早奥陶世扬子台地边缘的海绵-瓶筐石-微生物礁

This article was downloaded by: [University of Saskatchewan Library] On: 08 February 2014, At: 06:08 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK GFF Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/sgff20 Early Ordovician sponge-Calathium-microbial reefs on the Yangtze Platform margin of the South China Block Qijian Lia, Yue Lib & Wolfgang Kiesslinga a University of Erlangen-Nürnberg, Section Palaeoenvironmental Research, Loewenichstr. 28, Erlangen91054, Germany; , b Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, No. 39 East Beijing Road, Nanjing210008, P.R. China; Published online: 05 Feb 2014. To cite this article: Qijian Li, Yue Li & Wolfgang Kiessling , GFF (2014): Early Ordovician sponge-Calathium-microbial reefs on the Yangtze Platform margin of the South China Block, GFF, DOI: 10.1080/11035897.2013.862852 To link to this article: http://dx.doi.org/10.1080/11035897.2013.862852 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. 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GFF, Vol. 00 (Pt. 1, ), pp. 1–5. q Geologiska Fo¨reningen. doi: http://dx.doi. org/10.1080/11035897.2013.862852. Abstract: Early Ordovician (late Tremadocian to early Floian) reefs on the Yangtze Platform margin of the Hunghuayuan Formation at Zhangzhai section in southeastern Guizhou, South China show three types of reefs in two broad categories: microbial-dominated (stromatolite and lithistid sponge- Calathium-calcimicrobial) reefs and metazoan-dominated (lithistid sponge-Calathium) reefs. These reef types represent different communities controlled by varying environmental settings. Stromatolites tended to develop in the shallow subtidal zone, whereas unlaminated calcimicrobial mounds and metazoan- dominated reefs thrived at greater depths. This is the first report on metazoan-dominated reefs at the platform margin of the Hunghuayuan Formation. Keywords: metazoan-dominated reefs; Yangtze Platform margin; calathid; microbialite; Early Ordovician. 1University of Erlangen-Nu¨rnberg, Section Palaeoenvironmental Research, Loewenichstr. 28, Erlangen 91054, Germany; qijianli@hotmail.com, wolfgang.kiessling@fau.de 2Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, No. 39 East Beijing Road, Nanjing 210008, P.R. China; yueli@nigpas.ac.cn Manuscript received 3 June 2014. Revised manuscript accepted 3 November 2013. Introduction Fossil reefs document Paleozoic marine ecosystem evolution and are among the most important source of paleoecological information. Several studies show that there was a transition from microbial to metazoan-dominated reefs in a stepwise fashion during the Ordovician period (Webby 2002). Earliest Ordovician reefs were mainly microbial dominated, and the subsequent emergence of new reef types (bryozoan reefs and lithistid sponge-Calathium reefs) represented significant turning points (Li et al. 2004; Adachi et al. 2011). We studied Early Ordovician sponge-Calathium-microbial reefs from southeast- ern Guizhou, South China for a more detailed assessment of reef systems on platform margins during this transitional period. Geological setting The South China Block comprises three regions: Yangtze Platform, Jiangnan Slope, and Zhujiang Basin (Chen & Rong 1992). The Yangtze Platform was extensively covered by an epeiric sea during a transgressional interval from the Early to the Middle Ordovician. Shallow-marine carbonates prevailed in offshore areas (Zhan & Jin 2007). The study section in Yuping County was situated on the southeastern margin of the platform (Fig. 1A–C). The outcrop (27827033.2100N; 109802035.6200E) located 0.6 km northwest of the Zhangzhai village is the most accessible and continuous exposure (over 120m thick) of the Lower Ordovician rocks in this area. The reef-bearing Hunghuayuan Formation is about 82m thick and consists of dark-gray to gray skeletal grainstones and packstones with peloidal limestones. It conformably overlies the middle Tremadocian Tongtzu Formation and is overlain by the middle Floian Zitai Formation (Fig. 1D). The age of the formation is assigned to the late Tremadocian to early Floian (Zhen et al. 2009), which belongs to TS1d–2a of the Ordovician time-slice system (Webby et al. 2004). Types and features of the reefs The small-scaled reefs are generally domical or lenticular in shape, ranging in height from tens of centimeters to several meters, and in width from less than a meter to more than 10m. They are chiefly built by benthic microbes, lithistid sponges, the quasi-sponge