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The fabrics and origins of peloids immediately after the end-Permian extinction, Guizhou Province, South China
Sedimentary Geology 164 (2004) 161 – 178 www.elsevier.com/locate/sedgeo
The fabrics and origins of peloids immediately after the end-Permian extinction, Guizhou Province, South China Natsuko Adachi a,*, Yoichi Ezaki a, Jianbo Liu b a
Department of Geosciences, Graduate School of Science, Osaka City University, Sugimoto 3-3-138, Sumiyoshi, Osaka 558-8585, Japan b School of Earth and Space Sciences, Peking University, Haidian, Beijing 100871, People’s Republic of China Received 12 December 2002; received in revised form 2 September 2003; accepted 8 October 2003
Abstract Peloids are major contributors to modern and ancient limestones, although their origins are not yet comprehensively understood. Upper Permian to Lower Triassic carbonate successions are well preserved at the Bangeng area in Guizhou Province, South China, where microbialites (thrombolites), coccoidal microbes and peloids are preserved together. These peloids are classified into three types (Peloid-A, B, C) that typically occur together along with coccoidal microbes and restricted bioclasts in a variety of microenvironments. Peloid-A is most common and exhibits close relationship with coccoidal microbes. Peloid-A1 has spherical grain and a size range typical for coccoidal microbes (10 – 60 Am in diameter), whereas Peloid-A2 is marked by larger spherical and subrounded grains with diffuse margins, ranging from 70 to 200 Am in diameter. Peloid-A1 is in part produced by the complete micritic filling in coccoidal microbes by their metabolic activities, whereas Peloid-A2 is related to the calcification of a colony of coccoidal microbes by their metabolic activities, or simply to the aggregation of individual peloids and coccoidal microbes. Peloids of this type might also originate from other carbonate precipitation processes. Peloid-B is derived merely by micrite filling in ostracodes and gastropods skeletons, whereas Peloid-C is of micritized-bioclast origin. These peloidal origins do not explain those of all environments and ages. Peloids are polygenetic in origin, irrespective of the presence of coccoidal microbes. However, it is emphasized that the peloids formed after the end-Permian extinction are dominated by microbes related ones. Those kinds of peloids might have occurred repeatedly and predominantly, especially in severely deteriorated environments such as those after the end-Permian mass extinction. D 2003 Elsevier B.V. All rights reserved. Keywords: Carbonate; Extinction; Microbe; Microbialite; Peloid; Permian – Triassic
1. Introduction Peloids are among the most important constituents of shallow-marine carbonate sediments in both modern and ancient environments. Peloids are spherical,
* Corresponding author. Fax: +81-6-6605-2522. E-mail address: [email protected] (N. Adachi). 0037-0738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2003.10.007
ellipsoidal, or angular grains, composed of microcrystalline carbonate, but with no internal structure (Tucker, 2001). Peloids have various origins (e.g., Flu¨gel, 1982; Macintyre, 1985; Tucker and Wright, 1990), and the following hypotheses have been proposed; (1) fecal pellets (e.g., Land and Moore, 1980), (2) micritized grains (e.g., Bathurst, 1971), (3) intraclasts (e.g., Fa˚hraeus et al., 1974), (4) a precipitate origin (e.g., Macintyre, 1985), and (5) a microbially mediated
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precipitate (e.g., Chafetz, 1986; Reitner, 1993). Evidence supporting an origin related to microbial activity has been documented for modern peloids (Monty, 1976; Chafetz, 1986; Reitner, 1993; Kaz´mierczak et al., 1996). Monty (1976) and Kazmierczak et al. (1996) have observed peloids associated with cyanobacterial mats. In addition, Chafetz (1986) has shown that the nuclei of marine peloids originated within and around active clumps of bacteria. In contrast, Reitner (1993) detected no bacterial remains in peloids and concluded that the cores of marine peloids were formed within organic mucilage. Moreover, in the geological record peloids occasionally occur as an irregular, dome-shaped micritic coating on bioclasts or fill confined cavities. The fabrics are inferred to result from in situ growth products generally related to microbial activity (e.g., Reid, 1987; Sun and Wright, 1989; Neuweiler, 1993). Coniglio and James (1985) reported early Palaeozoic peloids derived from the breakdown of the filamentous calcified cyanobacterium Girvanella, whose open spaces were filled in with micrite. Skeletal reef-building organisms apparently disappeared for a lengthy interval following the end-
Permian mass extinction, during which microbialcarbonate buildups predominated (e.g., Sano and Nakashima, 1997; Lehrmann et al., 1998, 2001, 2003; Lehrmann, 1999; Kershaw et al., 1999, 2002; Ezaki et al., 2003). However, peloids continued on as common constituents, even in limestones, immediately after the extinction episode. Peloids in this particular geological interval are thought to have had their origins minimally influenced by skeletal organisms and are therefore of special interest. Carbonate sequences after the end-Permian extinction are well preserved in the Bangeng area of Guizhou Province, South China (Lehrmann et al., 1998, 2001, 2003; Lehrmann, 1999), where microbialites (thrombolites), coccoidal microbes as well as peloids are typically present. These limestones are important for understanding the genetic relationship between microbes and peloids. Therefore, we examined in great detail the peloidal fabrics from the Tianwan section in the Bangeng area, in order to elucidate peloid origins, using both optical and scanning electron microscope (SEM). We first describe the features (fabrics) of peloids as they relate to microenvironments, including the
Fig. 1. Index map showing the study area at the Tianwan section in the Bangeng area of Guizhou Province, South China (modified from Lehrmann et al., 2003).
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cryptic spaces, intraskeletal cavities and matrices within both microbialites and skeletal limestones. Special attention is paid to the interrelationship of peloids and coccoidal microbes. We then classify peloids and summarize their distribution. Finally, we discuss plausible origins for each type of peloids.
2. Geological setting Permian – Triassic shallow-marine successions are well preserved in the Bangeng area in Guizhou Province, South China (Lehrmann et al., 1998, 2001, 2003; Lehrmann, 1999). The study section is located at Tianwan, about 25 km north of Luodian, south Guizhou (Fig. 1). This area was situated in the interior of an isolated carbonate platform (Great Bank of Guizhou in the Nanpanjiang Basin sensu Lehrmann et al., 1998) during Triassic time (Fig. 1). A wide variety of carbonate facies, representing different sedimentary settings, accumulated to a great thickness in this area. Lehrmann et al. (1998) and Lehrmann (1999) first remarked on the intercalations of Early Triassic calcimicrobial mounds. Strata that span the Permian/Triassic (P/T) boundary in the Tianwan section are briefly described below, in ascending order (Fig. 2). Uppermost Permian limestones are characterized by medium- to thick-bedded skeletal grainstone that includes abundant skeletal organisms such as calcisponges, fusulinids (Palaeofusulina), smaller foraminifers, bryozoans, rugose corals, echinoderms, and dasycladacean algae. They are overlain by alternating beds of microbialites and skeletal limestones. The latter include bioclasts of thin-shelled bivalves, gastropods, ostracodes, serpulid worms, echinoderms, and smaller foraminifers, which exhibit a reduced biotic diversity. Microbialites are marked mostly by clotted fabrics (thrombolites). Thrombolite beds range from a few tens of centimeters to several meters in thickness, in places forming a distinct domal structure that is buried laterally by skeletal limestone. The alternating beds of thrombolites and skeletal limestone are overlain by thinly bedded, argillaceous, lime mudstone and mudstone. The conodonts Clarkina changxingensis, C. subcarinata, C. deflecta, and Hindeodus typicalis that are indicative of the C. changxingensis Zone of Late
Fig. 2. Stratigraphic column across the Permian/Triassic boundary in the Tianwan section, occurrences of conodonts and conodont biozones.
Permian age, were discovered in skeletal grainstone immediately above the lowermost thrombolite bed (Fig. 2). Hindeodus parvus that is diagnostic of the very earliest Triassic H. parvus Zone, was first detected in skeletal packstone 3.4 m above the lowermost microbialite, whereas Isarcicella isarcica and I. staeschei of the second conodont zone of the Triassic I. isarcica Zone, were then found in skeletal grainstone about 8 m above. The only conodont Hindeodus sp. was recovered from thinly bedded, argillaceous, lime mudstone. From a biostratigraphic point of view, the P/ T boundary is defined only by the first appearance of H.
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parvus (Fig. 2). The age of microbialites in the study section is provisionally considered to range from latest Permian to earliest Triassic.
3. Methods This study is based on field observations and laboratory analysis of polished slabs and thin sections. Oriented specimens, cut perpendicular to bedding (along a growth direction), were then polished and prepared for making thin sections. Peloids and coccoidal microbes in particular were observed in detail by
using optical microscope and SEM. The results from all observations were compared, in order to thoroughly describe the fabrics and discuss their origin. The size of the peloids was measured and graphed to learn their relationship to microenvironments. About 260 large thin sections (5 7.5 cm) and auxiliary small thin sections (2.8 4.6 cm) were prepared for opticalmicroscope analysis. The thin sections used for SEM observations were relatively thick. They were polished with 1 Am diamond dust and etched for 25 seconds with 2% formic acid prior to coating with gold. SEM observations were made at 17 kV with a JEOL JSM5500 in Osaka City University.
Fig. 3. Mesoscopic and microscopic features of thrombolites. (a) Polished surface of a thrombolite, which exhibits clotted mesostructure. Bright gray colour corresponds to mesoclots (large arrow), whereas dark gray corresponds to cryptic spaces and matrices (small arrows). Scale bar = 2 cm. (b) Detail of area in (a) (black rectangle). Sparitic parts are framework of thrombolites, which are composed by small clots (white arrows). For full details, see (c). Large black arrows show cryptic spaces, whereas small black arrows show matrices containing some bioclasts. Scale bar = 1 cm. (c) Spherical to ellipsoidal small clots are amalgamated with each other. Some clots are fringed with coccoidal microbes (white arrow). Scale bar = 0.25 mm. (d) Spherical and ellipsoidal coccoidal microbes with dark micritic envelopes are partly preserved at the framework, as shown by black arrows. Scale bar = 0.2 mm.
