Presentation Information
[O12-P63]A Study of the Boundary Between the Izumi and Osaka Formations in the Izumisano Area
*Hanatsu Okido1, Daisuke Oya1, Kento Okumura1, Yuka Nishida1 (1. Osaka Prefectural Tennoji High School)
Keywords:
the Upper Cretaceous Izumi Group,the Quaternary Osaka Group,Fossil
1 Background and Purpose
In the southern part of Osaka Prefecture, the Upper Cretaceous Izumi Group and the Quaternary Osaka Group are widely distributed. Previous studies (Oka, 1978; Miyata et al., 2012) have provided detailed reports on the boundaries in surrounding areas; however, descriptions of the area in between, particularly around the city, remain limited.
In this study, field investigations were conducted at four locations within Izumisano City (indicated by red circles in Fig. 1). Based on lithofacies, fossils, minerals, and statistical analysis of structural data at each site, the aim was to constrain the boundary between the two groups.
2. Data and Methods
From March to December 2025, outcrop surveys were carried out at four locations in Izumisano City (around Takinoya, Inunakisan, Nakasho, and Daitaidai-minami). At each site, the following were conducted: (1)observation of lithofacies, (2) collection of fossil and characteristic mineral samples, and (3) measurement of strike and dip using a clinometer (10 measurements per outcrop). The obtained strike and dip data were statistically analyzed using Python. Specifically, Rayleigh tests were applied to strike data after doubling the angles to determine the presence of preferred orientation, and Watson–Williams tests were conducted to assess significant structural
differences between sites.
3. Results
The characteristics of each site are summarized as follows:
Izumi Group (3 sites): At Takinoya, Inunakisan, andDaitaidai-minami,consolidated mudstone and sandstone were distributed. Notably, fossils of the bivalve Nuculana (Ezonuculana) mactraeformis and the gastropod
Pleurotomaria sp. were obtained from the mudstone at Takinoya. These are consistent with the depositional age and paleoenvironment of the Izumi Group in other regions. In addition, dawsonite, characteristic of the Izumi Group, was identified at Takinoya and Inunakisan.
Osaka Group (1 site): At Nakasho, unconsolidated brownish volcanic ash layers and clay layers were observed, consistent with the lithological characteristics of the Osaka Group.
Structural analysis: Statistical analyses revealed a certain degree of regularity in strike and dip within each site, but significant differences were detected between sites. In particular, a marked inconsistency was observed between the stratigraphic relationships inferred from lithofacies and the measured strike data.
4. Discussion and Future Work
Based on lithofacies and fossil evidence, the boundary between the Izumi Group and the Osaka Group in this region is estimated to lie within a ~4.8 km range between the Nakasho and Daitaidai-minami sites.
Regarding the structural inconsistencies observed in the strike and dip data, uncertainty in identifying measurement surfaces was considered; however, no definitive interpretation was reached in this study. Therefore, structural data were used only as supplementary indicators for stratigraphic discrimination.
Future work will focus on resolving the inconsistencies in structural data and conducting further geomorphological analysis and additional field investigations within this 4.8 km uncertain zone to refine the boundary position and clarify the contact relationship between the two groups.
In the southern part of Osaka Prefecture, the Upper Cretaceous Izumi Group and the Quaternary Osaka Group are widely distributed. Previous studies (Oka, 1978; Miyata et al., 2012) have provided detailed reports on the boundaries in surrounding areas; however, descriptions of the area in between, particularly around the city, remain limited.
In this study, field investigations were conducted at four locations within Izumisano City (indicated by red circles in Fig. 1). Based on lithofacies, fossils, minerals, and statistical analysis of structural data at each site, the aim was to constrain the boundary between the two groups.
2. Data and Methods
From March to December 2025, outcrop surveys were carried out at four locations in Izumisano City (around Takinoya, Inunakisan, Nakasho, and Daitaidai-minami). At each site, the following were conducted: (1)observation of lithofacies, (2) collection of fossil and characteristic mineral samples, and (3) measurement of strike and dip using a clinometer (10 measurements per outcrop). The obtained strike and dip data were statistically analyzed using Python. Specifically, Rayleigh tests were applied to strike data after doubling the angles to determine the presence of preferred orientation, and Watson–Williams tests were conducted to assess significant structural
differences between sites.
3. Results
The characteristics of each site are summarized as follows:
Izumi Group (3 sites): At Takinoya, Inunakisan, andDaitaidai-minami,consolidated mudstone and sandstone were distributed. Notably, fossils of the bivalve Nuculana (Ezonuculana) mactraeformis and the gastropod
Pleurotomaria sp. were obtained from the mudstone at Takinoya. These are consistent with the depositional age and paleoenvironment of the Izumi Group in other regions. In addition, dawsonite, characteristic of the Izumi Group, was identified at Takinoya and Inunakisan.
Osaka Group (1 site): At Nakasho, unconsolidated brownish volcanic ash layers and clay layers were observed, consistent with the lithological characteristics of the Osaka Group.
Structural analysis: Statistical analyses revealed a certain degree of regularity in strike and dip within each site, but significant differences were detected between sites. In particular, a marked inconsistency was observed between the stratigraphic relationships inferred from lithofacies and the measured strike data.
4. Discussion and Future Work
Based on lithofacies and fossil evidence, the boundary between the Izumi Group and the Osaka Group in this region is estimated to lie within a ~4.8 km range between the Nakasho and Daitaidai-minami sites.
Regarding the structural inconsistencies observed in the strike and dip data, uncertainty in identifying measurement surfaces was considered; however, no definitive interpretation was reached in this study. Therefore, structural data were used only as supplementary indicators for stratigraphic discrimination.
Future work will focus on resolving the inconsistencies in structural data and conducting further geomorphological analysis and additional field investigations within this 4.8 km uncertain zone to refine the boundary position and clarify the contact relationship between the two groups.