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 graded deposits, locally deformed (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 (Steinmanniporella sp. or Dissocladella sp.) (Fig. 7E). In the wackestone places of MF7, sponge spicules (Fig. 7B) as well as very rare radiolarians (both recrystallized into calcite) were also observed. The foraminifers association is represented by Agathammina? sp. (Fig. 9: 29), Endotriada? sp., Duostominidae (Fig. 9: 27), and rare Nodosariids. No volcanic grains are observed in MF7.
5 – Facies interpretation
Hereafter are presented the interpretation of each microfacies in term of depositional environment. Those 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 on the Table 2.
136 5.1 – Lagoonal environment: MF1 and MF2
MF1, dominated by mud and characterized by the presence of few bivalve and echinoderm debris, as well as intraclasts of MF2, has been interpreted as 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 corresponds to minor isolated parts
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 places of MF7.
E. Steinmanniporella sp. or Dissocladella sp. Scale bars: A, B, D: 5 mm; C, E: 500 µm.
137
FaciesSamplesLocalitiesBiotic content and foraminifers assemblageOther clasts and sedimentary featuresFacies interpretation MF1 Debris-peloids wackestoneGP77, GP265, GP267Pippu area Debris of echinoderms and bivalves and rare indet. calcimicrobes / No foraminifer observed (Note that in the sample GP77,Agathammina austroalpinawas osbserved in a intraclast of MF2)
Peloids, intraclasts of MF2, rare micritized ooids / Rare burrowsProtected lagoon MF2 Peloidal-bioclastic packstone to grainstone
GP76A, GP76B, GP76C, GP79, GP251, GP252, GP253, GP254, GP261, GP262A, GP262B, GP262C, GP263, GP270-1, GP270-2
Pippu area
Echinoderm ossicles, rare ostracods and gastropods, microproblematica (Baccanella floriformis, Plexoramea cerebriformis, Radiomura cautica, debris of Tubiphytes spp. and indet. micritic tubes), Cayeuxia sp. and other indet. calcimicrobes, rare ?Soleporacean red algae and green algaeHolosporella sp. / Agathammina austroalpina, Agathammina iranica, Arenovidalina chialingchiangensis, Aulotortus sp., Diplotremina sp., Endotriada kuepperi, Gsollbergella spiroloculiformis, Ophthalmidium exiguum, Ophthalmidium exiguum?, Ophthalmidium aff. O. ubeyliense, Ophthalmidium?n. sp., Ophthalmidiumsp., Paraophthalmidium carpathicum, Planiinvoluta carinata, Turriglomina carnica, Variostoma cf. V. turboidea, Variostoma sp., Lamelliconus multispirus, Miliolata indet., Ammodiscidae, Duostominidae, Endotebidae gen. sp. indet., Nodosariid Abundant volcanic grains, peloids (fecal pellets, reworked mud grains and micritized clasts), aggregate grains, coated grains and rare micrtizied ooids / _
Open lagoon MF3 Tubiphytes-calcimicrobe rudstoneGP269, GP-270Pippu area
Tubiphytes spp., echinoderm ossicles, Uvanella sp., ?Solenoporacean red algae, microproblematica (Baccanella floriformis, Plexoramea cerbriformis, Radiomura cautica), rare serpules, bryozoan debris, microbial crust, and recrystallized frame buidlers (i.e., corals and spongiomorphids) / Ophthalmidium sp., Dustominidae Reworked mud grains, micritized grains, coated grains, aggregate grains / _
Peri-reef MF4 Oolitic-bioclastic grainstoneGP255, GP256, GP257, GP258, GP259, GP260Pippu areaEchinoderms ossicles, calcimicrobes indet., debris of bryozoans and green algae / No foraminifer observed
Spheroidal radial-concentric ooids (200μm to 2mm in diameter, single, compound, broken and regenrated), aggregate and coated grains, micritized grains and intraclats of MF2/MF3 / _
Sandbar MF5 Echinoderm-peloid packstone to rudstoneGP213, GP218, GP223, GP238, GP239, GP241Esashi area Loc. 1, Loc. 2
Large echinoderm ossicles, derbis of bivalves, bryozoans and calcimicrobes indet.; remobilized framebuilders (i.e., recristallized corals and spongiomorphids) / Endotriadellacf. E. wirzi?,Gaudryina triadica, foraminifers indet.
