The Ladinian-Carnian boundary succession in Spiti (Tethys Himalaya) and its bearing to the definition of the GSSP for the Carnian stage (Upper Triassic)

Autor(en)

Marco Balini, Leopold Krystyn, Alda Nicora, Valentina Torti

Abstrakt

The Triassic succession of Spiti Valley area (Tethys Himalaya, India) is known since the XIX century for its rich fossil content described and illustrated in several outstanding monographs by Diener (1895, 1897, 1907, 1908). Such a large amount of information unfortunately lack of detail concerning the bed-by-bed distribution of the taxa, as a consequence its usefulness for the definition of modern bio-and chronostratigraphic scales is scarce. In the last years four expeditions have been carried out with the purpose to revise the biostratigraphy of the Middle Triassic part of the succession. In particular the Upper Ladinian/Lower Carnian succession of the Kaga Fm. and Chomule Fm. has been investigated with special care. Five stratigraphic sections have been sampled in Pin and Lingti Valleys for ammonoids, pelagic bivalves and conodonts. The bulk of the faunas described by Diener has been found, together with several new findings. The ammonoid record is unusually complete and allow accurate dating of the sections; it also represents the reference for the calibration of the conodonts and bivalves distribution. At present four ammonoid faunas have been recognized: Meginoceras, Frankites, Daxatina and Trachyceras. The critical interval for the L/C boundary falls in the lower part of the Chomule Fm., where Frankites to Trachyceras faunas have been found. The range of Frankites slightly overlaps the range of Daxatina, as the LO of Frankites is recorded in the same bed containing the FO of Daxatina. The FO of Trachyceras is difficult to correlate between the sections. Daonella and Halobia are very common in the sections, with relatively few species (D. Pichleri, D. tyrolensis, D. lommeli, H. zitteli, H. fascigera) showing a relatively long range. The most important event in the development of the pelagic bivalves seems to be the FO of Halobia, which is recognized in the middle part of the Frankites range. The distribution of conodonts is also characterized by long range of the species, and the only event useful for the definition of the L/C boundary is the FO of polygnathiformis. In the studied sections this event is coeval with the FO of Halobia. The new data allow the drawing of the following conclusions: (1) the studied sections show a unique fossil record, at least within the Tethys, because all the three traditional biostratigraphic tools for dating and correlating Triassic marine rocks (ammonoids, conodonts and pelagic bivalves) can be applied; (2) the most important events in the development of pelagic bivalves and conodonts are not coeval with any of the most important events in the distribution of ammonoids; (3) the FO of Daxatina, which has been proposed as a marker for the L/C boundary, cannot be accurately recognized neither with conodonts nor with pelagic bivalves. The correlation with the GSSP candidate for the Carnian boundary (Prati di Stuores, Dolomites) demonstrates that Spiti Valley sections are complementary to Prati di Stuores, where the pelagic bivalves are lacking and the conodonts distribution is somewhat problematic (FO of polygnathiformis in the Aon Zone), but where magnetostratigraphy works well.

Organisation(en)

Institut für Paläontologie

Externe Organisation(en)

Università degli Studi di Milano-Bicocca

Journal

Journal of Asian Earth Sciences

Band

19

Seiten

3-4

Anzahl der Seiten

2

ISSN

1367-9120

Publikationsdatum

2001

ÖFOS 2012

105118 Paläontologie

Sustainable Development Goals

SDG 14 – Leben unter Wasser

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/6b78073b-1c1c-4f0c-afe6-37d6898a14c9