fossils Calathium as well as bryozoans and are usually surrounded by skeletal/intraclastic grainstones and packstones. Skeletal fragments of ostracods, trilobites, gastro- pods, brachiopods, nautiloids, and echinoderms are found within the reefs and surrounding sediments. Three reefal types are recognized. Stromatolite reefs Stromatolite reefs are common in the lower part of the formation (Fig. 1D). They are small dome-shaped mounds (varying from 80 to 135 cm in width and 55 to 85 cm in thickness), which GFF, 2014 http://dx.doi.org/10.1080/11035897.2013.862852 Article D ow nl oa de d by [U niv ers ity of Sa sk atc he wa n L ibr ary ] a t 0 6:0 8 0 8 F eb ru ary 20 14 interfinger laterally with or are onlapped by bioclastic layers. The reefs consist either entirely of stromatolites or contain a low proportion of sessile metazoans. They grew typically on peloidal/bioclastic packstones or wackestones containing sporadic lithistid sponges and Calathium. The shape of the stromatolites ranges from convex (or columnar) (10–40 cm wide and 10–55 cm high) to stratiform microbial laminites of variable thickness. Several stromatolites are often laterally linked building a framework-like structure (Fig. 2A). Inter- stromatolite spaces are filled with bioclastic packstones/ Fig. 1. Locations and lithologic column of the Zhangzhai section in Guizhou Province, South China. A. Location of the South China Block (shadowed). B. Early to Middle Ordovician paleogeographic reconstruction after Zhan & Jin (2007). C. Index map of Zhangzhai section (enlargement of the area indicated by a rectangle inA).D. Lithologic column of the Zhangzhai section. M, mudstone; W, wackestone; P, packstone; G, grainstone. 2 Q. Li et al.: Early Ordovician sponge-Calathium-microbial reefs GFF 00 (2014) D ow nl oa de d by [U niv ers ity of Sa sk atc he wa n L ibr ary ] a t 0 6:0 8 0 8 F eb ru ary 20 14 wackestones containing a small number of patchy calcified sponges and Calathium fragments. Stromatolites consist of (1) a dark layer with fenestrate texture consisting almost entirely of Girvanella and (2) a light layer with cloudy texture dominated by micrite. The two layers are irregularly stacked on top of each other to create an interbedded accumulation which is the basal unit for the layered deposits. Micrite is ubiquitous not only in the light layers but also in inter- Girvanella spaces of the dark layers (Fig. 2B). Besides the dark layers, sporadic Girvanella can be recognized everywhere in various forms: dispersed, enrolled, or tangled tubes as well as intraclasts of cemented tubes. Fragments of sponges, trilobites, Calathium, and echinoderms, which float in the micrite, are more common in the light layer. These stromatolite reefs are similar to other buildups reported from the Hunghuayuan Formation in Anhui (Adachi et al. 2009) and Fenhsiang Formation in Hubei (Zhu et al. 2006). All the stromatolites are essentially “fine-grained stromatolites” (Riding 2011) composed mainly of fine-grained (micrite) and filamen- tous microfabrics. The filamentous microfabrics (dark layers) represent lithified microbial mats formed predominantly by in situ calcified Girvanella, whereas the fine-grained micro- fabrics probably originated from trapping and precipitation of the benthic microbial communities (Riding 2000). Benthic microbes, especially Girvanella, are the dominant builders, while lithistid sponges and Calathium are limited in volume (average 12–18% altogether) and are rarely found in growth position. Fig. 2. Photographs of Early Ordovician reefs in the Zhangzhai section. A. Stromatolites with rare Calathium (arrows) at the base and in inter-stromatolite spaces. Ruler (rectangle) ¼ 5 cm. B. Photomicro- graph of the dark layers in stromatolite, showing the filamen- tous microfabrics (arrows). C. Reef core of the lithistid sponge- Calathium-calcimicrobial reef. D. Photomicrograph of the lithistid sponge-Calathium-calcimicrobial reef, showing spicular networks and clotted microbial fabrics. E. Photomicrograph of the lithistid sponge-Calathium-calcimicrobial reef, showing Girvanella accumu- lations attach themselves between Calathium and bryozoan fragment. F. Reef core of the lithistid sponge- Calathium reefs. Ruler (rectangle) ¼ 5 cm. G. Inside view of lithistid sponge-Calathium frame. Ruler (rectangle) ¼ 5 cm. H. Photomicro- graph of the lithistid sponge- Calathium reefs, showing lithistid sponge-Calathium frame. Ca, Calathium; Sp, lithistid sponge; Mi, microbialite; Na, Nautiloid; Br, bryozoan; MI, micrite. Photos