Peloids (reworked mud grains our micritized bioclasts), intraclasts of MF2, aggregate and coated grains, rare ooids of MF4 and locally deformed / _
Upper slope MF6 Peloid-ooid packstone to grainstone
GP219A, GP219B, GP220, GP221, GP222, GP224, GP226, GP227, GP228, GP235 Esashi area Loc. 1, Loc. 2, Loc. 4 Debris of echinoderms (locally coated) and alcimicrobes indet. / Planiinvoluta carinata, encrusting Tolypammina gregaria, foraminifers indet.
Ooids from MF4, micritized grains, coated grains and calcispheres indet. / Grains arrangement (unidirectional flow)
Upper slope MF7 Laminated mudstone to packstone
GP214, GP215, GP229A, GP229B, GP230, GP231, GP234, GP236, GP237 Esashi area Loc. 2, Loc. 3, Loc. 4 Debris of calcimicrobes indet. and green algae (Steinmanniporella sp. or Dissocladella sp.), sponge spicules and rare radiolarians / Agathammina? sp., Endotriada? sp., Duostominidae, Nodosariid, foraminifers indet.
Microbial peloids, micritized grains and ooids from MF4 (locally micritized) / Laminations (locally deformed), grading and stromatactis
Middle to lower slope
Table 2 – Microfacies content and environmental interpretations.
138 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 mud or cement–supported facies, which is characteristic of environments defined by variable hydrodynamics. The presence of green algae indicates a 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 marker of the link with open environments such as oolitic shoals. The foraminifers association dominated by Miliolids and in MF2 particularly by the family Ophthalmidiidae, is also indicative of open–lagoon, hypersaline environments (Chablais et al., 2010a,b, 2011; Decarlis et al., 2013; Gaetani et al., 2013; Gale, 2012; Gale et al., 2012, 2014;
Gazdzicki, 1983; Haas et al., 2010; Igo & Adachi, 1990; Kamoun et al., 1994; Mancinelli et al., 2005; Michalik et al., 1993; Mircescu et al., 2019; Onoue et al., 2009; Parente & Climaco, 1999). Note that most of the clasts are micritized or coated by a micritic rim, which suggests a strong algal or fungi activity in shallow–water environments with low sedimentation rates (Kendall & Alsharhan, 2011; Reid & Macintyre, 2012; Swinchatt, 1969). Similar microfacies are widely described from synchronous carbonate platforms from Panthalassa (Chablais et al., 2010b; Kiessling & Flügel, 2000; Peybernes et al., 2016b; Peyrotty et al., 2020a), supporting our interpretation. MF2 also contains numerous volcanic grains indicating a setting close to a volcanic edifice in erosion.
5.2 – Reef and peri–reef environments: MF3
In both Esashi and Pippu areas, no proper reefal facies have been found, but some evidences indicate that small reefs/bioherms probably developed in places in the carbonates.
Indeed, MF3 display 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.
These bioclasts occur mainly as cement–supported, reworked or broken grains, and no reefal framework is present. We can therefore be assumed that MF3 is a peri–reefal facies, deposited in an open environment and dominated by reef/bioherm debris with high abundance of calcimicrobes, typical of such Upper Triassic environments (Chablais et al., 2010b; Kiessling
& Flügel, 2000; Peybernes et al., 2016b; Peyrotty et al., 2020a). The existence 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 the presence of sessile or encrusting foraminifers, typical of reef/back–reef environments (i.e.,
139 Planiinvoluta carinata, Tolypammina gregaria) (Berra & Cirilli, 1997; Chablais et al., 2010a;
Michalik & Jendrejakova, 1978; Peybernes et al., 2015; Roniewicz et al., 2007; Russo, 2007), were observed in slope and open–lagoon settings (see Table 2 and sections 5.1 and 5.4), thus confirming the presence of reefs/bioherms, partially dismantled and transported in the surrounding parts of the platform. Similar biotic associations were precisely described in analogous system from the Tethyan and Panthalassa domains (Martindale et al., 2010, 2012, 2015; Peybernes et al., 2015, 2016b; Reid & Ginsburg, 1986; Russo et al., 1997). However, the general low preservation of our facies, as well as the scarcity of outcrops, do 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 as 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, 1997). Such facies are widely reported from modern and fossils systems (Esrafili–Dizaji & Rahimpour–Bonab, 2014;
Friedman, 1995; Lokier and Fiorini, 2016; Peyrotty et al., 2020a; Rankey & Reeder, 2009, 2011; Qiao et al., 2016). However, according to Flügel (2004), radial–concentric ooid shapes are typical of moderate water energy in an intermittently agitated setting with normal marine salinity. MF4 might consequently corresponds to a sand bar controlled by tidal currents on the edge of the depositional system. MF4 exhibit 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 points out that 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 & Alsharhan, 2011;
Reid & 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, enhance 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.,
140 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 of MF2 to MF4, including echinoderms, ooids, calcimicrobes, peloids, coated grains, 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 foraminifers 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, Endotriadella cf. E. wirzi
?, see Table 2) (Berra & Cirilli, 1997; Chablais et al., 2010a; Chablais et al., 2011; Lakew, 1990; Michalik & Jendrejakova, 1978; Mircescu et al., 2019; Peybernes et al., 2015; Roniewicz et al., 2007; Russo, 2007) supporting the near location 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 upper slope environment (see large and non–coated specimens, Fig. 6B) (Chablais et al., 2010b; Gale et al., 2014; Peybernes et al., 2016b; Preto, 2012; Reijmer et al., 1991). MF6 is characterized by MF2 and MF4–derived grains, which suggest that MF6 is set up on the upper part of the slope, below open–lagoon and sandbar environments. However, we note that MF6 has been locally observed 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 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, locally graded, and are interpreted as debris–flows occurring on middle to lower slope settings.
