The fabric of microbialites refers to the size, shape and arrangement of their minerals. The variety of fabric features can reflect different hydrodynamic conditions, deposition rates, types of microbial communities, seawater temperature and saturation, and other environmental conditions (Turner et al. 1997). The fabrics of microbialites can be observed and described on different scales. At present, the classification of fabrics on mesoscale (e.g., polylobate, mesoclots, etc.) and microscale (e.g., peloids, filamentous microbial cements, spherulites, etc.) of Shapiro (2000) are widely used. The mesoscale fabrics are cm-sized and are visible to the naked eyes, and consist of mesoclots and the lighter-colored patches between them, or dark-colored laminae and the alternative lighter-colored laminae. The microscale fabrics (< 1 mm) are the microscopic petrographic characteristics of the mesoclots that can only be observed under microscopes. Based on mesoscale features, Riding (2011) divided microbialites into dendrolites, stromatolites, thrombolites and leiolites.
The study of fabrics on the microscopic scale is important to explore their genetic mechanism and forming environments. Laminated fabric and clotted fabric are the most common two types of fabrics. In addition to them, there are skeletal fabric, binding fabric and other types (Wu et al. 2018). From thin section examination, two main fabrics are identified: a) the laminated and clotted fabrics, which are the most common and variable features in this formation, and b) skeletal fabrics (Fig. 2). Researchers have made preliminary studies on the microbialites in this formation (Song et al. 2012; Huang et al. 2016), but they are all on mesoscale, lacking detailed description and classification of microscale fabrics. In this study, the features and classification of microscale fabrics are detailed, the clotted fabrics are further divided into four subtypes: vermicular clotted fabric, reticular clotted fabric, spotted clotted fabric and clumpy clotted fabric, and the laminated fabric is subdivided into two subtypes: wavy lamination and compound lamination.
5.1 Clotted fabric
Clot is the basic component of thrombolites, being dark in color, having distinct and irregular edges, and is composed of micrite or dolomicrite. It was considered to be the framework of the thrombolites (Aitken 1967), and formed from microbially-induced calcification or sediment-trapping. Clots can be divided into mesoclots and microclots based on scales. In the past 50 years, the studies on ancient microbialites focused on the interpretation and description of mesoclots. According to morphology, Du (1992) divided mesoclots into porous clots, mottled clots and reticular clots. Riding (2000) divided the thrombolite into seven categories in terms of their genesis, including calcified microbial thrombolite and coarse agglutinated thrombolite. Harwood Theisen and Sumner (2016) classified Cambrian thrombolites in the Great Basin into eight types, including hemispherical microdigitate, bushy, coalescent columnar and massive fenestrated.
Except for the description of mesoscale characteristics of microbialites in the Xiaoerblak Formation, the microscale features are the prime focus in this paper. It is worth noting that the term ‘clotted fabric’ used in this article refers in particular to microclot fabrics. Our microscopic examination reveals that the thrombolitic microclots in the Xiaoerblak Formation is internally composed of dolomicrite or powder crystalline dolomite, and are in different shapes (e.g., vermicular, elliptical, irregular, reticular, etc.). The light-colored material between microclots is composed of fine dolomites, medium dolomites, or coarse dolomites, and are called bright spots. The bright spots have relatively smooth edges, and are distinct from the dissolution pores which generally have bay-shaped outlines, and may represent the primary pores formed in the growth stage, but filled with cement during the diagenesis stage. The primary pores can have two origins: the pores left by the microbes that had functioned as builders during the formation of the microbialites, but decomposed after their death, and the remaining voids among the microclots. The former type is usually small in size and require microscope study. On the basis of the morphology of microclots, the clotted fabrics are further classified into the four fabric subtypes: vermicular clotted fabric, reticular clotted fabric, spotted clotted fabric and clumpy clotted fabric.
