Integrated Ladinian bio-chronostratigraphy and geochrononology
of Monte San Giorgio (Southern Alps, Switzerland)
Rudolf Stockar
•
Peter O. Baumgartner
•
Daniel Condon
Received: 18 November 2011 / Accepted: 14 March 2012 / Published online: 15 May 2012
Ó Swiss Geological Society 2012
Abstract New biostratigraphic data significantly improve
the age assignment of the Ladinian succession of Monte
San Giorgio (UNESCO World Heritage List site, Southern
Alps, Switzerland), whose world-famous fossil marine
vertebrate faunas are now dated to the substage and zone
levels. High-resolution single-zircon U–Pb dating was
performed using ID-TIMS and chemical abrasion (CA)
pre-treatment technique on volcanic ash layers intercalated
in the biostratigraphically-defined intervals of the Meride
Limestone. It yielded ages of 241.07 ± 0.13 Ma (Cava
superiore beds, P. gredleri Zone), 240.63 ± 0.13 Ma
(Cassina beds, P gredleri/P. archelaus transition Zone) and
239.51 ± 0.15 Ma (Lower Kalkschieferzone, P. archelaus
Zone). Our results suggest that the time interval including
the vertebrate-bearing Middle Triassic section spans
around 4 Myr and is thus significantly shorter than so far
assumed. The San Giorgio Dolomite and the Meride
Limestone correlate with intervals of the Buchenstein
Formation and the Wengen Formation in the reference
section at Bagolino, where the Global boundary Stratotype
Section and Point (GSSP) for the base of the Ladinian was
defined. The new radio-isotopic ages of the Meride
Limestone are up to 2 Myr older than those published for
the biostratigraphically-equivalent intervals at Bagolino
but they are consistent with the recent re-dating of the
underlying Besano Formation, also performed using the
CA technique. Average sedimentation rates at Monte San
Giorgio are by more than an order of magnitude higher
compared to those assumed for the Buchenstein Formation,
which formed under sediment-starved pelagic conditions,
and reflect prevailing high subsidence and high carbonate
mud supply from the adjoining Salvatore/Esino platforms.
Finally, the high-resolution U–Pb ages allow a correlation
of the vertebrate faunas of the Cava superiore/Cava infe-
riore beds with the marine vertebrate record of the Prosanto
Formation (Upper Austroalpine), so far precluded by the
poor biostratigraphic control of the latter.
Keywords Ladinian Monte San Giorgio
Meride Limestone Buchenstein Formation
Biostratigraphy Geochronology U–Pb Ash bed
Zircon
Abbreviation
MCSN Museo cantonale di storia naturale, Lugano,
Switzerland
1 Introduction
The Middle Triassic succession of Monte San Giorgio
(Southern Alps, Switzerland/Italy; Fig. 1) has been inscri-
bed on the UNESCO World Heritage List because of its
unique palaeontological value and it is, in particular,
world-famous for the exceptionally well-preserved fossil
fishes and marine reptiles. Although this section is a rich
Editorial Handling: Daniel Marty.
R. Stockar (&) P. O. Baumgartner
Institut de Ge
´
ologie et Pale
´
ontologie, Universite
´
de Lausanne,
Anthropole, 1015 Lausanne, Switzerland
e-mail: rudolf.stockar@ti.ch
R. Stockar
Museo Cantonale di Storia Naturale, Viale Cattaneo 4,
6900 Lugano, Switzerland
D. Condon
NERC Isotope Geosciences Laboratory, British Geological
Survey, Nottingham NG12 5GG, England, UK
Swiss J Geosci (2012) 105:85–108
DOI 10.1007/s00015-012-0093-5
archive for the depositional environment of thanatocoe-
noses, which had (and still have) a major impact on the
Middle Triassic palaeontology, the lack of sufficiently
precise bio- and chronostratigraphic constraints still pre-
cludes a quantified assessment of both the basin and biota
evolution. A unique exception is the upper Anisian-low-
ermost Ladinian Besano Formation, which provided a
detailed age-diagnostic ammonoid and bivalve record and
also yielded high-resolution radio-isotopic age data (Brack
and Rieber 1993; Brack et al. 2005; Mundil et al. 2010). By
contrast, the scarce age-diagnostic fossils so far reported
from the overlying San Giorgio Dolomite and Meride
Limestone formations provide only an uncomplete bio-
stratigraphic framework while reliable radio-isotopic ages
are not available at all. Consequently, conclusions on
topics, which are crucial for this Lagersta
¨
tte, such as the
assessment of evolutionary rates of its fossil vertebrate
faunas, remain highly speculative.
