A cross-section was made to reconstruct the geology in the subsurface. The topographic profile was constructed perpendicular to the strike or trend of the tectonic structures. The geology of the surface was added to this profile, including rock types, unit boundaries, and bedding dips and faults derived from field data and/or geological maps. Surface data were extrapolated in deep areas. Reconstructed deformations were subsequently retro-deformed to a restored section (manually). The restored sec-tion was realized by preserving the rock volume and maintaining a constant bed thickness and equal lengths for all units. To un-deform the cross-section, several pin lines were established, which serve as marker horizons for measuring bed length (Dahlstrom, 1969; Woodward et al., 1989).
DATA
The study area exhibits folded units, comprising continuous succession of carbonate rocks ranging in age from the Devonian to the Triassic (Fig. 2A). The dataset originated from three stratigraphic sections (Fig. 2B). Two of these sections were measured along the ‘new’ coral-bearing bioconstruc-tions, including three small reefs (6–16 m thick and 10–20 m wide, section 1; Fig. 3) and an extended reef (up to 50 m high and 250 m wide, section 2; Fig. 4), respectively located near Longjiangdong and Xiadong villages (Langping, Guangxi). The last section (section 3) comprises a long sequence of limestones (slope succession), dated as Late Devonian–Early Permian and located close to Gan-dongzi village (Langping, Guangxi).
DATING
The stratigraphy was constrained using two independent methods, namely Sr isotopes (87Sr/86Sr) and fusulinid assemblages (Fig. 5).
Figure 2: (A) Geological map of the Langping area (Tianlin, Guangxi, southern China) with the location of the three studied sections, modified from Gong et al. (2012). (B) Lithofacies successions record from the three measured sections.
Figure 3: Outcrop of the three small coral reefs located near Longjiangdong village (Langping). The coral reefs range from 6 to 16 m high and 10 to 20 m wide. Section 1 crosses successively the base, core and cover of each coral reef. (A) Reef substrates are dominated by coarse skeletal grain limestones. (B) Reef cores are composed of branching colonial corals.
Figure 4: Outcrop of the extended Xiadong coral reef. The coral reef measures up to 50 m high and 250 m wide. Section 2 crosses vertically the core and cover of the large coral reef.
Figure 5: Distribution of the Sr isotope (87Sr/86Sr) and fusulinid samples along the three measured sections. Sr values are detailed in Table 1 and foraminiferal assemblages are summarized in Tables 2 and 3.
Chemostratigraphy
The Sr isotope compositions of selected samples are summarized in Table 1. Samples were selected in order to have an optimized and consistant stratigraphic resolution. The 87Sr/86Sr ratios for sections 1, 2, and 3 are 0.70771–0.707785, 0.707721–0.707824, and 0.707708–0.708203, respectively. Based on the Phanerozoic strontium curve, strontium signals of selected samples correlate with several stratigraphic intervals (Howarth and McArthur, 1997; McArthur et al., 2001). Because the study area exhibits a rock succession ranging in age from the Devonian to Triassic, 87Sr/86Sr values lead
Biostratigraphy
Foraminiferal (especially fusulinid) zonations are reliable biostratigraphic markers and are com-monly used to identify Carboniferous chronostratigraphic divisions (Davydov et al., 2012). In sections 1 and 2 (Table 2) and the lower part of section 3 (interval 1, Table 3), the fauna includes small fusulinids
Section Interval Sample Location (m)
87Sr/86Sr Ages
min average max min average max
Section 1 1 119 1 0,707785 343.35 342.75 342.15 329.95 330.15/20 330.45
Viséan Viséan
2 130 16 0,707756 342.15 341.50 340.75
-Viséan
3 146 56.3 0,707716 340.05 338.75/85 337.25 331.40 331.80 332.30
Viséan Viséan
4 155A 78.4 0,707748 341.80 341.10 340.30 330.70 330.95 331.25
Viséan Viséan
5 161 94.6 0,707714 339.85 338.45/50 336.95 331.45 331.90 332.45
Viséan Viséan
Section 2 6 110 substrate 0,707824 344.65 344.20 343.65 329.10 329.35 329.60
Viséan Viséan
7 61 2.1 0,707753 342.05 341.35 340.60 330.60 330.80/85 331.10
Viséan Viséan
8 74 23.3 0,707786 343.35 342.80 342.20 329.90 330.15/20 330.40
Viséan Viséan
9 87 57.4 0,707721 340.40 339.30/35 337.75 331.25 331.60/65 332.10
Viséan Viséan
Section 3 10 189 1.1 0,708166 375.75 376.45 377.05
-Late Devonian
11 198 184.4 0,708202 374.30 375.20 375.90
-Late Devonian
12 207 231.6 0,707872 346.15 345.70 345.25
-Tournaisian
13 215 337.8 0,707763 342.45 341.85 341.15
-Viséan
14 233 418.7 0,707793 343.60 343.05/10 342.50 329.75 330.00/05 330.25
Viséan Viséan
15 254 535.8 0,707708 331.60 332.05/10 332.80
-Viséan
16 266 659.2 0,707997 325.30 325.65 326.00
-Serpukhovian
17 273 765.5 0,708203 317.30 318.00/05 318.75
-Bashkirian
to interpret sections 1 and 2 as Viséan, whereas the section 3 is estimated as Late Devonian–Early Permian. However, for the last section, Sr values could correspond to the several potential ages and additional investigations are needed to better constrain the adequate stage within the Carboniferous.
