An Italian ad hoc working group, co-ordinated by C. Broglio Loriga, proposes the Prati di Stuores/Stuores Wiesen section (Dolomites, Italy) as a GSSP candidate for the definition of the base of the Carnian stage. The Prati di Stuores/Stuores Wiesen area is famous thanks to the classical works by Münster (1834), Wissmann and Münster (1841), Klipstein (1845), Laube (1869), Mojsisovics (1869, 1882), Ogilvie Gordon (1893). Besides being a rich and famous fossiliferous locality, it also includes the type-section of the Cordevolian substage sensu Mojsisovics, Waagen and Diener (1895) and Urlichs (1974). The Prati di Stuores/Stuores Wiesen section lies in the neighbourhood of Pralongià, on the southern slope of the crest separating the Badia/Abtei Valley and the Cordevole Valley (Fig. 1).
The section supplies a rich ammonoid fauna which predates the first appearance of Trachyceras and documents the presence of genus Daxatina, a particularly significant taxon for global correlations (Mietto and Manfrin, 1995a, b). Due to its possibility of correlating different paleolatitudinal provinces, the presence of the cosmopolitan genus Daxatina suggests placing the Ladinian/Carnian boundary in a lower position than previously indicated in the same section (cf. Urlichs, 1974, 1994; Krystyn,1978).
The candidature of the Prati di Stuores/Stuores Wiesen section is here outlined in a synthetic form with the indications of its main stratigraphical, biostratigraphical and magnetostratigraphical features. Lithostratigraphy and sequence stratigraphy are by V. De Zanche and P. Gianolla (Padova) and C. Neri (Ferrara), magnetostratigraphy by W. Lowrie and G. Muttoni (Zürich), ammonoids and biochronostratigraphy by S. Manfrin and P. Mietto (Padova), conodonts by A. Mastandrea (Modena), palynomorphs by S. Cirilli (Perugia) and G. Roghi (Padova), isolated foraminifers by D. di Bari (Modena), foraminifers in thin section by C. Broglio Loriga (Ferrarra) and R. Rettori (Perugia), gastropods, bivalves and brachiopods by R. Posenato (Ferrara), microcrinoids and holothurians by G.F. Laghi and M. Rechichi (Modena).
The Prati di Stuores/Stuores Wiesen section was permanently stacked out in order to avoid any possible dissension between samplings carried out by workers at different times.
The section consists of two parts (Figs. 1 and 2). The westernmost one, herein named section 1, exactly corresponds to section SW4 in Neri et al. (1995) and to the section illustrated by De Zanche and Gianolla (1995) and by Mietto and Manfrin (1995 b). It extends downward for about 20 m below picket 0, which marks the base of the section studied by the team. The top of section 1 is correlatable to a short section (section 1 bis), lying just eastward, not stacked out and separated by faults with modest throw. Lastly, an eastern section (section 2) exists, which corresponds to the Stuores Wiesen section in Urlichs (1974, 1994) and to the section SW5 in Neri et al. (1995). Due to tectonic omission, the base of the section 2 cannot be correlated with the top of the section 1 bis. Further indications on the relationships between the Prati di Stuores/Stuores Wiesen section as intended herein and the section in Urlichs (1974, 1994) are reported in Neri et al. (1995). In the following text, the term section 1 is intended as the Prati di Stuores/Stuores Wiesen section.
The name Carnian was introduced in 1869 by Mojsisovics to define the stratigraphical interval corresponding to the Trachyceras aonoides Zone. The Author listed six localities, then under Austro-Hungarian rule, where these rocks crop out. However, the correct origin of the name Carnian (Karnische Stufe) is uncertain. According to Tozer (1967, 1984) the term derives from Carinthia (Kärnten), within which the locality of Raibl (ancient name of Cave del Predil), explicitly indicated by Mojsisovics, was administratively included. On the contrary, even though referring to the Raibl succession in Carnia, Gaetani (1995) thinks that the name Carnian derives from Carnian Alps, although the section actually belongs to the Julian Alps.
