1:45 PM - 3:15 PM
[O11-P28] Paleo-current directions of the Morozaki group near the Utsumi fault at the southern end of the Chita peninsula, Japan
Keywords:transverse erosional marks, cross-lamination, deformed load casts
1. Background and Purpose Miyakawa et al. (2021) observed that during the deposition of the Morozaki Group, the Utsumi Fault functioned as a normal fault, creating a fault cliff approximately 1,000 meters high. In addition, research by Nagoya High School Earth Sciences Club (2025) noted that the paleo-dip direction reconstructed from deformation of load casts does not match the paleo-dip direction indicated by cross-laminae. This suggests that the turbidity currents impinging on the fault cliff may have been deposited on the reverse slope. We conducted the additional investigation to increase the measurement of paleo-dip directions and paleo-current directions for every horizon, and to examine the characteristics of sedimentation under the deep sea nearby active fault.
2. Data and Analysis Methods Sedimentary structures in the Toyohama Formation of Miocene Morozaki Group, exposed at low tide along the coast of Toyohama Sakai in Minamichita -town, Chita District, Aichi Prefecture, were investigated. Using a geological compass and the Geoclino app for iPhone, we simultaneously recorded the strike and dip of the bedding plane and lineation of both direction of paleo-dip and direction of paleo-current. The paleo-current direction was determined by measuring cross-laminae and transvers erosional marks as described by Allen (1971) noting that the wider and deeper parts of transverse erosional marks indicate the upstream side of the flow. Furthermore, following Moretti et al. (2001), paleo-dip direction was the opposite at the direction in which the load casts slant diagonally downward.
3. Results Our observations reveal notable differences between outcrop areas. On the offshore side (lower), many cross-laminae indicate a predominantly southeastward paleo-current direction parallel to the Utsumi Fault. Conversely, on the national highway side (upper), cross-laminae commonly record a flow from eastward to east-southeast. The paleocurrent direction inferred from transverse erosional marks generally follows an eastward trend along the fault; however, one cross-lamina immediately above these marks displays a flow diverging from the fault. Similarly, the paleo-dip direction determined from deformation load scars trends away from the fault, yet an adjacent cross-lamina shows a flow parallel to it in an easterly direction. Other cross-laminae consistently exhibit an eastward paleo-current parallel to the fault, while some stratigraphic layers indicate a westward flow away from the fault. Spiral structures of Turritella pointing eastward along the fault further support the interpretation of a normal water flow from west to east. In addition, several glendonite concretions were identified in an outcrop located on private land near the national highway.
4. Discussion and Future Issues Two primary paleo-current directions emerged: one that follows the Utsumi Fault and another along the paleo-dip direction. This suggests that, during deposition, dominant water flows were directed along the fault, encountering instances where turbidity currents ascended and descended the fault cliff. The consistent paleocurrent orientation on the offshore side, which is in close proximity to the fault, contrasts with the greater variability observed on the national highway side, approximately 500 meters away. This spatial difference highlights the significant influence of the fault on flow direction, with more dispersed flow patterns occurring further from the barrier. During the March–April 2025 survey season, the Geoclino for iPhone application was used to gather paleocurrent data from more than 100 locations; however, integrating this dataset into our current column map presented some challenges. Moving forward, we plan to systematically organize these measurements to enhance the interpretability of our presentations. Furthermore, we intend to explore the use of stereo nets and rose diagrams for a more effective visualization of directional data. Notably, two stratigraphic horizons—one marked by transverse erosional marks and another by deformation of load casts scars—exhibit excellent continuity and may serve as key beds. Together with characteristic tuff layers, these markers could form the basis for establishing a standard stratigraphic framework for the Toyohama Formation.
Acknowledgments We gratefully acknowledge the cooperation of the private landowners, whose access to outcrops displaying transverse erosional marks and glendonite concretions was essential for our survey and sampling efforts.
2. Data and Analysis Methods Sedimentary structures in the Toyohama Formation of Miocene Morozaki Group, exposed at low tide along the coast of Toyohama Sakai in Minamichita -town, Chita District, Aichi Prefecture, were investigated. Using a geological compass and the Geoclino app for iPhone, we simultaneously recorded the strike and dip of the bedding plane and lineation of both direction of paleo-dip and direction of paleo-current. The paleo-current direction was determined by measuring cross-laminae and transvers erosional marks as described by Allen (1971) noting that the wider and deeper parts of transverse erosional marks indicate the upstream side of the flow. Furthermore, following Moretti et al. (2001), paleo-dip direction was the opposite at the direction in which the load casts slant diagonally downward.
3. Results Our observations reveal notable differences between outcrop areas. On the offshore side (lower), many cross-laminae indicate a predominantly southeastward paleo-current direction parallel to the Utsumi Fault. Conversely, on the national highway side (upper), cross-laminae commonly record a flow from eastward to east-southeast. The paleocurrent direction inferred from transverse erosional marks generally follows an eastward trend along the fault; however, one cross-lamina immediately above these marks displays a flow diverging from the fault. Similarly, the paleo-dip direction determined from deformation load scars trends away from the fault, yet an adjacent cross-lamina shows a flow parallel to it in an easterly direction. Other cross-laminae consistently exhibit an eastward paleo-current parallel to the fault, while some stratigraphic layers indicate a westward flow away from the fault. Spiral structures of Turritella pointing eastward along the fault further support the interpretation of a normal water flow from west to east. In addition, several glendonite concretions were identified in an outcrop located on private land near the national highway.
4. Discussion and Future Issues Two primary paleo-current directions emerged: one that follows the Utsumi Fault and another along the paleo-dip direction. This suggests that, during deposition, dominant water flows were directed along the fault, encountering instances where turbidity currents ascended and descended the fault cliff. The consistent paleocurrent orientation on the offshore side, which is in close proximity to the fault, contrasts with the greater variability observed on the national highway side, approximately 500 meters away. This spatial difference highlights the significant influence of the fault on flow direction, with more dispersed flow patterns occurring further from the barrier. During the March–April 2025 survey season, the Geoclino for iPhone application was used to gather paleocurrent data from more than 100 locations; however, integrating this dataset into our current column map presented some challenges. Moving forward, we plan to systematically organize these measurements to enhance the interpretability of our presentations. Furthermore, we intend to explore the use of stereo nets and rose diagrams for a more effective visualization of directional data. Notably, two stratigraphic horizons—one marked by transverse erosional marks and another by deformation of load casts scars—exhibit excellent continuity and may serve as key beds. Together with characteristic tuff layers, these markers could form the basis for establishing a standard stratigraphic framework for the Toyohama Formation.
Acknowledgments We gratefully acknowledge the cooperation of the private landowners, whose access to outcrops displaying transverse erosional marks and glendonite concretions was essential for our survey and sampling efforts.