growth took place synsedimentary, near or at the paleoseafloor,
making spheroidal concretions a potential timing
indicator for methane seepage events. In addition, their
restricted spatial distribution, forming e.g. the building stones
of m-diameter tubular concretions (i.e. the methane seepage
plumbing system) and grapestone-like structures in fault
zones, provides a viable spatial mapping tool for locations of
focused fluid ascent.
The onset of low-magnesium calcite precipitation, in
equilibrium with marine pore waters, was initiated at multiple
nucleation sites (pervasive nucleation) as a result of the
microbially-mediated anaerobic oxidation of rising methane,
which might have created localized microenvironments of
increased calcite supersaturation. Concretions > 0.5 cm
diameter often show regular concentric banding of alternating
cloudy calcite- and transparent calcite-cemented rings,
recording growth from the centre outward under changing
conditions of pore water geochemistry.
ISSN: 0016-7037
Publication status: published
KU Leuven publication type: IMa
Appears in Collections:Division of Geology
× corresponding author
# (joint) last author

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