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Volume 32 Issue 3
Jun.  2021
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Yong Yi Zhen. Middle Ordovician Conodont Biostratigraphy of Australasia. Journal of Earth Science, 2021, 32(3): 474-485. doi: 10.1007/s12583-020-1116-1
Citation: Yong Yi Zhen. Middle Ordovician Conodont Biostratigraphy of Australasia. Journal of Earth Science, 2021, 32(3): 474-485. doi: 10.1007/s12583-020-1116-1

Middle Ordovician Conodont Biostratigraphy of Australasia

doi: 10.1007/s12583-020-1116-1
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Middle Ordovician Conodont Biostratigraphy of Australasia

doi: 10.1007/s12583-020-1116-1

Abstract: Seven conodont biozones are recognized in the carbonate-dominated shelf-marine Middle Ordovician developed in the intracratonic sedimentary basins (Canning, Amadeus and Georgina) of central and north-western Australia, in the Lachlan and New England orogens of New South Wales, and in the Takaka Terrane of New Zealand. A separate scheme identifying seven conodont biozones spanning the Middle Ordovician has also been developed for siliciclastic sequences deposited in slope-basinal environments in the Lachlan Orogen in New South Wales and Victoria. This biozonal classification consisting of two parallel biostratigraphic schemes for the shelf-marine and deep-marine successions respectively has significantly increased precision in regional and global biostratigraphic correlation and laid a solid foundation for the Middle Ordovician chronostratigraphy of Australia and New Zealand. Recognition of short-ranging pandemic species as the eponymous species of the biozones also supports direct correlation with the classical conodont successions established in Baltoscandia and the North American Midcontinent, and with those of the major Chinese terranes (South and North China and Tarim). The Lachlan Orogen appears to be globally unique in enabling correlation of contemporaneous conodont faunas over a considerable spectrum of water depths and biofacies ranging from carbonate shelves, slopes to deep-water basins.

Yong Yi Zhen. Middle Ordovician Conodont Biostratigraphy of Australasia. Journal of Earth Science, 2021, 32(3): 474-485. doi: 10.1007/s12583-020-1116-1
Citation: Yong Yi Zhen. Middle Ordovician Conodont Biostratigraphy of Australasia. Journal of Earth Science, 2021, 32(3): 474-485. doi: 10.1007/s12583-020-1116-1
  • Conodonts are the most important biostratigraphic tool in dating and correlating carbonate-dominated Ordovician successions, and (together with graptolites) are crucial in finely calibrating the timescale and chronostratigraphy of the Ordovician. Commenced in the 1940s in Australia, studies on Ordovician conodonts have been particularly active in the past three decades and produced a significant amount of new taxonomic and biostratigraphic data from eastern Australian orogens in New South Wales, Tasmania and North Queensland, from several intracratonic sedimentary basins (e.g., Canning, Amadeus, Arafura and Georgina) in central and northwestern Australia, and from the South Island of New Zealand (Fig. 1). These studies have recently resulted in establishment of conodont biozone schemes for the Upper Ordovician (Zhen and Percival, 2017) and Lower Ordovician (Zhen et al., 2017, 2015). Middle Ordovician conodonts from Australia and New Zealand are taxonomically diverse and represented by faunas inhabited various depositional settings including near shore shallow water carbonates, distal carbonate platforms, slopes and turbidite basins. Therefore, analysis of the biofacies and their distribution will provide crucial data, enabling recognition of contemporaneous conodont biofacies over a considerable spectrum of water depths for more precise regional correlations. With completion of the major studies on Middle Ordovician conodonts from New Zealand (Zhen et al., 2011a, 2009), the Canning Basin of western Australia (Zhen, 2020, 2019b; Zhen et al., 2020a, c; Nicoll, 1993, 1992), the Amadeus Basin of central Australia (Zhen et al., in press; Zhen, 2019a), the Georgina Basin of northern Australia (Kuhn and Barnes, 2005; Zhang et al., 2004; Stait and Druce, 1993) and from the Lachlan Orogen of New South Wales (e.g., Zhen and Rutledge, 2020; Zhen et al., 2020b, 2004; Percival and Zhen, 2018, 2007; Zhen and Pickett, 2008; Zhen and Percival, 2004a, b; Pickett and Percival, 2001), the time is now opportune to present a regional synthesis and to formally establish a Middle Ordovician conodont biostratigraphic framework for Australasia, which is the focus of this contribution.

    Figure 1.  Map of Australia and New Zealand showing distribution of the Ordovician rocks (modified from Zhen and Percival, 2017; Webby et al., 1981). NT. Northern Territory; WA. Western Australia; SA. South Australia; QLD. Queensland; NSW. New South Wales; VIC. Victoria.

