The wide range of facies described and interpreted in this study is of great value to create for the very first time a conceptual depositional model for the Dalengorsk limestone system (Fig. 12). Each facies correspond to an accurate depositional environment within the platform, spanning from internal protected to basinal, with significant transition between restricted and open areas. All information collected so far, from field observation, microfacies description and literature, allows us to define this carbonate system as an “atoll–type carbonate platform”, developed on a basaltic seamount in the Panthalassa Ocean. All arguments to support this hypothesis are detailed below.
Geological settings – The Dalnegorsk limestone, of Late Triassic (Norian) age, is accreted within a Latest Jurassic to Early Cretaceous accretionary prism (Taukha
55 terrane), resulting in the accretion of Paleo–Pacific formations onto the Paleo–Asian continental margin (Khanchuk, 2006; Kemkin, 2006; Kemkin et al. 2016). Khanchuk et al. (1989) also interpreted the limestone units as fragments of paleo–guyot from the Panthalassa, characterized by volcanic rocks typical of hot spots. The studied limestone is represented by large isolated slabs surrounded by many lithologies of different ages and cropping out in a very limited geographic area (Fig. 2). In the entire Taukha terrane, this area records the only occurrences of shallow–water Triassic limestones (Kemkin et al., 1999; Khanchuk et al., 2016). These occurrences are, therefore, considered as having their origin as top–carbonate platforms on a single mid–oceanic system developed on volcanic substrate (e.g., seamount, oceanic plateau), accreted and tectonically scattered in a limited area. We can reasonably argue that, if Dalnegorsk limestone was originally represented by a swarm of smaller atolls or a wide attached platform, the mode of occurrence would be much more dispersed and spread over greater distances.
- Association with basaltic and deep oceanic rocks – As detailed in the section 4 of this chapter, limestone in the Taukha terrane occurs in a complex mélange of different oceanic lithologies and has also been observed in the field in tectonic contact with alkaline basalts (Khanchuk et al., 1989) (Fig. 3D). This lithological association is typical of Oceanic Plate Stratigraphy, defined as ancient oceanic floor sedimentary sequences (Isozaki et al., 1990; Kusky et al., 2013; Wakita & Metclafe, 2005). The presence of volcanic grains in the slope facies (F7), confirms that carbonates developed on a volcanic edifice.
- Absence of any terrigenous input – Over 128 collected samples, no clastic minerals (e.g., quartz, zircon, feldspars, etc.) or lithics have been observed. Limestone is made of only carbonate grains and mud, indicating an isolated system, far from any emerged land providing terrigenous inputs (Nakazawa, 2001; Soja, 1996). Note that apart for F7, no volcanic grains have been found within the other facies. At the time of the main carbonate production, the volcanic system was possibly immerged and not providing any influx of volcanics within the platform.
Abundance of lagoonal deposits – F1 and F2, interpreted as lagoonal deposits, are the most common facies in the Dalnegorsk limestone. Many outcrops are characterized by
56
Sampled Area Facies Sample Foraminifers Age of foraminifers
association
Stratigraphic distribution (previous studies)
Gorbusha block _ _ No formaminifers found ?Trias Never studied before
GP-116 "Trochammina" alpina
Niznii-Rudnik Mine F2 GP-166 Indet. foraminifer ?Trias Never studied before
GP-169-A Triasina hantkeni
GP-169-B Encrusting foraminifer, indet. Miliolids
GP-172-A Indet. foraminifer
GP-173 Indet. Miliolids
GP-176 Parvalamella sp., Trochosiphonia? sp., "Trochammina" jaunensis
GP-178-A Indet. Miliolids, ?Frentzenella frentzeni
GP-178-A-2 Indet. Miliolids, Lamelliconus semivacuus, Aulotortus sp.
