Advanced Search

Indexed by SCI、CA、РЖ、PA、CSA、ZR、etc .

Volume 32 Issue 3
Jun.  2021
Turn off MathJax
Article Contents

Yong Yi Zhen, Ian G. Percival, Phil Gilmore, Jodie Rutledge, Liann Deyssing. Conodont Biostratigraphy of Ordovician Deep-Water Turbiditic Sequences in Eastern Australia——A New Biozonal Scheme for the Open-Sea Realm. Journal of Earth Science, 2021, 32(3): 486-500. doi: 10.1007/s12583-021-1421-3
Citation: Yong Yi Zhen, Ian G. Percival, Phil Gilmore, Jodie Rutledge, Liann Deyssing. Conodont Biostratigraphy of Ordovician Deep-Water Turbiditic Sequences in Eastern Australia——A New Biozonal Scheme for the Open-Sea Realm. Journal of Earth Science, 2021, 32(3): 486-500. doi: 10.1007/s12583-021-1421-3

Conodont Biostratigraphy of Ordovician Deep-Water Turbiditic Sequences in Eastern Australia——A New Biozonal Scheme for the Open-Sea Realm

doi: 10.1007/s12583-021-1421-3
More Information
通讯作者: 陈斌, bchen63@163.com
  • 1. 

    沈阳化工大学材料科学与工程学院 沈阳 110142

  1. 本站搜索
  2. 百度学术搜索
  3. 万方数据库搜索
  4. CNKI搜索

Figures(7)

Article Metrics

Article views(123) PDF downloads(23) Cited by()

Related
Proportional views

Conodont Biostratigraphy of Ordovician Deep-Water Turbiditic Sequences in Eastern Australia——A New Biozonal Scheme for the Open-Sea Realm

doi: 10.1007/s12583-021-1421-3

Abstract: Ordovician conodonts representing 28 genera and 28 named and three unnamed species were identified from 740 chert and siliceous siltstone spot samples (> 3 000 thin sections) from deep-water turbiditic sequences of the Lachlan Orogen in central and southern New South Wales, Australia. Based on these faunas, a new conodont biozonal scheme has been established to divide the Ordovician turbiditic successions of the Lachlan Orogen into 12 superbiozones and biozones. They are (in ascending order) the Paracordylodus gracilis Superbiozone (including the Prioniodus oepiki Biozone), Periodon flabellum Superbiozone (including the Oepikodus evae Biozone in the lower part), Periodon hankensis Biozone, Periodon aculeatus Superbiozone (including the Histiodella labiosa, Histiodella holodentata, Histiodella kristinae, Pygodus serra and Pygodus anserinus biozones) and the Periodon grandis Biozone. The Pygodus anserinus Biozone is divided further into the lower and upper subbiozones. This new conodont biozonation scheme spanning the upper Tremadocian to middle Katian interval permits precise age-dating and correlation of deep-water siliciclastic rocks that characterize the Ordovician Deep-Sea Realm regionally and internationally.

Yong Yi Zhen, Ian G. Percival, Phil Gilmore, Jodie Rutledge, Liann Deyssing. Conodont Biostratigraphy of Ordovician Deep-Water Turbiditic Sequences in Eastern Australia——A New Biozonal Scheme for the Open-Sea Realm. Journal of Earth Science, 2021, 32(3): 486-500. doi: 10.1007/s12583-021-1421-3
Citation: Yong Yi Zhen, Ian G. Percival, Phil Gilmore, Jodie Rutledge, Liann Deyssing. Conodont Biostratigraphy of Ordovician Deep-Water Turbiditic Sequences in Eastern Australia——A New Biozonal Scheme for the Open-Sea Realm. Journal of Earth Science, 2021, 32(3): 486-500. doi: 10.1007/s12583-021-1421-3
  • Most studies of Ordovician conodonts focus on discrete specimens recovered by acid leaching of carbonate rocks. In comparison, conodonts from non-carbonate siliciclastic rocks are poorly known. In Australia, deeper-water conodonts are known from shale bedding plane assemblages (e.g., Stewart and Nicoll, 2004; Zhen et al., 2001; Nicoll, 1980) and from thin sections of Ordovician cherts and siliceous siltstones in turbiditic sequences of eastern Australia. This latter method, pioneered by Ian Stewart (Murray and Stewart, 2001; Stewart and Fergusson, 1995, 1988; VandenBerg and Stewart, 1992; Stewart and Glen, 1991, 1986; Stewart, 1988), opened a new window in establishing detailed biostratigraphic frameworks for deep-water successions, especially those lacking graptolites, which otherwise are typically poorly dated and correlated. During regional mapping programs conducted by the Geological Survey of New South Wales over the past three decades, Ordovician cherts and siliceous siltstones have been widely sampled in turbiditic sequences in central and southern New South Wales (e.g., Quinn et al., 2014; Percival, 2012; Percival and Quinn, 2011; Percival et al., 2011, 2003; Och et al., 2007; Percival and Zhen, 2007; Glen et al., 2004; Lyons and Percival, 2002). These studies recognized five conodont assemblage biozones including the Paracordylodus gracilis assemblage biozone, Oepikodus evae assemblage biozone, Paroistodus horridus-Spinodus spinatus assemblage biozone, Pygodus serra assemblage biozone and the Pygodus anserinus assemblage biozone (e.g., Bruce and Percival, 2014; Percival, 2012, 2006). Re-examination of conodonts preserved in 740 chert and siliceous siltstone samples (Fig. 1) from this region has resulted in the establishment of a new conodont biozonal scheme (Fig. 2). This biozonal classification has, for the first time, provided a detailed biozonal succession for the Ordovician (upper Tremadocian to middle Katian) deep-water siliciclastic sequences that is readily applicable in a global context (Fig. 3).

