What is the reason for the preservation of fish skeletons?

The generally excellent preservation of fish skeletons is ascribed to early cessation of decay after death, most probably due to the growth of microbial mats.

What is the thickness of the Cassina beds?

The Cassina beds bear a large number of discrete bentonite layers, derived from the alteration of volcanic ash, ranging in thickness from \\1 mm to 5 cm within the studied section.

What is the reason for the occurrence of microbial mats?

Proliferation and preservation of microbial mats may have been promoted by the absence of ecological competition, such as grazing and bioturbation by macrofaunal biota.

What is the main reason for the cyclicity of the Cassina beds?

The presence of microfabrics possibly related to benthic microbial mats suggests that also in the basin of the Cassina beds a major part of the organic matter was produced by bacteria.

What is the reason for the varying distribution of the plant remains?

The varying three-dimensional patchy (laterally and vertically within the sediment) distribution is likely to be due to the effects of varying microenvironments and reproduction.

How did the author benefit from the help of colleagues and students?

During the first 3 years of excavations in the upper part of the Cassina beds, the author benefited from the help of colleagues and students from Swiss, German and Italian universities.

Why does postmortem buoyancy not apply to Triassic marine fishes?

postmortem buoyancy hardly applies to Triassic marine fishes, owing to their heavy ganoid scales, which prevented the carcassesfrom rising due to the accumulation of decay gas (Tintori 1992).

What is the opportunistic behaviour of the ostracods?

the observed vertical distribution, marked by irregular occurrences and densities throughout the studied section, suggests an opportunistic behaviour.

What is the description of the conditions that have been suggested as having fostered the episodic?

Fluctuating anoxic to temporarily suboxic (sensu Tyson and Pearson 1991) conditions are suggested as having fostered the episodic colonization of the seafloor by the extremely lowoxygen tolerant thin-shelled foraminifers.

What is the common type of carbonate nodules?

These carbonate nodules are flattened, usually 2–4 cm in diameter, and are sometimes affected by patchy replacement by dolomite or chalcedonic quartz.

What types of lithofacies are found in the turbidite ?

This lithofacies includes calcarenites, micritic limestones and marly limestones, and ranges in thickness from 1 mm to 4 cm within the studied section.

What are the primary depositional structures of the fine-grained micritic and marly?

Primary depositional structures of the fine-grained micritic and marly limestones (wackestones to lime mudstones, less frequently packstones; Fig. 4g, h) are not always visible owing to recrystallization and diagenetic dolomite formation.

What can be done to improve the understanding of the palaeogeography of the Cassi?

Comparative taphonomical studies of the same fossil species throughout the entire Cassina beds can further characterize the biostratinomic evolution in relation to changing bottom-water conditions.

What is the skeletal structure of Archaeosemionotus?

Archaeosemionotus (Fig. 6a, c) is characterized by a peculiar skull composed of a complex mosaic of small bones, which make it especially vulnerable to decaying processes.

(Open Access) Facies, depositional environment, and palaeoecology of the Middle Triassic Cassina beds (Meride Limestone, Monte San Giorgio, Switzerland) (2010) | Rudolf Stockar

Q: What are the contributions in "Facies, depositional environment, and palaeoecology of the middle triassic cassina beds (meride limestone, monte san giorgio, switzerland)" ?

In 2006, the Museo Cantonale di Storia Naturale ( Lugano ) started a new research project focusing for the first time on microfacies, micropalaeontological, palaeoecological and taphonomic analyses.

Facies, depositional environment, and palaeoecology of the Middle

Triassic Cassina beds (Meride Limestone, Monte San Giorgio,

Switzerland)

Rudolf Stockar

Received: 18 June 2009 / Accepted: 20 January 2010 / Published online: 28 May 2010

