5 – Material and methods - Sedimentology, biostratigraphy and diagenesis of Upper Triassic shal

For this study, 150 thin sections (2.3 × 3.5 cm) from the four mentioned localities have been carefully observed using high resolution film scanner (Nikon CoolScan 4000 ED) and optical microscope. 38 thin sections from surrounding outcrops have been also observed but their bad preservation and sporadic occurrence preclude in depth analyses. A subset of 39 samples were selected for the preparation of uncovered and polished thin section for cathodoluminescence (CL) observations which were completed using an optical cold cathodoluminescence system (model Cathodyne built by NewTec Scientific) mounted on an

162

Fig. 3 – a Outcrop view of Loc. 1 and Loc. 2. b Contact between the limestone slab under the castle (Loc. 1) and the volcaniclastic unit (Loc. 2). c ‘ Sheared ’ limestone blocks in the transition zone between the slab and the volcaniclastic unit. d Limestone blocks and clasts in the volcaniclastic matrix. e Close up view of the contact zone.

163 Olympus BX41 microscope with beam conditions of 8–12 kV at 180–200 µA.. Samples were first described according to Dunham (1962) supplemented by Embry & Klovan (1971) and then attributed to microfacies (MF1–8) and depositional environments according to Peybernes et al.

(2016b). These authors described 8 microfacies (MF) from the Sambosan limestone in Shikoku and proposed a depositional model, which is used in this paper. Microfacies content and interpretations are summarized in Table 2.

Fig. 4 – Outcrop view of Loc. 3 and Loc. 4. a Massive limestone slab in the main part of the quarry, arrow indicates man for scale (Loc. 3). b Limestone block in siliceous mudstone matrix at the top of the quarry (Loc. 4). c Limestone blocks and clasts embedded in VCM in an adjacent smaller quarry (Loc. 4).

164

Fig. 5 – a Schematic columnar sections of the studied localities. b Interpretation based on the mode of occurence (MO), microfacies (MF) and biostratigraphic data, see discussion in the text.

165

6 – Results

6.1 – Microfacies

The massive limestone slab under the castle (Loc. 1) is composed of reef and peri–reef limestone (MF4 and MF3 respectively), that is locally dolomitized. Reef primary framework is mainly built by sphinctozoan, inozoan and chaetetid sponges (Fig. 6a). Stanley & Onoue (2015) reported corals from this outcrop but they are not common in our samples. Secondary framebuilders are microencrusters (mainly Radiomura cautica, Microtubus communis and Actinotubella gusici), associated with relatively thin, light microbial crusts. Interstitial reef sediments are peloid–ooid–bioclast grainstone to packstone. Peloidal and clotted micrite can also be present but is relatively rare. Additionally, reworked reef pieces, as well as other elements derived from the platform margin, also occur in bioclast‑intraclast rudstone‑floatstone (MF3) that characterized fore reef deposits (Fig. 6e). Near the contact between limestone slab and the volcaniclastic unit (Fig. 3e), reef facies appears more diversified. Some samples are coral boundstone (Fig. 6b) while others display thick microbial crust succession with numerous encrusting organisms and microproblematica such as

Microfacies type Sedimentary grains Facies interpretation

MF1: Radiolarian–filament

166

MF6: Peloidal–echinoderm packstone–grainstone

Major : peloids (including

microcoprolites), echinoderms, coated grains, aggregates

Subordinate : Ooids, oncoids, calcimicrobes, intaclasts, bioclasts, foraminifers

Open lagoon, shoal and sand bar

MF7: Involutinid wackestone–

packstone

Major : involutinids, locally megalodont shells

Subordinate : gastropods, dasycladacean, echinoderms, peloids, microcoprolites, bioclasts

Open lagoon

MF8:Bioclast–microcoprolite wackestone–mudstone

Major : bioclasts, microcoprolites Subordinate : peloids

Restricted lagoon

Table 2 – Microfacies characteristics (modified after Peybernes et al. 2016b).

167

Fig. 6 – Microfacies of the massive limestone slab under the castle (Loc. 1, Upper Carnian–

Norian/Rhaetian?) a Reef limestone (MF4, Reef Type 2) with recrystallised sponges (S), thin microbial crusts (mc) and grainstone interstitial sediment (g) ; sample CP–7. b Coral boundstone (MF4, Reef Type 2) with recrystallised corallites (Co) and encrusting organisms (arrows) ; sample CP212. c Microproblematica boundstone (MF4, Reef Type 2) Radiomura cautica (Rc) Baccanella floriformis (Bf) and Celyphia–like (Ce) organisms embedded in light–colored microbial crusts ; sample CP–214. d Thick light–colored microbial crust (MF4, Reef Type 2) ; sample CP–214. e Intraclast bioclast rudstone (MF3) note the dissolution voids (V) infilled by micrite ; sample GP–5.

