Chrono-, litho- and conodont bio-stratigraphy of the Rauchkofel Boden Section (Upper Ordovician–Lower Devonian), Carnic Alps, Austria

An updated stratigraphy of the Rauchkofel Boden Section, a classical reference section for the Carnic Alps that exposes rocks from the Katian (Upper Ordovician) to the Pragian (Lower Devonian) is here presented, following latest developments in conodont taxonomy and biostratigraphy, as well as in chrono stratigraphy, and the recent introduction of a new lithostratigraphic outline of the Carnic Alps. The original conodont collection of the ’70s and ʼ80s was restudied and complemented by a detailed resampling in order to achieve a more precise conodont biostratigraphic assignment. Twenty-five conodont Zones are now documented. The lithostratigraphy is precisely fixed to the new lithostratigraphic scheme of the Pre-Variscan sequence by definition of seven distinct formations. Finally, the position of chronostratigraphic boundaries is discussed.

new sampling throughout the section.Lithostratigraphy is further implemented by the recognition of seven formations, and the position of the chronostratigraphic boundaries is discussed.

Geological Settings
The Carnic Alps are located on either side of the Italian-Austrian border.Here, one of the best exposed and most complete Palaeozoic successions in the world, ranging from the Middle Ordovician to the Upper Permian, is exposed.
During the early Palaeozoic the Carnic Alps belong to those group of terrains (Galatian terranes; von Raumer and Stampfli 2008), that detached from the north-ern Gondwana margin within the Lower Ordovician, and moved northward faster than the main supercontinent.The drift from about 50°S in the Late Ordovician, to 35°S in the Silurian and to tropical belt in the Devonian (Schönlaub 1992) is reflected by distinct litho-and biofacies patterns.
Rocks from the Middle Ordovician to the lower Pennsylvanian, that were affected by the Variscan orogeny during the late Bashkirian and Moscovian (Venturini 1990, Schönlaub andForke 2007) constitute the so-called Pre-Variscan sequence.The lithostratigraphy of this sequence was recently revised and 36 formations were finally discriminated in the Pre-Variscan sequence of the Carnic Alps (Corradini and Suttner 2015).For a recent description of the geology of the Carnic Alps, refer to Corradini et al. (2015e, 2016.

Fig. 2.
Chronostratigraphy and lithostratigraphy of the Rauchkofel Boden Section.Stratigraphic log modified after Schönlaub (1980).Lines on the right side of the log indicate the parts of the section illustrated in Figs.3-6. eschweizerbart_xxx

The Rauchkofel Boden Section
The Rauchkofel Boden Section is located on the southwestern slope of Mt.Rauchkofel, at coordinates N 46°36Ј 54Љ, E 12°52Ј 30Љ, and an altitude of 2175 m (Fig. 1).It is easily accessible along the trail running from the Lake Wolayer to the top of Mount Rauchkofel.About 65 m of calcareous rocks documenting the Upper Ordovician -Lower Devonian are exposed .A significant gap is present at the Ordovician/Silurian boundary, where possibly part of the Hirnantian and the Llandovery are missing.