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4. Constituents and microenvironments 4.1. Thrombolites The term thrombolite was coined by Aitken (1967) and has been used by later workers (e.g., Kennard and James, 1986; Burne and Moore, 1987; Riding, 2000). Shapiro (2000) recently revised the definition as follows: a thrombolite is a microbialite composed of a clotted mesostructure. As seen in mesoscopic (polished hand-specimen) and optical-microscopic views, most parts of thrombolites exhibit mesoclots (Fig. 3a,b). They are composed of millimeter- and centimeter-size sparitic masses that have a sporadic distribution. Round to ellipsoidal small clots, 100– 500 Am in diameter, are present in places, and they have a darker rim at their periphery and are filled in with mosaic calcite and dolomite (Fig. 3c). Small clots are fringed in part with coccoidal microbes (Fig. 3c) and are merely due to closely packed coccoidal microbes. These small clots amalgamate with each other to various degrees to form frameworks of thrombolites with irregularly outlined fenestral fabrics (cryptic
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spaces) that are filled with peloids in their lower halves (Figs. 3b and 4a). Thrombolites in some cases grade laterally and vertically into skeletal limestone. 4.2. Coccoidal microbes Spherical and ellipsoidal structures with micritic envelopes are similar to those that have been reported in the earliest Triassic microbialites from the Huaying area of Sichuan Province, South China (Ezaki et al., 2003). Those cellar structures are regarded as calcified coccoid microbes. Coccoidal microbes are scattered and/or clustered, especially within frameworks of thrombolites (Fig. 3d). They are also present in cryptic spaces, intraskeletal cavities and matrices of thrombolites. Coccoidal microbes range in diameter from 10 to 60 Am, but 80 % fall into the 20– 40 Am range, and showing good sorting (Fig. 5). Each coccoidal microbe is surrounded by a micritic envelope (on average 2 –5 Am thick) and contains a hollow space 10 – 20 Am in diameter that is filled in with sparitic calcite and dolomite (Figs. 3d and 6c –f). Seen under an SEM, the envelope consists of fine micrite, and some
Fig. 4. Schematic diagram showing different types of microenvironments (cryptic spaces, intraskeletal cavities and matrices) in both microbialite (a) and skeletal limestone (b).
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prise densely accumulated clots, amalgamations of these, and coccoidal microbial remains. Cryptic spaces are formed within thrombolite frameworks and are small, laterally elongate cavity spaces up to 5 mm wide (Figs. 3b and 4a), whereas interframeworks of thrombolites contain intraskeletal cavities and matrices. In skeletal limestones, intraskeletal cavities, and matrices are identified (Fig. 4b). Intraskeletal cavities refer to sheltered spaces within individual bioclasts and/or beneath reworked skeletal debris related with thin shelled bivalves and gastropods (Fig. 4a,b).
5. Features of peloids
Fig. 5. Size frequency distribution of coccoidal microbes.
of the micritic envelopes may show one or more concentric or eccentric laminae (Fig. 7c – f).
Peloidal sediments are ubiquitous within thrombolites and skeletal limestones, and peloidal fabrics are described below, arranged by microenvironments (cryptic spaces, intraskeletal cavities and matrices). 5.1. Microbialites
4.3. Peloids Peloids are generally micritic spherical and ellipsoidal grain. Although micrite has crystal sizes of 1 –4 Am (Folk, 1959), it as used here includes crystals as large as 5 – 10 Am in diameter as seen under an SEM. If the coccoidal microbes are completely filled in with micrite, they are treated as peloids. Detailed features are mentioned in following section. 4.4. Microenvironments Various microenvironments are recognized within microbialites (thrombolites) and skeletal limestones, and these include cryptic spaces, intraskeletal cavities, and matrices (Fig. 4). Thrombolite frameworks com-
Thrombolites are overwhelmingly present and are roughly divided into sparitic parts and micritic matrices (s.l.). The sparitic parts further consist of frameworks and cryptic spaces, whereas the micritic parts are composed of bioclasts that contain intraskeletal cavities, and matrices (s.s.) (Fig. 4a). 5.1.1. Cryptic spaces Cryptic spaces are mostly filled in with peloids in their lower parts and exhibit a geopetal texture (Fig. 6a –c,e). Each peloid is most distinct in outline within the upper part of the geopetal-sediment filling, whereas its outline becomes indistinct in the lower part of the sediment filling, owing to compaction into peloidal micrite (Fig. 6g).
Fig. 6. Photomicrographs of peloidal fabrics in cryptic spaces of thrombolites. (a) The aggregations of small clots form the framework, and cryptic spaces are filled in with peloidal sediment. Scale bar = 1 mm. (b) Detail of area in (a) (white rectangle). Peloids show diffuse margins and are well sorted. White rectangle is depicted in Fig. 7a and black rectangle is depicted in Fig. 7b. Scale bar = 0.5 mm. (c) Cryptic spaces are filled in with well-sorted peloidal sediment. Scale bar = 0.5 mm. (d) Detail of area in (c) (white rectangle). Peloids, which have comparatively welldefined margins and dense micritie, together with coccoidal microbes are clustered (black arrow). See also Fig. 7c. Scale bar = 0.25 mm. (e) Cryptic spaces are filled in with variously sized peloids (arrow). Small white rectangle is depicted in Fig. 7d and black rectangle in Fig. 7g. Scale bar = 1 mm. (f) Detail of area in (e) (large white rectangle). Coccoidal microbes are scattered among peloids and/or are included within peloid (black arrow). Scale bar = 0.25 mm. (g) Outlines of peloids are clearer in upper parts of cryptic spaces (black arrow), whereas its fabrics are altered into peloidal micrites in lower parts, owing to close packing (white arrow). Scale bar = 1 mm. (h) Peloids are amalgamated in part with neighbouring peloids, and exhibit a clotted texture. Scale bar = 0.5 mm.
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Peloids in most cryptic spaces are extremely wellsorted, with 80 % ranging from 20 to 60 Am in diameter and averaging 51 Am (Fig. 8a). This size range of peloids is similar to that of coccoidal microbes (Fig. 5). These peloids are mostly spherical in shape. These peloids may partly amalgamate to produce clotted fabrics (Fig. 6h). Isolated and/or clustered coccoidal microbes are scattered among the peloids. Some of the peloids show somewhat distinct margins and comparatively dense micrite, and look like coccoidal microbes in size and shape (Fig. 6c,d). In other cryptic spaces, peloids are poor-sorted, with 25% ranging from 20 to 60 Am in diameter and averaging 90 Am (Fig. 8a). These peloids are spherical to ellipsoidal in shape. Isolated and/or clustered coccoidal microbes are included within each peloid, and are scattered among the peloids (Fig. 6f). Pyrites occur as raspberry-like, spherical aggregates of equigranular crystals (framboidal pyrite) and as single euhedral or subeuhedral crystals from 1 up to 100 Am in diameter. Tiny framboidal pyrites are closely aggregated in part. Similar pyrites are also present in intraskeletal cavities and matrices of thrombolites and skeletal limestones. 5.1.2. Intraskeletal cavities Intraskeletal cavities made up of thin-shelled bivalves and gastropods, are mostly filled in with peloids, micrite and rare bioclasts, and ordinarily exhibit a geopetal texture (Fig. 9a). Peloids range in diameter from 30 to 150 Am, averaging 71 Am (Fig. 8b). Each peloid is spherical to ellipsoidal with diffuse margins. Peloids occasionally exhibit clotted textures by becoming amalgamated with each other. Intraskeletal cavities are rarely filled in with well-sorted peloids, as seen in the cryptic spaces. Coccoidal
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microbes are interspersed among the peloids and in places are contained within the peloids (Fig. 9b). 5.1.3. Matrices Matrices are filled in with peloids and micrite. Peloids are so closely packed in most cases as to form peloidal micrites, where the outlines of the peloids are not clearly discernible. Each peloid with spherical to ellipsoidal in shape ranges from 40 to 180 Am in diameter, with an average of 90 Am (Fig. 8b). Coccoidal microbes are rarely scattered among the peloids. Comparatively large peloids with spherical to ellipsoidal shapes, 200 – 700 Am in diameter, are sometimes closely packed together with ostracodes (Fig. 9c). These peloids have distinct margin, and some of them preserve the remains of ostracodes carapaces at their peripheries. 5.2. Skeletal limestone Skeletal limestones (skeletal grainstone and packstone) include low-taxonomic-diversity bioclasts of ostracodes, bivalves, gastropods, serpulid worms, echinoderms, and smaller foraminifers. Coccoidal microbes seen in microbialites are absent here, although peloids are still abundant. 5.2.1. Intraskeletal cavities The peloids in intraskeletal cavities that formed within bivalves or gastropods ordinarily have a geopetal texture (Fig. 9d,e). The outline of each peloid is more clearly defined in the upper parts of the geopetal sediment infilling. The peloids are spherical to ellipsoidal in shape, ranging from 20 to 300 Am in diameter, averaging 90 Am (Fig. 8c). Sorting is poor compared with peloids in cryptic spaces within thrombolites.