  • Ordovician sedimentary rocks are known from surface outcrops and subsurface drilling data from all eight states and territories of Australia and New Zealand (Fig. 1). Rocks of the Middle Ordovician age are more widely distributed than those of either the Upper or Lower Ordovician, with conodonts reported from some 41 stratigraphic units in Australia and three in New Zealand (Table 1). Carbonate-dominated shelf-marine successions are most extensively preserved in the Canning, Amadeus, and Georgina basins, with smaller remnants of shelves flanking oceanic volcanic islands mainly preserved in New South Wales. Deep-water turbidites widely distributed in the Lachlan Orogen in New South Wales and Victoria with siliciclastic-dominated deposits of Middle Ordovician age were also reported from Tasmania and New Zealand. Studies of Middle Ordovician conodonts, particularly from New South Wales and the Canning and Amadeus basins have provided essential data for the establishment of the two parallel conodont biozonal schemes presented herein, which represent respectively two distinctively different depositional systems, i.e., the carbonate-dominated shelf-marine settings and siliciclastic deep-marine slopes and basins (Fig. 2).

    Table 1.  List of stratigraphic units yielded Middle Ordovician conodonts in Australia and New Zealand

    Figure 2.  Conodont biozones recognized in the Middle Ordovician (Dapingian and Darriwilian) from the carbonate-dominated shelf-marine successions and the siliciclastic-dominated slope-basinal successions in Australia and New Zealand. Fm. Formation; NZ. New Zealand.

    Confirmed upper Darriwilian rocks have not been recorded from the intracratonic sedimentary basins of central and northwestern Australia, but sporadic data are available from New Zealand (Zhen et al., 2011a, 2009) and central New South Wales (Zhen et al., 2004; Pickett and Percival, 2001). The Wahringa Limestone Member of the Fairbridge Volcanics covering a stratigraphic interval from the upper Darriwilian to upper Sandbian is so far the best to represent the upper Darriwilian as a regional reference section (Zhen et al., 2004, figs. 1, 2 therein). However, the biostratigraphic resolution of this section on conodonts is still low and further studies are required to focus on the middle part of this limestone member that has the potential to reveal the Middle/Upper Ordovician boundary level in the carbonate successions in Australia.

    Some 72 conodont species have now been recorded from the Middle Ordovician rocks in Australia and New Zealand (Fig. 3). They form a firm basis for the regional biostratigraphy (Fig. 3). The conodont biozones recognized respectively from the carbonate-dominated shelf-marine and deep-water slope-basinal successions in Australia and New Zealand are discussed in the following sections.

    Figure 3.  Composite range chart of selected conodont species from the Middle Ordovician of Australia and New Zealand (data sources from Zhen et al., 2021, 2020a, b, 2011a, 2009; Zhen, 2020, 2019a; Zhen and Rutledge, 2020; Zhen and Pickett, 2008; Zhen and Percival, 2004a, b; Pickett and Percival, 2001); D. Drepanoistodus, P. Plectodina, and Fl. Florian.

  • Conodonts of confirmed Dapingian age have been reported from only a few localities in Australia, such as from the Willara Formation (Zhen et al., 2020c;Nicoll, 1993) in the Canning Basin, the upper part of the Horn Valley Siltstone (Zhen et al., in press; Zhen, 2019a; Nicoll, 1992; Cooper, 1981) in the Amadeus Basin and possibly in the Coolibah Formation of the Georgina Basin (Stait and Druce, 1993). Conodonts of Darriwilian age are much widely reported, being documented from a number of stratigraphic units (Table 1) in New South Wales (Zhen and Pickett, 2008; Zhen et al., 2004; Pickett and Percival, 2001), from the Canning (Zhen, 2020, 2019b;Zhen et al., 2020a; Watson, 1988), the Amadeus (Zhang et al., 2004) and Georgina (Kuhn and Barnes, 2005; Stait and Druce, 1993; Hill et al., 1969) basins in central-northwestern Australia, and from the Takaka Terrane of New Zealand (Zhen et al., 2011a, 2009). Seven conodont biozones are recognized in the carbonate-dominated successions of the Middle Ordovician developed in several intracratonic sedimentary basins (e.g., Canning, Amadeus and Georgina) and on remnant island shelves preserved in the Lachlan Orogen in central New South Wales (Fig. 2).

