**4.1. Geology of the Yuda Basin**

The Yuda Basin is located in the axis of the northern part of the Ou Backbone Range (Figure. 1). This portion of the Backbone Range is divided into western and eastern sectors, the altitudes of which are up to 1,000 m and 9,00 m, respectively. The two sectors are bounded by three fault systems; the Senya (bounding the western margin of the western sector), the Kawafune-Warikurayama (dividing the two sectors) and the Eastern Marginal Fault systems, from west to east (Figure. 4). The Kawafune-Warikurayama and Eastern Marginal Fault systems were originally formed as normal faults bounding eastern margins of half grabens during Early - Middle Miocene rifting/backarc opening. These normal faults have been reactivated (inverted) as thrusts during post-rift stages, resulting in a "pop-up" uplift of the present Ou Backbone Range (Figure. 4) [40]. The Yuda Basin is a depression between the eastern and western sectors (Figure. 4), and is 15 km long in a N-S direction and 5 km wide in the E-W direction. Pre-Tertiary basement rocks and Early – Middle Miocene syn-rift volcanic rocks are distributed in the axis of the western and eastern sectors, while Middle Miocene to Pleistocene marine and non-marine deposits are distributed in the surrounding lowlands including the Yuda Basin (Figure. 4).

in the western margin of the Yuda Basin (Core 41PAW-1 in Figure. 7) and in the western sector

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**Figure 5.** Generalized stratigraphy of the Yuda Basin with approximate ages and tectonic stages (I – VI). Key tuff bed names: SS: Sasoh tuff, SN: Sawanakagawa tuff, TS: Torasawa tuff, OW: Ohwatari tuff, TN; Tsukano tuff. F: planktonic

foraminifer zonation after [41]. N: nannofossil zonation after [42]. Modified from [43].

(Figure. 7).

The stratigraphy of the study area is divided into the Oishi, Kotsunagizawa, Kurosawa, Sannai, Hanayama and Yoshizawa Formations in ascending order (Figure. 5). The basin structure shows a simple synclinal structure bounded by the Kawafune-Warikurayama Fault at its western margin (Figure. 6). The deposits become younger toward the basin center, reflecting a synclinal structure.

The Oishi Formation consists of syn-rift volcanics, volcaniclastic rocks and mudstone. This formation had been formed by syn-rift felsic volcanism within half-grabens bounded by the Kawafune-Warikurayama and Eastern Marginal Fault systems in the Kuroko graben during the syn-rift stage (16 – 13.5 Ma) (Figure. 3).

The Kotsunagizawa Formation consists of mudstone, fine-grained sandstone and felsic tuffs. The formation conformably overlies the Oishi Formation and intercalates the Okinazawa basalt member, consisting of basaltic tuff breccia and lapilli stone in the middle part of the formation (Figures 5, 7). The uppermost several meters of the formation grades from mudstone to sandstone upward and is overlain by the Ochiai volcanic breccia bed (OB: 7 m thick)[43]. The age of the Kotsunagizawa Formation spans from 13.5 Ma to ca. 12 Ma based on biostra‐ tigraphy and fission-track datings (Figure, 8)[30-31, 45].

The Kurosawa Formation consists of shallow marine sandsone and tuffaceous mudstone with intercalations of felsic tuffs and conglomerate. Three key tuff beds, the Tsukano Tuff beds (TN: 20–50 m thick), Ohwatari Tuff beds (OW: 25 m thick) and the Torasawa Tuff bed (TS: 3–15 m thick) are intercalated in the middle and upper parts of the formation, respectively [30, 43] (Figures. 5-7). The Kurosawa Formation unconformably overlies the Kotsunagizawa Forma‐ tion in the eastern margin of the Yuda Basin. The age of the Kurosawa Formation in the Yuda Basin was dated to be 9-6.5 Ma based on fission-track dating with a notable age gap of 2 – 3 Ma at the unconformity between the Kotsunagizawa and the Kurosawa formations (Figure. 8) [31, 43]. The Kurosawa Formation is, however, continuous from the Kotsunagizawa Formation in the western margin of the Yuda Basin (Core 41PAW-1 in Figure. 7) and in the western sector (Figure. 7).

**4.1. Geology of the Yuda Basin**

(Figure. 4).

a synclinal structure.

the syn-rift stage (16 – 13.5 Ma) (Figure. 3).

tigraphy and fission-track datings (Figure, 8)[30-31, 45].

