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Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 325^346 www.elsevier.com/locate/palaeo

Biological, depositional, microspherule, and geochemical records of the Frasnian/Famennian boundary beds, South China X.P. Ma  , S.L. Bai Department of Geology, Peking University, Beijing 100871, PR China Accepted 6 December 2001

Abstract The investigation of shallow subtidal, inter-reef depression, and inter-reef platform sections from central Hunan of China shows that a transgression^regression cycle occurred in the Late Frasnian (linguiformis Zone?). The lowest triangularis Zone to Middle triangularis Zone deposits are either very thin or variously missing in some shallow-water sections of Hunan. The uppermost Frasnian (upper linguiformis zone) black-shale interval (including black shale and carbonates) was not ubiquitously developed and probably formed in a regressive, shallower water and anoxic to dysoxic conditions suggested from higher Ce/La ratios, penecontemporaneous dolomitization of carbonates, and low faunal diversity. Two steps of the Frasnian/Famennian (F/F) mass extinction are postulated. The first is the extinction of the benthos near the end of the Frasnian which was probably caused by the sudden onset of anoxic environments. Shallow-water rugose corals and ostracods experienced sudden biomass loss right at the deposition of the black shale. The second is the extinction of pelagic fauna at the end of the Frasnian. Microspherules of probable impact origin are found in several layers in the Xikuangshan F/F boundary section, including two major peaks, respectively, at the Upper rhenana Zone and upper linguiformis zone, and two minor peaks, respectively, near the top of the linguiformis Zone and Lower triangularis Zone. The microtektites are white or brownish in color, characterized by high contents of Si, Al, and intermediate CaO, MnO, and FeO, and minor K2 O and Na2 O. Occurrences of these microtektites do not seem to be directly related to the F/F mass extinction. From the Lower rhenana Zone through the Upper crepida Zone there are four elemental anomalous layers. These elemental anomalies are interpreted to have resulted from the contemporaneous active rifting process and hydrothermal activities in South China. 8 2002 Elsevier Science B.V. All rights reserved. Keywords: Upper Devonian; Frasnian/Famennian boundary; Kellwasser event; microtektites; mass extinction; South China

1. Introduction The Devonian Frasnian/Famennian (F/F) biocrisis is one of the most prominent extinction

* Corresponding author. Fax: +86-10-6275-1187. E-mail address: [email protected] (X.P. Ma).

events during the Phanerozoic (see McGhee, 1996; Walliser, 1996 for general discussion). Different terrestrial and extraterrestrial mechanisms have been suggested to explain the bio-crisis including cold water (Copper, 1986), episodic climatic warming (Thompson and Newton, 1988), anoxic water induced from an impact (Goodfellow et al., 1988), sea-level lowering (Johnson

0031-0182 / 02 / $ ^ see front matter 8 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 4 8 4 - 9

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and Sandberg, 1988) or transgression^regression (T^R) couplet (House, 1985), intensi¢cation of rifting-hydrothermal process (Bai et al., 1994), tectonically triggered rifting and its consequences (Racki, 1998). Although there have already been many studies devoted to the F/F extinction mechanism, basic biotic, sedimentological, physical and geochemical data remain elusive or poorly known (Racki, 1999a). In addition, many data, especially geochemical analyses, are open to alternative interpretations. In China, most F/F event-related studies concentrate on a few major shallow-water and pelagic sections of the South China plate (e.g. Hou et al., 1988, 1992, 1996; Ji, 1989, 1994; Wang et al., 1991; Bai et al., 1994; Ma and Bai, 1996, among others). However, many problems like faunal distribution and extinction pattern as well as the paleoenvironmental evolution across the F/F boundary have to be resolved. For example, there are di¡erent opinions regarding the depositional environment of the Late Frasnian black shale (Nandong shale of Bai et al., 1994). Muchez et al. (1996) considered it to be the result of a marine transgression comparable to the Upper Kellwasser horizon of Europe. Ma and Bai (1996) regarded the black-shale horizon to be the result of a shallowing of the environment. So far, microtektite-like spherules have been reported from above or at the F/F boundary in South China and Belgium (Wang, 1992; Claeys et al., 1992; Claeys and Casier, 1994). These microspherules either postdate the F/F extinction horizon or their occurrence is biostratigraphically not well de¢ned. Bai et al. (1994) and Ma and Bai (1996) found a few microspherule layers both below and above the F/F boundary. However, these microspherules need further study in terms of morphology and geochemistry before their origin can seriously be discussed.

2. Palaeogeographic setting During the Devonian, the South China plate comprised the Yangtze platform and the Southeast Caledonian Belt. From the Late Givetian (Middle varcus Zone), the Guangxi area broke,

forming a series of intersected rifts, giving the Late Devonian rift system of South China (Fig. 1). The basinal-rift facies developed mainly in the Guangxi Autonomous Region and it also extended northwards into the Hunan area. Several well-known deeper water and rift-marginal F/F boundary sections are located in Guangxi, for example, the Maanshan section of Jia et al. (1988), the Baqi section of Wang and Bai (1988), the Xiangtian ( = Luoxiu) section of Wang et al. (1991) and Yan et al. (1993) and the Nandong section of Bai et al. (1994). The platform facies was well developed northward in Hunan Province, where four major facies types during the Frasnian may be recognized (Yu et al., 1990): inner littoral, outer littoral (we would call it shallow subtidal in this paper), reef platform or inter-reef platform, and inter-reef depression facies. The inner littoral facies is characterized by sandstones across the F/F boundary. The shallow subtidal facies, as developed in the Xikuangshan section, is characterized by limestones and calcareous shales, with hematite-rich beds deposited in the Early Famennian. Abundant brachiopods are present both below and above the F/F boundary, locally with abundant and diverse corals below the boundary. The inter-reef depression facies (previously also known as ‘basinal’ facies) is characterized by marls, shales and various limestones. The Jiangjiaqiao and Chongshanpu sections (Ma, 1998) and the Shetianqiao section (Ma et al., 2002) are located in this facies. Abundant and diverse brachiopods are present both below and above the F/F boundary in this facies. Rugose corals occur just below the F/F boundary in the northern part of this facies and Buchiola (bivalves) and cephalopods (nautiloids and ammonoids) occur southwards in the Shetianqiao and Jiangjiaqiao sections. The inter-reef platform facies is characterized by shallow-water, thick-bedded carbonates, including various grainstones, micrites, and dolostones, with patch reefs made up of stromatoporoids and corals. Brachiopods are very rare in the Frasnian but may be abundant in the Famennian. In this facies the F/F boundary is easily de¢ned lithologically in most sections with oncolitic limestone yielding Yunnanellina hanburyi above and

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thick to massive-bedded siliceous-nodular Amphipora limestone below, yielding some corals and minor brachiopods. During the Famennian shallow subtidal, interreef depression, and inter-reef platform facies do not have much di¡erence in lithologic associations except that the outer littoral facies is characterized by deposition of a thin iron bed and that the inter-reef depression facies is characterized by having some nautiloids.