Radiomura cautica, Baccanella floriformis and Celyphia–like organisms (Fig. 6c, d). Even if these samples come from blocks that are not directly connected with the main cliff, their microfacies are strongly similar to the massive limestone unit, thus these blocks are attributed to Loc. 1. All reef facies found at Loc. 1 corresponds to Reef Type 2 in Peybernes et al. (2016a).

Loc. 2 is characterized by the occurrence of dark grey limestone clasts and blocks set in VCM. Limestone clasts from this unit are usually well preserved and display diversified microfacies. The most interesting one is a Tubiphytes–sponge–coral boundstone (MF4, Fig. 7) that have been found in particular on the right side of the outcrop. Abundant sponges, Tubiphytes sp. and Tubiphytes–like organisms (i.e., Isnella misiki, Plexoramea cerebriformis and Plexoramea gracilis) constructed a reef framework that defines voids and cavities filled with thick isopachous and blocky cements (Fig. 7b–c, e). Corals can also be present as primary framebuilders but are less common. Numerous microproblematica and small peloids occupy

Fig. 7 – Microfacies of limestone clasts under the castle (Loc. 2) a–e Sponge–Tubiphytes boundstone (MF4, Reef Type 1, Ladinian? –Lower Carnian). Note the presence of abundant Tubiphytes sp. (arrows), Uvanella irregularis (Ui), the inozoan ?Peronidella sp. (P) and other undetermined sponges (S), Bryozoan (B) and voids filled with isopachous and blocky cements (V), note the fracture (red line) ); a sample CP–263B; b sample CP–265B; c sample CP–265F; d sample CP–20B; e sample CP–265C.

168 the interstitial sediment along with microbialite. Biota from this microfacies has been described by Peybernes et al. (2015). In addition to previously reported taxa, we observed Uvanella irregularis, ?Peronidella and an undetermined bryozoan. This microfacies corresponds to Reef Type 1 in Peybernes et al. (2016a). Another type of boundstone has also been found at the same outcrop. This microfacies (attributed to MF4) is here called Reef Type 3 and is described in detail for the first time at Mt Sambosan. It is composed of hypercalcified sponges that are encrusted by algae, numerous microproblematica and thick dark microbial crusts (Fig. 8).

Interstitial sediment between this reef framework consists of peloidal and clotted micrite, unattached microproblematica, small peloids and sometimes the dasycladacean algae Griphoporella curvata (Fig. 8b). Isopachous and blocky cements are locally abundant. One sample also displays thrombolitic columns that are more than 3cm high (Fig. 8e). Bioclast–

intraclast rudstone–floastone (MF3) is also present in the same VCM breccia unit (Fig. 9a–b).

Intraclasts are mostly fragments of the sponge–algae boundstone (Reef Type 3). Most bioclasts are sponges, corals and algae fragments, echinoderms and bivalves shells. Additional grains are calcimicrobs and peloids. The dark micritic matrix contains abundant ostracods and tiny bioclasts. In an especially well preserved sample (CP–28) a peculiar foraminifer assemblage (Fig. 10) has been found in this dark micritic matrix. The association is composed of small spirillinids (?Tethysiella pilleri, Paalzowella sp., Turrispirillina sp., Spirillina sp.) and small involutinids (Coronipora etrusca and other undetermined forms), associated with Lenticulina sp. and undetermined porcellaneous foraminifers. Additionally, peloid–echinoderm grainstones

169

Fig. 8 –Microfacies of limestone clasts under the castle (Loc. 2). a–c Sponge–algae boundstone (MF4, Reef Type 3, Norian–Rhaetian). Note the presence of dasycladacean Griphoporella curvata (D), Solenoporacean (So), various encrusting algae (ea) including Norithamnium madoniensis, sponges (S) and thick dark colored microbial crusts (mc) ; a sample JC–325E ; b sample CP–261 ; c sample CP–259B. d–e Microfacies associated with sponge–algae boundstone (Reef Type 3). d Dasycladacean Griphoporella minuta (D) in grainstone facies ; sample CP–259A. e Thrombolithic column composed of clotted micrite, note the inverse geopetal structure between the two columns (arrow) ; sample CP–266B.