The Yuda Basin is located in the axis of the northern part of the Ou Backbone Range (Figure. 1). This portion of the Backbone Range is divided into western and eastern sectors, the altitudes of which are up to 1,000 m and 9,00 m, respectively. The two sectors are bounded by three fault systems; the Senya (bounding the western margin of the western sector), the Kawafune-Warikurayama (dividing the two sectors) and the Eastern Marginal Fault systems, from west to east (Figure. 4). The Kawafune-Warikurayama and Eastern Marginal Fault systems were originally formed as normal faults bounding eastern margins of half grabens during Early - Middle Miocene rifting/backarc opening. These normal faults have been reactivated (inverted) as thrusts during post-rift stages, resulting in a "pop-up" uplift of the present Ou Backbone Range (Figure. 4) [40]. The Yuda Basin is a depression between the eastern and western sectors (Figure. 4), and is 15 km long in a N-S direction and 5 km wide in the E-W direction. Pre-Tertiary basement rocks and Early – Middle Miocene syn-rift volcanic rocks are distributed in the axis of the western and eastern sectors, while Middle Miocene to Pleistocene marine and non-marine deposits are distributed in the surrounding lowlands including the Yuda Basin

160 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

The stratigraphy of the study area is divided into the Oishi, Kotsunagizawa, Kurosawa, Sannai, Hanayama and Yoshizawa Formations in ascending order (Figure. 5). The basin structure shows a simple synclinal structure bounded by the Kawafune-Warikurayama Fault at its western margin (Figure. 6). The deposits become younger toward the basin center, reflecting

The Oishi Formation consists of syn-rift volcanics, volcaniclastic rocks and mudstone. This formation had been formed by syn-rift felsic volcanism within half-grabens bounded by the Kawafune-Warikurayama and Eastern Marginal Fault systems in the Kuroko graben during

The Kotsunagizawa Formation consists of mudstone, fine-grained sandstone and felsic tuffs. The formation conformably overlies the Oishi Formation and intercalates the Okinazawa basalt member, consisting of basaltic tuff breccia and lapilli stone in the middle part of the formation (Figures 5, 7). The uppermost several meters of the formation grades from mudstone to sandstone upward and is overlain by the Ochiai volcanic breccia bed (OB: 7 m thick)[43]. The age of the Kotsunagizawa Formation spans from 13.5 Ma to ca. 12 Ma based on biostra‐

The Kurosawa Formation consists of shallow marine sandsone and tuffaceous mudstone with intercalations of felsic tuffs and conglomerate. Three key tuff beds, the Tsukano Tuff beds (TN: 20–50 m thick), Ohwatari Tuff beds (OW: 25 m thick) and the Torasawa Tuff bed (TS: 3–15 m thick) are intercalated in the middle and upper parts of the formation, respectively [30, 43] (Figures. 5-7). The Kurosawa Formation unconformably overlies the Kotsunagizawa Forma‐ tion in the eastern margin of the Yuda Basin. The age of the Kurosawa Formation in the Yuda Basin was dated to be 9-6.5 Ma based on fission-track dating with a notable age gap of 2 – 3 Ma at the unconformity between the Kotsunagizawa and the Kurosawa formations (Figure. 8) [31, 43]. The Kurosawa Formation is, however, continuous from the Kotsunagizawa Formation

**Figure 5.** Generalized stratigraphy of the Yuda Basin with approximate ages and tectonic stages (I – VI). Key tuff bed names: SS: Sasoh tuff, SN: Sawanakagawa tuff, TS: Torasawa tuff, OW: Ohwatari tuff, TN; Tsukano tuff. F: planktonic foraminifer zonation after [41]. N: nannofossil zonation after [42]. Modified from [43].

**Figure 7.** Correlation of lithologic columns in the Yuda Basin (modified from [43]). Numbers in open circles denotes

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The Sannai Formation consists of alternating hard mudstone and black shale associated with felsic tuffs. The formation interfingers with the Kurosawa Formation (Figure. 7) and is distributed in the western sector and in the southern margin of the Yuda Basin, while it is absent in the eastern margin of the Yuda Basin (Figure. 7). This formation is equivalent to the Onnagawa Formation, a siliceous shale unit in the center of the Akita Basin (Figures. 2&3).