were taken in the field at the Zhangzhai section on September 2010, and the samples are housed in the Nanjing Institute of Geology and Palaeon- tology, Chinese Academy of Sciences, China. GFF 00 (2014) Q. Li et al.: Early Ordovician sponge-Calathium-microbial reefs 3 D ow nl oa de d by [U niv ers ity of Sa sk atc he wa n L ibr ary ] a t 0 6:0 8 0 8 F eb ru ary 20 14 Lithistid sponge-Calathium-calcimicrobial reefs Lithistid sponge-Calathium-calcimicrobial reefs are the most prevalent type (Fig. 1D). Individual reef bodies are 0.75–2.15m in width and 0.7–1.1m in height. They are typically surrounded and underlain by bioclastic packstones. These reefs are generally lacking recognizable lamination and characterized by a moderate density of in situ sessile metazoans (e.g. lithistid sponge and Calathium) encrusted with stacks of microbial fabrics. Calathium is conspicuous in the outcrop for its unique double-walled structure with cone shape in an autochthonous position (Fig. 2C). It accounts for no more than 20% of volume in the reef core. Lithistid sponges constitute up to 11% of the total reef volume. There is no clear evidence showing that lithistid sponge and Calathium attach upon each other. A small number of bryozoans, echinoderms, brachiopods, nautiloids, and gastropods occur randomly, which together comprise ,8% of the reef volume. The microbialite is made up of massive and non-laminated microbial fabrics, in most cases without a macroscopic clotted (or mottled) texture. Overall, microbial micrite constitutes the bulk of the boundstones, whereas skeletal remains of metazoans make a small contribution (about 10–28%). Microbial fabrics exhibit various forms: (1) accumulation of dispersed Girvanella floating in the micrite; (2) intraclasts of dense Girvanella; and (3) tangled filaments encrusting bioclasts or lithoclasts (Fig. 2D, E). Besides Girvanella, the possible alga Nuia is recognized as straight or curved short tubes with a central canal (Fig. 2D). Nuia tends to make up a significant volume near the base facies; however, it is rare in boundstones from the core parts. Although conspicuous in outcrops, Calathium is relatively rare. Lithistid sponges are common but usually limited to isolated spicules and relic spicular networks. There is a large amount of Girvanella encrusting between and within lithistid sponges and Calathium fossils (Fig. 2E). These reefs imply a greater community complexity than the stromatolite reefs, but the total volume of sessile metazoans is still low (,30%). The lithistid sponges and Calathium are “floating” in the microbial matrix rather than forming a rigid framework. Microbial fabrics contribute to the reef accretion in a variety of ways, not only as binders, precipitators and sediment contributors but also as “stabilizers” for the skeletons. The microbial mats overlay and encrust the surfaces of skeletal organisms, trapping and binding bioclasts at the same time. As a result, the skeletal remains are connected and bounded together by the microbial filaments (e.g. Girvanella) such that a complicated reef framework arose. Lithistid sponge-Calathium reefs The lithistid sponge-Calathium reefs are found in the upper part of the formation (Fig. 1D). They tend to grow larger than the previous two types. Individual reef bodies are usually oblong or lenticular in shape with a maximum basal diameter of 12m and a maximum thickness of 4.5m, characterized by a high growth density of lithistid sponges and Calathium with microbial boundstones and other bioclasts (Fig. 2F). Stacked Calathium fossils account for over 40% of the reef core (Fig. 2G), creating a striking frame-like cluster structure. Most of Calathium individuals are upright or slightly curved with a goblet-shaped structure. Lithistid sponges are also abundant reaching up to 20% in volume. The massive microbialite with various bioclasts fills up the spaces between the metazoan skeletons. Nautiloids appear occasionally in the reefs. Scattered bryozoan, echino- derm, and brachiopod debris constitute up to 10% of core facies and increase in percentage downward in the reefs. The base of the reefs consists mostly of crinoidal packstones/grainstones containing rare Calathium. Microbial fabrics are still common in the massive biolithite. Besides plenty of well-preserved specimens, patchy Calathium hardparts and sponge spicules are widespread in thin sections. Rigid frameworks are formed by Calathium and lithistid sponges together with bryozoans and microbial boundstone (Fig. 2H). Although bryozoans are not abundant (no more than 6% in volume), they were important epibionts encrusting Calathium and lithistid sponges. Filamentous Girvanella is easily recognized in dark clotted fabrics or encrustations clinging on the surface of bioclasts. Nuia, as a major component in some burrows, is also common. Trilobite debris, echinoderm ossicles, brachiopod shells, and ostracods are scarce (7–8%). The lithistid sponge-Calathium reef is notably dominated by in situ metazoans. The dense growth of Calathium and lithistid sponges was sufficient to inhibit hydrodynamic removal and sorting of sediment (Riding 2002). Calathium and sponges were not only key framework builders but also important sediment contributors as well as bafflers. The microbial fabrics remain important stabilizers. Girvanella grew on the surfaces of skeletons to enhance the stability of the skeletal framework. Nevertheless, the microbial contribution to reef growth was far less important. Discussion The sponge-Calathium-microbial reefs are dominated either by metazoans (lithistid sponge and Calathium) or by calcified microbes (laminated or unlaminated fabrics). The broad spectrum in the proportion between metazoans and microbes makes them similar to early (Series 2) Cambrian reefs. Furthermore, Calathium, as the most import metazoan builder, has certain aspects in common with solitary archaeocyathans. Both Calathium and archaeocyathans have a porous double- wall, holdfasts at the base and an open upper end leading to the central cavity, which are suitable for water circulation (Church 1991). This suggests that the main metazoan reef builders were functionally similar in the early Cambrian and in the Early Ordovician. The Early Ordovician transition from microbial- to metazoan-dominated reefs was accompanied by the rise of novel sessile metazoans. However, none of the key builders are new in the sponge-Calathium-microbial reefs. The upward change of reef types in Zhangzhai is probably not a direct consequence of the diversification of skeletal reef organisms. Instead, they may represent different communities controlled by varying environ- mental setting. The mesostructure of microbialites will be particularly susceptible to even small fluctuation of sea-level change. Early Ordovician bioherms from the western coast of Newfoundland show a distinct bathymetric zonation for the cryptalgal structure (Pratt & James 1982): stromatolites tend to have developed in the intertidal and supratidal zones (sometimes in shallow subtidal zones), whereas unlaminated cryptalgal structures formed subtidal mounds with more diversified metazoans. The reef pattern here is in accordance with these coeval counterparts. The stromatolites may have grown in the uppermost subtidal zone. The decrease of stromatolites upwards is interpreted as a consequence of sea-level rise. The unlaminated calcimicrobial 4 Q. Li et al.: Early Ordovician sponge-Calathium-microbial reefs GFF 00 (2014) D ow nl oa de d by [U niv ers ity of Sa sk atc he wa n L ibr ary ] a t 0 6:0 8 0 8 F eb ru ary 20 14 reefs containing a moderate amount of metazoans become the only type of microbial-dominated buildups in the rest of the formation. Lithistid sponges and Calathium turned into more significant reef builders at deeper depths higher up in the section. The lithistid sponge-Calathium reefs from the upper part showcase a rare example of Early Ordovician metazoan- dominated reefs from the platform margin. Sessile metazoans flourished in habitats with higher plankton availability and more suitable physical conditions for suspension feeders. They are closely adjacent or even in contact in the reef core to build a bundle-like framework (primary framework). Irregular cavities among the primary framework are populated by microbes (or bryozoans occasionally) which are typical cryptic fauna and build secondary frameworks. Conclusions (1) Based on the main reef builders and their roles in it, the sponge-Calathium-microbial reefs of the Hunghuayuan Formation at Zhangzhai exhibit three ecological types as stromatolite reef, lithistid sponge-Calathium-calcimicro- bial reef, and lithistid sponge-Calathium reef. (2) The first two types are microbial dominated and are prevalent communities at the margin of the platform, whereas lithistid sponge-Calathium reefs display an uncommon case of metazoan-dominated reefs at the platform margin. (3) The ecological differentiation was probably controlled by variable environmental settings. Stroma