Section 1
Interval Foraminiferal assemblage Age
A Endothyrid: Endothyra sp., Endothyranopsis sp., globivalvulinid, Janischewskina sp., Mediocris sp., Omphalotis sp., palaeotextulariid, Palaeotextularia sp., Spinothyra sp., Tetrataxis sp.
Fusulinid: Eostaffella sp., Novella sp.
Late Viséan-Serpukhovian
Section 2
Interval Foraminiferal assemblage Age
B Endothyrid: Bradyina sp., Climacammina sp., Endothyra sp., Endothyranopsis sp., globivalvulinid, Ja-nischewskina sp., Mediocris sp., palaeotextulariid, Palaeotextularia sp., Tetrataxis sp.
Fusulinid: Eostaffella sp., Pseudoendothyra sp.
Late Viséan-Serpukhovian
Table 2: Synthesis of foraminiferal assemblages identified in sections 1 and 2.
Section 3
Interval Foraminiferal assemblage Age
C Archaediscid; Endothyrid: Climacammina sp., Endothyra sp., Janischewskina sp., Mediocris sp., Palaeo-textularia sp., Spinothyra pauciseptata; Fusulinid: Eostaffella sp.
Viséan-Serpukhovian
D Archaediscid: Asteroarchaediscus sp.; Endothyrid: palaeotextulariid; Fusulinid: Eostaffella sp., Plecto-staffella sp., (primitive?) PseudoPlecto-staffella sp., SemiPlecto-staffella sp., Pseudoendothyra sp.
Early Bashkirian
E Endothyrid: Palaeotextularia sp.; Fusulinid: Eoschubertella sp., Nankinella sp., Ozawainella sp., primi-tive Profusulinella sp., Pseudoendothyra sp.
Late Bashkirian
F Fusulinid: Eofusulina sp., Neostaffella sp., Ozawainella sp., Profusulinella sp., Pseudostaffella sp. Earliest Moscovian (Vereian-early Kashirian?)
G Endothyrid: Climacammina sp., Palaeotextularia sp., Tetrataxis sp.; Fusulinid: Beedeina sp., Eostaffella sp., Fusulinella sp., Ozawainella sp., Pseudoendothyra sp., Schubertella sp.
Late Early-early Late Moscovian
(late kashirian-Podolskian)
H Fusulinid: Fusulina sp., Kanmeraia sp., Quasifusulinoides sp., Quasistaffella sp., Pseudostaffella sp. Latest Moscovian (Myachkovian)
I Fusulinid: Fusulinella sp., Protriticites sp. Moscovian/Kasimovian
transition
J Fusulinid: Fusiella sp., Fusulina sp., Quasifusulinoides sp., Ozawainella sp., earliest schwagerinid Early Kasimovian
K Fusulinid: Primitive schwagerinid (Montiparus sp., Triticites sp.), Staffella sp., Nankinella sp. Middle Kasimovian
L Endothyrid: Climacammina sp.; Fusulinid: Schwagerinids (Rauserites sp.) Late Kasimovian (? and Gzhelian)
Table 3: Synthesis of foraminiferal (mainly fusulinid) assemblages identified in section 3.
(Eostaffella sp. and Mediocris sp.) and dominant smaller foraminifera (such as Janischewskina sp., Endothyranopsis sp., Endothyra sp., Spinothyra sp., Omphalotis sp., and several paleotextulariids).
Foraminiferal assemblage suggests a late Mississippian age (Late Viséan–Serpukhovian) for this interval (e.g. Einor, 1996; Kulagina et al., 2003; Cózar and Somerville, 2004; Somerville and Cózar, 2005; Somerville, 2008). Along section 3, several other subdivisions can be defined using specific fusulinid assemblages listed in Table 2. The occurrence of (1) Pseudostaffella sp. and Semistaffella sp., (2) Eofusulina sp., Profusulinella sp., Fusulinella sp., Beedeina sp., Kanmeraia sp., Quasifu-sulinoides sp., and Quasistafella sp., and (3) Fusiella sp., QuasifuQuasifu-sulinoides sp., Montiparus sp., and schwagerinids (Triticites sp. and Rauserites sp.) respectively, indicate Bashkirian, Moscovian, and Kasimovian (to Gzhelian) ages (e.g. Rauzer-Chernousova et al., 1951; Einor, 1996).
Based on the fusulinid assemblage and 87Sr/86Sr chemostratigraphy, the ages of coral reefs, pre-viously estimated Late Bashkirian-Middle Kasimovian (section 1) and Viséan (section 2; Gong et al., 2012; Yao and Wang, 2016), are revised as Late Viséan-Serpukhovian. In addition, foraminiferal assemblage identified in section 3 allows to better dating of the different stratigraphic levels (Table 3). The correlation between Sr isotopes and foraminiferal assemblage suggests that four Sr isotope values deviate from the global trend (sample n°275, 295, 307, 318; Table 1), a matter that requires more investigations.