Fig. 7. SEM images of peloidal fabrics. (a) SEM photomicrograph of area in Fig. 6b (white rectangle). Each peloid consists of fine crystals (2 – 6 Am in diameter) (arrows) that are surrounded by coarser crystals of subhedral and equant. Scale bar = 50 Am. (b) SEM photomicrograph of area in Fig. 6b (black rectangle). Each peloid consists of a cluster of calcite crystals smaller in the centre, but becoming progressively larger outward (arrow). Scale bar = 50 Am. (c) SEM photomicrograph of area in Fig. 6d (arrow). Spherical peloids (small arrows) occur together with coccoidal microbes (large arrows). Peloids consist of fine calcite, whereas coccoidal microbes consist of coarser calcite rimmed by fine calcite (1 – 4 Am in diameter). Scale bar = 50 Am. (d) SEM photomicrograph of area in Fig. 6e (white small rectangle). Each peloid preserves remains of coccoidal microbes (arrows). Black arrow shows a coccoidal microbe filled with mesh-like microdolomite. Scale bar = 100 Am. (e) Detail of area in (d) (large white arrow). Coccoidal microbe with concentric laminae. Scale bar = 20 Am. (f) Detail of area in (d) (two small white arrows). Coccoidal microbes are composed of coarser calcite in the centre and may be further surrounded by micrite. Scale bar = 20 Am. (g) SEM photomicrograph of area in Fig. 6e (small black rectangle). Many peloids have irregular shapes. They consist of fine, spherical and subhedral crystals (3 – 10 Am in diameter) (black arrow) and are surrounded by coarser calcite (white arrow). Scale bar = 100 Am. (h) SEM photomicrograph of area in Fig. 9e (black arrow). Each peloid shows an irregular shape and consists of comparatively coarser calcite (arrow). Scale bar = 50 Am.
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Fig. 8. Frequency distribution of peloid diameter. (a) Cryptic spaces in the different thrombolite samples (cryptic spaces-1, -2). Note size distribution of coccoid microbes and Peloid-A. (b) Intraskeletal cavities and matrices in thrombolites. Note size distribution of coccoid microbes and Peloid-A. (c) Two different microenvironments (intraskeletal cavity and matrix) within skeletal limestone. Note size distribution of coccoid microbes and Peloid-A, B and C.
5.2.2. Matrice Matrices are filled in with peloids, micrite and sparitic calcite (Fig. 9d). Peloids within matrices sometimes grade into peloidal micrite, due to being densely compacted. These peloids are spherical to ellipsoidal in shape, ranging from 40 to 500 Am in diameter with an average of 160 Am, indicating poor sorting (Fig. 8c). Relatively large peloids with spherical to ellipsoidal shapes, ranging from 200 to 500 Am
(rarely 700 Am) in diameter, are characteristically present, where ostracodes carapaces and/or gastropods shells filled in with sparite and micrite are present. The peloidal occurrence is the same as that in matrices of microbialites (Fig. 9c). Cortoids formed by the micritization of bioclasts, such as bivalves, are found. Densely micritized peloids, 150– 400 Am in diameter, are also present (Fig. 9f). Those peloids are distinct in margin and spherical to ellipsoidal in shape.
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Fig. 9. Photomicrographs of peloidal fabrics in intraskeletal cavity (a, b) and matrices (c) of thrombolites, and intraskeletal cavity and matrices of skeletal limestone (d – f). (a) Intraskeletal cavity is filled in with peloidal sediments (lower part) and calcite cements (upper part). Scale bar = 1 mm. (b) Detail of area in (a) (black rectangle). Coccoidal microbes are found abundantly among and within peloids (arrow). Scale bar = 0.25 mm. (c) Ellipsoidal peloids with a wide size range (200 – 700 Am in diameter). Some peloids still preserve ostracodes carapaces (arrow). Scale bar = 1 mm. (d) Thin-shelled bivalves are abundant. Matrix is mostly occupied by peloids and micrite. Scale bar = 5 mm. (e) Detail of area in (d) (white rectangle). Intraskeletal cavities are filled in with peloidal sediments (lower part) and calcite cement (upper part). Black arrow is depicted in Fig. 7h. Scale bar = 0.5 mm. (f) Photomicrograph showing micritized bioclasts, cortoids (arrows), and peloids which are made up of dense micrite and have definite margin. Scale bar = 1 mm.
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6. Types of peloids and their distributions The peloids described above can be classified on the basis of their fabric (size, shape and marginal features), mode of occurrence, and the presence or absence of coccoidal microbes. Three types of peloids can be distinguished: Peloid-A, B, and C. Peloid-A is the most common type in microbialites and skeletal limestones. It ranges from 20 to 200 Am in diameter, and is divided into subtypes A1 and A2, based on size. Peloid-A1 falls within the size-range characteristic of coccoidal microbes. Peloid-A1 is further subdivided into Peloid-A1.1 (Figs. 6c,d and 7c) and A1.2 (Figs. 6a –c and 7a,b), based on the presence or absence, respectively, of coccoidal microbes and their identifying microscopic features. Peloid-A1.1 is distinguished from Peloid-A1.2 by having distinct margin and comparatively dense micrite. Peloid-A2 is characterized by spherical and ellipsoidal grains with diffuse margin, and it differs from Peloid-A1 by its larger size (70 – 200 Am). Peloid-A2 is subdivided into two types, Peloid-A2.1 (Figs. 6f and 7d) and A2.2 (Figs. 6e, 7g,h and 9e), based on the presence or absence of coccoidal microbes, respectively. Further detailed features of Peloid-A are summarized in Table 1.
Peloid-B is characterized by a spherical to ellipsoidal shape with distinct margin, a width of about 200 – 700 Am. Peloids of this type are sometimes found together with ostracodes and/or gastropods whose inner spaces are filled in with sparitic calcite or micrite (Fig. 9c). Peloid-C is composed of dense micritic grain with definite margin, spherical, ellipsoidal to angular in shape, and ranges from 150 to 400 Am in diameter (Fig. 9f). These peloids co-occur with lots of cortoids that are related to micritization on the surfaces of bioclasts such as bivalves. Distributions of each type of peloids are different according to microenvironments (Fig. 10). PeloidA1.1 is especially abundant in cryptic spaces within thrombolites, where it co-occurs with coccoidal microbes. It is rare in intraskeletal cavities and matrices of thrombolites, and is absent from skeletal limestones. Cryptic spaces, intraskeletal cavities, and matrices are never completely filled with only this type of peloid, but always contain other types such as Peloid-A1.2 and Peloid-A2. Although Peloid-A1.2 occurs throughout microbialites and skeletal limestones, it is particularly abundant in cryptic spaces within thrombolites, where in some cases it may be the only peloid type present. It is also common in
Table 1 Detailed features of Peliod-A under optical microscope and SEM Peloid-A Peloid-A1
Diameter Shape Margin Other characters
Components under SEM
Peloid-A2
Peloid-A1.1
Peloid-A1.2
Peloid-A2.1
Peloid-A2.2
20 – 60 Am spherical somewhat distinct same size and shape as coccoidal microbes; comparatively dense micrite; commonly cluster or co-occur with coccoidal microbes fine micrite (1 – 4 Am)
20 – 60 Am spherical diffuse sometimes produce clotted fabrics
70 – 200 Am spherical to ellipsoidal diffuse coccoidal microbes abundant within peloids and distribute sporadically among them
70 – 200 Am spherical to ellipsoidal diffuse variable in shape and poorly sorted
fine micrite (2 – 6 Am); a cluster of micrite, sometimes with the smallest ones in the centre and becoming progressively larger outward
polygonal to spherical micrite (3 – 10 Am)
spherical to polygonal micrite (3 – 10 Am); micrite sometimes becomes progressively larger in diameter outward
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Fig. 10. Distribution and relative abundance of three types of peloids (Peloid-A, B, C) and coccoidal microbes according to differences in microenvironments (cryptic space, intraskeletal cavity, and matrix). See text for details.
intraskeletal cavities of thrombolites, and is present in the matrices of thrombolites, and intraskeletal cavities and matrices of skeletal limestones. Peloid A2.1 is especially abundant in cryptic spaces within thrombolites, along with abundant coccoidal microbes. It is present in intraskeletal cavities and matrices of thrombolites, and absent from skeletal limestones. PeloidA2.2 is common in the matrices, but comparatively rare in the cryptic spaces and intraskeletal cavities of thrombolites, and skeletal limestones. Peloid-B is volumetrically small throughout, being comparatively common only in the matrices of skeletal limestones where ostracodes are densely packed. In matrices of thrombolites, this type of peloids is rarely packed closely. Peloid-C is rare, and is recognized only in the matrices of skeletal limestones. It may be present in
matrices where there is abundant bioclasts whose surfaces are intensively micritized.