MF7 sediments are defined as micrite associated to microbial activity (i.e., microbial peloids and stromatactis) consistent with quiet environments below waves actions. 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) (see Table 2 and 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
141 assumed to be linked to slope movement during the deposition or very early, before the 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 lagoon (Flügel, 2004; Peyrotty et al., 2020a). Note that similar slope deposits were precisely described in synchronous systems from the Panthalassa Ocean (Chablais et al., 2010b; Peybernes et al., 2016b).
6 – Biostratigraphy
Pippu and Esashi carbonates were previously studied in the 70’s from a biostratigraphic point of view. Igo et al. (1974) reported Upper Triassic (Carnian) Conodont (i.e., Paragondolella polygnathiformis) from the Pippu limestone no more precise description or pictures were given for this species. On the other hand, bryozoans of Ladinian–Carnian affinity were identified in the Esashi limestone (Ishizaki, 1979). This author described two species: (1) Psezadobatostome kobayashii, first reported by Sakagami (1972) from a lower Carnian horizon of the Upper Triassic Kochigatani Formation in Shikoku Island (Japan), (2) Dyscritella hidakensis, first described from a limestone block in the Kerimai River to the north of Motourakawa Region (Hokkaido, NAC) by Sakagami & Sakai (1979). Note that both species were only observed in Japan (Schafer, 1994) and their stratigraphic extension rely on conodonts and bivalve identifications (Sakagami & Sakai, 1979; Sakagami, 1972; Hu, 1984). Only few debris of bryozoans were found in our facies and 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 does not permit precise identification for biostratigraphy.
However, a well–diversified and preserved foraminifers assemblage was found in the Pippu limestone. Foraminifers belonging to the same association are also present, even rare, in the Esashi limestone, where they were resedimented after being exported from the platform. The foraminifers allow us to refine and precise the stratigraphic extension of the Pippu and Esashi limestone, respectively. Over the past ten years, Upper Triassic benthic foraminifers from the Panthalassa Ocean were precisely identified and their paleobiogeographic significance is of high importance for the characterization of this huge ocean (Chablais et al., 2010b,c; Peybernes et al., 2015, 2016a,b; Peyrotty et al., 2020a; Rigaud et al., 2010, 2012, 2013a,b, 2015a,b, 2016;
Rigaud & Martini, 2016). So far, no studies about Triassic foraminifers were conducted on the mid–oceanic carbonates from Hokkaido and the presented data are therefore essential for the
142 understanding of life colonization and evolution across oceans during the Late Triassic. The foraminiferal assemblage of Pippu and Esashi limestone and their related stratigraphic extension are presented hereafter.
6.1 – Foraminifer association of the Pippu limestone
The Pippu limestone is characterized by a diversified, biostratigraphicaly significant foraminiferal content (Figs. 8, 9 and Table 2 for 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 to Ophtalmidium exiguum, characterized by a small tests with thin wall, and Ophtalmidium aff.
