Watanabe et al.
Progress in Earth and Planetary Science(2023) 10:40
https://doi.org/10.1186/s40645-023-00567-x

Progress in Earth andPlanetary Science

Open Access

REVIEW

Geological history of the land area
between Okinawa Jima and Miyako
Jima of the Ryukyu Islands, Japan,
and its phylogeographical significancefor the terrestrial organisms of theseand adjacent islands
Nana Watanabe1, Kohsaku Arai2 , Makoto Otsubo3 , Mamoru Toda4, Atsushi Tominaga5 , Shun Chiyonobu6,
Tokiyuki Sato6, Tadahiro Ikeda7 , Akio Takahashi8 , Hidetoshi Ota7 and Yasufumi Iryu1*   

Abstract
The modern and Late Pleistocene terrestrial fauna of Miyako Jima and adjacent islands (the Miyako Islands)
in the southern Ryukyu Islands, southwestern Japan, includes some endemic taxa or genetically unique populations
that exclusively have closest allies in the more isolated Okinawa Jima and adjacent islands (the Okinawa Islands)
than in the Yaeyama Islands, which are located southwest of the Miyako Islands with much narrower intervening
straits. Those taxa or populations include representatives of lineages that have physiologically highly limited ability
for over-sea dispersal and the Miyako Islands are currently separated from the Okinawa Islands by at least 300 kmof open water; therefore, the formation of this phylogeographical pattern is perplexing. In this study, we review
the late Cenozoic geology of the Miyako Islands, southern Okinawa Jima, the Okinawa–Miyako submarine plateau(OMSP; a plateau located between Okinawa Jima and Miyako Jima), and the Kerama gap, which is a depression
between the OMSP and Okinawa Jima. We then consider the origin of the modern and Late Pleistocene terrestrial
animals, including a number of non-volant vertebrates on the Miyako Islands. Finally, we propose a new hypothesis(the OMSP hypothesis) to explain the enigmatic composition of modern and Late Pleistocene terrestrial vertebrate
fauna of the islands. Southern Okinawa Jima was uplifted and emerged after ca. 2 Ma and was temporarily connectedto the OMSP, which is likely to have emerged earlier than southern Okinawa Jima, to form a large island extending
from Okinawa Jima to the Miyako Islands with a NE–SW direction of ~ 400 km. Subsequently, Okinawa Jima became
separated from the OMSP when the Ryukyu Group—which is composed of Quaternary reef and associated fore-reefand shelf deposits—began to accumulate around the island at 1.7–1.4 Ma. During the interval from 2.0 to 1.7–1.4 Ma,
numerous terrestrial animals, including flightless vertebrates, extended their distribution to the OMSP. Although
the Miyako Islands repeatedly underwent complete submergence during deposition of the main part of the Ryukyu
Group (1.25–0.4 Ma), they were uplifted and emerged to become a land area after ca. 0.4 Ma. In contrast, the OMSP*Correspondence:
Yasufumi Iryu
yasufumi.iryu.d8@tohoku.ac.jp
Full list of author information is available at the end of the article© The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
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subsided after ca. 0.4 Ma and was almost completely submerged after 0.27 Ma. During ca. 0.4–0.27 Ma, terrestrial
animals migrated from the OMSP to the Miyako Islands.
Keywords Ryukyu Islands, Kerama gap, Okinawa–Miyako submarine plateau, Pleistocene, Shimajiri Group, Ryukyu
Group, Vertebrate, Phylogeography, OMSP hypothesis

1 Introduction
The Ryukyu Islands are a chain of islands extending
1200 km from Tanegashima in the northeast (30° 44′ N,
131° 0′ E) to Yonaguni Jima in the southwest (24° 27′ N,
123° 0′ E), and they represent a typical island-arc–trenchsystem, which has been generated by subduction of thePhilippine Sea Plate along the Ryukyu Trench beneaththe Eurasian Plate (Fig. 1). The Ryukyu Islands are geographically divided into the North, Central, and South
Ryukyus by two major faults underlying the Tokara gap
and the Kerama gap. The Okinawa Trough is a backarc
basin that separates the island arc from the East China

Fig. 1 Topography and bathymetry of the Ryukyu Island Arc. Map of the topography and bathymetry of the Ryukyu Island Arc and its surroundings.
A Aguni Jima, KG Kerama gap, MS Mikako saddle, OMSP Okinawa–Miyako submarine plateau. The red triangle indicates the location of MITI
Miyakojima-Oki well. The map was prepared using Generic Mapping Tools (Wessel et al. 2019; available at https://​www.​gener​ic-​mappi​ng-​tools.​org;
accessed 1 Nov 2021). Elevation and depth data are from SRTM15+V2.4 (https://​doi.​org/​10.​1029/​2019E​A0006​58; accessed 1 Dec 2021)

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Sea Shelf. The Kuroshio Current, a major warm current emerging to the east of Luzon Island, enters theEast China Sea through the strait between Taiwan andYonaguni Jima, flows northeastward along the island
chain, and then bifurcates to the southwest of Yakushima (Fig. 1). The main current changes its direction andexits to the Pacific through the Tokara Strait (the strait
between Amamioshima and Yaku Shima), and a subsidiary current flows northward along Kyushu. Although the
Ryukyu Islands are located at relatively high latitudes in
the coral reef province, most of the islands and islets arefringed by coral reefs owing to the predominant flow of
this warm current along them.
The Ryukyu Islands are often called “the Galapagos of
the Orient” because their modern terrestrial fauna andflora are characterized by a high proportion of endemic
taxa, such as Pentalagus furnessi (Amami-rabbit), Gallirallus okinawae (a flightless rail named Yambaru-kuina),
Protobothrops flavoviridis (a venomous snake namedHabu), and Cheirotonus jambar (a long-armed scarabbeetle named Yanbaru-tenagakogane) (Government ofJapan 2019). Several hypothetical scenarios have been
proposed to explain this current biogeographic pattern by assuming their ancestors’ migration to and isolation within the past Ryukyu Islands in response toradical temporal changes in the configuration of land
there. Kizaki and Oshiro (1977), for example, attempted
to reconstruct the geological evolution of the Ryukyu
Islands and invoked temporal changes in the extent andshape of each component island to explain the origin
and migration pathway of terrestrial fauna of the Ryukyu
Islands. Those authors assumed that the Ryukyu Islands
were uplifted after deposition of the Upper Miocene–
Lower Pleistocene Shimajiri Group, which is composed
mostly of siliciclastic rocks, forming a land bridge fromthe Eurasian continent to the current Tokara Strait during the Early Pleistocene, and that the terrestrial animals
migrated from the continent via this land bridge. The
hypothesis of the Pleistocene land bridge was subsequently repeatedly proposed by various studies but withsome modifications (e.g., Ujiié 1986; Koba 1992; Kimura
1996, 2002; Oshiro and Nohara 2000; Ujiié and Kaneko
2006). These paleogeographic reconstructions werebased primarily on the classical theory of orogeny, lacking the perspective of plate tectonics and the expansionof the Okinawa Trough, which would have had a significant influence on the paleoenvironment of the Ryukyu
Islands were not considered (for comparison of the paleogeographic maps of those previous studies, see Furukawa and Fujitani 2014).
Recent molecular phylogenetic and evolutionary studies of various lineages of terrestrial organisms on the
Ryukyu Islands have yielded results that are clearly
inconsistent with these hypotheses (e.g., Dedeine et al.
2016; Xu et al. 2016). For example, a number of amphibian and reptile lineages occurring in the present-day
Central Ryukyus have been isolated from the Eurasian continent and the South and North Ryukyus since
the Pliocene or earlier (Ota 1998; Okamoto 2017). To
resolve these contradictions, we integrate the latest geological, paleontological, and phylogeographical data andpropose a new hypothesis (the OMSP hypothesis) to
explain temporal changes in the configuration of islands(OMSP = Okinawa–Miyako submarine plateau; Fig. 1).
The present study introduces this more plausible hypothesis and presents a model to explain the enigmatic modern and Late Pleistocene terrestrial vertebrate fauna of
the Miyako Islands, Miyako Jima and its adjacent islands(Ogami Jima, Ikema Jima, Kurima Jima, Irabu Jima, andShimoii Shima).
Although the Miyako Islands belong to the South
Ryukyus, modern and Late Pleistocene terrestrial vertebrates, the latter of which have been found in fissure,cavity, and cave deposits (Fig. 2a), include not only taxa
common to other islands of the South Ryukyus but also
those that are endemic to the Miyako Islands and areotherwise common to the Central Ryukyus (e.g., Ota
1998, 2003; Tominaga et al. 2019) (see Sects. 3.6 and
3.7). The occurrence of taxa exclusive to the Central
Ryukyus or of populations genetically closest to those(See figure on next page.)
Fig. 2 Outcrop photographs of upper Cenozoic deposits on Miyako Jima and Okinawa Jima. a Cavity-filling deposits (arrowed) with terrestrial
vertebrate fossils at an outcrop near Nishihara, Miyako Jima. S Shimajiri Group, R Ryukyu Group. Scale bar = 1 m. b Stratigraphic relationshipbetween the Shimajiri Group and the Chinen Formation at the Urizun outcrop in Chinen, southern Okinawa Jima. Dark-gray siltstone (S)of the Shinzato Formation (Shimajiri Group) overlain by light-gray fine-grained siltstone of the lower Chinen Formation (LC), which in turn is overlain
by beige bioclastic grainstone of the upper Chinen Formation (UC). Limestone of the Naha Formation (Ryukyu Group) occurs at the top of thisoutcrop. However, its contact with the Chinen Formation is not observed. Scale bar = 5 m. c Limestone of the Ryukyu Group (R) unconformably
overlying siltstone (S) of the Yonahama Formation (Shimajiri Group) at an outcrop near Nishihara, Miyako Jima. Scale bar = 2 m. d Limestonesof the Ryukyu Group (the Itoman and Naha formations) unconformably overlying siltstone of the Shimajiri Group at an outcrop ~ 500 m southeastof Yamagusuku, Itoman City, Okinawa Jima. N, Naha Formation; I, Itoman Formation; S, Shinzato Formation. Scale bar = 1 m. e Diageneticallyaltered limestone of the Itoman Formation (I) overlain by limestones of the Naha Formation (N) at Maehira Quarry near Maehira. The altered
limestone is abundant in siltstone blocks derived from the Shimajiri Group. Scale bar = 2 m. f Well-sorted detrital limestone (= bioclastic grainstone)of the Minatogawa Formation (M) unconformably overlying detrital limestone (= bioclastic packstone) of subunit 1R of the Naha Formation (N)at an outcrop located to the southeast of Aragusuku, Yaese Town, Okinawa Jima. Scale bar = 1 m

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in the Central Ryukyus is enigmatic because the Miyako
Islands are separated from Okinawa Jima, the nearest major island of the Central Ryukyus, by ~ 300 kmof sea, which is a large barrier for taxa with limited or
non-existent over-sea dispersal ability. Indeed, a distinct discontinuity in nonvolant terrestrial fauna hasgenerally been recognized between the Miyako Islands
and the Okinawa Islands (e.g., Ota 2000; Komaki 2021).
On the basis of geological, paleontological, and molecular phylogeographical data, this study shows that the
OMSP emerged sometime between 5.53 and 0.27 Ma

Fig. 2 (See legend on previous page.)Watanabe et al. Progress in Earth and Planetary Science(2023) 10:40

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and acted as a land bridge for these animals to migratefrom the Miyako Islands to the Okinawa Islands.
Results of deep-sea tow and submersible surveys haveled some authors to suspect that Okinawa Jima andMiyako Jima were connected by a land bridge sometime
during the period from 0.24 Ma to 20 ka, followed by
rapid subsidence (Monma et al. 1991; Kimura et al. 1991,
1992). However, this hypothesis has not been widely supported because it assumes the occurrence of large-scale
subsidence (up to 2000 m) during a geologically shortperiod (ca. 20 kyr) and because the sedimentary environments and chronology of the upper Cenozoic deposits on these islands were not well constrained at thattime. In this paper, we show that a land bridge between
Okinawa Jima and Miyako Jima did not exist and that thepresumed rapid subsidence did not occur during such
periods (0.24 Ma–20 ka and < 20 ka, respectively) by analyzing and interpreting sedimentological and chronological–biochronological data collected by Kimura et al.
(1991, 1992).
Geographical names used here follow the Gazetteer ofJapan 2007 (Geographical Survey Institute of Japan andJapan Coast Guard 2007). However, in this paper, theword “Island” is removed from the official name of eachisland for the sake of brevity. For example, Miyako Jima
Island is described as “Miyako Jima”.