Within their chronostratigraphical resettling of the Triassic, Mojsisovics, Waagen and Diener (1895), re-defined the Carnian stage and subdivided it into three substages, Cordevolian, Julian and Tuvalian, each of them respectively corresponding to the Trachyceras aon, T. aonoides, Tropites subbullatus ammonoid zones. In agreement with this subdivision, Urlichs (1974) considers the Prati di Stuores/Stuores Wiesen section as the type-section of the Cordevolian. On the basis of the major event philosophy, Krystyn (1978) considered the distinction between Aon Z. and Aonoides Z. of less importance and lowered them to the rank of subzones of the Aonoides Zone. As a consequence, he proposed a subdivision of the Carnian stage into two parts: the lower one corresponding to the Julian substage, obviously including the Cordevolian and comprising the Aonoides, Austriacum and "Sirenites" zones, the upper one corresponding to the Tuvalian. This proposal was rejected by Bizzarini and others (1986) and Urlichs (1994). On the basis of the ammonoid biostratigraphy in the Southern Alps, and particularly in the Prati di Stuores/Stuores Wiesen area, Urlichs considered the original zonal statement by Mojsisovics to be valid and, therefore, asserted the opportunity of maintaining the Cordevolian substage.
Leaving aside the not greatly differing opinions of various authors, up to now the general rule for locating the base of the Carnian was the first appearance of the genus Trachyceras. In the Tethyan domain, this event is believed to occur at the base of the Aon Subzone/Zone, while in North America it lies within the Desatoyense Zone (cf. Tozer, 1967, 1984, 1994).
Recently, Mietto and Manfrin (1995a, b)
have defined a stratigraphical interval, characterized by species of Trachyceras
different from T. aon and associated with Daxatina and
Clionitites, which, in the Prati di Stuores/Stuores Wiesen section,
is placed below the traditional base of the Carnian. The earlier occurrence
of Trachyceras and the concomitant occurrence of a cosmopolitan
genus such as Daxatina, encouraged the two authors to suggest the
lowering of the Ladinian/Carnian boundary. In this way, the lowermost Carnian
should include the upper part of the Frankites regoledanus Zone (Krystyn
in Zapfe, 1983), usually considered Ladinian and correlated with the Frankites
sutherlandi Zone in North America.
The area is placed to SW of the village
of San Cassiano/St. Kassian and belongs to the type-area of the San Cassiano
Fm., defined by Wissmann (in Wissmann and Münster, 1841) as Schichten
von St. Cassian and then named Cassianer Schichten. The S. Cassiano
Fm. is a basinal unit, subdivided by Ogilvie Gordon (1900) into two members:
"lower" (UCS, Unterer Cassianer Schichten) and "upper" (OCS, Oberer
Cassianer Schichten) on the basis of lithological features and paleontological
contents. According to this subdivision, the UCS overlie the La Valle Fm.
(although their lower boundary is not well defined) and consist of alternating
siltstones, claystones, calcarenites, marly limestones and turbiditic volcanoarenites,
while the OCS, which overlie the highest volcanoarenitic layer (tuffsandstein
in Urlichs, 1974), are mainly made up of muddy beds, siltstones and marls,
with some limestone intercalations. The distinction between UCS and OCS
on the basis of volcanic debris content was debated by De Zanche et al.
(1993), De Zanche and Gianolla (1995) and Neri et al. (1995): they prefer
not to consider the volcanic debris content as a diagnostic tool because
it is paleogeographically controlled. In agreement with Richthofen (1860),
the La Valle Fm./San Cassiano Fm. boundary (at 1.5 m from the bottom in
section 1) is marked by the occurrence of oolitic turbidites, which also
testify to the progradation of a carbonate platform (Cassian Dolomite).
In conclusion, in the lower part of the section, mainly grey and dark grey
marls with volcanoturbiditic intercalations crop out (La Valle Fm.), while
in the upper part brown and yellowish marls with carbonate turbidites prevail
(San Cassiano Fm.).
The Prati di Stuores/Stuores Wiesen section is illustrative of the basinal stratigraphy in the western Dolomites after the volcanic acme. The upper part of the La Valle Fm. records a strong decrease in sedimentary supply and, therefore, it consists of calcilutites, marls and claystones. Upwards, a progressive increase in neritic carbonate content and a new increase in volcanosedimentary supply follow (San Cassiano Fm.). In the upper part of the section, the latter is gradually replaced by carbonate neritic turbidites. The latter also document the progradation of a carbonate platform (Cassian Dolomite) and of a terrigenous coastline.