  • The Jumudontus gananda Biozone was proposed by Nicoll (1992, fig. 3) to encompass the middle part of the Horn Valley Siltstone with its base marked by the FAD (First Appearance Datum) of Jumudontus gananda (Figs. 4n4r) and the top by the FAD of H. altifrons. It covers the stratigraphic interval extending from upper Floian to lower Dapingian, represented in the Horn Valley Siltstone of the Amadeus Basin (Zhen et al., in press; Zhen, 2019a; Cooper, 1981) and in the lower part of the Willara Formation of the Canning Basin (Zhen et al., 2020c, 2018). Jumudontus gananda was a pandemic species widely reported predominantly from shallow-water shelf settings in Australia, North China, North America, and Argentine Precordillera. Among the 39 species recovered from the J. gananda Biozone of the Horn Valley Siltstone in the Amadeus Basin, two other species, Microzarkodina russica and Periodon flabellum, are also age- diagnostic and crucial for regional and international correlation (Zhen, 2019a; Fig. 6). More specifically, M. russica was reported previously only from the Baltoniodus triangularis Biozone (basal Dapingian) in Baltoscandia (Männik and Viira, 2012). The pandemic species P. flabellum is the eponymous species of the P. flabellum Superbiozone recognized in the slope-basinal succession in the Lachlan Orogen of New South Wales representing a stratigraphic interval of upper Floian to lower Dapingian (Figs. 2, 3). Co-occurrence of J. gananda and O. evae reported in North America (Landing, 1976) and the Argentine Precordillera (Serpagli, 1974) also confirms correlation between the lower part of the J. gananda Biozone and the O. evae Biozone. The latter is widely recognized in deep-water turbidite successions of late Floian age in New South Wales and Victoria (Fig. 2).

    Figure 4.  Biozonal index conodont species from the Middle Ordovician carbonates in Australia and New Zealand. (a) Pygodus anserinus Lamont and Lindström, 1957; Pa element, MMMC2541 (sample C0828), from lower part of the Billabong Creek Limestone of the Gunningbland area in central New South Wales. (b)–(d) Pygodus serra (Hadding, 1913); (b) Pb element, CNP1339 (sample CN553), (c) (CNP1340, sample CN553) and (d) (CNP1341, sample CN487), Pa element, from Sluice Box Formation of Takaka Terrane in New Zealand. (e) Pygodus anitae Bergström, 1983; Pa element, CNP1336 (sample CN578) from Sluice Box Formation of Takaka Terrane in New Zealand. (f) Eoplacognathus suecicus Bergström, 1971; (f) Pa element, CNP1243 (sample CN578) from Sluice Box Formation of Takaka Terrane in New Zealand. (g) Eoplacognathus pseudoplanus (Viira, 1974); Pb element, CPC44048 (sample 479–480 m) from upper part of the Goldwyer Formation of the Canning Basin, western Australia. (h) Yangtzeplacognathus foliaceus (Fåhræus, 1966); Pb element, CNP1246 (sample CN579) from Sluice Box Formation of Takaka Terrane in New Zealand. (i), (j) Histiodella holodentata Ethington and Clark, 1982; Pa element, (i) (CPC44097, sample 519–520 m) and (j) (CPC44096, sample 549–550 m) from Goldwyer Formation of the Canning Basin in western Australia. (k) Histiodella serrata Harris, 1962; Pa element, GSWA F54263 (sample 225104) from basal part of the Goldwyer Formation of the Canning Basin in western Australia. (l) Histiodella minutiserrata Mound, 1965; Pa element, CPC42941 (sample 85-3054) from top of the Horn Valley Siltstone in the Amadeus Basin. (m) Histiodella altifrons Harris, 1962; Pa element, CPC42939 (sample 85-3054) from top of the Horn Valley Siltstone in the Amadeus Basin. (n)–(r) Jumudontus gananda Cooper, 1981; (n), (o) Pa element, CPC42997 and CPC43006; (p) Pb element, CPC43003; (q) Sd element, CPC42992; (r) Sb element, CPC42986; from Horn Valley Siltstone. Scale bars=100 μm.