isolated. Molecular phylogenetic techniques have beenused to address whether vicariance or dispersal has been
the main mechanism of establishing island populations.
The approach using these techniques attempts to determine the phylogenetic relationships and divergence times
between extant island taxa and their ancestors and compare them with the geological ages of the islands to infer
whether the island taxa arrived via a land bridge or by
over-sea dispersal.
Island biogeography–phylogeography and geology arecomplementary. For example, the land bridge and dispersal (over-sea) theories have both been proposed to
explain the island taxa on the Galapagos Islands. However, the formation of a land bridge around the islands
has been ruled out by increased knowledge of oceanfloor geology. As another example, the biogeographyof the Malay Archipelago is highly complex, and faunalboundaries such as the Wallace Line and the Weber Line
have been drawn. This complex biogeography is known
to have resulted from the location of the archipelago at
the junction of four major plates in combination withpast sea-level changes (Ali and Heaney 2021). These areexamples where the distribution of organisms can be reasonably explained by the geology of the islands and theirsurroundings. Conversely, the distribution of organisms
may provide constraints on reconstructing Earth’s history. It is well known that Alfred Wegener (1929) citedthe geographic distribution of Mesosaurus (a Permian
aquatic reptile), Cynognathus and Lystrosaurus (Triassic terrestrial reptiles), and Glossopteris (a Permian seedfern) as essential pieces of evidence supporting the Theory of Continental Drift.
A land bridge connecting two-thirds of the Ryukyu
Islands to the Eurasian continent was assumed to haveformed during the Quaternary (Kizaki and Oshiro 1977).
However, phylogenetic studies of amphibians and reptiles
have refuted this interpretation (Ota 1998), and this view
is supported by geological evidence (Iryu et al. 2006). It
is thus concluded that the integration of phylogeography
and geoscientific knowledge is the best way to elucidate
the geological development of islands and the establishment and maintenance mechanisms of island biota.

2 Island biogeography–phylogeography
and geology
Islands are where the theory of evolution emerged. It is
well known that Darwin and Wallace’s attempts to understand island biota led to the formulation of the new concept of biological evolution (Darwin 1859; Wallace 1863)
Island biogeography is a branch of biogeography that
focuses on islands (e.g., Whittaker and Fernandez-Palacios 2007). Depending on perspective, biogeography canbe broadly divided into historical biogeography and ecological biogeography. Historical biogeography attempts
to reconstruct the origin, dispersal, and extinction of taxa
and biota. In contrast, ecological biogeography aims to
explain geographic variations in the diversity and spatiotemporal distribution of taxa in terms of their interactions with the physical and biological environment. The
study of island biogeography follows this division.
A controversial topic in historical biogeography iswhether island organisms have migrated over a landbridge that was connected to a relatively large land mass(vicariance) or across oceans (dispersal) (de Queiroz
2005; Cowie and Holland 2006; Yoder and Nowak 2006;
Grandcolas et al. 2008). One reason for the prolonged
controversy is that even organisms on continental islands
once connected by a land bridge may have migrated toislands across the sea after they were separated and3 Review
3.1 Geology of the Miyako Islands
3.1.1 Shimajiri GroupThe Cenozoic succession on the Miyako Islands consistsof the Pliocene to Lower Pleistocene Shimajiri Groupconsisting of siliciclastic rocks and the Pleistocene
Ryukyu Group composed chiefly of carbonates formedin coral reefs and associated fore-reefs to shelves. Deep
drilling of the MITI Miyakojima-Oki well (Figs. 1 and
3) has revealed that the Shimajiri Group is underlain byWatanabe et al. Progress in Earth and Planetary Science(2023) 10:40

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Fig. 3 Correlation of upper Cenozoic successions. Upper Cenozoic successions on and around the Miyako Islands, on the OMSP, and on Okinawa
Jima, together with their correlations and ages. Paleobathymetric changes and tectonic movements of southern Okinawa Jima are also shown.
Upper Pleistocene and Holocene deposits are not illustrated. Data: Miyako Jima, Sato et al. (2002), Hanagata and Nobuhara (2015); MITI
Miyakojima-Oki well, Tsuburaya and Sato (1985); Offshore Okinawa 1 − X well, Aiba and Sekiya (1979), Sato et al. (2022); Okinawa Jima, Tanaka
and Ujiié (1984), Hanagata (2004), Sato et al. (2004), Odawara et al. (2005b), Chiyonobu et al. (2009), Fujita et al. (2011), Imai et al. (2017). Biozones:a CN zones, calcareous nannofossil zones of Okada and Bukry (1980); b PL zones, planktic foraminiferal zones of Berggren et al. (1995); c N zones:
standard tropical–subtropical planktic foraminiferal zones of Blow (1969). Paleobathymetry: 1, Mollusks, Ogasawara and Masuda (1983); 2, Mollusks,
Noda 1980, 1988; 3, Ostracods (Nohara 1987); 4, Benthic foraminifers (Nakagawa et al. 2001); 5, Benthic foraminifers (Matsumoto et al. in press).
Note that the vertical lengths of the bars for biozones do not denote actual chronological ranges. *sublittoral; **upper shelf slope (100–300 m
water depth); ***upper continental slope (< 500 water depth); ****lower continental slope (500–1500 water depth). Tectonic movement: Faulting 1,
formation of normal faults cutting the Shimajiri Group; Faulting 2, formation of faults cutting the Shimajiri Group and the main body of the Ryukyu
Groupthe Lower Miocene Yaeyama Group (Tsuburaya and Sato
1985). The lower interval of the Yaeyama Group is composed of quartz-rich sandstone and black mudstone witha shallow-marine limestone bed at the top. The lower
interval correlates to the Lower Miocene via planktic
foraminiferal zones N6–N8 (Blow 1969) and calcareous
nannofossil zones CN2–CN3? (Okada and Bukry 1980).
The upper interval consists mainly of partly calcareous,
fine-grained sandstone alternating with mudstone, intercalated with coal beds. No age-diagnostic fossils havebeen found in the upper interval. The Yaeyama Grouphas been discovered in the Miyako R-1 Exploratory Well

drilled on Miyako Jima (Kato 2016). The Yaeyama Groupis interpreted to have been deposited in non-marine andshallow-marine environments on the basis of lithological characteristics (e.g., the occurrence of coal beds andshallow-marine carbonate rocks).
Several lithostratigraphic frameworks have been proposed for the Shimajiri Group on the Miyako Islands.
These frameworks are essentially similar but vary interms of stratigraphic nomenclature (Doan et al. 1960;
Ujiié and Oki 1974; Yazaki 1978; Yazaki and Oyama 1979;
Nakamori 1985). The lithostratigraphy of the ShimajiriGroup is outlined here following Nakamori (1985). The

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Shimajiri Group includes the Oura, Onogoshi, Yonahama, and Minebari formations (Fig. 3). The Oura Formation (> 600 m thick) is composed mainly of sandstone,siltstone, and their alternations, together with granule
to cobble conglomerates. Oyster (Crassostrea gigas) andlignite beds are intercalated in the lower interval of thisformation, whereas the upper interval yields mollusks
and solitary corals. Lithological characteristics and fossil assemblages imply that the depositional environmentsof the lower and upper intervals were brackish (intertidal
zone of an inner bay) and open-marine shallower than
50–60 m water depth, respectively (Ogasawara and Masuda 1983). The occurrence of a fossil elephant, Trilophodon (which is currently considered a junior synonym ofGomphotherium), possibly from the Oura Formation(Hasegawa et al. 1973), suggests that there was a landarea around the Miyako Islands during the deposition ofthis formation. The Onogoshi Formation (600 m thick)comprises massive or weakly bedded siltstone and mudstone yielding mollusks and solitary corals. The lower
interval of the formation is considered to have been
deposited in an open-marine environment shallower
than 50–60 m water depth, like that of the upper intervalof the Oura Formation. In contrast, the upper interval of
the Onogoshi Formation is thought to have accumulated
on a deeper (lower sublittoral to upper bathyal) muddy
bottom (Ogasawara and Masuda 1983). The main lithology of the Yonahama Formation (700 m thick) is tuffaceous siltstone with interlayers of tuff. The individualinterlayers measure several centimeters to 1 m thick andcontain abundant pumice. This formation yields mollusks
and solitary corals and is more abundant in planktic foraminifers and calcareous nannofossils than the other threeformations, suggesting a more pelagic and deeper setting than that of the Onogoshi Formation. The molluskan
assemblages consist of upper bathyal taxa together withthose reworked from shallow (upper sublittoral) deposits(Ogasawara and Masuda 1983). The Minebari Formation(500 m thick) consists predominantly of rhythmic alternations of sandstone and siltstone with tuff interlayers.
A single set of sandstone and siltstone is 6–40 cm thick.
Although mollusks and solitary corals have been found in
this formation, they are less common than in the otherformations of the Shimajiri Group. The lithology and fossil assemblages are thought to indicate a return to a lower
sublittoral environment (Nakamori 1985). In contrast,
Nobuhara (2001) reported bathyal molluskan assemblages with high species diversity (more than 80 species)
from this formation.
The geological age of the Shimajiri Group on Miyako
Jima is uncertain, with two different views having beenpresented. Planktic foraminiferal biostratigraphy has
been used to determine an age range from the Late

Miocene to the Calabrian for the Shimajiri Group (Ujiié
and Oki 1974; Yazaki 1978). Kato (2016) measured strontium (Sr) isotopes of foraminifers, supporting the Late
Miocene initiation of sedimentation of the group (as earlyas 7 Ma). Hanagata and Nobuhara (2015) demonstrated
that the stratigraphic interval comprising the upper partof the Onogoshi Formation to the Minebari Formation
correlates to zones PL 1 (Messinian to Zanclean) to PL
5 (Piacenzian to Gelasian) defined by Berggren et al.
(1995). It has not been possible to place chronologicalconstraints on the Oura Formation because Globorotalia tumida, an age-diagnostic planktic foraminifer for PL
1, is a deep thermocline species and is extremely rare ordoes not occur in the Oura Formation, which consists ofshallow-marine sedimentary rocks. However, planktic
foraminiferal and calcareous nannofossil biostratigraphy
and magnetostratigraphy have been used to constrain
the age of the Shimajiri Group to the Zanclean–Gelasian(Nakagawa et al. 1976; Nakamori 1985; Sato et al. 2002).
Nannofossils indicating a Late Miocene age have notbeen found at the site from which Ujiié and Oki (1974)
reported age-diagnostic planktic foraminifers for the Late
Miocene (Sato et al. 2022). Sato et al. (2002) showed thathorizons stratigraphically higher than those investigatedby Hanagata and Nobuhara (2015) in the Minebari Formation are as young as ca. 2 Ma. Consequently, all ormost of the Shimajiri Group is Zanclean to Gelasian inage.
3.1.2 Ryukyu GroupThe Ryukyu Group (MacNeil 1960), formerly termed the
Riukiu Limestone (Yabe and Hanzawa 1930), is composedof Pleistocene carbonate deposits that were formed incoral reefs and associated fore-reefs to shelves and coeval
non-marine siliciclastic sedimentary rocks. The groupis distributed over most of the islands of the Central
and South Ryukyus, where the deposits are found up to~ 200 m in elevation. The Ryukyu Group on the Miyako
Islands has been divided into the Miyakojima Limestone
and the Shimojijima Limestone (Nakamori 1985). The
former constitutes the main body of the Ryukyu Group,
occurs throughout the Miyako Islands, and is cut by
numerous normal faults that strike NNW–SSE and are
downthrown to the east, exhibiting cuesta-like landforms (Fig. 5). In contrast, the latter comprises younger
reef deposits with limited horizontal and vertical distribution and is found only on Shimoji Shima and Irabu
Jima, where it unconformably overlies the Miyakojima
Limestone. The Miyakojima Limestone is interpreted as
being represented by three cycles of reef deposits, eachof which begins with detrital or larger foraminiferal limestones and is terminated by coral limestone. Because
the abovementioned studies were conducted before theWatanabe et al. Progress in Earth and Planetary Science(2023) 10:40

