Re-assessment of ammonoid specimens from the Early Carboniferous Protocanites Beds of the Badenweiler–Lenzkirch Zone (Schwarzwald, Central Variscan Belt): age constraints for a lithostratigraphic key bed

The Protocanites Beds of the Black Forest Massif (Germany) form an important lithostratigraphic key bed, as the index fossils contained in this formation enable correlation within large areas of the internal zone of the central Variscan Belt of Europe. Furthermore, the formation provides significant information on the geodynamic evolution of the Moldanubian Zone. The stratigraphic age of the formation has been a subject of controversy in the past; a Devonian to Viséan age has been proposed. To determine the age of the Protocanites Beds, two ammonoid specimens from the Protocanites Beds of Schönau are re-described here. They belong to Becanites abnobensis and Pericylcus princeps and fix the stratigraphic position of the Protocanites Beds to the early Late Tournaisian.


Introduction
Deciphering the geological framework within the internal zone of the central European Variscan Belt is very difficult (for the latest review, see Franke et al. 2000), as reliable biostratigraphic age determinations are rare and lithostratigraphic correlation is often questionable due to the lack of unambiguous marker beds and intensive tectonic dislocations. One of the scarce marker horizons within the Moldanubian Black Forest Massif is the Protocanites Beds with their type locality at the city of Schönau im Wiesental; this formation was proven to serve as a lithostratigraphic key. Geochemical analysis together with sedimentological and petrographic investigations of this formation (Güldenpfennig 1997(Güldenpfennig , 1998 led to a plate tectonic model (Loeschke et al. 1998) for the Moldanubian Black Forest Massif, in which the Badenweiler-Lenzkirch Zone was interpreted as a major Variscan suture Zone. However, the age of the Protocanites Beds and consequently the onset of subduction was controversial when taking the plate tectonic model proposed by Loeschke et al. (1998) into account.
Usually, the Protocanites Beds were interpreted as Early Carboniferous in age (Burgath and Maass 1973;Güldenpfennig 1997 with additional references therein; see Sittig 1961). However, Hann and Sawatzki (1998) suggested a general Devonian age of the formation on the basis of palynological investigations of this important marker horizon. The considerable contrast between the classical stratigraphic age assignment and new age determinations makes it necessary to reinvestigate and re-evaluate the name-giving ammonoid from the type locality of the Protocanites Beds.
The Protocanites Beds (also termed Protocanites Greywackes) are part of the non-metamorphic volcanosedimentary sequences of the Badenweiler-Lenzkirch Zone. They have a thickness of more than 500 m (Kneidl et al. 1982) and occur within the entire, marine parts of the zone where they can serve as an important lithostratigraphic marker formation. The formation can be correlated within the Badenweiler-Lenzkirch Zone as well as with the timeequivalent deposits of the Moldanubian Southern Vosges Basin (France) and the north-eastern part of the French Massif Central (Eisele et al. 2000;Leloix et al. 1999;e.g., Maass 1988;Maass et al. 1991). Wilser (1933) coined the term for relatively immature, medium grain-sized greywackes, which yielded the index ammonoid (first determined Prolecanites cf. Lyoni by Spiegelhalter 1910) at the quarry near the forestry office of Schönau im Wiesental. While earlier workers (Metz and Rein 1958;Schäfer 1957;e.g., von Bubnoff 1919) compared and correlated the Protocanites Beds with the metamorphosed succession of the Gschwend-Sengalenkoopf Series (sensu Altherr and Maass 1977), Sittig (1963Sittig ( , 1969 was the first who recognized their significance as an independent lithostratigraphic element.
Detailed petrographic and geochemical investigations of the Protocanites Beds and the tectonically emplaced Late Devonian shales (Kneidl et al. 1982;Weyer 1962) were carried out by Güldenpfennig and Loeschke (1991) and Güldenpfennig (1997Güldenpfennig ( , 1998 concluding that the Protocanites Beds are deposits representing only a limited time interval. The restricted period of their occurrence together with the remarkable lateral distribution emphasize their importance as a lithostratigraphic key horizon.