    Figure 5.  Biozonal index and age-diagnostic conodont species from the Middle Ordovician deep-water turbiditic sequences of the Lachlan Orogen, central-southern New South Wales. (a), (b) Pygodus anserinus Lamont and Lindström, 1957; Pa element, a (MMF 46057a-01) from Nattery Chert Member of the Abercrombie Formation and b (MMF 45548a-01) from Ballast Formation. (c)–(f) Pygodus serra (Hadding, 1913); (c) (MMF 46805b-01), (d) (MMF 46805c-07), Pa element, (e) (MMF 46805a-15), S element, (f) (MMF 46805c-11), Pb element, all from undifferentiated Abercrombie Formation. (g) Pygodus anitae Bergström, 1983; Pa element (MMF 45455d-01), from the Lang Formation. (h), (i) Periodon aculeatus Hadding, 1913; (h) (MMF 46331c-01), Pa element, from Nattery Chert Member of the Abercrombie Formation; and (i) (MMF 46805b-09), M element, from undifferentiated Abercrombie Formation. (j)–(m) Histiodella kristinae Stouge, 1984; Pa element, j (MMF 46216c-03), (k) (MMF 46216c-01), and (l) (MMF 46216c-10) from undifferentiated Abercrombie Formation; and (m) (MMF 46330a-02) from Peach Tree Chert Member of the Abercrombie Formation. (n) Histiodella labiosa Bauer, 2010, Pa element (MMF 46028a-01) from Peach Tree Chert Member of the Abercrombie Formation. (o) Histiodella holodentata Ethington and Clark, 1982; Pa element (MMF 46277b-01), from Peach Tree Chert Member of the Abercrombie Formation. (p)–(s) Periodon hankensis Stouge, 2012; (p) (MMF 46191d-01) and (q) (MMF 46191b-01), Pa element, (r) S element (MMF 46191c-03), and (s) (MMF 46191a-02), M element, all from the undifferentiated Abercrombie Formation. (t), (u) Paroistodus horridus (Barnes and Poplawski, 1973); S element, (t) (MMF 46277c-01b) and (u) (MMF 46094b-02) from the Peach Tree Chert Member of the Abercrombie Formation. (v) Histiodella wuhaiensis Wang, (Bergström, Zhen, Zhang, Wu and Chen, 2013, Pa element (MMF 46112b-03) from the Peach Tree Chert Member of the Abercrombie Formation. (w) Histiodella sp. cf. H. kristinae Stouge, 1984; Pa element (MMF 46216c-13) from the undifferentiated Abercrombie Formation. (x) Periodon flabellum (Lindström, 1954); M element (MMF 46096a-04) from the Peach Tree Chert Member of the Abercrombie Formation. Scale bars=100 μm.

    Figure 6.  Conodont-based correlation of the Australasian Middle Ordovician with those of South China (Wang et al., 2019; Zhang et al., 2019), Baltoscandia (Bergström and Ferretti, 2017; Männik and Viira, 2012) and North American Midcontinent (Bauer, 2010; Goldman et al., 2007; Webby et al., 2004).

  • Histiodella altifrons (Fig. 4m) was reported from the Amadeus Basin of central Australia and the Canning Basin of western Australia (Nicoll, 1993, 1992), North America (Bauer, 2010; McHargue, 1982) and the Argentine Precordillera (Lehnert, 1995). In Australia, the H. altifrons Biozone has been recognized in the Amadeus Basin and the Canning Basin defined by the FAD of H. altifrons at the base and the FAD of Histiodella serrata at the top based on the faunas recently documented from the Horn Valley Siltstone of the Amadeus Basin (Zhen et al., in press, 2018) and from the upper part of the Willara Formation of the Canning Basin (Zhen et al., 2020c; Nicoll, 1993). The H. altifrons Biozone recognized in Australia has an age ranging from the late Dapingian to the earliest Darriwilian, and correlates to the H. altifrons and the succeeding Histiodella minutiserrata (Fig. 4l) biozones and the basal part of the Histiodella sinuosa Biozone, that are well-known in the North American Midcontinent succession (Fig. 6).

  • This biozone was defined by the FAD of Histiodella serrata (Fig. 4k) as the base and the FAD of H. holodentata as the top and is recognized in the lower part of the Goldwyer Formation of the Canning Basin in western Australia (Zhen et al., 2020a). It was also represented in the upper part of the Willara Formation of the Canning Basin (Zhen et al., 2020c, fig. 2) and in the Nora Formation of the Georgina Basin of far western Queensland (Hill et al., 1969) based on the occurrence of the eponymous species. Histiodella serrata was originally described from North America (Harris, 1962) and was also recorded in North China (An et al., 1983). In the Joins and Oil Creek formations of southern Oklahoma, Bauer (2010) recognized six species of Histiodella (namely H. altifrons, H. minutiserrata, H. sinuosa, H. serrata, H. labiosa and H. holodentata), which he suggested represented an inferred evolutionary lineage with H. altifrons as the most primitive form. The morphological features of these species showed several evolutionary trends, particularly the reduction of the cusp and stronger development of serrations or denticles on the Pa elements in the more derived species. Histiodella serrata, which might be directly evolved from H. sinuosa, makes its first appearance in the lower part of the H. sinuosa Biozone (at top of the Joins Formation) in the Oklahoma succession (Bauer, 2010). Considering the stratigraphic ranges of these six species of Histiodella recorded from southern Oklahoma (Bauer, 2010, table 1) and their phylogenetic relationships, co-occurrence of both H. minutiserrata and H. serrata near the base of the Goldwyer Formation in the Canning Basin supports correlation of the base of the H. serrata Biozone in Australia to the lower part of the H. sinuosa Biozone of the North American Midcontinent succession (Bauer, 2010; Fig. 6).

