5:15 PM - 6:30 PM
[HCG28-P02] Relationship between magnetic anisotropy and hydraulic conditions in modern flood deposits
Keywords:magnetic anisotropy, modern deposits, X-ray CT analysis
Introduction: The magnetic susceptibility measurement is widely used in paleocurrent reconstruction because of its high efficiency, but the relationship between the magnetic anisotropy and hydraulic conditions has not been well studied. In this paper, we discuss the relationship between magnetic anisoropy and paleocurrent / hydraulic conditions, based on the comparison of these analysis; observation of the modern flood sediments, the magnetic susceptibility measurement of each unit, and the particle orientation analyze using X-ray CT images.
Subject / Method: The location of the outcrop was at the east end of Toyama O-hashi on the right bank of the Jinzu River, Toyama Prefecture. The outcrop of mound-like deposits found here is 1.6 m high and 6.3 m wide, and distributed along a slope that dips 15° west toward the river channel. A 7cc plastic cube was used to collect samples in a fixed position, and magnetic susceptibility measurements, X-ray CT imaging, and particle analysis were performed to compare the results. ImageJ and R were used to analyze the particle orientation.
Results of field observation: The beds can be divided into three main units. The lowest unit, Unit A, consists of a 20cm thick, alternating layer of well-sorted very fine sand and fine sand, including plant fragments. The sand layer forms a foreset lamination along the slope. The upper unit, Unit B, consists of 22cm thick, well-sorted fine sand that eroding the Unit A. The sand layer has planar-cross lamination that is parallel to the channel associated with current ripple movement. The uppermost unit, Unit C, consists of 18cm thick, well-sorted fine sand with foreset lamination along the slope as in Unit A. Unit C contains no plant fragments.
Results of X-ray CT image analysis: The maximum axis of unit A is mainly towerd to the left bank or upstream direction with low angle (about 15°), showing a radial distribution with large dispersion. The maximum axis of unit B is dominated by the upstream to downstream direction with low angle (about 15°), and the dispersion is the largest. The maximum axis of unit C is towerd to the left bank with low angle (about 15°), and the dispersion is the narrowest.
Results of magnetic susceptibility measurement: The shape of the anisotropy was all closer to oblate. The direction of the minimum axis of the anisotropy showed good concentration in each unit. In Unit A, the minimum axis is almost vertical, and the maximum axis has little variation and points to a very low angle (less than 10°) on the left or right bank. In Unit B, the minimum axis is inclined by 30°to 44° in the direction between the upstream and the left bank, and the maximum axis is highly variable and refers to the lower angle (less than 26°) than the left bank. In Unit C, the minimum axis is almost vertical, the maximum axis is highly variable, and refers to the upstream, downstream right bank, and the very low angle (less than 8°) of the right bank.
Discussion: Since the components of the particles do not change significantly with each unit and are not intercalated by temporal gaps such as mud drapes or organic thickening layers, this formation is considered to have been deposited in a single flood. The direction of deposition is basically lateral accretion along the slope. Considering the sedimentary structure and the upward coarsening of the units, the hydraulic conditions can be regarded as a lower flow regime for Units A / B and an upper flow regime for Unit C. X-ray CT image analysis of the particle orientation is consistent with this interpretation. When Unit A was deposited, the maximum axis was oriented toward the left bank due to down-slope gravity, and when Unit B was deposited, the downstream flow at the water level lowering was dominant, and the maximum axis was oriented in the upstream-downstream direction. When Unit C was deposited, the down-slope flow was more dominant than the downstream flow, and the maximum axis was concentrated in the left bank direction. The results of the magnetic susceptibility measurement indicate that it is difficult to completely reconstruct the paleocurrent direction and particle orientation from magnetic anisotropy alone, and that at least a detailed description and analysis of the facies is necessary. On the other hand, the increase in the dispersion of the maximum axis from unit A to unit C may reflect the increase in the traction force from the lower flow regime to the upper flow regime, suggesting that the magnetic anisotropy and hydraulic conditions correspond to some extent.
Acknowledgemnt: This work was supported by JSPS KAKENHI Grant Number 20K21055.
Subject / Method: The location of the outcrop was at the east end of Toyama O-hashi on the right bank of the Jinzu River, Toyama Prefecture. The outcrop of mound-like deposits found here is 1.6 m high and 6.3 m wide, and distributed along a slope that dips 15° west toward the river channel. A 7cc plastic cube was used to collect samples in a fixed position, and magnetic susceptibility measurements, X-ray CT imaging, and particle analysis were performed to compare the results. ImageJ and R were used to analyze the particle orientation.
Results of field observation: The beds can be divided into three main units. The lowest unit, Unit A, consists of a 20cm thick, alternating layer of well-sorted very fine sand and fine sand, including plant fragments. The sand layer forms a foreset lamination along the slope. The upper unit, Unit B, consists of 22cm thick, well-sorted fine sand that eroding the Unit A. The sand layer has planar-cross lamination that is parallel to the channel associated with current ripple movement. The uppermost unit, Unit C, consists of 18cm thick, well-sorted fine sand with foreset lamination along the slope as in Unit A. Unit C contains no plant fragments.
Results of X-ray CT image analysis: The maximum axis of unit A is mainly towerd to the left bank or upstream direction with low angle (about 15°), showing a radial distribution with large dispersion. The maximum axis of unit B is dominated by the upstream to downstream direction with low angle (about 15°), and the dispersion is the largest. The maximum axis of unit C is towerd to the left bank with low angle (about 15°), and the dispersion is the narrowest.
Results of magnetic susceptibility measurement: The shape of the anisotropy was all closer to oblate. The direction of the minimum axis of the anisotropy showed good concentration in each unit. In Unit A, the minimum axis is almost vertical, and the maximum axis has little variation and points to a very low angle (less than 10°) on the left or right bank. In Unit B, the minimum axis is inclined by 30°to 44° in the direction between the upstream and the left bank, and the maximum axis is highly variable and refers to the lower angle (less than 26°) than the left bank. In Unit C, the minimum axis is almost vertical, the maximum axis is highly variable, and refers to the upstream, downstream right bank, and the very low angle (less than 8°) of the right bank.
Discussion: Since the components of the particles do not change significantly with each unit and are not intercalated by temporal gaps such as mud drapes or organic thickening layers, this formation is considered to have been deposited in a single flood. The direction of deposition is basically lateral accretion along the slope. Considering the sedimentary structure and the upward coarsening of the units, the hydraulic conditions can be regarded as a lower flow regime for Units A / B and an upper flow regime for Unit C. X-ray CT image analysis of the particle orientation is consistent with this interpretation. When Unit A was deposited, the maximum axis was oriented toward the left bank due to down-slope gravity, and when Unit B was deposited, the downstream flow at the water level lowering was dominant, and the maximum axis was oriented in the upstream-downstream direction. When Unit C was deposited, the down-slope flow was more dominant than the downstream flow, and the maximum axis was concentrated in the left bank direction. The results of the magnetic susceptibility measurement indicate that it is difficult to completely reconstruct the paleocurrent direction and particle orientation from magnetic anisotropy alone, and that at least a detailed description and analysis of the facies is necessary. On the other hand, the increase in the dispersion of the maximum axis from unit A to unit C may reflect the increase in the traction force from the lower flow regime to the upper flow regime, suggesting that the magnetic anisotropy and hydraulic conditions correspond to some extent.
Acknowledgemnt: This work was supported by JSPS KAKENHI Grant Number 20K21055.