According to De Zanche et al. (1993) and Gianolla (1995), the whole Prati di Stuores/Stuores Wiesen section belongs to the Car 1 3rd order depositional sequence (DS). In the Stuores area, the Car 1 DS includes the following units: La Valle Fm., lower part of the San Cassiano Fm., Cassian Dm. 1 and, locally, basic volcanics. The lowstand systems tract (LST) mostly consists of more or less coarse-grained volcano-sedimentary deposits (e.g. Marmolada Conglomerate, Pachycardientuffe). Layers including karstified carbonate olistholiths ("Cipit limestones" p.p.), probably deriving from the destroying of previous carbonate buildups, are characteristic. TST deposits are defined by the strong decrease in terrigenous content and by the appearance of fine-grained terrigenous-carbonate facies (upper part of the La Valle Fm.). In the Prati di Stuores/Stuores Wiesen section, the maximum flooding surface (mfs) seems to lie close to La Valle Fm./San Cassiano Fm. boundary; it supplied ammonoids belonging to the topmost Regoledanus Subzone (sensu Mietto and Manfrin, 1995b). In the basinal areas the Car 1 HST corresponds to the lower part of the San Cassiano Fm., interfingered with the Cassian Dm. 1 (De Zanche et al., 1993). In carbonate shelf areas, the next SB is placed at the top of the eroded and karstified Cassian Dm. 1; in basinal areas, the Car 2 SB falls at the strong increase of clastic supply which is also recorded in the upper part of the Prati di Stuores/Stuores Wiesen (cf. Urlichs 1974, 1994), that is above biocalcarenitic and oolitic turbidites (Car 1 HST).
According to De Zanche et al. (1993) and to Gianolla et al. (1998), the Car 1 DS has a suprabasinal value. On the contrary, Mastandrea, Neri and Russo (1997) think that the complex stratigraphic setting of the western Dolomites during Ladinian-Carnian is so strongly controlled by tectonics that the eustatic signal is overcome. Therefore, according to the authors, a sequence stratigraphic framework defined in the Dolomites cannot be considered as a global standard. Moreover, they suggest that the post-volcanic succession in the region should include more carbonate platforms than those indicated in De Zanche et al. (1993) and Gianolla et al. (1998).
Although at present research on ammonoids,
conodonts and palynomorphs is still in progress, available data allow a
biochronostratigraphical framework to be presented.
The definition of a new subzonal unit below the Aon Subzone is the most important result of the biostratigraphical analysis in the Prati di Stuores/Stuores Wiesen section. Mietto and Manfrin (1995 a, b) named it as the Daxatina cf. canadensis Subzone. Its base (at 45 m) is placed at the appearance of Daxatina cf. canadensis (Whiteaves), which corresponds to the FAD of Daxatina; the genus Clionitites appears at 52.2 m, while the genus Trachyceras [i.e. Trachyceras bipunctatum (Münster)] is found beginning from 61.5 m. As shown in Fig. 2, ammonoid fauna lying below 45 m perfectly fits with horizon C of the Regoledanus Subzone (sensu Mietto and Manfrin 1995 b). Note that Frankites apertus (Mojsisovics) is present beginning from this horizon upwards at least till 79 m. The upper subzone boundary crops out in the section 1 bis (Fig. 2) at 194.3 m in which a specimen of a probable Trachyceras aon (Laube) occurs. Due to the high sedimentation rate, in the Prati di Stuores/Stuores Wiesen section the Daxatina cf. canadensis Subzone is 149 m thick. It directly underlies the Aon Zone in Urlichs (1974, 1994), at whose base the Author placed the Ladinian/Carnian boundary.
The specimens of Daxatina considered here cannot be confused with the exemplars of Daxatina in Urlichs (1974), which later (1994) the author correctly considered to be juvenile stages of Trachyceras. However, as illustrated in Mietto and Manfrin (1995b: fig. 6), at a whorl height of 1 cm they show an unequivocal ceratitic suture, with entire saddles and well rounded tips. Due to their morphological features, ornamentation and suture line, exemplars of Daxatina in the Southern Alps are well comparable with those splendidly illustrated by Tozer (1994) in Canada as D.canadensis.
The genus Daxatina is a cosmopolitan
taxon. It was pointed out in the Boreal domain (e.g. Bjørnøya,
Arctic Ocean), where Böhm (1903, 1904) defined it as Dawsonites;
later emended in Daxatina by Strand (1929). Its type-species, Trachyceras
canadensis Whiteaves (1889), comes from the Liard River (British Columbia;
cf. also Smith, 1927; Mc Learn, 1947a, b; Tozer, 1967, 1994), therefore
from a mid-latitude domain. Known also in Alaska (Martin, 1916), it is
believed to be typical of the mid-high latitudes (Arkell, Kummel and Wright,1957).
