The Miocene PYB, one of the Tertiary sub-basins in the western margin of the East Sea (Sea of Japan), has experienced pull-apart style back-arc extension and subsequent extension, related to the clockwise rotation of the southwestern part of Japanese Island10. The active tectonic movement resulted in rapid subsidence of the basin from shallow marine to deep sea11 (max.: 700 m/Ma). The basin was filled with more than 1 km-thick non-marine to deep-marine sediments originating from six coarse-grained fan-delta systems along the western margin of the basin12,13 (Fig. 1). The Doumsan fan-delta is one of the largest gravelly fan-delta complexes. The western margin of the fan-delta is bounded by Cretaceous sedimentary and Eocene volcanic rocks which are represented by NNE-SSW trending normal and relatively high relief NW-SE trending transfer fault, while the eastern part is surrounded by local basement highs: Chilpo13 (northeastern part) and Guryongpo14 (southeastern part) and the Youngil Bay developed between them (Fig. 1). The bay is where the thickest sediment was deposited, implying that the Youngil Bay is the deepest part of the PYB13. The Doumsan fan-delta shows a whole spectrum of basin-margin sedimentary characteristics deposited in alluvial fan, braided stream, submarine Gilbert-type topset, foreset, toeset, prodelta, slope apron, and basin plain environments12,13,15 (Fig. 2). The exposed foreset is more than 250 m in height with a depositional angle of more than 30˚, and prograde about 2 km from the fan apex, showing a radial distribution pattern13. The vertical and lateral variations in sedimentary facies in outcrop and cores revealed that the fan-delta experienced four stages of evolution controlled by the tectonic subsidence and sediment supply; Stage P1: deposition of shallow marine fan-delta, late Early Miocene (ca. 17.5 Ma - 16.5 Ma); Stage P2: rapid subsidence of the basin and deposition of Gilbert-type fan-delta, latest Early Miocene (ca. 16.5 Ma - 16.0 Ma); Stage P3: deposition of fine-grained fan-delta, Middle Miocene (ca. 16.0 Ma - 13.0 Ma); Stage P4: deposition of diatomaceous mudstone, late Middle Miocene (ca. 13.0 Ma - 11.5 Ma)(Fig. 2)13. During Stage P3, the fan-delta experienced significant modification with a sudden decrease in coarse-grained sediment supply, resulting in the deposition of fine-grained sediments on the steeply-inclined slope, base of slope, prodelta, and basin plain environments. In these environments, coarse-grained sediments moved mainly by subaqueous sediment gravity flows with sheet, channel, and lobe geometries13,16,17. Hwang et al. (2021) also recognized two basin-wide megaturbidites and named the thick, laterally extensive sandstone as Pohang Megaturbidite (Fig. 2B; P3.m1, ca. 16 Ma) and HCM (Fig. 2B; P3.m2, ca. 13 Ma). The Pohang Megaturbidite occurs in subsurface cores with thickness ranging from 1.6 m to 12 m. The megaturbidte (Fig 2B; P3.m1) followed the general depositional slope, toward the east. Contrarily, the HCM (Fig 2B; P3.m2) flowed transversely to the northeast, irrespective of the general depositional slope.
Deposit characteristics
The HCM is exposed across the Doumsan fan-delta from the southeast (proximal) to the northeast (distal) direction (Fig. 2). The exposed outcrop is over 70 m thick in the proximal part, and less than 4 m thick in the distal part. The total volume of the HCM was estimated to be more than 2 km3 13. The succession is composed of three units; pebble- to cobble-conglomerate (Unit 1), pebble-rich conglomerate (Unit 2), and very coarse sandstone to mudstone (Unit 3), in ascending order (Fig. 3A). The pebble- to cobble-conglomerate overlies steeply-inclined foreset conglomerates (> 30˚ in slope angle) that were deposited during Stage P2 and steeply-inclined grayish mudstones (> 15˚ in slope angle) that were deposited during Stage P3 with sharp or erosive boundaries (Fig. 3B, Fig 4A). The conglomerate ranges in thickness from 1 m to 14 m, and gradually thins and terminates toward the northeast in the toeset region (Fig. 5). This unit is composed of rounded or sub-rounded pebble- to cobble-sized volcanic, granitic, and sedimentary rock clasts, as well as rip-up mud or calcite-cemented sandstone blocks. Sub-angular to sub-rounded boulder-sized clasts with maximum diameters more than 1 m, commonly occur (Fig. 6C). The conglomerate is characterized by disorganized and matrix-supported fabric with a poorly-sorted muddy sand matrix. Some parts are inversely graded and crudely stratified (Fig. 3B, Fig. 4A). Sedimentary structures suggest deposition from surge-type debris flow with variable strength rather than sluggish debris flow18.
