5.1 – General observations
For this study, eleven outcrops were studied, described and extensively sampled. Ten of them showed a relatively good preservation (grains and matrix preserved). The Gorbousha block (Loc. 1) is the only intensely recrystallized outcrop. Samples from this area only show sparry calcite and are therefore not exploitable for this study. Most large bioclasts (i.e., sponges, corals, gastropods and bivalves) appear recrystallized or dissolved and reprecipitated (see sparry cement in Fig. 6B, E). A major event of moldic dissolution was observed for each sampled facies from all outcrops. As the majority of observed organisms had skeletons
31 originally composed of aragonite (metastable in atmospheric and burial conditions) most of them have not been preserved. The identification is based on their general shape and very rarely preserved internal structures. Few samples show dolomitization, represented by displacive dolomite rhombs precipitated along the main joints (fractures, veins, and stylolites). These crystals are not fabric–destructive and attributed here in to a late event of dolomitization, not significant for this work.
Fig. 3 – A. Typical mode of occurrence of an Upper Triassic carbonates in the Dalnegorsk area: massive limestone slabs forming topographic highs. B. Sampled wall in the Rudnik–Nikolaevsky Quarry. C.
Plurimetric dark patch in the Karyernaya Quarry, noticeably richer in organic matter than the surrounding carbonates. D. Basaltic rocks (pillow lava?) found in contact with sampled limestone in the Verkhnii–Rudnik Quarry.
5.2 – Facies description
Hereinafter, the sedimentary structures, microfacies and fossil contents are described in details. The presented facies are a combination of lithofacies, biofacies and microfacies, including therefore the sedimentary structures, the mineralogy and the biotic and abiotic content. The microfacies are classified according to the classification of Dunham (1962) and subsequent improvements by Embry & Klovan (1971). Eight main facies have been defined from field observations and from 128 samples collected in the Dalnegorsk area. As mentioned
32 in the previous section, the mode of occurrence of this limestone is governed by the process of accretion. Consequently, they do not present a lateral continuity or preserved bedding. Thus, in the following descriptions, the lateral and vertical facies distributions are described, but, when inferred, might not correspond to an actual stratigraphic or sedimentary context. In this geological setting, facies associations are defined by facies co–occurrences in a same outcrop area, presenting no faults or signs of shearing. The descriptions below are not exhaustive: the exact content of each facies and associated sampled locality are presented in Table 2.
F1 – Oncoid–microbial mudstone to wackestone (Fig. 4). Dark grey limestone locally disturbed by dark centimetric width burrows (Fig. 9B, I, J) filled with the F3 or F6B. In the Partizansky area, F1 is laterally associated to pluri–metric megalodontid bioherms (i.e., Megalodontid boundstone organized as pluri–metric mounds, Fig. 4G, H) corresponding to facies F2. F1 is essentially composed of dark micrite associated with oncoids, cyanoids, micritized ooids, calcimicrobes Cayeuxia sp. and microproblematica Baccanella floriformis.
Rare echinoderms, thin encrusted shell debris (Fig. 4A), ostracods, fecal pellets and large isolated megalodontids occur (Fig. 9). Cavities (stromatactis structures and moldic porosity) are filled with geopetal infillings or microbial peloids. Few samples contain also small rounded radial ooids with a regular size close to 200 µm in diameter (Fig. 4A). The foraminiferal association is represented by rare ?Trochosiphonia sp. and other undeterminable foraminifers.
F1, abundant in the Dalnegorsk area, has been found in the Verkhnii–Rudnik outcrops, the Sakharnaya Mountain, and the Partizansky and Sadovy Mine outcrops. The latter has never been studied before.
F2 – Involutinid–Megalodontid wackestone to rudstone (Fig. 4). Dark grey limestone with some darker areas (Fig. 3C), possibly richer in organic matter or more intensely metamorphosed. Large unevenly shaped burrows are very common and are filled with identical facies, F3 or F6 (Fig. 9I, J). F2 contains thick megalodontid shells, broken or in life position, isolated or locally accumulated in the form of pluri–metric size bioherms (megalodontid boundstone, Fig. 4G, H). Gastropods and involutinid foraminifers, mainly represented by Parvalamella friedli, are the second major components. Other carbonate clasts observed in this facies are rare oncoids, echinoderms, bivalves and Dasycladacean green algae debris.
Gastropod–echinoderm debris–rich, laminated wackestone to packstone (Fig. 4B), is rarely associated with F2, but is included in this facies as a localized higher energy deposit. The facies F2 is very abundant, especially in Partizansky and Karyernaya outcrops.
