Shallow-marine sedimentary records of the Eocene-Oligocene greenhouse-icehouse transition (Italy, Switzerland and France)
DOKPE
- Jaramillo-Vogel, David University of Fribourg
- Strasser, André (Degree supervisor) University of Fribourg
- 2013 ; Fribourg (Suisse) : Département de géosciences, sciences de la terre, Université de Fribourg
1 ressource en ligne (176 pages) ; 1 fichier pdf
PhD: Université de Fribourg (Suisse), 2013
English
During the Middle Eocene to Early Oligocene, the Earth suffered the most important climatic transition of the Cenozoic era characterized by a prominent global cooling and the onset of the Antarctic glaciation, marking the beginning of the present-day ice-house world. The Eocene–Oligocene transition (EOT; ~34 Ma) represents the culmination of this climatic change. While deep-marine sediments of this time interval have been well studied in recent years, little is known about the EOT in shallow-marine carbonate depositional systems. On one hand, shallow-marine sections spanning this time interval are scarce. On the other hand, the correlation of shallow benthic biozones with stable-isotope, magneto-, and calcareous-plankton stratigraphy established in pelagic sections remains poorly constrained. Therefore, the potential of environmentally sensitive shallow-marine depositional settings for recording the evolution of this climate transition remains largely unexplored. The aim of this study is to identify the EOT in shallow-marine sections and study the expression of climatic events within these deposits.
In the course of this thesis, rocks of the Late Eocene shallow-marine Calcare di Nago Formation (Nago and San Valentino sections; northern Italy), the Sanetsch Formation (Tsanfleuron, Sanetsch Buvette, and Flaine sections; Switzerland and France) and the deep-marine Scaglia Variegata and Scaglia Cinerea Formations (Massignano and Monte Cagnero sections, central Italy) were studied. Due to the exceptionally good exposure and continuous record of Late Eocene to Early Oligocene rocks within the Calcare di Nago Formation, most of the work was performed on the Nago and San Valentino sections. The Nago section consists of a 150 m thick tropical to subtropical shallow-marine carbonate succession, which is easily accessible. These bioclastic carbonates are mainly composed of light-dependent coralline red algae, larger benthic foraminifera, and corals. We present a correlation between the Nago section and the nearby San Valentino section, both deposited within the photic zone, and three pelagic sections from the Tethys (Massignano), Southern Indian Ocean (ODP Site 744), and Tanzania (composite TDP Sites 12 and 17). Our correlation is based on stable carbon isotopes, Sr-dating, and bio-stratigraphic tie points. This platform-to-basin correlation makes it possible to integrate palaeoecological and sequence-stratigraphical observations made in the shallow-marine carbonates with geochemical (e.g., oxygen isotopes) and micropalaeontological records obtained from the pelagic sections.
The carbon-isotope based correlation between the Nago section and ODP Site 744 shows a 0.4‰ negative shift in δ18O starting shortly before the onset of carbon-isotope segment 1 (here defined). Interestingly, this pronounced negative shift coincides with a large-scale transgressive surface and a rapid deepening of the environment in Nago. Therefore, it represents strong evidence of glacio-eustasy, implying the partial melting of at least medium-scale continental ice sheets before the Eocene-Oligocene transition. In Nago, the positive oxygen-isotope shift leading to the EOT-1 (visible in TDP Sites 12 and 17) does not correlate with a sharp sequence boundary as recognized elsewhere. It rather corresponds to stepwise shallowing of the depositional environment. The earliest Oligocene isotope event 1 (Oi-1), identified only in San Valentino, coincides with the most pronounced facies change recorded in the studied deposits. This facies shift is characterized by a drastic change from larger benthic foraminifera and red algal-dominated limestones to bryozoan-dominated limestones. These bryozoan beds locally contain up to 86% bryozoans, while coralline algae as well as larger benthic foraminifera are absent. Coralline algae and nummulitid foraminifera recover in the upper part of the bryozoan beds, whereas orthophragminids do not recover. Shifts in carbonate-producing communities provide evidence of environmental changes in the neritic realm. The bryozoan beds are interpreted to result not necessarily from a deepening of the depositional environment but to be a shallow-water record of a cooling pulse of at least regional scale, as bryozoan beds occur in several Italian localities around the Eocene-Oligocene boundary. They may thus represent an analogue of modern cool-water carbonates. Total phosphorus content increases in the bryozoan beds, suggesting that the cooling was accompanied by an increase in nutrient supply. This phosphorous peak is coeval with the globally recognized increase in productivity around the Oi-1.
High-amplitude and high-frequency climatic changes have been described from Middle and Late Eocene deposits at several deep-sea localities. The well age-calibrated Middle to Late Eocene shallow-marine carbonate deposits of the Nago section represent an excellent opportunity to test whether or not this climatic variability is somehow recorded in shallow-marine successions. Based on detailed logging and microfacies analysis, two orders of sequences (large- and small-scale) have been recognized in Nago. Large-scale sequences are characterized by a transgressive-regressive-transgressive trend. Small-scale sequences are bound by sharp transgressive surfaces and characterized by the superposition of lithofacies deposited under diverse environmental conditions within the photic zone, interpreted to result from high-frequency relative sea-level fluctuations in the order of a few tens of meters. The comparison of the Nago section with the deep-water δ18O record of ODP Site 744 suggests that these sea-level changes had an important glacio-eustatic component. This implies that build-up of the Antarctic glaciation during the Late Eocene, and most probably already during the Middle Eocene, was modulated by episodes of waxing and waning of continental ice sheets, most probably controlled by high-frequency climate changes induced by orbital cycles. In Massignano and Monte Cagnero, small-scale sequences are composed of limestone-marl alternations. Some of them correlate well with the sequences defined in Nago while others don’t, suggesting that not all small-scale sequences have the same duration. The sections of Flaine, Tsanfleuron and Sanetsch Buvette record a continuous transgressive trend from the Middle/Late Eocene to the Early Oligocene. The passage from shallow-marine photozoan carbonates of the Sanetsch Formation to the deep-marine Globigerina Marls is characterized by transitional beds. These transitional beds may be the expression of global oceanographic changes occurring at the EOT, which together with relative sea-level rise, lead to the switch-off of the carbonate platform.