The pebble-rich conglomerate (Unit 2) is distributed in the foreset and toeset regions (> 5˚ in slope angle). On the foreset region, this unit overlies the pebble- to cobble-conglomerate (Unit 1, Fig. 3C) with diffuse, sharp, or erosive lower boundaries (Fig. 3D, Fig. 4B). On the toeset, however, this unit overlies grayish mudstone that was deposited during Stage P3 with an erosive lower boundary. Large-scale syn-depositional deformation structures, such as giant flame structures and recumbent folds commonly occur (Fig. 4C). The conglomerate is up to 17 m in thickness and gradually thins and grades laterally or vertically into gravelly sandstone (Unit 3) toward the northeast (Fig. 5). This unit is dominantly composed of sub-rounded pebble-sized volcanic, granitic, and sedimentary rock clasts with subordinate amounts of cobble clasts. Several meters long rip-up mud blocks (Fig. 3C), shallow marine fossil-bearing calcite-cemented sandstone blocks (Fig. 4D), and angular to sub-rounded exotic granite boulder-sized clasts with maximum diameter more than 3 m also occur (Fig 4E, Fig. 6A, 6B). This unit is represented by disorganized and clast-supported fabric with poorly-sorted granule to sand matrix, and commonly shows variable grading patterns, such as coarse-tail normally grading and basal inverse grading with abundant water escape structures. Sedimentary structures are indicative of deposition from gravelly high-density turbidity current19.
The uppermost sandstone and mudstone (Unit 3) crop out from the foreset to the basin plain regions. On the foreset and toeset regions, this unit overlies the pebble-rich conglomerate (Unit 2) with a gradational lower boundary (Fig. 3D), and is up to 50 m thick on the toeset (Fig. 3E). On the prodelta to basin plain regions, this unit overlies the grayish mudstone deposited during Stage P3 with sharp or erosive lower boundary. The sandstone gradually thins toward the northeast and is commonly normally graded from gravelly coarse sand at the base to silt or mud at the top (Fig. 3F, Fig. 5). The coarse-grained lower part is either generally massive or crudely stratified with abundant water escape structures (Fig. 4F, Fig. 5), whereas the fine-grained upper part has well-developed tractional structures, such as horizontal, cross and backset laminae, antidune, and scour-and-fill structures (Fig. 4G, Fig. 5). These units also contain abundant lignite fragments (Fig. 4G), pebble-sized rip-up mud clasts (Fig. 4G), and meter-scale rip-up mud blocks or calcite-cemented sandstone blocks (Fig. 3E, Fig. 4F). Sedimentary structures suggest a transitional change from high-concentration supercritical turbidity current20 to low-density subcritical turbidity current21. Although the HCM was deposited by debris flow, gravelly high-density turbidity current, and high- and low-concentration turbidity currents, we use the term “turbidite” because the debris flow deposit comprises ~ 5 % of total volume of the HCM and occurs only in the proximal part.
Reconstruction of dispersal patterns
Detailed measurements of paleocurrent direction on the Doumsan fan-delta system were conducted using clast imbrications and the trends of chute or channel axes12,17, and anisotropy of magnetic susceptibility (AMS) analysis for sandstone and mudstone22. These studies revealed radial distribution of sedimentary facies and paleocurrent patterns from the fan apex, west of the Doumsan Mountain (Fig. 7). Contrarily, the dispersal pattern and paleocurrent direction of the HCM are different from those of the underlying and overlying successions (Fig. 7). In the foreset, the HCM occurs as a narrow northeast-trending belt on the southeastern part of the fan apex (Fig. 5, Fig. 7). Here, clast imbrication of the pebble- to cobble-conglomerate (58˚ - 69˚) and pebble-rich conglomerate (60˚ - 71˚) in HCM shows northeastward paleocurrent patterns (Fig. 5, Fig. 7). However, the underlying conglomerates and overlying sand-mudstone couplets show southeastward paleocurrent directions (Fig. 7). In the toeset, the HCM displays thick northeast-trending lobate geomorphology on the eastern part of the fan apex (Fig. 5, Fig. 7). The clast imbrication of the pebble-rich conglomerates (25˚ - 60˚) and the AMS analysis of the coarse-grained sandstone (26˚ - 72˚) in the HCM suggest northeastward paleocurrent patterns (Fig. 5, Fig. 7), although the underlying gravel channel lobe and overlying sand-mudstone couplets show east or partly southeastward paleocurrent directions (Fig. 7). In the prodelta and basin plain, the HCM occurs as a sheet-like sandstone on the northeastern part of the fan apex (Fig. 5, Fig. 7). Clast imbrication and AMS analysis of the coarse-grained sandstone show northeastward paleocurrent patterns (Fig. 5, Fig. 7). The underlying and overlying sand-mudstone couplets also show northeastward paleocurrent directions (Fig. 7). As mentioned above, the grain size and bed thickness decrease toward northeast, showing elongate and lobate geomorphology. These characteristics suggest that the HCM temporary flowed to the northeast, irrespective of the local depositional slope (Fig. 7).