 Swiss Geological Society 2010

Abstract The Ladinian Cassina beds belong to the fos-

siliferous levels of the world-famous Middle Triassic

Monte San Giorgio Lagersta

tte (UNESCO World Heritage

List, Canton Ticino, Southern Alps). Although they are a

rich archive for the depositional environment of an

important thanatocoenosis, previous excavations focused

on vertebrates and particularly on marine reptiles. In 2006,

the Museo Cantonale di Storia Naturale (Lugano) started

a new research project focusing for the ﬁrst time on

microfacies, micropalaeontological, palaeoecological and

taphonomic analyses. So far, the upper third of the

sequence has been excavated on a surface of around 40 m

and these new data complete those derived from new

vertebrate ﬁnds (mainly ﬁshes belonging to Saurichthys,

Archaeosemionotus, Eosemionotus and Peltopleurus),

allowing a better characterization of the basin. Background

sedimentation on an anoxic to episodically suboxic seaﬂoor

resulted in a ﬁnely laminated succession of black shales

and limestones, bearing a quasi-anaerobic biofacies, which

is characterized by a monotypic benthic foraminiferal

meiofauna and has been documented for the ﬁrst time from

the whole Monte San Giorgio sequence. Event deposition,

testiﬁed by turbidites and volcaniclastic layers, is related to

sediment input from basin margins and to distant volcanic

eruptions, respectively. Fossil nekton points to an envi-

ronment with only limited connection to the open sea.

Terrestrial macroﬂora remains document the presence of

emerged areas covered with vegetation and probably

located relatively far away. Proliferation of benthic

microbial mats is inferred on the basis of microfabrics,

ecological considerations and taphonomic (both biostrati-

nomic and diagenetic) features of the new vertebrate ﬁnds,

whose excellent preservation is ascribed to sealing by

bioﬁlms. The occurrence of allochthonous elements allows

an insight into the shallow-waters of the adjoining time-

equivalent Salvatore platform. Finally, the available bio-

stratigraphic data are critically reviewed.

Keywords Cassina beds  Middle Triassic 

Monte San Giorgio  Facies  Palaeoecology 

Depositional environment

Introduction and geological setting

The Middle Triassic sequence at Monte San Giorgio

(Figs. 1, 2), inscribed in 2003 on the UNESCO World

Heritage List because of its unique palaeontological

value, consists of several distinct formations, overlying a

pre-Permian metamorphic basement and a Permian vol-

canic succession. The lowermost formation is the Bellano

Formation, a ﬂuvio-deltaic sequence dated to the late

Anisian (Illyrian) on the basis of its palynological content

(Sommaruga et al. 1997). During the late Anisian a pro-

gressive transgression of a shallow epicontinental sea

from the east took place, in consequence of which car-

bonate platforms began to grow (Furrer 2001a) north of

an emerged land buried today under the Po Plain (Brusca

et al. 1981; Picotti et al. 2007). Dolomitized microbial

Editorial handling: Daniel Marty & Andreas Wetzel.

R. Stockar (&)

Museo Cantonale di Storia Naturale, Viale Cattaneo 4,

6900 Lugano, Switzerland

e-mail: rudolf.stockar@ti.ch

R. Stockar

Institut de Ge

ologie et Pale

ontologie, Universite

de Lausanne,

Anthropole, 1015 Lausanne, Switzerland

Swiss J Geosci (2010) 103:101–119

DOI 10.1007/s00015-010-0008-2

limestones, characterized by stromatolitic laminations

(Lower Salvatore Dolomite), were deposited in a shallow

subtidal to intertidal environment. During the latest Ani-

sian and Ladinian, a 30–100 m deep intra-platform basin

with restricted circulation developed in the Monte San

Giorgio area (Bernasconi 1994; Furrer 1995), whereas in

the north shallow-water sedimentation continued (Middle

and Upper Salvatore Dolomites; Zorn 1971). Extensional

tectonics have been suggested as a controlling factor for

the intra-platform basin (e.g. Zorn 1971; Furrer 1995)but

conclusive evidences are missing in the Monte San

Giorgio/Monte San Salvatore area, where no Triassic

tectonic structures are recognizable (Bernasconi 1994). In

the Monte San Giorgio area, the Besano Formation was

deposited on top of the Lower Salvatore Dolomite

(‘‘Grenzbitumenzone’’; Frauenfelder 1916), and consists

of an up to 16 m thick alternation of black shales and

dolomites, including in its uppermost part the Anisian–

Ladinian boundary (Ro

hl et al. 2001). A volcanic ash

layer lying a few metres below this boundary yielded an

U–Pb minimum age of 241.2 ± 0.8 Ma (Mundil et al.