Lithostratigraphy
The base of the Rauchkofel Boden Section is represented by a more than 100 m thick unfossiliferous clastic sequence named the Himmelberg Formation (Schönlaub 2015).It comprises massive to well bedded greyish to greenish sandstones and interbedded arenaceous shales showing locally cross-bedding, ripples and conglomeratic intercalations indicating a shallow marine environment.This formation is tentatively assigned to the Upper Ordovician and more clearly to the Katian Stage.Based on the heavy minerals zircon, tourmaline and rutile for these clastics, a source area of acid plutonic rocks (granites, pegmatites) has been inferred (Schnabel 1976).
The Himmelberg Formation is sharply overlain by the 10 to 15 m thick Wolayer Formation, the varying thickness of which depends on the amount of erosion upon its deposition (Schönlaub and Ferretti 2015).The Rauchkofel Boden Section represents the type section for this formation.The massive limestone of the unit is indistinctly bedded and rich in cystoid debris or complete cystoid thecae, bryozoans, rare corals, brachiopods, ostracods, trilobites and conodonts (Ferretti and Histon 1997, Ferretti et al. 1999, 2004, 2012b, Ferretti 2005, Brett et al. 2009).The grain-sized and rudstone fabric indicates a dominant allochthonous accumulation of echinoderm debris and other bioclasts possibly deriving from shallow water high-energy crinozoan mounds (Dullo 1992).On the basis of conodonts, the Wolayer Fm. is dated to the late Katian-?basal Hirnantian (Am.ordovicicus Zone; Ferretti and Schönlaub 2001).
The contact to the overlying Kok Formation (Ferretti et al. 2015a) is represented by an up to 5 mm thick irregular clayish stylolitic seam indicating a distinct disconformity (Fig. 3C).Locally, a limestone-lime- stone-contact is developed.Stromatolite-like structures along discontinuity surfaces have been associated to a peculiar microbial activity (Ferretti 2005, Ferretti et al. 2012b).The conodont biostratigraphy and sedimentology of the Kok Formation was studied in detail by Schönlaub (1970Schönlaub ( , 1971Schönlaub ( , 1977)), Ferretti and Histon (1997), Ferretti et al. (1999, 2004), Histon (1999), Ferretti (2005) and Brett et al. (2009).According to Ferretti (2005), hematitic to manganese-rich crusts and thin oolitic grainstones infill small pockets on the upper irregular surface of the underlying Wolayer Formation.The infillings have yielded conodonts of the upper part of the Pt.a. amorphognatoides Zone, corresponding to the basal Wenlock.Apparently, most (if not all) of the Hirnantian Stage and the Llandovery Series are missing (Ferretti 2005, Brett et al. 2009, Ferretti et al. 2012b).
The Kok Formation is 3.50 m thick and is represented by pinkish to greyish nautiloid-rich packstones and wackestones of Wenlock age in the lower part, followed by indistinctly bedded encrinitic, bioclastic and oolitic grainstones with iron-rich shaly partings of early Gorstian age.The uppermost part comprises grayish and pinkish wacke-/packstones with abundant juvenile and adult partly oriented nautiloids associated with articulate brachiopods, bivalves and gastropods (Ferretti and Histon 1997, Ferretti et al. 1999, 2004, 2012b, Ferretti 2005, Brett et al. 2009).
The overlying 40-50 cm thick Cardiola Formation (Ferretti at al. 2015b) was excavated during World War I as a trench and later covered by soil and loose rocks that were easily dug for current sampling.Black bituminous shales interfinger with lenses of dark micritic limestones yielding nautiloids, bivalve representatives of the genus Cardiola (Kříž 1979(Kříž , 1999) ) and conodonts of the P. siluricus Zone.The fauna is dominated by nautiloids embedded in a matrix of bioclasts which are frequently coated by micritic envelopes (Ferretti and Histon 1997, Ferretti et al. 1999, 2004, Ferretti 2005, Brett et al. 2009).
With a sharp boundary, the Cardiola Fm. is overlain by 18 m thick limestones assigned to the Alticola Formation (Ferretti et al. 2015c) of upper Ludfordian to basal Lochkovian age.Its upper part forms a steep southward facing mostly grass-covered slope which ends up at the Silurian/Devonian boundary.The limestone sequence is composed of massive pink to gray wackestones/packstones with locally rich occurrences of large nautiloids, trilobites and solitary rugose corals in the middle part (Ferretti and Histon 1997, Ferretti et al. 1999, 2004, Ferretti 2005, Brett et al. 2009).The Ludlow/Přídolí boundary is drawn in the uppermost part of the steep slope, just below the upper boundary of the Oz.crispa Zone.Towards the top, the Přídolían part of the limestone sequence is represented by dark grey, massive to coarse-bedded wackestones and packstones rich in echinoderms including Scyphocrinites debris and even loboliths (Ferretti and Histon 1997, Ferretti et al. 1999, 2004, Ferretti 2005, Brett et al. 2009).The Silurian/Devonian boundary is drawn in the uppermost part of the unit, about 40 cm below its top, where the basal Devonian conodont Icriodus woschmidti Walliser was recovered.
The following Rauchkofel Formation (Corradini et al. 2015d) is extremely condensed and consists of 1.80 m thick thin-bedded limestone beds interbedded with black shales of Lochkovian age.
The flat area south of the steep meadow is represented by 18 m thick limestones of the La Valute Formation (Corradini et al. 2015c) which was previously named "Bodenkalk" by Schönlaub (1985).It is composed of grey, coarse-bedded, very compact cephalopod limestones (mudstones to wackestones).The Rauchkofel Boden Section is the type section of the La Valute Fm.As in other areas (i.e., Mt.Zermula: Pondrelli et al. 2015, Corradini et al. 2016) the upper part the La Valute Fm. becomes more marly and nodular and gradually passes into the overlying Findenig Fm. (Corriga et al. 2011, Spalletta et al. 2015).This unit is represented by 20 m of reddish nodular mudstones and wackestones.Orthoceratid nautiloids and hardly visible dacryconoarids (Alberti 1985) are the only fossils observable in the field.The Lochkovian/ Pragian boundary is drawn just above the formation boundary by the occurrence of the dacryoconarid Nowakia acuaria (Richter) at the base of the Findenig Fm. (Schönlaub 1980, Alberti 1985).

Conodont fauna
The Schönlaub conodont Collection from Rauchkofel Boden is stored at the Austrian Geological Survey in Vienna.It includes 108 samples, mainly collected between 1969 and 1979, with a few integration on selected intervals in the early '80s.This material was restudied and updated by MGC and CC in 2015, according to the recent taxonomic and biostratigraphic novelties.We complemented the original Schönlaub Collection with 36 new samples collected by AF in the Ordovician and Silurian part of the section, mainly in the Wolayer, Kok and Cardiola formations, and 41 picked by CC and MGC in the Ludlow to Lochkovian part.The additional Ordovician samples are stored at the Palaeontological Museum of the University of Modena and Reggio Emilia (IPUM code), and the Silurian and Devonian ones in the Palaeontological and Geological Museum "Domenico Lovisato" of Cagliari University (MDLCA code).
Conodonts are in general quite abundant and relatively well preserved throughout the section, but with great differences from level to level.Best preserved and richest associations are derived from the lower part of the section, in the sector morphologically above the steep slope: the associations from the Wolayer to the lower part of the Alticola formations (Katian to Ludfordian) are particularly good.Samples from the steep slope (central and upper part of Alticola Fm.) yielded very scarce and/or poorly preserved conodont elements and a precise biostratigraphy within the Přídolí was preliminarly attempted.Faunas from the uppermost part of the Alticola Fm. (uppermost Přídolí) to most of the La Valute Fm. (middle Lochkovian) are well preserved and relatively abundant, and they suddenly became very scarce in the upper part of the section, where many samples are barren of conodonts.

