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Palaeoecology of the Conodonts Hindeodus and Clarkina During the Permian±Triassic Transitional Period

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Palaeoecology of the Conodonts Hindeodus and Clarkina During the Permian±Triassic Transitional Period http://www.paper.edu.cn Palaeogeography, Palaeoclimatology, Palaeoecology 171 )2001) 63±72 Palaeoecology of the conodonts Hindeodus and Clarkina during the Permian±Triassic transitional period Xulong Laia,b,*, Paul Wignallc, Kexin Zhanga aFaculty of Earth Sciences, China University of Geosciences, Wuhan, Hubei 430074, People's Republic of China bDepartment of Geology, University of Leicester LE1 7RH, UK cDepartment of Earth Sciences, University of Leeds, LS2 9JT UK Received 9 March 2000; accepted for publication 26 March 2001 Abstract Detailed investigation of the distributions of the Hindeodus and Clarkina )Neogondolella) conodont faunas has been under- taken on the P/T boundary strata of the Meishan section, Zhejiang Province, South China, in association with detailed facies analysis. This reveals that Hindeodus increased and Clarkina sharply declined in abundance during a phase of relative sea-level rise which began at the end-Permian )Bed 25) and extended into Early Triassic. This was associated with the development of anoxic conditions. This distribution does not accord with previous suggestions that Hindeodus was a nearshore, shallow water taxon. The decline of the supposed deeper water genus Clarkina is also somewhat surprising but it may relate to the inhibition of a nektobenthic genus by dysoxic±anoxic bottom waters. The widespread facies and geographic distribution of Hindeodus suggests it was a pelagic type unaffected by anoxic bottom waters. Hence, the Hindeodus lineage provides a reliable criterion for identi®cation of the Permian±Triassic boundary. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Permian±Triassic boundary; Conodonts; Palaeoenivironment; Palaeoeoclogy 1. Introduction versial. Orchard )1996) considered Clarkina to be a deep-water genus and Hindeodus a shallow-water Yin et al. )1988) proposed the ®rst appearance of genus. Therefore, Baud )1996) proposed the use of the conodont Hindeodus parvus as the marker for the the deeper water Clarkina instead of Hindeodus to basal Triassic; an idea that has been widely accepted identify the basal Triassic. During the past two )e.g. Paull and Paull, 1994). As the main fossils used decades, numerous studies of the biofacies and for correlation of the Permian±Triassic )P/T) bound- palaeoecology of conodonts during the P/T transi- ary, conodonts play an important role in the study of tional period have been published. To date, most this interval, with the genera Clarkina )Neogondo- conodont specialists consider Clarkina )or Neogondo- lella) and Hindeodus being particularly important. lella) to be an offshore, outer shelf or basinal, deep- However, the palaeoecology of these taxa is contro- water taxon )Clark, 1974, 1981; Carr et al., 1984; Clark and Hatleberg, 1983; Clark and Carr, 1984; Hatleberg and Clark, 1984; Hirsch, 1994; Wardlaw * Corresponding author. Address: Faculty of Earth Sciences, and Collinson, 1984; Tian, 1993a,b; Wang, 1996; China University of Geosciences, Wuhan, Hubei 430074, People's Republic of China. Wang and Wang, 1997; Orchard, 1996; Orchard and E-mail address: [email protected] )X. Lai). Krystyn, 1998; Krystyn and Orchard, 1996; Baud, 0031-0182/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0031-0182)01)00269-3 转载 中国科技论文在线 http://www.paper.edu.cn 64 X. Lai et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 171 52001) 63±72 1996). However, the biofacies of Hindeodus is much 2.1. Palaeoenvironmental analyses more controversial. Some consider it limited to near- shore, shallow marine facies )Wardlaw and Collinson, Three systems tracts have been recognized in the 1984; Tian, 1993a; Hirsch, 1994; Wang, 1996; Orch- uppermost Permian and lowermost Triassic at ard, 1996; Krystyn and Orchard, 1996; Baud, 1996), Meishan )Fig. 1). while others regard it as a widespread genus both in Bed 24d consists of three parasequences; each term of biofacies and/or geographic occurrence parasequence begins with a 1 cm thick dark-grey )Clark, 1981; Clark and Hatleberg, 1983; Hatleberg siliceous-mud bearing limestone. The middle part of and Clark, 1984; Rexroad, 1993; Wang and Zhong, each parasequence is comprised of a 4 cm thick, 1995; Kozur et al., 1996; Kozur, 1996; Lai, 1997; Lai bioclastic packstone, and the upper part is a 5 cm et al., 1998). New investigations of conodont biofa- thick packstone. From bottom to top within each para- cies have been undertaken on the P/T boundary strata sequence, the colour becomes lighter, bioclastic of the Meishan Section, Zhejiang Province, South grains become bigger and the bioclasts change China )Yin et al., 1996) Ð the global stratotype systematically