paleoenvironmental distribution of ordovician ... - PALAIOS

PALAIOS, 2017, v. 32, 462–489 Research Article DOI: http://dx.doi.org/10.2110/palo.2016.054

PALEOENVIRONMENTAL DISTRIBUTION OF ORDOVICIAN CALCIMICROBIAL ASSOCIATIONS IN THE TARIM BASIN, NORTHWEST CHINA LIJING LIU,1 YASHENG WU,1 HONGXIA JIANG,1 NAIQIN WU,2

AND

LIANQI JIA1

1

Key Laboratory of Petroleum Resources Research, CAS, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 2 Key Laboratory of Cenozoic Geology and Environment, CAS, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China email: [email protected]

ABSTRACT: The stratigraphic and facies distribution of 20 calcimicrobial genera (including calcified cyanobacteria and associated problematic calcified microfossils) are reported for the entire Ordovician succession in the Tarim Basin in northwestern China based on examination of drill cores and 8500 thin sections from 64 wells from the Tabei, Bachu, Tazhong, and Tadong uplifts. A total of ten calcimicrobial associations are recognized in the Lower to Upper Ordovician based on taxonomic composition and distribution within four paleoenvironment types: reef (marginal and patch reef), open platform/bank (marginal and patch bank), lagoon, and tidal flat. The temporal distribution of the calcimicrobial genera closely follows changes in sedimentary environments; an extensive literature survey reveals similar relationships in much of the Paleozoic and Mesozoic. Based on their paleoenvironmental preferences, calcimicrobes can be classified into five paleohabitat types: (1) reef-adapted (Acuasiphonoria, Razumovskia, Phacelophyton , Gomphosiphon, Epiphyton, Renalcis, and Izhella); (2) open platform/bank-adapted (Subtifloria and Bevocastria); (3) both reef and open platform/bank-adapted (Bija, Apophoretella, Rothpletzella, and Wetheredella); (4) lagoon-adapted (Hedstromia, Cayeuxia, Zonotrichites, Ortonella, and Garwoodia), and (5) not only reef and open platform/bank-adapted but also tolerant of tidal flat conditions (Girvanella and Proaulopora). The occurrences of these calcimicrobes in strata not only can indicate ancient sedimentary facies but also can reveal paleoecological parameters of ancient seas, such as nutrient levels (e.g., N and P), predation pressure, and sea level, especially in strata absence of other well-studied facies fossils.

INTRODUCTION

The term ‘‘calcimicrobes’’ encompasses calcified cyanobacteria fossils and associated problematic calcified microfossils, and is commonly used by paleontologists in its broadest sense to include distinct skeletal features regardless of their systematic affinities (James and Bourque 1992; Wood 1999; Copper 2002; Nose et al. 2006). Calcimicrobe fossils are conspicuous components in most Neoproterozoic, Paleozoic, and Mesozoic marine carbonate strata (Riding 1982; Riding 2011; Konhauser and Riding 2012), where they occur in deposits from open sea, lagoon, and reef environments (Wray 1977; Flügel 2004). In all these settings, they play important sedimentological roles in building reefs and producing finegrained and fragmentary materials (Chuvashov and Riding 1984; Riding 1991; Flügel 2004). Strong ecological relationships have been identified between specific calcimicrobes and the environments they inhabit, as shown by the recurrent associations of calcified microbes in the Silurian of southeastern Alaska (Soja and Riding 1993). Although microbial communities are considered to be potential indicators of environmental change and biotic events (Shen et al. 2005), calcimicrobes have often been overlooked in previous paleoecological studies because of their microscopic size, unfamiliar morphology, uncertain taxonomic affinities, and obscurity in recrystallized samples (Wray 1977; Soja and Riding 1993). Although descriptions of calcimicrobes from the Ordovician are relatively sparse (Nitecki et al. 2004), their role as reef-builders in various kinds of reefs (Pitcher 1964; Kapp 1974; Nikitin et al. 1974; Antoshkina 1998; Bian and Zhou 1990; Fang et al. 1993; Chen 1996; Ye et al. 1995; Webby et al. 1997; Pratt and Haidl 2008; Wang et al. 2009; Zhang et al. 2009a; Adachi et al. 2009, 2014a, 2014b; Zhang et al. 2014; Rong et al. 2014; Li et al. 2015),

oncoid and fragment producers in banks and open platforms (Chuvashov and Riding 1984; Riding and Fan 2001), and fragment producers in restricted environments (Walker 1972; Bian and Zhou 1990; Pratt and Haidl 2008; Kwon et al. 2012) has become increasingly widely recognized. Nevertheless, knowledge on the relationship between specific calcimicrobial taxa and the particular environments they inhabited remains very limited. Recently, an extremely diverse Ordovician calcimicrobial flora (including calcified cyanobacteria and associated microbial fossils) consisting of at least 20 genera was discovered in the Tarim Basin (Liu et al. 2016a). Based on a preliminary study, their occurrences in the reef, open platform, and lagoon deposits of the region appear to be closely linked to depositional environment type (Liu et al. 2016a, 2016b). Previous research revealed a close paleoecological relationship between the rivulariacean-like calcified cyanobacteria (Hedstroemia, Ortonella, Zonotrichites, and Cayeuxia) and a restricted lagoon environment in the Upper Ordovician Lianglitag Formation of the Bachu–Tazhong Platform (Liu et al. 2016b). In this study, the sedimentary facies distribution of all 20 calcimicrobial genera are investigated based on material obtained from 64 wells in the Tarim Basin. In addition, a literature survey of their occurrences in other regions and geological times is compiled with the aim of revealing their paleohabitat preferences and understanding their potential significance in paleoenvironmental analysis. GEOLOGICAL SETTING

The Tarim Basin spans a total area of 530,000 km2 and is located in Xinjiang, Northwest China. It is bordered by four mountain ranges: Kunlun to the south and southwest, Altun to the southeast, Tianshan to the

Published Online: July 2017 Copyright Ó 2017, SEPM (Society for Sedimentary Geology) 0883-1351/17/032-462/$03.00

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FIG. 1.—Regional setting of the Tarim Basin and locations of 64 wells used in this study. (Modified from Liu et al. 2016a, based on Jia et al. 1995).

northwest, and Kuluketak to the northeast (Fig. 1). The basin is cut by a series of faults (Yang et al. 2010) that divide it into eight tectonic units or sub-units (Jia et al. 1995) (Fig. 1). During the Ordovician, the Tarim plate was located near the equator (Cocks and Torsvik 2002) and was the site of several carbonate platforms on the the Tazhong, Bachu, Tabei, and West Tadong uplifts, resulting in successions 2000 to 6000 m thick. This study is based on 64 drilling wells that penetrate the Ordovician carbonate sequence in these areas (Fig. 1). Ordovician Carbonate Platform Stratigraphy The Ordovician deposits in the studied areas are divided into six formations from bottom to top: Penglaiba (O1p), Yingshan (O1-2y), Yijianfang (O2y), Tumuxiuk (O3t), Lianglitag (O3l), and Sangtamu (O3s), which in total span almost the entire Ordovician (Fig. 2). However, the Yijianfang and Tumuxiuk formations are absent in most parts of the Tazhong and Bachu uplifts (Fig. 2), and the Sangtamu Formation is markedly diachronous in the Bachu–Tazhong and Tabei platforms. The stratigraphy and lithology of the Ordovician in the studied areas based on previous work as well as the current study is summarized here (Fig. 2) (Zhou et al. 1990; Ni et al. 2001; Gu et al. 2005; Xiong et al. 2006; Zhao et al. 2006; Wang et al. 2007; Cai and Li 2008; Cai et al. 2008; Yang et al. 2009; Li et al. 2009; Zhao et al. 2010; Cai et al. 2012; Gao et al. 2014; Jia et al. 2015; Liu et al. 2016a; Jia et al. 2016 ). Sedimentary History of Ordovician Carbonate Platforms The Ordovician was a critical period for development of the Paleo-Asian Ocean (He et al. 2007). During the mid-Early Ordovician, the Tarim Block

