Hinderer, Matthias (1991)
The Fleons Formation in the western Carnic Alps: Ordovician or Carboniferous?
In: Giornale di Geologia, seria 3a, 53 (1)
Artikel, Bibliographie

Kurzbeschreibung (Abstract)

The Volcaniclastic Fleons Formation in the Western Carnic Alps - Sedimentology, Petrography and Geochemistry Abstract A psammitic to psephitic volcaniclastic sequence, known as the Fleons Formation, occurs between phyllitic schists (Visdende Formation) in the western Carnic Alps. It consists of quartzitic schists, quartzites, volcaniclastic greywackes, and conglomerates. Up to recent time an investigation of the Fleons Formation was missing despite a discussion about its role in the evolution of the Carnian Basin during the Variscan cycle. This paper provides new data on the sedimentology, petrography, and geochemistry of the Fleons Formation based on investigations in the area of its typ e 10c a lit Y (Monte Fleons/Raudenspitz). The sedimentary environment of the Fleons Formation can be reconstructed because sedimentary structures and sequences are fairly well preserved. They indicate an interaction of a wave-dominated shoreline environment with a locally evolved fandelta environment. The shoreline environment is petrographically documented in a 0uartzitic Series (equivalent to the "Tscharrknollenfazies" west of the investigated area) whereas the Greywacke Series ("Fleonsgrauwacken") originates from the coarser and immature clastics of the fan-delta environment. Accordingly, the Ouartzitic Series originates from reworking of the Greywacke Series. The Visdende Formation can be considered as a deeper basin facies. The close relationship of both formations is documented by the interdigitation of both facies types. The modal analysis of the detritic components shows that the greywackes can be divided in a quartz-feldspar-poor and a quartz-feldspar-rich type. The first type is dominated by acidic volcanic detritus, the second type by basic volcanic detritus. Further, the latter includes a higher amount of detritus from a metamorphic basement. The provenance of the Greywacke Series can be considered as a three component system: basic and acidic volcanic rocks which are developed on a basement including mainly metasediments, subordinate granitoids and ultramafites. Microprobe analysis of clastic pyroxenes indicates the presence of mildly alkaline basalt volcanism, which is documented by sills in the area nearby. The acidic volcaniclastic input to the Fleons Formation can probably originate from the porphyroids of the western Carnic Alps. This relationship is supported by garnet microprobe analysis, bulk chemistry, and field observations. An identical garnet type could be found as phenocryst in the porphyroids and as clast in the Fleons Formation. The plate-tectonic position of the Fleons Formation is best described as a backarc or post collisional rifting situation. The stratigraphic position of the Fleons Formation is still controversial. An Ordovician or Carboniferous age is discussed, but, biostratigraphic evidence is missing. The new investigations carried out support an Ordovician age. The arguments are as follows: 1) The sedimentary environment of the Fleons Formation and Visdende Formation corresponds to the environment of the Ordovician clastic sediments of the central and eastern Carnic Alps. 2) Acidic volcanic detritus of the Greywacke Series of the Fleons Formation is most probably derived from the Ordovician porphyroids. 3) Finally, no traces of an input of plant remains were found, which should be expected to occur in a Carboniferous deltaiccoastal environment of the Southalpine realm.

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