    Figure 1.  (a) Map of Australia showing the location of map (b); (b) simplified geological map of central and southern New South Wales, Australia (modified from Colquhoun et al., 2020), showing the distribution of the main packages of Ordovician metasedimentary rocks (dominated by turbiditic successions) in central and southern New South Wales and locations of the 740 samples studied (the mapped extents of the Hensleigh Siltstone, Weemalla Formation, Goonumbla Volcanics and Oakdale Formation are too small to be shown at this scale). NT. Northern Territory; WA. Western Australia; SA. South Australia; QLD. Queensland; NSW. New South Wales; TAS. Tasmania; VIC. Victoria.

    Figure 2.  Ordovician conodont biozone succession, stratigraphic ranges of the selected age-diagnostic species recognized in the turbidites, and biostratigraphic framework established based on the conodont occurrences in the 740 samples collected from central and southern New South Wales, Australia. Biostratigraphic codes: 1. Sandbian- Katian (13 samples with occurrence of Periodon grandis, Scabbardella sp. indet.); 2. Sandbian (2 samples with co-occurrence of P. grandis+P. aculeatus); 3. middle Darriwilian-Sandbian (106 samples from the P. aculeatus Superbiozone); 4. upper Darriwilian–lower Sandbian (30 samples with occurrence of Pygodus sp. indet.); 5. lower Sandbian (14 samples from the upper subbiozone of the P. anserinus Biozone); 6. uppermost Darriwilian (9 samples from the lower subbiozone of the P. anserinus Biozone, occurrence of P. anserinus+P. serra); 7. upper Darriwilian (88 samples from the Pygodus serra Biozone); 8. middle Darriwilian (5 samples with occurrence of Histiodella sp. indet., Dzikodus sp. indet.); 9. Histiodella kristinae Biozone (5 samples); 10. Histiodella holodentata Biozone (6 samples); 11. lower–middle Darriwilian (19 samples with occurrence of Paroistodus horridus); 12. lower Darriwilian (4 samples from the H. labiosa Biozone); 13. upper Dapingian–lower Darriwilian (45 samples from the P. hankensis Biozone); 14. upper Floian–lower Dapingian (47 samples from the P. flabellum Superbiozone); 15. upper Floian (74 samples from the O. evae Biozone); 16. upper Tremadocian–middle Floian (16 samples from the P. gracilis Superbiozone); 17. lower Floian (3 samples from the P. oepiki Biozone); 18. Middle– Upper Ordovician (undifferentiated, 6 samples); 19. Lower–Middle Ordovician (undifferentiated, 20 samples); 20. Ordovician (undifferentiated, 198 samples).

    Figure 3.  Correlation of the conodont biozones established in the turbiditic sequences of New South Wales with the deep-water basinal succession (cherts and siliceous siltstones) recognized in Kazakhstan of the Central Asian Orogenic Belt (Tolmacheva, 2019, 2014) and the slope successions recognized in South China (Wang et al., 2019; Zhang et al., 2019) and marginal Laurentia (Stouge et al., 2017; Stouge, 2012, 1984; Cooper et al., 2001; Pohler, 1994); H. Histiodella; P. Periodon; Pr. Prioniodus; Y. Yangtzeplacognathus; Hir. Hirnantian.

