5:15 PM - 7:15 PM
[SEM16-P05] Establishment of Paleomagnetic Stratigraphy of the lacustrine deposits (Green River Fm) for reconstructing the terrestrial climate during the Eocene (Preliminary results)
Keywords:Paleomagnetism, Remanent magnetization, Eocene, Hyperthermal, Paleomagnetic stratigraphy, Lacustrine deposits
The early Eocene is known as a "hothouse" period during which repeated rapid warming events, called "Hyperthermal," occurred (Westerhold et al., 2020). However, the response of terrestrial climate to Hyperthermal events remains largely unknown. This study aims to elucidate terrestrial climatic changes in response to rapid warming by investigating the lower-middle Eocene lacustrine deposits (Green River Formation) in Utah, USA.
The Green River Formation extends across Utah, Wyoming, and Colorado, serving as a crucial record that indicates the presence of a vast lake in mid-latitude of central USA during the early Eocene (Tanavsuu-Milkeviciene, & Sarg, J., 2012). The formation covers the long-term depositional record from the early to middle Eocene (~54-44 Ma), based on 40Ar/39Ar dating of interbedded tuff layers (Smith et al., 2008). However, the lower part of the formation (49-53 Ma) lacks interbedded tuff layers, preventing the construction of a detailed age model. Therefore, this study aims to construct a paleomagnetic stratigraphy of the Green River lacustrine deposits and correlate it with the geomagnetic polarity timescale to develop a detailed age model.
In June 2024, a field investigation was conducted at the Indian Canyon section in Utah, USA, where paleomagnetic samples were collected from 148 stratigraphic levels across a 500-meter interval in the lower part of the formation. To examine the magnetic minerals that retain primary and secondary magnetization, a three-axis IRM (Isothermal Remanent Magnetization) thermal demagnetization experiment was conducted based on Lowrie (1990). As a result, the disappearance of magnetization was observed at 580-600 degrees Celsius in all samples, suggesting that the primary magnetization is carried by magnetite. Additionally, the Curie temperatures corresponding to minerals such as goethite, pyrrhotite, and maghemite were observed in many samples, suggesting that the Green River Formation has acquired secondary magnetization components due to the alteration of magnetic minerals.
Next, stepwise AFD (Alternating Field Demagnetization) and stepwise ThD (Thermal Demagnetization) analysis were performed on samples from 117 stratigraphic levels. The ChRM (Characteristic Remanent Magnetization) component, which demagnetized linearly towards the origin, was extracted from 48 stratigraphic levels. Among the extracted ChRM, those with the MAD (Maximum Angular Deviation) of less than 30 degrees were found in 44 stratigraphic levels. Currently, due to measurements being conducted on a sample-by-sample basis, a discussion based on average orientation is not possible. However, assuming that the ChRM represents primary magnetization, the VGP (Virtual Geomagnetic Pole) were calculated. By determining the polarity from the latitude, a paleomagnetic polarity stratigraphy was constructed. As a result, six geomagnetic reversal boundaries corresponding to the C22n/C22r, C22r/C23n, and C23n/C23r boundaries were identified from the lower part of the Green River Formation (a stratigraphic thickness of 500 meters). By comparing the geomagnetic timescale of the early Eocene, it is suggested that fluvial channel layers developed in the 100-500 meter interval of the formation may correspond to Hyperthermal events during the early Eocene "hothouse" period.
The Green River Formation extends across Utah, Wyoming, and Colorado, serving as a crucial record that indicates the presence of a vast lake in mid-latitude of central USA during the early Eocene (Tanavsuu-Milkeviciene, & Sarg, J., 2012). The formation covers the long-term depositional record from the early to middle Eocene (~54-44 Ma), based on 40Ar/39Ar dating of interbedded tuff layers (Smith et al., 2008). However, the lower part of the formation (49-53 Ma) lacks interbedded tuff layers, preventing the construction of a detailed age model. Therefore, this study aims to construct a paleomagnetic stratigraphy of the Green River lacustrine deposits and correlate it with the geomagnetic polarity timescale to develop a detailed age model.
In June 2024, a field investigation was conducted at the Indian Canyon section in Utah, USA, where paleomagnetic samples were collected from 148 stratigraphic levels across a 500-meter interval in the lower part of the formation. To examine the magnetic minerals that retain primary and secondary magnetization, a three-axis IRM (Isothermal Remanent Magnetization) thermal demagnetization experiment was conducted based on Lowrie (1990). As a result, the disappearance of magnetization was observed at 580-600 degrees Celsius in all samples, suggesting that the primary magnetization is carried by magnetite. Additionally, the Curie temperatures corresponding to minerals such as goethite, pyrrhotite, and maghemite were observed in many samples, suggesting that the Green River Formation has acquired secondary magnetization components due to the alteration of magnetic minerals.
Next, stepwise AFD (Alternating Field Demagnetization) and stepwise ThD (Thermal Demagnetization) analysis were performed on samples from 117 stratigraphic levels. The ChRM (Characteristic Remanent Magnetization) component, which demagnetized linearly towards the origin, was extracted from 48 stratigraphic levels. Among the extracted ChRM, those with the MAD (Maximum Angular Deviation) of less than 30 degrees were found in 44 stratigraphic levels. Currently, due to measurements being conducted on a sample-by-sample basis, a discussion based on average orientation is not possible. However, assuming that the ChRM represents primary magnetization, the VGP (Virtual Geomagnetic Pole) were calculated. By determining the polarity from the latitude, a paleomagnetic polarity stratigraphy was constructed. As a result, six geomagnetic reversal boundaries corresponding to the C22n/C22r, C22r/C23n, and C23n/C23r boundaries were identified from the lower part of the Green River Formation (a stratigraphic thickness of 500 meters). By comparing the geomagnetic timescale of the early Eocene, it is suggested that fluvial channel layers developed in the 100-500 meter interval of the formation may correspond to Hyperthermal events during the early Eocene "hothouse" period.