experienced regional extension to slight compression, but this switched to regional compression during the Late Ordovician (Gao and Fan 2014). The Tarim Block records a pattern of sea level change (Fig. 2) similar to that of the global curve (Haq and Schutter 2008): transgression during the Tremadocian to the middle Dapingian, regression in the late Dapingian, transgression in the Darriwilian to Sandbian, and regression in the Katian (Jiang et al. 2001; Bao et al. 2006; Gao et al. 2006; He et al. 2007; Li et al. 2009; Yang et al. 2010). The radiation of calcareous skeletal metazoans in the Middle and Late Ordovician led to a switch from microbial-dominated reefs in the Early and Middle Ordovician to stromatoporoid, coral, and calcareous algae-dominated reefs in the Late Ordovician (Wood 1999; Webby 2002). Lithofacies maps of the studied area during the Ordovician period (Fig. 3) based on regional seismic and well data correlations (Zhang et al. 2007; Feng et al. 2007; He et al. 2007; Zhao et al. 2009; Lin et al. 2011; Gao et al. 2014; Gao and Fan 2015; Liu et al. 2016b) have been updated based on additional observations reported in this paper. Early Ordovician.—During the Early Ordovician, East Tarim was an under-compensated deepwater basin, while most of West Tarim was a large, uniform carbonate platform (Zhao et al. 2009) (Fig. 3A, 3B). During the Tremadocian, an open platform developed with a narrow platform margin and widespread tidal flat facies in the platform’s interior. Laminated dolomites of the Penglaiba Formation were deposited in this area (Fig. 3A; He et al. 2007; Cai and Li 2008; Zhao et al. 2009). During the Floian Stage, an open platform setting developed in the platform’s interior owing to transgression, and packstone and grainstone from the

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FIG. 2.—Stratigraphy and lithology of the Ordovician succession in the study area. (Modified from Liu et al. 2016a, based on Zhou et al. 1990; Ni et al. 2001; Cai et al. 2008; Cai and Li 2008; Li et al. 2009; Yang et al. 2009; Zhao et al. 2010; Gao et al. 2014).

lower part of the Yingshan Formation were deposited (Fig. 3B; He et al. 2007; Cai and Li 2008). Middle Ordovician.—During the Dapingian, most of the uniform platform was still an open platform setting, but some patch banks had developed and packstones and grainstones from the upper part of the Yingshan Formation were also deposited (Fig. 3B; He et al. 2007; Cai and Li 2008; Gao and Fan 2015). During the Darriwilian, the uniform platform split into the Tabei and Bachu-Tazhong Platforms (Zhang et al. 2007; Zhao et al. 2009; Gao and Fan 2014; Gao and Fan 2015). A transition from extension to compression occurred in the mid-Early Ordovician, which caused the uplift and erosion of most parts of the Bachu and Tazhong Platforms with only some margin areas accumulating carbonate deposits (Fig. 3C–3E). During the early Darriwilian, the northwest margin of the Bachu Platform developed tidal flats, while the Tabei Platform remained wide open platform (Li et al. 2007) (Fig. 3C); during the middle-late Darriwilian, marginal reefs built by Calathium and marginal banks appeared on both the Tabei and Bachu-Tazhong Platforms (Fig. 3D; Zhou et al. 1990; Gu et al. 2005; Zhu et al. 2006; Li and Yang 2006; Li et al.

2007; Cai and Li 2008; Li et al. 2009). The occurrence of the Calathium reef in the Yijianfang Formation suggests that the Tarim plate was the last and sole refuge of Calathium at that time (Li et al. 2007), because Calathium-bearing reefs that had thrived globally during the early Ordovician collapsed in the Middle Ordovician (Webby et al. 2002; Li et al. 2007; Wang et al. 2012). Late Ordovician.—During the Late Ordovician, marine transgression occurred and sea level reached its highest point during the Sandbian (Gao and Fan 2014) (Fig. 2), causing the platforms to be drowned and the development of condensed, red, argillaceous limestone of the Tumuxiuk Formation (Fig. 3E; Li et al. 2009). During the early Katian, shallow water reefs built by stromatoporoids, corals, and calcareous algae (Gu et al. 2005; Cai et al. 2008; Yang et al. 2009; Li et al. 2009; Wang et al. 2013; Ma et al. 2014) began to develop (even though the same reefs had developed since the earliest Sandbian in other places; Webby et al. 2002; Wang et al. 2012), and banks formed along the margins of the Bachu-Tazhong Platform. Restricted tidal flat and lagoon environments developed in the interior of the Bachu-Tazhong Platform (Fig. 3F; Yang et al. 2010; Gao et al. 2014),

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FIG. 3.—Changes in paleogeographic distribution of lithofacies in the Tarim Basin during the Ordovician (summarized and modified from Zhang et al. 2007; Feng et al. 2007; He et al. 2007; Zhao et al. 2009; Lin et al. 2011; Gao et al. 2014; Gao and Fan. 2015; Liu et al. 2016b) .

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FIG. 4.—Ordovician calcimicrobe taxa of the Tarim Basin (summarized from Liu et al. 2016a).

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TABLE 1.—Depositional environments types in Tarim Ordovician platform cores. Depositional environment

Sub-environment

Platform margin

Marginal reef

Open platform

Restricted platform Drowned platform Mixed open platform

Margin bank Patch reef Patch bank Open platform Tidal flat Lagoon

Features lack of bedding, presence of in situ sessile benthic organisms, and especially bound or interconnected growth structure grainstone, includes various bioclastics, oncoids similar to marginal reef similar to marginal bank packstone and wackestone fine-grained lime mudstone to wackestone containing fenestral structure micrite, monotonous biota argillaceous wackestone and some mudstone deposits, containing echinoderms, brachiopods, trilobites, and ostracods calcareous mudstones and siltstones interbedded with argillaceous limestone, and containing some dasycladaceans, bryozoans, and corals

whereas an open platform and some patch reefs simultaneously formed on the Tabei Platform (Fig. 3F). Following this, the marginal reef and bank zones of the Bachu-Tazhong Platform disappeared, probably due to a small rise in relative sea level, and the entire Bachu-Tazhong Platform became an open platform facies with some patch reefs (Fig. 3G, Gao et al. 2014; Liu et al. 2016b). In addition, the Tabei Platform began to accumulate the mixed carbonate–siliciclastic deposits of the Sangtamu Formation (Fig. 3G; Zhao et al. 2009; Yang et al. 2011; Gao and Fan 2014). Subsequently, the Bachu-Tazhong Platform became a mixed-deposit platform and the Tabei Platform became a mixed shelf (Fig. 3H; Zhao et al. 2009) that were eventually buried by terrigenous sediments (Feng et al. 2007; Zhang et al. 2007; Cai and Li 2008). MATERIALS AND METHODS

Ordovician drill cores from 64 wells scattered among the Tazhong, Bachu, Tabei, and Tadong uplifts of the Tarim Basin were systematically examined and sampled (Fig. 1, Online Supplementary Material Table 1). More than 8500 large thin sections (5 3 7 cm) were prepared and studied using transmitted light microscopy and microphotography. Identification of calcimicrobes, which include 20 genera, was made in accordance with Liu et al. (2016a). Other calcareous fossils, such as calcareous algae, bryozoans, corals, and stromatoporoids, were identified in accordance with Liu et al. (2012), Chang et al. (2011a, 2011b), Jiang et al. (2011, 2013), and Yang et al. (2012, 2015). Determination of sedimentary facies in drill cores was based on lithological features, sedimentary structures, and community components. In this study, a reef is defined as a laterally confined, biogenic, rigid structure that developed from the growth or activity of sessile benthic organisms with calcified skeletons, and which exhibits topographic relief (Flügel and Kissling 2002). Reef deposits can be recognized in drill cores based on features such as a lack of bedding, presence of in situ sessile benthic organisms, and in particular, a bound or interconnected growth structure. However, both reefs and biostromes are constructed by in situ organisms, but only the former have a topographic relief. Because relief is difficult to observe in drill cores (Zhang et al. 2009a; Wang et al. 2013), some researchers have suggested that both reefs and biostromes should be combined in a single reef facies (Webby 2002), which is the choice made in this study. CALCIMICROBE FLORA IN THE TARIM ORDOVICIAN

