Upper Triassic shallow–water carbonates from the Naizawa Accretionary Complex, Hokkaido (Japan): New insights from Panthalassa

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Article

Reference

Upper Triassic shallow–water carbonates from the Naizawa Accretionary Complex, Hokkaido (Japan): New insights from

Panthalassa

PEYROTTY, Giovan, et al .

Abstract

Due to their accretion on the Circum–Pacific area during the Jurassic and the Cretaceous, Upper Triassic car- bonates from the Panthalassa occur very scarcely and with relatively poor preservation in accretionary com- plexes. However, they represent a unique opportunity to improve our knowledge of the depositional conditions in tropical regions of the Panthalassa.

Recognized as a cradle of life, shallow–water carbonates are also of great importance to understand of how life evolved out of the Tethyan domain during the Triassic. Since 2007, several sedimentological and biostratigraphic studies, focused on the Upper Triassic shallow water limestone from the Circum–Pacific area, have been carried out as part of the REEFCADE project at the University of Geneva. Carbonates were thus reported in the southern part of Japan and in the Russian Far East. Hokkaido Island, in northern Japan, represents the missing link between those two areas. To fill this gap, five limestone outcrops, so far poorly described in the literature, were identified and sampled in the Pippu and Esashi areas (central and northern part of Hokkaido [...]

PEYROTTY, Giovan, et al . Upper Triassic shallow–water carbonates from the Naizawa

Accretionary Complex, Hokkaido (Japan): New insights from Panthalassa. Palaeogeography, Palaeoclimatology, Palaeoecology , 2020, vol. 554, no. 109832

DOI : 10.1016/j.palaeo.2020.109832

Available at:

http://archive-ouverte.unige.ch/unige:136522

Disclaimer: layout of this document may differ from the published version.

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Palaeogeography, Palaeoclimatology, Palaeoecology

journal homepage:www.elsevier.com/locate/palaeo

Upper Triassic shallow–water carbonates from the Naizawa Accretionary Complex, Hokkaido (Japan): New insights from Panthalassa

G. Peyrotty

^a,^⁎

, H. Ueda

^b

, C. Peybernes

^a

, R. Rettori

^c

, R. Martini

^a

aDepartment of Earth Sciences, University of Geneva, Rue des Maraîchers 13, CH–1205 Genève, Switzerland

bDepartment of Earth and Environmental Sciences, Hirosaki University, 3 Bunkyo–cho, Hirosaki, Amori 036–8561, Japan

cDipartimento di Fisica e Geologia, University of Perugia, Via Pascoli, 06123 Perugia, Italy

A R T I C L E I N F O Keywords:

Benthic foraminifers Microfacies Sedimentology Limestone Biostratigraphy

A B S T R A C T

Due to their accretion on the Circum–Pacific area during the Jurassic and the Cretaceous, Upper Triassic carbonates from the Panthalassa occur very scarcely and with relatively poor preservation in accretionary complexes. However, they represent a unique opportunity to improve our knowledge of the depositional conditions in tropical regions of the Panthalassa. Recognized as a cradle of life, shallow–water carbonates are also of great importance to understand of how life evolved out of the Tethyan domain during the Triassic. Since 2007, several sedimentological and biostratigraphic studies, focused on the Upper Triassic shallow water limestone from the Circum–Pacific area, have been carried out as part of the REEFCADE project at the University of Geneva.

Carbonates were thus reported in the southern part of Japan and in the Russian Far East. Hokkaido Island, in northern Japan, represents the missing link between those two areas. To fill this gap, five limestone outcrops, so far poorly described in the literature, were identified and sampled in the Pippu and Esashi areas (central and northern part of Hokkaido Island, respectively). Their related microfacies are presented in detail as well as their foraminiferal associations. The obtained age, based on foraminiferal biostratigraphy from both areas, is defined as Carnian and the facies similarities, associated with specific modes of occurrence, identify the outcrops from the two areas as part of the same depositional system. Based on microfacies interpretations, a hypothetical depositional model is presented. It corresponds to a intra–oceanic depositional system developed on the flanks of an emergent volcanic seamount. The strong similarity with synchronous systems from the Sambosan Accretionary Complex (southwestern Japan) is discussed.

1. Introduction

Upper Triassic shallow–water carbonates from the Panthalassa are today accreted on the Circum–Pacific area (i.e., Philippines, Japan, Russian Far East, USA and Mexico) and constitute the best archive of the environmental conditions prevailing in this vast ocean during the Late Triassic. Over the past 13 years, the great majority of these systems have been precisely studied from different points of view, i.e., sedimentology, biostratigraphy and diagenesis (Bucur et al., 2020accepted;

Chablais et al., 2010a, 2010b, 2010c;Heerwagen and Martini, 2018, 2020;Khalil et al., 2018;Onoue et al., 2009; Peybernes et al., 2015, 2016a, 2016b, 2020accepted;Peyrotty et al., 2020a, 2020baccepted;

Rigaud et al., 2010, 2012, 2013a,b, 2015a,b, 2016;Rigaud and Martini, 2016;Sano et al., 2012;Senowbari–Daryan et al., 2010). The obtained results thus provide an accurate picture of the Late Triassic

shallow–water limestone settings from the Panthalassa, with their related depositional conditions, biological content and post–depositional evolution. However, despite reports of occurrences of Upper Triassic limestone in a few studies (Igo et al., 1974;Ishizaki, 1979;Sakagami and Sakai, 1979), Triassic carbonates from the Hokkaido Island (northern Japan) remain so far unknown in term of petrosedimentary and depositional study. On the Asiatic margin of the Pacific, Upper Triassic limestones from Panthalassa have been observed in Late Jur- assic/Early Cretaceous accretionary complexes, namely: (1) the Sam- bosan Accretionary Complex (SAC) in Kyushu and Shikoku islands (Japan) (Chablais et al., 2010a, 2010b, 2010c;Peybernes et al., 2015, 2016a, 2016b, 2020accepted); (2) the Taukha Terrane in the Russian Far East (Kemkin et al., 2016;Punina, 1997;Peyrotty et al., 2020a); (3) the Oshima Belt in Hokkaido Island (Japan) and the North Kitakami Belt in northeastern Honshu Island (Japan) (Sakagami et al., 1969;Sano

https://doi.org/10.1016/j.palaeo.2020.109832

Received 1 May 2020; Received in revised form 27 May 2020

⁎Corresponding author.

E-mail addresses:giovan.peyrotty@unige.ch(G. Peyrotty),ueta@geo.sc.niigata-u.ac.jp(H. Ueda),peybernes.camille@unige.ch(C. Peybernes), roberto.rettori@unipg.it(R. Rettori),rossana.martini@unige.ch(R. Martini).

Available online 30 May 2020

T

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et al., 2009); and (4) the Palawan Block in the Philippines (Kiessling and Flügel, 2000). From the 1970s to 1990s, several authors identified Upper Triassic limestone blocks in Hokkaido, based on bryozoan and conodont biostratigraphy (Igo et al., 1974; Ishizaki, 1979; Sakagami and Sakai, 1979), from the Naizawa and Horobetsugawa tectonic units, which belong to the Sorachi–Yezo belt located in the central part of the island. These tectonic units correspond to Early and Late Cretaceous accretionary complexes, respectively. As a starting point for this study, these locations were extensively explored and sampled, and the best–- preserved facies were identified in the Naizawa Accretionary Complex (NAC) where several limestone blocks occur in the Pippu and Esashi areas (Igo et al., 1974;Ishizaki, 1979). Microfacies of carbonates from both areas are precisely described and age constraints are proposed on the basis of foraminiferal assemblage. Finally, the relationships between the limestones from these two areas, today 100 km apart, is discussed and a coherent, albeit hypothetical, depositional model is proposed to reconstruct the morphology of the original depositional system. The obtained results are of great significance for the understanding of carbonate development in Panthalassa, as the Hokkaido Island is a strategic point of study. It is indeed considered as the missing link between the shallow–water carbonate limestone found in the southern part of Japan and in the Russian Far East. Therefore, the results are compared with analogous and/or similar depositional systems from Panthalassa to highlight differences or similarities and to discuss the potential relationships between their provenance and depositional conditions.