  • Histiodella holodentata (Figs. 4i, 4j) was a pandemic and morphologically distinctive species that predominantly inhabited offshore (distal shelf) to deep-water environments. It has been widely utilized as a biozonal index species spanning the middle Darriwilian (Fig. 6). Eoplacognathus pseudoplanus (Fig. 4g) predominantly inhabited shallow water proximal shelf settings. Therefore, conodont biozones based on these two eponymous species are coeval in time, but represent different biofacies (Zhen et al., 2020c; Fig. 6). Recent revision of the fauna from the Canning Basin of western Australia confirmed that specimens assigned to Eoplacognathus suecicus by Watson (1988) should be reassigned to E. pseudoplanus, which suggested a correlation with the upper part of the H. holodentata-E. pseudoplanus Biozone (Zhen, 2020, 2019b; Zhen et al., 2020c, fig. 8; Figs. 2, 5). In the Canning Basin of western Australia, the stratigraphic range of E. pseudoplanus does not overlap with that of H. holodentata (Zhen et al., 2020a; Watson, 1988), apparently as result of facies changes indicating shallowing upwards from the middle part of the Goldwyer Formation.

    In Australasia, the H. holodentata-E. pseudoplanus Biozone is correlated with the H. holodentata Biozone recognized in the deep-water turbiditic succession in New South Wales (Fig. 6). Conodonts representing this biozone have been reported from the Goldwyer Formation and the top of the underlying Willara Formation of the Canning Basin in western Australia (Zhen et al., 2020a, c; Watson, 1988) and from an unnamed limestone lens within the Goonumbla Volcanics exposed at Kirkupwest of Parkes in New South Wales (Zhen and Pickett, 2008). Conodonts of the H. holodentata Biozone were also reported from allochthonous limestones possibly derived from the Summit Limestone deposited on the Takaka Terrane in the northern part of the South Island, New Zealand (Zhen et al., 2009).

    A small fauna from limestones (both autochthonous and allochthonous) in the Haedon Formation in the New England Orogen of northern New South Wales includes Ansella jemtlandica, Oistodus lanceolatus, Periodon aculeatus and gen. et sp. indet. (Furey-Greig, 2004), suggesting a broad Darriwilian age. The fragmentary material assigned to gen. et sp. indet. by Furey-Greig (2004, pl. 1, figs. 20, 21) likely belongs to Loxodus dissectus, which is a distinctive species reported from the H. holodentata Biozone in North China (An et al., 1983) and the Canning Basin of western Australia (Zhen et al., 2020a; Watson, 1988).

  • As discussed by Zhen et al. (2011a, p. 297, 300), the species definition of Eoplacognathus suecicus Bergström, 1971 and its closely related and possibly direct ancestor species Eoplacognathus pseudoplanus (Viira, 1974) were interpreted rather differently by conodont specialists in the 1970s to 1990s, resulting in significantly varied time spans for this biozone. More specifically, following Löfgren (1978) in treating E. pseudoplanus as a junior synonym of E. suecicus, the time span of the E. suecicus Biozone (or the E. pseudoplanus Biozone of others) was significantly expanded to accommodate the entire middle Darriwilian (e.g., Wang et al., 1996; An et al., 1985). This rather confusing situation has subsequently been resolved by detailed taxonomic revision of these two species based on abundant material from Sweden and South China (Löfgren, 2004; Löfgren and Zhang, 2003; Zhang, 1998a, b, c, d; Fig. 6). These latter studies have clearly established the E. pseudoplanus Biozone and the succeeding E. suecicus Biozone in the Baltoscandian succession and in equivalent strata on the Yangtze Platform of South China.

    In Australasia, E. suecicus (Fig. 4f) was confirmed from the Sluice Box Formation of the Takaka Terrane of the Nelson Province in New Zealand (Zhen et al., 2011a). Co-occurrence of E. suecicus with Pygodus anitae (Fig. 4e) and Yangtzeplacognathus foliaceus (Fig. 4h) in two samples (CN579 and CN578) and association with Pygodus serra in another sample (CN487) from the Sluice Box Formation suggested that this fauna was recovered from the stratigraphic interval covering the upper E. suecicus Biozone and the basal Pygodus serra Biozone (Zhen et al., 2011a, table 1; Fig. 6). A small fauna including Pygodus anitae from limestone clasts enclosed in porphyritic lavas of the Goonumbla Volcanics between Parkes and Gunningbland in central New South Wales might also come from strata correlative to the upper part of the E. suecicus Biozone (Percival, 1999).