A total of 20 genera and 32 species have been identified from the Tarim Ordovician (Liu et al. 2016a) and the main generic characteristics are summarized (Fig. 4). The calcimicrobes referred to in this paper are considered on a generic level because there are no apparent differences in

the paleoenvironmental distribution among different species of the same genera. Almost all these calcimicrobes have previously been considered to have cyanobacterial affinity (Bornemann 1886; Pollock 1918; Pia 1927; Korde 1973; Luchinina 1975; Copper 1976; Hofmann 1975; Riding 1977). Cyanobacterial calcification is not obligate and is mediated by environmental factors as well as biological processes (Golubić 1973; Pentecost and Riding 1986); carbonate saturation state and availability of dissolved inorganic carbon (DIC) are two key external factors influencing its occurrence (Thompson and Ferris 1990; Merz 1992; Kempe and Kaz´mierczak 1994; Riding 2009). Cyanobacteria in present-day oceans do not calcify or only weakly calcify, probably due to a decline in seawater saturation of CaCO3 minerals (Riding 1993; Kempe and Kaz´mierczak 1994), but their calcification is well developed locally in calcareous streams and lakes (Golubić 1973; Pentecost and Riding 1986; Pentecost 2005). In terms of calcification and morphology, many calcimicrobes can be compared with modern cyanobacteria: for example, Girvanella with Plectonema; Subtifloria with Microcoleus; Razumovskia with Phormidium, Ortonella, Hedstroemia, Cayeuxia; Zonotrichites with Rivularia; Phacelophyton and Gomphosiphon probably with Calothrix; and Proaulopora probably with Dichothrix (Liu et al. 2016a; Fig. 4). However, although the cyanobacterial affinities of Renalcis, Epiphyton, Rothpletzella, Wetheredella, and Garwoodia are still upheld by many researchers (Pratt 1984; Kaz´mierczak and Kempe 1992, 2004; Turner et al. 2000), there is no confirmed evidence from modern calcified cyanobacteria, and these affinities are not firmly established (Chafetz and Guidry 1999; Laval et al. 2000; Stephens and Sumner 2002; Luchinina and Terleev 2008; Luchinina 2009; Woo and Chough 2010; Feng et al. 2010; Liu et al. 2016a). SEDIMENTARY FACIES OF THE TARIM ORDOVICIAN

Five depositional environments and nine sub-environments (Armstrong 1974; Wilson 1975; Jin et al. 2016) are recognized based on the available cores (Table 1). The platform margin represents the zone of the platform located below mean low tidal base and above normal wave base. It has relatively high water energy and consists of marginal reef (Fig. 5) and marginal bank (Fig. 6A). The open platform is located below normal wave base, is characterized by a good connection with the open sea, and has moderate water energy; it can be subdivided into open platform (Fig. 6B), patch reef, and patch bank. The restricted platform has a poor connection with the open sea due either to the presence of barriers or its breadth; it can be divided into lagoon (Fig. 6C), developed in the sheltered intertidal zone, and tidal flat (Fig. 6D), developed in the supralittoral zone and intertidal zones. Drowned platform (Fig. 6E) refers to a carbonate platform drowned by a sudden and large-amplitude rise in relative sea-level rise, and mixed-

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deposit open platform (Fig. 6F) refers to a carbonate platform with abundant inputs of siliciclastic sediments. The recognizable sedimentary features of these depositional environments are listed in Table 1. Six types of reef facies have been recognized in the study area (Gu et al. 2005; Zhou et al. 2009; Yang et al. 2009, 2010; Zhang et al. 2009a; Wang et al. 2011; Wang et al. 2013; Ma et al. 2014) (Table 1): Calathium-microbial reefs (Fig. 5A, 5B), Calathium reefs (Fig. 5C), stromatoporoid reefs (Fig. 5D, 5E), calcareous red algal reefs (Fig. 5F–5H), coral reefs (Fig. 5I, 5J), and microbial-calcareous green algal reefs (Fig. 5K, 5L) (here, groups with lower abundance appear before those with higher abundance). Although calcimicrobes are not referred to within the names of all of these reef facies types, they are present in almost all. RELATIONSHIPS BETWEEN PALEOENVIRONMENTS AND CALCIMICROBES

There is no record of calcimicrobes in the drowned platform deposits of the Tumuxiuk Formation, but they are widely distributed in marginal and patch reef, open platform, margin and patch bank, and lagoon, and are locally present in mixed open platform and tidal flat deposits of the Yingshan, Yijianfang, and Lianglitag formations. In addition, some calcimicrobial taxa are always closely associated with certain facies and recur in many wells (Figs. 7, 8); these are defined as associations, of which 10 with distinct taxonomic composition and facies distribution are recognized throughout the whole Ordovician (Fig. 9). Each association consists of two to eight genera and is named after the main taxonomic components (with the group with low abundance appearing before that with higher abundance) (Fig. 10). There are no apparent differences in calcimicrobial associations between marginal reefs and patch reefs or among open platforms, margin banks, and patch banks; thus, it can be concluded that calcimicrobes are mainly related to the following four paleoenvironment types: (1) reef (including marginal and patch reefs); (2) open platform/bank (including marginal and patch banks); (3) lagoon; and (4) tidal flat. Calcimicrobial Associations in Reef Environments Calcimicrobes are represented in all six reef environments (Table 1, Fig. 5), but their taxonomic composition and abundance differ among them. Calathium-Microbial Reefs.—Calathium-microbial reef deposits are found in the Yijianfang Formation of well GC4, (Fig. 3D; Wang et al. 2011) with a Renalcis–Epiphyton association (Fig. 11A–11C) consisting of abundant Epiphyton, some Renalcis, and rare Girvanella as the main reef-builders (Fig. 5A, 5B). Epiphyton is the framework-builder and Renalcis fills the interstices of the skeletal and microbial framework (Fig. 11A–11C). Calathium is a secondary reef-builder (Wang et al. 2011) and encruster with faintly microbial laminae (Fig. 5B). Only a few echinoderms occur as attached organisms. Calathium Reefs, Stromatoporoid Reefs, Calcareous Red Algal Reefs.—Calathium reefs are recognized in the Yijianfang Formation of wells TK1, HA902, HD17, HD13, and LN50, with Calathium and lithistid