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4. Biostratigraphy and extinctions of benthic and pelagic faunas

ning^Guilin Rift (Fig. 1). Its sediments represent an episialic basinal facies. In the Nandong section, the Frasnian consists mainly of lydite (Liujiang Formation), while the Famennian as well as the Upper 17 m of the Frasnian is mainly composed of nodular limestones (Wuzhishan Formation; Bai et al., 1994). In the Yangdi section, the Frasnian is composed of three stratigraphic units (Ji, 1994). The Fuhe Formation comprises the Lower part of the Frasnian (from the Middle falsiovalis through the Lower hassi Zones), which is about 40 m thick and composed of gray, mediumto thick-bedded dolomitic and marly limestones. The 30^40 m thick Lazhutai Formation comprises the Middle part of the Frasnian (from the Upper hassi through the jamieae Zones) which is composed of alternating gray shales, black lydite, and minor grayish yellow siltstones. The overlying Xiangtian Formation is about 18 m thick, composed of thin- to thick-bedded micritic and marly limestones of the Lower rhenana through linguiformis Zones. The Famennian, as in the Nandong section, is mainly composed of nodular limestones. Conodonts of the Nandong section have been described in Bai et al. (1994). In this section, the base of the linguiformis Zone, Lower triangularis Zone, Middle triangularis Zone and Upper triangularis Zone are marked by the occurrence of Palmatolepis linguiformis, Palmatolepis triangularis, Palmatolepis delicatula platys and Palmatolepis tenuipunctata, respectively. This zonation coincides well with the current standard zonation (Ziegler and Sandberg, 1990). The F/F boundary is de¢ned by the ¢rst occurrence of P. triangularis, which was accepted internationally as a marker species for the boundary (Klapper et al., 1993). Conodonts of the Yangdi section have been investigated by Ji (1994). Their distribution in the F/F boundary interval is shown in Fig. 2. The Lower rhenana through the Upper triangularis Zones are marked by individual diagnostic conodonts of each zone.

4.1. Deeper water sections: Nandong and Yangdi

4.2. Xikuangshan section

During the Late Devonian, these two sections were within the long and narrow Longzhou^Nan-

At the Xikuangshan section, the Frasnian succession may be divided into three parts (Wang et

3. Methods Trace element contents were analyzed with an ICP (JA-ICAP 9000 manufactured by the Fisher Scienti¢c Company) at the Department of Geology Laboratory Center, Peking University. Two methods have been adopted: (1) whole-rock analysis. The procedure is as follows : 0.1 g powder sample was ¢rst dissolved in 10 ml HF (analytical reagent), dry, a few drops of H3 ClO4 , dry, 2 ml HNO3 , dry, 25 ml 10% HCl. Chinese standard sample GSR 6 was used for limestone samples and GSR 5 was used for shale and mudstone samples. (2) HCl-soluble component analysis. The procedure is as follows: 0.1 g powder sample was ¢rst dissolved in 5 ml HCl (analytical reagent), dry, 25 ml 10% HCl. Chinese standard sample GSR 6 was used for both limestone and shale/mudstone samples. The error is within S 10% for all analyses. Electronic microprobe analyses were performed on polished microspherules using an EPM-810Q connected to an Oxford energy spectrometer. Standard samples SPI 16, SPI 34 and SPI 41 of the American SPI supplies were used. Working conditions were set under 15 kV, 3 nA, and Co 2000 cps. The error of analyses is within S 2%.

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Fig. 1. Lithofacies paleogeographic map of South China in the Late Devonian (B after Hou et al., 1988, partly revised based on Yu et al., 1990; Ji, 1994; Bai et al., 1994). Localities of sections: 1, Leimingdong; 2, Xikuangshan; 3, Daping; 4, Chongshanpu; 5, Shetianqiao; 6, Jiangjiaqiao; 7, Jingtouwei; 8, Xiangtian; 9, Baqi; 10, Maanshan; 11, Nandong; 12, Xianghualing; 13, Qiziqiao; 14, Yangdi.

al., 1986; Bai et al., 1994). The Lower part is composed of 51 m of thin-bedded clastics including sandstones and shales, which may be equivalent to the Lungkouchong bed of the Qiziqiao section ; the Middle part consists of 194 m dark gray thin- to thick-bedded limestone (Qilijiang Member) (Fig. 3); and the Upper part is composed of 49.5 m thick calcareous shales with argillaceous limestone interbeds that are equivalent to the Lower part of the Changlongjie Mb. ( = Changlungchieh Shale of Tien, 1938). The F/F boundary interval of the Xikuangshan section has been described by Ma (1993) and Bai et al. (1994). Zonal conodont species of the linguiformis and triangularis Zones have never been found in the platform facies of Hunan. However, Polygnathus webbi as well as colonial rugose corals have been

found in bed L5 and below (Fig. 2) which indicate an age no later than the latest Frasnian (Ziegler and Sandberg, 1990). Icriodus iowaensis iowaensis and Icriodus deformatus have been found in bed L9 (sample L9/0.25) (Fig. 2, also see Bai et al., 1994 for illustrations). I. deformatus is associated with the ¢rst occurrence of Palmatolepis triangularis in the Yangdi section (Ji, 1994), suggesting an age of the Lower triangularis Zone for bed L9 of the Xikuangshan section. Ji (1989) reported the conodont Palmatolepis crepida from a horizon that was interpreted as the ‘base of the Tuzitang Member’, which was considered to be equivalent to the Middle part of the Changlongjie Formation (Tan et al., 1996; = Middle part of the Upper Changlongjie Member sensu Tien, 1938). Further analysis shows that the Ji (1989) ‘base of the Tuzitang Member’ should be correlated with our bed

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Fig. 2. Distribution of important conodonts^corals^brachiopods (Hunanotoechia) and microtektites (Si and Si^Al microspherules) in the Yangdi (data from Ji, 1994) and Xikuangshan sections. The occurrence of Palmatolepis crepida in the Xikuangshan section is adopted from Ji (1989). Spherule abundance is expressed by number of spherules per 100-g sample.

L23 of the Upper Changlongjie Mb. This bed should approximately represent the base of the crepida Zone, which is supported by the occurrence of Polygnathus brevilaminus in bed L20 and various beds below, indicating an age no later than the crepida conodont Zone for bed L20 and its underlying strata. The distribution of conodonts con¢nes the F/F

boundary within an interval of only 1.15 m thickness (from bed L6 through bed L9a; see 5. Sedimentology across the F/F boundary in the Xikuangshan section for stratigraphic division) in the Xikuangshan section (Ma, 1993). In the black-shale interval (bed L6), some brachiopods have been discovered in the siliceous carbonate intercalation which belong to the same species