O. ubeyliense, showing a flattened test and a regular and evolute planispiral coiling. A third population, named here Ophtalmidium? n. sp. was not known until now in the Triassic. These are forms, robust in axial section (multichambered?), displaying an early streptospiral stage followed by a sigmoidal coiling with four number of whorls. It 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, genus Hemigordius). Igo & Adachi (1990) made this same reflection concerning the ophthalmidid foraminifers of the San Juan Island, which resembles 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. In the Triassic Miliolata, pillars are recorded in the, thus far, only known species of the porcelaneous Middle Triassic Chinese genus Paratriasina He, 1980, type–species P.
jiangyouensis He, 1980. The genus Partriasina is considered by Zaninetti 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 of Paratriasina is 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 Agathammina austroalpina, Agathammina iranica, Arenovidalina chialingchiangensis, Planiinvoluta carinata and Turriglomina carnica. Miliolids are associated to other, much less represented forms in the
143 Pippu limestone, such as Lamelliconus mutlispirus, Aulotortus sp., Variostoma cf. V. turboidea, Variostoma sp., Endotriada kuepperi and a new, unknown Endotebidae gen. sp. indet.
Representative 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 Aulotortidae are abundant and well diversified in the Upper Triassic lagoonal limestone of the Sambosan Accretionary Complex (Kyushu and Shikoku islands; (Chablais et al., 2010b,c, 2011; Peybernes et al., 2016b). The foraminifers 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, not typical species of Norian–Rhaetian (e.g., Aulosina oberhauseri, Triasina hantkeni, Lamelliconus semivacuus, Frentzenella frentzeni, Kristantollmanna truncata, Wallowaconus oregonensis) were present in our samples. Igo (1979) dated the Pippu limestone as Carnian on the basis of Conodont and the stratigraphic extension obtained with the foraminifers assemblage therefore fully confirms this age.
6.2 – Foraminifer association of the Esashi limestone
In comparison with the Pippu limestone, the Esashi one does not present a rich foraminiferal content. Esashi facies were indeed interpreted as slope deposits (see section 5.4) and therefore do not constitute on 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) and are Agathammina austroalpina, Agathammina? sp., Gaudryina triadica, Planiinvoluta carinata,Endotriadella cf. E. wirzi?, Endotriada? sp., encrusting Tolypammina gregaria.
Duostominidae and Nodosariids also occur. These species, considered as a whole, are indicative of a Late Triassic age, which is consistent with the Carnian age given by foraminifers for the Pippu limestone. Note that Ishizaki (1979) identified bryozoans with Ladinian–Carnian affinity from the Esashi limestone. With the above, we consider the foraminifers of Esashi limestone to be part of the same foraminifers association found in Pippu limestone. Moreover, some species are present in both localities and typical Norian–Rhaetian forms are absents. Accordingly, a Carnian age for the Esashi limestone is proposed.
144
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:
GP–270–1; 3: GP–263). 4, 5, 7, 8. Ophthalmidium exiguum Koehn–Zaninetti, 1969 (4: GP–76–A; 5: GP–
253; 7, 8: GP–270–2). 6. Ophthalmidium exiguum? Koehn–Zaninetti, 1969 (GP–263). 9, 13. Ophthalmidium aff. O. ubeyliense Dager, 1978 (9: GP–76–C; 11–13: GP–270–1). 10–12, 14. Ophthalmidium sp. (10, 14:
GP–252; 11, 12: GP–270–1). 15–23. Ophthalmidium? n. sp. (15: GP–270–1; 16–18, 20: GP–253; 19, 21–23:
GP–252). 24. Paraophthalmidium carpathicum Samuel & Borza, 1981 (GP–253). 25–31. Agathammina austroalpina Kristan–Tollman & Tollman, 1964 (25, 26: GP–76–B; 27–29: GP–76–C; 30: GP–77; 31: GP–
76–A). 32. Gsollbergella spiroloculiformis (Oravecz–Scheffer, 1968) (GP–262–A). Scale bars are 50 µm.
145
146
Fig. 9 – Pictures 1 to 24 represent foraminifers from the Pippu limestone. Pictures 25 to 30 are foraminifers from the Esashi limestone. 1–5. Agathammina iranica Zaninetti, Brönnimann, Bozorgnia & Huber, 1972 (1, 2: GP–76–B; 3: GP–76–A; 4: GP–263; 5: GP–252). 6. Miliolata indet. (GP–252); note the pillar–type
Fig. 9 – Pictures 1 to 24 represent foraminifers from the Pippu limestone. Pictures 25 to 30 are foraminifers from the Esashi limestone. 1–5. Agathammina iranica Zaninetti, Brönnimann, Bozorgnia & Huber, 1972 (1, 2: GP–76–B; 3: GP–76–A; 4: GP–263; 5: GP–252). 6. Miliolata indet. (GP–252); note the pillar–type