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sponges as the main reef builders (Fig. 5C), with some laminated microbial encrusters and echinoderms as dwellers. The taxonomic affinity of Calathium has been debated for over a century, and it has recently been considered as a kind of hypercalcified sponge closely related to archaeocyathans (Li et al. 2015). A similar reef type also occurs in outcrop in the Bachu area (Fig. 3D) (Li and Yang 2006; Li et al. 2007; Cai et al. 2008; Rong et al. 2014; Li et al. 2016), where bryozoans encrust the walls of Calathium (Li et al. 2016). Stromatoporoid reefs are recognized from the Lianglitag Formation in wells TZ822, TZ82, and M401 and include Pachystylostroma (Fig. 5D), Clathrodictyon (Fig. 5E), Cystistroma, Labechia, Rosenella, and Ecclimadictyon (identifications based on Yang et al. 2012; Jiang et al. 2011), which occur in various growing forms (laminar, tabular, anastamosing laminar and tabular, domal, and bulbous) and probably functioned as the main reef builders. Some corals, such as Favosites, Heliolites, and Pachypora (identifications based on Jiang et al. 2013), and green algae, such as Vermiporella, also contributed to reef building, with echinoderms and bryozoans as the main dweller organisms. Calcareous red algal reefs are recognized in the Lianglitag Formation in wells M401, TZ822, TZ70, and TZ242. Red algae, such as Solenopora (Fig. 5F–5H), Parachaetetes, and Petrophyton (identified based on Liu et al. 2012), often occur as crustose or articulated (Fig. 5F–5H) and are also the main reef builders in the Late Ordovician stromatoporid-coral-red algal patch reefs of South China, Jiangxi (Fang et al. 1993). Calcareous green algae, such as Vermiporella, also occur as a secondary reef builder, and echinoderms and bryozoans are the main dweller organisms. The Wetheredella–Rothpletzella association, consisting of abundant Rothpletzella, Wetheredella, and sparse Girvanella, is not only locally distributed in Calathium reefs, but is also widely present in the red algal and stromatoporoid reefs (Fig. 11D–11I). Rothpletzella and Wetheredella occur as the main builders in these reefs, and encrust the surfaces of skeletal organisms, growing and accreting upwardly and laterally, and contributing to the formation of the reef framework (Adachi and Ezaki 2007, Fig.11D–11I). Microbial-Calcareous Green Algal Reefs.—Microbial-calcareous green algal reefs are recognized in the Lianglitag Formation in wells TZ42,TZ822, LG36, LN14, LN63, JF127, and LG391 (Figs. 7, 8). The main reef builders are diverse branching calcareous green algae, such as Vermiporella, Dasyporella, Kazakhstanelia, Aphroporella, and Palaeoporella (Liu et al. 2012), which generally compose the framework (Fig. 5K, 5L), with echinoderms as the dwellers. This reef type is similar to the calcareous green algal reefs in the Lianglitag Formation from the Bachu area (Cai et al. 2008; Wang et al. 2009; Zhang et al. 2014), the Upper Ordovician reef in the Sanqushan Formation in Jiangxi Province, South China (Bian and Zhou 1990), and the Upper Ordovician reef in the Chu Ili Mountains of Kazakhstan (Nikitin et al. 1974). The Renalcis–Phacelophyton association consists of abundant Girvanella, Phacelophyton, Renalcis, and Izhella, some Acuasiphonoria, and rare Razumovskia, Gomphosiphon, and Rothpletzella (Fig. 12), which occur as the main reef builders (Fig. 5K, 5L). Acuasiphonoria (Fig. 12G, 12K) and Phacelophyton (Fig. 12A, 12E, 12G, 12I) are always responsible for constructing the reef framework, together with some types of green algae, such as

FIG. 5.—Photographs of various reef deposits in cores. Abbreviations: Epip. ¼ Epiphyton; Cala. ¼ Calathium; Pach. ¼ Pachystylostroma; Sole. ¼ Solenopora; Echi. ¼ echinoderm; Amsa. ¼ Amsassia; Tetr. ¼ Tetradium; Un. ¼ unidentified organism; Verm. ¼ Vermiporella. A, B) Calathium-microbial reef, GC4-6-43-33, O2y; framework of calcimicrobe Epiphyton, photograph of core (A) and framework of calcimicrobe Epiphyton micrograph (B). C) Calathium reef, Calathium framework, TK1-14-5-15, O2y, photograph of core. D, E) Stromatoporoid reef; stromatoporoid Pachystylostroma framework, TZ822-8-108-85, O3l, photograph of core (D) and stromatoporoid Clathrodictyon framework, M401-24-70-53, O3l, micrograph of thin section (E). F–H) Calcareous red algal reef; framework of Solenopora (longitudinal and cross section), M401-23-68-23, O3l, photograph of core (F); framework of Solenopora (longitudinal and cross sections), M401-23-68-23, O3l, micrograph of thin section (G); framework of Solenopora (longitudinal and cross sections), M401-23-68-22, O3l, micrograph of thin section (H). I, J) Coral reef, Amsassia and Tetradium framework, TZ822-12-66-47, O3l, photograph of core (I) and Amsassia and unidentified organism framework (J). K, L) Microbial-calcareous green algal reef, framework of Vermiporella, TZ42-2-49-11, O3l, photograph of core (K) and framework of Vermiporella, TZ42-2-49-46, O3l, micrograph of thin section (L).

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FIG. 6.—Photographs of bank, open platform, restricted platform, drowned platform, and mix platform deposits in cores. Abbreviations as in Figure 5 and: onco. ¼ oncoids; Quep. ¼ Quepora. A) Marginal bank, bioclastic grainstone, containing oncoids and bioclastics, TZ822-1-43-22, O3l, photograph of core. B) Open platform, wackestone, YM201-47-78-58, O1-2y, photograph of core. C, D) Restricted platform, lagoon, micritic limestone, containing coral Tetradium, 5090.41 m of TZ72, O3l, micrograph (C) and tidal flat, micritic limestone, with fenestral structure, TZ166-8-22-51, O3l, photograph of core (D). E) Drowned platform, argillaceous packstone and wackestone, containing echinoderms (echi.) and trilobites, LG34-1-67-37, O3t, micrograph of thin section. F) Mixed open platform, argillaceous wackestone containing corals, such as Quepora HE39-43-6, O3s, photograph of core.

Vermiporella. Izhella and Renalcis fill in the interstices of the skeletal and microbial frameworks, thereby enhancing rigidity (Adachi and Ezaki 2007; Fig. 12C, 12D, 12F–12H). The rare Rothpletzella (Fig. 12H), Razumovskia (Fig. 12J), and Gomphosiphon (Fig. 12B) form both microbial crusts and framework (Fig. 12G, 12K). Girvanella, Renalcis, Izhella, together with rare Rothpletzella, Subtifloria, and Ortonella are also recorded in the microbial-green algal reef of the Bachu Platform (Wang et al. 2009; Zhang et al. 2014).

Coral Reefs.—Coral reefs are recognized in the Lianglitag Formation in wells TZ242, TZ24, and TZ822. The corals Amsassia and Tetradium (Fig. 5I, 5J), as well as an unidentified organism (Fig. 5J), are the main framework builders (Fig. 5I, 5J). Amsassia is also the main reef builder in the Middle Ordovician reefs of the Jinghe Formation in north-central China (Lee et al. 2014a). Amsassia is generally regarded as a tabulate coral, although some researchers have recently considered it to be an alga (Sun et al. 2014). Tetradium is also generally considered to be a tabulate coral, although some

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FIG. 7.—Stratigraphic and sedimentary facies distribution of calcimicrobes in wells from the Tabei and Bachu Platforms.

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FIG. 8.—Stratigraphic and sedimentary facies distribution of calcimicrobes in wells from Tazhong Platform.

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FIG. 9.—Occurrences of ten Ordovician calcimicrobial associations in specific wells and stratigraphic and sedimentary facies.

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FIG. 10.—Taxonomic composition of each calcimicrobial association.

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FIG. 11.—Micrographs of Renalcis–Epiphyton and Wetheredella–Rothpletzella associations. Abbreviations as in Figure 5 and: Rena. ¼ Renalcis; Roth. ¼ Rothpletzella; Weth. ¼ Wetheredella; Rose. ¼ Rosenella; Girv. ¼ Girvanella. A–C) Renalcis–Epiphyton association from Calathium–microbial reef, framework of Epiphyton, GC4-6-48-33, O2y (A, B) and framework of Renalcis (picture from Wang et al. 2011), GC4, O2y (C). D–I) Wetheredella–Rothpletzella association: (D–F) from Calathium reef: framework of Calathium, Rothpletzella as encrusters, HA902-3-71-43, O2y (D); framework of Calathium, Wetheredella as encrusters, TK1-13-22-16, O2y (E); Rothpletzella and Wetheredella as encrusters in Calathium reef, TK1-15-35-10, O2y (F); G–I) from red algal and stromatoporoid reef: framework of stromatoporoid Rosenella and red algae Solenopora, Wetheredella, and Rothpletzella as encrusters, M401-24-70-25, O3l (G); framework of red algae Solenopora and some green algae Vermiporella, Rothpletzella and Girvanella as encrusters, TZ822-8-108-53, O3l (H); and framework of stromatoporoid Pachystylostroma, Wetheredella, and Rothpletzella as encrusters, echinoderm attached, 5614.32 m of TZ822-8-108-53, O3l (I).

