Sedimentological and palynological constraints on the basal Triassic sequence in Central Switzerland

Abstract

In Central Switzerland, Mesozoic sedimentation began after erosion and peneplainisation of the Hercynian relief and late Paleozoic continental deposition in SW-NE striking pull-apart basins. The first Triassic sedimentary sequence overlaying a weathered crystalline basement consists of a relatively thin (<10 m), lithologically highly variable unit with coarse-grained siliciclastic deposits at the base, grading into a mixed sandstone/shale-dolomite sequence followed by well-bedded dolomites with chert nodules.Sedimentary texture analyses and petrological investigations revealed four different sedimentary units starting at the base with a regolith unit that represents the weathered crystalline basement. It is overlain by terrestrial plain deposits, followed by mixed siliciclastic-carbonaceous sediments and a sequence of dolomites, deposited between the supralittoral and eulittoral zones of a tidal flat (Mels-Formation), and the eulittoral to sublittoral zones of a carbonate tidal flat environment (Röti-Dolomit), respectively.Palynological data from four localities in Central Switzerland indicate a heterochronous early Anisian age (Aegean -Bithynian/Pelsonian) for the supra-to eulittoral mixed siliciclastic-carbonaceous sediments. These new biostratigraphic ages suggest that the first Triassic marine transgression in Central Switzerland is time equivalent with those of the basal Wellendolomit in Northern Switzerland but slightly older than in the Germanic Basin. Consequently, Central Switzerland was located at this time at the northern shoreline of the Tethys and not on the southern limit of the Germanic Basin. ZUSAMMENFASSUNGDie mesozoische Sedimentation begann in der Zentralschweiz nach einer erosiven Phase des herzynischen Gebirges und der Ablagerung spätpaläozoischer klastisch-kontinentaler Sedimente in SW-NE gerichteten transpressiven Grä-ben. Die frühesten triadischen Ablagerungen, die den verwitterten kristallinen Untergrund überdecken, sind eine meist geringmächtige (< 10 m) Sequenz von grobkörnigen Siliziklastika, gefolgt von einer Wechsellagerung von Sandsteinen, Tonsteinen und Dolomit, die durch gut gebankte Dolomite mit Hornstein-Konkretionen überlagert werden.Detaillierte sedimentologische und petrographische Analysen führten zur Unterteilung der Sequenz in 4 Einheiten, beginnend mit einem basalen Regolith, dem Verwitterungsgrus des kristallinen Untergrundes. Im Hangenden folgt als erstes eine terrestrische, siliziklastische Einheit, die von einer suprabis eulitoralen siliziklastisch-karbonatischen Wechselfolge einer Wattenfläche (Mels-Formation) überlagert wird. Im Hangenden treten gut geschichtete Dolomite des eu-bis sublitoralen Bereichs eines Karbonatwatts (Röti-Dolomit) auf.Palynologische Daten von vier Lokalitäten in der Zentralschweiz deuten auf eine heterochrone marine Transgression (Aegean bis Bithynian/Pelsonian) der supra-bis eulitoralen, siliziklastisch-karbonatischen basalen Triassedimente hin. Diese neuen biostratigraphischen Daten zeigen, dass die erste marine Transgression in de...

Palynofacies patterns, acritarch diversity and stable isotope signatures in the Lower Muschelkalk (Middle Triassic) of N Switzerland: Evidence of third-order cyclicity

“…2) with marine ingression into the western part of the Germanic Basin from the South via a central alpine gate (Western Gate of Szulc 2000; Alpenrhein-Depression of Einsele & Schönenberg 1964) and continuing northwards into the Hessian Depression (Götz & Gast 2007). This hypothesis is corroborated by Gisler et al (2007) who investigated early Triassic deposits from the central part of Switzerland. However, further studies are necessary to clarify the position and temporal influence of a western gate area during the Anisian.…”

New stratigraphic data from the Lower Penninic between the Adula nappe and the Gotthard massif and consequences for the tectonics and the paleogeography of the Central Alps

“…For the Helvetic realm, in the Mont Blanc area the arkoses are Lower to early Middle Triassic (Avanzini and Cavin, 2009) and in the Aar massif they are Anisian (Gisler et al, 2007).…”

The Teggiolo zone: a key to the Helvetic–Penninic connection (stratigraphy and tectonics in the Val Bavona, Ticino, Central Alps)

“…The Teggiolo Triassic may also contain thick intercalations of anhydrite such as those encountered in the Simplon tunnel in the overturned limb of the Antigorio nappe (Schardt 1903 similarities with the Triassic of the Helvetic sensu lato domain (e.g. Frey 1968;Mégard-Galli and Baud 1977;Epard 1989;Gisler et al 2007). It has no affinity with the Briançonnais Triassic.…”