Exotic granitic boulders
In the HCM, more than 1 m long (max.: 3 m in long diameter) outsized granitic boulders commonly occur as a single clast or patches in Unit 1 and Unit 2 (Fig. 6A, B, and C). These boulder clasts are exotic in that the outsized boulder clasts occur only in the HCM among the 1 km thick succession of the Doumsan fan-delta deposited during the Miocene (17.5 Ma – 11.5 Ma). In the Doumsan fan-delta, the Stage P1 (17.5 Ma – 16.5 Ma) deposits consist of alluvial fan, shallow marine and Gilbert-type foreset conglomerate and sandstone in the outcrop12, and slope apron and basin plain mudstone in the cores that were drilled in the basin center13. Here, the conglomerates are mostly composed of pebble to cobble-sized clasts of Cretaceous sedimentary, Cretaceous to Paleocene granite, and Paleocene to Eocene volcanic rocks with rare boulder-sized clasts (max.: < 50 cm in long diameter, Fig. 6D). The Stage P2 (16.5 Ma – 16 Ma) deposits consist of shallow marine Gilbert-type topset and steeply-inclined (> 30˚ in slope angle) foreset in the outcrop12, and slope apron and basin plain deposits in the cores13. Here, clast composition is similar to the underlying Stage P1 deposits, where the largest clasts occur at the base of foreset (max.: < 1 m in long diameter, Fig. 6E). The Stage P3 (16 Ma – 13 Ma) deposits consist of fine-grained foreset, toeset, prodelta, slope apron, and basin plain deposits in the outcrop and cores where the sediments are mostly composed of mudstone and sandstone12,16. Some channelized and lobate conglomerate contains pebble to cobble-sized clasts of volcanic, sedimentary, and granitic rocks with a few boulder-sized clasts (max.: < 50 cm in long diameter). The similarity of clast composition between the HCM and Stage P1, P2, and P3 deposits, also, suggests that they originated from the same alluvial feeder system, within 8 km west of the fan apex23.
Triggering event of the HCM
Event-driven deposits rarely show unique distribution pattern in stratigraphic record and contain exotic sediments, providing valuable information to identify their triggering event as well as mechanism1,9. The transversely emplaced dispersal pattern and distinctive paleocurrent direction in the HCM are distinctly different from the underlying and overlying sediment gravity flow deposits. The northeastward distribution pattern, regardless of local topography, indicates that the HCM was deposited as a result of exceptionally high velocity and voluminous movement of the sediment gravity flow, which is sufficient to surmount or erode out the local topography24,25. The extreme movement was also recorded in the HCM with abundant water escape structures, meter-scale rip-up mud blocks, and giant mud flames (Fig. 3B, Fig. 4C, F). According to a correlation between outcrop and subsurface cores in the PYB, the HCM occurs just above the foreset in the proximal part, deposited during Stage P2 (ca. 16 Ma). However, it occurs ~ 10 m below the diatomaceous mudstone in the distal part, deposited during Stage P4 (later than 13 Ma) (Fig. 2)13. The stratigraphic position of the HCM suggests a large-scale collapse of the fine-grained foreset ( >15˚ in slope angle) that was deposited during 3 Ma. Although, the absence of fine-grained forest deposits in the proximal part may also be responsible for the frequent slide/slumps, depositing the various sediment gravity flow deposits and the Pohang Megaturbidite13,17, more than several hundred meters thick fine-grained foreset sediments may have been reworked in the proximal part. The total volume of the HCM (more than 2 km3)13 also suggest the occurrence of large-scale collapse in the fine-grained foreset.