The Hanayama Formation distributed along the syncline in the axis of the Yuda Basin is divided into a main part and the Arasawa Tuff Member distributed in the southern Yuda Basin (Figure. 7). The main part of the formation unconformably overlies the Kurosawa Formation, and consists of fluvial, marsh, deltaic and shallow marine deposits [30-31]. The Hanayama Formation includes four 3rd order depositional sequences overlapped by numerous high frequency sequences (Figure. 7). Two key tuff beds, the Sawanakagawa Tuff beds (SN: 4–30 m thick) and the Sasoh Tuff bed (SS: 10–50 m thick) are intercalated in the middle and upper Hanayama Formation (Figures. 5-7)[30]. The Arasawa Tuff Member chiefly consists of dacite pumice tuff associated with dacite lava and conglomerate. This member interfingers with upper part of Kurosawa Formation and also with the main part of the Hanayama Formation in the southern Yuda Basin (Figures. 5-7). The age of the main part of the Hanayama Formation

locations of columns shown in Figure 10.

**Figure 6.** Detailed geological map and geological cross-sections of the Yuda Basin. A-A', B-B' and C-C' are lines of cross-sections. 41PAW-1 denotes the location of the drilling hole [44] in the southwestern part of the Yuda Basin. Modified from [31]

**Figure 7.** Correlation of lithologic columns in the Yuda Basin (modified from [43]). Numbers in open circles denotes locations of columns shown in Figure 10.

The Sannai Formation consists of alternating hard mudstone and black shale associated with felsic tuffs. The formation interfingers with the Kurosawa Formation (Figure. 7) and is distributed in the western sector and in the southern margin of the Yuda Basin, while it is absent in the eastern margin of the Yuda Basin (Figure. 7). This formation is equivalent to the Onnagawa Formation, a siliceous shale unit in the center of the Akita Basin (Figures. 2&3).

The Hanayama Formation distributed along the syncline in the axis of the Yuda Basin is divided into a main part and the Arasawa Tuff Member distributed in the southern Yuda Basin (Figure. 7). The main part of the formation unconformably overlies the Kurosawa Formation, and consists of fluvial, marsh, deltaic and shallow marine deposits [30-31]. The Hanayama Formation includes four 3rd order depositional sequences overlapped by numerous high frequency sequences (Figure. 7). Two key tuff beds, the Sawanakagawa Tuff beds (SN: 4–30 m thick) and the Sasoh Tuff bed (SS: 10–50 m thick) are intercalated in the middle and upper Hanayama Formation (Figures. 5-7)[30]. The Arasawa Tuff Member chiefly consists of dacite pumice tuff associated with dacite lava and conglomerate. This member interfingers with upper part of Kurosawa Formation and also with the main part of the Hanayama Formation in the southern Yuda Basin (Figures. 5-7). The age of the main part of the Hanayama Formation

**Figure 6.** Detailed geological map and geological cross-sections of the Yuda Basin. A-A', B-B' and C-C' are lines of cross-sections. 41PAW-1 denotes the location of the drilling hole [44] in the southwestern part of the Yuda Basin.

162 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

Modified from [31]

was estimated from 6.5 to less than 3 Ma based on fission-track dating (Figure 8) [30, 45]

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The Yoshizawa Formation overlies the Hanayama and Kurosawa formations with a notable angular unconformity and is distributed in the western part of the Yuda Basin along the Kawafune-Warikurayama Fault (Figure. 4). This formation consists of alternation of conglom‐ erate, sandstone and mudstone associated with lignite seams and peat. This formation may have been deposited on alluvial fans that were formed along the eastern margin of the western sector [30, 45]. The Yoshizawa Formation may be early Pleistocene in age based on the stratigraphic relationships between this formation and the Hanayama Formation and fluvial

Three major unconformities were formed around 10 Ma, 6.5 Ma and after 3 Ma in the eastern margin of the Yuda Basin. These unconformities have significant implications for post-rift tectonics not only in the Ou Backbone Range but also in Northeast Japan and thus the styles

The unconformity between the Kotsunagizawa Formation and the Kurosawa Formation at around 10 Ma is a partial unconformity within the basin and shows significant horizontal change in terms of the style [31]. The unconformity apparently eroded the Kotsunagiza‐ wa Formation at two hills in the eastern margin of the Yuda Basin (Figures. 5-7). At the road cut section north of the Kotsunagizawa (Figure. 6), the unconformity eroded the upper part of the Kotsunagizawa Formation and the Okinazawa basalt member and the shallow marine sandstone of the Kurosawa Formation overlies the Okinazawa basalt member with the basal conglomerate bed composed of boulders of basalts eroded from the underlying Okinawaza basalt member (Figure. 9)[43]. In this section, the upper 40 meters of the Kotsunagizawa Formation and the Ochiai volcanic breccia bed were eroded as well (Figure 9). The lower Kurosawa Formation in this section yields fission-track ages of 9.2 and 7.1 Ma (Figure. 10), significantly younger age than the top of the Kotsunagizawa Formation dated at around 12-11 Ma (Figure 8)[31]. This suggests an age gap of 3 – 2 Ma interval at