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as present in the uppermost Frasnian beds L4 and L5 (Bai et al., 1994, p. 91, ¢g. 7^14 therein). This black-shale horizon can be correlated with the F/F boundary shale (Nandong Shale) which is widespread in the Nandong and Xiangtian areas. On the basis of conodonts and the black-shale horizon, the F/F boundary can be drawn at the top of bed L8 (i.e. the base of bed L9) at Xikuangshan. The Lower and Middle triangularis zonal boundary may be set at the top of bed L9, which is in agreement with that of Hou (1991, p. 65), representing a transgression surface. The Yunnanellina brachiopod lineage morphotypes 1 and 2 of Ma (1995) may be approximately correlated with the Middle and Upper triangularis Zones (Fig. 2). 4.3. Jiangjiaqiao section The Frasnian succession exposed in the Jiangjiaqiao section is about 550 m thick. According to ammonoids found in bed C1 which are similar to Manticoceras drevermanni Wedekind, 1913, Ma (1998) interpreted bed C1 to be dated within the linguiformis Zone (Fig. 4). Based on the ¢rst occurrence of Yunnanellina hanburyi, the F/F boundary is assumed in the vicinity of bed C5. Data from the Shetianqiao section which is only about 35 km northeast of the Jiangjiaqiao section support this conclusion. Both sections are located in the same facies realm (Fig. 1). The Frasnian succession of the Shetianqiao section is about 800^900 m thick (Yu et al., 1990). About 295 m below the F/F boundary Palmatolepis gigas, Ancyrodella ioides, and Ancyrognathus triangularis occur. This association indicates an age of the Early rhenana Zone (Ziegler and Sandberg, 1990). Therefore the Upper 295 m thick sequence of the Shetianqiao Fm. should belong to the Lower rhenana Zone through the linguiformis Zone. Thus it would be logical to consider the uppermost 20^40 m interval of the Frasnian in the Jiangjiaqiao section as belonging to the linguiformis Zone, though the precise base of the linguiformis Zone as well as ranges of other conodont Zones is di⁄cult to determine. Detailed sequence stratigraphy and chemostratigraphy need to be carried out to make correlations between pelagic and shallow-water sections.

4.4. Daping section Based on the lithology and faunal composition, the F/F boundary in the Daping section (about 15 km south of the city of Shaoyang) is placed at the top of bed D4 (Fig. 4). This boundary represents an erosional surface. The lowermost Famennian may be missing because the brachiopod Yunnanellina occurs right at the base of the Famennian oncolite layer. In Hunan, Yunnanellina occurs in most sections at about 5^10 m above the F/F boundary (Ma, 1995). The Jingtouwei section, located 80 km south of the Daping section (Fig. 1), is characterized by a comparable lithological association, with massive cherty limestone below and oncolitic limestone above the boundary. The oncolitic limestones in the Jingtouwei section are relatively thick (up to 100 m thick). According to Shen (1982), the oncolite layer is placed within the Lower part of the crepida Zone. However, in the Daping section, the basal Famennian oncolite layer yields also Platyspirifer sp. and Sinospirifer subextensus which are associated with Yunnanellina hanburyi morphotypes 1 and 2 in the Jiangjiaqiao section. Further correlation with the Xikuangshan section based on the range of the Yunnanellina lineage suggests an age no earlier than the Middle triangularis Zone for the oncolite layer in the Daping section. Its exact age (Middle or Late triangularis Zone) needs further study of the Yunnanellina lineage in that section. 4.5. Correlation of the platformal sections Ma (1995) and Ma and Day (1999) correlated the Xikuangshan section with the Jiangjiaqiao section by means of the Yunnanellina lineage, i.e. the L10^L14 interval may be correlated with C6^ C8; L15^L18 can be correlated with part of C9 (Fig. 4). Bed D7 of the Daping section is very similar to beds L15^L18 of the Xikuangshan section. D1 and D2 of the Daping section may be correlated with C1 of the Jiangjiaqiao section through sea-level change pattern in both sections. 4.6. Extinction of benthic and pelagic faunas In South China atrypid and gypidulid brachio-

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Fig. 3. Major stratigraphic divisions of the Xikuangshan section and correlation with the Nandong section, showing major physical and geochemical events. Letter T in the Events column indicates the start of marine T^R cycles. For fossil control: 1, Palmatolepis rhomboidea and Polylophodonta con£uence were found at 154 m above the base of the Magunao Mb. (total thickness 266 m; Wang et al., 1986); 2, later form of Palmatolapis quadrantinodosalobata has been found in the Upper range of the brachiopod Yunnanellina (see Wang and Bai, 1988); the conodont indicates an age of the Late crepida Zone; 3, Palmatolepis crepida (see Ji, 1989); 4, Icriodus iowaensis and Icriodus deformatus; 5, Polygnathus webbi and last occurrence of Frasnian corals; 6, correlated with the Lower rhenana Zone of the Shetianqiao section where Ancyrodella ioides, Ancyrognathus triangularis, and Palmatolepis gigas are present (Yu et al., 1990). Black bars in the lithological column represent gray and black shales. For other lithological symbols, see Fig. 2.

pods are very rare near the top of the Frasnian (upper linguiformis zone). However, atrypids are abundant at about 20 m below the boundary (Ma, 1998). A signi¢cant biomass and diversity loss of the atrypids occurred well below the F/F boundary (Ma et al., 2002). Earlier reports of occurrences of atrypids in the Lower Famennian in Guangxi (e.g. Wang and Bai, 1988; Jia et al., 1988) are interpreted to be the result of reworked deposition (Bai et al., 1994; Ma, 1998). Hunanotoechia was the only abundant rhychonelloid brachiopod genus in central Hunan both in the subtidal and inter-reef depression facies and became extinct just below the black-shale horizon (Ma,

1993). In the topmost Frasnian (linguiformis Zone) there are probably only three Cyrtospirifer species in central Hunan, including Cyrtospirifer cf. whitneyi, ‘Cyrtospirifer archiaciformis’ (Grabau) and ‘Tenticospirifer gortani’ (Pellizzari) (Ma, 1994). They did not pass the F/F boundary. Whereas hermatypic corals and benthic ostracods were very abundant and diverse near the top of the Frasnian and were seen to disappear quite suddenly ‘at a bedding plane’, e.g. in the Chongshanpu and Xikuangshan sections (Ma and Bai, 1996; Ma, 1998), which may be reconcilable with ‘a large impact’ hypothesis of some authors (e.g. McLaren and Goodfellow, 1990). This sudden

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Fig. 4. Stratigraphy, sedimentological and faunal evolution of three shallow-water sections of di¡erent sub-facies from South China.

biomass loss occurred right at the base of the uppermost Frasnian black-shale interval (Fig. 5). The corals are most abundant at the top 20-cm interval of bed L5 of the Xikuangshan section. Equivalent to this is bed T3 of the Chongshanpu section (Ma, 1998). These corals include Disphyl-

lum, Frechastrea?, Phillipsastrea, Peneckiella, Pseudozaphrentis, Temnophyllum, Sinodisphyllum, Sinopora and Syringopora (Ma, 1994). Extinction of manticolepid conodonts is welldocumented in the basinal facies of Guangxi, e.g. in the Nandong and Yangdi sections. In the

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matolepis gigas, and Palmatolepis linguiformis (Walliser et al., 1989), coincident with extinction of pelagic organisms in the pelagic sections of Guangxi (Bai et al., 1994; Ji, 1994; Hou, 2000).