1996; Brack et al. 2007). Most of the spectacular verte-

brate fossils (reptiles and ﬁshes) together with important

index fossils, including ammonoids and daonellid

bivalves, come from this formation (Rieber 1969, 1973;

Kuhn-Schnyder 1974;Bu

rgin 1992). The Besano Forma-

tion grades upwards into the approximately 60 m thick

San Giorgio Dolomite followed by the 400–600 m thick

Meride Limestone (Frauenfelder 1916; Furrer 1995). The

Besano Formation, the San Giorgio Dolomite and the

Meride Limestone were deposited in the same basin,

whose east–west extension is estimated to have been

about 10 km or even 20 km if they were settled in the

same basin as the Perledo-Varenna Formation (Gianotti

and Tannoia 1988; Bernasconi 1994). As the Meride

Limestone is considered to be the source rock of the oil

of Trecate, Villa Fortuna and Gaggiano ﬁelds (Italy;

Picotti et al. 2007) the intra-platform basin where it was

deposited extended southwards for over 60 km. The

Besano Formation and the dasycladalean-rich Middle

Salvatore Dolomite are roughly coeval, representing a

basinal and a shallow-water facies, respectively. The San

Giorgio Dolomite and the Meride Limestone are time-

equivalent to the Upper Salvatore Dolomite, which shows

the same intertidal to shallow subtidal facies as the

Middle Salvatore Dolomite from which it is distinguished

by a marker horizon bearing small bivalves (Zorn 1971).

The Ladinian Lower Meride Limestone, 90 m (Wirz

1945) to 150 m thick (Furrer 1995) consists of well-

bedded limestones and marlstones with very rare biotur-

bation structures. As a whole, the Lower Meride

Limestone is interpreted as a sequence of lime mud

turbidites deposited under anoxic to dysoxic conditions

(Bernasconi 1994). Three levels (Cava inferiore beds,

Cava superiore beds and Cassina beds), consisting of

ﬁnely laminated limestones and black shales with

Lower Salvatore Dolomite

San Giorgio Dolomite

Besano Formation

50m

Meride

Limestone

Switzerland

Italy

Arzo

Meride

Besano

Serpiano

Porto Ceresio

Poncione d'Arzo

Monte San Giorgio

Lake Lugano

Cassina beds / locality

Cassina

45°54' N

E '

Fig. 1 Map of the Monte San

Giorgio area showing the

Middle Triassic carbonate

sequence and the location of the

excavation site of the Cassina

beds near Cassina. Scale bar

1km

102 R. Stockar

intercalated volcanic ash layers, are present in the upper

part and yield different vertebrate assemblages. Organic

carbon content averages around 1% TOC (Picotti et al.

2007) but it reaches up to 20% in thin black shale lay-

ers (Bernasconi 1994). The top of the Lower Meride

Limestone is deﬁned by a dolomite marker bed

(‘‘Dolomitband’’; Frauenfelder 1916), reaching a thickness

of about 30 m (Wirz 1945; Furrer 1995). The overlying

Upper Meride Limestone is a sequence of alternating

well-bedded limestones and marlstones with an increasing

clay content towards the top. The uppermost part is the

120 m thick ‘‘Kalkschieferzone’’, made up of thin-bedded,

laminated marlstones and claystones with peculiar ﬁsh

faunas, crustaceans and arthropods. It represents the late

evolution of the intra-platform basin, recording strong

seasonal variations of salinity and water level and

cissairT etaLcissairT elddiMnaimreP

Liassic

Anisian

Rhyolithe and associated volcaniclastics

Nothosauria

Chondrichthyes

Reptiles

Ichthyosauria

Placodontia

Crossopterygii

Actinopterygii

Saurichthys

(other than Saurichthys)

Ticinosuchus

Protorosauria

Bellano Fm.

Cassina beds

Cava superiore beds

Cava inferiore beds

San Giorgio Dolomite

Kalkschieferzone

Pizzella Marls

Meride Limestone

Dolomia Principale

Tremona Series

Macchia Vecchia / Broccatello

Moltrasio Fm.

