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- Title
Paleoecological interpretation of brachiopod assemblages in tectonic crevices of the Upper Triassic Dachstein Formation.
- Authors
Huemer, Julian; Kiel, Steffen; Peckmann, Jörn; Zuschin, Martin
- Abstract
Crevices filled with red sediment that contain high-abundance and low-diversitybrachiopod assemblages were found within the Late Triassic Dachstein limestonefacies in the Dachstein Formation, located in the Steinernes Meer Massif, Salzburg,Austria. The origin of the crevices, which strike similarly with a mean orientationof 238˚, is interpreted as tectonic. The crevices and their surroundings were ahabitat for brachiopod communities of the genus Sulcirostra, which ranges from theNorian to the Pliensbachian. The assemblages were studied regarding a possiblerelation to hydrocarbon seepage. The aim of this study was the testing of this workinghypothesis through microfacies analysis, stable carbon and oxygen isotopes, andconsideration of size-frequency distribution-related population dynamics of brachiopods.Samples were taken from four locations. Thin sections were examined with 20-foldmagnification, using transmitted light microscopy. Most brachiopod shells are wellpreserved with many individuals being articulated, thus not showing any sign ofexposure to mechanical stress and transportation. Most size-frequency distributions ofbrachiopods are right skewed, reflecting dominance of smaller individuals, indicatingparautochthonous deposition of the assemblages. Geopetal structures inside brachiopod shellsare similarly oriented. Therefore, the brachiopod assemblages are interpreted asparautochthonous deposits. Stable carbon and oxygen isotope compositions weremeasured from powder samples. The samples were taken from (1) brachiopod shells,(2) their internal cement, (3) the red, fission-filling background sediment and (4)the embedding Dachstein limestone facies. Cements in brachiopod shells frommethane-seep paleoenvironments have been reported to inherit the extremely depleted δ13Ccomposition of methane, which ranges as low as –100‰ (V-PDB). A previous studyon a limestone deposit with a mass occurrence of Sucirostra from eastern Oregonreported δ13C values of authigenic carbonates as low as –24‰ agreeing with achemosynthesis-based ecosystem nourished by hydrocarbon-rich fluids. With a meanvalue of 2.63 (δ13C) and little scatter in the stable carbon isotope composition ofcements, an adaption to seeps is not supported in the case of the Alpine occurrences ofSulcirostra. Instead, it is concluded that the shell accumulations are parautochthonousdeath assemblages of brachiopods that dwelled in a normal marine environment.
- Subjects
SALZBURG (Austria); OXYGEN isotopes; CARBON isotopes; STABLE isotopes; POPULATION dynamics; MICROSCOPY; LIMESTONE; SOCIAL dominance
- Publication
Geophysical Research Abstracts, 2019, Vol 21, p1
- ISSN
1029-7006
- Publication type
Article