full-scale introduction of a new sedimentological framework into the stratigraphic scheme of the Ryukyu Group,
the stratigraphic division (cycle) is the opposite of thatused presently (unit). The Shimojijima Limestone was
not confirmed by Sagawa et al. (2001), and thus its existence is uncertain.
Since the 1980s, extensive investigations of biota andsediments have been conducted in coral reefs and associated shallow lagoons and fore-reefs to island shelves
around the Ryukyu Islands (e.g., hermatypic corals,
Nakamori 1986; nongeniculate coralline algae, Iryu 1992;
rhodoliths, Matsuda and Iryu 2011), and these haveshown the ecological and sedimentological specificity ofmany benthic organisms and sediments and have identified characteristic assemblages and sediments in eachof the topographic zones and sub-areas there (e.g., Iryu
et al. 1995). On the basis of the findings of those studies, a classification for carbonate rocks of the Ryukyu
Group was proposed initially by Nakamori (1986) andsubsequently refined by Nakamori et al. (1995) and Iryu
et al. (1998). Application of this classification has enabled the lithostratigraphy of numerous islands and areas
to be established (e.g., Yoron Jima, Odawara and Iryu
1999; Irabu Jima and Shimoji Shima, Sagawa et al. 2001;
Tokunoshima, Yamada et al. 2003; Yomitan (OkinawaJima), Odawara et al. 2005b; Itoman (Okinawa Jima),
Kaneko and Ujiié 2006; Sagae et al. 2012; Motobu Peninsula (Okinawa Jima), Yamamoto et al. 2006). In thosestudies, the stratigraphic succession of the Ryukyu Groupwas divided into several units. A single unit is defined
as a sequence that was initiated during a sea-level lowstand, developed through a subsequent transgression
and highstand, and terminated in a regression (Yamamoto et al. 2006; Fig. 4). Vertically, a single unit consistsof shallow-water reef deposits (mostly coral limestone)
in proximal (inland and higher-elevation) areas and of
relatively deep-water off-reef deposits (rhodolith, Cycloclypeus-Operculina, and detrital limestones) in distal(coastal and lower elevation) areas. Between the proximaland distal areas, a single unit is composed of coral limestone grading upward into deeper limestones overlain bycoral limestone. The uppermost limestone is observed inonly a few cases. Laterally, the proximally situated corallimestone is surrounded by the distally located rhodolith,
Cycloclypeus-Operculina, and detrital limestones.
Iryu et al. (2006) established the correlation of the
Ryukyu Group on islands from Kikai Jima in the north toYonaguni Jima in the south. We updated the correlationof Iryu et al. (2006), demonstrating the unit-level correlation of the Ryukyu Group on Okinawa Jima and the
Miyako Islands and correlated the units of the main bodyof the Ryukyu Group with Marine Isotope Stages (MISs;
Emiliani 1955; Lisiecki and Raymo 2005; Ahn et al. 2017).

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Fig. 4 Stratigraphic succession of limestones within the Ryukyu
Group. Schematic figure showing stratigraphic succession
and the spatial arrangement of limestones within the Ryukyu
Group (modified from Yamamoto et al. 2006). Cy.-Op.,
Cycloclypeus-Operculina

We correlated units of the Ryukyu Group and MISs,
assuming that a single unit was formed during a period
from one even-numbered MIS (during a lowstand ofsea level and a following transgression) to a subsequent
odd-numbered MIS (during a highstand and a following
regression).
The Miyako Islands are covered entirely by the Ryukyu
Group (Figs. 2c and 5). No formal description based on
the new sedimentological framework has been published
to date on the lithostratigraphy of the main body (up to
60 m thick) of the Ryukyu Group on Miyako Jima. Yamada (2002) reported that the main body consists of fiveunits (Fig. 5). According to Anai et al. (2017), these units
were deposited during the period from 0.99 Ma (firstoccurrence (FO) of Gephyrocapsa parallela; hereafter,
numerical ages of Upper Pliocene to Quaternary calcareous nannofossil datums follow Sato et al. 2009) to 0.45 Ma(last occurrence (LO) of Pseudoemiliania lacunose), andWatanabe et al. Progress in Earth and Planetary Science(2023) 10:40

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Fig. 5 Geological map of the Ryukyu Group on Miyako Jima. The Ryukyu Group on this island consists of five units (modified from Yamada 2002). Ni
Nishihara; 1, Tsuzupisuki-Abu cave; 2, Tanahara cave; 3, Pinza-Abu cave; 4, Mumyo-no-ana cave; 5, Amaga cave

the Brunhes–Matuyama boundary is located in MY-Unit
4 (Fig. 6). Considering these chronological constraints
and a conformable stratigraphic succession of the fiveunits, MY-Units 1, 2, 3, 4, and 5 are thought to have been
deposited during MIS 26–25, 24–23, 22–21, 20–19, and18–17 (0.96–0.65 Ma), respectively.
The Ryukyu Group on Irabu Jima and Shimoji Shimareaches 100 m in thickness and is composed of 13 units(Figs. 6 and 7; Sagawa et al. 2001). Of these, the four lowest units (1–4) consist of reddish, diagenetically altered
limestones. The main body of the Ryukyu Group (units
1–12) forms a stratigraphic architecture of aggradation
and retrogradation. Accordingly, the islands and theirenvirons are inferred to have subsided during the deposition of the main body. In contrast, unit 13 is up to 12 m
thick and consists chiefly of coral limestone that extends
seaward (offlapping) over the main body and is limited inits distribution to submarine cores drilled to the south of
Irabu Jima (i.e., to the west of Miyako Jima). The spatialconfiguration of unit 13 and its stratigraphic relationship with the main body indicates a rapid tectonic transition in this area, whereby subsidence ceased after thedeposition of unit 12 and uplift of the area began (Sagawa
et al. 2001). Four calcareous nannofossil datums are present in the main body (Fig. 6): LO of large Gephyrocapsa
spp. (1.18 Ma) in unit 2, FO of G. parallela (0.99 Ma) inunit 4, LO of Reticulofenestra asanoi (0.85 Ma) in unit 7,
and LO of Pseudoemiliania lacunosa (0.45 Ma) in unit 12.
Bio- and magnetostratigraphic data (Sagawa et al. 2001;
Anai et al. 2017) allow the units of the Ryukyu Group on
the Miyako Islands to be correlated with each other andwith MISs (Fig. 6). This correlation indicates that coral
reef formation started earlier on Irabu Jima and Shimoji
Shima (at 1.25 Ma) than on Miyako Jima (at 0.96 Ma) andthat the uppermost unit (unit 13) was formed after the
rapid transition in tectonic movement from subsidence
to uplift (at ca. 0.4 Ma). However, units 3–4, 6 and 8–11
are not well correlated with MISs. Because the negative
δ18O excursion of MIS 32 was relatively small, the reef
and off-reef deposits that formed during MISs 34–31,
which should have been separated into two units, werelikely described as a single unit (unit 3) by Sagawa et al.
(2001). It is also the case for unit 6. Conversely, because
MIS 15 is characterized by double peaks, two units ofWatanabe et al. Progress in Earth and Planetary Science(2023) 10:40

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Fig. 6 Correlation of the Ryukyu Group. Correlation of the Ryukyu Group on the Miyako Islands and three areas of Okinawa Jima with oxygenisotope stratigraphy and calcareous nannofossil biostratigraphy established at Integrated Ocean Drilling Program Site U1308 in the North
Atlantic Ocean (Sato et al. 2009). Numbers on the oxygen isotope stratigraphy denote Marine Isotope Stages. We correlated units of the Ryukyu
Group and Marine Isotope Stages, assuming that a single unit was formed in response to a single cycle of glacioeustatic sea-level change
and corresponding to deep‐sea benthic foraminiferal δ18O values. Dark-blue successions represent shallow-water reef deposits (mostly corallimestone) that have been altered by meteoric diagenesis to show reddish staining. They include the lower part of the Ryukyu Group on Irabu Jimaand the Itoman Formation and its equivalents on Okinawa Jima

reef and off-reef deposits (units 10 and 11) might haveformed during this stage.
3.2 Geology of Okinawa Jima
3.2.1 Shimajiri GroupThe Shimajiri Group is distributed on southern Okinawa
Jima, where it is divided into three formations; i.e., theTomigusuku, Yonabaru, and Shinzato formations, inascending stratigraphic order (Fig. 3; Natori 1976). The
Tomigusuku Formation is composed of alternating bedsof sandstone and mudstone, identified as turbidites (Ujiié
and Kaneko 2006). The formation is divided into 13 units(T1 to T13, from youngest to oldest; Fukuta et al. 1970;
Okinawa Natural Gas Research Group 1971), with odd

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Fig. 7 Geological cross-sections through the Ryukyu Group on Irabu Jima and Shimoji Shima. The Ryukyu Group on the two islands is up to 100 min thickness and is divided into 13 units (modified from Sagawa et al. 2001)

numbers corresponding to sandstone-dominated intervals. The basal unit (T13) is dominated by sandstone withbasal conglomerates (30 m thick) and correlates to the
Oroku Sandstone Member (Makino and Higuchi 1967).
The Yonabaru and Shinzato formations consist mainly of
mudstones with thin tuff and sandstone interlayers. The
latter formation is distinguished from the former by its

more numerous interlayers of tuff. The three formations
appear to constitute a conformable sequence.
On the basis of their paleontological analysis of molluskan assemblages and a review of previous studies on
assemblages of mollusks and brachiopods (MacNeil
1960; Fukuta et al. 1970), Ogasawara and Masuda (1983)
suggested deepening of the depositional environmentWatanabe et al. Progress in Earth and Planetary Science(2023) 10:40

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starting from a sublittoral zone (~ 50 m water depth) during deposition of the lower interval of the Yonabaru Formation to a bathyal zone (~ 1000 m water depth) during
deposition of the Shinzato Formation (Fig. 3). The environment during deposition of the Tomigusuku Formation has not been specified. Noda (1980, 1988) reported
abyssal mollusk species from the Shinzato Formation,suggesting a deeper setting for the deposition of this formation than had previously been considered. Ostracod
assemblages show that the deepening occurred earlier
than mollusks suggest (Nohara 1987): sublittoral during
deposition of the lower Tomigusuku Formation; upper
bathyal during deposition of the middle Tomigusku tolower Shinzato formations; and outer sublittoral during
deposition of the upper Shinzato to lower Chinen formations. Nakagawa et al. (2001) reported two benthic
foraminiferal assemblages from the Shinzato Formation,
both of which were interpreted to indicate a shelf-slope
environment with a water depth of > 250 m on the basisof a comparison with benthic foraminiferal assemblages
from around the Miyako Islands (Kodato and Nakagawa
1993). Benthic foraminiferal assemblages at the Urizun
outcrop (Sato et al. 2004) indicate a deeper setting (lower
continental slope, 500–1500 m water depth) for theupper Shinzato Formation (Matsumoto et al. in press).
In contrast, Ujiié and Kaneko (2006) proposed a bathyal
depositional environment for the Shimajiri Group on thebasis of the abundant occurrence of “Globocassidulina
subglobosa and var.” throughout the group and the overlying Chinen Formation. Overall, we infer the following
reconstruction of the depositional environment of the
Shimajiri Group: sublittoral during deposition of thelower Tomigusuku Formation; bathyal during depositionof the middle Tomigusku to lower Shinzato formations;and lower sublittoral to upper bathyal (200–300 m water
depth) or deeper during deposition of the upper Shinzato
Formation.
Planktic foraminiferal and calcareous nannofossil
biostratigraphy indicates that the Shimajiri Group on
Okinawa Jima correlates to the Upper Miocene (Tortonian) to Lower Pleistocene (Gelasian); i.e., the N17–
N21 and CN9a–CN12d zones (Fig. 3; Tanaka and Ujiié
1984; Hanagata 2004; Sato et al. 2004; Imai et al. 2017).
Ujiié and Kaneko (2006) correlated the lower Tomigusuku Formation on Okinawa Island with the CN9a Zone
and suggested that the N16/N17 boundary is locatedwithin this zone. However, according to recent microfossil biostratigraphy, the N16/N17 zone boundary is
lower than the CN9 Zone (Raffi et al. 2020). Therefore,
the biostratigraphy of Ujiié and Kaneko (2006) is inmarked contrast to the currently proposed biostratigraphy. Although Tanaka and Ujiié (1984) reported that thestratigraphic position of the shift from dextral to sinistralcoiling preference in Pulleniatina and the CN12d/CN13aboundary both lie in the upper Shinzato Formation, theinterval including these two events corresponds to thelower Chinen Formation of Nakagawa et al. (2001) (Satoet al. 2004). On the basis of the above data, the depositional age of the Shimajiri Formation is estimated to beca. 8 to ca. 2 Ma.
3.2.2 Chinen Formation