from deeper water types )including section and point )GSSP) of the Permian±Triassic slender non-fusunilind foraminiferas, spicules and Boundary )PTB) and are presented here. A new ecolo- echinoderm) to shallower water types )including gical model for Hindeodus and Clarkina is proposed, thicker-shelled brachiopods, calcareous algae and and the data from several P/T boundary sections else- bigger non-fusulinid foraminiferas). There is a wavy where in the worldwide are re-evaluated. boundary at the top of Bed 24d where a thin bed of Note that the genus name for the gondolellid cono- iron oxide-rich calcareous mud occurs. The truncation donts near the P/T boundary is controversial. Kozur fossil at a wavy surface also occurs at the top of Bed )1989) proposed the genus Clarkina, and several 24d. Microfacies change sharply across the wavy authors accept this genus name instead of Neogon- boundary, which Zhang et al. )1996) considered to dolella. However, Orchard and Rieber )1999) be a type II sequence boundary. reconstructed the multielement taxonomy of Neogon- Bed 24e consists of single upward shallowing dolella, and placed Clarkina in synonymy with parasequence. The lower part is a 1 cm thick, dark- Neogondolella. Most work on the Meishan section grey, siliceous, muddy limestone with horizontal uses Clarkina instead of Neogondollea )e.g. The bedding. The middle part is a 4 cm thick, wavy- Permian±Triassic Boundary Working Group, 1999; bedded bioclastic wackestone and the upper part is a Yin and Tong, 1998; Mei et al., 1998, Wardlaw and 5 cm thick bioclastic wackestone showing reverse Mei, 1998), and we have maintained this `convention' grading. The bioclastics mainly consists of fusulinids, here, although we acknowledge that the validity of non-fusulinid foraminifers and brachiopods. this genus is doubtful. Beds 25 and 26 are the P/T boundary claystone. Generally, Bed 25 has been called the `white clay bed' and Bed 26 the `black clay bed' )Yin et al., 2. Palaeoenvironmental changes and conodont 1996). According to previous studies, both beds are evolution in the Meishan section composed of montmorillonite-illite claystones with subordinate kaolinite, and are of volcanic origin As the one of four candidates for the GSSP, the )Yin et al., 1992; Yin and Zhang, 1996). There are Meishan section has received detailed multidisciplinary pyrite laminae at the bottom and top surfaces of Bed analysis during the past two decades. It is one of the 25. Bed 26 is organic-rich, dark in colour, with ®ne best section for the study of conodont palaeoecology lamination, although Planolites burrows also occur. across the Permian±Triassic transitional period. The taxa are dominated by pelagic types mainly Based on systematic analyses of lithofacies, microfa- consisting of ophiceratid ammonites. A few dysaero- cies, biofacies and sea-level changes, the palaeoenvir- bic benthic taxa are presented including non-fusulinid onmental changes can be related to the distribution of foraminifera and tiny, thin-shelled brachiopods. the conodonts Hindeodus and Clarkina shown in Bed 27 is a 16 cm thick grey, argillaceous micrite Fig. 1. with scattered pyrite grains and a pervasively 中国科技论文在线 http://www.paper.edu.cn X. Lai et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 171 52001) 63±72 65 Fig. 1. Palaeoenvironmental changes and conodont evolution across the Permian±Triassic boundary in the Meishan section, Zhejiang, China. 1. claystone; 2. marl; 3. mud-bearing micrite; 4. siliceous micrite; 5. bioclastic micrite; 6. anoxic environment; 7. horizontal bedding; 8. normal graded bedding; 9. high temperature quartz; 10. zircon; 11. microspherule; 12. fusulinid; 13. non-fusulinid foraminifer; 14. conodont;15. calcareous alga; 16. brachiopod; 17. bivalve; 18. ammonite. burrowed fabric. Based on analysis of the microfacies, The minor omission surface between beds 24d and fauna, and in comparison with other sections )e.g. 24e may mark a preceeding phase of relative sea-level Shangsi section in Sichuan, Lai et al., 1996), Bed 27 lowstand, as interpreted by Zhang et al. )1996), or at Meishan records a highly condensed record. In alternatively a phase of non-deposition caused by a conodont sampling both Zhang )Zhang et al., 1995) sudden pulse or deepening. Regionally, the ±Tr strata and Wang )1994) subdivided this 16 cm bed into four mark a major phase of onlap in China )Jin et al., 1994) equally thick partitions. and elsewhere )Wignall et al., 1996; Hallam and Bed 28 is a 4 cm thick illite-montmorillonite mixed Wignall, 1999). This transgression was associated claystone, with abundant pyrite crystals, together with the development of oxygen-poor deposition at hexagonal bipyramid quartz, zircon and micro- Meishan, as indicated by the presence of abundant spherules of volcanic origin. pyrite, the dysaerobic paper-pecten genus Claraia Bed 29 is grey, medium-bedded,
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