The amount of erosion varies over a short distance, but tends to increase where the boundary shifts to the east, towards the uplift axis of the eastern sector (Figures. 6 & 10). The maximum amount of the erosion attains more than 100 m at another hill west of Yugawa (Figure. 6), where the unconformity eroded the entire Kotsunagizawa Formation and the Kurosawa Formation directly overlies dacite tuff breccia of the Oishi Formation (Figure. 10). At the boundary, the Kurosawa Formation onlaps the Oishi Formation with lower angle dip of 8° W than the underling Oishi Formation with a dip of 16° W (Figure. 6)[43]. This indicates angular uncon‐ formity between the Oishi and Kurosawa formations. The basal 5 m of the Kurosawa Forma‐

although the upper boundary has not yet been precisely dated.

**4.2. Styles and origins of unconformities in the Yuda Basin**

and origins of these unconformities are described and discussed herein

terraces [30].

*4.2.1. 10 Ma unconformity*

the unconformity (Figure 8) [43].

**Figure 8.** Age-thickness diagram in the Yuda Basin after [31].

was estimated from 6.5 to less than 3 Ma based on fission-track dating (Figure 8) [30, 45] although the upper boundary has not yet been precisely dated.

The Yoshizawa Formation overlies the Hanayama and Kurosawa formations with a notable angular unconformity and is distributed in the western part of the Yuda Basin along the Kawafune-Warikurayama Fault (Figure. 4). This formation consists of alternation of conglom‐ erate, sandstone and mudstone associated with lignite seams and peat. This formation may have been deposited on alluvial fans that were formed along the eastern margin of the western sector [30, 45]. The Yoshizawa Formation may be early Pleistocene in age based on the stratigraphic relationships between this formation and the Hanayama Formation and fluvial terraces [30].

## **4.2. Styles and origins of unconformities in the Yuda Basin**

Three major unconformities were formed around 10 Ma, 6.5 Ma and after 3 Ma in the eastern margin of the Yuda Basin. These unconformities have significant implications for post-rift tectonics not only in the Ou Backbone Range but also in Northeast Japan and thus the styles and origins of these unconformities are described and discussed herein

#### *4.2.1. 10 Ma unconformity*

**Figure 8.** Age-thickness diagram in the Yuda Basin after [31].

164 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

The unconformity between the Kotsunagizawa Formation and the Kurosawa Formation at around 10 Ma is a partial unconformity within the basin and shows significant horizontal change in terms of the style [31]. The unconformity apparently eroded the Kotsunagiza‐ wa Formation at two hills in the eastern margin of the Yuda Basin (Figures. 5-7). At the road cut section north of the Kotsunagizawa (Figure. 6), the unconformity eroded the upper part of the Kotsunagizawa Formation and the Okinazawa basalt member and the shallow marine sandstone of the Kurosawa Formation overlies the Okinazawa basalt member with the basal conglomerate bed composed of boulders of basalts eroded from the underlying Okinawaza basalt member (Figure. 9)[43]. In this section, the upper 40 meters of the Kotsunagizawa Formation and the Ochiai volcanic breccia bed were eroded as well (Figure 9). The lower Kurosawa Formation in this section yields fission-track ages of 9.2 and 7.1 Ma (Figure. 10), significantly younger age than the top of the Kotsunagizawa Formation dated at around 12-11 Ma (Figure 8)[31]. This suggests an age gap of 3 – 2 Ma interval at the unconformity (Figure 8) [43].