7. Origins of peloids As a result of the end-Permian biotic crisis, thrombolites together with peloids and only a limited bioclasts occur at the Tianwan section. Coccoidal microbes together with other microbes might have played an important role in the formation of microbialite frameworks after the end-Permian extinction (Ezaki et al., 2003). Microbial activities can lead to CaCO3 precipitation. Cyanobacterial calcification is associated with raised alkalinity through photosynthesis (e.g., Merz, 1992). It occurs upon and within mucilage sheaths, closely associated with the organic
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macromolecules of cell envelopes (Riding, 1977, 1991; Pentecost and Riding, 1986; Merz-Preiß, 2000). Other workers have also described carbonate precipitation connected with dead cyanobacteria, where bacteria decompose the cyanobacteria to lead to post-mortem calcification (Chafetz and Buczynski, 1992; De´farge et al., 1996; Merz-Preiß, 2000). Kaz´mierczak et al. (1996) found that peloids formed within calcified coccoidal cyanobacterial mats. Those mats are not very coherent and easily disintegrate into subglobular aragonitic peloidal bodies representing the permineralized groups of coccoids. Apart from their presence in frameworks, coccoidal microbes are common in cryptic spaces, and present in intraskeletal cavities and matrices of thrombolites (Fig. 10). The patterns of distribution and relative abundance of both Peloid-A1.1 and Peloid-A2.1 are similar to those of coccoidal microbes within microbialites. Peloid-A1.1 exhibits a distinct margin and has the same morphological features and size as coccoidal microbes. It co-occurs with coccoidal microbes. This peloid is filled in with fine micrite, whereas coccoid microbe is filled in with sparitic calcite in a central hollow space. Coccoidal microbes might have been preserved as a result of calcification on and within the sheath by microbial activities. After the death of the cell, the cell might be decomposed and leave a void, which is subsequently filled in with sparitic calcite, through inorganic and/or microbemediated cementation (Fig. 11-1-5). Peloid-A1.1 may have originated simply from the complete micritic infilling of coccoidal microbes as a consequence of post-mortem calcification by bacterial activities (Fig. 11-6). Peloid-A1.2 typically occurs as a dense accumulation in cryptic spaces where peloids are extremely well sorted (Fig. 8a). Coccoidal microbes and Peloid-A1.1 co-occur with PeloidA1.2. Peloid-A1.2 is also closely related to coccoidal microbes in relative abundance. Peloid-A1.2, although lacking microbial remains, might have been formed through the same microbial process with Peloid-A1.1 (Fig. 11-7). Peloid-A1.2 is distinguished from the Peloid-A1.1 by having a diffuse margin, and that may merely reflect the difference in degree of calcification. Peloid-A2.1 is obviously larger than coccoidal microbes and might have been formed as a result of a colony of coccoidal microbes being calcified (Fig. 11-10-12). However, some larger
peloid-like bodies might be formed by the aggregation of smaller individual peloids. In fact, since irregular, aggregate masses made up of individual peloids are present in specimens treated here, it is probably that Peloid-A2.1 also was generated by the aggregation of peloids (Peloid-A1) and coccoidal microbes (Fig. 1114). Similarly, Peloid-A2.2 might have been formed by induced calcification in a primary assemblage (colony) of coccoidal microbes and/or by the aggregation of smaller individual peloids, as in Peloid-A1.2 (Fig. 11-13 and 15). Micrite infilling within individual or colony of coccoidal microbes were explained as a consequence of coccoid microbial and bacterial activities. Micritization has often been inferred to result from intensive microboring (e.g., Bathurst, 1971), but recently Reid and Macintyre (1998), Reid et al. (1992) have reexamined micritization by recrystallization, based on the crystal textures seen in SEM and mineralogical analyses. They suggest that micritization by recrystallization is a pervasive diagenetic process in the shallow tropical seas. The causes of early carbonate recrystallization probably involve biological processes, such as organic decomposition leading to carbonate dissolution and precipitation, as well as inorganic processes (Reid and Macintyre, 1998). It is therefore possible that some peloids might have resulted directly from coccoid microbes that were micritized by the recrytallization (Fig. 11-6, 7, 9, 12, 13 and 16). We infer that Peloid-A1.2 and Peloid-A2.2 in part are related to the calcification of individuals and colonies of coccoidal microbes and/or aggregations of peloids. However, it should be noted that peloids with the same size and shape as those found in microbialites also occur in skeletal limestone that are free of observed coccoidal microbes. This implies that the origin of such peloids is not always directly related to the coccoidal microbes alone. It is well known that peloids occur as crusts on bioclasts and as cavities fill precipitation. These peloids are often inferred to be related in origin to bacterial activities (e.g., Chafetz, 1986; Reid, 1987; Sun and Wright, 1989; Reitner, 1993). Chafetz (1986) and Reitner (1993) explained the relations between bacterial activities and the formations of peloids in modern reefs. Chafetz (1986) has shown that the nuclei of marine peloids originated as a fine-grained precipitate of high-magnesian calcite within and around active
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Fig. 11. Plausible origins of Peloid-A and their mutual interrelationships with respect to microbes. 1 – 5: Precipitation of CaCO3 occurs on (1) and within (2) the sheath of the individual coccoidal microbes as a consequence of activities of coccoidal microbes. After the death of the cell, cell is completely decomposed (3, 4) and remaining spaces are filled with sparite calcite, forming calcified coccoidal microbes (5). 6: When coccoidal microbe is decomposed by bacterial activity, coccoidal microbe subsequently undergoes postmortem calcification, and is completely filled in with micrite, forming Peloid-A1.1. 7: Peloid-A1.2 is formed by the same process as Peloid-A1.1, but coccoidal microbe is not preserved well. 10 – 12: Precipitation of CaCO3 occurs on the colony of the coccoidal microbial sheath (10) and within the mucus sheath (11). Cells decompose completely and the remaining voids are filled with sparite calcite and/or bacterial activities induce secondary precipitation of CaCO3, resulting in the formation of Peloid-A2.1 (12). 13: Peloid-A2.2 is formed by the same process as Peloid-A2.1, but coccoidal microbes are not preserved well. 14 – 15: Peloid-A2.1 and A2.2 are also formed by aggregations of Peloid-A1 and/or coccoidal microbes, resulting in the formation of Peloid-A2.1 (14) and Peloid-A2.2 (15). 6, 7, 9, 12, 13, 16: Micrite filling within individual coccoidal microbes might be resulted from recrystallization, forming Peloid-A1.1 (6, 9) and A1.2 (7, 9). Peloids-A2.1 and A2.2 are also formed by micritization resulted from recrystallization of colony of coccoidal microbes (12, 13, 16). 8, 17: Peloids are formed in relation to (nanno)bacteria and organic matter, which is bacterial metabolic products, resulting in the formation of Peloid-A1.2 (8) and Peloid-A2.2 (17).
clumps of bacteria, and that the bacterial activity influenced precipitation of the calcite. Additionally, Reitner (1993) noted that the cavities are partly filled with mucus substances and that the peloids grow within the mucus, which exhibits a clumpy structure. Such clumps, which may be a result of decaying of organic slimes, are the starting points of peloid
formation. The decaying process via bacteria leads to the precipitation of carbonate that is related to ammonification (Berner, 1968, Reitner, 1993). The sulphate-reducing bacterial activity also leads to the formation of carbonate (Wright, 1999). The presence of framboidal pyrites in the framework of thrombolites treated here might suggest sulphate-reducing
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bacterial activity and the formation of peloids as byproducts. Folk (1993) found bacterial and nannobacterial remains in Palaeozoic and Mesozoic limestones, and Kirkland et al. (1998) interpreted clusters of small spherical bacteria (nannobacteria) (0.1 Am in diameter) in the cores of peloids within microbialite. Such remains were not recognized within the peloids treated here. However, we cannot underestimate (nanno)bacterial role in the formation of peloids described here. Peloid-A1 and Peloid-A2 in microbialites and skeletal limestones might have been also produced through a series of processes mentioned above (Fig. 11-8 and 17). In contrast, peloids related to bioclasts (Peloid-B) originate only as homogenous micrite filling in mainly ostracodes and rarely gastropods whose skeletons have been completely dissolved. Since ostracodes are common in cryptic spaces, intraskeletal cavities, and matrices, this type of peloid is also present there. Peloid-C only occurs in matrices where bioclasts are abundant. Peloid-C is quite variable in shape, size and sorting, and is often accompanied by abundant bioclasts and cortoids. Peloid-C is thus related to micritized grains such as shell fragments. Peloid-A is a main component in limestones after the end-Permian extinction and is in part clearly related in origin to coccoidal microbes, whether or not direct microbial remains are now present. We infer that this peloid type originated by carbonate precipitation around the nuclei of the primary and secondary products of microbial metabolic activity. A mutual relationship between peloids and responsible microbes has not yet been fully resolved in the geological record. However, limestones following the end-Permian extinction, are characterized by microbialites that include remains of coccoidal microbes and numerous peloids. The presence of coccoidal microbes implies that microbial activities influenced the formation not only of microbialites but also of concurrent peloids. Hypotheses of peloidal formation related to coccoidal microbes in this study do not apply to the origin of all other peloids in the geological record. However, this study emphasizes the importance not only of primary microbial activity in the formation of peloids, but also secondary activity by means of organic decomposition. The direct mechanisms for the formation of peloids have not yet been fully specified. They
are polygenetic in origin. And the direct remains of nannobacteria have not yet been found in this study. Folk’s (1993) method provides a basis for recognizing bacterial remains and their plausible genetic relation to peloidal formation. Further research, especially in the areas of nanno-scale petrology, is needed to establish the relationship between the microbes and the nannobacteria, and to elucidate how the latter might have induced calcification and have been closely involved in the formation of peloids.
8. Conclusions (1) Microbialites after the end-Permian extinction are characterized by the dominance of thrombolites that include coccoidal microbes and peloids. Thrombolites exhibit various microenvironments, including cryptic spaces, intraskeletal cavities, and matrices, where both coccoidal microbes and peloids typically occur. (2) Peloids are divided into three types (Peloid-A, B, C), based on their fabric (shape, size and marginal features), mode of occurrence and the presence or absence of coccoidal microbes. Peloid-A is most common and is further subdivided into Peloid-A1 and Peloid-A2 based on size. Peloid-A1 is in part derived simply from the complete micritic filling in coccoidal microbes as a result of microbial metabolic activity, whereas Peloid-A2 results from calcification of a colony of coccoidal microbes due to their metabolic activity, or simply from aggregations of peloids (Peloid-A1) and coccoidal microbes. Some Peloid-A originates from an individual and a colony of coccoidal microbes micritized by recrystallization. Peloid-A also originates from other carbonate precipitation processes. Peloid-B is derived simply by micrite filling in ostracodes and gastropods skeletons, and Peloid-C has originated from micritized bioclasts. (3) Peloids are polygenetic in origin and ubiquitous in limestones of various ages and environments. However, coccoidal microbes-related peloids may have occurred repeatedly and predominantly in severely deteriorated environments such as those after the end-Permian mass extinction. Further nanno-scale petrological research will reveal how (nanno)bacteria might have interacted with each other and induced calcification leading to the formation of peloids.
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Acknowledgements The authors wish to thank Akira Yao of Osaka City University and Hao Wei-cheng of Peking University for invaluable discussions. Yang Shou-ren of Peking University is thanked for identification of conodonts. The authors also thank Noel P. James of Queen’s University and Ian G. Macintyre of Smithsonian Institution for reviewing earlier versions of this paper. This work was supported by the Special Scholarship for Doctor Course Students of Osaka City University, grants (no. 13440149, 14540440) from the Scientific Research Fund of the Japan Society for the Promotion of Science, and the Special Funds for Major State Basic Research Project (G200077700) of P. R. China.