The distinctive coarse-grained sediments of the HCM, encased in fine-grained sediments (Fig. 3B, 3G), suggest that the HCM was deposited by a less frequent event3,4,26. The abundant shallow-marine fossils and lignite fragments indicate that sediments were originated from the reworking of the shallow marine or coastal sediments13,16. Particularly, the occurrence of exotic sub-angular boulder-sized granitic clasts and the similarity of clast composition to that of the basement rocks, more than 1 km west of the present fan apex, strongly suggest that there was a sea-level run-up27,28, which eroded the western border drainage system of the Doumsan fan-delta and transported the boulders to the deep sea. According to detailed monitoring of offshore slope failure29, large-scale flood may cause erosion of the alluvial feeder system and slope failure. However, this possibility is discarded because the exotic granite boulders occur only in the HCM among the 1 km succession of the Doumsan fan-delta deposited during approximately 6 Ma (17.5 Ma - 11.5 Ma)13. Furthermore, if the exotic sub-angular boulder clasts were transported by a millennium-scale flood, the boulders would move radially, following the local depositional slope. Only catastrophic sea-level run-up, in addition to less frequent events such as a giant tsunami wave, are inferred to have been recorded in the HCM13.
Possibility of tsunami generation
Although the physical aspects of the generation mechanism of tsunami remain obscure, here, we review some possible scenarios that could have formed giant tsunamis in the PYB without considering the bolide impact. The East Sea, formed by pull-apart style back-arc spreading and subsequent multi-phase tectonic reactivation during the Cenozoic, has been tectonically active30. In this type of basin, tectonic earthquake (fault activity), volcanic eruption, and major slide/slump on the high-gradient slope may have occurred anywhere. Initially, the extensional movement was related to the two major strike-slip principal displacement zones on the eastern (i.e., Japanese side: the Hidaka Shear Zone, Tanakura Tectonic Line) and western (i.e., Korean side: the Ulleung Fault) margins of the East Sea10,31 (Fig.1A). During the Middle Miocene (ca. 18 Ma - 12 Ma), collision of the Izu-Bonin Arc to the southwestern Japanese Islands resulted in the clockwise and counterclockwise rotation of the southwestern and northeastern parts of the Japanese Islands, respectively31. Subsequently, the tectonic regime of the East Sea changed from extension to compression31,32. Particularly, the eastern margin including offshore faults, related to the Hidaka Shear Zone and Tanakura Tectonic Line, is regarded as a major source region of distant tsunamis, based on the historical records and modern observation of earthquakes and tsunamis33,34 (Fig. 1A). According to seismic observation, the N-S and NNE-SSW trending high-angle reverse faults were reactivated from extension to compression associated with the rotation of northeastern Japan34. The reactivation led to frequent tectonic movements, causing tsunami waves. Modern earthquake-induced tsunamis, several meters in height, have been commonly recorded in the western margin of East Sea (i.e. the Korea Peninsula)33,34.
Another source of tsunami events could have been the volcanic activities in the East Sea. During the Middle Miocene (ca. 18 Ma - 12 Ma), the collision of the Izu-Bonin Arc against the southwestern Japanese Islands rapidly weakened the extensional stress in the back-arc region, and hence, the back-arc opening in the East Sea ceased until about 12 Ma32. Under the weakened extensional or incipient compressional regime, limited volcanic eruptions have occurred with chain-like volcanic sea mounts across the northern Ulleung Basin35 (Fig 1A) and the northeastern margin of the Yamato Basin30 (Fig 1A). The volcanic eruptions in the East Sea may also be another possible mechanism for the generation of giant tsunamis, although the eruption time (ca. 12 Ma), based on volcano-stratigraphic age control, was slightly later than that of the deposition of the HCM (later than 13 Ma).
According to a tsunami simulation based on seismic reflection data from the western and southwestern margin of the Ulleung Basin, the other source of tsunami event could have been the slide/slump on the steeply-inclined slope around the East Sea37. Along the entire shelf margin and basin plain of the Ulleung Basin, various mass transport deposits (MTDs) have been deposited38,39,40. The MTD of the western and southwestern margins of the Ulleung Basin exhibit different characteristics during the Middle Miocene (ca. 16 Ma - 12 Ma). The western margin is characterized by a small amount of sediment input via a few small-scale streams41, and the MTDs of the western margin have a few km3 and limited runout38. Contrarily, the southwestern margin is characterized by a high-sedimentation rate with several regressive sequences of a shelf-slope system42. The MTDs of the southwestern margin are one order of magnitude larger than that of the western margin and may have traveled several tens of kilometers to the basin plain38. Although the larger MTDs of the southwestern margin are potentially more likely to cause giant tsunami waves, Urgeles et al. (2019) reported that the tsunamis that were generated in this area may have had a limited impact on the PYB, because the Guryongpo basement high may have restricted the tsunami propagation towards the Youngil Bay (Fig. 1B). Thus, we think that the giant tsunami which affected the PYB during the Middle Miocene (later than 13 Ma) may be related to the fault movement or volcanic eruption in the northeastern part of the East Sea.