Conodont biostratigraphy
The biostratigraphic assessment is based on the conodont zonation schemes in use for the Upper Ordovician to the Lower Devonian.Bergström and Ferretti (2016) have recently re-tuned the conodont biostratigraphic schemes in use for the Ordovician.The scheme by Cramer et al. (2011) was followed for the Silurian, with the emendations by Corradini and Corriga (2012) and Corradini et al. (2015a).However, the scheme by Corradini and Serpagli (1999) was utilized for the Wenlock, as the detailed subdivision of the Wenlock by Cramer et al. (2011) revealed to be unfitting for the Carnic Alps, as already pointed out by Corradini et al. (2016).The zonation schemes provided by Corradini and Corriga (2012) and Valenzuela-Ríos et al. (2015) are applied for the Lochkovian, with the variations suggested by Corriga et al. (2016) in the lower Lochkovian.Finally, the scheme by Slavík ( 2004) is adopted for the lower Pragian.
The studied conodont fauna allows the discrimination in the Rauchkofel Boden Section of twenty-five biozones documenting an interval ranging from the Katian (Upper Ordovician) to the lower Pragian (Lower Devonian) .However, as reported above, possibly the Hirnantian and the Llandovery are missing (Fig. 2).The conodont zones are briefly discussed below.For each Zone, its original definition, relative interval in the Rauchkofel Boden Section, occurrence of the most characteristic taxa, and a few comments, if necessary, are provided.The complete conodont distribution data are provided in Figures 4-6.Main taxa are illustrated in Plates 1-3.

Amorphognathus ordovicicus Zone, Bergström (1971)
The Am. ordovicicus Zone was defined by Bergström (1971) as corresponding to the total range of the marker index Am.ordovicicus (Branson and Mehl).For a review of the Amorphognathus evolutionary lineage, on which the Late Ordovician conodont biozonation is based, refer to Ferretti et al. (2014) and Bergström and Ferretti (2015).The population of Amorphognathus present in the Rauchkofel Boden Section includes both Am.ordovicicus and Am.duftonus.The Zone is documented in the Wolayer Fm. (Fig. 4).Conodonts are abundant in the upper part of the unit (samples 309 and 309 top), where the fauna is dominated by coniform elements of Walliserodus, associated with numerous elements of Amorphognathus and rare Hamarodus and Plectodina.

Pterospathodus amorphognathoides amorphognathoides Zone, Walliser (1964)
The Pt. am.amorphognathoides Zone as defined by Walliser (1964) corresponds to the total range interval of the index Pt.am.amorphognathoides Walliser.This interval was later considered a "Zonal group" by Jeppsson (1997), who subdivided it into three zones.
In the Rauchkofel Boden Section the Pt.am.amorphognathoides Zone is discriminated at the very base of the Kok Fm. (Fig. 4) in millimetric carbonatic infillings of small pockets on the irregular erosive surface of the underlying Wolayer Fm.The conodont association includes Pt. p. procerus (Walliser), Distomodus staurognathoides (Walliser) and coniform taxa, and does not allow to recognize any of the zones proposed by Jeppsson.Therefore we refer to the Pt.am.amorphognathoides Zone by Walliser.However, since all these taxa range up to the top of the Zone, the missing of taxa whose range is limited to the Llandovery and due to the continuity in the sedimentation with the overlying beds, the interval can be likely attributed to Chrono-, litho-and conodont bio-stratigraphy of the Rauchkofel Boden Section 451 eschweizerbart_xxx the uppermost part of the Zone, just above the Llandovery/Wenlock boundary.

Kockelella ranuliformis Interval Zone, Corradini and Serpagli (1999)
The K. ranuliformis interval Zone is defined as the interval between the LAD of Pt. am.amorphognathoides and the FAD of Oz. s. rhenana Walliser (Corradini and Serpagli 1999).Jeppsson (1997) subdivided this interval into two biozones at Gotland, and this scheme is accepted in various papers in Baltica and Laurentia (i.e., Cramer et al. 2010, Männik et al. 2014), but this subdivision is not applicable in the condensed sequence of the Carnic Alps.The named Zone is discriminated at the Rauchkofel Boden Section in the lower part of the Kok Fm. (Fig. 4), in the 40 cm thick interval between samples 310 and 312, where the fauna is dominated by coniform elements of genera Dapsilodus, Panderodus and Pseudooneotodus.