The amount of erosion varies over a short distance, but tends to increase where the boundary shifts to the east, towards the uplift axis of the eastern sector (Figures. 6 & 10). The maximum amount of the erosion attains more than 100 m at another hill west of Yugawa (Figure. 6), where the unconformity eroded the entire Kotsunagizawa Formation and the Kurosawa Formation directly overlies dacite tuff breccia of the Oishi Formation (Figure. 10). At the boundary, the Kurosawa Formation onlaps the Oishi Formation with lower angle dip of 8° W than the underling Oishi Formation with a dip of 16° W (Figure. 6)[43]. This indicates angular uncon‐ formity between the Oishi and Kurosawa formations. The basal 5 m of the Kurosawa Forma‐

tion in this section contains glauconite sandstone (Figure. 10), indicating that significant hiatus

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In other parts of the eastern margin of the Yuda Basin, however, this boundary is a paracon‐ formity. Bathyal mudstones in the upper part of the Kotsunagizawa Formation grade upward into shallow marine sandstones in the top ~10 m of the formation. The Ochiai volcanic breccia bed [43] of up to 15 m in thickness rests on sandstones at the top of the formation and is a distinct key correlation bed in the uppermost Kotsunagizawa Formation (Figure. 10). The breccia bed was covered by a fine tuff bed (<1 m in thickness), which is intensely burrowed and bioturbated. This fine tuff bed is sharply overlain by shallow-marine, massive sandstone of the basal Kurosawa Formation (Figure. 10). The contact between intensely burrowed fine tuff at the top of the Kotsunagizawa Formation and the shallow marine sandstone of the basal Kurosawa Formation can be traced at the same stratigraphic position <1 m above the Ochiai breccia bed over 10 km from the south to the north except for the two hills where the uncon‐ formity incised deeply the underlying strata (Figure. 10). Although no significant erosion at the boundary was suggested, the significant age gaps between two formations indicate hiatus

In the western margin of the Yuda Basin (Core 41PAW-1 in Figures. 6, 7) and in the western sector (Figure. 7), the boundary between the Kotsunagizawa and Kurosawa formations is continuous. Correlation between the Yuda Basin and the westerm sector (Figure. 7) indicates that the equivalent time interval of unconformity in the eastern Yuda Basin corresponds to the deposition of the lower Kurosawa Formation in the western sector. Based on the above consideration, there was a westward submarine paleo-slope in the basin during ~12–9 Ma. In summary, the unconformity dated around 10 Ma was attributed to the uplift and westward tilting of the eastern sector, starting around 12 Ma and followed by the cessation of subsidence until 9 Ma and by subsequent onlapping of the Kurosawa Formation from west because of the resumption of the subsidence. The overall observation suggests that the eastern sector started to uplift and emerge earlier than the western sector. However, the western sector may have also slightly uplifted in this stage because the middle bathyal Kotsunagizawa Formation changed upward into the upper bathyal to outer shelf Kurosawa Formation (Figure 7).

The base of the main part of the Hanayama Formation was defined by the first occurrence of gravelly sandstone deposited in fluvial channels. The relationship between the Kurosawa and Hanayama formations is thus unconformity formed by fluvial erosion. Nakajima et al. [30] interpreted the unconformity as a sequence boundary formed by a relative sea-level lowering. Although the erosion by the unconformity is not significant (Figure. 7) and no age gap was suggested between two formations [30-31], this unconformity may have reflected major tectonic changes at around 6.5 Ma for the following reasons [43]. After the 6.5 Ma unconformity was formed, 4th or 5th order high frequency depositional sequences comprising of fluvial, deltaic and shallow water deposits with significant amount of conglomerate were deposited [30]. This contrasts with the Kurosawa Formation, which consist totally of shallow marine sandstones. This suggests that supply of coarse sediments began to exceed the accommodation

or low sedimentation rate event took place at the time of the unconformity[46].

or slow deposition at the boundary.

*4.2.2. 6.5 Ma unconformity*

**Figure 9.** A photo showing the unconformity between the Kotsunagizawa and Kurosawa formations in column K of Figure 10. White circles denote basal conglomerates derived from the Okinazawa Basalt Member of the Kotsunagiza‐ wa Formation. Modified from [43].

**Figure 10.** Detailed correlated columns showing the unconformity between the Kotsunagizawa and Kurosawa forma‐ tions in the Yuda Basin. Inset shows locations of lithologic columns in Figure 7. Modified from [31].

tion in this section contains glauconite sandstone (Figure. 10), indicating that significant hiatus or low sedimentation rate event took place at the time of the unconformity[46].