The Chinen Formation is a transitional lithofacies
between the Shimajiri and Ryukyu groups. Its distribution is limited to southern Okinawa Jima, where it is
exposed along the eastern coast from Uezato via Chinen
to Katsuren Peninsula and sporadically on the western
coast at Naha Port (Fig. 8; Nakagawa et al. 2001; Chiyonobu et al. 2009; Fujita et al. 2011). Differing definitionsof the Chinen Formation (for a review, see Chiyonobu
et al. 2009) had complicated study of the stratigraphy
and chronology of the upper Cenozoic succession on
Okinawa Jima until Nakagawa et al. (2001) establishedthat this formation consists of lower and upper intervals.
The lower Chinen Formation is composed of calcareoussiltstone that is similar in lithology to mudstone of the
Shinzato Formation but slightly coarser and more poorly
bedded (Fig. 2b). The upper Chinen Formation comprises
well-bedded detrital limestone (bioclastic packstone)
consisting mainly of benthic foraminifers (including large
benthic foraminifers), mollusks, brachiopods, bryozoans,and echinoids. Corals and coralline algae are absent or
occur only very rarely.
Ogasawara and Masuda (1983) inferred rapid shallowing during the deposition of the Chinen Formation, as themolluskan assemblage in the lower interval of this formation includes many bathyal taxa that are also common in
the Shinzato Formation and because those in the middle
and upper intervals comprise upper sublittoral species(Fig. 3). Ostracod assemblages indicate that the shallowing started during deposition of the upper Shinzato
Formation and that the lower and upper intervals of theChinen Formation were deposited in the outer sublittoralzone and reefs (Nohara 1987). This reconstruction agrees
roughly with that deduced from benthic foraminifers by
Nakagawa et al. (2001), who identified four assemblages,
all of which indicate that the shelf-slope environment
ranging in water depth from 100 to 300 m. On thebasis of paleontological analysis of benthic foraminiferal assemblages of the Shinzato and Chinen formations
and geochemical analysis of dolomite concretions at theboundary between the two formations, Matsumoto et al.
(in press) concluded that the Shimajiri Basin was rapidly uplifted by 250 ± 100 m at ca. 2 Ma and termed thistectonic event the “Chinen Disturbance” event. Thoseauthors suggested that the relative sea-level fall caused by

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Fig. 8 Geological map of the Ryukyu Group on southern Okinawa Jima. Geological map and cross-sections of the Ryukyu Group on southern
Okinawa Jima (after Sagae et al. 2012). Geological map: Ar Aragusuku, M Maehira, U Uezato, Y Yamagusuku, YD Yoza-dake. Index map in the left
panel: M Maeda, N Nishihara

this uplift destabilized methane hydrate in sediments of
the Shimajiri Group, drastically increasing the methane
supply and flux, which led to the formation of methanederived dolomite concretions at the boundary. The uplift
was generated by the vertical component of the right-lateral strike-slip faulting that formed the Kerama gap (Araiand Inoue 2022).
Calcareous nannofossil biostratigraphy indicates that
sedimentation of the Chinen Formation started andended earlier in Uezato and Chinen than on Katsuren
Peninsula and at Naha Port. In Chinen, the sedimentation
started slightly before 1.99 Ma (LO Discoaster brouweri)
and ended at 1.71–1.39 Ma (1.71 Ma, FO Gephyrocapsa
oceanica; 1.39 Ma, FO of large Gephyrocapsa spp.) (Satoet al. 2004; Odawara et al. 2005a). The Chinen Formation on Katsuren Peninsula ranges in age from 1.99 to
1.76 Ma (1.76 Ma, FO of Gephyrocapsa caribbeanica) to
1.39–1.18 Ma (1.18 Ma, LO of large Gephyrocapsa spp.)(Chiyonobu et al. 2009). The age of the Chinen Formation
at Naha Port ranges from 1.71 to 1.39 Ma (Fujita et al.
2011). These findings suggest that the upper interval of

the Chinen Formation corresponds to the lower intervalof the Naha Formation.
3.2.3 Ryukyu GroupThe Ryukyu Group is distributed on southern and central Okinawa Jima and is exposed up to 170 m above sea
level (Fig. 8). The group, which unconformably overlies the Shimajiri Group (Fig. 2d) and unconformably/
conformably overlies the Chinen Formation, consists of
the Naha Formation and its correlatives, which exhibit
a common stratigraphic succession but vary in terms of
the stratigraphic nomenclature, as well as younger limestones (Fig. 6). The Naha Formation and its correlatives
constitute the main body of the Ryukyu Group and the
younger limestones expand seaward, offlapping overthe main body, with relatively limited distribution. The
Naha Formation and its correlatives are composed of up
to five units. Unit 1 is composed of proximal coral limestone and distal detrital limestone. The reddish, diagenetically altered coral limestone is assigned to anotherlithostratigraphic unit, the Itoman Formation (Fig. 2e;

Watanabe et al. Progress in Earth and Planetary Science(2023) 10:40

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Kaneko and Ito 2006). Locally, conglomerates and calcareous sandstones were deposited in an embayment on
northern Motobu Peninsula (i.e., the Guga and Nakoshi formations, respectively; Yamamoto et al. 2006). In
contrast, the distal detrital limestone shows no indications of episodic subaerial exposure. Units 2–4 consistof coral limestone that grades upward into deeper-water
facies consisting of rhodoliths, Cycloclypeus-Operculina,
and detrital limestones. Unit 5 is known only from the
Yomitan (Odawara et al. 2005b) and consists exclusivelyof coral limestone. The drastic change in sedimentation
between unit 1 and units 2–4 is considered to have beencaused by an increased amplitude of sea-level change at
ca. 0.8 Ma, possibly related to the Middle Pleistocene
Transition (Yamamoto et al. 2006). Unit 4 unconformably overlies unit 3 on southern and central Okinawa Jima
(Itoman, Yomitan-Maeda, and Motobu Peninsula) and Ie
Jima (Fig. 8; Muraoka et al. 2005; Odawara et al. 2005b;
Takeuchi et al. 2006; Yamamoto et al. 2006; Sagae et al.
2012). This unconformity is found on Okinoerabu Jimaand Tokunoshima (Iryu et al. 1998; Yamada et al. 2003)
and is considered to have been caused by a sea-level fall
during the MIS 16 glacial period. The sea level fall during
this period was greater than that during other preceding and subsequent glacial periods if it is assumed that
δ18O time series of deep-sea benthic foraminifers (Fig. 6)
indicate eustatic sea-level changes (Miller et al. 2005).
Although this interpretation is based on the foraminiferal
δ18O time series from IODP Site U13081 in the North
Atlantic Ocean (Fig. 6; Sato et al. 2009), it holds evenwhen using the LR04 stack (Lisiecki and Raymo 2005)
and the Prob-stack (Ahn et al. 2017) as the time series.
Units 1–5 form a stratigraphic architecture of aggradation and retrogradation, indicating that subsidence
continued during the deposition of these units (Fig. 3)
to produce accommodation space for reef deposits thatformed during subsequent sea-level fluctuations.
The Ryukyu Group on Okinawa Jima is chronologicallyconstrained by calcareous nannofossil biostratigraphy
like that on the Miyako Islands. The FO of large Gephyrocapsa spp. (1.39 Ma) and LO of R. asanoi (0.85 Ma)
are located in a lower horizon of unit 1 and immediately
below unit 2, respectively (Fig. 6). The FO of G. oceanica(1.71 Ma) and LO of P. lacunosa (0.45 Ma) have not been
detected. The lithostratigraphy and calcareous nannofossil biostratigraphy of the Ryukyu Group allow these units
to be correlated with MISs (Fig. 6), whereby unit 1 andunits 2–4 were deposited during 1.7–0.8 and 0.8–0.5 Ma,
respectively. This interpretation agrees well with Sr isotope ages (1.6–1.3 Ma) obtained from the Itoman Formation, which were reported by Kaneko and Ito (2006) andrecalculated in this study based on “The Geologic Time
Scale 2020” (Gradstein et al. 2020).

The younger limestones have not been chronologicallyconstrained because of the lack of age-diagnostic fossils.
However, Nakamori et al. (1999) argued that the Minatogawa Formation (Fig. 2f ), which comprises younger
limestone on southern Okinawa Jima, must have been
deposited during the last or penultimate interglacial
stage, as deduced from the Holocene uplift rate (0.4 mm/years) and the resulting terrace altitude.
The main body of the Ryukyu Group on southern
Okinawa Jima is cut by numerous normal faults striking
ENE–WSW and ESE–WNW and downthrown predominantly to the north (Fig. 8). Several minor faults cut the
Minatogawa Formation.
3.3 Geology of the Okinawa–Miyako submarine plateau

A submarine plateau lies between Okinawa Jima andMiyako Jima (Hamamoto et al. 1979), which was referred
to as the “500 m Deep Island Shelf ” by Ujiié (1980) and is
described in the present study as the OMSP. The OMSP
is approximately rectangular in plan view, extending
NE–SW with a long side of ~ 135 km and a short side of~ 35–75 km (Fig. 9). The northeastern end of the OMSP
is separated from Okinawa Jima by the Kerama gap, andthe southwestern end is separated from Miyako Jima by
the Miyako saddle. The OMSP consists of three flat plainsthat shallow successively from east to west. The shallowest water depths of the eastern and middle plains are
137 m (Higashi-Daikyu-Sone) and 100 m (Nishi-DaikyuSone), respectively. The western plain is known as the
Miyako-Sone submarine platform (Arai et al. 2014), andits shallowest depth is 21 m at the Zyuho-Sone. This shallowing trend is due to displacement on NW–SE-trending
normal faults that are downthrown to the northeast. The
southeastern and northwestern margins of the OMSP are
truncated by the shelf slopes down to the Ryukyu Trench
and the Okinawa Trough, respectively.
Litho- and biostratigraphic analyses have revealed
that drillcore deposits from the offshore Okinawa 1 − xwell drilled at 215.5 m water depth on the northeastern
OMSP (Fig. 9) consist mostly of thick (~ 2700 m) siltstone with fine- and very fine-grained sandstone of the
Shimajiri Group underlain by phyllite and overlain by~ 50 m-thick limestone that may be assigned to the Shimanto Belt and the Ryukyu Group, respectively (Aibaand Sekiya 1979; Sato et al. 2022). Discoaster quinqueramus and D. berggrenii, which define the top and bottomof the calcareous nannofossil zone NN11 (Martini 1971;= CN9), occur continuously in the Shimajiri Group succession, indicating that the group in this well correlates to
the Tortonian–Messinian (Late Miocene) between 8.10
and 5.53 Ma. Although calcareous nannofossils are rare
and poorly preserved in the Ryukyu Group, a drillcore