  • Our project involved examination of nearly 4 000 individual conodont elements preserved in over 3 000 chert and siliceous siltstone thin sections prepared from 740 spot samples of the turbiditic successions exposed in central and southern New South Wales (Fig. 1). Thin sections approximately 50 microns in thickness were cut parallel to the bedding plane of each chert or siliceous siltstone sample. Conodont elements in these sections vary from rare to abundant and are intersected as randomly orientated two dimensional shapes. They were photographed using an Olympus compound microscope (BX3M) mounted with a digital image system (DP74 with 20 mp resolution). All the materials studied, including the chert thin sections illustrated in Figs. 46, are deposited in the paleontology collection of the Geological Survey of New South Wales, housed at the W. B. Clarke Geoscience Centre at Londonderry in outer western Sydney, Australia.

    Figure 4.  Lower Ordovician biozonal index species of the Prioniodus oepiki Biozone (a)–(i), Paracordylodus gracilis Superbiozone (j)–(l) and Oepikodus evae Biozone (m)–(w) recovered from the deep-water turbiditic sequences in central and southern New South Wales. (a)–(g) Prioniodus oepiki (McTavish, 1973); (a) and (b), M element, (a) MMF 46388k-04, (b) MMF 46457c-01; (c)–(f) S element (likely Sa), (c) MMF 46388c-02, (d) MMF 46388I-02, (e) MMF 46388k-01, (f) MMF 46388c-01; (g) P element, MMF 46388g-06; (h), (j)–(l), (w) Paracordylodus gracilis Lindström, 1954, Selement, (h) MMF 46257d-02, (j) MMF 46389a-01, (k) MMF 45483c-03, (l) MMF 46109h-01, (w) MMF 46091a-03; (i), (v) Paroistodus proteus (Lindström, 1954), (i) Sc element, MMF 46257d-03, (v) M element, MMF 45963b-05; (m)–(r) Oepikodus evae (Lindström, 1954); (m)–(o) P element, (m) MMF 46091c-06, (n) MMF 46091b-04, (o) MMF 46133a-05; (p) and (r) S element, (p) MMF 46133a-03, (r) MMF 45963h-01; (q) M element, MMF 45588c-02; (s)–(u) Periodon flabellum (Lindström, 1954), (s) and (t) Pa element, (s) MMF 46208b-02, (t) MMF 46218c-03, (u) M element, MMF 46091c-04. Scale bars=100 μm.

    Figure 5.  Middle Ordovician biozonal index species of the Periodon flabellum Superbiozone (unnamed upper biozone) (a)–(e), Periodon hankensis Biozone (f)–(j), and Periodon aculeatus Superbiozone (k)–(ag) recovered from the deep-water turbiditic sequences in central and southern New South Wales. (a)–(e) Periodon flabellum (Lindström, 1954); (a) and (b) M element, (a) MMF 46228c-02, (b) MMF 46248a-01; (c) Pa element, MMF 46238b-02; (d) Pa element, MMF 46238c-03; (e) S element, MMF 46238b-03; (f)–(j) Periodon hankensis Stouge, 2012; (f) M element, MMF 46191d-02; (g)–(i) Pa element, (g) MMF 46191d-01, (h) MMF 46195c-01, (i) MMF 46125d-03; (j) S element, MMF 46191d-04; (k)–(o) Periodon aculeatus (Hadding, 1913); (k) and (l) M element, (k) MMF 45515b-01, (l) MMF 45452a-07; (m) and (n) Pa element, (m) MMF 45452c-03, (n) 45515b-03; (o) S element, MMF 45444b-01; (p) Histiodella labiosa Bauer, 2010 (Histiodella labiosa Biozone), Pa element, MMF 46028a-01; (q) Histiodella holodentata Ethington and Clark, 1982 (Histiodella holodentata Biozone), Pa element, MMF 46277b-01; (r) Histiodella kristinae Stouge, 1984 (Histiodella kristinae Biozone), Pa element, MMF 46216c-10; (s) Histiodella sp. cf. H. kristinae Stouge, 1984 (Histiodella kristinae Biozone), Pa element, MMF 46068a-01; (t)–(z) Pygodus serra (Hadding, 1913) (Pygodus serra Biozone); (t)–(v) Pa element, (t) MMF 45977a-01, (u) MMF 45977b-01, (v) MMF 45997a-02; (w) Pb element, MMF 45997d-03; (x)–(z) S element, (x) MMF 45997c-01, (y) MMF 47997a-03, (z) MMF 45997a-01; (aa)–(ag) biozonal index species of the lower subbiozone of the Pygodus anserinus Biozone; (aa)–(ad) Pygodus anserinus Lamont and Lindström, 1957, Pa element; (aa) MMF 46399b-02, (ab) MMF 46057a-01, (ac) MMF 45966b-01, (ad) MMF 46391b-01; (ae)–(ag) Pygodus serra (Hadding, 1913), Pa element, (ae) MMF 46399a-01, (af) MMF 46057d-01, (ag) MMF 46391c-01, (ah) Pseudobelodina sp. indet. (Pygodus serra Biozone), S element, MMF 45625a-03; (ai) Belodina sp. A (Pygodus serra Biozone), S element, MMF 45531d-03; (aj) Microzarkodina sp. indet. (lower–middle Darriwilian), P element, MMF 45381d-04. Scale bars=100 μm.