5. Sedimentology across the F/F boundary in the Xikuangshan section 5.1. Upper part of the Upper rhenana Zone: LA^LD interval Bed LA consists of gray shelly limestones with abundant brachiopods dominated by cyrtospiriferids (Fig. 4). Bed LB is composed of gray oolitic sparitic limestones (Fig. 6A) with abundant ostracods, some rugose corals, bryozoans and rare brachiopods, conodonts and crinoid stems. The ooids are generally 1 mm in diameter, re£ecting a radial structure in the interior and a tangential structure in the outer part. Fig. 5. Ostracod species diversity and percentage of various conodont types in the Xikuangshan F/F boundary section. See Fig. 9 for lithological legends.

Yangdi section, this extinction occurs at the top of the linguiformis Zone (Fig. 2). Nearly all of the manticolepids (species of Palmatolepis with downward-inclined posterior platform) became extinct (Ji, 1994). These two-step extinctions of benthic and pelagic faunas also seem true for some sections in Europe. For example, at both Steinbruch Schmidt and Aeketal sections of Germany, the benthic trilobite Palpebralla brecciae disappeared just below the Upper Kellwasser horizon (Groos-U¡enorde and Schindler, 1990), coincident with extinction of benthic organisms in the Xikuangshan section. Similarly, in the Luoxiu section of Guangxi (Fig. 1), benthic organisms such as atrypids, corals etc. also became extinct below the black interval (Hou, 2000, ¢g. 5 therein). The pelagic fauna became extinct at the top of the Upper Kellwasser horizon, including Polygnathus webbi, Ancyrognathus asymmetricus, Ancyrognathus tsieni, Ancyrognathus ubiquitus, Ancyrodella curvata, and manticolepids Palmatolepis rhenana, Palmatolepis subrecta, Pal-

5.2. Upper part of the linguiformis Zone: L1^L4 interval The interval above bed LE is mostly covered, but probably re£ects the same lithology as the L1^L4 interval. The latter is characterized by alternating shale/mudstones and minor bioclastic micrites. Lamination is well-developed in the mudstone and shale which generally lack any fossils. Brachiopods, conodonts, sponge spicules (Fig. 6B), bryozoans, ostracods, bivalves and gastropods become relatively common in the limestones of the Upper part. 5.3. Abundant and diverse corals: L5 interval Bed L5 is composed of gray, medium- to thickbedded limestones, mainly micritic and partly sparitic. A main feature of this bed is the occurrence of abundant corals and oncoids (Fig. 6E), together with ostracods, brachiopods, conodonts, bryozoans and gastropods. A diverse coral fauna occurs at the top of this bed, just below the blackshale horizon. The corals include Disphyllum, Pseudozaphrentis, Temnophyllum, Frechastrea?, Peneckiella, Sinodisphyllum, Sinopora, Syringo-

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pora etc. Disphyllum is the predominant coral, which is widely distributed in central Hunan during the latest Frasnian. Ostracods become extremely abundant and diverse towards the top of this bed (Fig. 5) and are characterized by the Polytilites^Roundyella ? assemblage including Fabalicypris sp., Samarella sp. in addition to the nominal taxa (Ma et al., 2002). 5.4. The top of the Frasnian: L6^L8 interval Bed L6 is a black-shale horizon, with an organic carbon content of 0.15^0.2% (for mineral composition, see Table 1). Some terrestrial gravels and land plant fragments were seen on hand specimens. Cross-lamination is observed in thin section microscopy (Fig. 6D). Hou et al. (1996) reported sandy layers in this black-shale unit. There are only terrestrial spores, some acritarchs, and minor scolecodonts in the shale. An intercalated siliceous carbonate layer (L6/0.55, Fig. 6F) shows abundant branched bryozoans, common brachiopods encrusted by bryozoans and Spirobis and rare ostracods. The bryozoans and brachiopods (almost all disarticulated) are oriented parallel to the bedding plane and are interpreted to be the result of transportation. Bed L7 is dolomitic bryozoan sparitic^micritic limestone (Fig. 6G), with abundant bryozoans, some brachiopod fragments, rare gastropods, trace fossil Palaeophycus and microfossils including extremely abundant ostracods and some conodonts (almost all coniform elements). Table 1 Mineral composition (in %) in selected samples of the Xikuangshan section Samples

L6/0.1

L6/0.55

L7

Illite Chlorite Kaolinite Talc Quartz Plagioclase Calcite Ankerite Pyrite

12 5 42

5 35

4 19

40

35

11

25

47 17 2

1

L9b

L10/0.6

2 2 10 80

14 4 18 4 25 3 31

2

1

Analyses of mineral composition were performed on powdered samples using a BD86 automatic X-ray di¡ractometer.

Bed L8 is black shale with a thick intercalated argillaceous bioclastic limestone layer, similar to the siliceous intercalation observed in bed L6 (L6/ 0.55) in faunal composition. The fossils are mainly bryozoans, brachiopods and ostracods. The shale is lithologically comparable to bed L6 containing abundant spores, acritarchs, and scolecodonts. The limestone varies in thickness, sometimes accounting for 80^90% of the entire thickness of L8. 5.5. The Lower triangularis Zone: L9a^L9c interval L9a comprises the Lower 24 cm of bed L9, composed of ferrigenous bioclastic micritic to sparitic dolomitic limestone (Fig. 6H), with about 30% bioclasts including abundant bryozoans and common to rare ostracods, gastropods, bivalves and brachiopods, etc. Conodonts are very rare. Bed L9b is a 75 cm thick, medium- to thinbedded bioclastic sparitic limestone with abundant ostracods (but low diversity, Fig. 5), bivalves, rare brachiopods, conodonts, gastropods and bryozoans. Bioclasts may account for up to 50% (Fig. 6I). In comparison to bed 9a, bed 9b does not show any evidence of dolomitization. Ripple marks are observed at 25 cm above the base of L9b. L9c is a 35 cm thick, gray oolitic grainstone (Fig. 6J), with about 50% ooids. The ooids are generally 0.5 mm in diameter, mainly with a radial structure but also a tangential structure. Bioclasts include common ostracods, rare brachiopods, gastropods and conodonts. 5.6. Middle to Upper triangularis Zones (part): L10^L19 interval Interval L10 is characterized by gray calcareous mudstones and shales. Lamination is well developed. In the Lower part, there are a few thin shelly layers of usually less than 1 cm in thickness, with poorly preserved brachiopods. Beds of L11 and L12 are a shell layer with abundant brachiopods, but shells in the Upper part of bed L12 were almost completely dissolved, only leaving various types of molds.

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Beds of L13 and L14 are characterized by calcareous shales and mudstones with thin intercalated limestone and shell beds which show rare brachiopods and ostracods. Bed L14 shows desiccation cracks. The interval from L15 to L18 is composed of argillaceous laminated limestone and intraformational conglomerates. The limestone band is generally 2 cm thick, commonly with some bioclastics of various contents. The argillaceous band is 0.5 cm thick, which is composed of quartz silts, calcite grains and minor ostracods. The intraformational grains are generally 1 cm in size, with an overall range of 0.2^5 cm in size. They appeared without a preferred orientation.