GP-178-B Indet. Miliolids, Lamelliconus semivacuus
GP-178-B-2 Indet. Miliolids
GP-179-A Parvalamella friedli
GP-179-B Aulotortus sp., Parvalamella friedli
F2 GP-186-B Parvalamella friedli, Aulotortus sp., Aulosina oberhauseri, Trochosiphonia josephi, Variostomatidae
F1 GP-187 Trochosiphonia? sp.
Sadovy Mine F6 GP-190 ?Duostominidae + Variostomatidae Anisian-Rhaetian Never studied before
Primorsky Massif F4 GP-192 Indet. Miliolids ?Trias Punina, 1997, corals:
Late Norian
Vokzalnaya Massif F2 GP-195 Parvalamella friedli Late Ladinian-Rhaetian Punina, 1997, corals:
Upper Norian to Rhaetian
F2 GP-199 Aulotortus sp., Trochosiphonia josephi
F4 GP-205 "Trochammina" jaunensis
GP-206-A Aulotortus sp., Parvalamella sp., Trochosiphonia josephi GP-206-B Aulotortus sp., Parvalamella sp., Trochosiphonia josephi, Tubulastella comans GP-206-C Aulotortus sp., Parvalamella friedli, Aulosina oberhauseri, Tubulastella comans
GP-210 Aulotortus sp., Parvalamella friedli (Ladinian only in the lower layers of Verkhnij massif)
Late Carnian (middle and upper Tuvalian)
dark grey homogenous carbonates, locally presenting large megalodontid bioherms (Fig. 4G) and open lagoon facies are widely represented in the Dalnegorsk area. This abundance of protected facies is also in agreement with an isolated context of the system
Table 4 – List of identified foraminifers from Dalnegorsk limestone and associated facies. Age is based on foraminifer association of each sampled area.
57 - Association with basaltic and deep oceanic rocks – As detailed in the section 4 of this chapter, limestone in the Taukha terrane occurs in a complex mélange of different oceanic lithologies and has also been observed in the field in tectonic contact with alkaline basalts (Khanchuk et al., 1989) (Fig. 3D). This lithological association is typical of Oceanic Plate Stratigraphy, defined as ancient oceanic floor sedimentary sequences (Isozaki et al., 1990; Kusky et al., 2013; Wakita & Metclafe, 2005). The presence of volcanic grains in the slope facies (F7), confirms that carbonates developed on a volcanic edifice.
- Absence of any terrigenous input – Over 128 collected samples, no clastic minerals (e.g., quartz, zircon, feldspars, etc.) or lithics have been observed. Limestone is made of only carbonate grains and mud, indicating an isolated system, far from any emerged land providing terrigenous inputs (Nakazawa, 2001; Soja, 1996). Note that apart for F7, no volcanic grains have been found within the other facies. At the time of the main carbonate production, the volcanic system was possibly immerged and not providing any influx of volcanics within the platform.
- Abundance of lagoonal deposits – F1 and F2, interpreted as lagoonal deposits, are the most common facies in the Dalnegorsk limestone. Many outcrops are characterized by dark grey homogenous carbonates, locally presenting large megalodontid bioherms (Fig. 4G) and open lagoon facies are widely represented in the Dalnegorsk area. This abundance of protected facies is also in agreement with an isolated context of the system (Soja, 1996), developed on an immerged volcanic edifice. In such settings, lagoons are represented by large, more or less deep topographic depressions, surrounded and protected by open marine sediments (i.e., reefs, shoals) developed on a narrow area, at the platform edge. These areas are therefore filled by sediments originating from the external part of the system, with the favor of waves, swells and sediment abrasion (Trower et al., 2019) together with sediments produced in–situ. Consequently, they hold a greater amount of sediment compare to the other parts of the platform.