"Dolomitband"

Lower Salvatore Dolomite

Besano Fm.

"Grenzbitumenzone"

Rhaetian

Carnian

Ladinian

Norian

Age diagnostic fossils

241.2 +/- 0.8 Ma

(U-Pb age; Mundil et al. 1996)

Arpadites arpadis (Wirz 1945; Furrer 2001a;

Furrer et al. 2008)

Daonella moussoni (Wirz 1945; Schatz 2001, 2005b)

Echinitosporites iliacoides (Scheuring 1978)

100 m

0 m

Scattered/poorly documented occurrences

Fishes

Fig. 2 Stratigraphic section of

the Triassic sediments in the

Monte San Giorgio area

(modiﬁed and updated after

Furrer 1995). Vertebrate-

bearing beds and index fossils

of the Meride Limestone are

indicated

Cassina beds of the Monte San Giorgio 103

progressively buried by an increasing input of siliciclastic

material (Furrer 1995).

On the Swiss side of Monte San Giorgio, from 1994 to

2005 a team from the Palaeontological Institute and

Museum of the University of Zurich (PIMUZ), led by

H. Furrer, studied a section of the Kalkschieferzone and

both the Cava inferiore and the Cava superiore beds (Furrer

1995, 1999a, b, 2001a, 2003). Another team from the

University of Milan (UNIMI), led by A. Tintori, carried out

excavations in the Kalkschieferzone between 1997 and

2003 (Krzeminski and Lombardo 2001; Lombardo 2002;

Lombardo and Tintori 2004; Tintori and Lombardo 2007).

All these projects were co-ordinated and ﬁnancially sup-

ported by the Museo Cantonale di Storia Naturale

(Dipartimento del territorio del Cantone Ticino), the

institution in charge of the management of the research

activities as well as the housing of fossil specimens from

the Monte San Giorgio UNESCO WHL site.

After conclusion of these projects, in 2006 the Museo

Cantonale di Storia Naturale (Lugano) opened a new

excavation in the Cassina beds, in order to investigate bed

by bed the fossiliferous sequence. The new site (Swiss

National Coordinates: 716’990/85’150; Fig. 1) is located

about 50 m north of the outcrop excavated in the 1970s by

the PIMUZ (see further below). Here, the Cassina beds

represent an almost 3 m thick sequence, the upper third of

which has so far been excavated on a surface of around

40 m

, passing upwards into thick-bedded dolomitic lime-

stones and dolomites. The studied section mainly consists of

interbedded ﬁnely laminated, organic-rich shales and

limestones with intercalated thicker micritic limestones and

tephra layers. The preliminary results of the ﬁrst 3 years are

presented here. The collected material is deposited at the

Museo Cantonale di Storia Naturale, Lugano (MCSN).

Historical remarks and previous ﬁnds

The Cassina beds are named after the locality lying to the

south of the Monte San Giorgio summit, where they were

discovered in 1933 by the PIMUZ technical assistant Fritz

Buchser. A ﬁrst excavation was started in the same year by

the PIMUZ, which carried out further excavations in 1937,

in 1971–1973 and in 1975 (Furrer 2003). The excavations

yielded reptile fossils belonging to four species (Kuhn-

Schnyder 1974): many pachypleurosaurids ascribed to

Neusticosaurus edwardsii (Carroll and Gaskill 1985;

Sander 1989), three specimens of the large nothosaurid

‘‘ Ceresiosaurus’’ lanzi (Rieppel 1998, 2007;Ha

nni 2004)

and two protorosaurs belonging to Macrocnemus bassanii

(Peyer 1937) and to Tanystropheus meridensis (Wild

1980), respectively. The latter, a largely incomplete juve-

nile specimen, was regarded as conspeciﬁc with

Tanystropheus longobardicus by Nosotti (2007). The

associated ﬁsh fauna turned out to be dominated by

the basal actinopterygian Saurichthys (S. curionii and

S. macrocephalus; Rieppel 1985). In addition, smaller

actinopterygians ascribed to three different genera were

also reported: Peltopleurus sp. (a single specimen descri-

bed in Bu

rgin 1992), Archaeosemionotus sp. nov. and

Macrosemiidae gen. et sp. nov. (both cited in Bu

rgin 1999

but undescribed and not ﬁgured so far). Reported micro-

fossils include ostracods, foraminifers (Glomospira sp.,

Trochammina sp., Dentalina sp.; Wirz 1945) and palyno-

morphs (Scheuring 1978).