5.1.1 Vermicular clotted fabric
The vermicular microclots are expressed by an undirected arrangement of worm-like dark clots in the bright matrix (Fig. 7a, b, c). They are irregularly curved, and are composed of dolomicrites. In this paper, three parameters are used here to constrain the morphology of the microclots, which are length, width and angle of curvature, and we take the total length of the straight segments that comprise a sinuous microclot as its summed length. The length of the vermicular microclots in the thrombolites of this formation varies a lot, ranging from 0.1 mm to 0.8 mm with no conspicuous patterns, while the width is relatively uniform, ranging between 0.1 mm and 0.15 mm. Most of the worm-like microclots are slightly curved with the bending angle concentrated at 150°–170°, even some seem like straight rods. All of the vermicular microclots are disorderly distributed within the spar-filled groundmass, most being evenly isolated spacing at an average distance of less than 0.5 mm, but few contacted with each other producing a chaotic area which made the vermicular outline blurred. The cement among the microclots is usually made up of two kinds of dolomites: one is the fine–medium crystalline hoary dolomites. These dolomites are distributed around the microclots usually in irregular shape and showing light-gray shadow on the surface. The other type is the medium to coarse dolosparite which are close to the hoary dolomites and far away from microclots. Bright dolosparite often forms a regular rhombic shape, and is larger than hoary dolomites, which hints at the existence of enough space to grow. There are also some voids between some dolosparite, and they are considered to be the result of burial dissolution for special bay-shaped outlines. Overall, the area ratio of the dolomicrites to the cements almost reaches 1:1. The special arrangement of microclots and cements naturally produces a gradual transition of color from dark to bright. However, due to recrystallization, some microclots have been altered from black dolomicrite to yellowish-brown powder crystalline dolomite, making the edge of the vermicular microclots blurred and difficult to separate from the surrounding cements. The vermicular fabric mainly occurs in the Interval IV associating with the reticular clotted fabric, spotted clotted fabric and even laminated fabrics.
5.1.2 Reticular clotted fabric
In this fabric the microclots are in irregularly reticular shape, and are composed of dark dolomicrites or fine dolomites (Fig. 7d, e, f). The reticular clotted fabric is the main subtype of the clotted fabric in the Xiaoerblak Formation, and consists of two parts: the meshes and the fillings. The meshes consist of dolomicrite, and have a width of about 0.01 mm. The fillings in the meshes are composed of fine, medium or coarse crystalline dolomite, and are in irregular shape. The diameter of the fillings ranges from 0.01 mm to 0.5 mm. This kind of fabric mainly occurs in Interval II, Interval IV and the lower part of Interval V. The reticular clotted fabric in Interval II is mainly composed of fine dolomite, and the fillings are mainly medium dolomite, with a diameter of 0.05–0.13 mm (0.1 mm on average). The reticular clotted fabric in the Interval IV and the lower part of Interval V is mainly composed of dolomicrite, and the fillings are mainly fine dolomite, with average diameter of 0.05 mm. It often appears in combination with the spotted clotted fabric and vermicular clotted fabric. The reticular clotted fabric in the middle part of Interval V mainly exists in the bright laminae of the laminated fabric, with an average diameter of 0.1–0.5 mm. The reticular clotted fabric has various forms in this interval, but its content is limited, about 20%–30%, gradually decreasing upwards.
5.1.3 Spotted clotted fabric
In this microfabric, the microclots are spot-like and in dark color, composed of dolomicrite or powder crystalline dolomite (Fig. 8a, b), and have a diameter of about 0.01 mm. Some microclots are scattered and isolated, at an average distance of more than 0.05 mm, some are gathered in clusters, and are about 0.05 mm in diameter or larger. The spotted microclots usually have unobvious edges, and are cemented by recrystallized powder and fine crystalline dolomites or spars. The spotted microclots in this formation may have two origins. Firstly, they can be a kind of original microbial microclots formed by microbially-induced precipitation of coccoidal aggregates during their metabolic activities (Riding 2000). Secondly, they may be the remaining parts of previous vermicular or reticular microclots due to recrystallization. Under the second circumstance, the spotted microclots are generally surrounded by fine or medium dolomites, and the trace of dolomicrite left by the original microclots is preserved in the fine or medium dolomite. The spotted fabric is mainly distributed in the Lower Member and the microbial mounds in Interval IV. Its content is limited to only about 10% due to its small volume and alteration by strong diagenesis. It is not the main fabric of the microbialites in this formation, and often occurs in combination with the vermicular clotted fabric, reticular clotted fabric and laminated clotted fabric.
5.1.4 Clumpy clotted fabric
In this microfabric, the microclots are in clumpy shape, are composed of dark dolomicrite, and vary in size, ranging in diameter from 0.5 mm to 1.5 mm. The dolomicrite of the clumpy microclots are mostly recrystallized into powder crystalline dolomite, and only the black edges are preserved. The interspaces between the clumpy microclots aerially account for 20% to 25%, and have been filled by coarse dolomite (Fig. 8c, d). Some clumpy microclots are isolated and scattered, with smooth edges, and others are in close contact, and can reach 70% in content, with sharp outlines. The transition from angular irregular clumpy microclots to round grains is present in some thin sections, indicating that some clumps were broken into grains by water actions. This fabric is limited in distribution in the Xiaoerblak Formation, and is not continuous in vertical distribution. Most clumpy microclots have been fragmentized. This kind of fabric occurs in the middle and lower part of Interval V, generally in association with intraclastic dolostones. The content of the clumpy microclots decreases upward the formation, coupling with the increase in the content of intraclastic grains.