2. Geological setting

Five main tectonic units, generally defined as Jurassic to Paleogene accretionary complexes and extending in a north–south direction (Ueda, 2016), characterize Hokkaido Island (Fig. 1). The investigated areas are located in the central and northern parts of Hokkaido Island, on the Sorachi–Yezo Belt (Ueda, 2016) (Fig.1B). The latter is characterized by i) accretionary complexes (i.e., the Idonnappu Zone) in the east, ii) ultramafic and high P/T metamorphic rocks in the central part (i.e., the Kamuikotan Zone) and iii) a Cretaceous forearc basin in the west (i.e., the Yezo Group) (Fig.1B). The Idonnappu Zone is composed of an ophiolitic mélange (i.e., the Oku–Niikappu Complex) and of two accretionary complexes, namely: the NAC, of Early Cretaceous age, and the Horobetsugawa Complex, of Late Cretaceous age (Ueda, 2016).

These two complexes are in contact, and locally separated by the Oku–Niikappu Complex. The studied carbonates, previously identified byIgo et al. (1974)andIshizaki (1979), are located on the NAC, for- merly known as the Kitami–Esashi Complex in the north of Hokkaido (Suzuki et al., 1997). The NAC, extending northward across the island, is composed of Permian to Triassic shallow–water limestone and Triassic to earliest Cretaceous cherts, both associated with hemipelagic mudstone and terrigenous clastic rocks of middle Early Cretaceous age.

Basaltic rocks, presenting different chemical types, were also identified and defined as depleted tholeiite and oceanic island basalt (OIB) (see Ueda, 2016for more details about the NAC). This basaltic complex is in contact with either the ophiolitic Horobetsu Accretionary Complex (i.e., the Poroshiri Ophiolite of mid–Cretaceous age) or the Hidaka metamorphic belt to the east and the Yezo Group to the west (Fig. 1B). The Idonnappu Zone is defined as unique to Hokkaido Island, with a probable prolongation to the Sakhalin Island to the north. It is noted that no equivalent units were found in SW Japan (Ueda, 2016).

3. Studied areas and mode of occurrence

The two investigated areas are located in the central and northern parts of Hokkaido Island (Fig. 1B) and both belong to the Naizawa Accretionary Complex (NAC). Their limestone outcrops were extensively sampled in 2016 and 2018. The localities are described herein and the outcrops' locations (GPS coordinates), thicknesses, and a list of

collected samples, are presented inTable 1.

3.1. Pippu area

The Pippu area consists of a single outcrop locality set to the northeast of the Asahikawa city, on the Tosshozan Mount. The sampled carbonates, referred to in this paper as the Pippu limestone, are part of the Tosshozan Formation (Fig. 2A), belonging to the NAC. This formation consists of the studied limestone bounded by acidic tuff and associated with marine rocks such as cherts, siliceous shales and len- ticular basaltic pillow lavas (seeKato and Iwata, 1989for a complete description of the Tosshozan Formation). In the 1970s,Igo et al. (1974) identified the Carnian conodont?Paragondolella polygnathiformisfrom the Pippu limestone. This work remains, so far, the only study conducted on these carbonates. Note thatKawamura and Sugai (2011)also mentioned the Pippu limestone, with minor microfacies description, in an abstract for the Annual Meeting of the Geological Society of Japan.

Ishizuka et al. (1984)identified radiolarians of Valanginian age from the siliceous shales associated with the limestone and therefore suggested that the latter is an exotic block belonging to a Lower Cretaceous trench–fill system (see also Kato et al., 1986). The Pippu limestone outcrops as a 15 to 20 m high cliff (Fig. 3A) and extends over a few hundred meters along the Pippu River on the eastern basal part of the Tosshozan Mount (Fig. 2A). In this locality, 41 samples of various microfacies were collected in July 2016 and 2018.

3.2. Esashi area

The Esashi area is defined by 4 outcrop localities (Loc. 1 to 4, Fig. 2B) situated in the Esashi Mountains, about 18 km to the west of the Esashi coastal city (Fig. 2B). Loc. 1 is situated circa 3 km to the east of road 120, in the Nakano River. It consists of a small, isolated limestone outcrop (5 m wide, 2 m high) partially covered by vegetation (Fig. 3B,Table 1). Loc. 2 to 4 are distributed along the prefectural road 120 and the Peichan River, 3 to 5 km south of Loc. 1. Loc. 2 corresponds to a large, intensely fractured, limestone cliff (40 m wide, 20 m high) (Fig. 3C,Table 1), lying on conglomeratic deposits composed of plur- icentimetric clasts of limestone, cherts and mudstone embedded in a volcaniclastic matrix. The northern part of the cliff is in contact with bedded sandstone and mudstone. Loc. 2 also includes a smaller outcrop, 200 m to the south of the cliff, on the other side of the Peichan River.

This outcrop is an isolated smaller cliff (10 m wide, 10 m high) (Fig. 3D, Table 1), overlooking the river, and almost fully covered by vegetation.

Both outcrops at Loc. 2 were defined as the same limestone block, which extends on both sides of the Peichan River (Ishizaki, 1979) (Fig. 2B). Loc. 3 is a limestone outcrop, 3 m wide, 2 m high (Fig. 3E, Table 1), partially covered with vegetation and located to the right of the start of a forest path. This path leads to Loc. 4 (after 800 m from road 120), which is a small abandoned quarry composed of the targeted limestone (20 m wide, 12 m high; Table 1). This carbonate unit is embedded in a conglomeratic deposit (Fig. 3F) consisting of plur- icentimetric clasts of cherts, mudstone and sandstone in a volcaniclastic matrix. The four investigated outcrops, referred to as Esashi limestone in this paper, appertain to the Heian Formation. The latter belongs to the NAC and is composed of the targeted limestone, mudstone, sandstone and tuff and lava, both of diabasic texture (Ishizaki, 1979).

Based on radiolarian data,Igo et al. (1987) dated the Heian For- mation to be Valanginian to Barremian. Therefore, the Heian Formation is equivalent, in age and composition, to the Tosshozan Formation in the Pippu area. Until this study, the sampled limestone outcrops in the Esashi area were only identified and studied from a biostratigraphic point of view (bryozoans) byIshizaki (1979). This author reported two species of bryozoans of Ladinian–Carnian affinity (i.e.,Psezadobatos- tome kobayashiiandDyscritella hidakensis), and suggested a Ladinian to Carnian age for the limestone. Note that other limestone outcrops were identified byIshizaki (1979)but our field investigations revealed that

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they are today covered by dense vegetation or consist of highly recrystallized carbonates and therefore of low interest for our study. In the Esashi area, 36 samples of various microfacies were collected in July 2018.