Lastly, the discovery of Daxatina at low latitude in the Tethys
realm allows is to be considered a useful tool for global correlation.
The whole section is characterized by conodonts of the Budurovignathus group and by two long-range species belonging to genus Gladigondolella: G. tethydis (Huckriede) and G. m. malayensis Nogami. The Budurovignathus group is represented by B. mungoensis (Diebel), B. longobardicus (Kovacs), B. diebeli (Kozur and Mostler) and B. mostleri (Kozur). All these taxa are typical of the diebeli Assemblage Zone in Krystyn (1983), referred by the author to the Regoledanus Zone (= uppermost Ladinian).
As shown in Fig. 2, the first occurrence of conodonts of the Budurovignathus group is documented at about 26 m. Unfortunately, the interval between 26 and 85 m is barren in conodonts: it contains the FAD of Daxatina cf. canadensis (at 45 m).
The upper part of the section (above 85 m) still contains Budurovignathus and long-ranging gondolellas (i.e. G. tethydis, G. m. malayensis). B. diebeli and B. mostleri occur until about 177 m, together with a broken specimen of Gondolella that, due to its bad preservation, had an uncertain classification (G. inclinata vel G. polygnathiformis ).
G. polygnathiformis occurs together
with Trachyceras aon near the base of section 2 (Fig.
1) which corresponds to the section in Urlichs (1974) as the stratotype
of the Cordevolian substage. On the basis of data from sections in the
Stuores area, it seems that the FAD of Pseudofurnishius murcianus
murcianus is located slightly below the first occurrence of G.
polygnathiformis. Moreover, in the lower samples of section 2, B.
diebeli occurs together with G. polygnathiformis, confirming
the overlap of the ranges of these taxa shown by a number of stratigraphic
sections within Alpine Europe (Mastandrea et al. in progress).
The vertical distribution of conodonts and ammonoids across the Ladinian/Carnian boundary was discussed by Krystyn (1983) in the Epidaurus section and, more recently, by Kovacs et al. (1991) with regard to the Füred Limestone in the Balaton Highland. The above quoted papers present two contrasting sets of data.
According to Krystyn (1983), in the Epidaurus section the conodont assemblage characterized by Budurovignathus diebeli, marker of the Diebeli Zone, is correlatable with the Regoledanus Zone sensu Krystyn (1983). The first occurrence of Gondolella polygnathiformis is considered by Krystyn (1983) to coincide with the base of the Aon Subzone. Unfortunately, the "Carnian" tract of the section is quite poor in ammonoids, only represented in the Aonoides Zone sensu Krystyn (1978) by specimens of Trachyceras sp. The ranges of Budurovignathus diebeli, B. mungoensis and B. mostleri do not show any overlap with the range of G. polygnathiformis, although subsequently, on the basis of other stratigraphic sections (i.e. the Mayerling section), Krystyn reached the conclusion that the range of the Budurovignathus group goes upward into the Aonoides Zone (Gallet et al., 1998).
Clear evidence of overlap between the ranges of Budurovignathus and G. polygnathiformis are supplied by a number of stratigraphic sections, including the Prati di Stuores/Stuores Wiesen section and outcrops from the Apuseni Mts. (Rumania, Kozur, 1980; 1989). The real question is if the overlap is due mainly to a downward extension of the range of G. polygnathiformis, below the FAD of Trachyceras aon or, on the contrary, to the upward extension of the range of the Budurovignathus group into the Aonoides Zone.
In the Ladinian-Carnian succession of the Balaton Highland, conodonts of the Budurovignathus group (including B. diebeli) occur together with G. polygnathiformis from the base of the Füred Limestone (Kovacs et al., 1991); the lowermost part of the formation contains Frankites sp., and has been referred by the above quoted authors to the Regoledanus Zone. The conclusion of Kovacs et al. (1991) is that the base of the Carnian stage has to be located at the first appearance of G. polygnathiformis and thus they include the Regoledanus Zone in the Carnian.