Ozarkodina sagitta rhenana Zone, Aldridge and Schönlaub (1989)
The Oz. s. rhenana Zone is defined as the interval between the FAD of the index taxon Oz. s. rhenana and the FAD of Oz. s. sagitta Walliser (Aldridge and Schönlaub, 1989).Jeppsson (1997) subdivided this interval into seven zones, that are impossible to discriminate in the Rauchkofel Boden Section.However, two of them (K.patula Zone and K. o. ortus Zone) were recognized in the Cellon Section by Corradini et al. (2015a).At Rauchkofel Boden, the Oz.s. rhenana Zone is discriminated in the lower part of the Kok Fm. by the entry of Oz. s. rhenana in sample 312, and is 30 cm thick only (Fig. 4).In the Carnic Alps the Zone has been documented a few km to the East in the La Valute area in a different facies represented by an alternation of shales and limestone, and is about 3 m thick (Corradini et al. 2016).

Ozarkodina sagitta sagitta Zone, Aldridge and Schönlaub (1989)
The Zone corresponds to the interval of total range of Oz. s. sagitta (Jeppsson 1997, Jeppsson andCalner 2002).In the Rauchkofel Boden Section the Zone is discriminated from sample 313 to 322/2 by the occurrence of Oz. s. sagitta (Walliser), and is 45 cm thick (Fig. 4).Within the Zone, the index taxon largely dominates the association.Ferretti and Schönlaub (2001).(Walliser), that normally has a longer range is here documented only from the upper part of the Zone.

Ozarkodina bohemica Interval Zone
The interval between the LAD of Oz. s. sagitta (Walliser) and the FAD of K. ortus absidata Barrick and Klapper is assigned to the Oz.bohemica Interval Zone.This interval was named Oz.b. longa Zone by Calner and Jeppsson (2003), who subdivided the former Oz.bohemica Zone by Aldridge and Schönlaub (1989) into two parts, the Oz.bohemica longa and K. o. absidata zones respectively.However, since Oz.bohemica bohemica (Walliser) and Oz. b. longa Jeppsson have not been found in the Carnic Alps so far, and because the latter taxon is not present in the lowermost part of the Zone (Calner and Jeppsson 2003), it looks more appropriate to name this interval as Oz.bohemica Interval Zone.
In the Rauchkofel Boden Section the Oz.bohemica Interval Zone is tentatively detected in the short (20 cm thick) not sampled interval between samples 322/2 and 323/1 (Fig. 4).

Kockelella ortus absidata Zone, Calner and Jeppsson (2003)
The K. o. absidata Zone represents the interval between the FAD of K. o. absidata and the FAD of K. crassa (Walliser), and includes the top Homerian strata (Cramer et al. 2011).At Rauchkofel Boden the Zone is detected in the narrow interval of sample 323/1 by the entry of the index taxon (Fig. 4).

Kockelella crassa Zone, Walliser
The K. crassa Zone corresponds to the interval of the total range of the marker K. crassa (Corradini and Serpagli 1999).The base of this Zone coincides with the base of the Ludlow series (Cramer et al. 2011).
In the Rauchkofel Boden Section the Zone is discriminated in a 40 cm interval in the central part of the Kok Fm., from sample 313/3 to 314 (Fig. 4).Kockelella v. variabilis Walliser enters within the Zone, and Coryssognathus dubius (Rhodes) in its upper part.

Kockelella variabilis variabilis Interval Zone, Cramer et al. (2011)
The K. v. variabilis Interval Zone corresponds to the interval between the LAD of K. crassa and the FAD of Ancoradella ploeckensis Walliser (Cramer et al. 2011).

Ancoradella ploeckensis Zone, Walliser (1964)
The A. ploeckensis Zone is defined as the interval between the FAD of A. ploeckensis and the FAD of Polygnathoides siluricus Branson and Mehl.However, as pointed out by other authors (i.e.: Corradini and Serpagli 1999, Slavík 2014), A. ploeckensis is a rare species, which has not been found also in the Rauch kofel Boden Section.The base of the Zone is here recognized by the entry of Kockelella o. sardoa, which has its FAD coincident with the FAD of A. ploeckensis (Serpagli and Corradini 1999).The A. ploeckensis Zone is discriminated in the uppermost 90 cm of the Kok Fm. (Fig. 4).Wurmiella inflata Walliser occurs only in this Zone, whereas elsewhere it appears in older strata (Corradini andSerpagli 1999, Corriga et al. 2009).Wurmiella?posthamata Walliser has its only occurrence in samples 325 and X, both collected in the uppermost bed of the Kok Fm.Kockelella v. ichnusae Serpagli and Corradini and Wurmiella sp.A, characterized by an asymmetrical P1 element, enter in the upper part of the Zone, in the same level where C. dubius has its last occurrence.

Polygnathoides siluricus Zone, Walliser (1964)
This Zone corresponds to the interval of total range of P. siluricus and is one of the zones with widest distribution in the Silurian: it has been indicated in all published zonal schemes and everywhere its boundaries are defined on the same criteria.

Ancyrodelloides trigonicus
Within this Zone a few anomalous elements, represented mainly by ramiforms with branched processes, are present.Such specimens are documented in various intervals of the Silurian and Lower Devonian, but are particularly abundant in the P. siluricus Zone (e. g., Corradini et al. 1996, 2015a, 2016, Slavík et al. 2010, Corriga et al. 2014).