U–Pb isotopic dating using isotope dilution thermal
ionization mass spectrometry (ID-TIMS) on single zircon
grains from volcaniclastic layers intercalated within fossil-
bearing marine sediments provides at present the best
means of directly cross-correlating biostratigraphic and
geochronologic data. We apply this technique, coupled
with the annealing/chemical abrasion (CA) pre-treatment
procedure of Mattinson (2005), to the Ladinian sequence of
Monte San Giorgio, which, in addition, has been con-
strained by new biostratigraphic data.
The main goals of this study are the following:
1. to re-define the biostratigraphic subdivisions of the
Ladinian sequence at Monte San Giorgio and to
correlate it with the reference section at Bagolino
(northern Italy), bearing the GSSP for the base of
Ladinian,
2. to resolve the age of the world-famous marine
vertebrate faunas of the Meride Limestone to the
substage and ammonoid zone level,
3. to correlate the fossil vertebrate faunas of the Meride
Limestone with analogous faunas from the Upper
Australpine realm which, although lacking a sufficient
biostratigraphic control, are well-constrained by high-
resolution radio-isotopic dating,
4. to assess the average sedimentation rates of the basinal
deposits composing the San Giorgio Dolomite and the
Meride Limestone,
5. to provide new high-resolution radio-isotopic ages for
the Ladinan Stage and to compare them with the
available geochronologic data.
2 Geological setting
The Monte San Giorgio belongs to the Southern Alps, a
fold-and-thrust belt extending over an area of 500 km (east
to west) by 50–150 km (north to south) and bounded by the
Lake Lugano
Lake Lugano
45°54' N
1 Km
1 0 Km
8°57' E
RS09/5a
Ca31
RS09/4
a
Porto Ceresio
Besano
Serpiano
Meride
Monte San Giorgio
a
b
Va
l Serrat
a
Va
l Porina
Val Mara
Fig. 1 a Simplified map of Monte San Giorgio showing the Middle Triassic carbonate succession and the location of the radio-isotopically dated
samples. b Location of the Middle Triassic sections mentioned in the text
86 R. Stockar et al.
Insubric/Periadriatic Lineament to the north and west. To
the east, the Southern Alps continue into the external
Dinarides, whereas, to the south, the front of the South-
Alpine wedge is buried below the post-orogenic deposits of
the Po Plain. In Middle Triassic times, the South-Alpine
domain was situated on a passive continental margin open
to the tropical western Neo-Tethys (e.g. Stampfli and Borel
2002), which was progressively submerged by a long-term
transgression from the east. The marine ingression reached
the eastern South-Alpine domain in the Late Permian and
the westernmost (i.e. west of Lake Como) South-Alpine
domain in late Anisian times. The palaeogeographic sce-
nario was complicated by intensive Middle Triassic
transtensive–transpressive tectonics, which resulted in a
structural compartmentalization consisting in repeatedly
uplifted and subsiding blocks. The geodynamic signifi-
cance of these events is still debated and as puzzling as the
character of the time-equivalent magmatism (see further
below). However, even if unquestionably affecting the
crustal structure of the Southern Alps, this tectonic activity
does not seem to be related to the Late Triassic–Early
Jurassic rifting of the Alpine Tethys and the opening of the
Ligurian Ocean (Bertotti et al. 1993). The increasing dif-
ferentiation of depositional environments occurring from
late Anisian times onwards resulted in the development of
a rapidly changing pattern of carbonate platforms, locally
anoxic intraplatform basins and open-marine pelagic
basins (Brack and Rieber 1993). Close to the base of the
E. curionii Amm. Zone (earliest Ladinian), water depths
reached maximum values (around 800 m) in the Dolomites
(eastern Southern Alps), while they were likely shallower
towards the western Southern Alps (Brack and Rieber
1993; Brack et al. 2005).
2.1 The Middle Triassic sequence of Monte San
Giorgio
The Monte San Giorgio basin is located at the western
termination of the South-Alpine domain described above.