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Fig. 9 Bathymetric map of the Okinawa–Miyako submarine plateau. Numbers on the index map indicate banks developed on the platform: 1,
Kita Zyuho-Tai; 2, Zyuho-Sone; 3, Hozan-Sone; 4, Minami Hozan-Tai; and 5, Higashi Zyuho-Tai. This map was prepared using Generic Mapping Tools(Wessel et al. 2019). Elevation and depth data are from SRTM15+V2.4 (https://​doi.​org/​10.​1029/​2019E​A0006​58; accessed 1 Dec 2021). KF Kerama
fault, Tk Tokashiki Jima in the Kerama Islands

sample from the base of the Ryukyu Group contains
Gephyrocapsa parrallela, which first emerged at 0.99 Ma.
The age of the lower boundary of the Shimajiri Groupin the offshore Okinawa 1 − x well (ca. 8 Ma) is similar tothat on southern Okinawa Jima (Imai et al. 2017). In contrast, the sedimentation of this group started ca. 2.7 Myrlater on Miyako Jima and off its eastern coast (MITI
Miyakojima-Oki well; Fig. 9) compared with the northeastern OMSP and Okinawa Jima (Fig. 3). The upperboundary of the Shimajiri Group in the offshore Okinawa
1 − x well (> 5.53 Ma) is at least 3.5 Myr older than thaton Miyako Jima and off its eastern coast and Okinawa
Jima (ca. 2 Ma). Considering the transition from a siltstone-dominant lithology to a lithology dominated bycalcareous fine- and very fine-grained sandstone occupying the uppermost ~ 220 m of the Shimajiri Group in theoffshore Okinawa 1 − x well and the large sedimentary
gap between the Shimajiri and Ryukyu groups, it is likelythat the OMSP was uplifted to form a land area during
the Late Miocene at the earliest or the Plio-Pleistocene(but not later than 2 Ma), either when deposition of the
Shimajiri Group ceased here earlier than in other regions

or when the upper part of the Shimajiri Group (ca.
5.5–2 Ma) was emerged and eroded. The reported age of
the Ryukyu Group (< 0.99 Ma) does not necessarily meanthat this group dates back to 0.99 Ma on the OMSP but
instead reflects a lack of tight chronological constraints
owing to the low abundance and poor preservation of
calcareous nannofossils. Considering that < 0.27 Ma (agecorresponding to the FO of Emiliania huxleyi; Sato et al.
2009) carbonate rocks extend over the western marginof the OMSP (Arai et al. 2014) and the Kerama saddle(Fig. 9), a topographic high in the Kemama gap that isinterjacent between Okinawa Jima and the OMSP (seeSect. 3.4; Kimura et al. 1991, 1992), they might also cover
the eastern margin of the OMSP.
Itaki et al. (2013) collected marine sediments containing fossilized barnacles and molluskan shells along withrounded gravels perforated by boring shells from thenorthern margin of Higashi-Daikyu-Sone (25° 56′ N, 126°
29′ E; 762 m water depth). The mollusks include two species (Arca boucardi and Cardita leana) that commonly
inhabit the intertidal to upper subtidal zone (0–20 m
water depth; Okutani 2017). Calcareous nannofossils

Watanabe et al. Progress in Earth and Planetary Science(2023) 10:40

Page 16 of 30

retrieved from mollusk-bearing mudstone indicate an ageof 1.71–1.39 Ma (denoted here as 1.7–1.4 Ma). This age
corresponds to that of the lowest horizon of the Ryukyu
Group on Okinawa Jima. Although it is uncertain fromwhich part of Higashi-Daikyu-Sone the mudstone containing these fossils was derived, the occurrence of thesefossils suggests that the OMSP may have emerged andexpanded in land area until 1.7–1.4 Ma, after depositionof the Shimajiri Group.
Bathymetric mapping and observations of the seafloorusing a remotely operated vehicle (Hyper-Dolphin 3 K)on the slopes of the Miyako saddle in the southwestern
margin of the OMSP indicate the presence of terraces atwater depths of 140, 330, 400, and 680 m and a number
of NW–SE-trending faults that extend perpendicular to
the axis of the Ryukyu Island Arc (Arai et al. 2014). The
surveys revealed that well-indurated carbonate rocks
are exposed in terrace margins and on upper slopes andthat the lower slopes are covered with modern carbonate sediments. The well-indurated rocks are composed
exclusively of off-reef to shelf–shelf-slope carbonates
consisting chiefly of bioclastic packstone and rhodolith
floatstone. Calcareous nannofossils from the well-indurated carbonate rocks indicate a latest Early–Late Pleistocene age (< 0.85 Ma), which suggests that the rocks
correlate to the Ryukyu Group. With increasing distancewestward from the western plain of the OMSP, coeval
well-indurated carbonate rocks are found at progressively
lower levels owing to displacement on normal faults withdownthrow to the west. These fault blocks correspondto the abovementioned terraces. As the younger carbonates (0.27–0 Ma) are displaced (Arai et al. 2014, Fig. 8),
the faulting must have continued after 0.27 Ma. Calcareous nannofossil biostratigraphic ages and rock sample
lithologies (mostly bioclastic packstone and rhodolith
floatstone) show that 0.45–0.27 and < 0.27 Ma limestones
offlap/overlie 0.85–0.45 Ma limestones on the slope and< 0.27 Ma limestones extend over the western plain of the
OMSP (Arai et al. 2014). This evidence indicates that the
OMSP, or at least its southeastern margin, began to subside after 0.45 Ma and mostly became sufficiently deepfor deposition of the abovementioned offshore facies
after 0.27 Ma.

Kato et al. 1982) occur on the trench slope of the Ryukyu
Trench. The Kerama gap is believed to be underlain by
a left-lateral strike-slip fault that displaces pre-Neogene
basement of the Ryukyu Islands (Kizaki 1985). Submarine topographic and geological investigations haverevealed that the Kerama gap was formed by the activityof a group of NW–SE-trending normal faults during the
Pleistocene (Kimura et al. 1991, 1992; Matsumoto andKimura 1991; Kimura 1996; Matsumoto et al. 1996).
The Kerama saddle (Kimura et al. 1991) is a NE–SWtrending ridge in the southeastern Kerama gap, truncatedon the top by a flat, eroded plane extending ~ 40 km
from northeast to southwest (Figs. 9 and 10a). Kimura
et al. (1991, 1992) made observations of submarine
topography and geology and performed sedimentological and chronological analyses of rock samples collected from the Kerama saddle using a multibeam echosounder (Sea Beam), a towed deep-sea camera, and asubmersible (SHINKAI 2000). The stratigraphic succession in this saddle was interpreted to comprise the Shimajiri Group and overlying Ryukyu Group. Calcareous
nannofossils and planktic foraminifers were retrieved
from Shimajiri Group rock samples collected from theKerama saddle, 16–50 km east of the offshore Okinawa
1 − x well (Fig. 9). However, because Kimura et al. (1992)
observed calcareous nannofossils under a light microscope at magnifications of 400× and 600×, which is nothigh enough to identify them at the species level (Sato2013), their chronological interpretation is questionable.
Kimura et al. (1991) reported calcareous nannofossil species Discoaster berggrenii and D. quinqueramus fromthe Shimajiri Group at Site DT-26. The occurrence of
these two species indicates an age of 8.10–5.53 Ma (CN9). Ceratolithus acutus was reported from sites DT-27,
DT-28, DT-29, and DT-30(b). The occurrence of this species is limited to the period between 5.36 and 5.04 Ma(Raffi et al. 2020). Although these calcareous nannofossil biostratigraphic ages (8.01–5.04 Ma) are inconsistent
with planktic foraminiferal biostratigraphic ages (N22;< 1.93 Ma), as discussed by Kimura et al. (1991), the former accord well with the chronology of the ShimajiriGroup established from the offshore Okinawa 1 − x well
(8.10–5.53 Ma; Sato et al. 2022) and indicate a large sedimentary gap between the Shimajiri and Ryukyu groups,
which is consistent with the interpretation that the
OMSP was uplifted to form a land area during the Late
Miocene or later period.
The Ryukyu Group in the Kerama saddle has been
divided into the Ryukyu Limestone and the overlying Kerama Formation (Kimura et al. 1991, 1992). The
Ryukyu Limestone is subdivided into the Naha Limestone equivalent and the Minatogawa Limestone equivalent. The samples assigned to the Naha Limestone

3.4 Geology of the Kerama gap and Kerama saddle

The Kerama gap is a NW–SE-trending structural rift
between Okinawa Jima and the OMSP (Figs. 9 and 10).
This gap has a steep slope on the southwestern side anda gentler slope on the northeastern side, reaches a maximum water depth of 2100 m (Kato et al. 1982), and isconnected to the Okinawa–Miyako Deep Sea Terrace(Fig. 1; Matsumoto et al. 1996; Okamura et al. 2017).
Numerous submarine canyons (the Kerama Canyons;Watanabe et al. Progress in Earth and Planetary Science(2023) 10:40

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Fig. 10 Seismic sections of the Kerama gap. a Submarine topography of the Kerama gap. This map was prepared using Generic Mapping Tools(Wessel et al. 2019). Elevation and depth data are from SRTM15+V2.4 (https://​doi.​org/​10.​1029/​2019E​A0006​58; accessed 1 Dec 2021). b and c Seismic
sections along lines 1020 and 1012, respectively. Note that the Ryukyu Group equivalent increases its thickness from the basin side toward the fault
scarp along the northeastern margin of the OMSP (arrowed). A, Shimajiri Group equivalent; B, Ryukyu Group equivalent; C, Sediment younger
than the Ryukyu Group; Bs, Acoustic basement

equivalent have yielded calcareous nannofossils characteristic of the CN15 Zone (< 0.27 Ma). This chronological constraint was considered by those authors tobe supported by an electron spin resonance (ESR) age

of 0.24 Ma measured for a bivalve (Amusium pleuronectes). In contrast, relatively porous and weakly consolidated limestones were assigned by those authors to
the Minatogawa Limestone equivalent and consideredWatanabe et al. Progress in Earth and Planetary Science(2023) 10:40

Page 18 of 30

to be formed during the last interglacial period (MIS5e), following Kawana (1988).
The Kerama Formation unconformably overlies the
Ryukyu Limestone and is composed of the Kerama Limestone and the overlying Kerama Conglomerate. The
Kerama Limestone is distinguished from the Minatogawa
Limestone equivalent mainly on the basis of its younger
ages of approximately 100–30 ka, as determined using
accelerator mass spectrometry (AMS) 14C dating of pteropod shells and uranium-series dating by non-destructive γ-ray spectrometry of bulk carbonate samples. The
Kerama Conglomerate consists of clasts derived from the
Shimajiri Group and the Ryukyu Limestone with a matrixof unconsolidated medium- to coarse-grained sand thatyields foraminifers, calcareous nannofossils, and mollusks, from which biogenic carbonate has yielded AMS
14
C ages of 20–19 ka.
Kimura et al. (1992) argued that the Kerama gap intermittently emerged and formed part of a land bridge that
extended to the Eurasian continent sometime during
the period 0.24 Ma to 20 ka. This argument was basedon two essential assumptions, although no supporting
data exist for them. First, the Kerama Formation unconformably overlies the Ryukyu Limestone, and there is asedimentary gap between these two stratigraphic units,
which was caused by tectonic uplift and the subsequent
emergence and erosion of the Kerama saddle. Second, theMinatogawa Limestone equivalent and the Kerama Formation are composed of shallow-water deposits. However, no evidence was presented by Kimura et al. (1992)
for the unconformable relationship between the Ryukyu
Limestone and the Kerama Limestone. According tohand-specimen and thin-section observations by S. Matsuda (pers. comm., 2021), the limestones collected by
Kimura et al. (1991, 1992) from the Kerama saddle were
exclusively pelagic limestone (bioclastic packstone withplanktic foraminifers), and no shallow-water carbonate rocks were collected. This view is supported by theoccurrence of pteropods, a common component in shelf
to shelf-slope sediments around the modern Ryukyus(e.g., Tsuji 1993), in the Kerama Limestone (Kimura et al.
1992). These observations suggest that the Kerama saddle
area was not located in a shallow-water environment but
in a sufficiently and consistently deep-water environment
for the formation of pelagic limestone during depositionof the Ryukyu Limestone and the Kerama Formation in
the sense of Kimura et al. (1991, 1992). In other words,
the Kerama saddle area has been flooded for the last
< 0.27 Myr; therefore, the hypothesis that the Kerama
gap emerged intermittently during the period 0.24 Ma to
20 ka and then rapidly subsided is unlikely.
The marine geological map by Arai and Inoue (2022)shows that the Shimajiri Group equivalent (SGE) is

exposed in the Kerama saddle and that its overlying unit (Ryukyu Group equivalent; RGE) extends into
the Kerama Canyons located on the trench side of theKerama gap. The occurrence of the RGE has been confirmed by grab-sampled rocks that correlate to theCN14a Zone [Calabrian to Chibanian (Early to Middle
Pleistocene), between 0.99 and 0.45 Ma]. In Sect. 3.5, we
discuss the formation of the Kerama gap, referring to theabovementioned data.
3.5 Marine geology of the Kerama gap and surrounds