4. Microfacies and biotic content

As presented in section 2, all of the investigated outcrops are part of a Lower Cretaceous accretionary complex (the NAC).Igo et al. (1974) Fig. 1.Geological setting. A. Geographic position of Hokkaido Island. Red square corresponds to B. B. Geological map of the Sorachi–Yezo Belt (see text for details) with the location of the two investigated areas (i.e., Pippu and Esashi), highlighted by red arrows. ON: Oku–Niikappu Complex.Modified afterUeda, 2016. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1

Thicknesses and coordinates of the studied outcrops and collected samples.

Studied outcrop Thickness GPS coordinates Collected samples

Pippu limestone 15 to 20 m Lat. 43°50′36.46”N

Long. 142°27′32.27″E GP76 - > GP80

GP251 - > GP270

Esashi limestone Loc. 1 2 m Lat. 44°56′0.27”N

Long. 142°20′31.45″E GP238 - > GP241

Loc. 2 Large cliff:

20 m Small cliff:

10 m

Large cliff:

Lat. 44°54′32.67”N Long. 142°21′13.24″E Small cliff:

Lat. 44°54′27.12”N Long. 142°21′10.26″E

Large cliff:

GP213 - > GP 222 Small cliff:

GP223 - > GP228

Loc. 3 2 m Lat. 44°53′46.31”N

Long. 142°21′12.24″E GP229 - > GP232

Loc. 4 12 m Lat. 44°53′27.89”N

Long. 142°21′3.68″E GP233 - > GP237

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Fig. 2.Location of the investigated outcrops. A. Pippu limestone and Tosshozan Formation on the Tosshozan Mount. B. Esashi area (within the Esashi Mountains) with the four sampled localities in the Heian Formation (Loc. 1 to Loc. 4). GPS coordinates of each outcrop are given inTable 1.

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andIshizaki (1979)proposed that these carbonates are of Middle/Late Triassic age and are therefore defined as exotic systems originating from the Panthalassa and incorporated within the East Asia margin.

During accretion, limestone is subject to intense tectonic stresses, associated with relatively high temperature burial conditions and/or impacted by volcanic intrusions. The resulting outcrops are consequently highly fractured or faulted, partially recrystallized and do not present any bedding or sedimentary structures. Moreover, as presented in section 3, they occur as isolated blocks of various sizes, without any spatial continuity (Figs. 2, 3). In order to establish a coherent depositional model, and to reconstruct the zonation of the original facies, we can only rely on the characteristic microfacies of all outcrops. The pe- culiar mode of occurrence of allochthonous limestone in accretionary complexes and the related work approach for sedimentological studies are detailed inPeyrotty et al. (2020a). Here, we present the microfacies and biotic content from the Pippu and Esashi limestone. The observations were made from 77 thin sections (2.3 × 4 cm) under an optical microscope (Zeiss Axioskop). The microfacies are classified according to the classification ofDunham (1962)and subsequent improvements byEmbry and Klovan (1971).

4.1. Pippu limestone

MF1 – Debris–peloid wackestone (Fig. 4).MF1 is essentially composed of dark micrite associated with peloids and intraclasts of MF2. Likewise, rare burrows filled by MF2 were observed. Bioclasts are mostly represented by debris of echinoderm and bivalve debris, locally coated.

Rare calcimicrobes as well as micritized ooids can also occur. No foraminifers were observed in MF1. Note that this microfacies is poorly represented among all outcrops (Table 2).

MF2 – Peloid–bioclast packstone to grainstone (Fig. 4).MF2 is characterized by peloids of various origins (fecal pellets, reworked mud grains and micritized clasts) associated with several different bioclasts, mostly large echinoderms ossicles (Fig. 4B). Other significant bioclasts are microproblematica [i.e., undetermined micritic tubes,Plexoramea cerebriformis (Fig. 4C), Radiomura cautica, Baccanella floriformis (Figs. 4E, F)],Cayeuxiasp. and other undetermined calcimicrobes together with rare ostracods and gastropods,?Soleporacean red algae (Fig. 4G) and green algae (Holosporellasp.) (Fig. 4H). In places, mi- croproblematicaTubiphytesspp. debris, aggregates and coated grains and rare micritized ooids also occur. The foraminiferal association (see Table 2for the list of identified forms and references to figures) is dominated by the miliolid family Ophthalmidiidae, which occur in Fig. 3.Overview of the sampled outcrops. See text for details. A. Pippu limestone cliff. B. Loc. 1, Esashi area. C. Loc. 2 (large cliff), Esashi area. D. Loc. 2 (small cliff), Esashi area. E. Loc. 3, Esashi area. F. Loc. 4, Esashi area. Note the specific mode of occurrence of the limestone, which is embedded in a conglomeratic deposit. GPS coordinates of each outcrop are given inTable 1.

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abundance in MF2 and is represented by three species:Ophtalmidium exiguum, Ophtalmidiumaff.O. ubeyliense, Ophthalmidium? n. sp., Oph- talmidium sp. andGsollbergella spiroloculiformis. Note that the association is characterized by a remarkable diversity of other Miliolids, such asAgathammina austroalpina,Agathammina iranica,Arenovidalina chia- lingchiangensis, Planiinvoluta carinata, Turriglomina carnica, Para- ophtalmidium carpathicum, in association with robertinids (Variostoma cf. V. turboidea, Diplotremina sp.), endothyrids (Endotriada kuepperi, Endotebidae gen. sp. indet.) and rare involutinids (Lamelliconus multi- spirus,Aulotortussp.). MF2 is not a pure limestone as it is characterized by the presence of rounded to sub–angular volcanic grains, from 100 μm to 5 mm in size (Fig. 4D), locally coated by a micritic rim. This facies is the most represented in the Pippu limestone.

MF3 – Tubiphytes–calcimicrobe rudstone (Fig. 5).MF3 is dominated byTubiphytesspp., locally in the form of debris (Fig. 5C), and associated with large echinoderm ossicles (Fig. 5A). Poorly preserved calcimicrobes are also abundant (Fig. 5A) as well as reworked mud grains, micritized clasts and coated or aggregate grains. Other bioclasts are?- Soleporacean red algae, microproblematica (i.e., Plexoramea cere- briformis, Radiomura cautica, Baccanella floriformis) and the sponge Uvanellasp. (Fig. 5B). Serpulids and bryozoan debris are rare. Microbial crusts (Fig. 5D) were observed on a recrystallized framebuilder (not identifiable). Some intergranular cavities are filled by crystal silt that occurs after the precipitation of an isopachous dogtooth cement. The foraminifers are rareOphtalmidiumsp. and Duostominidae. No volcanic grains are incorporated within the MF3. As for MF1, MF3 is poorly represented in our samples (Table 2).

MF4 – Ooid–bioclast grainstone (Fig. 5).MF4 is mainly composed of well–sorted cement–supported ooids, associated with various bioclast debris (Fig. 5E). Ooids are spheroidal (200 μm to 2 mm in diameter), of radial–concentric type, and can be single or compound. They are defined by concentric micritic or sparitic layers locally obliterated by radial fibrous micritic structures extending from the core to the external part of the ooid (Fig. 5F). Large parts of the ooids are broken and regenerated (Fig. 5G). The cores of ooids are characterized by micritized grains, echinoderms, broken ooids, or sparite identified as recrystallized bioclasts. Other components of MF4 are aggregates and coated grains, diverse micritized grains and intraclasts of MF2 and MF3. Bioclasts are debris of organisms observed in MF2 and MF3 such as undetermined calcimicrobes, Tubiphytes spp. (Fig. 5E), green algae, bryozoan and echinoderms. No foraminifers were observed in MF4. Clasts are cemented by isopachous dogtooth cement followed by mosaic blocky.