This unique ‘‘remote’’ location resulted in a peculiar
sedimentary succession and in at least temporarily severe
dysoxic to anoxic bottom water conditions (Kuhn-Schnyder
and Vonderschmitt 1954; Zorn 1971; Rieber 1973a;
Bernasconi 1994; Furrer 1995, 2003;Ro
¨
hl et al. 2001; Etter
2002; Stockar 2010). The Middle Triassic succession at
Monte San Giorgio (Fig. 2) starts with fluvio-deltaic
deposits dated to the late Anisian (Bellano Formation,
Illyrian; Sommaruga et al. 1997), unconformably overlying
a Lower Permian volcanic basement. The upper Anisian
sediments testify to the progressive transgression of a
shallow sea from the east and to the initiation of carbonate
platform growth (Salvatore Dolomite/Esino Limestone;
Zorn 1971) north of a land area buried today below the Po
plain (Picotti et al. 2007). Dolomitized microbial lime-
stones, characterized by stromatolitic lamination, were
deposited in a shallow subtidal to intertidal environment
(Lower Salvatore Dolomite). While in the north and in the
east shallow-water sedimentation continued, during the
latest Anisian and Ladinian in the Monte San Giorgio area
the formation of an intraplatform basin with restricted
circulation resulted in the deposition of the Besano For-
mation, the San Giorgio Dolomite and the Meride
Limestone (Bernasconi 1994; Furrer 1995, 2003). The
Besano Formation (‘‘Grenzbitumenzone’’; Frauenfelder
1916) directly overlies the Lower Salvatore Dolomite and
is composed of a 16 m thick alternation of black shales and
laminated dolostones. Its uppermost part includes the
Anisian/Ladinian boundary (=base of the E. curionii Amm.
Zone; Brack and Rieber 1993; Brack et al. 2005). Most of
the spectacular vertebrate fossils (reptiles and fishes)
together with important index fossils including ammonoids
and daonellid bivalves come from this formation (e.g.
Rieber 1969, 1973b; Kuhn-Schnyder 1974;Bu
¨
rgin et al.
1989). The Besano Formation grades upwards into the San
Giorgio Dolomite and the Meride Limestone, all together
constituting an around 600 m thick sequence.
Fig. 2 Stratigraphic section of the Middle Triassic sediments in the
Monte San Giorgio area with the main vertebrate-bearing levels
(modified and updated from Furrer 1995). Radio-isotopically dated
volcanic ash layers are indicated. RS09/4, RS09/5a and Ca31: this
paper. MSG.09: Mundil et al. (1996, 2010)
Ladinian of Monte San Giorgio 87
The base of the San Giorgio Dolomite is defined at layer
187 of the standard profile at locality Mirigioli (Site P. 902;
Rieber 1973b; Bernasconi 1994) where the Besano For-
mation was excavated bed by bed between 1950 and 1968
and yielded exceptional vertebrate fossils (Kuhn-Schnyder
1974). The section is no longer accessible, but thickness
and lithofacies are laterally consistent enough to allow a
reliable correlation of single beds at different localities of
the Monte San Giorgio area. The transition from the
Besano Formation to the San Giorgio Dolomite is marked
by the disappearance of black shales, by a reduction of
laminated dolostones and by a major decrease in the
organic matter content (Bernasconi 1994). Sections in Val
Porina and Val Serrata (Fig. 1a) show that the San Giorgio
Dolomite results from early and late diagenetic dolomiti-
zation, the latter cutting across stratification and affecting
the original limestones in an irregular pattern up to a major
volcaniclastic bed (‘‘Val Serrata tuff’’) lying around 165 m
above the base of this unit (Fig. 2).
The Lower Meride Limestone chiefly consists of well-
bedded limestones. Most of the carbonate was likely derived
from the adjacent highly productive Salvatore/Esino car-
bonate platforms providing a constant supply of carbonate
mud to the basin. Three intervals (Cava inferiore beds, Cava
superiore beds and Cassina beds), mainly consisting of
finely laminated limestones with intercalated volcanic ash
layers, are present in the upper part of the Lower Meride
Limestone and yielded different vertebrate fossil assem-
blages (Peyer 1931; Sander 1989; Furrer 1995, 2003;
Stockar 2010). The top of the Lower Meride Limestone is
defined by a very irregular discontinuous dolomite horizon
(‘‘Dolomitband’’; Frauenfelder 1916), averaging around
30 m in thickness (Wirz 1945; Furrer 1995). The overlying
Upper Meride Limestone is a sequence of alternating well-
bedded limestones and marlstones, mainly representing
dilute lime mud turbidites. The uppermost part comprises
the 120 m thick ‘‘Kalkschieferzone’’ (Senn 1924), made up
of thin-bedded, mostly laminated, limestones and marl-
stones with peculiar faunas of fishes, reptiles, crustaceans
and insects. It represents the latest stage of the intra-plat-
form basin, recording strong seasonal variations of salinity
and water level, which was progressively buried by an
increasing input of siliciclastic material (Furrer 1995).