- Relative stability of the system over a very long time period – In this work, the Dalnegorsk limestone is considered as Norian in age, a stratigraphic stage that extends from 227 to 208.5 Ma (ICC, v2019/05). Even if the platform is possibly limited to the early–middle Norian, as our foraminiferal data suggest, the Dalnegorsk limestone
58 notably presents extremely limited facies changes, with very similar biotic and sedimentary content over a long time period. Despite the huge amount of carbonate, which suggests an important thickness for the original deposits, depositional conditions remained similar. This relative homogeneity indicates a continuously subsiding system, possibly affected by minor eustatic variations (see section 6 of this chapter and Haq, 2018). The Norian would indeed be a ?polar ice cap–free period (Preto et al., 2010).
This depositional context is also typical of mid–oceanic carbonates (Soja, 1996). Those systems are indeed only governed by constant thermal subsidence controlling the accommodation space, in a very simple tectonic setting, not subject to environmental stress related to continental variations (e.g., clastic inputs associated to tectonic or climatic changes), allowing a continuous and homogeneous carbonate development over a long time–period (Nakazawa, et al., 2009).
- Strong early cementation – Mid–oceanic carbonate systems are subject to active water circulation due to constant action of waves and swells. This circulation induces strong early cementation of open facies (Aissaoui et al., 1986 ; Harris et al., 1985). In the Dalnegorsk limestone, F4, F5 and F6 are characterized by large isopachous and blocky cement in voids, reef cavities and intergranular space (Fig. 6B, Fig. 7). This cementation occurs before any other diagenetic event or early compaction, confirming the rapid cementation. Moreover, no compaction pattern within the ooid facies (F6B) has been observed (i.e., concavo–convex contacts, collapses), and ooids are cemented and separated by large isopachous and blocky cements, signifying rapid cementation before burial. Furthermore, cements in F4, F5 and F6 are non–luminescent in cathodoluminescence (diagenetic work in progress), usually typical of early marine oxidizing conditions.
- Similar atoll–type systems described in the literature – The Taukha terrane is a part of the south–north continuity of Jurassic to Cretaceous accretionary complexes, going from the Philippines to the Sakhalin Island (Russian Far East). Accreted Upper–Triassic atoll–type systems from Panthalassa, belonging to these prisms in Japan and the Philippines, have been described in detail before (Chablais et al., 2010a,b, 2011;
Kiessling & Flügel, 2000; Onoue et al., 2009; Peybernes et al., 2015, 2016a,b). The mode of occurrence of these systems, as well as the biotic and sedimentary content, is very similar to those of Dalnegorsk limestone, suggesting an equivalent origin and geo–
59
Fig. 12 – Depositional model of Dalnegorsk limestone. FWWB: Fair Weather Wave Base.
60 –dynamic evolution (i.e., development in the Panthalassa Ocean on basaltic seamounts and later accretion during the Late Jurassic to Early Cretaceous). Foraminiferal assemblages, however, are closer to those observed in North American Panthalassan terranes of Oregon (Rigaud et al., 2013b, 2015b) and Yukon (N. del Piero, pers. comm.
2019), suggesting a more eastward origin in the Panthalassa.
Due to accretionary processes, most Dalnegorsk limestone presents neither bedding, nor polarity or spatial continuity. Accurate reconstructions of the original depositional settings are therefore impossible and herein the presented model is based on field observations, facies interpretation and a literature review and is partly speculative. Comparison with similar systems is consequently pivotal to create a coherent model. Chablais et al. (2010b) and Peybernes et al.
(2016b) defined detailed depositional models for coeval Upper Triassic atoll–type platforms, respectively in Honshu and Shikoku islands (Japan). Accreted in the Late Jurassic to Early Cretaceous on the Sambosan accretionary complex, they are the closest analogs for the Dalnegorsk limestone studied so far. Given that the platform geometry is governed by numerous patterns (e.g., wind and wave direction, nutrient supplies, temperature and salinity variations, topography and eustatic variations), the authors widely discussed the issues of comparison with modern and ancient analogs to build their models. The depositional settings presented in this work partly relies on their considerations and comparison with ancient and modern analogs. In addition, several fundamentals, characteristics of Upper Triassic atolls have been considered to define the proposed depositional model, such as:
- The absence of major glacio–eustatic variations in a ?polar ice–cap free setting (Preto et al., 2010; Sellwood & Valdes, 2006; Simms & Ruffell, 1990). Minor sea–level changes during the Late Triassic (Haq, 2018) are supposed to have a slight impact on the platform growth compared to recent systems controlled by glacio–eustatism (Taviani, 1998; Wright, 1992; Yokoyama et al., 2006).