(MF6) have been found in two limestone clasts at Loc. 2. The first clast (CP–29) is a well sorted grainstone containing abundant peloids and echinoderms (mainly crinoids) and interestingly displays ooids with basaltic grains acting as nucleus (Fig. 9c). Porosity is primarily filled by thin isopachous cements and then by syntaxial and blocky cements. The second clast (CP–17) is more heterogeneous (Fig. 9d) and contains numerous microcoprolites that are attributed to Parafavreina thoronetensis (Peybernes et al., 2016b). Isopachous cements are still visible but suffered some dissolution (see section 6.2).

170

Fig. 9 – a–b Intraclast–bioclast floatstone (MF3). Note the presence of reef derived intraclasts (i) and recrystallised bioclasts (B) embedded in ostracod–rich dark depositional micrite (m) ; a sample CP–264 ; b sample CP–16. c–d peloid–echinoderm grainstone (MF6). c Grainstone rich in ooids with basaltic nucleus (arrows) ; sample CP–29. d Grainstone with microcoprolite Parafavreina thoronetensis (arrows) ; sample CP–17.

Further north, quarries carved in the eastern flank of Mt Sambosan exhibit massive limestone slab surrounded by breccia units and highly deformed siliceous mudstone. Sampling along the limestone slab in the main quarry (Loc. 3) shows that this unit is relatively homogenous. Most samples are bioclastic packstone to floatstone that can be referred to fore reef deposits (MF3, Fig. 11a). Additionally, some zones are bioconstructed and contain in particular in situ hypercalcified sponges (Fig. 11b). Remarkably, Thaumatoporella parvovesiculifera, which is reported for the first time from Mt Sambosan, also occurs in association with chaetetid sponges (Fig. 11c).

The massive limestone slab in the quarry is surrounded by several outcrops of breccia (grouped in Loc. 4) that yield limestone clasts and blocks (MO2 and MO3). These limestone clasts show a high microfacies diversity and record every platform top depositional environments. Reefal facies is composed of sponges and encrusting organisms (MF4 and MF3, Fig. 12a).

Surprisingly, microbial crusts are absent and this is unusual in the Sambosan reef limestone.

Microtubus communis is present in the interstitial sediment between framebuilders (Fig. 12f).

Breccia at Loc. 4 also contains oncoid–calcimicrob grainstone (MF5, Fig. 12b) and peloid–

ooid–aggregate grainstone (MF6, Fig. 12c) that are interpreted as back reef and open lagoon sediments. The occurrence of involutinid wackestone (MF7, Fig. 12d) and microcoprolite–

bioclast wackestone–mudstone (MF8, Fig. 12e) also demonstrates the presence of more restricted, low energy environment in the platform interior. It is important to note that these lagoonal microfacies contains the involutinid foraminifer Triasina hantkeni (Fig. 12g), marker of the Late Norian (Sevatian) to Rhaetian, and the microcoprolite Payandea japonica and Parafavreina sp. (Senowbari–Daryan et al., 2010a).

Carbonate units at Mt Sambosan mainly record platform top and upper slope deposits.

On the other hand, deep water limestone and lower slope carbonate sediments are relatively rare. However, deep water radiolarian–filament mudstone–wackestone (MF1) is reported from several small outcrops along the road (Peybernes et al., 2016b). MF2, which corresponds to an alternation of radiolarian mudstone and bioclast–echinoderm–peloidal packstone–grainstone deposited on the slope (Peybernes et al., 2016b) has not been found at Mt Sambosan.

171

Fig. 10 – Foraminifer association from the ostracod–rich dark micritic matrix ; sample CP–28. a–

b ?Tethysiella pilleri. c–d Paalzowella sp. e–f Turrispirillina sp. g–j Spirillina sp. k–l Coronipora etrusca.

Fig. 11 – Microfacies of the limestone slab in the quarry (Loc. 3). a Bioclast floastone (MF3) with echinoderms gastropods and other shell fragments ; sample CP–274A. b Sponge boundstone (MF4, Reef Type 2) ; sample 274A. c Thaumatoporella parvovesiculifera (arrows) and Chaetitids (C) ; sample CP–275.

172 6.2 – Diagenesis

In the SAC, limestone is usually strongly recrystallized and fractured (Chablais et al., 2010a; Peybernes et al., 2016a,b) and early diagenetic features are often destroyed by burial and accretion processes. However, at Mt Sambosan, the relatively good limestone preservation allows to distinguish several diagenetic events, especially at Loc. 1 and Loc. 2.

