Abstract
The Carnian Stuttgart-Formation (Schilfsandstein) of the Central European Basin contains relics of Triassic volcanic detritus in form of euhedral zircon grains and authigenic analcime. Multiple LA-ICP-MS spot analyses of single zircon crystals from an outcrop near Heilbronn (SW Germany) yielded weighted average 206Pb/238U ages between 250 and 230 Ma, providing evidence for tephra fallout in the southern part of the Central European Basin related to Olenekian, Anisian–Ladinian and Carnian volcanic activity. The tephra was probably transported by monsoonal circulations from volcanic centres of the NW Tethys to the Central European Basin. The four youngest zircon crystals gave a weighted average 206Pb/238U age of 231.1 ± 1.6 Ma (10 analyses), which is interpreted to date syn-depositional tephra fallout into the fluvial Lower Schilfsandstein Member of the Stuttgart Formation. This new maximum depositional age provides the first evidence that deposition of the Stuttgart Formation, which represents the type-example of the mid-Carnian episode, a global episode of enhanced flux of siliciclastic detritus and related environmental perturbations, occurred during the Tuvalian 2 substage at ca. 231 Ma, about 3 million years later than suggested by previous correlations. Zircon grains with weighted average 206Pb/238U ages of 236.0 ± 1.2 Ma (n = 17) and 238.6 ± 1.5 Ma (n = 6) and 206Pb/238U ages between 241 ± 6 and 250 ± 3 Ma point to the presence of tephra in early Carnian to Olenekian strata of the Keuper to Buntsandstein Groups. Traces of these reworked tephra were incorporated into the Stuttgart Formation due to fluvial erosion in the southern Central European Basin and at its margins.
Highlights
Correlations of the terrestrial-marine Keuper Group of the Germanic Upper Triassic with stages of the international Triassic standard are based on biostratigraphic control using spinicaudatans and palynomorphs (Heunisch, 1999; Kozur and Weems, 2007; Kozur and Weems, 2010; Geyer and Triassic Tephra in Stuttgart FormationKelber, 2018; Heunisch and Wierer, 2021), partly supplemented by magnetostratigraphy and sequence-stratigraphy (e.g., Zhang et al, 2020)
We present the first set of Triassic U-Pb ages obtained from euhedral zircon grains of the Stuttgart Formation (Schilfsandstein) from the classical locality Jägerhaus quarry near Heilbronn in SW Germany
Weighted mean 206Pb/238U ages obtained from these zircon grains provide the first precise radiometric constraints on the timing of pre- to syndepositional tephra fallout within the southern part of the Central German Basin (CEB), and on the maximum depositional age of the Stuttgart Formation
Summary
Correlations of the terrestrial-marine Keuper Group of the Germanic Upper Triassic with stages of the international Triassic standard (overviews in Bachmann and Kozur, 2004; Franz et al, 2018a) are based on biostratigraphic control using spinicaudatans (conchostracans) and palynomorphs (Heunisch, 1999; Kozur and Weems, 2007; Kozur and Weems, 2010; Geyer and Triassic Tephra in Stuttgart FormationKelber, 2018; Heunisch and Wierer, 2021), partly supplemented by magnetostratigraphy and sequence-stratigraphy (e.g., Zhang et al, 2020). Exact depositional ages of individual formations remain vague owing to the apparent absence of tuff horizons with datable minerals like zircon, commonly used for precise dating of sedimentary successions This absence is surprising considering the fact that intense Triassic volcanic activity is known from the NW Tethys realm (e.g., Brusca et al, 1982; Cassinis et al, 2008; Marocchi et al, 2008; Beltran-Trivino et al, 2016; Bianchini et al, 2018; Dunkl et al, 2019; Lustrino et al, 2019; Storck et al, 2019), and postulated for the northern margin of the CEB (e.g., Fisher and Mudge, 1990; Ziegler, 1990; van Bergen and Sissingh, 2007; Larsen et al, 2008). As the Stuttgart Formation represents the typeexample of the mid-Carnian episode, a global phase of environmental perturbations and related ecosystem responses (reviews by Arche and López-Gómez, 2014; Ogg, 2015; Ruffell et al, 2015; Dal Corso et al, 2018; Dal Corso et al, 2020, and references therein), the results have implications for the interpretation of the mid-Carnian episode, in particular for its timing, duration, and international correlation, as will be discussed here
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