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| Titel |
Diagenetic evolution and stable isotopes of Lower Permian platform marginal carbonates (Trogkofel Limestone, Carnic Alps, Austria) |
| VerfasserIn |
Maria Schaffhauser, Karl Krainer, Diethard Gerald Sanders, Christoph Spötl |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038635
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| Schlagwörter |
Karbonatplattform, Trogkofelkalk, Tarviser Brekzie, Diagenese, Stabile Isotope, Perm, Paläozoikum, Südalpin |
| Geograf. Schlagwort |
Österreich, Kärnten, Hermagor (Bezirk), Karnische Alpen |
| Blattnummer |
198 [Weißbriach] |
| Blattnummer (UTM) |
3116 [Sonnenalpe Naßfeld] |
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| Zusammenfassung |
| The Trogkofel massif in the Carnic Alps, Austria/Italy, consists of a succession up to 400 m
thick of limestones deposited along a platform margin (Trogkofel Limestone; Artinskian).
The top of the Trogkofel Limestone is erosively overlain by the Tarvis Breccia. Up-section,
the Trogkofel Limestone consists of well-bedded shallow-water bioclastic limestones with
intercalated mud mounds, overlain by thick-bedded to unbedded limestones (bioclastic
grainstones, packstones, rudstones) and cementstone mounds rich in phylloid algae,
Tubiphytes, bryozoans and Archaeolithoporella. In the cementstone mounds, bioclasts are
coated by thick fringes and botryoids of fibrous calcite, and of calcite spar that probably
represents calcitized aragonite. Primary and intrinsic pores are filled by microbialite, and/or
by mudstone to bioclastic wackestone. Shallow-water bioclastic grainstones are
cemented by isopachous fringes of fibrous calcite, or by sparry calcite. Throughout the
succession, evidence for meteoric-vadose dissolution is present. The Trogkofel
Limestone is riddled by palaeokarstic dykes and caverns filled by (a) isopachous cement
fringes up to a few decimetres thick, and/or (b) by red, geopetally-laminated lime
mudstone to bio-lithoclastic wackestone; geopetal laminasets locally display convolute
bedding. Small dissolution cavities are filled by grey internal sediment, or by crystal
silt. Brecciated internal sediments overlain by unbrecciated, geopetally-laminated
infillings record deformation during or after deposition of the Trogkofel Limestone.
Polyphase fractures cemented by calcite may cross-cut both internal sediments and host
rock.
In the Trogkofel Limestone, local dolomitization is common. Replacement dolomites
show a wide range of shapes and fabrics, including: (a) fine-crystalline anhedral xenotopic
fabric, (b) coarse-crystalline subhedral to euhedral, hypidiotopic to idiotopic fabric of turbid
or optically zoned crystals, and (c) saddle dolomite as replacement and filling of fractures.
Closely below the erosional surface at the top of the Trogkofel Limestone, the dolomite is
characterized by vuggy porosity. The Tarvis Breccia, which represents coarse alluvial fan
deposits, in turn, is thick-bedded, poorly sorted, typically clast-supported, and consists of
angular lithoclasts embedded in a matrix of former lime mudstone. Both, matrix as well as
lithoclasts, are dolomitized.
Various types of cement (isopachous, botryoidal, microbialite, calcite spar), karstic cavity
fills (isopachous cements, internal sediments), and replacement dolomites of the Trogkofel
section, as well as of the Tarvis Breccia were analysed for their stable isotopic composition.
δ18O and δ13C data produced so far allow to differentiate between replacement dolomites and
saddle dolomite of the Trogkofel Limestone and the Tarvis Breccia. Saddle dolomite shows
the most depleted oxygen isotope values, suggesting formation during relatively
high temperatures. Carbon isotope values are invariably positive in all dolomite
types indicating lacking influence of organic diagenesis on the alkalinity of the
deep-burial pore water. Matrix dolomite from the Tarvis Breccia shows slightly positive
δ18O values. Calcite cements show a wide range in δ18O values (ca. -1 to -7 permil
VPDB), which overlaps the composition of unaltered brachiopod shells (ca. -3
permil VPDB). Oxygen isotope values of calcite cements reveal a trend towards
depleted δ18O values. This trend is reflecting most likely increasing temperature. |
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