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The fabrics and origins of peloids immediately after the end-Permian extinction, Guizhou Province, South China Natsuko Adachi a,*, Yoichi Ezaki a, Jianbo Liu b a
Department of Geosciences, Graduate School of Science, Osaka City University, Sugimoto 3-3-138, Sumiyoshi, Osaka 558-8585, Japan b School of Earth and Space Sciences, Peking University, Haidian, Beijing 100871, People’s Republic of China Received 12 December 2002; received in revised form 2 September 2003; accepted 8 October 2003
Abstract Peloids are major contributors to modern and ancient limestones, although their origins are not yet comprehensively understood. Upper Permian to Lower Triassic carbonate successions are well preserved at the Bangeng area in Guizhou Province, South China, where microbialites (thrombolites), coccoidal microbes and peloids are preserved together. These peloids are classified into three types (Peloid-A, B, C) that typically occur together along with coccoidal microbes and restricted bioclasts in a variety of microenvironments. Peloid-A is most common and exhibits close relationship with coccoidal microbes. Peloid-A1 has spherical grain and a size range typical for coccoidal microbes (10 – 60 Am in diameter), whereas Peloid-A2 is marked by larger spherical and subrounded grains with diffuse margins, ranging from 70 to 200 Am in diameter. Peloid-A1 is in part produced by the complete micritic filling in coccoidal microbes by their metabolic activities, whereas Peloid-A2 is related to the calcification of a colony of coccoidal microbes by their metabolic activities, or simply to the aggregation of individual peloids and coccoidal microbes. Peloids of this type might also originate from other carbonate precipitation processes. Peloid-B is derived merely by micrite filling in ostracodes and gastropods skeletons, whereas Peloid-C is of micritized-bioclast origin. These peloidal origins do not explain those of all environments and ages. Peloids are polygenetic in origin, irrespective of the presence of coccoidal microbes. However, it is emphasized that the peloids formed after the end-Permian extinction are dominated by microbes related ones. Those kinds of peloids might have occurred repeatedly and predominantly, especially in severely deteriorated environments such as those after the end-Permian mass extinction. D 2003 Elsevier B.V. All rights reserved. Keywords: Carbonate; Extinction; Microbe; Microbialite; Peloid; Permian – Triassic
1. Introduction Peloids are among the most important constituents of shallow-marine carbonate sediments in both modern and ancient environments. Peloids are spherical,
* Corresponding author. Fax: +81-6-6605-2522. E-mail address: [email protected] (N. Adachi). 0037-0738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2003.10.007
ellipsoidal, or angular grains, composed of microcrystalline carbonate, but with no internal structure (Tucker, 2001). Peloids have various origins (e.g., Flu¨gel, 1982; Macintyre, 1985; Tucker and Wright, 1990), and the following hypotheses have been proposed; (1) fecal pellets (e.g., Land and Moore, 1980), (2) micritized grains (e.g., Bathurst, 1971), (3) intraclasts (e.g., Fa˚hraeus et al., 1974), (4) a precipitate origin (e.g., Macintyre, 1985), and (5) a microbially mediated
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precipitate (e.g., Chafetz, 1986; Reitner, 1993). Evidence supporting an origin related to microbial activity has been documented for modern peloids (Monty, 1976; Chafetz, 1986; Reitner, 1993; Kaz´mierczak et al., 1996). Monty (1976) and Kazmierczak et al. (1996) have observed peloids associated with cyanobacterial mats. In addition, Chafetz (1986) has shown that the nuclei of marine peloids originated within and around active clumps of bacteria. In contrast, Reitner (1993) detected no bacterial remains in peloids and concluded that the cores of marine peloids were formed within organic mucilage. Moreover, in the geological record peloids occasionally occur as an irregular, dome-shaped micritic coating on bioclasts or fill confined cavities. The fabrics are inferred to result from in situ growth products generally related to microbial activity (e.g., Reid, 1987; Sun and Wright, 1989; Neuweiler, 1993). Coniglio and James (1985) reported early Palaeozoic peloids derived from the breakdown of the filamentous calcified cyanobacterium Girvanella, whose open spaces were filled in with micrite. Skeletal reef-building organisms apparently disappeared for a lengthy interval following the end-
Permian mass extinction, during which microbialcarbonate buildups predominated (e.g., Sano and Nakashima, 1997; Lehrmann et al., 1998, 2001, 2003; Lehrmann, 1999; Kershaw et al., 1999, 2002; Ezaki et al., 2003). However, peloids continued on as common constituents, even in limestones, immediately after the extinction episode. Peloids in this particular geological interval are thought to have had their origins minimally influenced by skeletal organisms and are therefore of special interest. Carbonate sequences after the end-Permian extinction are well preserved in the Bangeng area of Guizhou Province, South China (Lehrmann et al., 1998, 2001, 2003; Lehrmann, 1999), where microbialites (thrombolites), coccoidal microbes as well as peloids are typically present. These limestones are important for understanding the genetic relationship between microbes and peloids. Therefore, we examined in great detail the peloidal fabrics from the Tianwan section in the Bangeng area, in order to elucidate peloid origins, using both optical and scanning electron microscope (SEM). We first describe the features (fabrics) of peloids as they relate to microenvironments, including the
Fig. 1. Index map showing the study area at the Tianwan section in the Bangeng area of Guizhou Province, South China (modified from Lehrmann et al., 2003).
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cryptic spaces, intraskeletal cavities and matrices within both microbialites and skeletal limestones. Special attention is paid to the interrelationship of peloids and coccoidal microbes. We then classify peloids and summarize their distribution. Finally, we discuss plausible origins for each type of peloids.
2. Geological setting Permian – Triassic shallow-marine successions are well preserved in the Bangeng area in Guizhou Province, South China (Lehrmann et al., 1998, 2001, 2003; Lehrmann, 1999). The study section is located at Tianwan, about 25 km north of Luodian, south Guizhou (Fig. 1). This area was situated in the interior of an isolated carbonate platform (Great Bank of Guizhou in the Nanpanjiang Basin sensu Lehrmann et al., 1998) during Triassic time (Fig. 1). A wide variety of carbonate facies, representing different sedimentary settings, accumulated to a great thickness in this area. Lehrmann et al. (1998) and Lehrmann (1999) first remarked on the intercalations of Early Triassic calcimicrobial mounds. Strata that span the Permian/Triassic (P/T) boundary in the Tianwan section are briefly described below, in ascending order (Fig. 2). Uppermost Permian limestones are characterized by medium- to thick-bedded skeletal grainstone that includes abundant skeletal organisms such as calcisponges, fusulinids (Palaeofusulina), smaller foraminifers, bryozoans, rugose corals, echinoderms, and dasycladacean algae. They are overlain by alternating beds of microbialites and skeletal limestones. The latter include bioclasts of thin-shelled bivalves, gastropods, ostracodes, serpulid worms, echinoderms, and smaller foraminifers, which exhibit a reduced biotic diversity. Microbialites are marked mostly by clotted fabrics (thrombolites). Thrombolite beds range from a few tens of centimeters to several meters in thickness, in places forming a distinct domal structure that is buried laterally by skeletal limestone. The alternating beds of thrombolites and skeletal limestone are overlain by thinly bedded, argillaceous, lime mudstone and mudstone. The conodonts Clarkina changxingensis, C. subcarinata, C. deflecta, and Hindeodus typicalis that are indicative of the C. changxingensis Zone of Late
Fig. 2. Stratigraphic column across the Permian/Triassic boundary in the Tianwan section, occurrences of conodonts and conodont biozones.
Permian age, were discovered in skeletal grainstone immediately above the lowermost thrombolite bed (Fig. 2). Hindeodus parvus that is diagnostic of the very earliest Triassic H. parvus Zone, was first detected in skeletal packstone 3.4 m above the lowermost microbialite, whereas Isarcicella isarcica and I. staeschei of the second conodont zone of the Triassic I. isarcica Zone, were then found in skeletal grainstone about 8 m above. The only conodont Hindeodus sp. was recovered from thinly bedded, argillaceous, lime mudstone. From a biostratigraphic point of view, the P/ T boundary is defined only by the first appearance of H.
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parvus (Fig. 2). The age of microbialites in the study section is provisionally considered to range from latest Permian to earliest Triassic.
3. Methods This study is based on field observations and laboratory analysis of polished slabs and thin sections. Oriented specimens, cut perpendicular to bedding (along a growth direction), were then polished and prepared for making thin sections. Peloids and coccoidal microbes in particular were observed in detail by
using optical microscope and SEM. The results from all observations were compared, in order to thoroughly describe the fabrics and discuss their origin. The size of the peloids was measured and graphed to learn their relationship to microenvironments. About 260 large thin sections (5 7.5 cm) and auxiliary small thin sections (2.8 4.6 cm) were prepared for opticalmicroscope analysis. The thin sections used for SEM observations were relatively thick. They were polished with 1 Am diamond dust and etched for 25 seconds with 2% formic acid prior to coating with gold. SEM observations were made at 17 kV with a JEOL JSM5500 in Osaka City University.
Fig. 3. Mesoscopic and microscopic features of thrombolites. (a) Polished surface of a thrombolite, which exhibits clotted mesostructure. Bright gray colour corresponds to mesoclots (large arrow), whereas dark gray corresponds to cryptic spaces and matrices (small arrows). Scale bar = 2 cm. (b) Detail of area in (a) (black rectangle). Sparitic parts are framework of thrombolites, which are composed by small clots (white arrows). For full details, see (c). Large black arrows show cryptic spaces, whereas small black arrows show matrices containing some bioclasts. Scale bar = 1 cm. (c) Spherical to ellipsoidal small clots are amalgamated with each other. Some clots are fringed with coccoidal microbes (white arrow). Scale bar = 0.25 mm. (d) Spherical and ellipsoidal coccoidal microbes with dark micritic envelopes are partly preserved at the framework, as shown by black arrows. Scale bar = 0.2 mm.
