As presented in section 2 of this chapter, all the investigated outcrops are part of a Lower Cretaceous accretionary complex (the NAC). As proposed by Igo et al. (1974) and Ishizaki (1979), those carbonates are of Middle/Upper Triassic age and are therefore defined as exotic systems coming from the Panthalassa Ocean and incorporated within the East Asia margin.
During the accretion, the 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 the section 3 of this chapter, 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 rely only on the characteristic microfacies of all outcrops. The peculiar mode of occurrence of allochthonous limestone in accretionary complexes, and the related work
130 approach for sedimentological studies, are detailed in Peyrotty et al. (2020a). Hereafter are presented the microfacies and biotic content from the Pippu and Esashi limestone. The observations were made from 77 thin sections (2.3 x 4 cm) under an optical microscope (Zeiss Axioskop).
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 echinoderms and bivalves, locally coated. Rare calcimicrobes as well as micritized ooids can also occur. No foraminifers were observed in MF1. Note that this microfacies is very poorly found among all outcrops.
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)}, Cayeuxia sp. and other indetermined calcimicrobes together with rare ostracods and gastropods, ?Soleporacean red algae (Fig. 4G) and green algae (Holosporella sp.) (Fig.
4H). In places, microproblematica Tubiphytes spp. debris, aggregate and coated grains and rare micritized ooids also occur. The foraminifers association (see Table 2 for the list of identified forms) is dominated by the miliolid family Ophthalmidiidae which occur in abundance in MF2 and represented by three identified species: Ophtalmidium exiguum (Fig.8: 4 to 8), Ophtalmidium aff. O. ubeyliense (Fig. 8: 9, 13), Ophthalmidium? n. sp. (Fig. 8: 15 to 23), Ophtalmidium sp. (Fig. 8: 10 to 12, 14) and Gsollbergella spiroloculiformis Fig. 8: 2, 3, 32).
Note that the association is characterised by a remarkable diversity of other Miliolids, such as Agathammina austroalpina (Fig. 8: 25 to 31), Agathammina iranica (Fig. 9: 1 to 5), Arenovidalina chialingchiangensis (Fig. 9: 10, 11), Planiinvoluta carinata (Fig. 9: 7 to 9), Turriglomina carnica (Fig. 9: 19), Paraophtalmidium carpathicum (Fig. 8: 24), in association with Robertinids {Variostoma cf. V. turboidea (Fig. 9: 17), Diplotremina sp.(Fig. 9: 20)}, Endothyrids {Endotriada kuepperi (Fig. 9: 23), Endotebidae gen. sp. indet. (Fig. 9: 21)} and
131 rare Involutinids {Lamelliconus multispirus (Fig. 9: 14, 15), Aulotortus sp.(Fig. 9: 16)}. 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 by Tubiphytes spp., locally on the form of debris (Fig. 5C), and associated with large echinoderms 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 cerbriformis, Radiomura cautica, Baccanella floriformis) and the sponge Uvanella sp. (Fig.
5B). Serpulids and bryozoan debris are rare. Microbial crusts (Fig. 5D) were observed on 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 rare Ophtalmidium sp. and Duostominidae. No volcanic grains are incorporated within the MF3.
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, single or compound, 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). Marshall & Davies (1975) defined this particular radial organization as a secondary radial structure due to diagenetic overprint of the original fibers. A large part of the ooids are broken and regenerated (Fig. 5G). The core of ooids is characterized by micritized grains, echinoderms, broken ooids, or sparite identified as recrystallized bioclasts. Others components of MF4 are aggregate 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.
132
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 pointed by the yellow arrows). C. Plexoramea cerebriformis in the MF2. D. Abundant volcanic grains (yellow arrows) in the MF2. Note that peloids are mostly micritized foraminifers. E, F. Baccanella floriformis in the MF2. G. ?Solenoporacean red algae in the MF2. H. Holosporella sp. in the MF2. Scale bars: A, B: 2,5 mm; C, E : 200 µm; D, G, H: 500 µm.
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 together with various clasts representative of MF2 to MF4, such as aggregate and coated grains, intraclasts of MF2 and rare radial–concentric ooids of single, compound or regenerated types. Note that ooids are similar
133
Fig. 5 – MF3 and MF4. A. MF3, Tubiphytes–calcimicrobe rudstone. Yellow arrows indicate Tubiphytes spp.
debris. Note the large echinoderm ossicle (bottom right) associated to poorly–preserved calcimicrobes (white arrows) and bryozoan debris (red arrow). B. Uvanella sp. in the MF3. C. Abundant debris of Tubiphytes spp.
in the MF3. D. Laminated microbial crust on recrystallized framebuilder in the MF3. E. MF4, ooid–bioclast grainstone. Note the presence of reworked debris of Tubiphytes spp. (yellow arrows). F. Large ooid with a micritic core. Note the concentric sparitic and micritic layers as well as radial fibrous structure. G. Broken and regenerated radial fibrous ooid. Scale bars: A, E: 2,5 mm; B, F, G: 500 µm; C, D: 1 mm.
134
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 pointed by yellow arrows) in stylolithic contact with recrystallized radiolarian mudstone (lower part of the picture). Scale bars: A, B, E, F: 2,5 mm; C: 500 µm; D: 200 µm.
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 (Fig. 9: 26, 28 respectively).
135 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, grain arrangement is linked to deposition under unidirectional flow (Fig. 6E). Debris of undetermined calcimicrobes and calcispheres occur, together with foraminifers Planiinvoluta carinata, and the encrusting Tolypammina gregaria (Fig. 9: 25, 30 respectively). Note that MF6 has been observed in stylolithic contact with a radiolarian packstone, strongly recrystallized (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 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.