Arai and Inoue (2022) discussed the formation and evolution of the Kerama gap with respect to marine geological data. To the west of Okinawa Jima, acoustic basement
is continuously exposed from east to west on the seafloor
from Okinawa Jima to the Kerama Islands (Fig. 10a). In
addition, exposed acoustic basement connecting Tonaki
Jima and Kume Jima from east to west extends northward to around Aguni Jima. These patterns suggest thatthe acoustic basement is essentially continuous fromeast (Okinawa Jima) to west (Kume Jima). During thetransgression from the Miocene to the Pliocene, muddy
deposits of the SGE onlapped unconformably over the
acoustic basement. The Shimajiri Group, which consistsof muddy forearc deposits, accumulated on southern
Okinawa Jima. The tectonic deformation that formed
the Kerama gap caused the demise of sedimentary basinsof the Shimajiri Group and its equivalent. Kimura et al.
(1991) divided the tectonic movement into two stages,
the first resulting in the Kerama fault (Fig. 9; Katoet al. 1982) and the second generating a group of normal faults in the Kerama saddle. Alternatively, Arai andInoue (2022) argued that a series of tectonic movements
formed these faults.
The SGE is well stratified in seismic sections, suggesting that it was deposited in a basin-floor setting. Normal faults cutting the SGE strike WNW to ESE, parallelto the steep northern slope of the northeastern OMSP
(Figs. 9 and 10a, b), and are downthrown to the north.
The occurrence of a positive flower structures on thebasin side of the faults suggests that they are predominantly right-lateral strike-slip faults rather than simple
normal faults (Fig. 11). The vertical component of thesefaults has resulted in more than 1000 m of displacementof the SGE, suggesting that large-scale tectonic movements occurred after deposition of the SGE (Fig. 10b).
The overlying RGE increases in thickness from the basinside toward the fault scarp along the northeastern marginof the OMSP (arrowed in Fig. 10b), indicating that thevertical displacement continued during deposition of the
RGE. In contrast, on the basin side, the flower structures
deform the seafloor topography and cut the RGE, again
suggesting that post-SGE tectonic movements continued

Watanabe et al. Progress in Earth and Planetary Science(2023) 10:40

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Fig. 11 Schematic diagram showing the formation of the Kerama gap (modified from Arai and Inoue 2022). Stage 1: Deposition of the Douzaki-oki
Group on basement rock. Stage 2: Strike-slip faulting, resulting in subsidence of the Kerama gap; Stage 3: Deposition of the Nishimesaki-Oki Groupand formation of a positive the flower structure in the Kerama gap; Stage 4: Local deposition of the Aguni-oki Group

with a more significant right-lateral slip component
during and after deposition of the RGE. These geological observations suggest that the right-lateral strike-slip
faulting that formed the Kerama gap also caused theassociated subsidence of this gap and uplift of southern
Okinawa Jima and the OMSP, which had been a sedimentary basin for the Shimajiri Group and its equivalent,
to form a land area extending from Okinawa Jima via
the OMSP to Miyako Jima. The occurrence of the topographic high (the Kerama saddle) in the Kerama gap anda group of normal faults located on the trench side of thesaddle are considered to be products of these tectonic
movements.
The Tokara and Kerama gaps were initially regarded ashaving been formed by left-lateral strike-slip faults characterized by the en echelon arrangement of zonal tectonic belts (e.g., Konishi 1965; Ujiié 1980). However, thedistribution and geological structure of the Shimajiri and

Ryukyu groups and their equivalents indicate that theKerama gap represents a right-lateral strike-slip fault thatwas generated by the opening of the Okinawa Trough,causing a trenchward displacement of the Okinawa Jimaside relative to the OMSP side. Owing to the trenchward displacement, the right-lateral faulting was associated with large vertical displacement only in the westernpart of the Kerama gap. In contrast, a group of normal
faults extending across the island arc was formed to thesouthwest of the Kerama Islands, corresponding to thetrench-side exit of the Kerama gap and the eastern side of
the strike-slip fault, on account of the effect of stretchingof the arc (Arai et al. 2018). These normal faults extend
mostly NW–SE and are downthrown to the southwest(Fig. 10c). The vertical displacement of each fault is~ 150 m (0.2 s two-way travel time), which clearly reflects
subsidence of the Kerama saddle.

Watanabe et al. Progress in Earth and Planetary Science(2023) 10:40

Page 20 of 30

Abundant vertebrate fossils have been found in fissure,cavity, and cave deposits on the Ryukyu Islands (e.g.,
Hasegawa 1980; Oshiro and Nohara 2000; Ota 2003).
These fossils provide important data about the original fauna on the islands before alteration from humanimpacts drove native taxa to extinction or favored theestablishment of exotic taxa. This is particularly evident
on Miyako Jima, where many vertebrate fossils have beenfound in cave deposits. The first record of mammalianfossils on the island was provided by Tokunaga (1936),
who mentioned the occurrence of deer antlers but without any additional information. Subsequently, Otuka(1941) reported fossils of elephants (identified in a recent
study as Mammuthus trogontherii, a species of mammoth known from the Eurasian continent and Taiwan;
Taruno and Kawamura 2007) and deer (Capreolus as C.
tokunagai).

Since World War II, numerous vertebrate fossils of Late
Pleistocene and Holocene age have been recovered fromcave deposits in Tanahara, Amaga, and Pinza-Abu caves,and most recently from those in Mumyo-no-ana andTsuzupisuki-Abu caves (e.g., Department of Education,
Okinawa Prefectural Government 1985; Miyakojima City
Board of Education 2015). These fossils range in age from25.8 or 26.8 to 8.7 ka (uncalibrated 14C dates; Kawamura
et al. 2016). The most scientifically successful excavations
were conducted at Pinza-Abu cave in 1982, 1983, and1984 (Department of Education, Okinawa Prefectural
Government 1985), from which 34 species of vertebrates(including Homo sapiens) were reported, according to thecompilation by Hasegawa (2012).
The Late Pleistocene–Holocene vertebrate assemblages
found in the caves of Miyako Jima are quite different incomposition from comparable assemblages from other
Ryukyu Islands, including Okinawa Jima and Ishigaki

3.6 Vertebrate fossils from Miyako Jima

Fig. 12 Modern and fossil vertebrates from Miyako Jima, supporting the OMSP hypothesis. a Hebius concelarus; b Calamaria pfefferi; c Takydromus
toyamai; d Capreolus miyakoensis (reconstructed skeleton housed in Okinawa Prefectural Museum and Art Museum); e Vertebra of fossil
Protobothrops sp. (Ikeda 2007) from Amaga cave (Late Pleistocene, 38,180 ± 180 cal yr BP; Otsuka et al. 2008). Scale bar = 2 mm

Watanabe et al. Progress in Earth and Planetary Science(2023) 10:40

Page 21 of 30

Jima (Ota 2003; Hasegawa 2012; Kawamura et al. 2016),
as the former include species unique to Miyako Jima [e.g.,
Capreolus miyakoensis (roe deer; Fig. 12d), Diplothrix (orRattus) miyakoensis (rat), Mauremys oshiroi (freshwater
turtle), vivarids (including an undescribed large-bodied
Protobothrops snake; Fig. 12e; Ikeda 2007), and two undescribed flightless birds (a crane and a Gallirallus rail);Matsuoka 2000; Hasegawa 2012; Kawaguchi et al. 2009;
Takahashi et al. 2015], or those shared exclusively witheastern Eurasia and Taiwan [e.g., Microtus fortis (vole);
Kaneko and Hasegawa 1995], besides the species foundfrom a broad range of the Late Pleistocene in the Southand Central Ryukyus [e.g., Manouria oyamai (tortoise):
Takahashi et al. 2018]. Of the Late Pleistocene Miyako
Jima endemics, C. miyakoensis and M. oshiroi have their
possible closest extant relatives in eastern Eurasia andmainland Japan, respectively (Hasegawa 2012; Takahashi
et al. 2015), whereas D. miyakoensis, Protobothrops sp.,and Gallirallus sp. are supposedly closest to D. legata, P.
flavoviridis, and G. okinawae, all of which are extant species endemic to the Central Ryukyus, respectively.
Because the Miyako Islands were submerged repeatedly during 1.25–0.4 Ma, these animals are inferred tohave migrated into the islands after ca. 0.4 Ma, when thetectonic movement of the islands changed from subsidence to uplift, making them emergent even during sealevel highstands (Sagawa et al. 2001). Most importantly,
these terrestrial Miyako Jima animals include those thatclearly had limited or no over-sea dispersal ability. It isthus highly likely that the occurrences of these animals
reflect the temporary existence of an exposed land area
between Okinawa Jima and Miyako Jima, which acted
as a land bridge or at least closely spaced islets for theirmigrations onto Miyako Jima from the Central Ryukyus.
However, as the OMSP has a flat top, it is more likely tohave formed a single flat island rather than a series ofislets. Some topographic highs, such as Higashi-DaikyuSone, Nishi-Daikyu-Sone, and Kita Zyuho-Tai, may haveexisted as hills above the flat top.

estimated by modern, objective methods in recent molecular studies (Kaito and Toda 2016; Tominaga et al. 2019).
The most closely related species and lineage for each are
the Okinawa lineages of H. pryeri and M. okinavensis,
respectively, both of which inhabit Okinawa Jima and its
neighboring islands. The divergence time between the
Okinawa lineages of H. pryeri and H. concelarus has been
estimated as 2.82 (1.98–3.68) or 2.74 (1.82–3.71) Ma(Kaito and Toda 2016). The divergence time between the
Okinawa and Miyako lineages of M. okinavensis has been
estimated as 4.67 (2.8–6.48) Ma (Tominaga et al. 2019).
Thus, both H. concelarus and the Miyako lineage of M.
okinavensis seem to have been maintained as independent lineages for at least the last 2–3 Myr. It is known thatthe estimates of divergence times for the animals vary
markedly. However, because the estimations of times inboth Kaito and Toda (2016) and Tominaga et al. (2019)
appear to have been made objectively, it is clear that both
H. concelarus and the Miyako lineage of M. okinavensis
have long evolutions extending back much farther thanca. 0.4 Ma, when uplift of the Miyako Islands began.
Assuming that the two species, as well as other terrestrial
endemics, evolved independently on the island that once
existed adjacent to Miyako Jima, their evolutionary histories are explainable.
A recent phylogenetic study of the operculate land snail
Cyclophorus turgidus has also suggested a close relationship between the Okinawa and Miyako lineages (Hirano
et al. 2022). According to that study, the most closely
related taxon to the Miyako lineage (C. turgidus miyakonis) is C. turgidus turgidus from northern Okinawa
Jima, with a divergence time of 0.92 (1.46–0.41) Ma.
Considering the range (95% confidence interval) for thetime, the distinction between the Okinawa and Miyako
lineages of C. turgidus also seems to be consistent with
our new hypothesis (the OMSP hypothesis, see Sect. 4)of the uplift and emergence of the OMSP during theLate Miocene or later period and its connection with
Okinawa Jima and subsequent separation between the
OMSP and Okinawa Jima during the Early Pleistocene(1.71–1.39 Ma).
Segawa (2011) conducted a molecular phylogenetic
study of the freshwater crab genus Geothelphusa andestimated the divergence time between G. miyakoensis on Miyako Jima and its sister species G. levicerevix of Tokashiki Jima (Fig. 9), an islet ~ 25 km west of
Okinawa Jima, to be no older than 0.16 Ma. This estimate
is much smaller than those for other terrestrial taxa/
lineages between Miyako Jima and the Okinawa Islands(see above), and even if the OMSP had been uplifted andemerged during the the Late Miocene or later period, thiswould be inconsistent with the above estimation. As inmany other studies that have estimated divergence times