Fig. 12 – Microfacies of the limestone clasts in the quarry (Loc. 4). a Bioclast floastone/boundstone (MF3?

MF4?) embedded in siliceous mudstone (sm), note the presence Chaetetid sponges (Ch) ; sample CP–269. b Calcimicrob–agggregate grainstone–rudstone (MF5), typical grains are calcimicrob (Ca) aggregate (A) ; sample CP–272B. c Ooid–peloid grainstone (MF6) ; sample JC–346E. d Involutinid wackestone (MF 7) containing Triasina hantkeni (arrow) ; sample CP–270B. e Microcoprolite wackestone (MF8), note the partially recrystallised microcoprolite (arrow) ; sample CP–273. f Microtubus communis (arrow) CP–269. g Close up view of Triasina hantkeni ) ; sample CP–270B. h involutinid wackestone intraclast in Calcimicrob–

agggregate grainstone–rudstone (MF5) ; sample JC–346E.

At Loc. 1 the central part of the outcrop is strongly dolomitized. Detailed sampling shows several dolomitization degrees (Fig. 13a–f). In less dolomitized samples, dolomite crystals are clearly situated along fractures (however, not all fractures display dolomite

173 crystals). In more dolomitized samples, fabric–destructive dolomite gradually obliterates other sedimentary features. CL reveals that scattered, highly luminescent rhombohedral dolomite crystals are unimodal and mainly euhedral (planar–e to planar–s). Highly dolomitized samples exhibit brown subhedral and light euhedral crystals with darker and less luminescent core.

Lastly, highly luminescent calcitic veins cut again the limestone fabric. At Loc. 3 some isolated dolomite crystals also occurs locally.

In the Tubiphytes–sponge boundstone (Reef Type 1) at Loc. 2, diagenetic silicification is frequent, especially in the central canal of Tubiphytes sp. (Fig. 13 g, h). CL observations show that cavities in this microfacies are partially filled with a succession of highly luminescent fibrous isopachous cement, followed by 2 stages of darker dogtooth cements (Fig. 13i).

Remaining space is filled with highly luminescent blocky cement.

At Loc. 2, sponge–algae boundstone (Reef Type 3), bioclast–intraclast rudstone–

floastone (MF3), and P. thorenetensis–rich grainstone (MF6) exhibit an identical diagenetic phase, revealed by CL (Fig. 13j–l). In these microfacies, bioclasts have been dissolved and the resulting moldic porosity was then filled by a rim of dogtooth cement that appears dark in CL, followed by highly luminescent blocky calcite. This diagenetic sequence suggests that these samples were all affected by a coeval dissolution event. Sponge–algae boundstone (Reef Type 3) and some intraclasts in MF3 also display large voids that are usually filled with the same sequence of dogtooth cements, crystal silt and blocky calcite (Fig. 14a). These cavities most probably result from an important event of dissolution, synchronous to the moldic porosity described above. Moreover, when these cavities are connected with the surrounding VCM, they are outlined by dogtooth cement and then filled with a mixture of carbonate lithoclasts, crystal silt (cement fragments), micrite and volcaniclastic material (Fig 14b). Furthermore, some intraclasts that occur in MF3 suffered a karstification/dissolution episode as demonstrated by their very irregular surface (e.g., CP–16, CP–28 and CP–260). These dissolution features are infilled by ostracod–rich dark micritic matrix containing the foraminifer association described above (Fig. 14c–e). When a remaining space is present, it was subsequently filled with dogtooth cements, crystal silts and blocky calcite (Fig. 14d–e), which is similar to the void–filling sequence described above.

In summary, the different microfacies recognized at Mt Sambosan record several early to late diagenetic phases. The paragenetic sequence of these events is compiled in Fig. 15. It is to be noticed that the different diagenetic events are not directly linked to the mode of occurrence (MO1–3), as different diagenetic pathways can be observed in the same locality (in Loc. 2 for example).

174 6.3 – Biotic content

Fossil assemblages from the Sambosan limestone are composed of corals (Okuda et al., 2005; Stanley & Onoue, 2015), sponges (Okuda et al., 2005; Chablais, 2010; Peybernes et al., 2015, 2016a,b), foraminifers (Chablais et al., 2011; Peybernes et al., 2015, 2016a,b), microproblematica (Peybernes et al., 2015, 2016a,b), microcoprolites (Senowbari Daryan et al.

