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Microfacies association D

Description. — Microfacies association D occurs at the base of Dujie section. It consists of bioclastic-lithoclastic packstone (D1) and bioclastic wackestone-packstone (D2), forming gray to dark-gray limestone. The bioclastic-lithoclastic packstone includes reworked colonial coral (Fig. 7a), solitary coral fragments, and scattered crinoids. Small lithoclasts ranging from 0.5 to 3 mm in size are recognized, along with crinoids, a few foraminifera, and reworked coral debris. The lithoclasts are subangular to subrounded and poorly sorted (Fig. 7b).

Bioclastic wackestone-packstone alternates with bioclastic-lithoclastic packstone in the lower part of the succession. Fossils in this facies are of low diversity. They are dominated by crinoid debris, foraminifers, brachiopod shells, and some broken ostracods (Fig. 7d). Rare coral fragments and lithoclasts occur. The matrix includes abundant peloids.

Interpretation. — The worn platform-derived bioclasts (e.g. reworked colonial coral) and sub-rounded lithoclasts in D1 suggest transport before deposition - widely found in platform marginal envrionment (Wilson 1975; Blomeier and Reijmer 2002; Flügel 2004). Microfacies types inferred is SMF6 (Flügel 2004). Microfacies D2 dominated by bioclasts including crinoids and foraminifers are indicative of an open-marine environment. The abundance of carbonate mud and the absence of coated grains suggests moderate hydrodynamic conditions. Thus, microfacies association D is inter-preted to be characteristic of a subtidal environment, likely below fair-weather wave base.

Microfacies association E

Description. — This microfacies association is composed of oncoid packstone-grainstone (D3),

Figure 7: Photographs of microfacies association D in Dujie section. (a) Field photograph of reworked colonial coral in bioclastic-lithoclastic packstone. (b) Photomicrograph of microfacies D1: crinoid (C), lithoclasts (L). (c) Field photograph of bioclastic wackestone-packstone. (d) Photomicrograph of bioclastic wackestone-packstone: crinoid (C), foraminifer (F), peloids (P).

aggregate-grain packstone-grainstone (D4), and peloid-ooid packstone-grainstone (D5).

The light-grey oncoid packstone-grainstone is characterized by well visible oncoids on weathered surfaces (Fig. 8a). Oncoids are locally abundant with various outlines and sizes ranging from 1 to 3 cm. Brachiopod shells and foraminifera are common nuclei of oncoid (Fig. 8b). Some microbial structures can be recognized in the oncoids. The matrix between the oncoids consist of diverse bio-clasts and small peloids. Biobio-clasts include common brachiopod shells, foraminifera, crinoids, and ostracods, along with some algal fragments.

Aggregate-grain packstone-grainstone comprise micritized aggregate grains (lumps) with sub-angular-subrounded shapes and abundant bioclasts including foraminifera, mollusc shells, crinoids, algae, rare corals, and Girvanella-like calcimicrobes. All skeletal grains exhibit micrite envelopes or biogenic encrustations (Fig. 8d). Peloids are common.

Peloid-ooid packstone-grainstone forms 30 to 50 cm-thick limestone beds. Cross-bedding is com-mon (Figs. 8e, f), displaying laminations rich in peloids and small ooids (Fig. 8f). Fossils are rare.

Interpretation. — The oncoids in D3 are common features in shallow-water environment with

Figure 8: Photographs of microfacies association E in Dujie section. (a) Field photograph of oncoid packstone-grain-stone: oncoid (red arrow). (b) Photomicrograph of oncoid packstone-grainstone. (c) Field photograph of aggregate-grain packstone-grainstone: aggregate-grain and coated grain (red arrow). (d) Skeletal grains with micrite envelopes (cortoids):

crinoid (C), foraminifer (F), micrite envelopes (ME). (e) Field photograph of peloid-ooid packstone-grainstone (lower part) with cross bedding (red arrow). (f) Photomicrograph of peloid-ooid packstone-grainstone with abundant ooids (red arrow).

periodic agitation (Flügel 2004). The presence of lumps in microfacies D4 suggest a shallow-marine environment with restricted or changing circulation (Flügel 2004). Cortoids from D4, the result of microboring by microbes, are common in shallow water, which suggest subtidal shoal environments with relatively higher hydrodynamic conditions (Bathurst 1966; Swinchatt 1969; Flügel 2004). The abundance of coated-grains, occurrence of cross-beds, and the reworking of oolitic limestones in MFA-E are indicative of a shallow and turbulent water in the photic zone, above the fair-weather wave base (Schauer and Aigner 1997; Flügel 2004; Qiao et al. 2016).