Triggering mechanism of the HCM
As highly energetic tsunami waves propagate from offshore to onshore, the propagation pathway is determined by bathymetry of the basin43. The topographic features, where the deepest part is located at the E–W trending Youngil Bay13, can facilitate the propagation of giant tsunami approaching from the east to west. Additionally, the local basement highs (Chilpo and Guryongpo) are located in the northern and southern parts of the Youngil Bay, respectively (Fig. 1B), and play a role in not only blocking tsunami waves entering the northeastern and southeastern parts of the basin but also directing the energy of tsunami waves into the Youngil Bay. Consequently, the directed tsunami waves rapidly amplified with overlapping waves, creating large-scale tsunami waves in Youngil Bay44.
As the amplified tsunami waves flood the coastal region on the fan-delta, they cause sea-level run-up on the onshore part of the fan-delta. The sea-level run-up can significantly be controlled by the onshore topography45,46,47,48. In the Doumsan fan-delta case, the shallow marine and onshore areas were surrounded by NNE-SSW trending normal and NW-SE trending transfer faults (Fig. 8A). The funnel-shaped geomorphology further confines the westward propagating tsunami waves, abruptly narrowing the shoreline (Fig. 8B). Thus, the energy of tsunami waves approaching the confined onshore area would be further amplified. The extremely amplified tsunami waves may be responsible for the giant run-up of sea-level, flooding the high-gradient drainage system of the fan-delta.
As the tsunami waves crash against the border faults, the waves can flood over the relatively low-relief fault or can be reflected by the steeply-inclined faults49. In the Doumsan fan-delta case, the NNE-SSW trending normal fault was relatively less steep than that of the NW-SE trending transfer fault, because the NNW-SSW fault experienced listric normal movement during the initial stage of basin formation. However, the NW-SE trending transfer fault underwent left-lateral strike-slip movement during and after the basin formation. Thus, the tsunami waves approaching the low-relief NNE-SSW normal fault could partly flood over or be reflected by NNE-SSW normal fault, whereas the tsunami waves approaching the high-relief steeply-inclined NW-SE transfer fault will mostly be reflected by the NW-SE transfer fault (Fig. 8C).
The direction of reflection depends on the incidence angle of tsunami waves and the strike and dip of the fault plane. The tsunami waves will reflect to the north as the westward approaching tsunami waves diagonally crash against the NE-SW trending transfer fault. Contrarily, the tsunami waves crashing against the NNE-SSW trending listric normal fault are likely to be reflected to the ESE. If the equivalent amount of tsunami wave energy hits the border faults, the amount of reflected tsunami wave energy from the NW-SE trending high-relief transfer fault can be more significant than that of the NNE-SSW trending low-relief normal fault (Fig. 8C). Hence, the general direction of reflected wave will be northward. As the reflected waves move offshore, the northward backflows will inevitably flow to the topographic low51. As the Doumsan fan-delta generally shows eastward-dipping topography with a radial distribution pattern12, the combined effect of northward reflected waves may be responsible for the northeastward paleocurrent direction of the HCM (Fig. 8D), which is different from the underlying and overlying sediment gravity flow deposits.
The backflow can form dendritic drainage patterns that coalesce into channels28 (Fig. 8D) and accelerate rapidly because the sea-level height during the backflow stage is lower than that of the tsunami inundation52. The accelerated flows are capable of directly reworking large volumes of sediments from the coastal region53. On a high-relief slope, a sudden decrease of water mass can result in temporary liquefaction of the loosely compacted fine-grained foreset sediments ( >15˚ in slope angle) which override the high-gradient coarse-grained foreset54,55 ( >30˚ in slope angle), causing collapse of several hundred meters thick, steeply-inclined, unstable, fine-grained foreset deposits1. In the Doumsan fan-delta case, the boundary between the fine-grained and coarse-grained foresets may have acted as a slip face for the giant failure of the steeply-inclined sediments (Fig. 8C, D).