Pedavis latialata -Ozarkodina snajdri
Interval Zone, Corradini et al. (2015a) This Zone corresponds to the interval between the Po.siluricus and the Oz.crispa zones (Corradini et al. 2015a).For the reasons of defining the Zone by the two more representative taxa refer to Corradini et al. (2015a, p. 60).At Rauchkofel Boden the Zone is discriminated in the lower part of the Alticola Fm. and is about 4 m thick.This interval documents the "Lau event" and the  "post-Lau recovery", with an evolution of faunas similar to that documented in other nearby areas (i.e., Prague Synform, Slavík and Carls 2012): in the lower part of the Zone the conodont diversity is scarce, and the association is dominated by coniform elements (Panderodus, Belodella and Dapsilodus).Pedavis latialata (Walliser) enters in the upper part of the Zone, and Oz.cf.snajdri (Walliser) and Wurmiella sp.B occur in the upper part, only.The latter species is characterized by a distinct enlargement of the blade in the P1 element, just below the insertion of denticles.

Ozarkodina crispa Zone, Walliser (1964)
The Oz. crispa Zone corresponds to the interval of total range of Oz. crispa Walliser (Corradini and Serpagli 1999).At Rauchkofel Boden it is detected in a 1.5 m interval in the lower part of the Alticola Fm., around the upper edge of the steep slope (samples 81/ 27-2I; Fig. 5).Pedavis latialata became extinct within the Zone; Panderodus recurvatus has its last occurrence in the section at the top of the Zone, whereas it normally ranges longer in the upper Přídolí (Corradini and Corriga 2012).

"Ozarkodina" eosteinhornensis s. l. Interval Zone, Corriga and Corradini (2009)
The "Oz." eosteinhornensis s. l.Interval Zone is defined as the interval between the last occurrence of Oz. crispa and the first occurrence of Oulodus elegans detortus (Walliser) (Corriga andCorradini 2009, Corradini andCorriga 2012).Conodonts are very rare in samples collected in the steep slope, and Oul.el.detortus enters only at a younger level.It appears therefore impossible to locate precisely the upper boundary of the "Oz." eosteinhornensis s. l.Interval Zone, which is tentatively drawn a couple of meters below the "Oz." eosteinhornensis s. s. horizon, in a position similar to other sections in the region, like the Cellon Section (Corradini et al. 2015a).

Lower Oulodus elegans detortus Zone, Corradini and Corriga (2012)
The lower boundary of the Lower O. e. detortus Zone is defined by the first occurrence of O. e. detortus and the upper boundary by the last occurrence of Dapsilodus obliquicostatus (Branson and Mehl) (Corradini and Corriga 2012).
In the Rauchkofel Boden Section the Zone has been discriminated along the lower part of the steep slope, but it not possible to precisely place its lower boundary.The index taxon Oul.el.detortus enters higher in the section, and other diagnostic taxa are missing.Therefore the base of the Zone is tentatively located about 2 m below the occurrence of "Oz." eosteinhornensis s. s. (Walliser), in a similar position than other sections in the Carnic Alps, like Cellon (Corradini et al. 2015).The upper boundary is placed just above sample 7 (Fig. 5), from where an incomplete specimen of D. ob liquicostatus was collected.
Conodonts are very rare in this interval, and mainly represented by W. excavata and coniform elements.In the central part of the Zone W. alternata Corradini and Corriga and "Oz." eosteinhornensis s. s. are present: the latter taxon always marks a well defined horizon, that can be used for correlations (see discussion in Corradini and Corriga 2012, p. 647).

Upper Oulodus elegans detortus
Zone, Corradini and Corriga (2012) The Upper O. e. detortus Zone is the interval between the LAD of D. obliquicostatus and the FAD of Icriodus hesperius Klapper and Murphy (Corradini and Corriga 2012).At Rauchkofel Boden this Zone has been discriminated in the upper part of the Alticola Fm. (beds of samples 7X-200): its lower boundary is placed just above the last occurrence of the index taxon, and the upper boundary by the first occurrence of Icriodus woschmidti (Walliser), that has its FAD together with the marker (Corradini and Corriga 2012).
Anomalously Oulodus el.elegans (Walliser) and Oul.el.detortus occur only in the lower part of this Zone, and Zieglerodina planilingua (Murphy and Valenzuela-Ríos) enters within this Zone, whereas elsewhere in the Carnic Alps these taxa have their first occurrences in the middle Přídolí (Oul.el.detortus and Z. planilingua) or in the uppermost Ludlow (Oul.el.elegans) (Corradini andCorriga 2012, Corradini et al. 2015a).Also, Zieglerodina zellmeri Carls et al. that is documented in most of the Přídolí in the Carnic Alps (Corradini and Corriga 2010, 2012, Corradini et al. 2015a, Corriga et al. 2016) and in the lower part of the Series in Bohemia (Carls et al. 2007) at Rauchkofel Boden occurs only in this Zone.These late first occurrences may be related to the scarcity of conodonts within the "steep slope" of the section, where the Alticola Fm. appears to have a more marly facies than the classical one of the unit.
In the upper part of the Zone the typical succession of events documented in other parts of the Carnic Alps (Corradini and Corriga 2010, 2012, Corradini et al. 2015, Corriga et al. 2016) occurs: last occurrence of Oz. confluens (Branson and Mehl) in sample 7Z, just followed by the entries of Z. remscheidensis (Ziegler) and Z. eladioi (Valenzuela-Rios) in sample 199, and by the last occurrence of Z. zellmeri (sample 7W).