6. Paleoenvironmental evolution and events at the F/F boundary interval 6.1. Paleoenvironmental evolution across the F/F boundary in central Hunan 6.1.1. Latest Frasnian The lithological succession of the whole Frasnian in the Xikuangshan section shows a general T^R phase. For the Upper part of the Frasnian (i.e. Lower part of the Changlongjie shale) in the Xikuangshan area, there are probably two T^R cycles, approximately equivalent to Upper rhenana and linguiformis Zones, respectively (Fig. 3). Both T-R cycles began with a suite of shale and marly limestone (hosting brachiopods, etc.). The ¢rst cycle ended with sparitic oolites and marls (bed LB, hosting corals, brachiopods, etc.). And the second cycle ended with reefal limestone, black shale, and dolomitic sparitic^micritic limestone at the top of the Frasnian. The deposition rate of bed L5 of the Xikuangshan section may sometimes be high as indicated from in¢lling of coral cavity by later diagenetic calcite which means a rapid burial of the coral. If the coral was not buried rapidly, its soft part would have been decomposed and the cavity would have been in¢lled with sediments. Sometimes the sedimentation rate may be low or even a hiatus existed as indicated by the presence of geopetal and microkarstic structures (Ma and

335

Bai, 1996, ¢g. 2 therein), or an erosional event with bioclastics above and micrite below (Fig. 6C). Overall this bed represents a shallower and slightly more agitated environment than that of bed L4. Generally speaking, the interval from L6 to L8 represents a restricted, anoxic to dysoxic environment with higher salinity, probably in an intertidal to supratidal Zone. This will be discussed in detail in 6.2. Palaeoenvironmental events of the F/F boundary interval. The environmental evolution of the Daping section on the inter-reef platform facies reveals a comparable facies development. In the Daping section bed D3 is characterized by thick-bedded to massive limestones with abundant Amphipora (Fig. 6K), minor rugose corals, gastropods, algal peloids, and locally algal oncoids. The thickbedded limestone bears abundant siliceous cherts of various morphologies (banded or isolated nodules of di¡erent shapes). This interval should represent a lagoonal normal-salinity environment. However, the top of D3 is composed of cryptalgal laminites or planar stromatolites (Tucker, 1981, p. 115) with birdseye structure, and bed D4 is composed of calcispheric-algal peloidal micritic limestone (Fig. 6L). These facies suggest a restricted lagoonal to intertidal depositional environment. 6.1.2. Earliest Famennian This interval is characterized by frequent sealevel oscillations. Bed L9a is lithologically similar to bed L7, suggesting a similar depositional condition. The lithology of beds L9b and L9c represent a carbonate shoal facies. The Upper bedding surface of bed L9c is undulating with current scouring structure, which marks a subsequent transgression represented by bed L10. The L11^L12 interval, especially the Upper part, probably represents a transported shellbank deposit. Then the sea regressed to a restricted lagoonal to intertidal environment (L13 and L14). The L15^L18 interval apparently represents a subtidal, sometimes high-energy, environment judging from their lithology, which marks the start of a subsequent marine transgression.

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6.2. Paleoenvironmental events of the F/F boundary interval 6.2.1. The bloom of siliceous organisms and cherts In the Jiangjiaqiao section (Fig. 4), bed C1 is made up of medium- to thick-bedded oncolitic limestone (Fig. 6M). Upwards a massive sponge limestone bed (bed C2) occurs and then again towards the end of the Frasnian (bed C4). In the Xikuangshan section there are a lot of sponge spicules, for example, in bed L3 (Fig. 6B) and a siliceous carbonate intercalation layer occurs in bed L6. In the Daping section the topmost Frasnian is characterized by massive limestone with abundant cherts. The bloom of siliceous biota near the F/F boundary in the Appalachia of eastern United States has been recognized before (see summary by McGhee, 1996). This siliceous biotic event seems to occur in most parts of the world and has been explained to be related to a climatic cooling (e.g. McGhee, 1996) and/or large-scale increase in volcano-hydrothermal activity (Racki, 1999b; see also Chen et al., 2001b). 6.2.2. Anoxic event There are a number of methods used for paleoredox analysis, for example, rare earth elemental ratio (Elder¢eld and Greaves, 1982; Wright et al., 1987; Holser et al., 1989; Bai et al., 1994), and trace metal elemental ratios (Jones and Manning, 1994). Holser et al. (1989) found that the Ce/La (both normalized to the North American Shale Composite Standard) ratio was high in pyritebearing beds and low in oolitic limestones. In oxic sea water, cerium occurs in the trivalent state and is fractionated by coprecipitation with metal-

337

lic oxides, especially iron oxide, thereby producing a negative cerium anomaly in sea water; whereas in anoxic sea water, no such fractionation occurs (Ahmad, 1989). These signals may be better recorded in biogenic and chemical precipitates such as shells and carbonates because trace elemental contents in clastic rocks may also be in£uenced by their original source compositions. Therefore the HCl-soluble method is suitable for this purpose. This is in agreement with our comparison of both the whole-rock method and the HCl-soluble method used in the analysis of about 1000 samples from the Devonian of South China (Bai et al., 1994). Ce/La ratios of various characteristic rock types show that in carbonates a Ce/ La ratio s 1.6 is characteristic of dysoxic to anoxic environments. The ratio is around 1.5 for beds L3^L5 and L9b^L9c (Fig. 9), suggesting an oxic (sometimes slightly dysoxic) environment which is supported from their lithologies. L7^ L9a have a Ce/La ratio ranging from 1.8 to 2.1 (average 2.0), re£ecting a dysoxic to suboxic environment. Bed L6 is black shale, considered to be anoxic by all previous workers. Its Ce/La ratio is 4.6 on average (ranging from 3.8 to 6.0 for ¢ve di¡erent samples). A dividing line between oxic and anoxic environments for shale/mudstone probably should be drawn somewhere between 2.1 and 3.8 because the gray shale of L10 has a Ce/La ratio of 2.1. L10 is probably a deposit under normal oxygen conditions. Mo/Al, V/Cr and Ni/Co ratios have been used for deducing paleoredox conditions (e.g. Jones and Manning, 1994). Ni/Co and Mo/Al ratio variations in the Xikuangshan F/F boundary interval are shown in Fig. 9. These two ratios basically

Fig. 6. Showing major lithologies which are most useful to paleoenvironmental explanation in the shallow-water facies of South China. Scale bar in B = 1 mm (same for E, I and J); scale bar in D = 1 mm (same for A, C, F, G, H and K); scale bar in L = 0.5 mm; scale bar in M = 1 cm. (A) Oolitic sparitic limestone of LB/0.9 (sample taken at 0.9 m above the base of bed LB). (B) Micrite with sponge spicules and minor other bioclasts (L3/0.2^0.4); (C) showing an erosional boundary near the top of L5 (L5/ 0.95) with abundant bioclasts above and minor bioclasts below. (D) Micro-cross-lamination in the black shale of L6/0.1 with common quartz silts and ¢ne sands at the top. (E) Oncoids in L5 (L5/0.6^0.8). (F) Thin bioclastic layer of L6/0.55 (mostly bryozoans and brachiopods, minor quartz grains are also seen) intercalated in the black shale of L6. (G) Bioclastic dolomitic limestone of L7, showing brachiopod, bryozoan, quartz and ostracod grains and dolomite crystals. (H) Bioclastic dolomitic limestone of L9/0.1 showing brachiopod, ostracod and bryozoan clasts and dolomite crystals. (I) Ostracod sparitic limestone of L9/0.65, with the bioclastic grains commonly coated with iron oxide. (J) Oolitic sparitic limestone of L9/t0.3. (K) Amphipora cross-section of D3-1. (L) Calcispheric and algal peloidal micrite. (M) Oncolitic limestone of C1-1.