    Figure 6.  Upper Ordovician biozonal index species of the upper subbiozone of the Pygodus anserinus Biozone (a)–(i) and Periodon grandis Biozone (j)–(l) from the deep-water turbiditic sequences in central and southern New South Wales and other representative species (m)–(q). (a)–(g) Pygodus anserinus Lamont and Lindström, 1957; (a)–(d) Pa element, (a) MMF 45551a-01, (b) MMF 45536b-02, (c) MMF 46123b-03, (d) MMF 45518e-01; (e) and (f) S element, (e) MMF 45726a-01, (f) MMF 45726d-01; (g) Pb element, MMF 45726c-01; (h), (i) Periodon aculeatus (Hadding, 1913), (h) M element, MMF 45536d-01, (i) S element, MMF 45517d-01; (j)–(l) Periodon grandis (Ethington, 1959), (j) M element, MMF 46501c-01, (k) and (l) Pa element, (k) MMF 45737b-01, (l) MMF 45737d-01; (m)–(o) Belodina sp. B, (m) M element, MMF 46560AA-02; (n), (o) S element, (n) MMF 46560T-01, (o) MMF 46560O-01; (p) Protopanderodus sp. indet., M? element, MMF 46560V-01; (q) Scabbardella sp. indet., short-based element, MMF 46560DD-01. Scale bars=100 μm.

  • Conodonts identified in these cherts and siliceous siltstones are assigned to 28 genera including 28 named and three unnamed species (Zhen and Rutledge, 2020). They are Ansella sp. indet., Acodus sp. indet., Baltoniodus sp. indet., Belodina sp. A, Belodina sp. B, Belodina sp. indet., Bergstroemognathus? keramis (Watson, 1988), B. extensus (Graves and Ellison, 1941), Bergstroemognathus sp. indet., Cornuodus longibasis (Lindström, 1954), Cooperignathus aranda (Cooper, 1981), C. nyinti (Cooper, 1981), Dapsilodus sp. indet., Drepanodus sp. indet., Drepanoistodus sp. indet., Dzikodus sp. indet., Eoplacognathus sp. indet., Histiodella holodentata Ethington and Clark, 1982, H. kristinae Stouge, 1984, H. labiosa Bauer, 2010, H. wuhaiensis Wang, Bergström, Zhen, Zhang, Wu and Chen, 2013, H. sp. cf. H. kristinae Stouge, 1984, H. sp. indet., Microzarkodina sp. indet., Oepikodus evae (Lindström, 1954), O. intermedius Serpagli, 1974, O. sp. indet., Oistodus sp. indet., Panderodus sp. indet., Paroistodus horridus (Barnes and Poplawski, 1973), P. originalis (Sergeeva, 1963), P. proteus (Lindström, 1954), Paracordylodus gracilis Lindström, 1954, Protopanderodus cooperi (Sweet and Bergström, 1962), Protopanderodus sp. indet., Periodon aculeatus Hadding, 1913, P. flabellum (Lindström, 1954), P. grandis (Ethington, 1959), P. hankensis Stouge, 2012, Periodon sp. indet., Prioniodus oepiki (McTavish, 1973), Protoprioniodus simplicissimus McTavish, 1973, P. yapu Cooper, 1981, Protoprioniodus sp. indet., Pseudobelodina sp. indet., Pygodus anitae Bergström, 1983, P. anserinus Lamont and Lindström, 1957, P. serra (Hadding, 1913), Pygodus sp. indet., Scabbardella sp. indet., Spinodus spinatus (Hadding, 1913), and Stiptognathus borealis (Repetski, 1982). All the 28 named species except for Bergstroemognathus? keramis are typical components of the Open-Sea Realm with cosmopolitan distribution (Zhen and Percival, 2003).