3.7 Extant endemic species of the Miyako Islands

The current endemic species/lineages of terrestrial animals on the Miyako Islands include Hebius concelarus(Miyako keelback snake ; Fig. 12a), Calamaria pfefferi(Pfeffer’s reed snake; Fig. 12b), Takydromus toyamai(Miyako grass lizard; Fig. 12c), Bufo gargarizans miyakonis (Miyako toad), the Miyako lineage of Microhyla
okinavensis (Okinawa rice frog), and Geothelphusa miyakoensis (Miyako freshwater crab). All of these animals
have difficulty in extending their distribution acrossoceans. The divergence times of H. concelarus and the
Miyako lineage of M. okinavensis from their respective most closely related species and lineages have beenWatanabe et al. Progress in Earth and Planetary Science(2023) 10:40

Page 22 of 30

in organisms using the molecular clock method, the adequacy of Segawa’s (2011) estimate is strongly influenced
by the adequacy of assuming a particular divergencetime for calibration, which is that the divergence time foranother pair of Geothelphusa species, one occurring on
the southwestern side and the other on the northeasternside of the broad gap between land, including the Tokara
Strait, corresponds to the inferred time of strait formation (1.6 Ma). However, neither of the two Geothelphusa
representatives occur on lands immediately adjacent to
the Tokara Strait; therefore, the value of divergence used
for calibration may be underestimated substantially. To
estimate the divergence time between G. levicerevix andG. miyakoensis with certainty, more reliable calibration
points for the genus are required.
Takydromus toyamai, Calamaria pfefferi, and Bufo
gargarizans miyakonis are also notable extant endemic
species of the Miyako Islands. The closest relatives orsubspecies of all three species are distributed in the eastern part of the Eurasian continent (mainland China) andTaiwan. However, the same or closely related species are
not distributed on Okinawa Jima and its neighboringislands or on the Yaeyama Islands (Ota 1998). The processes of the migration to and colonization of the Miyako
Islands by these endemic species remain unknown. Threemigration routes are assumed for the endemic species/
subspecies on the Miyako Islands with limited or noover-sea dispersal ability:(1) Ancestors of the endemic taxa were isolated frommainland China on the Ryukyu Islands by the openingof the Okinawa Trough (< 6 Ma; Kamata and Kodama
1999), survived in some islands of the Central and North
Ryukyu Islands, and migrated to the Miyako Islands via
the OMSP at < 0.4 Ma. They could survive only on the
Miyako Islands as endemic taxa. However, they wereunable to migrate to the Yaeyama Islands, located 130 km
west of the Miyako Islands and separated by sea.
(2) Because the distance between mainland China andthe Ryukyu Islands was much shorter than at present
during the early stage of the opening of the Okinawa
Trough (ca. 6 Ma), the ancestors of the endemic taxa
might have migrated from mainland China to someislands of the Ryukyu Islands, from where they migrated
to the Miyako Islands at < 0.4 Ma.
(3) The current endemic taxa or their ancestors reached
the Miyako Islands from southern China or Taiwan byusing intervening islands/islets as stepping stones (e.g.,
Yang et al. 2018).

the vertebrate fossils from fissure, cavity, and cave deposits on these islands are those of taxa that migrated to theislands after ca. 0.4 Ma when the islands were upliftedto become a stable land area. Some of these fossil vertebrates or their close relatives are found in the Central
Ryukyu Islands (e.g., Diplothrix miyakoensis, Protobothrops sp., and Gallirallus sp.; see Sect. 3.6). Moreover,some of the species/lineages that are currently confinedto the Miyako Islands, such as Hebius concelarus, theMiyako lineage of Microhyla okinavensis, and Geothelfusa miyakoensis, also have their closest relatives butwith slight morphological and genetic differences on
Okinawa Jima and/or adjacent Central Ryukyu Islands.
These groups of fossils and extant animals on the Miyako
Islands are likely to have migrated from the Central
Ryukyu Islands. However, Miyako Jima and Okinawa
Jima (or islets in the vicinity of the latter) are currently
separated by a distance of ~ 300 km over open sea, andthese taxa have limited or no ability for over-sea dispersal. To resolve this apparent discrepancy, we proposethe “OMSP hypothesis”, which is that the OMSP once
emerged to form a land area that temporarily acted as aland bridge or at least a series of islets for some terrestrial
animals to reach Miyako Jima from Okinawa Jima or its
vicinity. Details of this hypothesis are presented below inchronological order.

4 OMSP hypothesis
The Miyako Islands repeatedly underwent complete
submergence during deposition of the main body of the
Ryukyu Group (1.25–0.4 Ma). Therefore, it is certain that

4.1 Chronological snapshots
4.1.1 After deposition of the Shimajiri Groupto before deposition of the Ryukyu GroupThe uppermost Shimajiri Group in the offshore Okinawa
1 − x well drilled on the northeastern OMSP is correlated to the NN11 (= CN9) Zone [Tortonian to Messinian (Late Miocene), 8.10 to 5.53 Ma], which is older thanthat on the Miyako Islands and Okinawa Jima (ca. 2 Ma).
The OMSP appears to have been uplifted earlier than the
Miyako Islands and Okinawa Jima and existed as a terrestrial area from the Late Miocene or later period.
Uplift of the Miyako Islands and Okinawa Jima began
when deposition of the Shimajiri Group (Minebariand Shinzato formations, respectively) had ceased at
ca. 2 Ma (Fig. 3). On Okinawa Jima, the central axis of
the southern part of this island, which is defined as a
NE–SW-trending area with higher elevations extending
from present-day Nishihara to Itoman (Fig. 8), emergedat this time (Fig. 13a). In contrast, on the northwesternand southeastern sides of the central axis (present-day
Naha Port and Katsuren and Chinen peninsulas, respectively; Fig. 8), uplift started later than (and/or was not as
pronounced as) that along the central axis. The Chinen
Formation was deposited in a shelf environment on bothsides of the central axis. The Miyako Islands emerged atessentially the same time (ca. 2 Ma) as the central axis

Watanabe et al. Progress in Earth and Planetary Science(2023) 10:40

a ca. 2 Ma to 1.7–1.4 Ma

Page 23 of 30

b 1.7–1.4 Ma to 0.45 MaIs

c ca. 0.4 Ma

n

tio

a
ol

d ca. 0.4–0.27 Ma

n
io
at
r
ig

M

?

e Present
Terrestrial vertebrates
Reconstructed land area
Present land area
Coral reefs
Fig. 13 Changes in the paleogeography of the Okinawa–Miyako area and migration of terrestrial vertebrates. Chronological snapshotsof the Okinawa–Miyako area and the migration of terrestrial vertebrates to the Miyako Islands. The background map (topography and bathymetry)is derived from Fig. 1. a Period from after deposition of the Shimajiri Group to before deposition of the Ryukyu Group (ca. 2 to 1.7–1.4 Ma).
We assume that the areas where the Ryukyu Group unconformably overlies the Shimajiri Group and where the Chichibu and Shimanto
accretionary complexes occur were land areas during this period. b Period of interglacial highstands of sea level during deposition of the main
body of the Ryukyu Group (1.7–1.4 to 0.45 Ma). We assume that the area currently covered by the main body was submerged, where coral reefs
and associated fore-reefs and shelves were distributed. Note that reef formation started significantly later on the Miyako Islands (1.25 Ma on Irabu
Jima and 0.96 Ma on Miyako Jima) than on Okinawa Jima (1.7–1.4 Ma). c Period immediately after deposition of the main body of the Ryukyu
Group (ca. 0.4 Ma). d Period after deposition of the main body of the Ryukyu Group (ca. 0.4–0.27 Ma). The OMSP was almost completely submerged
after 0.27 Ma. e Present. The vertebrate symbols denote, from upper to lower, elephant, deer, wild boar, mouse, and snake. See the Sects. 3.6 and 3.7
for the relevant taxaof the southern part of Okinawa Jima. As far as southern Okinawa Jima, the Kerama saddle, and the OMSP,
this uplift was driven by the vertical component (up to
1000 m displacement) of the right-lateral strike-slip faulting that formed the Kerama gap, corresponding to theChinen Disturbance event (Matsumoto et al. in press).

During the interval between ca. 2 and 1.7–1.4 Ma(after deposition of the Shimajiri Formation and before
deposition of the Ryukyu Group), Okinawa Jima andthe OMSP (and even the Miyako Islands) were connected to form a single land area extending ~ 400 km
NE–SW (Fig. 13a). The animals and plants that

Watanabe et al. Progress in Earth and Planetary Science(2023) 10:40

Page 24 of 30

inhabited Okinawa Jima expanded their distribution
throughout this large island, which corresponds tothe Early Pleistocene land bridge that was assumed tohave existed during the period 2.0–1.1 Ma to explain,for example, the presence of Hebius concelarus, anendemic snake from Miyako Jima (Kaito and Toda2016).

Otherwise, we would be forced to assume an utterly
improbable migration scenario from Okinawa Jima to the
Miyako Islands that involved crossing 300 km of sea after
deposition of main body of the Ryukyu Group (< 0.4 Ma
on the Miyako Islands), despite the non-existent or limited over-sea dispersal ability of these animals. Some Late
Pleistocene and extant endemic species on the Miyako
Islands are thought to have diverged on the OMSP during this period. For example, the divergence time of H.
concelarus has been estimated at 3.7–1.8 Ma (Kaito andToda 2016). This estimate of divergence time is likely
slightly too old due to factors such as ancestral polymorphisms and geographic genetic variation in the ancestraltaxon. Thus, it is roughly compatible with the timingof the isolation of the OMSP from Okinawa Jima (1.7–
1.4 Ma) inferred from geological data. This compatibility
supports the OMSP hypothesis proposed in this paper.

4.1.2 During deposition of the main body of the Ryukyu
GroupThe main body of the Ryukyu Group began to be deposited between 1.7 and 1.4 Ma in southern and central
Okinawa Jima (Fig. 13b). Coral reefs were formed in theshallow waters surrounding the island, and bioclastic
sediments dominated locally by rhodoliths accumulated
in off-reef to shelf areas. These reef to shelf carbonates were deposited corresponding to Quaternary glacioeustatic changes in sea level, during which central tosouthern Okinawa Jima continuously subsided to produce accommodation space for reef deposits that formed
during subsequent changes in sea level. As a result, thecentral to southern of Okinawa Jima were submerged
repeatedly, meaning that this island was separated fromthe OMSP by sea.
Deposition of the Ryukyu Group started at 1.25 Ma on
Irabu Jima and 0.96 Ma on Miyako Jima, later than on
Okinawa Jima. No reef to shelf carbonates are known tohave been deposited after 0.65 Ma on Miyako Jima. However, the stratigraphy of the Ryukyu Group on Miyako
Jima has not been studied over the entire island, and it
is likely that deposition continued until ca. 0.4 Ma on
Miyako Jima, as on Irabu Jima. On the Miyako Islands,
reef to shelf carbonates were deposited corresponding
to Quaternary glacioeustatic sea-level changes, during
which the islands subsided. Because the Miyako Islands
are completely covered by the Ryukyu Group, the islands
are inferred to have been submerged repeatedly during its deposition. So far as we know, on the OMSP, thelimestones that formed during 0.85–0.27 Ma are distributed on the slope, and those that were deposited during
0.27–0 Ma extend over the plateau and the upper slope.
The abovementioned findings suggest that the plateau
emerged an island (or closely spaced islets) sometime
between 5.53 and 0.27 Ma (i.e., after deposition of the
Shimajiri Group and before deposition of < 0.27 Ma
limestone). We are convinced that many of the animals
which have been found in fissure, cavity, and cave deposits on Miyako Jima (such as those at Pinza-Abu cave) andhad limited or no over-sea dispersal ability are common
to or related to those on Okinawa Jima or more northerly islands and could have survived only on areas of
the OMSP exposed above sea level during this period.