According to two of us (S. Manfrin and P. Mietto), it is possible that the specimens of Frankites sp. in the Füred Limestone pertain to F. apertus rather than to F. regoledanus. This opinion is supported by a re-evaluation of the ammonoid fauna in Frech (1911) from the same stratigraphic unit: it includes "Trachyceras (Anolcites) Richthofeni Mojs.", classified, with some doubts, as F. apertus by Mietto and Manfrin (in progress). If this interpretation is correct, it may be possible to argue that the FAD of G. polygnathiformis falls within the Daxatina cf. canadensis Subzone and that its occurrence at the base of the Aon Subzone in the Prati di Stuores/Stuores Wiesen section 2 (Urlichs, 1974, 1994) is ecologically controlled. A significant environmental change from anoxic/dysoxic to fully oxygenated basin may occur near the base of Aon Subzone, as suggested by macro- and microbenthos assemblages. This fact may enforce the proposal to drawn the Ladinian/Carnian boundary at the base of the Daxatina cf. canadensis Subzone: its lower boundary falls into a tract of section not affected by significant paleoecological changes and thus may be regarded as "evolutionary"-controlled.
However, there is no agreement between the co-authors of the paper by Kovacs et al. (1991) about the real taxonomy of the conodonts classified as "G. polygnathiformis". According to Krystyn (personal communication to A. Mastandrea), G. polygnathiformis has never been found within the Regoledanus Zone (which, as intended by L. K., includes the Daxatina cf. canadensis Subzone).
The problem needs further discussion, in order to attest if the FAD of G. polygnathiformis is really more or less concomitant with the FAD of T. aon or, on the contrary, is within the upper Regoledanus Zone Auct. (= Daxatina cf. canadensis Subzone). Data from the Stuores area support the idea that the first appearance of G. polygnathiformis and T. aon are approximately coeval. This interpretation is congruent with data from Epidaurus; moreover, it is not in contrast with data from the Mayerling section (Gallet et al., 1998).
If the proposal to put the Ladinian/Carnian
boundary at the base of Daxatina cf. canadensis Subzone is accepted, it
must be taken into account that it most probably does not coincide with
significant events affecting the conodont assemblages.
In order to make easier the comprehension
of the paleontological characteristics of the interval including the Ladinian/Carnian
boundary, data both on palynomorphs, which could be important from the
biostratigraphic point of view, and on other fossils, which can give significant
environmental indications, are included.
Palynological analysis gave interesting preliminary results that have to be compared on a regional scale to be confirmed. Previous palynological studies (Van der Eem, 1983) on this area only concerned younger strata (cf. Urlichs levels). The palynological assemblages evolved, from the bottom to the top, as follow.
Assemblage A (from 0.5 to 45 m, Regoledanus Subzone): the palynological assemblage is highly diversified. In this interval, Enzonalasporites vigens Leschik occurs first. "Lueckisporites" cf. singhii Balme makes its first occurrence at just 50 cm below the Regoledanus/Daxatina cf. canadensis subzones boundary, and ranges up into the Daxatina cf. canadensis Subzone. Other important occurrences are Lunatisporites acutus Leschik and Neoraistrikia taylori Playford and Dettman. The most abundant species is the long ranging Ovalipollis pseudoalatus (Thiergart) Schuurman, which is present throughout the section.
Assemblage B (from 45 to 160 m, Daxatina cf. canadensis Subzone): at the Regoledanus/Daxatina cf. canadensis subzones boundary, a great diversification has been recorded. The Daxatina cf. canadensis Subzone is characterized by the first occurrence of Patinasporites densus Leschik and Vallasporites ignacii Leschik. Other important elements are Camerosporites secatus Leschik, Weylandites magmus (Bose and Kar) Van der Eem, Concentricisporites bianulatus (Neves) Antonescu, Converrucosisporites sp. B (cf. Van der Eem, 1983), Concavisporites sp. A (cf. Van der Eem, 1983) and Concentricisporites insignis Pautsch. In the uppermost part of the subzone, bad preserved specimens of Partitisporites maljawkinae (Klaus) Van der Eem also occurs for the first time. The long ranging species already present in the underlying subzone can also be regularly found together with new long ranging elements. The Triadispora group becomes more diversified towards the upper part of this subzone.