researchers have considered it to possibly be a red alga (Steele-Petrovich 2009), occurring as a minor reef builder or reef dweller (Fig. 5I, 5J). The Acuasiphonoria–Proaulopora association consists of abundant Proaulopora, some Acuasiphonoria and Gomphosiphon, sparse Apophoretella and Bija, and rare Girvanella (Fig. 13); this association is distributed throughout the reef. Proaulopora (Fig. 13A, 13B, 13G, 13H, 13J, 13K) and Gomphosiphon (Fig. 13C) are often abundantly attached on the surface of corals and the unidentified organisms. Acuasiphonoria commonly forms the reef framework (Fig. 13D, 13I), with some Apophoretella, Bija (Fig. 13E), and rare Girvanella (Fig. 13M) as dwellers. Calcimicrobial Associations in Open Platform/Bank Environments Paleoenvironment Interpretation.—Marginal and patch banks composed of grainstone are recognized in many formations in the study area. The grains are mainly intraclasts, trilobites, and echinoderms in the Yingshan Formation; Calathium, bryozoans, and echinoderms in the Yijianfang Formation; and corals, echinoderms, bryozoans, calcareous algae, ooids, and oncoids (also see Zhang et al. 2009a, 2009b; Fig. 6A) in the Lianglitag Formation (also see Zhang et al. 2015a), suggesting an agitated, open, shallow-water environment. Open platform deposits are characterized by packstone and wackestone, representing a moderately high water energy environment (Fig. 6B). There are no obvious differences between the bioclastic types from the open platform and the bank, which further evidences an open shallow-water environment. A mixed open platform is recognized in the lower part of the Sangtamu Formation in many wells and is characterized by calcareous mudstones and siltstones interbedded with argillaceous limestone, and some dasycladaceans, bryozoans, and corals (Fig. 6F), suggesting a turbid, open, shallow-water environment. Calcimicrobes.—The Girvanella–Proaulopora association, consisting of sparse amounts of Girvanella and some Proaulopora, occurs in the open platform/bank facies of the Yingshan Formation (Fig. 14A–14C) and occurs with echinoderms and brachiopods (Fig. 14A). Girvanella always forms small masses (Fig. 14A), and Proaulopora occurs as short and unbranched tubiforms, although their whorl-like external collars are often absent, probably because of abrasion in the shallow and agitated water (Fig. 14A–14C). The Girvanella–Subtifloria association, consisting of abundant Girvanella, Subtifloria, and rare Proaulopora, occurs in the open platform/bank facies of the Yijianfang Formation (Fig. 14D–14I), growing together with abundant echinoderms (Fig. 14D–14I), brachiopods, and some bryozoans. The occurrence of Subtifloria in the packstones and grainstones of the Yijianfang Formation has also been reported by previous researchers (Riding and Fan 2001; Rong et al. 2014). The Proaulopora– Bija association, consisting of abundant Bija and Proaulopora, locally

occurs in the open platform/bank facies of the Lianglitag Formation (Fig. 14J–14O), and has minimal association with other metazoans and algae; Proaulopora (Fig. 14L, 14M) and Bija (Fig. 14J, 14K, 14N, 14O) are often present as individual fragments. The Rothpletzella–Girvanella association, consisting of abundant Girvanella, Rothpletzella, and Wetheredella, some Apophoretella, rare Bevocastria, Garwoodia, and Ortonella, is widely distributed in open platform/bank deposits of the Lianglitag Formation (Fig. 15) in association with various organisms, such as corals, echinoderms, bryozoans (Fig. 15D), red algae (Fig. 15A), and green algae, indicating an open and shallow water environment. Girvanella often forms masses, or is associated with Wetheredella and Rothpletzella in oncoids (Fig. 15A, 15G), and sometimes Ortonella is involved (Fig. 15G). Some Apophoretella (Fig. 15B, 15F), rare Bevocastria (Fig. 15D), Garwoodia (Fig. 15C), and Zonotrichites (Fig. 15E) occur as individual fragments. Oncoids formed by Girvanella, Wetheredella, and Rothpletzella are also locally present in the mixed-deposit open platform of the Sangtamu Formation (Fig. 15H, 15I), which suggests that these genera could tolerate turbid water to some degree. Calcimicrobial Association in the Lagoon Environment Paleoenvironment Interpretation.—Lagoonal deposits are typically fine-grained with a dominant micritic composition and monotonous biota generally consisting of Tetradium (Figs. 6C, 16H),ostracodes, gastropds and several calcimicrobe genera (Fig. 16A–16H). The main community difference between lagoonal and marginal and open platform facies is its relatively low-diversity, and scarcity of echinoderms, bryozoans, brachiopods, dasycladaleans, red algae, stromatoporoids, oncolites, and reef encrusters formed by calcimicrobes, such as Girvanella, Wetheredella, and Rothpletzella, indicating a restricted back-reef environment with relatively low water energy and abnormal salinity and nutrients (Liu et al. 2016b). These features occur widely in the Lianglitag Formation in many wells from the interior of the Bachu-Tazhong Platform (Fig. 3F). Calcimicrobes.—This facies is dominated by the Hedstroemia– Cayeuxia association (Fig. 16A–16H; Liu et al. 2016b) characterized by abundant Hedstroemia, Ortonella, Zonotrichites, Cayeuxia, and some Garwoodia. These calcimicrobes occur in micritc and peloidal limestone in association with abundant Tetradium (Fig. 16H), an unidentified organism, ostracodes, gastropods, and a few green algae (Dimorphosiphonoides) (Liu et al. 2016b). Calcimicrobial Associations in the Tidal Flat Environment Paleoenvironment Interpretation.—Tidal flat deposits are characterized by fine-grained lime mudstone to wackestone with fenestral

! FIG. 12.—Micrographs Renalcis–Phacelophyton association from microbial-calcified green algal reef (A–C from TZ822, O3l; D–G from LN63, O3l; H–K) from calcified green algal reef of TZ42, O3l). Abbreviations as in Figure 5 and: Phac. ¼ Phacelophyton; Gomp. ¼ Gomphosiphon; Izhe. ¼ Izhella; Acua. ¼ Acuasiphonoria. A) Framework of Vermiporella and Phacelophyton, 12-66-57. B) Gomphosiphon, 12-66-41. C) Framework of Vermiporella, Renalcis, Izhella, Girvanella, 12-66-47. D) Framework of Vermiporella and Phacelophyton, 7-64-25. E) Framework of Phacelophyton and Renalcis, 7-64-23. F) Renalcis framework, 7-64-30. G) Acuasiphonoria and Renalcis framework, 7-64-30. H) Renalcis as frame builder, 4-53-14. I) Framework of Vermiporella and Phacelophyton, 4-53-22. J) Razumovskia as frameworker, 2-49-2. K) Framework of Acuasiphonoria, 4-53-49.

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FIG. 13.—Micrographs of Acuasiphonoria–Proaulopora association from coral reef (A–E from TZ822, O3l; F–I from TZ24, O3l; J–L from TZ241, O3l). Abbreviations as in Figure 5 and: Proa. ¼ Proaulopora; Apop. ¼ Apophoretella. A, B) Proaulopora attached on unidentified organism, 12-66-15. C) Gomphosiphon intergrowth with coral, 12-66-35. D) Acuasiphonoria framework, 5850.33 m. E) Bija as reef framework builder, with Apophoretella intergrowth, 12-66-32. F) Proaulopora as dweller with coral Tetradium, 14-35-34. G) Proaulopora attached on surface of coral Amsassia, 16-35-15. H) Proaulopora intergrowth with coral, 17-43-9. I) Acuasiphonoria as framework, 16-35-8. J) Proaulopora intergrowth with coral Tetradium, 4-36-27. K) Proaulopora attached on surface of coral Amsassia, 10-52-5. L) Unidentified organism, 13-45-18. M) Girvanella as dweller in reef, 10-52-10.