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4. Constituents and microenvironments 4.1. Thrombolites The term thrombolite was coined by Aitken (1967) and has been used by later workers (e.g., Kennard and James, 1986; Burne and Moore, 1987; Riding, 2000). Shapiro (2000) recently revised the definition as follows: a thrombolite is a microbialite composed of a clotted mesostructure. As seen in mesoscopic (polished hand-specimen) and optical-microscopic views, most parts of thrombolites exhibit mesoclots (Fig. 3a,b). They are composed of millimeter- and centimeter-size sparitic masses that have a sporadic distribution. Round to ellipsoidal small clots, 100– 500 Am in diameter, are present in places, and they have a darker rim at their periphery and are filled in with mosaic calcite and dolomite (Fig. 3c). Small clots are fringed in part with coccoidal microbes (Fig. 3c) and are merely due to closely packed coccoidal microbes. These small clots amalgamate with each other to various degrees to form frameworks of thrombolites with irregularly outlined fenestral fabrics (cryptic
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spaces) that are filled with peloids in their lower halves (Figs. 3b and 4a). Thrombolites in some cases grade laterally and vertically into skeletal limestone. 4.2. Coccoidal microbes Spherical and ellipsoidal structures with micritic envelopes are similar to those that have been reported in the earliest Triassic microbialites from the Huaying area of Sichuan Province, South China (Ezaki et al., 2003). Those cellar structures are regarded as calcified coccoid microbes. Coccoidal microbes are scattered and/or clustered, especially within frameworks of thrombolites (Fig. 3d). They are also present in cryptic spaces, intraskeletal cavities and matrices of thrombolites. Coccoidal microbes range in diameter from 10 to 60 Am, but 80 % fall into the 20– 40 Am range, and showing good sorting (Fig. 5). Each coccoidal microbe is surrounded by a micritic envelope (on average 2 –5 Am thick) and contains a hollow space 10 – 20 Am in diameter that is filled in with sparitic calcite and dolomite (Figs. 3d and 6c –f). Seen under an SEM, the envelope consists of fine micrite, and some
Fig. 4. Schematic diagram showing different types of microenvironments (cryptic spaces, intraskeletal cavities and matrices) in both microbialite (a) and skeletal limestone (b).
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prise densely accumulated clots, amalgamations of these, and coccoidal microbial remains. Cryptic spaces are formed within thrombolite frameworks and are small, laterally elongate cavity spaces up to 5 mm wide (Figs. 3b and 4a), whereas interframeworks of thrombolites contain intraskeletal cavities and matrices. In skeletal limestones, intraskeletal cavities, and matrices are identified (Fig. 4b). Intraskeletal cavities refer to sheltered spaces within individual bioclasts and/or beneath reworked skeletal debris related with thin shelled bivalves and gastropods (Fig. 4a,b).
5. Features of peloids
Fig. 5. Size frequency distribution of coccoidal microbes.
of the micritic envelopes may show one or more concentric or eccentric laminae (Fig. 7c – f).
Peloidal sediments are ubiquitous within thrombolites and skeletal limestones, and peloidal fabrics are described below, arranged by microenvironments (cryptic spaces, intraskeletal cavities and matrices). 5.1. Microbialites
4.3. Peloids Peloids are generally micritic spherical and ellipsoidal grain. Although micrite has crystal sizes of 1 –4 Am (Folk, 1959), it as used here includes crystals as large as 5 – 10 Am in diameter as seen under an SEM. If the coccoidal microbes are completely filled in with micrite, they are treated as peloids. Detailed features are mentioned in following section. 4.4. Microenvironments Various microenvironments are recognized within microbialites (thrombolites) and skeletal limestones, and these include cryptic spaces, intraskeletal cavities, and matrices (Fig. 4). Thrombolite frameworks com-
Thrombolites are overwhelmingly present and are roughly divided into sparitic parts and micritic matrices (s.l.). The sparitic parts further consist of frameworks and cryptic spaces, whereas the micritic parts are composed of bioclasts that contain intraskeletal cavities, and matrices (s.s.) (Fig. 4a). 5.1.1. Cryptic spaces Cryptic spaces are mostly filled in with peloids in their lower parts and exhibit a geopetal texture (Fig. 6a –c,e). Each peloid is most distinct in outline within the upper part of the geopetal-sediment filling, whereas its outline becomes indistinct in the lower part of the sediment filling, owing to compaction into peloidal micrite (Fig. 6g).
Fig. 6. Photomicrographs of peloidal fabrics in cryptic spaces of thrombolites. (a) The aggregations of small clots form the framework, and cryptic spaces are filled in with peloidal sediment. Scale bar = 1 mm. (b) Detail of area in (a) (white rectangle). Peloids show diffuse margins and are well sorted. White rectangle is depicted in Fig. 7a and black rectangle is depicted in Fig. 7b. Scale bar = 0.5 mm. (c) Cryptic spaces are filled in with well-sorted peloidal sediment. Scale bar = 0.5 mm. (d) Detail of area in (c) (white rectangle). Peloids, which have comparatively welldefined margins and dense micritie, together with coccoidal microbes are clustered (black arrow). See also Fig. 7c. Scale bar = 0.25 mm. (e) Cryptic spaces are filled in with variously sized peloids (arrow). Small white rectangle is depicted in Fig. 7d and black rectangle in Fig. 7g. Scale bar = 1 mm. (f) Detail of area in (e) (large white rectangle). Coccoidal microbes are scattered among peloids and/or are included within peloid (black arrow). Scale bar = 0.25 mm. (g) Outlines of peloids are clearer in upper parts of cryptic spaces (black arrow), whereas its fabrics are altered into peloidal micrites in lower parts, owing to close packing (white arrow). Scale bar = 1 mm. (h) Peloids are amalgamated in part with neighbouring peloids, and exhibit a clotted texture. Scale bar = 0.5 mm.
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Peloids in most cryptic spaces are extremely wellsorted, with 80 % ranging from 20 to 60 Am in diameter and averaging 51 Am (Fig. 8a). This size range of peloids is similar to that of coccoidal microbes (Fig. 5). These peloids are mostly spherical in shape. These peloids may partly amalgamate to produce clotted fabrics (Fig. 6h). Isolated and/or clustered coccoidal microbes are scattered among the peloids. Some of the peloids show somewhat distinct margins and comparatively dense micrite, and look like coccoidal microbes in size and shape (Fig. 6c,d). In other cryptic spaces, peloids are poor-sorted, with 25% ranging from 20 to 60 Am in diameter and averaging 90 Am (Fig. 8a). These peloids are spherical to ellipsoidal in shape. Isolated and/or clustered coccoidal microbes are included within each peloid, and are scattered among the peloids (Fig. 6f). Pyrites occur as raspberry-like, spherical aggregates of equigranular crystals (framboidal pyrite) and as single euhedral or subeuhedral crystals from 1 up to 100 Am in diameter. Tiny framboidal pyrites are closely aggregated in part. Similar pyrites are also present in intraskeletal cavities and matrices of thrombolites and skeletal limestones. 5.1.2. Intraskeletal cavities Intraskeletal cavities made up of thin-shelled bivalves and gastropods, are mostly filled in with peloids, micrite and rare bioclasts, and ordinarily exhibit a geopetal texture (Fig. 9a). Peloids range in diameter from 30 to 150 Am, averaging 71 Am (Fig. 8b). Each peloid is spherical to ellipsoidal with diffuse margins. Peloids occasionally exhibit clotted textures by becoming amalgamated with each other. Intraskeletal cavities are rarely filled in with well-sorted peloids, as seen in the cryptic spaces. Coccoidal
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microbes are interspersed among the peloids and in places are contained within the peloids (Fig. 9b). 5.1.3. Matrices Matrices are filled in with peloids and micrite. Peloids are so closely packed in most cases as to form peloidal micrites, where the outlines of the peloids are not clearly discernible. Each peloid with spherical to ellipsoidal in shape ranges from 40 to 180 Am in diameter, with an average of 90 Am (Fig. 8b). Coccoidal microbes are rarely scattered among the peloids. Comparatively large peloids with spherical to ellipsoidal shapes, 200 – 700 Am in diameter, are sometimes closely packed together with ostracodes (Fig. 9c). These peloids have distinct margin, and some of them preserve the remains of ostracodes carapaces at their peripheries. 5.2. Skeletal limestone Skeletal limestones (skeletal grainstone and packstone) include low-taxonomic-diversity bioclasts of ostracodes, bivalves, gastropods, serpulid worms, echinoderms, and smaller foraminifers. Coccoidal microbes seen in microbialites are absent here, although peloids are still abundant. 5.2.1. Intraskeletal cavities The peloids in intraskeletal cavities that formed within bivalves or gastropods ordinarily have a geopetal texture (Fig. 9d,e). The outline of each peloid is more clearly defined in the upper parts of the geopetal sediment infilling. The peloids are spherical to ellipsoidal in shape, ranging from 20 to 300 Am in diameter, averaging 90 Am (Fig. 8c). Sorting is poor compared with peloids in cryptic spaces within thrombolites.