  • Pygodus serra (Figs. 4b4d) had its distribution likely confined by a similar set of environmental factors as was Pygodus anserinus, i.e., restricted to distal platform to basinal settings. In the carbonate-dominated successions of Australasia, it was reported from the basal part of the Billabong Creek Limestone in the Goonumbla Volcanics (Pickett and Percival, 2001) and the lower-middle part of the Wahringa Limestone Member (Percival and Zhen, 2007; Zhen et al., 2004) in central New South Wales and from the Sluice Box Formation and Arthur Marble of the Takaka Terrane of New Zealand (Zhen et al., 2011a; Simes, 1980). In the basal part of the Billabong Creek Limestone, co-occurrence of both P. serra and P. anserinus suggesting a correlation with the lower part of the P. anserinus Biozone, but the upper part of the underlying Goonumbla Volcanics that should correlate to the P. serra Biozone is unfortunately not exposed in this section (Pickett and Percival, 2001, fig. 3). Co-occurrence of both typical P. serra and the advanced morphotype of P. serra (=Pygodus protoanserinus Zhang, 1998b) in the conodont fauna (assemblage A) from the basal part of the Wahringa Limestone Member confirms that this stratigraphic level correlates to the top part of the Pygodus serra Biozone (Zhen et al., 2004; Zhang, 1998b). Co-occurrence of both P. serra and Yangtzeplacognathus foliaceus (Fig. 4h) in one sample (CN553) from the Sluice Box Formation of New Zealand indicates that this sample was from the basal part of the Pygodus serra Biozone (Zhen et al., 2011a, table 1).

  • As its Pa element is morphologically distinctive and easy to recognize, Pygodus anserinus (Fig. 4a) has been widely utilized for identifying and correlating the stratigraphic interval (the P. anserinus Biozone) representing the uppermost Darriwilian to basal Sandbian. This pandemic species typically occurs in distal platform to basinal settings, making it well-suited for international correlation.

    Pygodus serra has a stratigraphic range from the base of the P. serra Biozone to the middle part of the succeeding P. anserinus Biozone. Therefore, the horizon characterised by the last co-occurrence of both P. serra and P. anserinus is considered to represent the topmost Darriwilian and was suggested to equate to a level immediately below the base of the Upper Ordovician where evidence for the boundary horizon defined by the diagnostic graptolite species Nemagraptus gracilis is otherwise lacking (Zhen and Rutledge, 2020; Zhen and Percival, 2017; Zhen et al., 2004). Co-occurrence of both P. anserinus and P. serra from the basal part of the Billabong Creek Formation of central New South Wales (Pickett and Percival, 2001) and from the top of the Arthur Marble in the Takaka Terrane of New Zealand (Simes, 1980) confirms that these two locations are correlative and represent the uppermost Darriwilian (lower part of the P. anserinus Biozone) known from Australia and New Zealand.

  • The overwhelming majority of conodont elements are recovered from carbonate rocks deposited on shelves and platforms in shallow water. In contrast, conodonts are difficult to recover from siliciclastic successions deposited in deep-water settings of the Open-Sea Realm (Zhen and Percival, 2003). In siliciclastic rocks, conodonts occasionally are found on bedding planes of shales and siltstones (Stewart and Nicoll, 2004; Nicoll, 1980), obtained by acetic-acid dissolution of calcareous siltstones or sandstones (Zhen and Percival, 2004b), or may be recovered from chert samples using chemical methods (Fowler and Iwata, 1995; Iwata et al., 1995) or by thin sectioning (e.g., Murray and Stewart, 2001; Stewart and Glen, 1991, 1986). Large scale chemical treatment (using HF) of chert samples is not practical largely due to workplace health and safety and environmental issues—the acid is extremely dangerous and requires careful handling in specialist laboratories and rigorous treatment prior to disposal. However, in the last 20 years, observation and identification of conodonts in chert and siliceous siltstone thin sections has been widely used to provide biostratigraphic support to the regional mapping of the Ordovician turbiditic sequences in New South Wales, Australia. It has been proved to be an excellent method of dating and correlating Ordovician turbidite successions of the Lachlan Orogen (Zhen and Rutledge, 2020; Percival et al., 2011; Och et al., 2007; Percival and Zhen, 2007; Lyons and Percival, 2002). Seven biozones have now been recognized to subdivide the Middle Ordovician turbiditic sequences in the Lachlan Orogen (Zhen and Rutledge, 2020). This new scheme has provided critical evidence for the age determination of those previously poorly- dated siliciclastic sediments in New South Wales, allowing direct correlations with the conodont succession established from the studies of the faunas from the carbonate-dominated successions in Australia and New Zealand, with graptolite biozonation of Victoria (Fig. 6) and with similar turbidite successions documented from Kazakhstan and central Asia (e.g., Tolmacheva, 2019, 2014; Tolmacheva et al., 2004).

