Aretz et al_2011.pdf - ORBi - Université de Liège

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Kölner Forum Geol. Paläont., 19 (2011)
M. ARETZ, S. DELCULÉE, J. DENAYER & E. POTY (Eds.)
Abstracts, 11th Symposium on Fossil Cnidaria and Sponges, Liège, August 19-29, 2011
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Upper Ordovician and Lower Silurian corals from the Pin Formation
(Spiti Valley, Northern India)
Thomas SUTTNER & Erika KIDO
Austrian Academy of Sciences (CPSA) c/o University of Graz, Institute for Earth Sciences (Geology &
Paleontology), Heinrichstrasse 26, A-8010 Graz, Austria; thomas.suttner@uni-graz.at,
erikakido07@yahoo.co.jp
The late Katian is known as the time when reefs flourished and expanded globally (compare WEBBY
2002) before the onset of the Hirnantian ice age, which tremendously affected biota world-wide (SHEEHAN
2001). This interval conforms to the Boda Event (FORTEY & COCKS 2005), which is considered as interglacial
phase during the late Ordovician ice-house. Faunal and floral climax conditions are documented especially
by increased abundance and a widening of the latitudinal distribution-belt of carbonate factories (see
WEBBY 2002) which produced limestone deposits at higher latitudes on the peri-Gondwana shelf
interrupting clastic sedimentation dominant at that time (e.g., LOI et al. 2010).
During the early Palaeozoic the Indian subcontinent was part of northern Gondwana inheriting a
palaeo-position of approx. 25-30°S. Among several localities, especially the succession in the Spiti Valley in
northern India (recent geographic position) is known for shallow marine carbonate deposits of Ordovician
to Silurian age yielding plenty of small patch reefs (Takche Member of the Pin Formation, compare Fig. 1).
One of these coral bearing units was referred to as “Silurian reefal buildups” of the Takche Formation (syn.
Pin Fm) by BHARGAVA & BASSI (1986). The age constrain of these bioclastic limestones was based on data
from macrofossils provided by REED (1912), who described most of the material from the Himalayan fossil
collection of Hayden and Krafft. Since then that unit remained assigned to Silurian age. In 2001 a first
attempt was made to apply modern microfossil biostratigraphy by dissolving several hundred kilos of
carbonate rock samples from the type section, which finally produced a quite diverse conodont fauna
referring to Katian age (SUTTNER 2003).
Actually, the stratotype of the Pin Formation is exposed in the Pin Valley (Spiti) near the small village of
Farka Muth (altitude: 3750m), Northern India. The Spiti region is part of the Himachal Pradesh district and
lies in the high north-western Himalayas. The Pin Formation comprises a 280 m thick sedimentary
succession of late Ordovician and Silurian rocks. HAYDEN (1904) divided the formation into horizon 1 to 8
(see Fig. 1). Some years later the Ordovician/Silurian boundary was fixed by REED (1912) between horizons
5 and 6 as he concluded that the assemblage of corals in Horizon 6 had affinities mainly with Silurian
species (Horizon 6 equates with the above mentioned “Silurian reefal buildups” of the Takche Formation
(syn. Pin Fm) by BHARGAVA & BASSI 1986). SUTTNER (2007) divided the Pin Formation into 17 lithological
units (P/1-17) in ascending order and assigned P/1-6, P/7-13 and P/14-17 to the Farka Muth, Takche, and
Mikkim members, respectively (Fig. 1). During the work on the lithostratigraphy and facies of this
formation also the position of O/S boundary within the Pin Formation could be revised by the application
of microfossil biostratigraphy and put between HAYDEN’S (1904) horizons 6 and 7 (unit P/13 and P/14
sensu Suttner 2007). It turned out that the entire Takche Member, which is correlated with HAYDEN’S
horizons 2 to 6, yield conodonts referring to the Amorphognathus ordovicicus Zone (Katian, Upper
Ordovician). However, age assignment for the other two members is still uncertain. Based on some few
ozarkodinid conodont specimens an early Silurian (Llandovery?) age is suggested for the Mikkim Member.
Hirnantian deposits are missing in this area, which is linked to the global lowstand. Solely a mixed
conodont fauna (Ordovician and Silurian species) is observed in the uppermost part of unit P/13 (compare
SUTTNER et al. 2007).
Apart from conodonts, abundant fossils, such as foraminifera, stromatoporoids, corals, gastropods,
orthocerids, trilobites, tentaculites, brachiopods, bryozoans and graptolites, have been known from the
Takche Member (e.g. REED 1912; BHARGAVA & BASSI 1986, 1998; SUTTNER & ERNST 2007). Quite well
preserved tabulate and rugose corals are documented from the units P/7, 8, 10, 11 and 13 (Takche Member)
and P/16 (Mikkim Member). The range chart in Fig. 1 shows that most of them appeared within the late
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