Icriodus hesperius Zone, Corriga et al. (2016)
The Icr. hesperius Zone is discriminated in a very short interval, 40 cm thick, across the boundary between the Alticola and the Rauchkofel Fm. (Fig. 5).The lower boundary is defined by the FAD of Icr.hesperius Klapper and Murphy, and the upper boundary by the FAD of Icr.postwoschmidti Mashkova (Corriga et al. 2016).In the Rauchkofel Boden Section these two species are not present, therefore the lower and upper boundaries were detected by the entries of Icr.woschmidti (Walliser) and Pandorinellina optima (Moskalenko), respectively: in many sections Icr.woschmidti enters at the same level of Icr.hesperius (Corradini and Corriga 2012).Ziegerodina sp.A Corriga et al. 2016, characterized by an alternate denticulation (Pl.3, Fig. 3), occurs in this Zone.

Icriodus postwoschmidti Zone, Corriga et al. (2016)
The I. postwoschmidti Zone is defined as the interval between the FAD of Icr.postwoschmidti and the FAD of Ancyrodelloides carlsi (Boersma).As Icr. postwoschmidti was not recovered in the Rauchkofel Boden Section, the lower boundary is tentatively aligned with the entry of Pandorinellina optima, that has been already used as zonal index in the lower Lochkovian of Bohemia (Slavík et al. 2012).However, the alignment of the FAD of Pand.optima with the FAD of Icr.postwoschmidti is still to be demonstrated.A new, more precise zonation for the upper part of the lower Lochkovian should be developed in the near future.The I. postwoschmidti Zone is tentatively suggested in a 80 cm thick interval in the lower part of the Rauchkofel Fm. (Fig. 5).Lanea omoalpha Murphy and Valenzuela-Ríos enters within the Zone.

Ancyrodelloides carlsi Zone, Corradini and Corriga (2012)
The Ad. carlsi Zone is defined as the interval between the FAD of Ad. carlsi and the FAD of Ad. transitans (Bischoff and Sannemann) (Corradini and Corriga 2012).Slavík (2011), Corradini and Corriga (2012) and Slavík et al. (2012) proposed the entry of the index taxon to be used to define the base of the middle Lochkovian.
At the Rauchkofel Boden Section the Zone is discriminated in the upper part of the Rauchkofel Fm. and in the lowermost part of the La Valute Fm. ."Ozarkodina" malladai Valenzuela-Ríos enters in the upper part of the Zone, just above the last occurrence of Z. planilingua.

Ancyrodelloides transitans Zone,
Valenzuela-Ríos (1994) The Ad. transitans Zone is defined as the interval between the FAD of Ad. transitans and the FAD of Ad. trigonicus Bischoff and Sannemann, and includes the Ad.transitans and L. eleanorae zones by Corradini and Corriga (2012).The latter taxon in fact was reported before the entry of Ad. transitans in the Pyrenees (Valenzuela-Ríos et al. 2015) and cannot be used for correlations.
In the Rauchkofel Boden Section the Zone occurs in the lower part of the La Valute Fm., in a 2 m interval between samples 208 and 211 (Fig. 6).The lower boundary is discriminated in sample 208 by the entry of Flajsella schulzei (Bardashev), that has its FAD in the lower part of the Zone (Valenzuela-Ríos et al. 2015), whereas Ad. trigonicus enters 30 cm higher in sample 209.Flajsella stygia Flajs, Lanea telleri (Schulze) and Wurmiella wurmi (Bischoff and Sannemann) enter within the Zone.In the Rauchkofel Boden Section the Zone is discriminated in a 6 m interval from samples 212 to 222, and corresponds to the range interval of the marker Ad. trigonicus (Fig. 6).Several species occur only within this interval: Fl. sigmostygia Valenzuela-Ríos and Murphy, Fl. streptostygia Valenzuela-Ríos and Murphy, Kimognathus delta (Murphy and Matti) and K. limbacarinatus (Murphy and Matti).All the late taxa of the genera Ancyrodelloides, Lanea and Flajsella became extinct within this Zone, as well as Z. remscheidensis and Z. eladioi.

Masaraella pandora β Zone,
Valenzuela-Ríos (1994) The Zone is detected in the upper part of the La Valute Fm., and is 3.2 m thick (Fig. 6).Because the index taxon is missing, the lower boundary is recognized by the last occurrence of Ad. trigonicus, which in the Carnic Alps has its LAD just below the entry of M. pandora β (Corradini and Corriga 2012).In this Zone the conodont abundance is lower than in the strata below.A single specimen of Pedavis robertoi Valenzuela-Ríos was collected from sample 224a, and the younger representatives of the genus Zieglerodina occur in the upper part of the Zone.

