The Spiti region (Tethys Himalaya) is well known for its Lower and Middle Triassic invertebrate faunal succession which is the most complete within the Tethys and therefore of outstanding biostratigraphic importance for this particular realm.
With the search of a GSSP for the Middle/Upper Triassic (i.e. Ladinian/Carnian) boundary in Northern Italy well in progress (Broglio Loriga et al., 1998), we decided to add the preliminary data obtained during our 1997 joint expedition to the Guling and Muth area of the Pin Valley (Spiti, for location see Krystyn & Orchard, 1996). Our results complement those of Broglio Loriga et al. (1998) as we report also distribution data of conodonts and halobiids, two fossil groups of specific importance for long distance correlations between different biogeographic (i.e. Tethys-North America) and paleoclimatic (i.e. Tethys-Boreal) realms.
According to the recently revised Triassic
lithostratigraphy of Spiti (Garzanti et al., 1995) the Ladinian-Carnian
boundary interval falls into the Hanse Group (Kaga, Chomule and Grey beds
Fm.), in particular between the upper part of the Kaga Fm. and the Chomule
Fm. For this interval, however, no new biostratigraphic data have been
provided so that at present the only available referring data are those
by early authors (Bittner, 1899, Krafft in Hayden, 1904; Diener, 1908 and
1912). These authors used a more traditional lithostratigraphical nomenclature
with "Formation" names based on the main occurring fossil groups (Daonella
Shales = Kaga Fm.; Daonella and Halobia Lmst. = Chomule Fm.). Dealing primarily
with the fossil content we continue to use their now informal names for
the greater chance to integrate new and old faunal records. Halobiids and
Upper Ladinian ammonoids are reported in the Daonella Shales, while in
the overlying Daonella Limestone Krafft (in Hayden, 1904) distinguished
a lower portion with Daonella and an upper part with Halobia
and the Ladinian-Carnian boundary in between. No ammonoids of stratigraphical
significance were reported.
The section was measured 500 m westward from the village of Guling, from the top of the Muschelkalk Member of the Tamba Kurkur Fm to the middle part of the Daonella Limestone (Fig. 1). A second segment covering the upper part of the Daonella Limestone to the Grey beds was not studied in detail. Pelagic bivalves of the genera Daonella and Halobia are common throughout the studied section, while stratigraphically significant ammonoids are restricted to distinct and widely spaced levels. Some conodont samples have also been processed. As the lithology is laterally constant over tens of kilometers, we have integrated into the Guling section some samples from the classical locality of Muth.
The fauna of the Daonella Shales is mostly
concentrated in the lenticular or bedded limestone levels that occur in
two intervals: 2 to 15 m and 24 to 43 m from the base. The interbedded
shales are difficult to sample because of cleavage. In the Daonella Limestone
fossils occur in both limestones and marly interbeds. The base of the Daonella
Limestone is characterized by a distinct 1-2 m thick stilolithic limestone
band ("Traumatocrinus Limestone").
The distinct ammonoid bearing horizons allow a quite good timing of the section. The Upper Ladinian age of the top of the Tamba Kurkur Fm. is demonstrated by Protrachyceras archelaus found in its last bed (M47) at Muth. At the base of the second limestone interval of the Daonella Shales a rich ammonoid fauna can be referred to the Meginae Zone on the basis of Meginoceras, Protrachyceras and Anolcites.
The base of "Traumatocrinus Limestone" provided "Celtites" epolensis, Protrachyceras, Rimkinites together with a specimen equivalent to Frankites in ribbing and style of the venter, while the thickness of the whorl is unusually greater. As the specimen is just a body chamber fragment it may belong either to a new genus or to a new species of the genus Frankites. If not condensed, the fauna could represent either the top of the Archelaus Zone s.l. (=Neumayri Zone sensu Mietto & Manfrin, 1995a) or the base of the Regoledanus Zone. A true Frankites (F. cf. regoledanus) has been found in Muth 1 m above the sample 97/179, but could not be extracted from the rock. Further collections are clearly necessary for a definite solution.