Microfacies association F

Description. — Six microfacies types were distinguished in microfacies association F: fenes-tral wackestone-packstone (D6), laminated peloid packstone (D7), peloid-intraclast packstone (D8), ooid-pisoid dolopackstone (D9), pisoid-peloid dolopackstone (D10), and laminated mudstone (D11).

Fenestral wackestone-packstone is characterized by fenestral fabrics (Figs. 9a, b) and a pelmi-critic framework. It is one important microfacies type of microfacies association F in Dujie section.

Agglutinated microbial peloids predominate. Fenestral fabrics are widespread (Fig. 9b). Fenestral wackestone-packstone commonly alternates with laminated peloid packstone or laminated mudstone with micro-tepees. No fossils except for some calcimicrobes can be recognized.

Laminated peloid packstone outcrops as thin layers, alternating with fenestral wackestone-pack-stone. This facies is dominated by peloids and calcimicrobes, cemented by micrite (Fig. 9b).

Peloid-intraclast packstone consists of light-gray, poorly sorted flat intraclasts of different sizes (0.5 - 20 cm) (Fig. 9c). The intraclasts include reworked microbial mats and laminated peloid pack-stone clasts. No cross-bedding occurs. Fenestral fabrics are abundant and occur as various shapes (Fig. 9d). Bioclasts are rare, except for a few calcimicrobes. In some beds, ooids, ranging from 0.2 to 1.5 mm with radial structure, can be observed, some of which are micritized.

Gray ooid-pisoid dolopackstone forms a 5 to 10 cm-thick interlayer in peritidal deposits. Pisoids ranging from 1 to 4 mm and broken ooids are main components (Figs. 10a, b). Mudstone clasts and broken ooids are the common nucleus.

Pisoid-peloid dolopackstone occurs as a very thin bed. An erosion surface has been identified at its base (Fig. 10a). This facies is dominated by dolomitic peloids ranging from 0.05 to 0.2 mm in size and some reworked pisoids (Fig. 10c).

Laminated mudstone forms light-gray or gray thin laminae (Fig. 10d). Micro-tepee structures developed in some beds (Fig. 10e). Fenestral fabrics and sheet cracks are widespread (Fig. 10f). No fossils are recognized in this facies.

Interpretation. — Microfacies association F with widespread fenestral fabrics, micro-tepee struc-tures, abundant microbial mat, intraclasts and virtually lacking fossils is interpreted as sediments deposited in a tidal flat environment (Hori et al. 2001; Lehrmann et al. 2001; Flügel 2004; Dalrymple and Choi 2007). Pisoids are abundant in microfacies D9 and D10, which are common constituents of the intertidal and supratidal members of shallowing-upward cycles (Purser and Loreau 1973; Assereto

Figure 9: Photographs of microfacies association F in Dujie section. (a) Field photograph of fenestral wackestone-pack-stone showing the abundant fenestral fabrics (red arrow). (b) Photomicrograph showing the alternation of laminated peloid packstone (red arrow) and fenestral wackestone-packstone (green arrow) with abundant fenestral fabrics (FS). (c) Field photograph of peloid-intraclast packstone with abundant light-gray intraclasts (red arrow). (d) Photomicrograph of peloid-intraclast packstone with abundant fenestral structures (FS) and intraclasts (red arrow).

and Folk 1976; Loucks and Folk 1976; Flügel 2004). The micro-tepee structures and sheet cracks in laminated mudstone are common, indicative of subaerial exposure in supratidal environments in a tropical to subtropical climate (Assereto and Kendall 1977; Kendall and Warren 1987; Schauer and Aigner 1997; Egenhoff et al. 1999; Flügel 2004). Thus, microfacies association F is considered to have formed during sea-level lowstand.

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