Review of the biostratigraphic setting

The Cassina beds are commonly regarded as early Ladinian

in age (e.g. Hellmann and Lippolt 1981; Carroll and

Gaskill 1985; Rieppel 1998) but the exact position of the

early/late Ladinian boundary within the Meride Limestone

is as yet unknown. Recently, Furrer et al. (2008, p. 602)

tentatively correlated the Cassina beds with the lowermost

Wengen Formation of the GSSP section at Bagolino

(P. archelaus zone, late Ladinian; Brack et al. 2005).

Some biostratigraphic considerations regarding the

Lower Meride Limestone are summarized here. Typical

age-diagnostic fossils such as ammonoids and conodonts,

requiring stenohaline surface waters, are rare within the

Meride Limestone, a situation usually observed for land-

ward basins restricted from open sea waters. In the

lowermost part of the sequence, below the Cava inferiore

beds, the following index fossils have so far been reported

by previous authors (Fig. 2).

• Daonella moussoni (Frauenfelder 1916, p. 279; Senn

1924, p. 563; Wirz 1945, p. 49; Schatz 2005a, p. 192;

Schatz 2005b, p. 100). Rieber (1969) attributed the

material from the Frauenfelder and Wirz collections to

Daonella aff. moussoni, whereas Schatz (2001, 2005b)

considered Daonella aff. moussoni and D. moussoni as

conspeciﬁc.

• Protrachyceras archelaus (Frauenfelder 1916, p. 279;

Wirz 1945, p. 53) and Protrachyceras cf. archelaus

(Senn 1924, p. 563). The material from the Wirz

collection was redetermined by H. Rieber as Protr-

achyceras cf. ladinum (Schatz 2001).

• Arpadites arpadis (Wirz 1945, p. 54).

In the Cava superiore beds, about 40 m above the

horizon mentioned previously and around 30 m below the

Cassina beds, specimens of Arpadites cf. arpadis were

collected by H. Furrer during the 1997–2005 excavations in

collaboration with the MCSN (Furrer 2001a; Furrer et al.

2008). In the Southern Alps, ammonoids belonging to

104 R. Stockar

Arpadites range from the P. gredleri to the P. archelaus

zone (Brack and Rieber 1993). In the GSSP section at

Bagolino (Buchenstein Fm.) A. arpadis occurs at 70.5–

75.6 m, that is below and above the dated Middle Pietra

Verde volcaniclastic layer (72.2 m, minimum U–Pb age of

238.8 ?0.5/-0.2 Ma), within the early Ladinian P. gredleri

zone (Mundil et al. 1996; Brack et al. 2005, 2007).

D. moussoni and Protrachyceras cf. ladinum indicate the

P. gredleri zone as well (Schatz 2001). Accordingly, the

Lower Meride Limestone up to and including the Cava

superiore beds can conﬁdently be attributed to the early

Ladinian P. gredleri zone.

Ammonoids and daonellids have so far not been reported

from the Cassina beds, making an age assignment more

difﬁcult. However, Scheuring (1978) reported two speci-

mens of the palynomorph Echinitosporites iliacoides, the

ﬁrst one from his sample 90, coming from the excavation

led at Cassina by E. Kuhn-Schnyder from 1971 to 1975, and

the second one from his sample 103/106 (lower Kalks-

chieferzone; Fig. 2). E. iliacoides is an important marker

fossil with a stratigraphic range restricted to the secatus–

dimorphus phase of Van der Eem (1983) and according to

recent data from the Seceda core (Buchenstein Fm.) it is

limited to the early late Ladinian P. archelaus zone

(Hochuli and Roghi 2002). Moreover, on the basis of the

occurrence of E. iliacoides, the Cassina beds correlate with

the uppermost Reiﬂing Formation (Northern Calcareous

Alps), which is assigned to the P. archelaus zone as well

(Kulm section; Bru

hwiler et al. 2007). In conclusion,

according to available biostratigraphic data an age assign-

ment of the Cassina beds to the early Longobardian

(P. archelaus zone; early late Ladinian) cannot be ruled out.