The lenticular limestones immediately above the "Traumatocrinus Limestone" yielded at Muth a small sized Muensterites (sample 97/175), while some meters above (sample 97/176) a specimen probably belonging to Daxatina has been found. The specimen shows the typical ventral morphology of Trachyceras, but the definite attribution is not sure because the specimen is a body chamber and misses the suture line.
Above this level some long ranging ammonoids
as Sirenotrachyceras, Joannites and Carnites floridus
are frequently found, but two levels (VT48 and VT52) provided well preserved
and undoubtful Trachyceras of the group of T. aon (here intended
as inclusive of T. (Brotheotrachyceras) Urlichs, 1994). A Trachyceras
aonoides fauna has been found in the lateral continuation of the outcrop
with a position just above the top of the section (sample I5).
At the base of the first limestone interval the Upper Ladinian speies Daonella pichleri occurs in one level (VTS16). Above this level D. indica and D. lommeli are very common. Especially D. lommeli shows a very long range, from the lower part of the Daonella Shales up into the Daonella Limestone. In general D. indica is less frequent than D. lommeli.
From the levels VT36 to VT 46 the bivalves show some modifications with respect to the typical D. lommeli morphology. In VT36 the specimens referred to D. lommeli group, show affinities with D. hagighiolensis, due to the presence of a smooth triangular posterior sector. In the literature (Krystyn & Gruber, 1974) this species is considered as stratigraphically younger than D. lommeli, beeing referred to the Sutherlandi Zone.
The specimens from sample VT46 are very similar to D. lommeli, but show some morphological characters typical of the genus Halobia, as for example the curved pattern of radial costae.
The first true Halobia comes from
level VT50, and a few meters above we have found H. zitteli. The
latter is a typical form of the Aon Zone and an excellent guide for cross-latitudinal
As just few samples have been collected, we cannot provide a complete range chart for the recognized species. Nevertheless, the transition from P. inclinata to Metapolygnathus polygnathiformis is recorded between the Muensterites level and the bed with the questionable Daxatina. Sample 97/175 contains exclusively P. inclinata, while in 97/180 we have found intermediate forms and just 30 cm above (sample D11) the first true representatives of M. polygnathiformis together with P. inclinata. The F.O. of M. polygnathiformis is therefore much lower in the Guling section than the one of Trachyceras gr. aon (respectively at 63 m and 88 m from the base).
The integrated analysis of our data shows
that between the Ladinian "Traumatocrinus Limestone" with Frankites
and the first Trachyceras gr. aon in the overlying Daonella
Limestone there is a 25 m thick rock interval without time-diagnostic fossils
in a strict sense. This boundary interval, which may represent the Daxatina
cf. canadensis Subzone sensu Mietto & Manfrin, 1995a and a part
of the Aon Zone sensu Urlichs, 1994, has to be carefully resampled
before final conclusions can be drawn.
Due to the scarcity of Ladinian/Carnian fossiliferous sequences in the Tethys the Guling section can be considered a good test for the criteria to define the base of the Carnian Stage. Till now most of the suggestions come from the ammonoids. We have the possibility to test halobiids and conodonts referred to an ammonoid record that is one of the most complete, beeing represented by a succession of up to 5 ammonoid zones across the boundary (Archelaus to Aonoides Zone).
At present two possible solutions can be discussed.
F.O. of Daxatina. This boundary has been proposed by Mietto & Manfrin (1995a and 1995b) because according to their data corresponds to a moment of major changes in ammonoid assemblages (F.A.D. of Daxatina and Clionitites almost coincident with F.A.D. of Trachyceras: Mietto & Manfrin, 1995b, p. 24). The boundary is potentially very useful because Daxatina has a worldwide distribution, but the synchronicity of its F.O. in the different paleobioprovinces has still to be demonstrated and its forerunner is also unknown. Moreover we see other problems. The first problem is, in some respects, formal. Daxatina is not easy to be identified, if the suture line is not visible (i.e., steinkern of body chamber or shelled specimens), so that it does not completely fulfil the requirements of a guide fossil. Recent literature demonstrates that a misinterpretation is possibile, also to experienced specialists. A possible solution could be the choice of the F.O. of Trachyceras that according to Mietto & Manfrin occurs slightly above the F.O. of Daxatina, but once again the worlwide synchronicity of this event has to be demonstrated. In North America for example Trachyceras appears far above Daxatina. As regard the appeareance of Clionitites, in Epidauros section this genus occurs already at the base of the Regoledanus Zone (sample A16 in Krystyn 1983, fig.3 p.244) then Mietto & Manfrin's sections record a F.O., not the F.A.D. of the genus.