The east–west extension of the Monte San Giorgio basin
is estimated to have been about 10 km or up to 20 km if it
was located in the same basin as the Perledo–Varenna
Formation outcropping to the east of Lake Como (Gianotti
and Tannoia 1988; Bernasconi 1994). Basin depths are
regarded as varying between 30 and 130 m (Monte San
Giorgio basin; Zorn 1971; Rieber 1973a; Bernasconi 1994)
and 160–260 m (Perledo–Varenna Formation; Gaetani
et al. 1992). Further to the east, the depositional setting of
the Perledo–Varenna Formation was, in turn, separated
from the pelagic deposits of the ‘‘Buchenstein’’ facies by
shallow-water barriers (Esino Platform, an equivalent to
the Salvatore Platform in the lake Como region), only a
few meters deep (Gaetani et al. 1992). Finally, as the
Meride Limestone is considered to be the source rock of
the Trecate–Villafortuna oil fields (Po Plain; Picotti et al.
2007), the intra-platform basin system probably extended
southwards for over 60 km.
For the compilation of the composite section illustrated
in this paper, stratigraphic successions have been studied at
the cm-scale at the following localities (approximated
Swiss National Coordinates refer to the centre of the sec-
tions): Val Porina (716
0
600/85
0
100), Valle della Cassina
(717
0
150/84
0
950), Val Sceltrich (717
0
000/84
0
400), Val
Serrata (717
0
850/84
0
150 and 717
0
900/83
0
700) and Fontana
Fredda–Val Mara (717
0
050/83
0
350). The widespread
occurrence of volcanic ash beds, sometimes associated
with weathering-resistant silicified horizons, as well as the
lateral persistence of bedding patterns allow an unambig-
uous correlation of the studied sections. A major
unresolved uncertainty occurs at the base of the Kalk-
schieferzone (Val Mara; Fig. 1a), where a fault with
unknown displacement precludes to observe the transition
to the underlying part of the Upper Meride Limestone.
2.2 The Middle Triassic volcanic ash layers
in the Southern Alps
Middle Triassic pelagic sediments containing zircon-bear-
ing acidic volcaniclastic horizons, usually with a greenish
colour, are widespread in the Southern Alps. Sedimento-
logic and petrographic features indicate an air-borne origin
of the sand- to silt-sized particles of these so-called ‘‘Pietra
Verde’’ layers, which Brack et al. (2005) suggested to have
probably originated from eruption centres mainly located
outside the present Southern Alps. Beyond the Southern
Alps, comparable volcaniclastic layers occur in the Eastern
Alps (e.g. Bru
¨
hwiler et al. 2007; Furrer et al. 2008), in the
Brianc¸onnais unit of the Western Alps (Caby and Galli
1964) and further afield (e.g. Balaton Highland, Hungary;
Pa
´
lfy et al. 2003). Current interpretations of this wide-
spread volcanic activity are highly divergent but chiefly
relate it to the evolution of the Maliac–Meliata Ocean to
the east. According to these models, the volcanic episode
could result from the extension associated with the opening
of this ocean in a general strike-slip scenario or from the
subduction of its young lithosphere (or of a segment of the
Palaeo-Tethys) generating a volcanic arc on continental
crust (see Bernoulli 2007 for a review).
Whatever the origin of the Middle Triassic volcanic
episode, due to their lateral persistence these tephra layers
proved ideal markers for precise correlations of sections up
to 200 km apart (Brack and Rieber 1993; Brack and
88 R. Stockar et al.
Muttoni 2000; Bru
¨
hwiler et al. 2007). Moreover, starting in
1996 (Mundil et al. 1996), several of these horizons have
been dated by U–Pb single zircon techniques, allowing a
geochronologic framework to be established.