5.2 Laminated fabric
The laminations generally have two forms: the straight lamination and the undulate lamination. The former is straight and generally formed by inorganic sedimentation. The undulate lamination, is irregularly curved, and is probably formed by precipitation of microbial mats. The laminations discussed below are all microbiogenic laminated fabric.
The laminated fabric refers to the alternation of the dark laminae composed of dolomicrite and the bright laminae composed of medium or coarse dolomite. Examination of all thin sections shows that the laminated fabrics in the Xiaoerblak Formation are all dolomicrites lacking calcified fossils, and are called dolomicritic laminated fabrics. It is noteworthy that the lighter colored rocks between the laminae are composed of clotted thrombolitic dolostone of powder to medium crystalline dolomite. The dolomicritic laminae are generally composed of dolomicrite, dark in color, and are wavy in shape. Based on the features of the dolomicritic laminae, the laminated fabrics are divided into two subtypes, the wavy laminated fabric, and the compound laminated fabric (Fig. 9). The compound lamination is the main type, and can account for more than 70%.
The wavy laminated fabric consists of thin dark-colored dolomicritic laminae and the alternating lighter-colored thrombolitic layers that are composed of powder, fine or medium crystalline dolomite (Fig. 10a, b, c). The dark laminae vertically contact with each other, some being imbricate, with thrombolitic fabrics filling the bright layers between the dolomicritic laminae. Comparing with the compound laminated fabric, the laminae in the wavy laminated fabric tend to be more curved with a wide range but mostly have poor continuity, with a length of less than 2 mm laterally. The thickness of a single dark lamina is less than 0.2 mm, while the clotted layers vary greatly in thickness, and are generally thicker than the dark laminae, ranging from 0.5 mm to 2 mm. There are numerous irregular and elongated pores filled with microspars or coarse dolomite in the bright layers or even lying near the dark laminae, which can be formed from dissolution and crystallization. The wavy laminated fabric is a rare type in the Xiaoerblak Formation, and mainly occurs in the bottom of the Interval V with low content.
The compound laminated fabric (Fig. 10d–i) consists of dark-colored dolomicritic laminae and the alternative lighter-colored thrombolitic layers between the laminae. The dark lamina can be considered as a kind of aggregation of several singular thin dark laminae. The dark laminae are slightly curved, 0.8–1.2 mm in thickness with single lamina less than 0.1 mm in thickness, usually thicker than the dark laminae in the wavy laminated fabric, and have better continuity. The bright layers are generally greater than 2 mm in thickness, and are composed of powder to fine crystalline dolomites, sometimes containing reticular, vermicular or spotted clotted fabric, and having bright spots composed of microspar and fine dolomite. There are also voids in the dark laminae, which are usually filled with powder crystalline dolomites. It is noted that the bright layers always contain more pores than the dark laminae, indicating the difference between biomineralization and inorganic mineralization. The compound laminated fabric is the dominated laminated type in the Xiaoerblak Formation, and mainly occurs in the Interval V, and in the Interval III.
5.3 Skeletal fabric
This microfabric has a framework consisting of Renalcis skeletons (Fig. 11a, b). The Renalcis skeletons are sparsely and irregularly distributed, with a size of (0.5–1 mm) × (0.5–2 mm), and consist of a string of hollow spherical chambers. Each chamber has a flower-like cavity, which is generally filled with coarse dolomite. The high variability in size demonstrates that the chamber is not an individual cell but a colony of numerous cells. The dark wall of the cavity comprises dense dolomicrite with distinct smooth exterior and crinkled blurred internal surface which is presumably inheritance of gelatinous colony morphology. To date, the systematic position of Renalcis is still uncertain. Some researchers considered it a calcified cyanobacterium related to chroococcaceae which experienced carbonate diagenesis involving obliteration of cell morphology (Hofmann 1975; Pratt 1984). Others suggested that it is formed by bacteria-induced precipitation (Stephens and Sumner 2002). This fabric is distributed in the upper part of Interval V. Since it contains fragments of Renalcis skeletons, and the overlying and underlying layers develop intraclastic fabric, this fabric is likely formed in shallow subtidal zone with relatively high water energy.