THE CASSINA BEDS (MIDDLE TRIASSIC, MONTE SAN GIORGIO)...Cassina, 1933 Photo PIMUZ Cassina, 1971...

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18th SwissSed Meeting, Fribourg, February 27, 2010 REFERENCES Koutsoukos, 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 verification 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 nodosariids Emerged area (land / island) covered with vegetation Shallow-water foraminifers Dasycladalean algae Saurichthys Eosemionotus Peltopleurus Archaeosemionotus PRELIMINARY CONCLUSIONS The studied section of the Cassina beds records a continuous background sedimentation (laminite lithofacies) mirroring fluctuating 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 floor, 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 seafloor by an extremely low-oxygen tolerant foraminiferal meiofauna. However, either oxygen values were too low or the sufficiently oxygenated periods were too short to allow colonization by a more diverse benthic macrofauna. Oxygen- deficient 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” effect). COMPARATIVE TAPHONOMY Within the laminite lithofacies, fish fossils display different 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 fish reached the sea bottom as complete bodies soon after death, and that all decaying processes occurred on the seafloor. 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” effect). Disarticulation pathways clearly vary between representatives of different 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 fish 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 seafloor. Preservation pathways in Archaeosemionotus Fully articulated specimen. Only skull elements crushed. MCSN 8073 MCSN 8009 mirror image MCSN 8084 MCSN 8086 Severe skull and scale disarticulation Nearly complete body disarticulation Skull disarticulation 1 cm 1 cm 1 cm 1 cm LITHOFACIES Three 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 reflects 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 stratification. The DOLOMITE LITHOFACIES, overlying the Cassina beds, is interpreted as a partly dolomitized counterpart of the turbidite lithofacies. 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) 2 Frequence on the bed surface (coprolites, nodules) or in thin section (microfossils) Saurichthys spp. Eosemionotus sp. Peltopleurus sp. Actinopterygii ind. Actinopterygii 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. 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 finer 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 sufficient to sustain large populations of foraminifers with low oxygen requirements. BACKGROUND SEDIMENTATION Preservation of the fine 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 fixed, 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 first time from Monte San Giorgio. 10 cm 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 Meride Limestone Switzerland Italy Arzo Meride Besano Serpiano Porto Ceresio Poncione d'Arzo Monte San Giorgio Lake Lugano Cassina beds / locality Cassina 1 km 45°54' N 8°57' E Permian Triassic Liassic Anisian Rhyolithe and associated volcaniclastics Bellano Fm. Cassina beds Cava superiore beds Cava inferiore beds San Giorgio Dolomite Kalkschieferzone Pizzella Marls Meride Limestone Dolomia Principale "Dolomitband" Lower Salvatore Dolomite Besano Fm. Rhaet Carnian Ladinian Norian 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 fish 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 m 2 , and these new data supplement those derived from new vertebrate finds (mainly represented by over 300 fish 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 five 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. Photo PIMUZ Cassina, 1933 Photo PIMUZ Cassina, 1971 Cassina, 2009 THE CASSINA BEDS (MIDDLE TRIASSIC, MONTE SAN GIORGIO) IN AN OXYGEN-DEPLETED ENVIRONMENT BACKGROUND AND EVENT SEDIMENTATION Rudolf Stockar 1, 2 Museo cantonale di storia naturale, viale Cattaneo 4, 6900 Lugano, Switzerland. 1 [email protected] Université de Lausanne, Institut de Géologie et Paléontologie, Anthropole, 1015 Lausanne, Switzerland. 2

Transcript of THE CASSINA BEDS (MIDDLE TRIASSIC, MONTE SAN GIORGIO)...Cassina, 1933 Photo PIMUZ Cassina, 1971...

  • 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

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    age

    MCS

    N 8

    084

    MCS

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    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

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    Present

    Common

    Abundant

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    Number of specimenson the bed surface (40 m )(vertebrates, plants)

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    Frequence on the bed surface(coprolites, nodules) or in thinsection (microfossils)

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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

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    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

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    Cassina

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    45°54' N

    8°57

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    Perm

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    Rhyolithe and associated volcaniclastics

    Bellano Fm.

    Cassina bedsCava superiore beds

    Cava inferiore beds

    San GiorgioDolomite

    Kalkschieferzone

    Pizzella Marls

    Mer

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    100 m

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    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

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    Cassina, 1933

    Phot

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    MU

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    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

    [email protected]

    Université de Lausanne, Institut de Géologie et Paléontologie, Anthropole, 1015 Lausanne, Switzerland. 2