Section 1
Section 1 crosses successively the substrate, core and cover of the three coral bioherms, located in Longjiangdong (Fig. 9A and 10). (1) The substrate of the respective bioherm is composed of bioclastic grainstone-rudstone or lithoclastic rudstone which provides hard surfaces for the initia-tion of colonial corals. The grain assemblage and texture of reef substrates suggest a high energy depositional environment. (2) The coral frameworks are exclusively built by medium-size (<60 cm) phaceloid colonies. Branching growth forms, commonly favored by rapid sedimentation and less frequent scouring events, are present (Chappel, 1980; Scrutton, 1999). Corals are commonly found in growth position and the enclosing sediment consists of fine-grained micrite with small and scarce skeletal fragments, which tend to reflect calm water conditions (Scrutton, 1999). Between coral col-onies, intervals of stromatactis-bearing mudstone, wackestone and floatstone commonly associated to bioclastic grainstone and rudstone intervals suggest fluctuations in water energy. The reef sizes, ranging between 6 and 16 m high, imply a limited accommodation space. Locally, the occurrence of light dependant organisms such as green algae points to a setting within the photic zone. (3) The covers are composed of bioclastic grainstone and rudstone, indicative of a high-energy environment.
Therefore, the features of the three coral biostromes suggest a moderate energy depositional envi-ronment, certainly close to the FWWB, in the photic zone.
Section 2
Section 2 crosses vertically the core and cover of the large coral reef, located in Xiadong (Figs.
9B and 11). The reef substrate was sampled in the other side of the valley, where it is better exposed, by extrapolation. (1) The substrate is composed of bioclastic packestone-rudstone providing a hard substrate for the initiation of coral communities. (2) The reef core is dominated by large branching corals (up to 80 cm in diameter) associated with scarce, small (<15 cm), and massive cerioid cor-al colonies. Branching colonicor-al corcor-als are commonly found in growth position and the enclosing sediment is composed of mudstone and wackestone with small-size skeletal fragments, which tend to reflect calm water conditions (Scrutton, 1999). Between coral colonies, intervals of lithoclastic grainstone and rudstone composed of reef-derived materials, and stromatactis-bearing mudstone, wackestone and floatstone are common. Rare and thin peloidal grainstone intervals occur. These in-tervals lacking coral frameworks do not form distinct, laterally consistent layers, and the reef growth
is globally continuous through time. The secondary reef contributors are dominated by stenohaline and filter-feeder organisms such as brachiopod and bryozoan, which suggest normal salinity condi-tions and oxygenated waters (Oertli, 1964; Wilson, 1975). The lack of green algae indicates a relative deep-water environment, where light is insufficient for photosynthesis (dysphotic zone). The large reef size (50 m, 250 m wide) implies a high accommodation space. (3) The reef cover is composed of bioclastic packstone, grainstone and rudstone. All reef features suggest a deeper setting at the platform margin, with the coral reef growing likely between the FWWB and the SWB, in the dysphotic zone.
Figure 9: Microfacies type successions recorded in the three measured sections. (A) Section 1, (B) Section 2, (C) Section 3.
Section 3
Section 3, from the Late Devonian to Early Permian, records a succession of various microfacies types, including coated-grain grainstone, stromatactis-bearing mudstone and wackestone, skeletal grain wackestone and packstone, crinoid-rich packstone, and lime mudstone (Fig. 9C).
The Tournaisian rock sequence is composed of ooid-pisoid grainstone and crinoid-rich packstone layers. The Lower Viséan deposits record alternating crinoid-rich packstone, cortoid grainstone, skeletal grain wackestone and packstone, and scarce dolomitized limestone beds. In this interval, small-scale patch reefs of Antheria-bryozoan occur (Gong et al., 2012). From the Mid-Late Viséan
Figure 10: Microfacies of Longjiangdong patch reefs. Reef substrates consist of either (A) bioclastic grainstone or (B) lithoclastic grainstone-rudstone. (C) Reef cores are characterized by branching coral framestone. The enclosing sediments consist of fine-grained micrite with scarce skeletal fragments. (D) In the reef complex, intervals of stromatactis floatstone occur. (F) The reef covers are composed of bioclastic grainstone.
Figure 11: Microfacies of Xiadong reef. (A) Reef substrate is composed of bioclastic rudstone. (B) Reef cover consists of bioclastic packstone-grainstone. (C-D) Reef core is formed by branching colonial corals. The enclosing sediments are composed of fine-grained mudstone with locally small-size bioclasts. (E-F) Between coral colonies (E) Stromatactis floatstone and (F) lithoclastic grainstone-rudstone intervals occur.
to Serpukhovian, rock succession is characterized by alternating cortoid grainstone layers, lime mud-stone, and skeletal grain packstone and wackestone. During the Bashkirian, limestones are commonly dolomitized. During the Moscovian, the rock sequence is composed of skeletal grain wackestone and packstone and crinoid-rich packstone layers. Small-scale bryozoan mounds occur in this time-in-terval. From the Late Kasimovian to Gzhelian, the rock sequence records exclusively skeletal grain wackestone and packstone.
Both microfacies features and field data allow to reconstructed changes in paleoenvironments throughout Carboniferous. The Bashkirian dolomitized limestones coincide with the global Mid-Car-boniferous sea-level drop, related to the major Gondwana glaciation (Wang et al., 2013, Tian et al., 2019; Huang et al., 2020). Therefore, the dolomite layers are interpreted as related to exposure events. The coated grain grainstone formed thick layers (Upper Viséan), which can be interpreted as in-place deposits formed in shallow-water and under high-energy conditions. Most of the skeletal grain packstone and wackestone form distinct layers that differ vertically in depositional texture, constituent composition, grain size, and grain packing (Fig. 12A and 12B). These interlayered beds result from fluctuations in water energy (periodic current activity). Small-scale, short-term changes in depositional texture are common in environments influenced by storm-induced currents (Wu, 1982; Wright, 1986). Stromatactis-bearing mudstone and wackestone, occurring in the Huanglong formation (Moscovian), commonly developed in a subtidal environment, on the flanks of Paleo-zoic mounds and reefs as well as on platform slopes (e.g. below wave base, Bourque et al., 1986;
below storm wave base, Bourque and Boulvain, 1993). The crinoid-rich packstone, characterized by high accumulation of crinoid fragments, is locally associated with chert nodules (Fig. 12C). The abundance and disarticulation of crinoids associated with the occurrence of nodular cherts suggest allochthonous sediments deposited in a subtidal environment by storm induced currents and turbid-ites (Tucker, 1969), certainly below the SWB due to the absence of wave-formed structures, such as hummocky cross-stratifications (Wright, 1986). The lime mudstone is commonly burrowed which suggest lagoonal sediments or most likely pelagic background sedimentation, deposited in a low-en-ergy environment below the FWWB and certainly below the SWB.
Therefore, throughout Carboniferous, the rock succession in section 3 records a wide range of depositional settings, from platform shoals (coated-grain grainstone) to slope (interlayered bioclastic packstone and wackestone). Calciturbidites (crinoid-rich packstone) are interpreted as slope depos-its. The mud-rich facies (lime mudstone) is interpreted as deposits of a quiet and deep environment.