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show a similar trend. They are high in the Upper part of bed L6 (black shale) but low in other samples of the same lithology (L6 and L8). A dramatic decrease of the Ni/Co ratio occurs from the top L6 to bed L8. In bed L9, the Mo/ Al ratio is also high, especially in L9a and the Upper part of L9c (Fig. 9). The black shale of L6 itself does not reveal any benthic organisms; however, the carbonate intercalations (L6/0.55, L7, part of L8, and L9a) contain abundant bryozoans, some brachiopods and rare ostracods. Trace fossils like Palaeophycus are observed in bed L7. It cannot be discounted that these bioclastic layers may be transported. Overall lithological, faunal and geochemical evidences show that the L6^L9a interval should represent deposits mainly under partly anoxic to dysoxic conditions and sometimes partly oxic conditions. It was this anoxic event combined with metal toxic in£ux that killed most benthic organisms in the shallow-water setting of South China. A possible source for this anoxia and metal toxic in£ux could be the intensi¢cation of rifting in Guangxi (Bai et al., 1994), which is probably reconcilable with the process proposed by Cathles and Hallam (1991) for the formation of anoxic bottom water during rapid rifting. The rift system also extends from Guangxi northwards into Hunan (Fig. 1). 6.2.3. The F/F black shale : sea-level rise or fall The Upper Kellwasser bed is widely distributed in the deeper water facies of Europe, which is considered by most workers to be anoxic and to result from a transgression (e.g. Sandberg et al., 1988; Joachimski and Buggisch, 1993). Some workers believe that the F/F black shale in South China also resulted from a marine transgression (see Muchez et al., 1996) and correlate this black interval in the Luoxiu ( = Xiangtian), Laojiangchong ( = Xikuangshan), and Lijiaping sections with the whole linguiformis Zone. It should be pointed out that Bai et al. (1994) placed the base of the linguiformis Zone about 3 m below the shale and the top of the Zone 1 m above the black-shale horizon in the pelagic Nandong section. Bai et al. (1994, p. 86) pointed out that the F/F boundary in the Xiangtian section should be placed 1 m above the black-shale horizon.

Investigation of the Xikuangshan shallow-water section shows that this black-shale interval (including L6^L8) was formed near the top of the linguiformis T^R cycle which probably began at about 15^20 m below the F/F boundary (Fig. 4). The black shales were deposited in a shallowwater environment. Evidences are as follows : (1) L7 is intercalated between two black-shale horizons (beds L6 and L8). Beds L7 and L9a are dolomitized with a restricted fauna: ostracod diversity in this interval is very low; conodonts are almost coniform (Fig. 5); megafossils are predominated by low-diversity bryozoans. These two beds are the only two dolomitized units in Upper Frasnian to Lower Famennian strata at this section. The dolomites are ¢nely crystalline and the dolomitization was non-selective, representing a penecontemporaneous process in an intertidal to supratidal regime. (2) Both shale and limestone intercalations bear terrestrial granules such as gravels, land-plant fragments, etc. Micro-cross-lamination and quartz sandy layers are present. (3) The L6^L9a interval is greatly varied laterally both in thickness and lithology. In the Xikuangshan area there are several F/F boundary sections exposed. About 20^30 m west of the Xikuangshan section on the other side of a small fault, bed L7 becomes 0.45 m thick which is composed of 20 cm thick yellowish gray limestone above and 25 cm thick yellowish gray shelly-bryozoan limestone below ; bed L8 becomes 0.25 m thick; bed L9, which is quite uniform in lithology, is only 0.7 m thick. Beds L7 and L8 are even absent in the Taotang area which is only a few kilometers northwards from the Xikuangshan section, and beds L6 (varicolored shale) and L9 become much thinner, 23 and 20 cm thick, respectively. The black-shale horizon is even absent in many shallow-water sections in central Hunan, e.g. in the Chongshanpu and Jiangjiaqiao sections, and in the deeper water Liujing and Du’an sections of Guangxi (Wu, 1997). In addition, in the Leimingdong section (Fig. 1), which is located in the littoral sandstone facies, the Frasnian represents a complete T^R cycle. It

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Fig. 7. SEM micrographs of the microspherules from the Xikuangshan section showing their external morphologies. (A) Big brownish translucent spherule with several di¡erent-sized small ones welded together. sample L9/t0.3. (B) Colorless to slightly brownish transparent spherule (specimen number 4-1 in Table 2) with attached small droplets and pits on the surface. L5/0.5. (C) Fe spherule with rough, dull surface. L5/0.5. (D) Brownish glassy spherule (3-2 in Table 2) with vesicles on the surface. B5/1.7. (E) Black spherule with protruding tips; surface has a metallic shine. L11. (F) An irregular spherule showing its typical form. L3/0.1. (G) Colorless transparent spherule (m004) with attached small droplets on the surface. L5/0.5. (H) Milky white spherule with two small spherules welded together; for composition, see specimen No. 11-1 and 11-2 in Table 2. Scale bar for (A) through (E), 80 Wm; for (F) and (H), 135 Wm; for (G), 125 Wm.

is composed of sandstones and shales at the base, shales and limestones with sandstone intercalations in the Middle part hosting abundant corals and brachiopods, and grayish white, medium- to thick-bedded sandstones intercalated with minor shales hosting terrestrial plant fossils in the Upper part. Thus, data from the uppermost Frasnian of central Hunan indicate a low-stand at the F/F boundary, contrary to a recent study by Hallam and Wignall (1999) who reported that ‘‘no evidence for regression during the Late Frasnian crisis interval, on the contrary it appears to have been an interval of highstand in Morocco’’. This latest Frasnian regression has been long recognized in many parts of the world by an erosional

surface or a hiatus of various degrees, e.g. in central and western North America (Day, 1998), in central Hunan (Daping section) of South China, etc. Of course, sea-water depth might increase in some areas primarily due to tectonic control as Late Devonian rifting was very active, e.g. in South China (Chen et al., 2001a,b).