In the course of this thesis, rocks of the Late Eocene shallow-marine Calcare di Nago Formation (Nago and San Valentino sections; northern Italy), the Sanetsch Formation (Tsanfleuron, Sanetsch Buvette, and Flaine sections; Switzerland and France) and the deep-marine Scaglia Variegata and Scaglia Cinerea Formations (Massignano and Monte Cagnero sections, central Italy) were studied. Due to the exceptionally good exposure and continuous record of Late Eocene to Early Oligocene rocks within the Calcare di Nago Formation, most of the work was performed on the Nago and San Valentino sections. The Nago section consists of a 150 m thick tropical to subtropical shallow-marine carbonate succession, which is easily accessible. These bioclastic carbonates are mainly composed of light-dependent coralline red algae, larger benthic foraminifera, and corals. We present a correlation between the Nago section and the nearby San Valentino section, both deposited within the photic zone, and three pelagic sections from the Tethys (Massignano), Southern Indian Ocean (ODP Site 744), and Tanzania (composite TDP Sites 12 and 17). Our correlation is based on stable carbon isotopes, Sr-dating, and bio-stratigraphic tie points. This platform-to-basin correlation makes it possible to integrate palaeoecological and sequence-stratigraphical observations made in the shallow-marine carbonates with geochemical (e.g., oxygen isotopes) and micropalaeontological records obtained from the pelagic sections.
The carbon-isotope based correlation between the Nago section and ODP Site 744 shows a 0.4‰ negative shift in δ18O starting shortly before the onset of carbon-isotope segment 1 (here defined). Interestingly, this pronounced negative shift coincides with a large-scale transgressive surface and a rapid deepening of the environment in Nago. Therefore, it represents strong evidence of glacio-eustasy, implying the partial melting of at least medium-scale continental ice sheets before the Eocene-Oligocene transition. In Nago, the positive oxygen-isotope shift leading to the EOT-1 (visible in TDP Sites 12 and 17) does not correlate with a sharp sequence boundary as recognized elsewhere. It rather corresponds to stepwise shallowing of the depositional environment. The earliest Oligocene isotope event 1 (Oi-1), identified only in San Valentino, coincides with the most pronounced facies change recorded in the studied deposits. This facies shift is characterized by a drastic change from larger benthic foraminifera and red algal-dominated limestones to bryozoan-dominated limestones. These bryozoan beds locally contain up to 86% bryozoans, while coralline algae as well as larger benthic foraminifera are absent. Coralline algae and nummulitid foraminifera recover in the upper part of the bryozoan beds, whereas orthophragminids do not recover. Shifts in carbonate-producing communities provide evidence of environmental changes in the neritic realm. The bryozoan beds are interpreted to result not necessarily from a deepening of the depositional environment but to be a shallow-water record of a cooling pulse of at least regional scale, as bryozoan beds occur in several Italian localities around the Eocene-Oligocene boundary. They may thus represent an analogue of modern cool-water carbonates. Total phosphorus content increases in the bryozoan beds, suggesting that the cooling was accompanied by an increase in nutrient supply. This phosphorous peak is coeval with the globally recognized increase in productivity around the Oi-1.
High-amplitude and high-frequency climatic changes have been described from Middle and Late Eocene deposits at several deep-sea localities. The well age-calibrated Middle to Late Eocene shallow-marine carbonate deposits of the Nago section represent an excellent opportunity to test whether or not this climatic variability is somehow recorded in shallow-marine successions. Based on detailed logging and microfacies analysis, two orders of sequences (large- and small-scale) have been recognized in Nago. Large-scale sequences are characterized by a transgressive-regressive-transgressive trend. Small-scale sequences are bound by sharp transgressive surfaces and characterized by the superposition of lithofacies deposited under diverse environmental conditions within the photic zone, interpreted to result from high-frequency relative sea-level fluctuations in the order of a few tens of meters. The comparison of the Nago section with the deep-water δ18O record of ODP Site 744 suggests that these sea-level changes had an important glacio-eustatic component. This implies that build-up of the Antarctic glaciation during the Late Eocene, and most probably already during the Middle Eocene, was modulated by episodes of waxing and waning of continental ice sheets, most probably controlled by high-frequency climate changes induced by orbital cycles. In Massignano and Monte Cagnero, small-scale sequences are composed of limestone-marl alternations. Some of them correlate well with the sequences defined in Nago while others don’t, suggesting that not all small-scale sequences have the same duration. The sections of Flaine, Tsanfleuron and Sanetsch Buvette record a continuous transgressive trend from the Middle/Late Eocene to the Early Oligocene. The passage from shallow-marine photozoan carbonates of the Sanetsch Formation to the deep-marine Globigerina Marls is characterized by transitional beds. These transitional beds may be the expression of global oceanographic changes occurring at the EOT, which together with relative sea-level rise, lead to the switch-off of the carbonate platform.
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- Faculté des sciences et de médecine
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- English
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- GeoFocus ; 33
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- https://folia.unifr.ch/unifr/documents/332634
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