As reported by several authors (Clement-Westerhof
et al., 1974; Schuurman, 1979; Scheuring 1978; Visscher et al., 1980; Visscher
and Brugmann, 1981; Van der Eem, 1983), in this section the first appearances
of E. vigens and "L". cf. singhii also occur respectively
in the upper (secatus-vigens phase, Longobardian) and uppermost
Ladinian. Differently, W. magmus, whose first occurrence
was recorded by Van der Eem (1983) at the base of the secatus-vigens
phase, first occurs in this section within the Daxatina cf. canadensis
Subzone. The first income of C. secatus, which is considered Late
Ladinian in age (Schuurman, 1979; Visscher and Krystyn 1978; Visscher and
Brugman, 1981; Van der Eem, 1983; Brugman et al., 1994) also occurs in
this section within the Daxatina cf. canadensis Subzone. The Carnian age
of other important elements such as P. densus and V. ignacii
is fully confirmed.
In order to obtain the most realistic assemblages, the study has been carried out both in thin section and washed material. Isolated foraminifers have been investigated from 33 samples of marls and clays. Foraminiferal assemblages from calcarenitic levels and muddy limestones have been studied in over 300 thin sections.
The foraminiferal assemblage evolves as follows:
A) 0 to 45 m (Regoledanus Subzone).
This segment ranges from the base of the section to the FAD of Daxatina
Isolated foraminifers - A rich fauna dominated by agglutinated foraminifers, such as Endothyra kuepperi Oberhauser, Palaeolituonella meridionalis (Luperto), Ammodiscus cf. infimus (Strickland), A. incertus (d'Orbigny), A. tenuissimus Gümbel, A. annulinoides Kristan-Tollmann, Glomospira perplexa Frank, Glomospirella facilis Ho, G. hemigordiformis (Tscherdynzew), Reophax eominutus Kristan-Tollmann together with the nodosariids Dentalina ex gr. subsiliqua Franke, D. cassiana Gümbel, Nodosaria primitiva Kübler and Zwingli, Pseudonodosaria obconica (Reuss), Kriptoseptida klebelsbergi (Oberhauser), Lenticulina cassiana (Gümbel), L. excavata (Terquem), L. bochardi (Terquem), L. karnica (Oberhauser) are present. The species Duostomina sp., D. biconvexa Kristan-Tollmann, D. alta Kristan-Tollmann, D. turboidea Kristan-Tollmann and Ophthalmidium cf. exiguum Köhn-Zaninetti are also present.
Thin section - In a mudstone layer at 9.10 m the appearance of the very important assemblage Gsollbergella spiroloculiformis (Oravecz-Scheffer) - Semimeandrospira ex gr. karnica -planispira (Or.-Scheff.) -Turriglomina carnica Dager was recorded. Piallina tethydis Rettori et al. first occurs in a fine grained level at 19.25 m. The appearance of Gordiospira triassica Urosevic was recorded at 34 m, but this species is scarcely present in the rest of the section. Gsollbergella spiroloculiformis is constant along the succession both in calcarenites and in muddy limestones, whilst T. carnica is more frequent from 29.60 m. In addition, just one specimen of Palaeolituonella meridionalis (Luperto) was recorded at 43.40 m. All taxa are small and, at present, are exclusively known in the "Carnian" of western and eastern Tethys, but very few species have been dated by ammonoids or conodonts.
Several species belonging to Ammobaculites, Gheorghianina, Glomospira, Glomospirella, Duostomina, Endoteba, Ophthalmidium, Reophax and Variostoma may be associated. Nodosariidae and Textulariidae also occur.
B) 46 to 145
m (Daxatina cf. canadensis Subzone p.p.)
Isolated foraminifers - Most foraminifers in segment A continue their presence, but the nodosariids become dominant and some new taxa, such as Schlagerina sp., Oberhauserella sp., Dentalina guembeli Schwager, Frondicularia sp. and Nodosaria nitidana Brand occur.
Thin section - Any peculiar change in foraminiferal assemblage was recorded in this segment except for the bloom of S. ex gr. carnica-planispira at 114 m. An increase in carbonate platform biota (Tubiphytes, bryozoans, Cayeuxia, calcareous sponges, thick shelled bivalves and encrusting foraminifers) become more abundant from 131.10 m.
C) 146 to 180 m (Daxatina
cf. canadensis Subzone p.p.)
Isolated foraminifers - The foraminiferal assemblage is dominated by duostominids and involutinids. The taxa mainly represented are: Duostomina biconvexa Kristan-Tollmann, Diplotremina astrofimbriata Kristan-Tollmann, Variostoma pralongense Kristan-Tollmann, V. exile Kristan-Tollmann, P. altoconica (Kristan-Tollmann), Papillaria sp., Lamelliconus ex gr. ventroplanus (Oberhauser), Aulotortus ex gr. sinuosus Weynschenk, Lamelliconus multispirus (Oberhauser). In addition, a few nodosariids and agglutinated foraminifers also occur.
