18th SwissSed Meeting, Fribourg, February 27, 2010
REFERENCESKoutsoukos, E. A. M., Leary, P.N., Hart, M. B. 1990. Latest Cenomanian–earliest Turonian low-oxygen tolerant foraminifera: a case-study from the Sergipe basin (N.E. Brasil) and the western Anglo-Paris Basin (southern England). Palaeogeography, Palaeoclimatology, Palaeoecology 77, 145–177.Mundil, R., Brack, P., Meier, M., Rieber, H. & Oberli, F. 1996. High resolution U-Pb dating of Middle Triassic volcaniclastics: time–scale calibration and veri�cation of tuning parameters for carbonate sedimentation. Earth and Planetary Science Letters 141, 137–151.Stockar, R. 2010. Facies, depositional environment, and palaeoecology of the Middle Triassic Cassina beds (Meride Limestone, Monte San Giorgio, Switzerland). Swiss Journal of Geosciences, 103, in press.Tyson, R.V. & Pearson, T.H. 1991. Modern and ancient continental shelf anoxia: an overview. In: Tyson, R.V. & Pearson, T.H. (Eds.): Modern and Ancient Continental Shelf Anoxia. Special Publication 58. Geological Society, London, 1–24.Wignall, P.B. & Hallam, A. 1991. Biofacies, stratigraphic distribution and depositonal models of British onshore Jurassic black shales. In: Tyson, R.V. & Pearson, T.H. (Eds.): Modern and Ancient Continental Shelf Anoxia. Spec. Publ. 58. Geol. Society, London, 291–309.
Not to scale!
Salvatore platform
Open sea Cassina beds
Thin-shelled nodosariidsEmerged area (land / island)covered with vegetation Shallow-water foraminifers
Dasycladalean algae
Saurichthys Eosemionotus
Peltopleurus Archaeosemionotus
PRELIMINARY CONCLUSIONSThe studied section of the Cassina beds records a continuous background sedimentation (laminite lithofacies) mirroring �uctuating but generally severely oxygen-depleted conditions on the bottom of a basin below wave base and adjacent to a shallow-water carbo-nate platform from which a recurrent carbonate supply reached the basin �oor, contributing to the sediment lamination.Episodic, short-lived depositional events occur randomly and are related to feeding from basin margins (turbidite lithofacies) and to volcanic activity (tephra lithofacies). Fluctuating anoxic to temporarily suboxic conditions are suggested to have fostered the tran-sient colonization of the sea�oor by an extremely low-oxygen tolerant foraminiferal meiofauna. However, either oxygen values were too low or the su�ciently oxygenated periods were too short to allow colonization by a more diverse benthic macrofauna. Oxygen-de�cient conditions are also consistent with the possible development of microbial mats which, in turn, may have contributed to pro-tect the vertebrate carcasses against disintegration (“microbial shroud” e�ect).
COMPARATIVE TAPHONOMYWithin the laminite lithofacies, �sh fossils display di�erent preservation patterns. Most of them are complete and preserved without, or with only partial, disarticulation. Even in the latter case, completeness of skeletons suggests that �sh reached the sea bottom as complete bodies soon after death, and that all decaying processes occurred on the sea�oor. As even anaerobic decay leads to disarticulation within short time, the prevailing articulated preservation may be due to the rapid growth of microbial mats and to the related bio-armouring of the carcasses (“microbial shroud” e�ect). Disarticulation pathways clearly vary between representatives of di�erent species but also within the same species. Archaeosemionotus is cha-racterized by a peculiar skull, composed of a complex mosaic of small bones, which makes it especially vulnerable to decaying processes, unlike the posterior part of the body, usually undisturbed. This genus proved particularly sensitive to biostra-tinomic processes and it is thus particularly suitable for comparative taphonomic analyses. Preliminary results show a transition from full skeletal articulation to partial and complete disarticulation. Ongoing investigations, requiring a large amount of �sh specimens to be mechanically prepared and cross-correlations with sediment analyses on single lamina scale, are expected to provide information about the relationships between preservation pathways and environmental condi-tions on the sea�oor.
Preser vation pathways in Archaeosemionotus
Fully articulated specimen. Only skull elements crushed.
MCS
N 8
073
MCS
N 8
009
mirr
or im
age
MCS
N 8
084
MCS
N 8
086
Severe skull and scale disarticulation
Nearly complete body disarticulation
Skulldisarticulation
1 cm 1 cm 1 cm 1 cm
LITHOFACIESThree lithofacies groups are intercalated throughout the studied section.LAMINITE LITHOFACIES. Finely lamina-ted organic-rich black shales and lime-stones constitute the main part of the section, and are characterized by a mi-crorhythmic pattern of irregular, wavy dark and bright laminae, 30 to 300 mi-crometres in thickness. This lithofacies re�ects the background sedimentation under severely oxygen-depleted condi-tions. In addition, it bears scattered car-bonate nodules composed of reworked shallow-water biota (foraminifers and dasycladalean algae).TEPHRA LITHOFACIES. Bentonite layers, derived from the alteration of volcanic ash and up to 5 cm thick, are easily de-tectable in outcrop since they weather to an orange colour. The absence of ad-mixed carbonate allochemical consti-tuents together with the widespread oc-currence of very thin (sub-mm) layers suggest an airborne origin.TURBIDITE LITHOFACIES. It includes cal-carenites, micritic limestones and marly limestones, ranging in thickness from 1 mm to 4 cm, and records event deposi-tion into the low-oxygen setting, mainly from dilute lime turbidity currents and detached lime mud-dominated turbidity currents due to water strati�cation.The DOLOMITE LITHOFACIES, overlying the Cassina beds, is interpreted as a partly dolomitized counterpart of the turbidite lithofacies.