Lithofacies description and interpretation

The classiﬁcation of Dunham (1962) is followed here,

keeping in mind that the greatest problem with this scheme

lies in its strong relation to the depositional character of the

textures, which may be affected by recrystallization and

replacement, sometimes obliterative (Flu

gel 2004). Three

distinct lithofacies groups are intercalated as thin to med-

ium beds throughout the studied section (Fig. 3). All these

are overlain by the dolomite lithofacies.

Laminite lithofacies

Description

Finely laminated, generally ﬁssile, organic-rich black

shales and limestones constitute the main part of the

studied section and reﬂect the background sedimentation

Marly limestones

Laminated organic-rich

black shales

Volcaniclastic layers

Calcarenites

Interbedded, laminated

organic-rich black shales

and limestones

Micritic limestones

Dolomitic limestones

Dolomite lithofacies

(above Cassina beds)

Laminite lithofacies

Turbidite lithofacies

Tephra lithofacies

0.1m

0.0m

1-2

3-5

6-15

>15

Present

Common

Abundant

Very abundant

Number of specimens

on the bed surface (40 m )

(vertebrates, plants)

Frequence on the bed surface

(coprolites, nodules) or in thin

section (microfossils)

Saurichthys

spp.

Eosemionotus sp.

Peltopleurus sp.

Actinopterygii ind.

Actinopterygii isolated scales and teeth

Neusticosaurus

isolated bones

Neusticosaurus sp.

Ceresiosaurus isolated teeth

Thin-shelled nodosariids

Ostracods

Land plants

Fish and reptile coprolites

Platform-derived nodules

Archaeosemionotus sp.

Fig. 3 Detailed sedimentological log of the investigated upper part

of the Cassina beds with indication of the distribution and abundance

of macro- and microfossils

Cassina beds of the Monte San Giorgio 105

Facies, depositional environment, and palaeoecology of the Middle Triassic Cassina beds (Meride Limestone, Monte San Giorgio, Switzerland)

Figures

Citations

Dolomite formation in the shallow seas of the Alpine Triassic

Integrated Ladinian bio-chronostratigraphy and geochrononology of Monte San Giorgio (Southern Alps, Switzerland)

Reassessment of the Middle Triassic rauisuchian archosaurs Ticinosuchus ferox and Stagonosuchus nyassicus

Death, decay and disarticulation: Modelling the skeletal taphonomy of marine reptiles demonstrated using Serpianosaurus (Reptilia; Sauropterygia)

Early Middle Triassic trace fossils from the Luoping Biota, southwestern China: Evidence of recovery from mass extinction

References

Classification of Carbonate Rocks According to Depositional Textures

Microfacies of Carbonate Rocks: Analysis, Interpretation and Application

Ecology and Applications of Benthic Foraminifera

Microfacies of Carbonate Rocks

Cycles and events in stratigraphy

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Frequently Asked Questions (16)

Q1. What are the contributions in "Facies, depositional environment, and palaeoecology of the middle triassic cassina beds (meride limestone, monte san giorgio, switzerland)" ?

Q2. What is the reason for the preservation of fish skeletons?

Q3. What is the thickness of the Cassina beds?

Q4. What is the reason for the occurrence of microbial mats?

Q5. What is the main reason for the cyclicity of the Cassina beds?

Q6. What is the reason for the varying distribution of the plant remains?

Q7. How did the author benefit from the help of colleagues and students?

Q8. Why does postmortem buoyancy not apply to Triassic marine fishes?

Q9. What is the average organic carbon content of the lower meride limestone?

Q10. What is the opportunistic behaviour of the ostracods?

Q11. What is the description of the conditions that have been suggested as having fostered the episodic?

Q12. What is the common type of carbonate nodules?

Q13. What types of lithofacies are found in the turbidite ?

Q14. What are the primary depositional structures of the fine-grained micritic and marly?

Q15. What can be done to improve the understanding of the palaeogeography of the Cassi?

Q16. What is the skeletal structure of Archaeosemionotus?