The first appearance of Ophthalmidium fusiforme (Trifonova) and Ammobaculites hiberensis Marquez and Trifonova occurs at the top of the section.
Thin section - At 172 m, the foraminiferal assemblage is at first dominated by involutinids (Lamelliconinae) mainly referred to Lamelliconus multispirus (Oberhauser) and L. procerus (Liebus) in association with G. spiroloculiformis, S. ex gr. karnica-planispira, T. carnica and P. tethydis already quoted in the previous segments.
Segments A and B are mainly characterized by autochthonous deep-water foraminifers. According to the relations between foraminiferal test morphology and energy-oxygen level suggested by Kahio (1989) and Boudiche and Ruget (1993), Dentalina, Pseudonodosaria and Nodosaria belong to elongated and cylindric morphotype; Frondicularia to the elongated and flattened one; Lenticulina to the flattened and planispiral type. Deep-water infaunal habits have been inferred for these taxa. Agglutinated and tubular tests of Glomospira, Glomospirella and Ammodiscus pertain to epifauna which inhabits shallow- to deep-water during Early Mesozoic. The association of all three morphotypes suggest disaerobic and low-energy conditions of the sedimentary environment. Moreover, the duostominid and involutinid dominated associations in segment C (upper part of the Daxatina cf. canadensis Subzone) records an increase in energy and oxigen level of the environment.
In conclusion, foraminifers allow the following remarks.
Bivalves in the Prati di Stuores/Stuores Wiesen section mainly consist of Posidonia wengensis (Wissmann), while no specimens of Daonella or Halobia have been found. Brachiopods are rare and only occur in the upper part of the section. P. wengensis is mainly contained in laminated dark grey clays and argillaceous marls alternated with tuffites and calcarenites. According to Fürsich and Wendt (1977), Posidonia is a typical element of the San Cassiano Fm. "pelagic assemblage". The latter is characterized by the absence of benthic and dominance of nektic forms and was deposited in the deeper parts of the Cassian basin (depocentre). Due to: (1) high turbidity, (2) high water-content of the bottom sediment (very soupy substrate) or (3) oxygen-deficient bottom-waters, this assemblage lacks benthic suspension feeders (Fürsich and Wendt, 1977). According to the new hypothesis concerning the tolerance of posidoniids to low-oxygen values and their benthic life habit (Wignall and Simms, 1990), the latter environmental factor seems to have played a dominant role in the composition of the "pelagic assemblage" occurring in the section.
Above 150 m, the disappearance of P. wengensis and the frequent occurrence of nuculoids (section 2) may suggest a change of the redox boundary which moved inside the sediments.
In conclusion, the depositional environment
of the Prati di Stuores/Stuores Wiesen section corresponds to a basin in
which prevailing anaerobic to disaerobic conditions of the bottom water
occurred in the lower part (from 0 to 105 m). As the sediment infilled
the basin, the water circulation increased. In the middle part (from 105
to 150 m), frequent aerobic events (Koninckina beds) alternated
with disaerobic events (Posidonia-bearing marls). In the upper part
of the section (from 150 to 180 m) more stable aerobic conditions allowed
the settlement of nuculoids, which occur frequently in section 2 (Aon Subzone).
In the middle-upper Triassic of the Tethys Realm many microcrinoid occurrences have been observed. Kristan-Tollman (1970) compared this phenomenon to the Jurassic Saccocoma facies. Many species belong to Somphocrininae Peck and particularly to the genus Osteocrinus Kristan-Tollman. Many holothurian sclerites are also documented.
In the section 1, the taxonomic diversity of the microcrinoids is very high. The following species can be recognized: Osteocrinus rectus Krist.-Toll., O. goestlingensis Krist.-Toll., O. saklibelensis Krist.-Toll., O. acus Krist.-Toll., O. sp., Ossicrinus reticulatus Krist.-Toll., Axicrinus alexandri Krist.-Toll., A. sp., Somphocrinus sp., Tulipacrinus sp. and Nodolanx multinodosa Krist.-Toll.
Osteocrinus rectus, the close species O. goestlingensis, and the hardly determinable O. saklibelensis, commonly have a much wider range with respect to the extension of the Prati di Stuores/Stuores Wiesen section. However, strictly concerning the latter, it is possible to make the following considerations:
The variations in taxonomic diversity can probably be referred to ecological factors.