In the reference section at Bagolino (eastern Lombardy,
northern Italy), bearing the GSSP for the base of the
Ladinian Stage (Brack et al. 2005), Pietra Verde layers
occur throughout the Buchenstein Formation section but
they are chiefly concentrated in three intervals (Fig. 3).
Pietra Verde layers first appear in the upper part of the
interval transitional from the pelagic Prezzo Limestone to
the siliceous nodular limestones of the Buchenstein For-
mation and follow a first stack of beige weathering tuff
beds (‘‘Ta-tuffs’’). Millimetre- to a few decimetre-thick
greenish acidic ash beds concentrate at the 56–62, 68–76
and 82–92 m intervals, constituting the so-called Lower,
Middle and Upper Pietra Verde respectively. At the top of
the Upper Pietra Verde, a major change in sedimentation
occurs which marks the abrupt switch to the siliciclastic
Wengen Formation. Pietra Verde layers constitute a pow-
erful tool to correlate Buchenstein-type sections across the
Southern Alps (Brack and Rieber 1993; Brack and Muttoni
2000; Muttoni et al. 2004) and could be laterally safely
traced as far as the Dolomites where, at Seceda, lies the
principal auxiliary section for the GSSP.
At Monte San Giorgio, intercalations of volcanic ash
layers occur throughout the upper Anisian–Ladinian
section, i.e. from the Besano Formation to the Kalkschie-
ferzone of the Upper Meride Limestone. Only in the
uppermost part (Upper Kalkschieferzone) they are missing.
Their lateral persistence makes them a reliable tool for bed-
by-bed correlations among partial sections. The ash layers
are air-borne tuffs altered to bentonite, without any detrital
admixture; at least the thicker ones show a distinct grada-
tion with a sand-sized basal part. Bentonites are usually
easily detectable in outcrop as they weather to an orange
colour. Thickness usually ranges from less than one milli-
metre to few centimetres. In the reference profile of the
Besano Formation (Rieber 1973b; Bernasconi 1994) 36 ash
layers have been recognized, while in the overlying San
Giorgio Dolomite and Meride Limestone over thirty ash
layers, between a few millimetres to 50 cm thick, have been
reported by Wirz (1945). Sub-mm yellowish bentonite
seams, however, are very frequent in the Meride Limestone
but they can be detected only on fresh bedding surfaces, as
highlighted by bed-by-bed excavations (e.g. Stockar 2010).
Somewhat different is the so-called ‘‘Val Serrata tuff’’
(Lower Meride Limestone), a tuffite with a carbonate
cement representing one of the most reliable marker beds of
the sequence. It consists of a lower and an upper bed, up to 4
and 3 m thick respectively, bracketing the 1.5-m-thick
fossiliferous sequence informally called ‘‘Cava inferiore
beds’’ (Sander 1989; Furrer 1999a). Composition of the
volcanic ash layers is very similar throughout the section.
They consist of a grey microcrystalline matrix of mixed-
layer illite-smectite clay minerals containing phenocrysts of
alkali feldspar, quartz and sometimes euhedral biotite.
Fig. 3 Simplified stratigraphic log of Bagolino section (GSSP for the
base of the Ladinian Stage) with U–Pb-zircon ages (BAG samples;
Mundil et al. 1996) and distribution of ammonoid taxa co-occurring in
the Ladinian section of Monte San Giorgio (Brack and Rieber 1993;
Brack et al. 2005; Schatz 2005). Also shown are the correlatable
U–Pb zircon ages (SEC samples; Mundil et al. 1996) and biostrati-
graphic data on palynomorphs (Hochuli and Roghi 2002; Bru
¨
hwiler
et al. 2007) and daonellids (Schatz 2005) from Seceda, the principal
auxiliary section in the Dolomites. The GSSP is defined by the lowest
occurrence of the ammonoid Eoprotrachyceras curionii (base of the
E. curionii Amm. Zone); it lies at the 63.25-m level of the Bagolino
reference column. Ammonoid zonal scheme according to Brack and
Rieber (1993), with the Fassanian Substage comprising both the
E. curionii and P. gredleri Amm. Zones (Brack et al. 2005). LPV, MPV,
UPV Lower, Middle and Upper Pietra Verde intervals respectively
Ladinian of Monte San Giorgio 89