In our section the comparison of ammonoid with bivalve distribution lead to conclude that the doubtful Daxatina is not accompanied by any event in the distribution of halobiids. In particular the typical Ladinian guide fossil Daonella lommeli occurs also above. Spiti data in this respect fit well with those from the Western Tethys where the D. lommeli group is also recorded up to Lower Carnian (Krystyn & Gruber, 1974). Finally as regard the conodonts our scarce data do not allow a definite solution. The early appearence of M. polygnatiformis within the Spiti boundary interval may be seen as a possible hint that the species is already present within the Tethyan "Daxatina beds". This unfortunately is not true for North America and the Arctic (Orchard & Tozer, 1998).
Base of the Aon Subzone (sensu
Krystyn, 1978). This traditional
boundary (see Krystyn, 1978 and Urlichs, 1994 for historical summary) can
be drawn at the F.O. of Trachyceras of the T. aon group.
Advantage of this solution is the easier identification of the index ammonoid
and the better integration with the F.O. of Halobia. The two bioevents
are not exactly coeval (F.O. of Halobia is a little younger), but
the approximation is very good. In fact Lower Carnian corresponds to a
radiation period for halobiids, with the extinction of genus Daonella
and the appeareance of genus Halobia. It must also be pointed out
that Halobia is climatic independant so that it could play an important
role for the correlation of Tethyan and Boreal successions (Krystyn, 1978).
No doubt the sequence described in the San Cassiano area by Broglio Loriga et al. (1998) is good and may have the potential for a GSSP. But its proper designation, in our opinion, seems to be premature for several reasons. The greatest problem is that the section presently lacks comparable data (except for ammonoids) from the overlying (basal) Aon Zone. This hampers seriously a thorough comparison of the pros and cons pointing for the former or the now proposed boundary.
More paleontological work is also needed to demonstrate the suitability of the new boundary as an international standard. The proposed GSSP for example is not sufficiently representative concerning such biostratigraphically significant groups as conodonts and halobiids. A possible paleoenvironmental or paleobiogeographical influence on the distribution of some taxa (i.e., lack of halobiids, "late" appeareance of Clionitites and M. polygnathiformis) should be tested.
Finally with regard to the ammonoids two serious problems have to be solved. At first the Trachyceras and Daxatina species from the Daxatina cf. canadensis Subzone have to be properly described and illustrated. The Trachyceras species in the Daxatina cf. canadensis Subzone could be a good reason for the new boundary, but at present the features of these species are unknown and the taxonomy does not seem to be stable: the only Trachyceras from the Subzone (Trachyceras muensteri: Mietto & Manfrin, 1995a, pl. 5, fig. 5; 1995b, pl. 2, fig. 16) is from debris, moreover it has been recently renamed as Daxatina cf. canadensis (Broglio Loriga et al., 1998, pl. 2, fig. 16).
Secondly, based on a careful study of both
the type material as well as the figures in Mietto & Manfrin (1995b)
and unpublished material from Epidauros we see no chance for a morphologic
(and taxonomic) separation of Frankites regoledanus and Frankites
apertus as done by the mentioned authors. When adopting the new zonation
this will lead to serious confusion in the future stratigraphic scale.
Field work has been supported
by CNR grant to Prof. M. Gaetani (MB & VT) and by "Austrian National
Committee for IGCP" - IGCP 359 (LK). Daniela Germani (Milano) helped during
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