33 F3 – Peloidal–bioclastic packstone to grainstone (Fig. 5). Light grey limestone locally punctuated with pluri–centimetric dark elongated burrows arranged in the same orientation and filled with the F6B. In the Verkhnii–Rudnik Valley, F3 is laterally associated to metric size coral–sponge bioherms (F5). In the Karyernaya outcrop, this facies is very locally represented
Fig. 4 – F1 and F2 A. F1, microbial wackestone characterized by encrusted shells and radial ooids (yellow arrows). B. F2, laminated gastropod–echinoderm rich wackestone–packstone. C, D. F2, Involutinid–
Megalodontid packstone to rudstone. Note the extraordinary abundance of Involutinids (yellow arrows), largely dominated by Parvalamella friedli. E, F. F2, Parvalamella friedli–rich facies. G. F2, outcrop view of Megalodontid bioherm in stylolitic contact with the surrounding facies, dominated by F1 and F2. H. Close up view on Megalodontid shell accumulation within the bioherm. Scale bars: A, B, C 2,5 mm ; D 1 mm ; E 200 µm ; F 500 µm.
in the burrows infilling of F2. F3 is dominated by peloids of various size and shapes (including fecal pellets, microbial peloids, reworked mud grains or micritized clasts), associated with coated shell debris (few preserved ooids and various other coated grains). Others significant
34 bioclasts such as echinoderms, serpulid colonies, Baccanella floriformis (Fig. 5E), very rare Parafavreina crustacean microcoprolites, calcimicrobes (Cayeuxia sp.) and microproblematica (?Plexoramea ceribriformis, Fig. 5) are also representative of this facies. Green algae (dasycladacean) floatstone also occurs locally. The complete bioclastic content is listed in Table 2. The foraminiferal association is dominated by Agathammina sp., “Trochammina” sp. and different Involutinida genera (see Table 4 for complete list of foraminifers). F3 is the most represented facies in the sampled areas. This facies has been found in every sampled outcrops, with the exception of Sadovy Mine and Primorsky Massif where outcrops are very limited in size.
F4 – Mollusc–Calcimicrobe packstone to grainstone (Fig. 5). Grey limestone represented by locally abundant mollusc debris. This facies is always found laterally associated with F3 and F5. The major components of F4 are coated calcimicrobes, represented herein by Plexoramea cerebriformis (Fig. 5F), Garwoodia sp., Cayeuxia sp. (Fig. 5D) and ?Hedstromia sp. The other main components are aggregate grains, intraclasts, oncoids and various micritic grains. Occasionally bivalve, gastropod, coral and sponge debris occur, always coated or encrusted by microbial crusts. Voids and stromatactis structures (Fig. 5C) are also very frequent and filled by sparry isopachous, blocky cements or geopetal infillings (Fig. 5C). The minor components of these facies are listed in Table 2. Few, but significant, foraminifers have been found in F4. They are mainly represented by agglutinated forms (“Trochammina” alpina, Endotriada sp.) and hyaline aragonitic multichambered forms (?Duostominidae, Variostominidae). This facies is not highly represented in the Dalnegorsk area. Most F4 samples have been found in the Verkhnii–Rudnik outcrops (valley and quarry).
F5 – Sponge–coral boundstone (Fig. 6). Very rare light grey fossil–rich limestone marked by numerous pluri–centimetric corals, sponges and large megalodontid and gastropod shells. F5 is always associated with F3 and/or F4. Primary frame builders attributed to phaceloid and solitary corals, chaetetid sponges and other undetermined calcareous sponges, locally encrusting corals or sponges, dominate this facies. The poor preservation of these organisms, however, does not permit their generic identification. The secondary frame builders are represented by microbial lamination encrusting the primary frame builders and numerous microproblematica assigned to Radiomura cautica, Microtubus communis, Baccanella floriformis and the sponge Uvanella sp. (Figs. 6 F, H, G). The interstitial sediment deposited
35 between the frame builders is dominated by F3 and F6 or, locally, by a dark homogeneousmicrite. Voids, defined as reef cavities, are filled by large isopachous and blocky
Fig. 5 – F3 and F4 A. F3, Peloidal–dasycladacean algae (recrystallized) floatstone. B. F4, Coated calcimicrobesgrainstone. Note the large isopachous and blocky cement filling the intergranular porosity (yellow arrows). C. F4, stromatactis filled by geopetal and blocky cement. Way–up is indicated by the yellow arrow. D. Cayeuxia sp. E. Baccanella floriformis. F. Plexoramea cerebriformis. Scale bars: A–C 2,5 mm ; D, F 500 µm ; E 200 µm.
cement (Fig. 6B). Undetermined Miliolids and encrusting forms represent the foraminiferal association. F5 is not a common facies in the Dalnegorsk limestone. It has only been observed and sampled in three localities, including the Rudnik–Nikolaevsky Quarry, that never studied before.
F6 – Ooid–bioclastic grainstone (Fig. 7). This facies can be subdivided in two sub–
facies (F6A and F6B), showing the same depositional characteristics but a very different biotic content:
36 F6A – Bioclastic grainstone (Fig. 7D). Light grey limestone only found in the embankments of the Sadovy Mine, made of basaltic, polygenic breccia and F6A–type limestone boulders. No sedimentary structure has been observed in this area. F6A is composed of bioclast debris represented by echinoderms, gastropods, and undetermined
Fig. 6 – F5, sponge–coral boundstone A. Outcrop view of reefal facies composed of phaceloids corals (white arrows), bivalve debris and laminated crusts (red arrow). Note the presence of reefal cavities, filled with dark cement (yellow arrows). B. Thin section of the reefal facies, characterized by recrystallizedframe builders, reef cavities filled with isopachous and blocky cements (yellow arrows) and laminated crust (white arrow).