A total of 96 paleomagnetic core samples
were analysed at the paleomagnetics laboratory of ETH Zürich. Stepwise
thermal demagnetization treatments and measurements of natural remanent
magnetization (NRM) were performed on all specimens in a shielded room.
Magnetic remanences were measured on a 2G Model 760 3-axis cryogenic magnetometer
mounting DC SQUID sensors. Least-square analysis was used to determine
the component directions of the NRM, chosen by inspection of vector end
point demagnetograms. Mean directions were determined with standard Fisher
statistics. The rock magnetic properties were investigated by means of
thermal unblocking characteristics of orthogonal-axes isothermal remanent
Paleomagnetic samples typically show the presence, in in situ coordinates, of a scattered low unblocking temperature component of viscous origin consistent with acquisition along the present-day field direction and/or induced at random during drilling or cutting. Above this low temperature component, interpretable paleomagnetic directions were obtained in 93% of the specimens. In particular, a bipolar characteristic component oriented in situ north and positive or south and negative was isolated in 72% of the specimens in the temperature range comprised usually between about 200°C and 450°C to 550/580°C. In 10% of the specimens the characteristic component could be followed up to maximum unblocking temperatures of 600 to 630°C. An additional 11% of the samples show in situ westerly stable-end-point trajectories interpreted as transitional directions associated with excursions of the Earth's magnetic field or acquired during a field polarity reversal. The bipolar and transitional characteristic component directions become shallower upon correction for bedding tilt.
The thermal unblocking characteristics of orthogonal-axes IRM show that all lithologies are dominated by a low coercivity and ca. 580°C maximum unblocking temperature phase interpreted as magnetite, maybe co-existing with subsidiary sulphurs characterized by ca. 320-
350°C maximum unblocking temperatures.
A higher coercivity and unblocking temperature phase, like haematite, may
be also present at places.
The latitude of the specimen virtual geomagnetic pole (VGP) with respect to the overall mean north paleomagnetic pole was used to delineate the magnetic polarity stratigraphy. VGP relative latitudes approaching +90°N (-90°N) are interpreted as recording normal (reversed) polarity. The latitude of the VGPs defines at Stuores a lower normal (S1n)-reversed (S1r)-normal (S2n)-reversed (S2r)-normal (S3n)-reversed (S3r)-normal (S4n) polarity sequence. Intermediate VGP latitudes are preferentially located at polarity transitions. The base of the Carnian, as defined by the base of the Daxatina cf. canadensis Subzone, falls towards the base of normal polarity interval S2n.
The Ladinian/Carnian boundary at Stuores can be tentatively correlated with the coeval Mayerling section in Austria from the literature (Gallet et al., 1998, fig. 10). We propose that the reversed polarity interval S1r in the Prati di Stuores/Stuores Wiesen section correlates with polarity interval MF- at Mayerling, and that normal polarity interval S2n at Stuores corresponds to interval MG+ at Mayerling (Fig. 3) [polarity nomenclature at Mayerling is here informally introduced after Gallet et al (1998) for clarity]. Towards the top the correlation becomes less clear. It is possible that the reversed polarity interval S2r at Stuores correlates at Mayerling with a small "excursion" located somewhat below meter 40 at a lithological transition, and here named MH-?. If so, the overlying normal polarity interval S3n at Stuores corresponds to interval MI+ at Mayerling and so forth. This correlation would suggest that the first occurrence of Gondolella polygnathiformis at Stuores, presently located at the base of section 2, may be lower, within normal polarity interval S3n.
The present paper proposes that the base of the Carnian stage should be placed at the base of the Daxatina cf. canadensis Subzone. This assumption is mainly supported by the occurrence of the genus Trachyceras below the FAD of T. aon, within the Daxatina cf. canadensis Subzone. Also due to its good magnetostratigraphic record, the Prati di Stuores/Stuores Wiesen section is proposed as the candidate stratotype for the Ladinian/Carnian boundary.
The proposal is supported by the following data:
This work was sponsored by
the C.N.R. [grants 96.00339.05, 97.00241.05, 98.00270.05 (Loriga) and 96.00364
(F. Russo); Centro di Studio per la Geodinamica Alpina - Padova], and by
M.U.R.S.T. [ex 40% (Cirilli); ex 40% (F. Russo); ex 60% (Rettori); ex 60%