Fig. 7. SEM images of peloidal fabrics. (a) SEM photomicrograph of area in Fig. 6b (white rectangle). Each peloid consists of fine crystals (2 – 6 Am in diameter) (arrows) that are surrounded by coarser crystals of subhedral and equant. Scale bar = 50 Am. (b) SEM photomicrograph of area in Fig. 6b (black rectangle). Each peloid consists of a cluster of calcite crystals smaller in the centre, but becoming progressively larger outward (arrow). Scale bar = 50 Am. (c) SEM photomicrograph of area in Fig. 6d (arrow). Spherical peloids (small arrows) occur together with coccoidal microbes (large arrows). Peloids consist of fine calcite, whereas coccoidal microbes consist of coarser calcite rimmed by fine calcite (1 – 4 Am in diameter). Scale bar = 50 Am. (d) SEM photomicrograph of area in Fig. 6e (white small rectangle). Each peloid preserves remains of coccoidal microbes (arrows). Black arrow shows a coccoidal microbe filled with mesh-like microdolomite. Scale bar = 100 Am. (e) Detail of area in (d) (large white arrow). Coccoidal microbe with concentric laminae. Scale bar = 20 Am. (f) Detail of area in (d) (two small white arrows). Coccoidal microbes are composed of coarser calcite in the centre and may be further surrounded by micrite. Scale bar = 20 Am. (g) SEM photomicrograph of area in Fig. 6e (small black rectangle). Many peloids have irregular shapes. They consist of fine, spherical and subhedral crystals (3 – 10 Am in diameter) (black arrow) and are surrounded by coarser calcite (white arrow). Scale bar = 100 Am. (h) SEM photomicrograph of area in Fig. 9e (black arrow). Each peloid shows an irregular shape and consists of comparatively coarser calcite (arrow). Scale bar = 50 Am.
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Fig. 8. Frequency distribution of peloid diameter. (a) Cryptic spaces in the different thrombolite samples (cryptic spaces-1, -2). Note size distribution of coccoid microbes and Peloid-A. (b) Intraskeletal cavities and matrices in thrombolites. Note size distribution of coccoid microbes and Peloid-A. (c) Two different microenvironments (intraskeletal cavity and matrix) within skeletal limestone. Note size distribution of coccoid microbes and Peloid-A, B and C.
5.2.2. Matrice Matrices are filled in with peloids, micrite and sparitic calcite (Fig. 9d). Peloids within matrices sometimes grade into peloidal micrite, due to being densely compacted. These peloids are spherical to ellipsoidal in shape, ranging from 40 to 500 Am in diameter with an average of 160 Am, indicating poor sorting (Fig. 8c). Relatively large peloids with spherical to ellipsoidal shapes, ranging from 200 to 500 Am
(rarely 700 Am) in diameter, are characteristically present, where ostracodes carapaces and/or gastropods shells filled in with sparite and micrite are present. The peloidal occurrence is the same as that in matrices of microbialites (Fig. 9c). Cortoids formed by the micritization of bioclasts, such as bivalves, are found. Densely micritized peloids, 150– 400 Am in diameter, are also present (Fig. 9f). Those peloids are distinct in margin and spherical to ellipsoidal in shape.
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Fig. 9. Photomicrographs of peloidal fabrics in intraskeletal cavity (a, b) and matrices (c) of thrombolites, and intraskeletal cavity and matrices of skeletal limestone (d – f). (a) Intraskeletal cavity is filled in with peloidal sediments (lower part) and calcite cements (upper part). Scale bar = 1 mm. (b) Detail of area in (a) (black rectangle). Coccoidal microbes are found abundantly among and within peloids (arrow). Scale bar = 0.25 mm. (c) Ellipsoidal peloids with a wide size range (200 – 700 Am in diameter). Some peloids still preserve ostracodes carapaces (arrow). Scale bar = 1 mm. (d) Thin-shelled bivalves are abundant. Matrix is mostly occupied by peloids and micrite. Scale bar = 5 mm. (e) Detail of area in (d) (white rectangle). Intraskeletal cavities are filled in with peloidal sediments (lower part) and calcite cement (upper part). Black arrow is depicted in Fig. 7h. Scale bar = 0.5 mm. (f) Photomicrograph showing micritized bioclasts, cortoids (arrows), and peloids which are made up of dense micrite and have definite margin. Scale bar = 1 mm.
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6. Types of peloids and their distributions The peloids described above can be classified on the basis of their fabric (size, shape and marginal features), mode of occurrence, and the presence or absence of coccoidal microbes. Three types of peloids can be distinguished: Peloid-A, B, and C. Peloid-A is the most common type in microbialites and skeletal limestones. It ranges from 20 to 200 Am in diameter, and is divided into subtypes A1 and A2, based on size. Peloid-A1 falls within the size-range characteristic of coccoidal microbes. Peloid-A1 is further subdivided into Peloid-A1.1 (Figs. 6c,d and 7c) and A1.2 (Figs. 6a –c and 7a,b), based on the presence or absence, respectively, of coccoidal microbes and their identifying microscopic features. Peloid-A1.1 is distinguished from Peloid-A1.2 by having distinct margin and comparatively dense micrite. Peloid-A2 is characterized by spherical and ellipsoidal grains with diffuse margin, and it differs from Peloid-A1 by its larger size (70 – 200 Am). Peloid-A2 is subdivided into two types, Peloid-A2.1 (Figs. 6f and 7d) and A2.2 (Figs. 6e, 7g,h and 9e), based on the presence or absence of coccoidal microbes, respectively. Further detailed features of Peloid-A are summarized in Table 1.
Peloid-B is characterized by a spherical to ellipsoidal shape with distinct margin, a width of about 200 – 700 Am. Peloids of this type are sometimes found together with ostracodes and/or gastropods whose inner spaces are filled in with sparitic calcite or micrite (Fig. 9c). Peloid-C is composed of dense micritic grain with definite margin, spherical, ellipsoidal to angular in shape, and ranges from 150 to 400 Am in diameter (Fig. 9f). These peloids co-occur with lots of cortoids that are related to micritization on the surfaces of bioclasts such as bivalves. Distributions of each type of peloids are different according to microenvironments (Fig. 10). PeloidA1.1 is especially abundant in cryptic spaces within thrombolites, where it co-occurs with coccoidal microbes. It is rare in intraskeletal cavities and matrices of thrombolites, and is absent from skeletal limestones. Cryptic spaces, intraskeletal cavities, and matrices are never completely filled with only this type of peloid, but always contain other types such as Peloid-A1.2 and Peloid-A2. Although Peloid-A1.2 occurs throughout microbialites and skeletal limestones, it is particularly abundant in cryptic spaces within thrombolites, where in some cases it may be the only peloid type present. It is also common in
Table 1 Detailed features of Peliod-A under optical microscope and SEM Peloid-A Peloid-A1
Diameter Shape Margin Other characters
Components under SEM
Peloid-A2
Peloid-A1.1
Peloid-A1.2
Peloid-A2.1
Peloid-A2.2
20 – 60 Am spherical somewhat distinct same size and shape as coccoidal microbes; comparatively dense micrite; commonly cluster or co-occur with coccoidal microbes fine micrite (1 – 4 Am)
20 – 60 Am spherical diffuse sometimes produce clotted fabrics
70 – 200 Am spherical to ellipsoidal diffuse coccoidal microbes abundant within peloids and distribute sporadically among them
70 – 200 Am spherical to ellipsoidal diffuse variable in shape and poorly sorted
fine micrite (2 – 6 Am); a cluster of micrite, sometimes with the smallest ones in the centre and becoming progressively larger outward
polygonal to spherical micrite (3 – 10 Am)
spherical to polygonal micrite (3 – 10 Am); micrite sometimes becomes progressively larger in diameter outward
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Fig. 10. Distribution and relative abundance of three types of peloids (Peloid-A, B, C) and coccoidal microbes according to differences in microenvironments (cryptic space, intraskeletal cavity, and matrix). See text for details.
intraskeletal cavities of thrombolites, and is present in the matrices of thrombolites, and intraskeletal cavities and matrices of skeletal limestones. Peloid A2.1 is especially abundant in cryptic spaces within thrombolites, along with abundant coccoidal microbes. It is present in intraskeletal cavities and matrices of thrombolites, and absent from skeletal limestones. PeloidA2.2 is common in the matrices, but comparatively rare in the cryptic spaces and intraskeletal cavities of thrombolites, and skeletal limestones. Peloid-B is volumetrically small throughout, being comparatively common only in the matrices of skeletal limestones where ostracodes are densely packed. In matrices of thrombolites, this type of peloids is rarely packed closely. Peloid-C is rare, and is recognized only in the matrices of skeletal limestones. It may be present in
matrices where there is abundant bioclasts whose surfaces are intensively micritized.