Locally, some intergranular voids are filled by crystal silt. Note that no volcanic grains are observed in MF4.

4.2. Esashi limestone

MF5 – Echinoderm–peloid packstone to rudstone (Fig. 6). MF5 is composed of large echinoderms and peloids, the latter being defined as micritized grains of various origin (i.e., reworked mud grains or micritized bioclasts) in a micritic matrix (Figs. 6A, B). Other bioclasts are debris of undetermined calcimicrobes (Fig. 6B), bivalves and bryozoans, associated with remobilized framebuilders (i.e., recrystallized corals and spongiomorphids) (Fig. 6A). Bioclasts occur Fig. 4.MF1 and MF2. A. MF1, debris–peloid wackestone. B. MF2, peloid–bioclast packstone to grainstone. Note the abundance of large echinoderms ossicles (some highlighted by the yellow arrows). C.Plexoramea cerebriformisin MF2. D. Abundant volcanic grains (yellow arrows) in MF2. Note that peloids are mostly micritized foraminifers. E, F.Baccanella floriformisin MF2. G.?Solenoporacean red algae in MF2. H.Holosporellasp. in the MF2. Scale bars: A, B: 2,5 mm; C, E: 200 μm; D, G, H:

500 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table2 Microfaciescontentandenvironmentalinterpretations. FaciesSamplesLocalitiesBioticcontentandforaminifersassemblageOtherclastsandsedimentaryfeatures Facies interpretation

MF1

Debris-peloids wackestone

GP77,GP265,GP267PippuareaDebrisofechinodermsandbivalvesandrareindet.calcimicrobes /Noforaminiferobserved (NotethatinthesampleGP77,Agathamminaaustroalpinawas osbservedinaintraclastofMF2)

Peloids,intraclastsofMF2,raremicritizedooids /RareburrowsProtectedlagoon MF2

Peloidal-bioclastic packstone

tograinstone

GP76A,GP76B,GP76C,GP79,GP251, GP252,GP253,GP254,GP261,GP262A, GP262B,GP262C,GP263,GP270–1, GP270–2

PippuareaEchinodermossicles,rareostracodsandgastropods,

microproblematica (Baccanellafloriformis,Plexorameacerebriformis,Radiomuracautica, debrisof Tubiphytesspp.andindet.micritictubes),Cayeuxiasp.andother indet.calcimicrobes, rare?SoleporaceanredalgaeandgreenalgaeHolosporellasp. / Agathamminaaustroalpina(Fig.8:25to31),Agathamminairanica (Fig.9:1to5),Arenovidalinachialingchiangensis(Fig.9:10,11), Aulotortussp.(Fig.9:16),Diplotreminasp.(Fig.9:20),Endotriada kuepperi(Fig.9:23),Gsollbergellaspiroloculiformis(Fig.8:2,3,32), Ophthalmidiumexiguum(Fig.8:4to8),Ophthalmidiumexiguum?, Ophthalmidiumaff.O.ubeyliense(Fig.8:9,13),Ophthalmidium?n.sp. (Fig.8:15to23),Ophthalmidiumsp.(Fig.8:10to12,14), Paraophthalmidiumcarpathicum(Fig.8:24),Planiinvolutacarinata (Fig.9:7to9),Turriglominacarnica(Fig.9:19),Variostomacf.V. turboidea(Fig.9:17),Variostomasp.,Lamelliconusmultispirus(Fig.9: 14,15),Miliolataindet.,Ammodiscidae,Duostominidae, Endotebidaegen.sp.indet.(Fig.9:21),Nodosariid

Abundantvolcanicgrains,peloids(fecalpellets, reworkedmudgrainsandmicritizedclasts), aggregategrains,coatedgrainsand raremicrtiziedooids /_

Openlagoon MF3

Tubiphytes-calcimicrobe rudstone

GP269,GP-270PippuareaTubiphytesspp.,echinodermossicles,Uvanellasp.,?Solenoporacean redalgae,microproblematica(Baccanellafloriformis,Plexoramea cerbriformis,Radiomuracautica), rareserpules,bryozoandebris,microbialcrust,andrecrystallized framebuidlers (i.e.,coralsandspongiomorphids) /Ophthalmidiumsp.,Dustominidae

Reworkedmudgrains,micritizedgrains, coatedgrains,aggregategrains/_Peri-reef MF4

Oolitic-bioclastic grainstone

GP255,GP256,GP257,GP258,GP259, GP260PippuareaEchinodermsossicles,calcimicrobesindet.,debrisofbryozoansand greenalgae /Noforaminiferobserved

Spheroidalradial-concentricooids(200μmto 2mmindiameter,single,compound,brokenand regenrated),aggregateandcoatedgrains, micritizedgrainsandintraclatsofMF2/MF3/_

Sandbar MF5

Echinoderm-peloid packstone

torudstone

GP213,GP218,GP223,GP238,GP239, GP241Esashiarea Loc.1,Loc.2Largeechinodermossicles,derbisofbivalves,bryozoansand calcimicrobesindet.;remobilizedframebuilders(i.e.,recristallized coralsandspongiomorphids)/Endotriadellacf.E.wirzi?(Fig.9:26), Gaudryinatriadica(Fig.9:28),foraminifersindet.

Peloids(reworkedmudgrainsourmicritized bioclasts),intraclastsofMF2,aggregateand coatedgrains,rareooidsofMF4andlocally

deformed /_

Upperslope MF6 Peloid-ooidpackstoneto grainstone

GP219A,GP219B,GP220,GP221,GP222, GP224,GP226,GP227,GP228,GP235Esashiarea Loc.1,Loc.2, Loc.4

Debrisofechinoderms(locallycoated)andalcimicrobesindet. / Planiinvolutacarinata(Fig.9:25),encrustingTolypamminagregaria (Fig.9:30), foraminifersindet.

OoidsfromMF4,micritizedgrains,coatedgrains andcalcispheresindet.

/ Grains

arrangement(unidirectionalflow)

Upperslope MF7 Laminatedmudstoneto packstone

GP214,GP215,GP229A,GP229B,GP230, GP231,GP234,GP236,GP237Esashiarea Loc.2,Loc.3, Loc.4

Debrisofcalcimicrobesindet.andgreenalgae(Steinmanniporellasp. orDissocladellasp.), spongespiculesandrareradiolarians / Agathammina?sp.(Fig.9:29),Endotriada?sp.,Duostominidae (Fig.9:27),Nodosariid, foraminifersindet.

Microbialpeloids,micritizedgrainsandooids fromMF4(locallymicritized)

/ Laminations

(locallydeformed),gradingand stromatactis

Middletolower slope

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together with various clasts representative of MF2 to MF4, such as aggregates and coated grains, intraclasts of MF2 and rare radial–concentric ooids of single, compound or regenerated types. Note that ooids are similar to those of MF4, but are locally intensely distorted (Figs. 6C, D). This distortion is characterized by a non–brittle deformation, and the detachment of the layers from the core of the ooid (Fig. 6C). Note that very rare, small volcanic grains also occur in MF5. The foraminifers Endotriadella cf. E. wirzi? and Gaudryna triadica are also present (Table 2).

MF6 – Peloid–ooid packstone to grainstone (Fig. 6).MF6 is defined by the association of ooids, peloids (i.e., micritized grains) and coated grains (Figs. 6E, F). Debris of echinoderms are also abundant and in places marked by a micritic rim. Ooids are similar to those of MF4, sometimes distorted (see MF5 for description), and some are micritized.