  • The Periodon flabellum Superbiozone was established to represent the stratigraphic interval from the upper Floian to lower Dapingian in the deep-water turbiditic sequences in central and southern New South Wales (Zhen and Rutledge, 2020; Figs. 2, 6). It can be further subdivided into the Oepikodus evae Biozone of the late Floian age in its lower part and the unnamed interval of the early Dapingian age in its upper part. Periodon flabellum (Fig. 5x) is the oldest species in the evolutionary lineage of Periodon reconstructed based on the material from New South Wales (Zhen et al., 2020b). Bagnoli and Stouge (1997) documented Periodon sp. A from the lower part of the Dapingian, which was subsequently utilized as a useful species to define the base of the Dapingian based on its co-occurrence with Baltoniodus triangularis within the lower member of the Dawan Formation at the GSSP (Huanghuachang Section) for the base of the Middle Ordovician (Wang et al., 2009). However, Zhen et al. (2020b) treated it as representing the advanced morphotype of P. flabellum because the data from chert samples in New South Wales did not warrant it as a separate species.

  • Elements of Periodon are the dominant forms revealed in the chert samples from the upper Lower to Upper Ordovician in New South Wales, and four species are recognized. They form an evolutionary lineage with Periodon flabellum as the oldest species, extending from the Oepikodus evae Biozone (upper Floian) to lower Darriwilian and with the youngest species Periodon grandis confined to the Upper Ordovician. The younger species P. aculeatus (Figs. 5h, 5i) occurs from the lower Darriwilian to Sandbian, and an intermediate species is assignable to Periodon hankensis Stouge, 2012 (Figs. 5p5s) that has a stratigraphic range extending from the upper Dapingian to the Darriwilian (Zhen et al., 2020b).

    Morphologically, P. hankensis is characterized mainly by having an M element bearing two denticles along the inner- lateral edge and a Pa element bearing two denticles on the anterior process. In comparison, the preceding species P. flabellum has no denticle (or rarely has one) on the inner-lateral edge of the M element and displays a single denticle on the short anterior process of the Pa element; the succeeding species P. aculeatus typically has two to four denticles along the inner-lateral margin of the M element and three or four denticles on the anterior process of the Pa element. Based on the study of the conodont faunas from western Newfoundland, Stouge (2012) originally defined the Periodon hankensis Biozone by the FAD of the eponymous species as the base and the FAD of P. aculeatus as the top to represent the stratigraphic interval of the late Dapingian and early Darriwilian. In the chert samples from New South Wales, the P. hankensis Biozone is recognized by the appearance of the eponymous species without association of P. aculeatus (Zhen and Rutledge, 2020; Fig. 6).

  • Bauer (2010) defined the Histiodella labiosa Biozone by the FAD of H. labiosa as the base and the FAD of H. holodentata as the top to represent a stratigraphic interval of the early Darriwilian age between the H. sinuosa Biozone and the H. holodentata Biozone in the middle and upper parts of the Oil Creek Formation of southern Oklahoma (Fig. 6). This biozone has been recognized based on the occurrence of the eponymous species in the Peach Tree Chert Member or middle part of the undifferentiated Abercrombie Formation of the turbiditic sequences (Fig. 5n) in New South Wales. Zhen et al. (2020c, fig. 2) also reported the appearance of H. labiosa at the top part of the Willara Formation (upper part of the H. serrata Biozone) in the Canning Basin in western Australia (Fig. 6).

  • In the deep-water turbiditic basinal successions of the Lachlan Orogen in New South Wales, the H. holodentata Biozone is represented by the occurrence of the eponymous species, from allochthonous limestones (derived from upper slope to shelf edge) in the Oakdale Formation (Zhen and Percival, 2004a) and from the basal part of the Weemalla Formation representing lower slope settings (Zhen and Percival, 2004b) in central New South Wales. Specimens assigned to H. kristinae by Zhen and Percival (2004a, p. 97, 98, figs. 14A–14L) from an allochthonous limestone in the Oakdale Formation (C1912) were subsequently reassigned to H. holodentata (see Zhen et al., 2011a p. 227). The morphotype of H. holodentata from that sample shows a rectangular outline of the Pa element with distal denticles on the anterior process nearly as high as the cusp tip, which represents the advanced form of H. holodentata (referred to as Histiodella sp. cf. H. holodentata by some authors e.g., Stouge 2012). Therefore, this allochthonous limestone (C1912) in the Oakdale Formation was derived from a stratigraphic interval correlating to the top of the H. holodentata Biozone. In the turbiditic siltstone, sandstone and shale of the Weemalla Formation, conodont samples were collected from autochthonous calcareous siltstone or sandstone which was dissolved in dilute acetic acid. Co-occurrence of H. holodentata and Dzikodus hunanensis in the basal part of the Weemalla Formation (sample B2) suggests correlation of this level with the lower part of the H. holodentata Biozone (see Zhang, 1998a; Fig. 2). This biozone, represented by the appearance of H. holodentata (Fig. 5o) has also been recorded from the Peach Tree Chert Member and middle part of the undifferentiated Abercrombie Formation in the turbiditic sequences of the Lachlan Orogen (Fig. 2). Other associated age-diagnostic species in the deep-water chert samples include Bergstroemognathus? keramis, Histiodella wuhaiensis (Fig. 5v), Paroistodus horridus (Fig. 5t), P. hankensis, P. aculeatus, and Spinodus spinatus (Fig. 3).