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FIG. 14.—Micrographs of Girvanella–Proaulopora, Girvanella–Subtifloria Proaulopora–Bija association from open platform and bank. Abbreviations as in Figure 5 and: Subt. ¼ Subtifloria; brac. ¼ brachiopod. A–C) Girvanella–Proaulopora association: packstone, containing Girvanella masses, echinoderms, and brachiopods, LG36-9-132-9, O1-2y (A); containing Proaulopora, LG36-10-128-3, O1-2y (B); and grainstone with Proaulopora, ZG16-1-94-21, O1-2y (C). D–I) Girvanella–Subtifloria association: bioclastic packstone and grainstone, Girvanella, Subtifloria and echinoderms intergrowing, YM1-Y-25, O2y (D); bioclastic packstone and grainstone with Girvanella, echinoderms, TK1-10-24-24, O2y (E); bioclastic packstone and grainstone with Subtifloria and echinoderms, TK1-12-16-15, O2y (F); bioclastic packstone and grainstone with Girvanella, Subtifloria, and Proaulopora, GC4-2-52-4, O2y (G); bioclastic packstone and grainstone with Subtifloria, HA902-3-71-5, O2y (H); and bioclastic packstone with Girvanella, Subtifloria and echinoderms, YM2-43, O2y (I). J–O) Proaulopora–Bija association: bioclastic grainstone containing Bija, TZ24-16-35-11, O3l (J); packstone containing Bija, LN621-2-52-22 (K); bioclastic grainstone and packstone with Proaulopora, JF127-14-58-9, O3l (L); bioclastic grainstone and packstone containing Proaulopora, from 5853.37 m of LN63, O3l (M); bioclastic grainstone and packstone containing Bija, LN621-2-52-22, O3l (N); packstone containing Bija, JF12713-20-12, O3l (O).

synsedimentary structures (Fig. 6D) related to temporary subaerial exposure and desiccation under the supralittoral and intertidal conditions; tidal flats are a hostile environment for most marine organisms. Tidal flat deposits are recognized in parts of the Penglaiba, Yijianfang, and Lianglitag formations in some wells from the Bachu-Tazhong Platform (Fig. 3F). Calcimicrobes.—There are no records of calcimicrobes in the dolostones of the Penglaiba Formation. Although rare Proaulopora and Girvanella, which compose the Proaulopora–Girvanella association, locally appear in the tidal flat facies of the Yijianfang and Lianglitag formations (Fig. 16I–16M), the lack of any other kind of associated fossil suggests a hostile supralittoral and intertidal environment. Calcimicrobes occur in lime mudstones and wackestones with fenestral structure (Fig. 16I, 16K), where Proaulopora commonly occurs as individual tubes and locally as branched tubes (Fig. 16K, 16M) and Girvanella occurs as very slender filaments (Fig. 16J, 16L). TEMPORAL AND PALEOENVIRONMENTAL DISTRIBUTION OF CALCIMICROBES IN THE ORDOVICIAN TARIM PLATFORM

During deposition of the Penglaiba Formation in the Tremadocian Stage, calcimicrobes were not present or they left no fossil record (Fig. 17A). When the Yingshan Formation was deposited in the Floian to Dapingian Stages, the Girvanella–Proaulopora association lived in widespread open platform facies (Fig. 17B). Subsequently, during deposition of the Yijianfang Formation in the Darriwilian Stage, many sedimentary environments developed, including marginal Calathium reefs, marginal Calathium-microbial reefs, open platforms/banks, and tidal flats; within these, the Wetheredella–Rothpletzella, Renalcis–Epiphyton, Girvanella– Subtifloria, and Proaulopora–Girvanella associations occurred (Fig. 17C). During the deposition of the Tumuxiuk Formation in the Sandbian Stage, the previous platforms were drowned and no calcimicrobes survived (Fig. 17D). Following this, when the Lianglitag Formation was deposited in the early and middle Katian Stage, many kinds of sedimentary environments developed and various microbial associations occurred within them: Wetheredella–Rothpletzella, Acuasiphonoria–Proaulopora, and Renalcis–Phacelophyton within the stromatoporoid reefs/calcareous red algal reefs, coral reefs, and microbial-calcareous green algal reefs, respectively;

Rothpletzella–Girvanella and Proaulopora–Bija in open platforms/banks; and Cayeuxia–Hedstroemia and Proaulopora–Girvanella in lagoons and tidal flats, respectively (Fig. 17E). During the middle and late Katian Stage, a mixed-deposit open platform formed, and most calcimicrobes disappeared, with only very a few Rothpletzella–Girvanella surviving (Fig. 17F). Temporal change of the calcimicrobial associations in the Tarim Ordovician platform is controlled by two main factors: global environmental conditions and local depositional environments (Lee et al. 2014b). Environmental influences on the calcification of cyanobacteria are partially reflected in changes in their abundance through time, ranging from rockforming to virtually absent (Riding 1992, 2006, 2009; Arp et al. 2001). Global environmental conditions include factors like microbe evolution, sea water carbonate saturation state, availability of dissolved inorganic carbon, and evolution of new Ordovician reefs that provided new ecological niches for calcimicrobes, resulting in more complex and diverse reefs from the Darriwilian onwards (Webby 2002). Previous research suggested that the abundance of calcimicrobes after the Middle Ordovician declined (Riding 1991, 1992; Riding and Fan 2001). However, our study found that calcimicrobes in the Late Ordovician Tarim Platform are much more diverse and abundant than Early and Middle Ordovician (Liu et al. 2016a and this study). Changes in microbial associations in the Ordovician of Tarim are also controlled by local depositional conditions. Changes in the diversity and abundance of calcimicrobial associations in the Ordovician Tarim Platform are closely related to changes in the diversity of ecological niches associated with changes in the abundance and distribution of reef, open platform, lagoon, and tidal flat environments. The absence of calcimicrobes from the Sandbian Stage is probably due to the increase in water depth at that time, and their significant reduction in the Hirnatian Stage is likely related to the input of siliciclastic sediments (Lee et al. 2014b). Furthermore, different calcimicrobial associations are tightly linked to particular sedimentary facies, which suggests a strong ecological relationship between specific calcimicrobes and their paleoenvironment (Fig. 17): the reef environment is related to Girvanella, Razumovskia, Acuasiphonoria, Phacelophyton, Gomphosiphon, Proaulopora, Bija, Apophoretella, Epiphyton, Renalcis, Izhella, Wetheredella, and Rothpletzella; the open platform/bank environment is related to Girvanella, Proaulopora, Bija, Apophoretella, Bevocastria, Subtifloria, Wetheredella,

! FIG. 15.—Rothpletzella–Girvanella association from open platform/bank. Abbreviations as in Figure 5 and: dasy. ¼ dasycladaceans; bryo. ¼ bryozoan; Bevo. ¼ Bevocastira; Orto. ¼ Ortonella. A) Packstone containing oncoids, Rothpletzella as encrusters and red algae solenopora as nucleus, TZ72-10-104-15, O3l. B) Apophoretella in packstone, TZ72-10-104-15, O3l. C) Garwoodia in packstone accompanied by echinoderms, T73, 4719.16 m, O3l. D) Bevocastira in wackestone accompanied by bryozoan, TZ23-9-47-5, O3l. E) Zonotrichities in packstone, TZ822, O3l. F) Apophoretella in packstone, TZ73-6-79-31, O3l. G) Packstone containing oncoid, Girvanella, Wetheredella, and Rothpletzella as encrusters, and Ortonella as nucleus, TZ822-5-139-85, O3l. H) Calcareous mudstone and argillaceous limestone, containing oncoid, Wetheredella, and Rothpletzella as encrusters, TZ35-9-38-38, O3s. I) Calcareous mudstone and argillaceous limestone containing oncoid, Rothpletzella as encrusters, and bryozoan as nucleus, HE3-9-43-3, O3s.