Grains are cement or micrite–supported. In some samples, grains appear to be arranged in a defined direction (Fig. 6E). Debris of undetermined calcimicrobes and calcispheres occur, together with foraminifersPla- niinvoluta carinata, and the encrustingTolypammina gregaria(Table 2).

Note that MF6 has been observed in stylolithic contact with a strongly recrystallized radiolarian packstone (Fig. 6F). As for MF5, very rare small volcanic grains also occur.

MF7 – Laminated mudstone to packstone (Fig. 7).MF7 is a laminated facies, ranging from mudstone to packstone. Mudstone is characterized by thin laminations made of micrite, microbial peloids and stromatactis filled by crystal silt and blocky cement (Figs. 7A, C). Laminations are either made of dark micrite punctuated by undetermined micritic grains or by microbial peloids cemented by sparite (Fig. 7C). Wackestone to packstone facies are defined by locally deformed graded deposits (Figs. 7B, D) dominated by echinoderm debris and peloids (i.e., micritized grains). Other clasts are ooids (locally micritized but not distorted) of the same type as MF4, undetermined calcimicrobes and green algae debris (Steinmanniporellasp. orDissocladellasp.) (Fig. 7E). In the wackestone portions of MF7, sponge spicules (Fig. 7B) as well as very rare radiolarians (both recrystallized into calcite) were also observed.

The foraminifer association is represented by Agathammina? sp.

(Table 2),Endotriada? sp., Duostominidae (Table 2), and rare Nodo- sariids. No volcanic grains are observed in MF7.

5. Facies interpretation

In this section, we present the interpretation of each microfacies in terms of their depositional environment. These assumptions rely on the sedimentary structures, the presence of specific organisms or clasts, as well as the mud fraction. The microfacies content and the related depositional environments are compiled inTable 2.

5.1. Lagoonal environment: MF1 and MF2

MF1, dominated by mud and characterized by the presence of some bivalve and echinoderm debris, as well as intraclasts of MF2, has been interpreted as being deposited in a low energy environment, in the lagoonal part of the carbonate platform. As MF1 is very poorly represented in the Triassic limestone of Hokkaido, we assume that this environment was not widely distributed in the original facies zonation.

It might correspond to minor isolated parts of the lagoon, away from any current, and protected within a topographic depression filled by mud and various debris from the surrounding facies. MF2 is a mud or cement–supported facies, which is characteristic of environments defined by variable hydrodynamics. The presence of green algae indicates deposition within the upper photic zone and the organism association, as well as the abundance of peloids, is typical of open–lagoon environments. Moreover, the rare micritized ooids are good indicators of a link with open environments such as oolitic shoals. The foraminiferal association, dominated by miliolids and, in MF2 particularly, by the family Ophthalmidiidae, is also indicative of an open–lagoon, hy- persaline environment (Chablais et al., 2010a, 2010b, 2011; Decarlis

et al., 2013; Gaetani et al., 2013;Gale, 2012; Gale et al., 2012,Gale et al., 2014a, 2014b; Gazdzicki, 1983; Haas et al., 2010; Igo and Adachi, 1990;Kamoun et al., 1994;Mancinelli et al., 2005;Michalík et al., 1993; Mircescu et al., 2019; Onoue et al., 2009; Parente and Climaco, 1999). Note that most of the clasts are micritized or coated by a micritic rim, which suggests strong algal or fungi activity in shallow–water environments with low sedimentation rates (Kendall and Alsharhan, 2011;Reid and Macintyre, 2012;Swinchatt, 1969). Similar microfacies are widely described in synchronous carbonate platforms from Panthalassa (Chablais et al., 2010b;Kiessling and Flügel, 2000;

Peybernes et al., 2016b;Peyrotty et al., 2020a), supporting our interpretation. MF2 also contains numerous volcanic grains, indicating a setting close to an eroding volcanic edifice (Table 2).

5.2. Reef and peri–reef environments: MF3

In both the Esashi and Pippu areas, no proper reefal facies have been found, but some evidence indicates that small reefs/bioherms probably developed in places within the carbonates. Indeed, MF3 displays organisms typical of Middle/Upper Triassic reefs (i.e., Tubiphytes and other microproblematica, microbial crusts and primary framebuilders such as sponges and corals, although recrystallized) associated with abundant calcimicrobes and a loose packing (Table 2). These bioclasts occur mainly as cement–supported, reworked or broken grains, and no reefal framework is present. We can therefore assume that MF3 is a peri–reefal facies, deposited in an open environment and dominated by reef/bioherm debris, with a high abundance of calcimicrobes, typical of such Upper Triassic environments (Chablais et al., 2010b;Kiessling and Flügel, 2000;Peybernes et al., 2016b;Peyrotty et al., 2020a). The ex- istence of small reefs/bioherms is supported by the presence, in MF5, of debris of primary framebuilders (i.e., corals and spongiomorphids), probably spread over the surrounding facies. Note that sessile or en- crusting foraminifers, typical of reef/back–reef environments (i.e., Planiinvoluta carinata,Tolypammina gregaria) (Berra and Cirilli, 1997;

Chablais et al., 2010a; Michalik and Jendrejakova, 1978; Peybernes et al., 2015; Roniewicz et al., 2007; Russo, 2007), were observed in slope and open–lagoon settings (seeTable 2and sections 5.1 and 5.4), thus confirming the presence of reefs/bioherms which had been partially dismantled and transported in the surrounding parts of the platform. Similar biotic associations were precisely described in an analogous system from the Tethyan and Panthalassa domains (Martindale, and Zonneveld, J.–P., Bottjer, D.J., 2010;Martindale et al., 2012, 2015;

Peybernes et al., 2015, 2016b;Reid and Ginsburg, 1986;Russo et al., 1997). However, the general poor preservation of our facies, as well as the scarcity of outcrops, does not allow us to make an accurate comparison of reefal and peri–reefal associations. The observations in the field, together with the microfacies analysis and their interpretations, suggest that reefs/bioherms occurred on the external part of the depositional system but were probably poorly represented since only a few related facies have been found.

5.3. Sandbar environment: MF4

MF4 is typical of an oolitic shoal or sandbar, dominated by cement–supported ooids and bioclasts, and governed by fair weather waves or tidal variations (Hine, 1977). Such facies are widely reported from modern and fossils systems (Esrafili Dizaji and Rahimpour Bonab, 2014;Friedman, 1995;Lokier and Fiorini, 2016;Peyrotty et al., 2020a;

Rankey and Reeder, 2009, 2011;Qiao et al., 2016). However, according toFlügel (2004), radial–concentric ooid shapes are typical of moderate water energy in an intermittently agitated setting with normal marine salinity. MF4 might consequently correspond to a sand bar controlled by tidal currents on the edge of the depositional system. Note that the particular radial fibrous micritic structures on the ooids (see section 4.1) might correspond to a secondary radial structure due to diagenetic overprint of the original fibers (Marshall and Davies, 1975). MF4

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Fig. 5.MF3 and MF4. A. MF3,Tubiphytes–calcimicrobe rudstone. Yellow arrows indicateTubiphytesspp. debris. Note the large echinoderm ossicle (bottom right) associated with poorly–preserved calcimicrobes (white arrows) and bryozoan debris (red arrow). B.Uvanellasp. in the MF3. C. Abundant debris ofTubiphytesspp. in MF3. D. Laminated microbial crust on recrystallized framebuilder in MF3. E. MF4, ooid–bioclast grainstone. Note the presence of reworked debris ofTubiphytesspp.