- Main reef builders (i.e., sponges, corals, algae) were not as diversified and abundant as today (Chappell, 1980; Huston, 2003; Rohwer et al., 2002) and had possibly not the capacity to construct robust frameworks.
- The flanks of volcanic seamounts and isolated platforms are generally steep and characterized by landslide and debris–flow deposits (Chablais et al., 2010b; Counts et al., 2018; Holcomb & Searle, 1991; Kenter, 1990; Normark et al., 2008; Peybernes et al., 2016b; Prat et al., 2016; Smoot, 1985; Staudigel & Schmincke, 1984). Facies
61 positioned by Chablais et al. (2010b) and Peybernes et al. (2016b) in a fore reef or upper slope have not been identified in the Dalnegorsk area. Edges of the platform are then supposed to be sharp and characterized by really steep flanks or cliffs. The related deposits correspond therefore to turbiditic and debris–flows sediments (F7).
- The geometry of the platform was probably controlled by dominant winds and waves (Chablais et al., 2010b; Gischler et al., 2014; Lee, 1974; Minero, 1991).
In the Dalnegorsk area, the slope of the carbonate system is represented by F7 and F8.
Those facies are typical of a deep environment, surrounding the volcanic seamount, regularly supplied by debris–flows originating from the platform to the favor of storms, swells, earthquakes, or slides and failures. The platform margin is represented by two different facies, whose layout possibly depends on leeward/windward orientation. F5, very poorly represented in the Dalnegorsk area, is dominated by fragile framebuilders and has been interpreted as a constituent of the leeward side of the platform due to its relative brittleness. On the other hand, F6 is a dominant facies interpreted mainly as shoal deposits located on the windward side and possibly alternating with patch reef on the leeward side. Tidal channels, ensuring the connections between inner and outer settings, also characterize the platform margin. F1 to F4 correspond to the platform interior, each one being characteristic of specific depositional environments. The protected part of the lagoon is dominated by mud–deposits (F1 and F2) associated locally with megalodontid bioherms. F3 and F4 represent the open–lagoon/back–
reef environments, as transition between open and protected environments, in variable water–
energy conditions. In particular, F4 is interpreted as being associated with reef deposits and thus probably located in a peri–reefal environment.
The speculative depositional model presented in this work (Fig. 12) is based on new observations of the Dalnegorsk limestone and allows to compare our results with models of coeval and similar systems from Panthalassa. Peybernes et al. (2016b) defined a carbonate bank model, whose margin are dominated by small patch reefs intercalated with low amount of ooid shoals, which correspond to the leeward side of our model. On the other hand, Chablais et al.
(2010b) defined a setting governed by dominant winds and characterized by shoal deposits on the leeward side and reef deposits on the windward side. This last setting is not represented in the Dalnegorsk limestone, where no proper reefs have been found. But again, Triassic reefs are usually extremely localized and chances to find such outcrops in a mélange are limited. The leeward oolitic facies in Chablais’s model corresponds to the windward side of the model in
62 this work. Shoals can occur in both leeward and windward sides of platforms. It is emphasized herein that in the three models, the lagoonal setting, probably displaying small patch reefs, is the dominant one in terms of size. This setting is in accordance to the detailed field observations.
However, unable to make a reliable estimate of the size of the Dalnegorsk limestone system, the presented model (Fig. 12) is not to scale and temporally corresponds to a highstand system tract during the Norian, when the carbonate production is supposed to be at its maximum (Schlager et al., 1994).