2010a, Peybernes et al. 2016b), polychaetes worm tubes (Peybernes et al., 2015, 2016a)

Fig. 13 – Diagenetic features. a–f Dolomitisation of the limestone slab at Loc. 1. a–c Polished hand sample showing different dolomitisation degree. d–f Corresponding cathodoluminescence view. a, d sample CP–8;

b, e sample CP–9; c, f sample CP–12. g–h Silicification of the central canal of Tubiphytes sp. in transmitted light and CL respectively ; sample CP–20–B. i Multiple isopachous cement precipitation filling the cavities in Tubiphytes–sponge boundstone ; sample CP–24–B. j–l Similar dissolution/recrystallisation event in biomolds from different microfacies, note the dark dogtooth cement followed by highly luminescent blocky calcite. j MF4 reef type 3 ; sample CP–261. k MF6 ; sample CP–17. l MF3 ; sample CP–30–B.

175 associated with algae, molluscs, brachiopods, bryozoans and calcimicrobs (Chablais, 2010;

Peybernes et al., 2016b). Conodonts have also been retrieved from several outcrops at and near

176

Fig. 14 – Diagenetic features. a–b Voids filled with dogtooth cement (dc), crystal silt calcite showing geopetal structure (arrows), blocky cement (bc), and limestone–VCM infill (L–VCM) ; a sample CP–261 ; b sample CP–15. c Intraclast–bioclast floastone (MF3) clast in VCM. Intraclasts display dissolution surfaces (red line) showing wavy boundary between reef derived intraclast (1) and ostracod–rich dark micritic matrix (2). d–e Karstic features in sponge–algae boudstone (MF4, Reef Type 3), cavities contain ostracod–rich micrite (2, yellow line), remaining space is filled by crystal silts and blocky cements (3, blue line), note the multiple infilling episodes with different geopetal orientations (arrows). f Table summarizing the diagenetic history of sponge–algae boundstone (MF4, Reef Type 3), Parafavreina thoronetensis–rich grainstone (MF6) and some bioclast–intraclast rudstone–floatstone (MF3) during the Rhaetien–Early Jurassic.

Mt Sambosan (Yamato Omine Research Group, 1981; Peybernes et al., 2016a). All taxa reported from Mt Sambosan limestone are compiled in Table 3. The taxonomic richness (up to 68 identified taxa) of the Sambosan limestone highlights the biotic diversity of mid oceanic shallow water ecosystems of the Panthalassa Ocean during the Late Triassic.

Fig. 15 – Paragenetic sequences of the different microfacies.

7 – Discussion

The Sambosan limestone and associated lithologies records the geological history of shallow water carbonate platform capping mid oceanic seamount from the Ladinian?–Early

177 Carnian to the Late Jurassic–Early Cretaceous (Fig. 16). Biotic assemblages indicate that the seamount(s) were situated in tropical Panthalassa during the Early Mesozoic (Onoue and Stanley, 2008; Chablais et al., 2010b; Peybernes et al., 2016a). Even if the original carbonate succession is destroyed (Fig. 5b), the seamount evolution can be reconstruct and subdivided into different stages reflecting the onset, growth, demise and collapse of the platform that covered the volcanic edifice.

7.1 – Platform onset

Sponge and microproblematica assemblages of Reef Type 1 indicate that carbonate sedimentation started during the Ladinian? Early Carnian (Peybernes et al., 2015) on volcanic edifice(s) composed of OIB basalt of hotspot origin (Onoue et al., 2004). Tubiphytes–sponge–

coral patch reefs, now exposed as boulders in the VCM at Loc. 2, correspond to pioneer communities settling on the seamount. During the deposition of these patch reefs, volcanic rocks were probably still emerged as suggested by the presence of basaltic nucleus in ooids of the associated grainstone facies (MF6), probably coming from the meteoric erosion of the volcanic edifice. Tubiphytes–sponge–coral boundstone has never been found in massive limestone units in the SAC. Therefore these Ladinian?–Lower Carnian carbonate build ups were probably dismantled during the Carnian and reworked on the seamount slope before the deposition of later platform carbonates. We suggest that these limestone boulders from these reefal environment were reworked downslope in volcaniclastic material which is the most common volcanic rock in shallow submarine volcanoes (Staudigel & Clague, 2010). Assuming this early mixing, these blocks can be compared to the famous Cipit Boulders from the Dolomites that were protected in the tuffaceous San Cassiano Formation (e.g., Tosti et al., 2014) or to the resedimented Ladinian?–Carnian reef limestone clasts from the Mufara Formation of Sicily (Martini et al., 1991) . It is to be noticed that, even if the limestone clasts in VCM (MO2) are better preserved than the limestone slabs (MO1), the preservation is not as good as in the Cipit boulders. Nevertheless, in Italy and in Japan as well, these shallow water limestone elements, reworked in deeper environments, are the unique witnesses of former Ladinian–Early Carnian carbonate platforms which are now dismantled or totally dolomitized.