Marly limestones
Laminated organic-richblack 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 specimenson the bed surface (40 m )(vertebrates, plants)
2
Frequence on the bed surface(coprolites, nodules) or in thinsection (microfossils)
Saur
icht
hys
spp
.
Eose
mio
notu
s sp
.Pe
ltopl
euru
s sp
.A
ctin
op
tery
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ind
.A
ctin
op
tery
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es a
nd
teet
h
Neu
stic
osau
rus
iso
late
d b
on
esN
eust
icos
auru
s sp
.
Cer
esio
saur
us is
ola
ted
teet
h
Thin
-sh
elle
d n
od
osa
riid
sO
stra
cod
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nd
pla
nts
Fish
an
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pti
le c
op
rolit
es
Plat
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-der
ived
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PALAEO-OXYGENATION. The background sedimentation completely lacks of benthic and nektobenthic macrofauna, and thus it can be assigned to the ORB 1 (Wignall & Hallam 1991), indicating anoxia sensu latu. On a �ner scale, the transiently occurring quasi-anaerobic biofacies suggests episodic suboxic bottom-water conditions (0.0–0.2 ml/l dissolved-oxygen range; Tyson & Pearson 1991). The upper limit of 0.2 ml/l is crucial because on the one hand it is the critical oxygen concentration below which macrofaunal bioturba-tion is suppressed, thus allowing the laminated fabric to be preserved, on the other hand it is still su�cient to sustain large populations of foraminifers with low oxygen requirements.
BACKGROUND SEDIMENTATIONPreservation of the �ne lamination (Fig. A) suggests a complete absence of both bioturbation and physical reworking. The laminated fabric may derive from benthic microbial mats generating a framework re-sembling that of a supple tissue. Fine platform-derived detrital particles, carried into the basin as su-spended load, could easily be trapped and �xed, con-tributing to the bright laminae (Fig. B).
Systematic micropalaeontological analyses of the background sedimentation revealed the episodic oc-currence of thin-shelled nodosariid foraminifers with elongate, randomly oriented, non-size-sorted tests (Figs. C-D). They are regarded as autochthonous, op-portunistic biota, tolerant to low oxygen concentra-tions. Such a quasi-anaerobic (sensu Koutsoukos et al. 1990) monotypic benthic biofacies is documented for the �rst time from Monte San Giorgio.
10 c
m
0.1 mm
0.1 mm
10 mm 0.1 mm
A
C
D
B
Polished section
Thin section, cut perpendicular to the lamination
Thin section, cut parallel to the lamination
Thin section
Lower Salvatore Dolomite
San Giorgio Dolomite
Besano Formation
50m
Mer
ide
Lim
esto
ne
Switzerland
Italy
Arzo
MerideBesano
Serpiano
Porto Ceresio
Poncione d'Arzo
Monte San Giorgio
Lake
Luga
no
Cassina beds / locality
Cassina
1 km
45°54' N
8°57
' E
Perm
ian
Tria
ssic
Liassic
An
isia
n
Rhyolithe and associated volcaniclastics
Bellano Fm.
Cassina bedsCava superiore beds
Cava inferiore beds
San GiorgioDolomite
Kalkschieferzone
Pizzella Marls
Mer
ide
Lim
esto
ne
Dolomia Principale
"Dolomitband"
Lower Salvatore Dolomite
Besano Fm.
Rh
aet
Car
nia
nLa
din
ian
No
rian
100 m
0 m
241.2 +/- 0.8 Ma(Mundil et al. 1996)
Tremona Series
This level is named after the locality lying to the south of the Monte San Giorgio summit, where it was discovered in 1933 by the PIMUZ (University of Zurich), which carried out subsequent excavations in 1937, in 1971–73 and in 1975. All these excavations focused on vertebrates, and particularly on marine reptiles but also an exceptionally preserved �sh fauna was brought to light.In 2006, the Museo Cantonale di Storia Naturale (Lugano) started a new research project focusing on microfacies, mi-cropalaeontological, palaeoecological and taphonomic analyses. So far, the upper third of the sequence has been ex-cavated on a surface of around 40 m2, and these new data supplement those derived from new vertebrate �nds (mainly represented by over 300 �sh specimens belonging to Saurichthys, Archaeosemionotus, Eosemionotus and Pel-topleurus), allowing a better characterization of the basin. Palynological data available in the literature suggest an Early Longobardian (early Late Ladinian) age, but a revision is currently in progress.
The “Cassina beds” are a three metre thick sequence belon-ging to the �ve vertebrate fossil-bearing levels of the Middle Triassic Monte San Gior-gio Lagerstätte (Canton Ticino, Southern Alps), inscribed in 2003 on the UNESCO World He-ritage List because of its unique palaeontological value.
Phot
o PI
MU
Z
Cassina, 1933
Phot
o PI
MU
Z
Cassina, 1971
Cassina, 2009
THE CASSINA BEDS (MIDDLE TRIASSIC, MONTE SAN GIORGIO)
IN AN OXYGEN-DEPLETED ENVIRONMENTBACKGROUND AND EVENT SEDIMENTATION
Rudolf Stockar 1, 2Museo cantonale di storia naturale, viale Cattaneo 4, 6900 Lugano, Switzerland.1
Université de Lausanne, Institut de Géologie et Paléontologie, Anthropole, 1015 Lausanne, Switzerland. 2
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