Pedavis gilberti Zone, Slavík et al. (2012)
The P. gilberti Zone, defined as the interval between the FAD of the index taxon, P. gilberti, and the FAD of Icr.steinachensis β Al Rawi, represents the uppermost part of the Lochkovian (Slavík et al. 2012).
The Zone is documented in the Rauchkofel Boden Section in the uppermost part of the La Valute Fm. by the occurrence of a single specimen of P. gilberti recovered in sample 224d.The conodont association is very scarce in this interval, and mainly represented by long-ranging taxa, therefore the upper boundary is approximatively placed just below the first evidence of the Pragian tentaculite Nowakia acuaria, documented by Schönlaub (1980) in the bed of sample 227, within the lithostratigraphic gradual transition between the La Valute and Findenig formations.

Icriodus steinachensis β Zone, Slavík (2004)
The lower and upper boundary of the Zone are defined by the FADs of Icr.steinachensis β and Pel.serratus Jentzsch, respectively.Slavík (2004) introduced a "steinachensis Zone" on the basis of the first occurrence of Icr.steinachensis η-morph to discriminate the earliest Pragian beds in Bohemia.Later Slavík et al. (2007) modified the definition of the base of the Zone with the entry of Icr.steinachensis β-morph, because it has been documented at the same level as the entry of Eognathodus sulcatus (Philip), the index taxon for the base of the Pragian.
In the Rauchkofel Boden Section the Zone is discriminated in the lower part of the Findenig Fm. (Fig. 6).Conodonts are very scarce in this interval, and many samples are barren.The lower boundary is indirectly placed by the entry of Nowakia acuaria, and the upper boundary by the first occurrence of Pel.serratus.A few poorly preserved specimens possibly attributed to Icr. steinachensis have been collected in the upper part of the Zone from sample 234-236.

Pelekysgnathus serratus
The Pel. serratus Zone is "determined by the first and last occurrence of taxa belonging of the Pelekysgnathus serratus group" (Slavík 2004, p. 62).At Rauchkofel Boden it is discriminated in the uppermost part of the section, by the entry of Pel.s. serratus in sample 237.Belodella devonica is the only other conodont taxon occurring in this Zone.

Chronostratigraphy
As reported above, the Rauchkofel Boden Section exposes an almost continuous sequence from the Katian (Upper Ordovician) to the Pragian (Lower Devonian), and therefore contains several chronostratigraphic boundaries.The conodont fauna allows to locate, or approximate, these boundaries, even if many of them are defined by the FAD of a graptolite species: -the Ordovician/Silurian boundary is drawn between the Wolayer Fm. and the Kok Fm.  and Corradini 2009), whereas elsewhere it enters a few centimetres above (i.e., 10 cm in the Cellon Section, Corradini et al. 2015, Corriga et al. 2016), and therefore its entry is a good tool to precisely approximate the boundary.-the Lochkovian/Pragian boundary is defined by the FAD of the conodont Eognathodus sulcatus in the Velka Chuchle Section (Czech Republic), but recent taxonomic revisions demonstrate that the FAD of the taxon is no more aligned with the GSSP (Slavík and Hladil 2004).Slavík (2004) indicates that the boundary can be detected by the entry of Icriodus steinachensis and this level was used to locate the base of the Pragian in the nearby Seekopf Section (Suttner 2007).In the Rauchkofel Boden Section conodonts are very scarce in the boundary interval and no diagnostic taxa have been recovered.Therefore the base of the Pragian is approximately traced around the transition from the La Valute Fm. to the Findenig Fm., where the dacryoconarid Nowakia acuaria is reported at level of sample 227 (Schönlaub 1980, Alberti 1985).However, this datum differs from other areas in the Carnic Alps (i.e., Mt.Zermula area), where the transition between the two formations is dated to the uppermost Lochkovian by conodonts (Corriga et al. 2011, Pondrelli et al. 2015, Corradini et al. 2016).

Conclusions
The main results of this paper can be summarized as follows: -the conodont association from the Rauchkofel Boden Section has been revised.Ninety-seven taxa (species and subspecies) were recognized.-the conodont fauna allows the discrimination of 25 biozones, from the Katian (Upper Ordovician) to the Pragian (Lower Devonian).-in terms of chronostratigraphy, all the Silurian and two Lower Devonian stage boundaries have been located in the section.However, the Llandovery series is completely missing, and possibly the upper Hirnantian strata, too.-the lithostratigraphy of the section has been updated according to the new lithostratigraphic scheme of the Carnic Alps.

Fig. 3 .
Fig. 3. Selected views of the Rauchkofel Boden Section.A) Lower part of the section from the upper part of the Wolayer Fm. to the lower part of the Alticola Fm.B) Orthoceras limestone in the uppermost bed of the Kok Fm.C) Sharp contact of the Wolayer and the Kok formations.D) Encrinitic limestone in the Wolayer Fm.E) Panoramic view of the upper part of the section from above the steep cliff.F) The steep cliff with the Přídolí part of the Alticola Fm., and the Rauchkofel (covered) and the lower part of the La Valute formations.G) Well bedded light gray limestone in the lower part of the La Valute Fm.H) View of the upper part of the section, with the transition between the La Valute and Findenig formations.