7. Microtektites from the Xikuangshan section There are a few types of microspherules present in the Xikuangshan section in addition to some organic^biogenic and diagenetic microspheres, e.g. apatite and pyrite spheres. (1) Fe spherules: are black in color, spherical, with a somewhat

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rough and dull surface (Fig. 7C). They are almost entirely composed of iron oxides. On the polished surface, small cavities may be present. This kind of spherule is very rare in the section. (2) Fe-rich spherules : are black in color, spherical and the surface has a metallic shine (Fig. 7E). Most of them are hollow or partly hollow. FeO abundance ranges from about 17 to 50%; SiO2 abundance about 40V50% or less; Al2 O3 abundances range from 2 to 17% and are negatively correlated with FeO contents. This kind of spherule is relatively common in the section. (3) Irregular microspherules: are also black in color. Most of them are spherical (Fig. 7F), but very irregular in outline such as elongate and £at, vase-shaped, dumbbell or brecciated ones. This type of spherule is quite abundant in the section. (4) Si- and Al-rich spherules: are characterized by high contents of Si and Al. As they possibly represent microtektites, the following description and discussion will exclusively concern this type of microspherule. 7.1. General morphology of the microspherules and their distribution The spherules are usually milky white to light brownish in color or transparent. Their size ranges from 50 to 300 Wm, generally 100 Wm in diameter. Shape is commonly perfectly spherical, solid or (partly) hollow; a few spherules consist of two or three spheres apparently fused together (Fig. 7A,B,G and H). The microspherule abundance is shown in Fig. 2. Two major peaks in the abundance of the microspherules are observed at 0.6 m above the base of bed LB (LB/0.6, Upper rhenana Zone) and at 0.5 m above the base of bed L5 (L5/0.5, upper linguiformis zone). Two other levels with this type of microspherule are the L7^L9a interval and the top 0.3 m of bed L9 (L9/t0.3). 7.2. Geochemical composition of the microspherules Al^Si spherules are generally yellowish or greenish brown to milky white in color, or are colorless ; transparent or non-transparent ; the shape is spherical, but two or more small spher-

Fig. 8. SEM micrographs of polished sections of some microtektites. (A) Back-scattered electron mode, showing the vesicles (black holes) in an Al^Si spherule (specimen No. 3-3 in Table 2). (B) Si X-ray mapping of the spherule in (A). (C, D) Si and Ca X-ray mapping showing the general isotropic character of a spherule (specimen No. 4-1 in Table 2); Al Xray mapping (not shown here) is similar, only slightly denser at the top and lighter at the bottom. Scale bar = 50 Wm.

ules may be attached to a bigger one (Fig. 7A, H). The brownish spherules are generally solid, whereas the white ones are mostly partly hollow. Al2 O3 abundance ranges from 25 to 35%; SiO2 abundance is about 50V55%; CaO and FeO contents are generally less than 10%, respectively. Si-rich spherules are brownish, glassy and transparent, spherical in shape. SiO2 contents are around 80%. These Si-rich spherules rarely occur. Polished sections examined under the scanning electron microscope (SEM) show that vesicles are present in the interior of some microspherules (Fig. 8). In others, bright, gray and sometimes dark areas may be seen with di¡erent compositions (Fig. 8A). The bright areas are mostly composed of SiO2 and Al2 O3 (Table 2), whereas the gray areas have a CaO content equal to or higher (even a few times higher) than SiO2 content; other components include S, Mg and Na or Al. This may suggest that there may be some inclusions of CaSO4 . X-ray elemental mapping of individual Si, Al and Ca shows that most spherules are generally isotropic (Fig. 8C,D).

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Table 2 Chemical compositions (in %) of various microtektites from the Xikuangshan section Horizon Specimen No.

SiO2

B5/1.7

3-2r

49.70 24.45

0.47

L9/t0.3

3-3

55.09 33.10

L9/t0.3 L5/0.5

m003 4-4

L5/0.5

Al2 O3 MgO K2 O

Na2 O CaO

TiO2

Cr

MnO Ni

FeO

Total Remarks

0.27

0.24

14.79

0.51

0

0.04

0.24

8.29

0.77

1.91

0.56

3.93

0.89

0.01

0.14

0.11

3.17

50.76 20.03 55.25 26.22

2.02 1.19

0.48 2.40

0.41 1.62

19.11 9.78

0.96 0.65

0.04 0.08

0 0.14

0.07 0.05

5.43 2.17

11-1

46.65 31.93

0.47

0.26

0.93

17.54

0.03

0.21

0.08

0

1.01

L5/0.5

11-2 m65

60.31 21.28 81.55 10.18

1.87 0.28

2.89 2.30

1.47 1.18

6.87 2.59

0.95 0

0.03 0

0.06 0

0.01 0

3.66 1.60

L5/0.5

m004

63.34 18.14

1.44

3.43

1.64

6.67

1.38

0

0

0.06

3.45

L5/0.5

m007

64.34 13.34

0.82

2.61

0.95

13.86

0.82

0.02

0

0

2.88

99.00 brownish, with vesicles (1) 99.68 light brown, with a small hole (2) 99.31 light brownish (4) 99.55 transparent, brownish core (3) 99.11 white, both with cavity 99.40 99.68 brownish, transparent (2) 99.55 transparent with vesicles (3) 99.64 transparent with darker core (2)

All Fe is expressed as FeO. Specimen number beginning with a letter ‘m’ means analyses on outer surface of microspherules; others were measured on polished section. B5/1.7 is from the Lower rhomboidea Zone of the Baqi section (reworked; see Bai et al. (1994) and Ma (1998) for stratigraphic data). Numbers in brackets of the Remarks column refer to number of analyses.

7.3. Origin of the Xikuangshan Si^Al and Si-rich microspherules The origin of microspherules has been the subject of debate over the past. Deep-sea microspherules, which include microtektites and microkrystites (crystal-bearing microspherules), are generally believed to be melted terrestrial material and ejected during the impacts of extraterrestrial bodies on earth (Glass, 1990). However, microspherules do occur in volcanic settings (e.g. Etna volcano in Italy; Lefe¤vre et al., 1986). However, these spherules are very small (around 3 Wm) and commonly rough-surfaced. They are chemically similar to the composition of volcanic rocks. The Xikuangshan spherules have a variety of morphologies (Fig. 7) di¡erent from the diagenetic spherules represented by Naslund et al. (1986) which are all spherical with rough or etched surfaces. The Xikuangshan Si^Al and Sirich microspherules display a morphology similar to some of the microtektites reported by Glass (1974), for example pitted surface, fused spherules, etc. The Xikuangshan microspherules possess some Ni in addition to high SiO2 and Al2 O3 . This is not reconcilable with a volcanic origin. It

should be noted that SiO2 abundance of the Xikuangshan spherules does not appear to be related to FeO, Al2 O3 or CaO abundance values. This is di¡erent from those spherules previously reported, where major elements commonly show an inverse correlation with SiO2 (as in Wang, 1992; Claeys and Casier, 1994; Koeberl, 1990), which probably suggests a di¡erent source for the Xikuangshan microspherules. The di⁄culty facing a volcanic origin for the Xikuangshan and South China F/F spherules is that volcanic microspherules are commonly rough-surfaced and smaller in diameter (e.g. Lefe¤vre et al., 1986), without showing diverse morphologies as those of the Xikuangshan spherules, and in addition Late Devonian explosive volcanic activity was not present in the Hunan area. Therefore, the Xikuangshan Al^Si and Si-rich microspherules (microtektites) are probably of impact origin judging from their morphologies and chemical composition. However, as reported for the Upper Eocene impacts by Wei (1995), some impacts may not have had any in£uence on the fauna as evidenced at both LB/0.6 and L5/0.5 horizons, where an impact might be present indicated from the abun-

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Fig. 9. Elemental ratio variations across the F/F boundary in the Xikuangshan section. Elemental abundance was measured with an ICP using the whole-rock method. Ce/La ratio of the HCl-soluble method is also given for comparison.

dance of microtektites, but the fauna did not show any signi¢cant turnover and did not become extinct until the top of bed L5 where the most severe benthic mass extinction occurred (Fig. 2). The anoxia-induced sudden biomass loss at the top of bed L5 may be a world-wide phenomenon, which should result from some sort of an abrupt event. The problem is that so far neither physical nor geochemical evidences of signi¢cance are found for a large impact as a main cause for this benthic biomass loss. It should be noted that there are also Fe and Fe-rich microspherules in the Xikuangshan section. Like the irregular microspherules, they are stratigraphically widely distributed but less abundant than those microtektites which are concentrated in a few layers. Recent study by Iyer et al. (1997) in the Central Indian Ocean Basin suggests a hydrovolcanic origin for their magnetite spherules. However, a cosmic dust origin for the Xikuangshan Fe and Fe-rich microspherules is also

probably more plausible judging from their longrange stratigraphic distribution and the absence of volcanic activity in the Hunan area.