The interstitial sediment is represented by dark micrite. C. Outcrop picture of recrystallized sponges and corals. D. Outcrop view of the bioherm facies. Phaceloid corals (yellow arrows) and sponges (white arrow).
Note the absence of reefal cavities. E. Sponge–coral floatstone characteristic of the bioherm facies. Partially encrusted corals (yellow arrows), sponges (white arrows) and echinoderms surrounded by typical open–
lagoon facies (F3). F. Radiomura cautica G. Uvanella sp. H. Microtubus communis. Scale bars: B,E 2,5 mm ; D 2 cm ; F 500 µm ; G, H 200 µm.
branched calcimicrobes. Numerous randomly shaped micritized grains and peloids also occur. The clasts are cemented by large isopachous and blocky cement. The
37 foraminiferal association is exclusively dominated by abundant ?Duostominidae and Variostomatidae.
F6B – oolitic grainstone (Fig. 7A, B, C). Dark grey limestone characterized by numerous ooids, which stand out at exposures. This facies locally occurs as an infilling of burrows in facies F1 and F3 and is laterally associated with F1 to F4. F6B is only composed of well–sorted cement–supported concentric ooids, compound or isolated, associated with very rare calcimicrobes and green algae debris. Few samples dominated by large oncoids and peloids also occur. Most ooids are micritized (laminations obliterated or absent) with no preserved nucleus (recrystallized). Their shape varies between rounded and elongated with some distorted forms (Fig. 7C). These ooids are cemented by large isopachous and blocky sparite. No foraminifers have been found in this facies. F6B is well represented in Partizansky and Karyernaya outcrops where it occurs in association with F1 and F2.
F7 – Laminated mudstone–wackestone and graded packstone–grainstone (Fig. 8).
Light grey laminated and graded limestone locally deformed by pluri–centimetric convolute structures and differential compaction features. This facies is laterally associated with F8 in the Niznii–Rudnik Mine and with F3 and F5 in Rudnik–Nikolaevsky Quarry. F7 is composed of mudstone randomly intercalated with centimetric graded coarser levels occasionally marked by flame structures (Fig. 8D). These levels are composed of uncemented bioclastic debris associated with angular to subrounded altered volcanic grains (Fig. 8C). Locally in the graded deposits, subrounded well–lithified limestone clasts (F3 or recrystallized facies) also occur (Fig.
8A). Bioclasts are dominated by shallow–water organisms (i.e., corals, sponges, echinoderms, shell debris). No foraminifers have been found in this facies. F7 is highly represented in the Rudnik–Nikolaevsky Quarry and has also been found in the Niznii–Rudnik Mine and Verkhnii–
Rudnik Quarry.
F8 – Radiolarian–filament mudstone to wackestone (Fig. 8E). Dark grey limestone without apparent sedimentary structures. This facies has been found laterally associated with F7. F8 is composed of dark micrite containing sparse bioclasts represented mainly by radiolarians (recrystallized) and filaments (very thin pelagic bivalve shells). Richer samples contain also rare shallow–water clasts represented by ooids, oncoids, brachiopods, echinoderms and ostracods. Rare lithified limestone clasts also occur. Foraminifers have not been found in
38 this facies. F8 occurs only in few samples from the Niznii–Rudnik Mine and Primorsky Massif.
It is the least represented facies in the Dalnegorsk area.
Fig. 7 – F6, ooid–bioclastic grainstone A, B. F6B, oolitic grainstone, well sorted. Note that ooids are largely micritized and the nucleus is not preserved. C. Distorted micritized ooids. D. F6A, bioclastic grainstone composed of micritized clasts (mostly calcimicrobes and shell debris), echinoderms and Duostominids (yellow arrows). Scale bars: A, B, D 2,5 mm ; C 500 µm.
F8 – Radiolarian–filament mudstone to wackestone (Fig. 8E). Dark grey limestone without apparent sedimentary structures. This facies has been found laterally associated with F7. F8 is composed of dark micrite containing sparse bioclasts represented mainly by radiolarians (recrystallized) and filaments (very thin pelagic bivalve shells). Richer samples contain also rare shallow–water clasts represented by ooids, oncoids, brachiopods, echinoderms and ostracods. Rare lithified limestone clasts also occur. Foraminifers have not been found in this facies. F8 occurs only in few samples from the Niznii–Rudnik Mine and Primorsky Massif.
It is the least represented facies in the Dalnegorsk area.
39
Fig. 8 – F7 and F8 A. Outcrop view of F7. Finely laminated facies, with recrystallized limestone clasts (yellow arrows). B. F7, Outcrop view of small–scale laminated layers. C. Thin section of F7, represented by grading sequences composed of shallow–water bioclasts (coral, yellow arrow ; shells, white arrows) mixed with volcanic grains (red arrows). D. Flames structures in F7. E. F8, composed of dark micrite, thin pelagic bivalve shells (yellow arrows) and recrystallized radiolarians (red arrows). Scale bars: A 5 cm ; C, D 2,5 mm ; E 1 mm.