(yellow arrows). F. Large ooid with a micritic core. Note the concentric sparitic and micritic layers as well as radial fibrous structures. G. Broken and regenerated radial fibrous ooid. Scale bars: A, E: 2.5 mm; B, F, G: 500 μm; C, D: 1 mm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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exhibits an association of ooids and debris of various bioclasts and intraclasts of MF2 and MF3 and is therefore in close contact with an open lagoon setting. This clearly shows that the debris of MF2 and MF3 are reworked in MF4 due to tidal currents. In oolitic shoals, ooids are generally early micritized due to long exposure at the water–sediment interface in a cyanobacteria–rich environment (Kendall and Alsharhan, 2011;Reid and Macintyre, 2012).Peyrotty et al. (2020a)ascribe this type of preservation to a low sedimentation rate in a turbulent environment. In MF4, ooids are not micritized and might have been deposited in a non–turbulent setting, nor constantly beaten by fair weather waves. A short exposure to light with a normal sedimentation rate, unfavorable to the action of cyanobacteria, enhances the depositional conditions of MF4. Regenerated ooids can be observed in both agitated and quiet environments (Flügel, 2004) and are consequently not a strong environmental proxy to consider. Note that such oolitic facies can also occur in beach deposits (Lloyd et al., 1987), but no

typical features of beachrock were observed (i.e., meniscus, bridging and gravitational cements, and parallel laminations).

5.4. Slope environment: MF5, MF6 and MF7

MF5 and MF6 consist of various bioclast debris and reworked grains of different origins, mainly from MF2 to MF4, including echinoderms, ooids, calcimicrobes, peloids, coated grains, as well as framebuilders and foraminifers that are cemented or embedded in micrite. MF5 is marked by the presence of remobilized angular broken framebuilders, indicating a setting close to a reef/bioherm on the upper slope or fore reef. Moreover, the foraminiferal association in MF5 and MF6 is dominated by forms typical of open–lagoon, back–reef and reef environments (i.e.,Planiinvoluta carinata,Tolypammina gregaria, Gaudryina triadica, Endotriadellacf.E. wirzi?, seeTable 2) (Berra and Cirilli, 1997;

Chablais et al., 2010a;Chablais et al., 2011;Lakew, 1990;Michalik and Fig. 6.MF5 and MF6. A. MF5, echinoderm–peloid packstone to rudstone. Note the presence of very large echinoderm ossicles (top left) and recrystallized corals (bottom right). B. MF5, dominated by echinoderm debris, peloids, coated grains and calcimicrobes (yellow arrows). C, D. Deformed ooids (see text for details). E.

MF6, peloid–ooid packstone to grainstone. F. MF6 with abundant ooids (some highlighted by yellow arrows) in stylolithic contact with recrystallized radiolarian mudstone (lower part of the image). Scale bars: A, B, E, F: 2.5 mm; C: 500 μm; D: 200 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Jendrejakova, 1978; Mircescu et al., 2019; Peybernes et al., 2015;

Roniewicz et al., 2007; Russo, 2007) supporting the proximity with facies from the top of the platform. Echinoderms are very abundant in MF5. They can be transported from the platform or be autochthonous to the upper slope environment (see large and non–coated specimens, Fig. 6B) (Chablais et al., 2010b;Gale et al., 2014a;Peybernes et al., 2016b;Preto, 2012;Reijmer et al., 1991). MF6 is characterized by MF2 and MF4–derived grains, which suggests that MF6 is located on the upper part of the slope, below open–lagoon and sandbar environments.

However, we note that MF6 has been locally observed to be in contact with radiolarian facies, which does not exclude deposition in deeper environments. In this case, in event of strong storms or swells and earthquakes, MF6 can be transported to deep environments, as a single gravitational debris–flow (Peyrotty et al., 2020a). The grain arrangement in MF6 (see section 4.2) is linked to deposition under unidirec- tional flow, and therefore also confirms the debris-flow nature of the deposit. The distortion of ooids observed in MF5 and MF6 is assumed to be linked to early burial deformation but further investigation might be necessary to precisely characterize this event. According to the above

observations, MF5 and MF6 are considered as upper slope deposits, below the edges of the carbonate system. MF7 shows laminated deposits, is locally graded, and is interpreted as forming from debris–flows occurring on middle to lower slope settings. MF7 sediments are defined as micrite associated with microbial activity (i.e., microbial peloids and stromatactis), consistent with quiet environments below wave action.

Muddy sediments occur in association with debris–flows (graded wackestone to packstone) with rare sponge spicules and radiolarians, confirming the deep–water setting. Graded parts are made of shallow–water derived grains (ooids and foraminifers, mainly Miliolids, from open–lagoon settings) (seeTable 2and sections 5.1 and 5.2) as well as echinoderm debris, and are interpreted as debris flows from the upper parts of the slope. The local deformation of the laminae is assumed to be linked to slope movement either during the deposition or very early, before compaction (i.e., slump). The absence of wave ripples and hummocky or swaley cross stratifications confirms that those deposits are not related to storm sediments within the lagoon (Flügel, 2004; Peyrotty et al., 2020a). Note that similar slope deposits were precisely described in synchronous systems from the Panthalassa Fig. 7.MF7. A. Laminated mudstone distinguished by stromatactis (yellow arrows) and microbial peloids. B. Graded wackestone to packstone. Note the presence of sponge spicules (yellow arrows). C. Microbial peloids from the muddy laminations in MF7. D. Deformed, graded deposit in the wackestone portions of MF7. E.

Steinmanniporellasp. orDissocladellasp. Scale bars: A, B, D: 5 mm; C, E: 500 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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(Chablais et al., 2010b;Peybernes et al., 2016b).

6. Biostratigraphy

The Pippu and Esashi carbonates were previously studied in the 1970's from a biostratigraphic point of view.Igo et al. (1974)reported Upper Triassic (Carnian) Conodont (i.e., Paragondolella poly- gnathiformis) from the Pippu limestone. No precise description or pic- tures were given for this species. On the other hand, bryozoans of La- dinian–Carnian affinity were identified in the Esashi limestone (Ishizaki, 1979). This author described two species: (1)Psezadobatos- tome kobayashii, first reported bySakagami (1972)from a lower Car- nian horizon of the Upper Triassic Kochigatani Formation in Shikoku Island (Japan); and (2) Dyscritella hidakensis, first described from a limestone block in the Kerimai River to the north of Motourakawa Region (Hokkaido, NAC) bySakagami and Sakai (1979). Note that both species are only observed in Japan (Schäfer, 1994) and their stratigraphic extension relies on conodont and bivalve identifications (Sakagami and Sakai, 1979; Sakagami, 1972;Zhao-Xun, 1984). Only small quantities of bryozoan debris were found in our facies and it is not possible to attribute them to the above–mentioned species. For this work, no conodont extraction was conducted and the general poor preservation of the organisms prevents precise identification for biostratigraphy. However, a well–diversified and preserved foraminiferal assemblage was found in the Pippu limestone. Foraminifers belonging to the same association are also present, although are rare in the Esashi limestone, where they were resedimented after being exported from the platform. The foraminifers allow us to refine the age of the Pippu and Esashi limestones. Over the past 10 years, Upper Triassic benthic foraminifers from the Panthalassa were precisely identified and their pa- leobiogeographic significance is of high importance for the character- ization of this huge ocean (Chablais et al., 2010b, 2010c;Peybernes et al., 2015, 2016a, 2016b;Peyrotty et al., 2020a;Rigaud et al., 2010, 2012, 2013a,b, 2015a,b, 2016;Rigaud and Martini, 2016). So far, no studies on Triassic foraminifers have been conducted on the intra–oceanic carbonates from Hokkaido and the presented data are therefore essential for the understanding of life colonization and evolution across oceans during the Late Triassic. The foraminiferal assemblage of the Pippu and Esashi limestones, and their related stratigraphic extension are presented, in the following subsections.