Fig. 5 .
Fig. 5. Distribution of conodonts in the central part (Cardiola, Alticola and Rauchkofel formations) of the Rauchkofel Boden Section.From left to right: system, series, stage, formation, lithological log (modified after Schönlaub 1980), samples, sample number, distribution of taxa (white dots indicate taxa identified with question), zones.Arrows at the end of distribution lines indicate that the taxon also occurs above/below the illustrated interval.Horizontal lines mark boundaries of chrono-/litho-/biostratigraphical units.For graphical reason not all the sample numbers are reported.Abbreviations: Card.= Cardiola; L.V. = La Valute; postwosch.= postwoschmidti; eosteinhorn.= eosteinhornensis; ploeck.= ploeckensis.

Fig. 6 .
Fig. 6.Distribution of conodonts in the upper part (La Valute and Findenig formations) of the Rauchkofel Boden Section.From left to right: system, series, stage, formation, lithological log (modified after Schönlaub 1980), samples, sample number, distribution of taxa (white dots indicate taxa identified with question), zones.Arrows at the end of distribution lines indicate that the taxon also occurs above/below the illustrated interval.Horizontal lines mark boundaries of chrono-/litho-/ biostratigraphical units.For graphical reason not all the sample numbers are reported.Abbreviations: RK. = Rauchkofel; steinachens.= steinachensis.
(Corradini and Serpagli 1999, Cramer et al. 2011, Melchin et al. 2012)rt or the whole Hirnantian and Llandovery.-theSheinwoodian/Homerianboundary can be traced in the lowermost part of the Oz.s.sagitta Zone, about 80 cm above the base of the Kok Fm.The in-dex taxon for the base of the Homerian is the graptolite Cyrtograptus lundgreni Tullberg, and its FAD is correlated with the lowermost part of the Oz.s.sagitta Zone(Corradini and Serpagli 1999, Cramer et al. 2011, Melchin et al. 2012).-the Wenlock/Ludlow boundary (= Homerian/Gorstian boundary) is placed 1.55 m above the base of the Kok Fm., where K. crassa is found in sample 313/3.The base of the K. crassa Zone is aligned with the FAD of Neodiversograptus nilssoni (Lapworth), the index taxon for the base of the Gorstian stage (Melchin et al. 2012).-the Gorstian/Ludfordian boundary can be tentatively traced at the base of the A. ploeckensis Zone, about 85 cm below the top of the Kok Fm.The boundary is defined by the FAD of the graptolite Saetograptus lentwardinensis (Hopkinson).According to Cramer et al. (2011, p. 194) "the position of the base of the A. ploeckensis conodont Zone with respect to the base of the Sa.leintwardinensis/Sa. linearis graptolite Zone, and the position of the base of either of these biozones with respect to the base Ludfordian GSSP remains uncertain and these three positions are tentatively correlated at the same level here".-the Ludlow/Přídolí boundary can be tentatively located in the lower part of the Alticola Fm., in the uppermost part of the steep slope, in the upper part of the range of Oz. crispa.In the Cellon Section, the index graptolite species Neocolonograptus parultimus Jaeger, occurs slightly below the upper boundary of the Oz.crispa conodont Zone (Corradini et al. 2015a).-the Silurian/Devonian boundary occurs in the uppermost part of the Alticola Fm., at level of sample 201, where the conodont Icr.woschmidti first appears.The base of the Devonian is defined by the FAD of the graptolite Monograptus uniformis Pribyl, and the conodont taxon "with wide distribution that appears closest to the Lower Devonian boundary" is Icr.hesperius (Carls et al. 2007, p. 157-158).In the Carnic Alps at places Icr.wo schmidti enters together with Icr.hesperius (i.e., Monte Cocco II section, Corriga Acknowledgements.Holger Gebhardt and Irene Zorn (Austrian Geological Survey) are deeply thanked for allowing the restudy by CC and MGC of the original conodont collections.Kathleen Histon and Monica Pondrelli helped during field work.Thanks are also expressed to Massimo Tonelli (CIGS, Modena) and Gabriele Cruciani (DSCST, Cagliari) for assistance during SEM investigations.Topographic map in Fig.1is based on maps 1:25000 printed by Tabacco (Tavagnacco, Udine).We acknowledge useful comments on the manuscript by Ladislav Slavík and an anonymous reviewer.Research by CC and MGC was partly supported by grants RAS and PRID (resp.C.Corradini), research by AF by grant UNIMORE-FAR2014 ROSAE andUNIMORE-FAR2016 PAsTIME (resp.A. Ferretti).This paper is a contribution to the IGCP Project 591 ʻThe Early to Middle Palaeozoic Revolution -Bridging the Gap between the Great Ordovician Biodiversification Event and the Devonian Terrestrial Revolutionʼ, the IGCP Project 596 ʻMid Palaeozoic climate and biodiversityʼ, and the IGCP Project 653 ʻThe onset of the Great Ordovician Biodiversity Eventʼ.