4.1.3 After deposition of the main body of the Ryukyu Group

The Miyako Islands and Okinawa Jima began to be
uplifted after deposition of the main body of the Ryukyu
Group (ca. 0.4 Ma on the Miyako Islands and < 0.45 Ma
on Okinawa Jima). The OMSP was temporarily connected to the Miyako Islands (Fig. 13c). The animals that
inhabited the OMSP migrated to the Miyako Islands
after ca. 0.4 Ma. NNW–SSE-trending normal faults
downthrown to the east formed on the Miyako Islands,
and Miyako Jima was separated from the OMSP by thisfaulting (Fig. 13d). In contrast, ENE–WSW- and ESE–
WNW-trending normal faults formed on Okinawa Jima.
At the same time, the OMSP subsided with the formation of NW–SE-trending normal faults downthrown to
the northeast, resulting in a submarine plateau topography that is higher in the southwest and lower in thenortheast. This faulting-induced subsidence began after
0.45 Ma, and the OMSP became mostly deep enoughfor the deposition of off-reef to shelf and/or shelf-slope
carbonates after 0.27 Ma (Fig. 13d and e), except for thesouthwestern part, which emerged during sea-level lowstands (Arai et al. 2016). In conclusion, the animals that
inhabited the OMSP migrated to the Miyako Islands after
the emergence of the Miyako Islands and before the submergence of the OMSP (0.4–0.27 Ma).
Several faults cut the Minatogawa Formation, although
the displacements along these faults are small (mostly
less than several centimeters, with an exception as large
as 1.5 m), suggesting that the faulting was almost finished before deposition of the Minatogawa Formationand its equivalent younger limestones (Fig. 6). However, the age of the Minatogawa Formation is not wellconstrained, and determination of its age is required infuture research.

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4.2 Tectonic implications

We discuss the subsidence of the area extending from
Okinawa Jima to Miyako Jima with reference to the formation of normal faults in the island areas of the Ryukyu
Island Arc (RIA) during the Quaternary. In most of
the island areas of the RIA, such as Okinawa Jima andMiyako Jima, the tension axis inferred from earthquake
mechanism solutions is parallel to the orientation of theisland arc (Kubo and Fukuyama 2003; Otsubo et al. 2008).
Anderson’s theory (Anderson 1951) predicts that thestress field with a tension axis parallel to the island arc
is expected to form normal faults that strike across theisland arc. The orientation of tension axis in the RIA isconsistent with the development of NW–SE-striking normal faults (Kizaki 1985; Matsumoto et al. 2009; Arai et al.
2014), some of which have been reported as active faults(Active Fault Research Group 1991). The development of
the normal faults was associated with the opening of the
Okinawa Trough (an active backarc basin) as “arc-parallel
stretching”, an elongational deformation that develops
parallel to the island arc owing to the overhang of theisland arc to the trench side caused by the opening of the
Okinawa Trough (Fabbri and Fournier 1999; Fournier
et al. 2001; Kubo and Fukuyama 2003). Given that these
normal faults cut the Ryukyu Group on Okinawa Jimaand Miyako Jima, the onset of the island-arc parallel
stretching is inferred as having occurred between postRyukyu Group deposition (after ca. 0.4 Ma) and the present. As the opening of the Okinawa Trough became
active at ca. 0.2 Ma (Sibuet et al. 1998), subsidence of
the OMSP below sea level was most likely driven by the
opening of the Okinawa Trough. This interpretation is inaccordance with the marine geological data described in
Sect. 3.5.
Insights into the Quaternary subsidence of the Kerama
gap and the OMSP can be gained from the pattern of
free-air gravity anomalies. A free-air gravity anomaly
with a long wavelength represents deviation from the isostatic state and indicates that tectonic forces are dynamically supporting the topography (Melosh and Raefsky1980). The free-air gravity anomaly pattern for the areaof the RIA is shown in Fig. 14 (Sawada et al. 2009). The
anomalies are positive around islands of the RIA (Matsumoto et al. 1996). In contrast, the anomalies are negativein the Kerama gap, breaking the positive anomaly patternaround the RIA (Matsumoto et al. 1996). These negative
anomalies indicate that the Kerama gap is deeper than
the state in which isostasy can be established. Free-air
gravity anomalies over the Kerama gap are more negative than those over the Okinawa Trough, which is currently undergoing rifting, and they are slightly positive
over the OMSP, although the seafloor of the OMSP (50–500 m water depth; Fig. 9) is shallower than that over the

Fig. 14 Free-air gravity anomaly pattern derived from observed
gravity and bathymetry data for the Ryukyu Island Arc (Sawada et al.
2009)

Kerama gap (2100 m water depth at the deepest point)(Sawada et al. 2009). The positive anomalies over the
OMSP are similar to those over the island areas around
Okinawa Jima and Miyako Jima and imply that the OMSP
has maintained isostasy under active normal faulting.
Considering the difference in seafloor depth between the
Kerama gap and the OMSP, we suggest that the differencein the free-air gravity anomalies represents the differencein subsidence rate of the Kerama gap and the OMSP after
deposition of the Ryukyu Group.
Furthermore, although Okinawa Jima, Miyako Jima,and the OMSP have positive free-air gravity anomalies,
the OMSP is slightly smaller than both Okinawa Jima andMiyako Jima (Fig. 14). The area between the Kerama gap
and the Miyako saddle, including the OMSP, is located at
the hinge of flexure of the Ryukyu Arc (Fig. 1). The locations of Okinawa Jima, Miyako Jima, and the OMSP may
have caused spatial differences in the amount of subsidence caused by NW–SE-trending normal faults owing toparallel stretching of the island arc. The differences may
be expressed as uplift of Okinawa Jima and Miyako Jimaand subsidence of the OMSP over the past 400 kyr.

5 Conclusions
In this paper, we have proposed a new hypothesis (theOMSP hypothesis) to at least partially explain the origin of modern and Late Pleistocene terrestrial vertebrates of the Miyako Islands and the timing andpathway of their migration to the islands. The terrestrial vertebrates include extant and fossil taxa thatare common with those on Okinawa Jima currently
separated by ~ 300 km of sea, despite their limited orWatanabe et al. Progress in Earth and Planetary Science(2023) 10:40

Page 26 of 30

non-existent ability for over-sea dispersal. Such fossil vertebrates are commonly found in fissure, cavity,and cave deposits. Southern Okinawa Jima was rapidly
uplifted and became a land area after deposition of the
Shimajiri Gruop had ceased at ca. 2 Ma. This land area
was connected to the OMSP, which had been upliftedand emerged during the Late Miocene or later period,
forming a large island extending ~ 400 km NE–SW.
The terrestrial animals and plants inhabiting Okinawa
Jima expanded their distribution throughout the large
island. Subsequently, Okinawa Jima subsided, to be
flooded and fringed by coral reefs at 1.7–1.4 Ma, when
the island was separated from the OMSP. Therefore, themigration of terrestrial organisms occurred during arather narrow interval at ca. 2 to 1.7–1.4 Ma. The subsidence lasted until < 0.45 Ma, during deposition of themain body of the Ryukyu Group. The Miyako Islands
were also uplifted after deposition of the group hadceased at ca. 2 Ma. The islands repeatedly underwent
complete submergence during deposition of the main
body of the Ryukyu Group (1.25–0.4 Ma). The islands
were then uplifted and emerged to become a landarea after ca. 0.4 Ma. In contrast, the OMSP began to
subside after 0.45 Ma and mostly became sufficiently
deep for deposition of the abovementioned offshore
facies after 0.27 Ma. During the interval from ca. 0.4 to
0.27 Ma, terrestrial vertebrates moved from the OMSP
to the Miyako Islands.
The OMSP hypothesis is a new hypothesis based on
an integration of geology and phylogeography. To verify and refine this hypothesis, it is essential to obtainbasic geological data for the OMSP, as reliable data are
currently limited for this plateau (e.g., Arai et al. 2014,2016) and only a single well has been drilled there (offshore Okinawa I − x well; Aiba and Sekiya 1979; Satoet al. 2022). There is also a need for ocean drilling at
multiple locations on the OMSP.

Museum for providing photographs (Figs. 2d and 12a, c and d, respectively).
The geological map of Miyako Jima (Fig. 5) is courtesy of S. Yamada. We thankH. Takayanagi and M. Arakane for preparing the figures. This manuscript was
improved by comments and suggestions of the Editor (H. Kawahata) andthree anonymous reviewers.

Abbreviations
AMS	Accelerator mass spectrometry
CN zones	Neogene coccolith zone of Okada and Bukry (1980)
EPL	Early Pleistocene land bridge
FO	First occurrenceLO	Last occurrence
MITI	Ministry of International Trade and Industry of Japan
N zones	Neogene planktic foraminiferal zones of Blow (1969)
NN zones	Neogene nannofossil zones of Martini (1971)
OMSP	Okinawa–Miyako submarine plateau
PL zones	Pliocene planktic foraminiferal zones of Berggren et al. (1995)
RGE	Ryukyu Group equivalent
SGE	Shimajiri Group equivalent
Acknowledgements
YI thanks all students affiliated to the Laboratory for Carbonate Sedimentology and Geochemistry Laboratory, Tohoku University, and who worked on
the stratigraphy of the Ryukyu Group. All authors express special thanks to
N. Kaneko of GSJ/AIST, S. Seki, and Okinawa Prefectural Museum and Art

Author contributions
NW, KA, HO, and YI conceptualized and designed this study. KA and MO
examined the OMSP hypothesis from the perspectives of marine geology andstructural geology, respectively. SC, TS, and YI performed sedimentologicaland biochronological analyses of the Ryukyu Group. HO, MT, AT, TI, and ATk
contributed to this study from the perspective of phylogeography. All authors
collaborated in the interpretation of data and the preparation of the manuscript. The final manuscript was read and approved by all authors.
Funding
This study was partly financially supported by Frontier Research in Duo (FRiD)
of Tohoku University to YI.
Availability of data and materials
The datasets used and/or analyzed during the current study are available fromthe corresponding author on reasonable request.

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Institute of Geology and Paleontology, Graduate School of Science, Tohoku
University, Sendai 980‑8578, Japan. 2 Research Institute of Geology and Geoinformation, Geological Survey of Japan/AIST, Tsukuba 305‑8567, Japan.
3
Research Institute of Earthquake and Volcano Geology, Geological Surveyof Japan/AIST, Tsukuba 305‑8567, Japan. 4 Tropical Biosphere Research Center,
University of the Ryukyus, Nishihara, Okinawa 903‑0213, Japan. 5 Facultyof Education, University of the Ryukyus, Nishihara, Okinawa 903‑0213, Japan.
6
Department of Earth Resource Science, Akita University, Akita 010‑8502,
Japan. 7 Institute of Natural and Environmental Sciences, University of Hyogo,
and the Museum of Nature and Human Activities, Sanda, Hyogo 669‑1546,
Japan. 8 Faculty of Biosphere‑Geosphere Science, Okayama University of Science, Ridaicho 1‑1, Kitaku, Okayama 700‑0005, Japan.
Received: 3 October 2022 Accepted: 29 June 2023

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