6.1. Foraminiferal association of the Pippu limestone

The Pippu limestone is characterized by a diversified, biostrati- graphicaly significant foraminiferal content (Figs. 8, 9andTable 2for the list of identified forms). Bilocular Miliolata, mostly represented by the Family Ophthalmidiidae, occur in high abundance in specific facies (Fig. 8: 1), interpreted as open–lagoon deposits. This interpretation is in accordance with these porcelaneous facies–controlled foraminifers, recognized in the packstone–grainstone facies (see section 5.1). Among the Ophthalmidiidae, two species have been identified and correspond toOphtalmidium exiguum, characterized by a small test with thin wall, and Ophtalmidium aff. O. ubeyliense, showing a flattened test and a regular and evolute planispiral coiling. A third population, referred here asOphtalmidium? n. sp. was not observed until now in the Triassic.

These forms are robust in axial section (multichambered?) and display an early streptospiral stage followed by a sigmoidal coiling with four whorls, and will be the subject of further systematic study, assuming we can find equatorial sections. It is interesting to mention that this morphology superficially resembles certain Permian taxa referable to the Paleozoic Cornuspiroidea (Hemigordiidae, genusHemigordius).Igo and Adachi (1990)made this same reflection concerning the ophthalmidid foraminifers of the San Juan Island, which resemble certain genera of primitive fusulinids. A foraminifer, attributed to Miliolata indet., shows curious structures, resembling pillars, in the last two whorls (Fig. 9: 6, arrows). Although the section is off-centre and oblique, and the

structures are difficult to interpret, it is interesting to mention them.

Indeed, in the Triassic Miliolata, pillars are recorded in the, thus far, only known species of the porcelaneous Middle Triassic Chinese genus ParatriasinaHe, 1980, type–speciesP. jiangyouensisHe, 1980. The genus Partriasina is considered byZaninetti et al. (1991) to belong to the evolutionary trend Arenovidalina–Paratriasina–Ophthalmidium. This phylogenetic trend is marked by structural modifications that occur through the development of internal pillars vs chambers development.

If the presence ofParatriasinais confirmed, it could be the first record of the co–occurrence of the three members of this phylogenetic lineage in the Carnian. Other significant miliolids are represented by Agatham- mina austroalpina, Agathammina iranica, Arenovidalina chia- lingchiangensis,Planiinvoluta carinataandTurriglomina carnica. Miliolids are associated to other, much less represented forms in the Pippu limestone, such asLamelliconus multispirus,Aulotortussp.,Variostomacf.

V. turboidea,Variostomasp.,Endotriada kuepperiand a new, unknown Endotebidae gen. sp. indet. Representatives of the aragonitic perforate, lamellar wall family Aulotortidae, typical of restricted lagoon environments, are very poorly represented as no wide protected lagoon was identified in the Pippu limestone. It is important to note that Au- lotortidae are abundant and well diversified in the Upper Triassic lagoonal limestone of the Sambosan Accretionary Complex (Kyushu and Shikoku islands;Chablais et al., 2010b, 2010c, 2011;Peybernes et al., 2016b). The foraminiferal association of the Pippu limestone is typical of the Late Triassic and can be constrained to the Carnian because of the presence of the Carnian markers Turriglomina carnica, Lamelliconus multispirus, Ophtalmidium aff. O. ubeyliense and Endotriada kuepperi.

Moreover, no typical species of Norian–Rhaetian (e.g.,Aulosina ober- hauseri, Triasina hantkeni, Lamelliconus semivacuus, Frentzenella frentzeni, Kristantollmanna truncata, Wallowaconus oregonensis) were present in our samples.Igo et al. (1974)dated the Pippu limestone as Carnian on the basis of conodont and the stratigraphic extension obtained with the foraminiferal assemblage therefore fully confirms this age.

6.2. Foraminiferal association of the Esashi limestone

Compared to the Pippu limestone, the Esashi limestone does not contain a rich foraminiferal assemblage (Fig. 9andTable 2for the list of identified forms). Indeed, the Esashi facies were interpreted as slope deposits (see section 5.4) and therefore do not constitute an adequate environment for the development of benthic foraminifers, which are circumscribed to the shallow top of the platform. Consequently, all the identified specimens come from the platform margin as part of debris–flows (see section 5.4). Despite a scarce foraminiferal content, significant forms were identified (Fig. 9):Agathammina austroalpina,Aga- thammina? sp.,Gaudryina triadica,Planiinvoluta carinata,Endotriadellacf.

E. wirzi?,Endotriada? sp., encrustingTolypammina gregaria. Duostomi- nidae and Nodosariids also occur. These species, considered as a whole, are indicative of a Late Triassic age, which is consistent with the Car- nian age, as given by foraminifers in the Pippu limestone. Note that Ishizaki (1979) identified bryozoans with Ladinian–Carnian affinity within the Esashi limestone.

Given the above, we consider the foraminifers of the Esashi limestone to be part of the same foraminiferal association found in the Pippu limestone. Moreover, some species are present in both localities and typical Norian–Rhaetian forms are absent. Accordingly, a Carnian age for the Esashi limestone is proposed.

7. Discussion

7.1. Pippu and Esashi carbonates originate from the same carbonate system Isolated outcrops of various sizes, without spatial continuity, characterize the mode of occurrence of allochthonous carbonate systems in accretionary complexes (Chablais et al., 2010b;Peybernes et al., 2016b;

Peyrotty et al., 2020a). Indeed, the tectonic process of accretion leads to

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Fig. 8.Foraminifers from the Pippu Limestone. 1. Overview of the abundance of the miliolid family Ophthalmidiidae in open–lagoon facies. 2, 3.Gsollbergella spiroloculiformis(Oravecz–Scheffer, 1968) (2: GP270–1; 3: GP263). 4, 5, 7, 8.Ophthalmidium exiguum(Koehn–Zaninetti, 1969) (4: GP76A; 5: GP253; 7, 8: GP270–2).

6.Ophthalmidium exiguum? (Koehn–Zaninetti, 1969) (GP263). 9, 13.Ophthalmidiumaff.O. ubeyliense(Dager, 1978) (9: GP76C; 11–13: GP270–1). 10–12, 14.

Ophthalmidiumsp. (10, 14: GP252; 11, 12: GP270–1). 15–23.Ophthalmidium? n. sp. (15: GP270–1; 16–18, 20: GP253; 19, 21–23: GP252). 24.Paraophthalmidium carpathicum(Samuel and Borza, 1981) (GP253). 25–31.Agathammina austroalpina(Kristan–Tollman and Tollman, 1964) (25, 26: GP76B; 27–29: GP76C; 30: GP77;

31: GP76A). 32.Gsollbergella spiroloculiformis(Oravecz–Scheffer, 1968) (262A). Scale bars are 50 μm.

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(caption on next page)

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the formation of mélange deposits where the original rocks are dismantled. As a result, rocks of similar geographic origin can outcrop miles apart. The Pippu and Esashi limestone are part of the NAC, and have therefore both been incorporated into the prism during the Early Cretaceous. Nowadays, these limestone outcrop about 100 km from each other. The detailed microfacies analysis of the Pippu and Esashi limestones bring to light strong similarities between these two areas.