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and Rothpletzella; the lagoon environment is related to Hedstromia, Cayeuxia, Zonotrichites, Ortonella, and Garwoodia; and the tidal flat environment is related to Girvanella and Proaulopora. PALEOENVIRONMENTAL DISTRIBUTION OF CALCIMICROBES AT OTHER TIMES AND DIVISION OF PALEOHABITAT TYPES

Previous researchers investigating the paleoenvironmental distribution of calcimicrobes have determined that the Epiphyton-group and Renalcisgroup commonly occurred in microbial or metazoan-bearing microbedominated reefs throughout geological time (Chuvashov and Riding 1984; Riding 1991). Girvanella often occurs as individual masses in the Paleozoic (Riding 1991), and is believed to have produced lime mud when its tubes fell apart (Pratt 2001). Girvanella, Wetheredella, and Rothpletzella are common in oncoids from the Ordovician to Carboniferous (Chuvashov and Riding 1984; Riding 1991). A literature survey by Liu et al. (2016b) revealed an ecological relationship between Hedstroemia, Cayeuxia, Ortonella, Zonotrichites, and Garwoodia and restricted environments from at least the Ordovician to the Cretaceous. Based on a new, extensive literature survey on the paleoenvironmental distributions of calcimicrobial associations throughout geological times (Table 2), most of the calcimicrobes in the Tarim Ordovician have the same, or similar, environmental distributions as many times in the Paleozoic and Mesozoic, although their association with special reef types is not as clear as in the Tarim. The twenty Ordovician calcimicrobial genera documented in the Tarim Platform can be classified into five paleohabitat types according to their main paleoenvironmental preferences (Fig. 17): (I) reef-adapted (Acuasiphonoria, Razumovskia, Phacelophyton, Gomphosiphon, Epiphyton, Renalcis, and Izhella); (II) open platform/bank-adapted (Subtifloria and Bevocastira); (III) both reef and open platform/bankadapted (Bija, Apophoretella, Rothpletzella, and Wetheredella); (IV) lagoon-adapted (Hedstromia, Cayeuxia, Zonotrichites Ortonella, and Garwoodia); and (V) not only reef and open platform/bank-adapted but also tolerant of tidal flat conditions (Girvanella and Proaulopora). PALEOENVIRONMENT SIGNIFICANCE

Modern tropical marine coastal ecosystems are characterized by specific cyanobacterial flora, such as Oscillatoria, Lyngbya, Microcoleus, Phormidium Calothrix, Schizothrix, and Scytonema (Hoffmann 1999). The highest biodiversity of cyanobacteria is observed in littoral zones, where they form intertidal and infralittoral mats, live as endoliths in carbonate substrates, or form symbiotic associations (Hoffmann 1999). Mangrove forests and coastal lagoons are inhabited by diverse cyanobacterial communities that reside on leaf litter, root litter, and live roots, and often form extensive mats on the surrounding sediments. Very high cyanobacterial community diversity is found near coral reefs, playing an essential role in the ecology of modern reef ecosystems by forming a major component of epiphytic, epilithic, and endolithic communities (Charpy et al. 2012). As photosynthetic organisms, cyanobacteria are important contributors to benthic and open ocean primary production, but their main role in tropical marine ecosystems appears to be as nitrogen fixers. Determining the distribution of modern marine cyanobacteria may assist in

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understanding the paleoenvironmental significance of calcified cyanobacteria fossils, although diversity in the less accessible infralittoral currently remains practically unknown. Ancient calcified rivulariaceans (such as Hedstroemia, Ortonella, Cayeuxia, and Zonotrichities) occurred abundantly in restricted lagoons and similar environments throughout geological time, but modern uncalcified rivulariaceans are also common in back-reef, lagoon, mangrove swamp, rocky shore, salt marsh, and saline lake environments. This is interpreted to be related to their ability to fix nitrogen and utilize organic phosphate in oligotrophic sea water (Liu et al. 2016b). Though rivulariacean-like cyanobacteria (such as Bija and Apophoretella) also locally appear to be nitrogen fixers in both open platform and reef environments, the abundant occurrence of other calcified cyanobacteria (such as Girvanella) in the same settings may reflect better N and P conditions in these environments than in restricted lagoons. The high diversity of calcimicrobes in reefs may be attributed to a sheltered habitat where predators can be avoided (Sheehan and Harris 2004), and the very low diversity of calcimicrobes on tidal flats suggest an environment too hostile for most organisms, due to subaerial exposure and desiccation (Jin et al. 2016), where only rare Girvanella and Proaulopora could survive. Overall, these relationships suggest that calcimicrobial fossils and their associations can be regarded as facies fossils, which can indicate ancient sedimentary facies and infer some paleoecological parameters of ancient seas, especially with significant potential in strata lacking other wellstudied facies fossils. CONCLUSIONS

The paleoenvironmental distribution of 20 calcimicrobial genera from the Ordovician carbonate platform of the Tarim Block demonstrate strong ecological relationships between calcimicrobe genera and the environment they inhabited. An extensive literature review shows that the paleoenvironmental distribution of most calcimicrobes are the same, or similar, throughout many different geological times and regions. Based on their main paleoenvironmental preferences, calcimicrobes can be subdivided into five paleohabitat types: (1) reef-adapted (Acuasiphonoria, Razumovskia, Phacelophyton, Gomphosiphon, Epiphyton, Renalcis, and Izhella); (2) open platform/bank-adapted (Subtifloria and Bevocastira); (3) both reef and open platform/bank-adapted (Bija, Apophoretella, Rothpletzella, and Wetheredella); (4) lagoon-adapted (Hedstromia, Cayeuxia, Zonotrichites Ortonella, and Garwoodia); and (5) not only reef and open platform/bank-adapted but also tolerant of tidal flat conditions (Girvanella and Proaulopora). The occurrences of these calcimicrobes in strata not only can indicate ancient sedimentary facies but also can reveal paleoecological parameters of ancient seas, such as nutrient levels (e.g., N and P), predation pressure, and sea level, especially in strata absence of other well-studied facies fossils. ACKNOWLEDGMENTS

We are very grateful to Robert Riding (USA), Jiasong Fan, and Yongding Dai (Beijing) for their very helpful suggestions and fruitful discussions on the study of both sedimentary facies and fossils identifications. Several anonymous reviewers are acknowledged for their constructive and helpful comments. This

FIG. 16.—Micrographs of Hedstroemia–Cayeuxia from lagoonal deposits and Girvanella–Proaulopora association from tidal flat deposits. Abbreviations as in previous captions (Figs. 5, 13) and: Heds. ¼ Hedstroemia; Zono. ¼ Zonotrichities. A–E) Hedstroemia–Cayeuxia association from lagoon: micritic limestone containing Hedstroemia, TZ72-12-84-10, O3l (A); micritic limestone containing Cayeuxia, M5-19-40-29, O3l (B); micritic limestone containing Ortonella, TZ83-3-50-23, O3l (C); micritic limestone containing Zonotrichities, O3l (D); micritic limestone containing Hedstroemia, TZ161, 4439.6 m, O3l (E). F) Micritic limestone containing Garwoodia, from 4475.63 m of TZ43, O3l. G) Micritic limestone, containing Hedstroemia, TZ23-9-47-36, O3l. H) Zonotrichites associated with Tetradium, O3l, from 4696.56 m of well TZ7. I–M) Proaulopora–Girvanella association from tidal flat: Girvanella occurrence in micritic limestone with fenestral structure, TK1-26-15-8, O2y (I, J); Proaulopora occurrence in micritic limestone with fenestral structure, TK1-22-19-17, O2y (K); Girvanella occurrence in micritic limestone with fenestral structure, TZ161-23-50-25, O3l (L); Proaulopora occurrence in micritic limestone with fenestral structure, TZ451-6-42-13, O3l (M).

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FIG. 17.—Diagrammatic sketch of temporal and paleoenvironmental distribution and division of paleohabitat types of Ordovician calcimicrobes in Tarim platform.