8. Geochemical signatures across the F/F boundary in South China 8.1. Lower rhenana Zone metal anomalies The Xikuangshan section is located in an antimony ore ¢eld which is the world’s largest. The stratabound ore deposit (Tu, 1988) is only present in the Upper Qilijiang limestone (Fig. 3). It is associated with strong silici¢cation and other elemental anomalies such as Ba, As and Hg. In the Nandong section bed K58 reveals Mn^ Ni anomalies (Bai et al., 1994). Elsewhere in the equivalent interval, manganese and barium mineral deposits were developed, e.g. at Mugui and Hongjiang of Guangxi.

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8.2. The F/F boundary interval: multiple elemental anomalies Towards the F/F boundary there are only some elemental anomalies which are mainly related to the paleoredox condition such as Ce/La, Ni/Co and Mo/Al ratios (Fig. 9). Above the F/F boundary in the Lower triangularis Zone of the Xikuangshan section there are two anomalous horizons: L9a and Upper part of L9b (Fig. 9). The anomalous elements include Mn, Sr, Zn, Cu, Ni, Fe, etc. The Fe abundance curve is almost identical with that of the Mn/Al curve except that the L9a anomaly is slightly bigger than the Upper L9b one. In the Nandong section, in the upper linguiformis to Middle triangularis Zones there are Mn, Ba, As and other elemental anomalies (Fig. 3), generally tens to hundreds of times the normal background value. 8.3. Upper crepida Zone metal anomalies In the Nandong section there are Mn, Zn and Ba anomalies in the Upper crepida Zone (sample K79/0, Bai et al., 1994, Appendix table 29 therein) which are about 10^20 times the background values for Mn and Zn and about 20^50 times the background values for Ba. In the Xikuangshan section, about in the equivalent interval, an iron-rich layer was deposited (Ningxiang type) which is widely distributed in Hunan and Hubei Provinces. 8.4. Origin of the elemental anomalies During the Devonian, hydrothermal deposits were widely developed in South China, especially in the Upper Devonian. Most mineral deposits such as Pb, Zn, Mn and Ba are thought to be of synsedimentary hydrothermal origin related to rifting activities (e.g. Chen and Gao, 1988; Chen et al., 1999; Huang et al., 2000). The Xikuangshan stratabound antimony ore deposit has also been thought recently to be of synsedimentary hydrothermal origin with later modi¢cation by most workers (e.g. Kuang, 2000). The above three elemental horizons are well correlated with major

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Upper Devonian mineral deposits of South China. Therefore they should have the same source as those mineral deposits, i.e. of hydrothermal origin. However, the Ningxiang-type stratabound iron-rich layer is generally thought to be sedimentary with terrestrial erosion as a source for the iron component (e.g. Zhao and Bi, 2000).

9. Conclusions Shallow-water rugose corals (both colonial and solitary) and ostracods experienced a sudden biomass loss at the onset of the latest Frasnian blackshale deposition. Any mechanism proposed as the cause of the F/F event should explain this suddenness. At least two of the three Late Frasnian Cyrtospirifer brachiopod species and the rhynchonelloid brachiopod Hunanotoechia became extinct at about the same time. This benthic faunal extinction may be a world-wide phenomenon, which occurred before the end of the Frasnian. Extinction of manticolepid conodonts occurs right at the end of the linguiformis Zone in South China, as in other parts of the world. Contrary to most workers’ point of view, lithological and faunal characteristics in the shallowwater facies of South China indicate that the F/F black-shale interval was probably formed in a regressive, shallower water setting. Following this was a hiatus or an erosional interval of di¡erent time spans in the triangularis Zone on the carbonate platform of South China. Higher Ce/La ratios and high contents of As and Ni/Co ratios co-occur in the black-shale interval (in both shales and carbonates), representing an anoxic to suboxic condition. It was probably this anoxic event plus hydrothermal toxicity in some regions that was responsible for the F/F benthic biomass loss in South China. The microtektite-like microspherules occur in several layers across the F/F boundary. These Si^Al microspherules are probably of impact origin. However there are no signi¢cant faunal changes associated with these microtektite layers in the Xikuangshan section. There are four major elemental anomalous intervals from the Lower rhenana through Upper

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crepida Zones. The Lower rhenana and Upper crepida Zones host several types of mineral deposits which have been mined for manganese, barite, and antimony. The end-linguiformis to earliest Famennian interval has multiple elemental anomalies including Mn, Ba, Zn, Fe, Ni, Cu and As. These elemental anomalies may have resulted from the contemporaneous active rifting process and hydrothermal activities in South China.

Acknowledgements This work was supported by the Foundation for University Key Teachers of the Chinese Ministry of Education, the National Natural Science Foundation of China (No. 40172006), the Major Basic Research Projects and the Chinese Stratigraphy Project of the Ministry of Science and Technology, China (G2000077700; 2001DEA20020-5), the Scientific Foundation for Returned Overseas Chinese Scholars, and a grant from Peking University for the microprobe analysis. F.T. Kyte of University of California (Los Angeles) and an anonymous reviewer critically read the manuscript and made a number of suggestions from which the paper has benefited. G.M. Shu, H.X. Shao, B. Yang, P.Y. Wang, and C.Y. Zhou of Peking University are thanked for analyses of electron microprobe, ICP trace elements, X-ray mineral composition, and printing part of the photographs. We also thank the guest editors, G. Racki and M. House, for useful discussions during the preparation of this paper. References Ahmad, R., 1989. Secular variations in global bottom seawater redox during Phanerozoic: evidence from cerium anomaly in biogenic apatite. Abstracts of the 28th International Geological Congress 1, Washington, DC, p. 20. Bai, S.L., Bai, Z.Q., Ma, X.P., Wang, D.R., Sun, Y.L., 1994. Devonian Events and Biostratigraphy of South China. Peking University Press, Beijing. Cathles, L.M., Hallam, A., 1991. Stress-induced changes in plate density, Vail sequences, epeirogeny, and short-lived global sea level £uctuations. Tectonics 10, 659^671. Chen, D., Tucker, M.E., Zhu, J., Jiang, M., 2001b. Carbonate sedimentation in a starved pull-apart basin, Middle to Late

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