7. Origins of peloids As a result of the end-Permian biotic crisis, thrombolites together with peloids and only a limited bioclasts occur at the Tianwan section. Coccoidal microbes together with other microbes might have played an important role in the formation of microbialite frameworks after the end-Permian extinction (Ezaki et al., 2003). Microbial activities can lead to CaCO3 precipitation. Cyanobacterial calcification is associated with raised alkalinity through photosynthesis (e.g., Merz, 1992). It occurs upon and within mucilage sheaths, closely associated with the organic
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macromolecules of cell envelopes (Riding, 1977, 1991; Pentecost and Riding, 1986; Merz-Preiß, 2000). Other workers have also described carbonate precipitation connected with dead cyanobacteria, where bacteria decompose the cyanobacteria to lead to post-mortem calcification (Chafetz and Buczynski, 1992; De´farge et al., 1996; Merz-Preiß, 2000). Kaz´mierczak et al. (1996) found that peloids formed within calcified coccoidal cyanobacterial mats. Those mats are not very coherent and easily disintegrate into subglobular aragonitic peloidal bodies representing the permineralized groups of coccoids. Apart from their presence in frameworks, coccoidal microbes are common in cryptic spaces, and present in intraskeletal cavities and matrices of thrombolites (Fig. 10). The patterns of distribution and relative abundance of both Peloid-A1.1 and Peloid-A2.1 are similar to those of coccoidal microbes within microbialites. Peloid-A1.1 exhibits a distinct margin and has the same morphological features and size as coccoidal microbes. It co-occurs with coccoidal microbes. This peloid is filled in with fine micrite, whereas coccoid microbe is filled in with sparitic calcite in a central hollow space. Coccoidal microbes might have been preserved as a result of calcification on and within the sheath by microbial activities. After the death of the cell, the cell might be decomposed and leave a void, which is subsequently filled in with sparitic calcite, through inorganic and/or microbemediated cementation (Fig. 11-1-5). Peloid-A1.1 may have originated simply from the complete micritic infilling of coccoidal microbes as a consequence of post-mortem calcification by bacterial activities (Fig. 11-6). Peloid-A1.2 typically occurs as a dense accumulation in cryptic spaces where peloids are extremely well sorted (Fig. 8a). Coccoidal microbes and Peloid-A1.1 co-occur with PeloidA1.2. Peloid-A1.2 is also closely related to coccoidal microbes in relative abundance. Peloid-A1.2, although lacking microbial remains, might have been formed through the same microbial process with Peloid-A1.1 (Fig. 11-7). Peloid-A1.2 is distinguished from the Peloid-A1.1 by having a diffuse margin, and that may merely reflect the difference in degree of calcification. Peloid-A2.1 is obviously larger than coccoidal microbes and might have been formed as a result of a colony of coccoidal microbes being calcified (Fig. 11-10-12). However, some larger
peloid-like bodies might be formed by the aggregation of smaller individual peloids. In fact, since irregular, aggregate masses made up of individual peloids are present in specimens treated here, it is probably that Peloid-A2.1 also was generated by the aggregation of peloids (Peloid-A1) and coccoidal microbes (Fig. 1114). Similarly, Peloid-A2.2 might have been formed by induced calcification in a primary assemblage (colony) of coccoidal microbes and/or by the aggregation of smaller individual peloids, as in Peloid-A1.2 (Fig. 11-13 and 15). Micrite infilling within individual or colony of coccoidal microbes were explained as a consequence of coccoid microbial and bacterial activities. Micritization has often been inferred to result from intensive microboring (e.g., Bathurst, 1971), but recently Reid and Macintyre (1998), Reid et al. (1992) have reexamined micritization by recrystallization, based on the crystal textures seen in SEM and mineralogical analyses. They suggest that micritization by recrystallization is a pervasive diagenetic process in the shallow tropical seas. The causes of early carbonate recrystallization probably involve biological processes, such as organic decomposition leading to carbonate dissolution and precipitation, as well as inorganic processes (Reid and Macintyre, 1998). It is therefore possible that some peloids might have resulted directly from coccoid microbes that were micritized by the recrytallization (Fig. 11-6, 7, 9, 12, 13 and 16). We infer that Peloid-A1.2 and Peloid-A2.2 in part are related to the calcification of individuals and colonies of coccoidal microbes and/or aggregations of peloids. However, it should be noted that peloids with the same size and shape as those found in microbialites also occur in skeletal limestone that are free of observed coccoidal microbes. This implies that the origin of such peloids is not always directly related to the coccoidal microbes alone. It is well known that peloids occur as crusts on bioclasts and as cavities fill precipitation. These peloids are often inferred to be related in origin to bacterial activities (e.g., Chafetz, 1986; Reid, 1987; Sun and Wright, 1989; Reitner, 1993). Chafetz (1986) and Reitner (1993) explained the relations between bacterial activities and the formations of peloids in modern reefs. Chafetz (1986) has shown that the nuclei of marine peloids originated as a fine-grained precipitate of high-magnesian calcite within and around active
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Fig. 11. Plausible origins of Peloid-A and their mutual interrelationships with respect to microbes. 1 – 5: Precipitation of CaCO3 occurs on (1) and within (2) the sheath of the individual coccoidal microbes as a consequence of activities of coccoidal microbes. After the death of the cell, cell is completely decomposed (3, 4) and remaining spaces are filled with sparite calcite, forming calcified coccoidal microbes (5). 6: When coccoidal microbe is decomposed by bacterial activity, coccoidal microbe subsequently undergoes postmortem calcification, and is completely filled in with micrite, forming Peloid-A1.1. 7: Peloid-A1.2 is formed by the same process as Peloid-A1.1, but coccoidal microbe is not preserved well. 10 – 12: Precipitation of CaCO3 occurs on the colony of the coccoidal microbial sheath (10) and within the mucus sheath (11). Cells decompose completely and the remaining voids are filled with sparite calcite and/or bacterial activities induce secondary precipitation of CaCO3, resulting in the formation of Peloid-A2.1 (12). 13: Peloid-A2.2 is formed by the same process as Peloid-A2.1, but coccoidal microbes are not preserved well. 14 – 15: Peloid-A2.1 and A2.2 are also formed by aggregations of Peloid-A1 and/or coccoidal microbes, resulting in the formation of Peloid-A2.1 (14) and Peloid-A2.2 (15). 6, 7, 9, 12, 13, 16: Micrite filling within individual coccoidal microbes might be resulted from recrystallization, forming Peloid-A1.1 (6, 9) and A1.2 (7, 9). Peloids-A2.1 and A2.2 are also formed by micritization resulted from recrystallization of colony of coccoidal microbes (12, 13, 16). 8, 17: Peloids are formed in relation to (nanno)bacteria and organic matter, which is bacterial metabolic products, resulting in the formation of Peloid-A1.2 (8) and Peloid-A2.2 (17).
clumps of bacteria, and that the bacterial activity influenced precipitation of the calcite. Additionally, Reitner (1993) noted that the cavities are partly filled with mucus substances and that the peloids grow within the mucus, which exhibits a clumpy structure. Such clumps, which may be a result of decaying of organic slimes, are the starting points of peloid
formation. The decaying process via bacteria leads to the precipitation of carbonate that is related to ammonification (Berner, 1968, Reitner, 1993). The sulphate-reducing bacterial activity also leads to the formation of carbonate (Wright, 1999). The presence of framboidal pyrites in the framework of thrombolites treated here might suggest sulphate-reducing
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bacterial activity and the formation of peloids as byproducts. Folk (1993) found bacterial and nannobacterial remains in Palaeozoic and Mesozoic limestones, and Kirkland et al. (1998) interpreted clusters of small spherical bacteria (nannobacteria) (0.1 Am in diameter) in the cores of peloids within microbialite. Such remains were not recognized within the peloids treated here. However, we cannot underestimate (nanno)bacterial role in the formation of peloids described here. Peloid-A1 and Peloid-A2 in microbialites and skeletal limestones might have been also produced through a series of processes mentioned above (Fig. 11-8 and 17). In contrast, peloids related to bioclasts (Peloid-B) originate only as homogenous micrite filling in mainly ostracodes and rarely gastropods whose skeletons have been completely dissolved. Since ostracodes are common in cryptic spaces, intraskeletal cavities, and matrices, this type of peloid is also present there. Peloid-C only occurs in matrices where bioclasts are abundant. Peloid-C is quite variable in shape, size and sorting, and is often accompanied by abundant bioclasts and cortoids. Peloid-C is thus related to micritized grains such as shell fragments. Peloid-A is a main component in limestones after the end-Permian extinction and is in part clearly related in origin to coccoidal microbes, whether or not direct microbial remains are now present. We infer that this peloid type originated by carbonate precipitation around the nuclei of the primary and secondary products of microbial metabolic activity. A mutual relationship between peloids and responsible microbes has not yet been fully resolved in the geological record. However, limestones following the end-Permian extinction, are characterized by microbialites that include remains of coccoidal microbes and numerous peloids. The presence of coccoidal microbes implies that microbial activities influenced the formation not only of microbialites but also of concurrent peloids. Hypotheses of peloidal formation related to coccoidal microbes in this study do not apply to the origin of all other peloids in the geological record. However, this study emphasizes the importance not only of primary microbial activity in the formation of peloids, but also secondary activity by means of organic decomposition. The direct mechanisms for the formation of peloids have not yet been fully specified. They
are polygenetic in origin. And the direct remains of nannobacteria have not yet been found in this study. Folk’s (1993) method provides a basis for recognizing bacterial remains and their plausible genetic relation to peloidal formation. Further research, especially in the areas of nanno-scale petrology, is needed to establish the relationship between the microbes and the nannobacteria, and to elucidate how the latter might have induced calcification and have been closely involved in the formation of peloids.
8. Conclusions (1) Microbialites after the end-Permian extinction are characterized by the dominance of thrombolites that include coccoidal microbes and peloids. Thrombolites exhibit various microenvironments, including cryptic spaces, intraskeletal cavities, and matrices, where both coccoidal microbes and peloids typically occur. (2) Peloids are divided into three types (Peloid-A, B, C), based on their fabric (shape, size and marginal features), mode of occurrence and the presence or absence of coccoidal microbes. Peloid-A is most common and is further subdivided into Peloid-A1 and Peloid-A2 based on size. Peloid-A1 is in part derived simply from the complete micritic filling in coccoidal microbes as a result of microbial metabolic activity, whereas Peloid-A2 results from calcification of a colony of coccoidal microbes due to their metabolic activity, or simply from aggregations of peloids (Peloid-A1) and coccoidal microbes. Some Peloid-A originates from an individual and a colony of coccoidal microbes micritized by recrystallization. Peloid-A also originates from other carbonate precipitation processes. Peloid-B is derived simply by micrite filling in ostracodes and gastropods skeletons, and Peloid-C has originated from micritized bioclasts. (3) Peloids are polygenetic in origin and ubiquitous in limestones of various ages and environments. However, coccoidal microbes-related peloids may have occurred repeatedly and predominantly in severely deteriorated environments such as those after the end-Permian mass extinction. Further nanno-scale petrological research will reveal how (nanno)bacteria might have interacted with each other and induced calcification leading to the formation of peloids.
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Acknowledgements The authors wish to thank Akira Yao of Osaka City University and Hao Wei-cheng of Peking University for invaluable discussions. Yang Shou-ren of Peking University is thanked for identification of conodonts. The authors also thank Noel P. James of Queen’s University and Ian G. Macintyre of Smithsonian Institution for reviewing earlier versions of this paper. This work was supported by the Special Scholarship for Doctor Course Students of Osaka City University, grants (no. 13440149, 14540440) from the Scientific Research Fund of the Japan Society for the Promotion of Science, and the Special Funds for Major State Basic Research Project (G200077700) of P. R. China.
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