In other parts of the eastern margin of the Yuda Basin, however, this boundary is a paracon‐ formity. Bathyal mudstones in the upper part of the Kotsunagizawa Formation grade upward into shallow marine sandstones in the top ~10 m of the formation. The Ochiai volcanic breccia bed [43] of up to 15 m in thickness rests on sandstones at the top of the formation and is a distinct key correlation bed in the uppermost Kotsunagizawa Formation (Figure. 10). The breccia bed was covered by a fine tuff bed (<1 m in thickness), which is intensely burrowed and bioturbated. This fine tuff bed is sharply overlain by shallow-marine, massive sandstone of the basal Kurosawa Formation (Figure. 10). The contact between intensely burrowed fine tuff at the top of the Kotsunagizawa Formation and the shallow marine sandstone of the basal Kurosawa Formation can be traced at the same stratigraphic position <1 m above the Ochiai breccia bed over 10 km from the south to the north except for the two hills where the uncon‐ formity incised deeply the underlying strata (Figure. 10). Although no significant erosion at the boundary was suggested, the significant age gaps between two formations indicate hiatus or slow deposition at the boundary.

In the western margin of the Yuda Basin (Core 41PAW-1 in Figures. 6, 7) and in the western sector (Figure. 7), the boundary between the Kotsunagizawa and Kurosawa formations is continuous. Correlation between the Yuda Basin and the westerm sector (Figure. 7) indicates that the equivalent time interval of unconformity in the eastern Yuda Basin corresponds to the deposition of the lower Kurosawa Formation in the western sector. Based on the above consideration, there was a westward submarine paleo-slope in the basin during ~12–9 Ma. In summary, the unconformity dated around 10 Ma was attributed to the uplift and westward tilting of the eastern sector, starting around 12 Ma and followed by the cessation of subsidence until 9 Ma and by subsequent onlapping of the Kurosawa Formation from west because of the resumption of the subsidence. The overall observation suggests that the eastern sector started to uplift and emerge earlier than the western sector. However, the western sector may have also slightly uplifted in this stage because the middle bathyal Kotsunagizawa Formation changed upward into the upper bathyal to outer shelf Kurosawa Formation (Figure 7).

### *4.2.2. 6.5 Ma unconformity*

**Figure 9.** A photo showing the unconformity between the Kotsunagizawa and Kurosawa formations in column K of Figure 10. White circles denote basal conglomerates derived from the Okinazawa Basalt Member of the Kotsunagiza‐

166 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

**Figure 10.** Detailed correlated columns showing the unconformity between the Kotsunagizawa and Kurosawa forma‐

tions in the Yuda Basin. Inset shows locations of lithologic columns in Figure 7. Modified from [31].

wa Formation. Modified from [43].

The base of the main part of the Hanayama Formation was defined by the first occurrence of gravelly sandstone deposited in fluvial channels. The relationship between the Kurosawa and Hanayama formations is thus unconformity formed by fluvial erosion. Nakajima et al. [30] interpreted the unconformity as a sequence boundary formed by a relative sea-level lowering. Although the erosion by the unconformity is not significant (Figure. 7) and no age gap was suggested between two formations [30-31], this unconformity may have reflected major tectonic changes at around 6.5 Ma for the following reasons [43]. After the 6.5 Ma unconformity was formed, 4th or 5th order high frequency depositional sequences comprising of fluvial, deltaic and shallow water deposits with significant amount of conglomerate were deposited [30]. This contrasts with the Kurosawa Formation, which consist totally of shallow marine sandstones. This suggests that supply of coarse sediments began to exceed the accommodation space possibly because of the uplift of surrounding backbone range. In addition, this uncon‐ formity marks the major changes from uniform sedimentation rate in north-south direction in the Kurosawa Formation to differential sedimentation rate in the Hanayama Formation (Figure. 7). Those changes in depositional style may have resulted from tectonic conversion from extension to compression as discussed later.

## *4.2.3. < 3 Ma unconformity*

The angular unconformity between the Hanayama and Yoshizawa formations is the most significant structure among three unconformities described herein. The Kurosawa and Hanayama formations form a syncline with dips of 20 – 30 ° and are unconformably overlain by almost flat Yoshizawa Formation (Figure. 6). The formation of syncline and subsequent unconformity may have reflected uplift of the eastern and western sectors of the Ou Backbone Range during and after deposition of the Hanayama Formation. The unconformity was formed after 3 Ma although the precise age has not been dated.

## **4.3. Evolution of the Yuda Basin since Middle Miocene**

Evolution of the Yuda Basin since Middle Miocene is summarized here, based on correlation of tuff beds and unconformities (Figures 7 & 10) and basin subsidence analysis (Figure. 11). Tectonic history of the Yuda Basin and the surrounding Ou Backbone Range was divided into six tectonic stages according to the basin subsidence pattern (Figure. 12)[31].