    The H. holodentata Biozone correlates with the E. pseudoplanus Biozone of the Baltoscandian succession (Löfgren, 2004, fig. 1; Zhen et al., 2020a; Fig. 6). This correlation is also supported by studies of the conodont faunas from the Dawangou Formation of the Tarim Basin and from the Kuniutan Formation of South China, where Yangtzeplacognathus crassus occurs immediately beneath H. holodentata (Zhen et al., 2011bChen et al., 2006; Zhang, 1998a).

  • In the Middle Ordovician conodont faunas from western Newfoundland, Histiodella kristinae and H. bellburnensis (considered as the direct descendants of H. holodentata) were recognized and taken as index species for two conodont biozones succeeding the H. holodentata Biozone (Stouge, 2012, 1984). Stouge (1984, table 3) correlated the H. kristinae Biozone to the E. suecicus Biozone of the Baltoscandian succession (Fig. 6). This was consistent with the suggestion of Bergström et al. (2009) that the top of the H. kristinae Biozone could approximate the base of the P. serra Biozone. Therefore, the H. bellburnensis Biozone should be correlated to the upper E. suecicus Biozone and the basal part of the P. serra Biozone (Y. foliaceus Subzone), the H. kristinae Biozone to the lower E. suecicus Biozone.

    In Australia, Histiodella kristinae (Figs. 5j5m), the eponymous species of this biozone, has been recorded only in chert samples from the Peach Tree Chert Member or middle part of the undifferentiated Abercrombie Formation in the Lachlan Orogen of New South Wales (Zhen and Rutledge, 2020; Fig. 2). Several chert samples (e.g., MMF 46198) from the Peach Tree Chert Member yielded both H. kristinae and H. holodentata, suggesting that they were from the basal part of the Histiodella kristinae Biozone. Co-occurrence of Histiodella sp. cf. H. kristinae (Fig. 5w) and Pygodus anitae (Fig. 3) from some samples (e.g., MMF 46043) in the Peach Tree Chert Member suggests a correlation with the upper part of the H. kristinae Biozone.

  • In the turbidite successions of the Lachlan Orogen, the Pygodus serra Biozone characterized by the appearance of the eponymous species (Figs. 5c5f) has been recognized in the 'Pittman Formation' (Nicoll, 1980, on shale bedding planes; now as part of the Abercrombie Formation) and from chert thin sections of the Nattery Chert Member or upper part of the undifferentiated Abercrombie Formation and various other time equivalent intervals in the turbiditic sequences of the Lachlan Orogen (Zhen and Rutledge, 2020; Percival et al., 2011; Fig. 2). Numerous P. serra specimens from these chert samples have made it possible to analyse morphometrics of the Pa element and the evolutionary trends of P. serra to more precisely correlate these turbidite rocks in New South Wales and Victoria.

  • Pygodus anserinus (Figs. 5a, 5b) is much less common in comparison with its direct ancestor, P. serra, in the deep-water cherts of the Lachlan Orogen in New South Wales and Victoria. The P. anserinus Biozone has been recognized in the Nattery Chert Member or the upper part of the undifferentiated Abercrombie Formation and various other time equivalent intervals in the turbiditic sequences of the Lachlan Orogen (Zhen and Rutledge, 2020; Fig. 2). The lower part of this biozone representing the uppermost Darriwilian is characterized by the occurrence of P. anserinus in association with P. serra, Periodon aculeatus and P. hankensis (Fig. 3).

  • The Middle Ordovician conodont biostratigraphic scheme recognised in Australia and New Zealand divides the carbonate- dominated shelf-marine successions and the deep-water slope and turbidite successions into seven biozones, respectively. This new biozonal classification is supported by extensive taxonomic and biostratigraphic studies carried out particularly in the last three decades in the Lachlan Orogen in New South Wales, the Amadeus and Canning basins of central and western Australia, and the Takaka Terrane of New Zealand.

    This framework serves as the regional standard and provides direct correlation with the classical conodont successions established in Baltoscandia and North American Midcontinent, and with those of the major Chinese terranes. It also provides much improved accuracy for calibrating and integrating with chronostratigraphy and chemostratigraphy of the Middle Ordovician in this region.

    This contribution promotes further integrated investigation, particularly between biozonations based on fossil groups from different depositional systems (e.g., conodonts and graptolites), and between biostratigraphy and other disciplines or methods in stratigraphy. Considerable scope remains for development of a fully integrated biostratigraphic framework for the Australasian region, enabling confident calibration against the well-established international biostratigraphic standard of the Middle Ordovician.

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