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770


Journal of Paleontology 89(5):768–790


environments, and the slope was likely relatively steep because deep-water shale and sandstone facies belts contact land margins directly; the southwest margin of the plate was also slope environments, but the slope was relatively gentle in that a shallow water small scale carbonate platform developed close to the land margin, and south of the carbonate platform were deep water slopes and basins of the ancient Qinling ocean (Feng et al., 1990, 1998, 1999, 2004). Ordovician rocks are well exposed and widely distributed


in the Wuhai area, and they constitute the main body of the Zhuozishan and Gangdel Mountains around the Wuhai City. The Ordovician System, a sequence of mixed carbonate and clastic facies, of the Wuhai area disconformably overlies the Late Cambrian Gushan Formation and is disconformably overlain by the Early Carboniferous Benxi Formation. After a review of the lithostratigraphic units proposed by Lu (1954), Guan and Che (1955),Mu (1959) and Zhang (1959), Chen et al. (1984) grouped the Ordovician strata of the Wuhai area into seven formations, namely the Sandaokan, Zhuozishan, Klimoli, Wulalike, Lashizhong, Gongwusu and Sheshan formations in ascending order. This lithostraigraphic scheme has been widely accepted (Wang and Luo, 1984; An and Zheng, 1990; Feng et al., 1990, 1998, 1999, 2004; Fu and Zheng, 2001; Li et al., 2012; Wang et al., 2013b, 2013c), and is also adopted by the present authors.


Lithology


The conodont samples investigated in this study were collected from two sections, the Wolonggang Section and the Hatuke Creek Section (Fig. 1C). These two sections constitute a continuous middle Darriwilian and lowermost Sandbian succession in the study area. The Wolonggang Section (39°22ʹ02ʺN, 106°52ʹ44ʺE), a


wide erosional stream-cut gorge near the Wolonggang trans- former substation, is about 8 km south of the town of Hainan. The Klimoli Formation, well exposed along the gorge, consists of 11 beds with a total thickness of 225.68m (Fig. 2). Its lower two-thirds consist primarily of alternating beds of black marl and calcareous mudstone, occasionally intercalated with thin-bedded packstone as well as medium-bedded wackestone. The upper one-third is dominated by black gray thin-bedded marl, except for the top 11.35 m, which is black graptolitic shale. The rocks strike NE (82°–86°) with a dip of 30°–33°. Wang et al. (2013b) subdivided the Klimoli Formation (approximately 65m thick) of the Dashimen Section (=the Hatuke Creek Section of this paper) into two parts: the Lower Klimoli Formation, approximately 43m thick, which is domi- nated by limestone; and the Upper Klimoli Formation, approximately 22m thick, of graptolitic black shale without limestone interbeds. An and Zheng (1990) found both the lithology and the thickness of the Klimoli Formation vary significantly from place to place in the Wuhai area. It is observed that the thickness of this formation ranges from 292m at the Wolonggang Section to 80mat Hatuke Creek section, and clay content dramatically increases at the latter section. The Klimoli Formation in the Hatuke Creek Section is, therefore, more condensed, making the Wolonggan Section the better section for biostratigraphic study.


The Hatuke Creek Section (39°28ʹ30ʺN, 106°49ʹ43ʺE) is


approximately 6 km northeast of the town of Hainan. The Ordovician rocks crop out along the dry Hatuke Creek to the ridge of a low hill. This more than 250m Ordovician section contains four lithostratigraphic units: the Zhuozishan, Klimoli, Wulalike and Lashizhong formations. Only strata from the upper part of the Klimoli Formation to the base of the Lashizhong Formation were measured and sampled in the present study (Fig. 3), and they strike 198°–200° and with dips that range from 15° to 40°. The upper part of the Klimoli Formation, which corresponds to the top of the lower Klimoli and the upper Klimoli of Wang et al. (2013b), displays a gradual lithological transition from marl upward to graptolitic black shale. The overlying Wulalike Formation, whose lower limit is drawn at the bottom of a widespread brecciated packstone, consists of about 40m of black shale in the lower part and about 100 m, of chiefly greenish yellow silty shale in the upper part. Several layers of thin-bedded marl, which yield a large number of conodonts, intercalate in this formation. At the top, the Wulalike Formation is capped by the sandstone-dominated Lashizhong Formation.


Conodont biostratigraphy


A total of 28 limestone samples were collected from the Wolonggang and Hatuke Creek sections. All samples, which weighed 2.5 kg on average, were treated with diluted acetic acid and heavy liquid to isolate conodonts. Forty-two species belonging to 26 genera were recovered and systematically studied (Table 1). The recovered conodonts are commonly excellently preserved and they occur in large numbers in some samples. Having a color alteration index (CAI) of 1–2, these conodonts show very little, if any, evidence of having been heated.


Three conodont zones and three subzones can be


discriminated in the Wolongang Section (Fig. 2). In ascending order, they are: the Dzikodus tablepointensis Zone, the Eoplacognathus suecicus Zone (including the Pygodus lunnensis Subzone and the Pygodus anitae Subzone), and the Pygodus serra Zone (only the Yangtzeplacognathus foliaceus Subzone). At Hatuke Creek, the following three zones and one subzone (Fig. 3) have been distinguished: the Eoplacognathus suecicus Zone (the Pygodus anitae Subzone), the Pygodus serra Zone, and P. anserinus Zone. Due to available data limitation, the upper or lower boundaries of some of these Zones/Subzones are not precisely defined.


Dzikodus tablepointensis Zone.—The lowest of the recognized zones ranges from 26m to 104m above the base of the Wolongang Section. The zone index species is associated with 17 other species, including Ansella crassa Bauer, 1994, A. jemtlandica (Löfgren, 1978), Baltoniodus sp., Coelocerodontus trigonius Ethington, 1959, Cornuodus longibasis (Lindström, 1955), Costiconus ethingtoni (Fåhræus, 1966), Dapsilodus viruensis (Fåhræus, 1966), Drepanoistodus basiovalis (Sergeeva, 1963), Periodon hankensis Stouge, 2012, P. macrodentatus (Graves and Ellison, 1941), P. flabellum (Lindström, 1955), Phragmodus polonicus Dzik, 1978, P. sp. Polonodus sp., Protopanderodus cooperi (Sweet and


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