Fig 1.
(A) Location of the Permian–Triassic boundary (PTB) sections in the Beibei area. (B) Location and regional geology of the study area (eastern Sichuan Basin), modified from [33]. (C) Global paleogeographic map, modified from [34]. (D) Early Triassic paleogeographic map of South China, modified from [35].
Fig 2.
(A) General lithological column of the lower Triassic (Griesbachian) succession at the Baimiaozi Section in Beibei. (B) Lithological column of the investigated microbial mounds.
Fig 3.
Outcrop images from the first member of the FXG Formation in the Baimiaozi Section.
The locations of the pictures are shown in Fig 2. (A) Lenticular bioclastic limestone found in the massive marl layer of lower Unit B. (B) Breccia in the marl of Unit B. (C) Light purple lamellar limestone containing the spots of recrystallized calcite in Unit B. (D) Bioclastic limestone containing breccia, interlayered by marl in Unit B. (E) Yellow claystone found between the grainstone layers in Unit C. (F) Grainstone with wave-caused crossbedding and stylolite structure in Unit C. (G) Boundary (fault) between the grainstone of Unit C and oolite of Unit D. (H) Spheroids in Unit D yielded from the mudstone deposited in the wave trough of the top surface of the massive oolitic limestone. (I) Massive and thick bedded oolite in the upper part of Unit D.
Fig 4.
Thin-section microphotographs from the first member of the FXG Formation in the Baimiaozi Section.
(A) Bivalve shell fossils are located mainly in the lenticular bioclastic limestone interlayered in the marl layers of lower Unit B. (B) Microphotograph of lamellar limestone showing impregnated clay material; the outcrop spot is composed of bivalve fossils and the is surrounded by recrystallized calcite (indicated by the yellow arrow) in Unit B. (C) Another thin section showing the light purple lamellar limestone of Unit B in which strongly recrystallized calcites are common. (D) Breccia with an irregular edge (indicated by the yellow arrow) found in the marl in Unit B. (E) Thin section of bioclastic limestone containing many stylolites in upper Unit B. (F) Peloids and recrystallized ooids surrounded by a micritization enclosure (indicated by the yellow arrows) in the oolite of Bed 10, Unit D.
Fig 5.
Outcrop images from the upper part (Units D and E) of the first and the lower second members of the FXG Formation.
(A) The complete sequence of upper Unit D and Unit E located at the footwall of the reverse fault. (B) Interbedded lamellar limestone (microbialite) showing aborted microbial mounds and mudstone of the microbial mound base. (C) A rudiment of a microbial mound within the box defined by the dashed yellow line in Bed 12. (D) Horizontal structures of microbialites within the box defined by the dashed red line in Bed 12. (E) Wavy structures of microbialites in Bed 12. (F) Image of Dome 1 and 2 in the outcrop. (G) Image of Dome 3 in the outcrop. (H) Image of Dome 4 in the outcrop. (I) Gray mound body of Dome 6, and the overlying marl and mudstone of the lower second member of the FXG Formation. (J) Image of Dome 7 in the outcrop. (K) Image of Dome 8 in the outcrop. (L) Net-like stylolites developed in Dome 8. (M) Shelly fossils on the surface of the bioclastic limestone of the mound cap of Dome 8. (N) Image of Dome 9 and 10 in the outcrop. (O) Photograph showing the lower part of the second member of the FXG Formation.
Table 1.
Sizes and characteristics of the microbial mounds at BMZ.
Fig 6.
Thin-section photographs showing internal fabrics of microbial mounds.
(A) Thin-bedded oolitic limestone of mound base facies in Bed 12 containing (1) recrystallized bioclasts, (2) ooids wrapped within a micrite envelope, and (3) the micritic matrix. (B) Microbialite (i.e., lamellar limestone) of mound base in Bed 12 showing (1) irregular micrite lumps and (2) stylolites. (C–E) The cores of the mound bodies in Dome 6 and Dome 8. (F) Microbialite in the middle of the microbial mound body in Dome 7 showing the branching structure of microbial aggregates. (G) Stromatolitic structures in Dome 7. (H) Bioclastic limestone of mound cap facies in uppermost Bed 13 showing (1) gastropod shells and (2) ostracod fossils. (I) Bioclastic limestone of microbial mound cap in uppermost Bed 13 at Dome 6 containing (1) gastropods, (2) ooids, and (3) the micritic matrix.
Fig 7.
Conodonts from the lower Triassic microbial mounds at Baimiaozi, Beibei.
A–F, Hindeodus cf. parvus (Kouzr and Pjatakova, 1976). A, lateral view; B, lateral view; C1, upper view; C2, lateral view; D, upper view; E1, upper view; E2, lateral view; F, lateral view. G–J, Isarclcella staeschei Dai and Zhang, 1989. G1, upper view; G2, lateral view; H1, upper view; H2, lateral view; I1, upper view; I2, lateral view; J1, upper view; J2, lateral view. Each scale bar equals 100 μm.
Fig 8.
Conodonts from the lower second member of FXG Formation at Baimiaozi, Beibei.
A, B, Hindeodus n. sp. A. A1, B1, aboral view; A2, B2, lateral view. C, D, E, F, G, Hindeodus sp. Indeterminate. C1, D1, E1, G1, aboral view; F1, oblique lower view; C2, D2, E2, F2, G2, lateral view. H, Hindeodus parvus? (Kozur and Pjatakova, 1976). H1, aboral view, H2, lateral view. Each scale bar equals 100 μm.
Fig 9.
Conodonts from the lower second member of FXG Formation at Baimiaozi, Beibei.
A, Hindeodus cf. sosioensis (Kozur, 1996). A1, oblique lower view; A2, lateral view. B, Hindeodus n. sp. A. B1, aboral view; B2, lateral view. C, D, E, F, I, J, Hindeodus sp. Indeterminate. C1, D1, E1, F1, I1, aboral view; J1, oblique lower view; C2, D2, E2, F2, I2, J2, lateral view. G, H, Hindeodus parvus? (Kozur and Pjatakova, 1976). G1, aboral view; H1, oblique lower view; G2, H2, lateral view. Each scale bar equals 100 μm.
Fig 10.
Model of the evolution of microbial mounds in the Upper Yangtze carbonate platform at Baimiaozi.
(A) Decimeter-level microbial mounds initially grew at the base of the microbial mounds locations in a low-energy environment with low terrestrial input. (B) Massive terrestrial material transported to the shallow sea area, together with a rapid rise of sea level caused the failure of microbial growth. (C) In a normal shallow sea environment, the adequate supply of nutrients allowed microbes to flourish and microbial mounds became well developed. (D) A large number of microorganisms improved the Early Triassic marine environment via photosynthesis. (E)Metazoans of single species such as ostracods and gastropods that fed on the microorganisms thrived to a certain extent. (F) Rapid transgression caused hypoxia and superfluous inputs of terrigenous clay minerals into the deep water increased the turbidity; these events led to the demise of the microbial mounds. A large number of intact shells of dead organisms were found on the microbial mounds.