7.2 – Mid–Carnian transition

Important changes in biotic composition between Ladinian?–Lower Carnian reefs and Upper Carnian–Norian–Rhaetian reefs are recognized worldwide (Flügel, 2002; Peybernes et

178

179 Jurassic, N : Norian, P: Permian, Ph: Phanerozoic, R: Rhaetian, Re: Recent, UT: Upper Triassic. x: presence;

–: absence; ?: presence uncertain. *Corals after Stanley & Onoue 2015, ¹: Conodonts after Yamato Omine Research Group (1981), ²: Conodonts after Peybernes et al. (2016).

180 al., 2016a; Martindale et al., 2017). At Mt Sambosan this transition is also expressed as a modification in mode of occurrence: small reef limestone blocks and pebbles in VCM record Ladinian?–Lower Carnian environments whereas Upper Carnian–Norian–Rhaetian reef assemblages occur in massive limestone slab. Previous authors already identified this striking difference between dark grey limestone clasts in VCM and the nearby massive limestone unit (Okuda et al., 2005; Stanley & Onoue, 2015; Peybernes et al., 2016a). The change in biotic content and mode of occurrence is broadly coeval with the Carnian Pluvial Episode (CPE), which happened during the Late Julian–Early Tuvalian (Dal Corso et al., 2015, 2018).

However, even if the CPE may explain the observed changes, additional temporal constraints are needed to reliably establish a causal link between the CPE and Sambosan carbonate platform evolution.

7.3 – Platform growth

The diversity of microfacies recorded in the limestone slabs and clasts at Mt Sambosan indicates that carbonate platform, including several depositional environments, developed at the seamount top during the Upper Carnian to Rhaetian (Peybernes et al., 2016b). According to the microfacies diversity and heterogeneity these platforms were probably wider than the Ladinian?– Lower Carnian ones. They were composed in particular of typical Norian–Rhaetian reef associations (Reef Type 2 and 3). Conodonts found in the massive limestone slab at Loc.

1 and in other outcrops of Mt Sambosan (Fig. 1c, Table 3) confirm that carbonate platform deposition lasted from the Late Carnian to Norian–Rhaetian (Yamato Omine Research Group, 1981; Peybernes et al., 2016a). The occurrence of Triasina hantkeni in lagoonal involutinid wackestone (MF7) also demonstrates that low energy environments were present during the Late Norian to Rhaetian. Involutinid wackestone intraclasts found in calcimicrob–aggregate grainstone–rudstone (MF5, Fig. 12h) indicate that this facies deposited slightly after MF7 but most probably still in the Norian–Rhaetian. The occurrence of the microcoprolite Parafavreina thoronetensis in some ooid–peloid grainstone (MF6) also suggests a Norian–Rhaetian age (Peybernes et al., 2016b). Griphoporella curvata is a typical Norian–Rhaetian algae (Barattolo et al., 1993; Senowbari–Daryan et al., 2011). Its occurrence in sponge–algae boundstone (MF4, Reef Type 3) indicates that the last reefal bioconstructions on the Sambosan carbonate platform took place during the Norian–Rhaetian.

181 7.4 – Platform demise

Diagenetic features described in section 6.2 shed light on the carbonate platform demise.

Indeed, P. thoronetensis–rich grainstone (MF6), sponge–algae boundstone (MF4, Reef Type 3) and also some intraclasts in MF3 are affected by a specific dissolution event as demonstrated by biomolds, large voids and karst–related features. The karstic features are first partially or totally filled with ostracod–rich dark micrite. In some samples the sediment infilling displays multiple geopetal features (Fig. 14e), indicating several stages of remobilization in a carbonate material. The remaining porosity is filled with non–luminescent dogtooth cements, common in meteoric phreatic environment, followed by luminescent blocky calcite, probably related to late marine precipitation (Halley & Harris, 1979; Moore, 1989; Flügel, 2004). This distinctive sequence of cement precipitation is also found in the biomolds and large voids mentioned

Dans le document Sedimentology, biostratigraphy and diagenesis of Upper Triassic shallow–water carbonates from Japan and Russian Far East: New investigations on the Panthalassa Ocean (Page 170-200)