Indeed, the Esashi limestone is represented only by slope deposits composed of grains of facies from open–lagoon, sandbar and reef/

buildup environments, all of which are identical to or characteristic of the deposits of the Pippu limestone (i.e., framebuilders, radial–concentric ooids, calcimicrobes, benthic foraminifers, coated grains).

Combining this crucial observation with the detailed microfacies observations, we propose that the Pippu and Esashi limestone were part of the same carbonate depositional system. The Pippu outcrop is defined by facies from the top of the carbonate system (i.e., lagoon, open lagoon, sandbar, reef/buildups) whereas slope deposits are preserved only in the Esashi area. Moreover, the biostratigraphic results, based on the foraminiferal assemblages, indicate that the Pippu and Esashi limestones are of Carnian age, which confirms our hypothesis.

7.2. Depositional model and comparison with similar carbonate systems The detailed microfacies study of the Pippu and Esashi limestones highlights different depositional environments, from shallow restricted to lower slope conditions. Since carbonates from both areas are defined as part of the same carbonate system (see section 7.1), we can propose a coherent, although hypothetical, single depositional model based on the combination of all data obtained from the Esashi and Pippu areas (Fig. 10). The studied limestone is interpreted as being deposited on the flanks of a volcanic edifice, partially emerged during the Carnian (Fig. 10) and which could progressively evolve towards an atoll–type system probably as early as the Norian. Indeed, Norian Panthalassic atoll–type carbonates are well–known in Japan (Chablais et al.,2010b;

Peybernes et al., 2016a, 2020) and the Russian Far East (Peyrotty et al., 2020a). Criteria supporting our interpretations for the depositional model are widely described inPeyrotty et al. (2020a)for a Norian atoll and some of them are pertinent for the investigated carbonates. The absence of any terrigenous input and the association with basaltic and deep oceanic rocks are indeed the main evidence which corroborate the intra–oceanic interpretation (see Peyrotty et al., 2020a for details).

Similar Upper Triassic intra–oceanic systems are well described in the literature, and their mode of occurrence as well as their facies content and distribution are equivalent to those of the Esashi and Pippu limestone (Chablais et al., 2010b;Peybernes et al., 2016b;Peyrotty et al., 2020a). The closest similarities are found

in coeval carbonates of the Sambosan Accretionary Complex (SAC) on Shikoku Island, where identical biota, distinguished by the abundance of Tubiphytes, have been reported (Reef Type 1, inPeybernes et al., 2016a). As for the NAC, the SAC is defined as an accretionary complex of Late Jurassic to Early Cretaceous age, and both contain very similar intra–oceanic carbonates as well as equivalent marine rock association (Matsuoka, 1992;Onoue and Sano, 2007). We can confidently suppose that shallow–water carbonates from the NAC and the SAC had a close paleogeographic position within the Panthalassa when they formed. In modern settings, numerous atoll constellations can be

observed in the Pacific. A similar configuration in the Panthalassa during the Late Triassic is most likely. In this case, carbonates of NAC and SAC could be relics of these tropical depositional systems.

In order to establish a coherent depositional model (Fig. 10), other parameters need to be taken into consideration. The studied limestones are characterized by a few facies typical of restricted lagoon. Con- versely, open–lagoon facies are widely represented, together with small reefs/buildups, suggesting that quiet lagoonal environments did not develop. The absence of a continuous reef barrier, associated with a tidal sand bar, can also indicate that the internal part of the system was not strongly protected from marine currents, thus avoiding the formation of large internal muddy deposits. However, the absence of quiet lagoonal sediments can be also due to the morphology of the substrate on which carbonates developed. MF2 is indeed marked by the input of abundant subangular to rounded volcanic grains, indicating that the carbonate system is established on the flanks of an eroding, emerged volcanic seamount. Such an environment, which currently exists in the islands of Hawaii and La Reunion, does not permit the development of vast, quiet lagoons, as is the case in the Maldives Islands, the Tuamotu Archipelago in French Polynesia or the Kiribati Islands. Indeed, the latter are formed on flat–toped topography with a large lagoon protected by reef barrier, or on a shoal and are documented, in the Upper Triassic fossil record, by the Dalnegorsk limestone (Peyrotty et al., 2020a). Sand bars, probably interspersed with small reefs/buildups, characterize the platform margin in the studied carbonates. Slope deposits are extensively represented and are characterized by grains derived from the platform margin and debris flows. The abundance of such deposits, the absence of erosional surfaces, flames or convolutes, and the association with possible slump structures are consistent with a gentle slope governed by low–energy debris–flows. Based on our observations and interpretations, a hypothetical depositional model has been established for the Pippu and Esashi limestone, deposited during the Carnianperiod (Fig. 10).

8. Conclusions

This work reports, for the first time, on microfacies, sedimentology and biostratigraphy of the Upper Triassic shallow–water intra–oceanic carbonates from Hokkaido Island (Japan). The Pippu and Esashi areas, situated in the Naizawa Accretionary Complex, and represented by 5 limestone outcrops, were explored and extensively sampled. Seven microfacies, characterized by specific fabrics and facies–depending organisms, have been defined and interpreted in term of their depositional environments. In the Pippu area, carbonates are attributed to lagoonal, sand bar and peri–reef environments. In the Esashi region, in contrast, limestone is characteristic of an upper to lower slope setting. It is composed of different grains, originating from the inner platform, margin and slope, identical to the facies found at Pippu. The foraminiferal assemblage has been accurately defined and illustrated in both areas, allowing us to refine and confirm the stratigraphic intervals previously identified using conodonts and bryozoans. The association of foraminifers is the same in the Pippu and Esashi limestones (i.e., in the facies of Esashi we only find the foraminifers reworked from the platform). The relatively well–diversified assemblage of foraminifers, dominated by bilocular Miliolata, mostly represented by the family Ophthalmidiidae, indicates a Carnian age for both localities. It is (Zaninetti, Brönnimann, Bozorgnia and Huber, 1972) (1, 2: GP76B; 3: GP76A; 4: GP263; 5: GP252). 6. Miliolata indet. (GP252); note the pillar–type structures (arrows) in the last part of the multilocular coil. 7–9.Planiinvoluta carinata(Leischner, 1961) (7: GP76B; 8, 9: GP270–1). 10, 11.Arenovidalina chialingchiangensis(Ho, 1959) (10: GP76B; 11: GP76A). 12.Ophthalmidium? n. sp. (GP252). 13.Ophthalmidiumsp. (GP252). 14, 15.Lamelliconus multispirus(Oberhauser, 1957) (14, 15:

GP262C). 16. MicritizedAulotortussp. (GP79). 17.Variostomacf.V. turboidea(Kristan–Tollmann, 1960) (GP76B). 18.Variostomasp. (GP262A). 19.Turriglomina carnica(Dager, 1978) (GP253).20.Diplotreminasp. (GP76C). 21. Endotebidae gen. sp. indet. (GP76B). 22, 23.Endotriada kuepperi(Oberhauser, 1960) (22: GP270–2;

23: GP263). 24. Nodosariid (GP251). 25.Planiinvoluta carinata(Leischner, 1961) (GP219B). 26.Endotriadellacf.E. wirzi? (Koehn–Zaninetti, 1969) (GP218). 27.

Duostominidae (GP236). 28.Gaudryina triadica(Kristan–Tollmann, 1964) (GP241). 29.Agathammina? sp. (GP236). 30. EncrustingTolypammina gregaria(Wendt, 1969) (GP222). Scale bars are 50 μm.