The stratigraphic age of the Protocanites Beds
There are a variety of age determinations for the Protocanites Beds of the Badenweiler-Lenzkirch Zone. Starting with Spiegelhalter (1910), the formation was attributed to the Early Carboniferous. Vöhringer (1960) suggested, in his new description of the subspecies "Protocanites supradevonicus abnobensis", an age of the younger part of the Gattendorfia Stufe (= Early Tournaisian) or a little younger. Thanks to the discovery of a specimen of Pericylcus princeps (de Koninck, 1844) in the Protocanites Beds at Schönau in 1961, a Tournaisian age of the greywackes could be confirmed (Sittig 1967). Later authors then used this stratigraphic date (e.g., Güldenpfennig 1997Güldenpfennig , 1998Maass 1988Maass , 2005Maass , 2006Güldenpfennig and Loeschke 1991;Maass et al. 1991;Huth and Zedler 2019;Kneidl et al. 1982). Hann and Sawatzki (1998) carried out palynological investigations on the Protocanites Beds and questioned the previous age determination of the Protocanites Beds. They recovered palynomorphs (acritarchs and chitinozoans) of Ordovician, Silurian, and also Devonian age from five samples, but only one of them (Präg, Sägentobel) yielded Devonian taxa. They concluded an undifferentiated Devonian time for the deposition of the Protocanites Beds, whilst the Ordovician and Silurian fossils were regarded as recycled (Hann and Sawatzki 1998, p. 186, 188). In subsequent papers, Hann et al. (2003), Vaida et al. (2004) and Hegner et al. (2005) regarded the Protocanites greywackes as Late Devonian to Early Carboniferous in age, while Brockamp et al. (2015), without providing evidence, suggested an early Viséan age. Already earlier, Schaltegger (2000) Kalt et al. 2000). BBZ Baden-Baden Zone, NSGM Northern Schwarzwald Granite Massif, CSGC Central Schwarzwald Gneiss Massif, BLZ Badenweiler-Lenzkirch Zone, SSGC Southern Schwarzwald Gneiss Massif. c Simplified geological framework of the Badenweiler-Lenzkirch Zone with particular emphasis on the city of Schönau with the type locality of the discussed Protocanites Beds (modified after Burgath and Maass 1973) to Viséan age of the Protocanites Greywacke that overlies Frasnian green and yellow shales.
The co-occurrence of the ammonoid genera Becanites and Pericyclus allows a precise age determination. While Becanites has a longer stratigraphic range from the Early to the Late Tournaisian, Pericyclus provides a constraint age of early Late Tournaisian. Becanites is known from the Gattendorfia Limestone of Dzikowiec in Lower Silesia (unpublished material), Middle Tournaisian strata such as the Bordalete Formation of South Portugal (Korn 1997) and the Oued Znaïgui Formation of the Anti-Atlas of Morocco (Korn et al. 2007) as well as early Late Tournaisian strata such as the Upper Kahla Formation of Gourara of western Algeria (Korn et al. 2010b) and the Teguentour Formation of the Mouydir of southern Algeria (Korn et al. 2010a).
The genus Pericyclus appears to be restricted to a limited early Late Tournaisian time interval. It was originally described from rare finds in the Calcaire de Calonne of Belgium (Delépine 1940;de Koninck 1844) and the Oued Znaïgui Formation of the Anti-Atlas of Morocco (Korn et al. 2003(Korn et al. , 2007Korn and Klug 2015). The most diverse assemblages with up to twenty species including several species of Pericyclus are in the Upper Kahla Formation of Gourara of western Algeria (Korn et al. 2010b) and the Teguentour Formation of the Mouydir of southern Algeria (Korn et al. 2010a). In summary, the co-occurrence of the two ammonoids allows only a stratigraphic attribution in the early Late Tournaisian, i.e., with a geochronological age of approximately 350 Ma according to the geological time scale (Davydov et al. 2012).
The second precise stratigraphic date in the lower portion of the greywacke, which is based on conodonts (Kneidl et al. 1982;Weyer 1962), provides an Early to Middle Famennian age. This means a geochronological age of approximately 365 Ma according to the geological time scale (Becker et al. 2012), i.e., much younger than the youngest zircons from the greywackes of the Badenweiler-Lenzkirch Zone, dated 384-371 Ma (Gruler et al. 1999).
Description. The single poorly preserved specimen GZG 190-1 (Geowissenschaftliches Museum, Georg-August-Universität Göttingen) with a conch diameter of about 47 mm is embedded on a large slab of shaley greywacke. It is slightly crushed and laterally deformed, so that the dimensions and proportions of the conch can only be estimated. The width Becanites abnobensis (Vöhringer, 1960), plaster casts of the holotype (before and after preparation of the suture line). c Pericyclus princeps (de Koninck, 1844), specimen GZG 190-1. Scale bar units = 1 mm of the umbilicus is one-third of the conch diameter. Only one volution can be studied; this bears about 45 rather sharp riblets. They cross the flanks in a slightly anterior direction and turn, in the ventrolateral area, further back to form a deep ventral sinus.
Description. Unfortunately, the original specimen could not be traced so far and may have been lost. Only two plaster casts (kept in the geological collection of the Albert-Ludwigs-Universität Freiburg) are available. They are from the same specimen of which Spiegelhalter (1910) had already published a photograph. The casts of the specimen were produced before and after the suture line preparation by Vöhringer (1960). The specimen had a conch diameter of 60 mm and was widely umbilicate (umbilicus approximately half of the conch diameter) with almost circular whorl cross-sections. The whorl overlap rate is very low. Smooth shell remains were attached. Since the suture line cannot be studied in the plaster casts, one relies on the figure provided by Vöhringer (1960) (reproduced here in Fig. 3b). In his suture line drawing, a slightly pouched, lanceolate external lobe can be seen, which is shorter than the adventive lobe. This adventive lobe and also the lateral lobe that is located on the flank, closely resemble the external lobe in their lanceolate shape. Only the sizes of the lobes are different, the adventive lobe being the deepest. All the three lobes and the two saddles in between them have the same width.

Conclusions
The Protocanites Beds of the Badenweiler-Lenzkirch Zone (southern Schwarzwald) are a stratigraphically important key horizon; the formation allows correlation within large areas of the internal zone of the central Variscan Belt of Europe and North Gondwana. However, their age classification has been the subject of controversy over the last 60 years. In order to clarify the stratigraphic position of the Protocanites Beds, we describe two ammonoid specimens that originate from the type locality of the formation in the quarry near the forestry office of Schönau im Wiesental: (1) Plaster casts of the probably lost holotype of Becanites abnobensis (Vöhringer, 1960), first described as Prolecanites cf. Lyoni by Spiegelhalter (1910), which are permanently kept in the geological collection of the Albert-Ludwigs-Universität Freiburg.
Funding Open Access funding enabled and organized by Projekt DEAL.
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