Girvanella, Phacelophyton Proaulopora, Razumovskia, Bija, Rothpletzella, Wetheredella, Epiphyton, Renalcis, Izhella (rare Apophoretella Ortonella, Zonotrichites, and Garwoodia)

Related genera

Association

Renalcis, Epiphyton Girvanella, Renalcis, Epiphyton, Tarthinia, Gordonophyton, Angulocellularia Epiphyton, Gordonophyton, Proaulopora, Razumovskia Epiphyton,Kordephyton,Bija, Tarthinia, Renalcis, Razumovskia Girvanella, Renalcis, Epiphyton,, Proaulopora, Bija, Apophoretella Proaulopora Girvanella, Epiphyton, Renalcis, Izhella, Proaulopora, Razumovskia Girvanella, Renalcis, Epiphyton, Kordephyton, Bija Girvanella, Epiphyton, Renalcis, Izhella, Kordephyton, Apophoretella-like

Girvanella Renalcis, Epiphyton

Rothpletzella, Wetheredella Girvanella, Rothpletzella Girvanella, Rothpletzella Proaulopora Razumovskia Girvanella, Rothpletzella

Girvanella, Renalcis

Rothpletzella, Wetheredella Rothpletzella Rothpletzella, Wetheredella, Epiphyton, Renalcis Razumovskia Zonotrichites-like, Girvanella-like Girvanella, Razumovskia, Renalcis, Phacelophyton, Proaulopora, Epiphyton Renalcis, Phacelophyton

Rothpletzella, Wetheredella

Girvanella, Ortonella Renalcis, Izhella, Rothpletzella, Wetheredella, Girvanella Epiphyton, Renalcis Renalcis, Izhella, Paraepiphyton, Rothpletzella, Wetheredella, Girvanella, Ortonella, Garwoodia Girvanella, Renalcis, Rothpletzella Rothpletzella, Wetheredella Girvanella, Rothpletzella, Wetheredella, Epiphyton, Renalcis, Izhella, Ortonella Rothpletzella, Wetheredella Rothpletzella, Wetheredella

Calcimicrobes

Reef facies

General

D3 D3 D3 D2 D1

Microbialites Calcimicrobial-stromatoporoid reef Microbial bindstone Stromatolite Red algal-microbial-stromatoporoid reef Microbial bindstones

O1 ]3, ]4 ]3 ]2 ]1 ]1 ]1 ]1 ]1

Microbial reef Microbial reef Microbial reef Microbial-archaeocyathid reef Microbial reef Microbial reef Archaeocyathid reefs

O1 O1

O3 O3 O3 O2 O2 O2

Hubei, South China

Hunan, South China

South China Siberia, Mongolia

Hunan, South China

Shandong, North China

Russia

Southern Oklahoma, USA Canada, USA, Russia, China, Iran

South China South China

Canada South China Kazakhstan Erdos Basin, North China Erdos Basin, North China New York and Vermont

Kazakhstan

South China

O3 O3

Zhejiang, South China North China North China

O3 O3 O3

Microbial reef Coral reef Coral-stromatoporoid-calcareous algal reef Calcareous green algalcalcimicrobial reef Calcareous green algalcalcimicrobial reef Calcimicrobial reef Coral-stromatoporoid reef Coral-stromatoporoid reef Coral reef Microbial reef Coral-stromatoporoid-calcareous algal reefs Microbial reef Receptaculitids-lithistid spongemicrobial reef Lithistid sponges-microbial reefs Microbial reef

Gotland Gotland Alexander Terrane, Alaska

S3 S1-2 S1

Eastern Australis

Guangxi, South China New South Wales, Australia

Canning Basin, Western Australia Sedimentary Basin, Western Canada Guilin, South China

Guilin, South China Guangxi, South China

Panthalassan Japan Queensland, Australia

Location

Microbialites Stromatoporoid-tabulate coral reef Microbial reef

D1

D3 D3

Microbial reef Microbial reef

Age C3 C2

Specific Microbial-algal reef Microbial reef

Environment

TABLE 2.—Paleoenvironmental occurrences of calcimicrobes in other regions and geological times.

Adachi et al. 2014a

Adachi et al. 2014b

Zhang and Yuan 1994 Korde 1973; Drosdova 1980

Sun et al.1985 Wang et al.1990

Zhuravlev 2001; Mei et al. 2005; Chen and Lee 2014 Lee et al. 2014b

Riding and Toomey 1972 Lee et al. 2015

Adachi et al. 2011 Adachi et al.2009

Pratt and Haidl 2008 Fang et al.1993; Chen 1996 Webby et al.1997 Ye et al.1995 Ye et al. 1995 Pitcher 1964; Kapp 1974

Nikitin et al.1974

Bian and Zhou 1990

Bian and Zhou 1990 Lee et al. 2014a Ye et al. 1995

Bao 1992 Adachi et al. 2007; Adachi and Ezaki 2007 Shen and Webb 2004; Adachi and Ezaki 2007 Calner 2005 Nose et al. 2006 Soja and Riding 1993

Wray 1967; Wood 2000 Bingham-Koslowski 2010 Shen et al. 2005; Shen et al. 2010

Shen et al.1997 Shen and Webb 2004

Sano and Kanmera 1996 Shen and Webb, 2008

Reference

PA L A I O S PALEOECOLOGY OF ORDOVICIAN CALCIMICROBES 485

Girvanella

Hedstroemia, Ortonella, Cayeuxia, Zonotrichites, Garwoodia

Girvanella, Rothpletzella, Wetheredella, Subtifloria, Proaulopora (rare Garwoodia, and Hedstroemia)

Related genera

Calcimicrobes

Girvanella

Hedstroemia Ortonella Ortonella, Zonotrichites, Garwoodia Hedstroemia, Ortonella Hedstroemia Girvanella

Subtifloria, Proaulopora Girvanella, Subtifloria, Proaulopora Hedstroemia, Ortonella, Cayeuxia, Zonotrichites, Garwoodia Ortonella, Cayeuxia, Garwoodia Cayeuxia Ortonella, Cayeuxia, Zonotrichites, Garwoodia Ortonella Ortonella, Garwoodia Garwoodia Hedstroemia

Girvanella Girvanella Girvanella, Wetheredella Girvanella, Rothpletzella Girvanella, Rothpletzella Wetheredella, Rothpletzella Rothpletzella, Hedstroemia, Garwoodia Girvanella Girvanella Girvanella Girvanella Girvanella Girvanella Subtifloria, Proaulopora

Association

Tidal flat

Restricted lagoon and similar environment

Open platform, Bank

General open platform open platform open platform open platform open platform platform open platform

Specific

P C1 D1 S2 S1-2 O3 O3 O3 O2-3 MP

Shelf-lagoon Lagoon Lagoon Restricted lagoon or back-barrier setting Back-reef environment Back-reef lagoon Restricted environment Restricted environment Restricted lagoon Tidal flat

NP

J3 J2 T3

Restricted lagoon Back-reef lagoon Back-reef lagoon

Tidal flat

J3-K1

]1 ]1

O3 O2 O1 ]3 ]1,]2 ]3, ]4 ]1 ]1

J T2-T3 D3 D3 S3 S3 S3

Age

Back-reef lagoon, restricted lagoon

Bank, grainstone, higher energy Deep and shallow ramp

Oncoid, open platform Oncoid, open platform Shallow water platform Oncoid, open platform Oncoid, open platform Oncoid, open platform Bank, high-energy settings

Oncoid, Oncoid, Oncoid, Oncoid, Oncoid, Oncoid, Oncoid,

Environment

TABLE 2.—Continued.

Williston Basin, Canada New York State Baffin and Bylot Islands, Arctic Canada Spitsbergen

Alexander Terrane, Alaska South China South China

Carnic Alps, Austria Williston Basin, Canada New York Gotland, Sweden

Germany, Romania, Greece, Italy, Northern Calcareous Alps Northern Alps, Paris Basin Scotland Northern Alps

Eastern Indiana Central Nevada, USA West Texas, USA Henan, North China South China and North China Menggu, North China Cantabrian Mountains, northern Spain Western Anti-Atlas, Morocco Germany

Northern Cotswolds Dolomite Alps, Northern Italy Montana and Utah, USA Western Canada Sedimentary Basin English Midland Gotland, Sweden Podolia, western Ukraine

Location

Knoll et al. 1993

Kah and Riding 2007

Liu et al. 2016b (review) and reference therein

´ Alvaro et al. 2006 Elicki 1999

Blackwell et al.1984 Kaya and Friedman 1997 Klement and Toomey 1967 Dai et al. 2016 Liu and Zhang 2012 Zhang et al. 2015b ´ Alvaro et al. 2000

Wethered 1890 Biddle 1983 Rodriguez and Gutschick 2000 Bingham-Koslowski 2010 Ratcliffe 1988 Calner 2005 Łuczyński et al. 2009

Reference

486 L.J. LIU ET AL.

PA L A I O S

PA L A I O S


study was supported by the National Natural Science Foundation of China (Grant No. 41502004, 41502148 and 41372121). SUPPLEMENTAL MATERIAL

Data are available from the PALAIOS Data Archive: http://www.sepm.org/ pages.aspx?pageid¼332. REFERENCES

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Received 26 May 2016; accepted 29 January 2017.