    Conodonts preserved in these chert and siliceous siltstone lithologies are remarkably low in diversity compared with those known from carbonate shelf or slope facies. Regardless, two distinctive assemblages of Floian and Darriwilian ages are dominant among the current samples and are more widely distributed regionally. The Darriwilian fauna is recognized based on the appearance of several species of Histiodella and Pygodus, Periodon aculeatus and Paroistodus horridus. Oepikodus evae, O. intermedius, Periodon flabellum, Paracordylodus gracilis, Prioniodus oepiki and Stiptognathus borealis characterize the Floian fauna (Fig. 2). Relatively higher diversity and abundance of these two disparate faunas coincided with the two most significant eustatic sea level rises during the Early and Middle Ordovician as depicted in data from the Baltoscandian paleocontinent (Nielsen, 2004, fig. 10.2). In contrast, conodonts from the Dapingian and the Upper Ordovician are relatively poorly represented and show a significantly reduced diversity and abundance. Similar diversity patterns throughout the Ordovician were also reported in conodont faunas identified in cherts and siliceous siltstones of the Central Asian Orogenic Belt (Tolmacheva, 2019, 2014) but with discernible differences. In Kazakhstan, conodont diversity reached its apex in the early Darriwilian (characterized by the abundance of Paroistodus horridus and Periodon aculeatus), whereas in New South Wales, the diversity peak sits in the Pygodus serra Biozone of late Darriwilian age (Fig. 2).

    Among the 28 genera recognized, Periodon occurs in 650 samples examined. Its domination is indicative of the Periodon Biofacies, which characterizes distal shelf to slope and basinal settings (Rasmussen and Stouge, 2018; Wu et al., 2014; Feltes and Albanesi, 2013; Pohler, 1994) of the Open-Sea Realm. Other common genera are Pygodus (present in 151 samples), Oepikodus (in 89 samples), Ansella (77 samples), Paroistodus (48 samples) and Paracordylodus (40 samples) (Zhen and Rutledge, 2020, table 3). In samples from the upper Darriwilian and Upper Ordovician, Belodina is common, recovered from 33 samples. The occurrence of Eoplacognathus, Microzarkodina and Pseudobelodina in the deeper water siliciclastic rocks is unexpected, as these genera typically inhabited shallow water settings—all are extremely rare, represented by a few specimens in only one or two of the total 740 samples. More specifically, Eoplacognathus and Microzarkodina are common in the Cold Domain (e.g., Baltoscandia) and Temperate Domain (e.g., South China) of the Shallow-Sea Realm, but are relatively rare in Australasia (Zhen, 2020; Zhen et al., in press, 2011). Belodina and Pseudobelodina are typical of the tropical domain and have been reported from Upper Ordovician faunas that inhabited shallow shelf to slope settings in New South Wales and other parts of eastern Australia (Zhen and Percival, 2017; Zhen et al., 1999; Trotter and Webby, 1994).

  • Ordovician turbidites in New South Wales are generally deeply weathered, although cherts and siliceous sandstones are more resistant, and outcrops are often discontinuous. Most samples dealt with in this study were collected as spot samples rather than from continuous stratigraphic sections. These factors have resulted in a biostratigraphic framework comprising three superbiozones, nine biozones and two subbiozones (Fig. 2) that is reconstructed solely on the occurrences of age-diagnostic conodonts in the studied samples. As conodont elements are preserved as randomly orientated two dimensional shapes in the thin sections, precise taxonomic identifications are difficult and in many cases impossible, particularly for the coniform taxa, most of which can only be assigned to generic level. Unevenness of the data also varies from area to area as the earlier mapping projects conducted by the Geological Survey of New South Wales did not systematically collect chert samples for conodonts. Once several distinctive conodont faunas were recognized, more thorough sampling was instigated.

    Age-diagnostic species include five species of Histiodella, four species of Periodon, three species of Pygodus, three species of Paroistodus, two species of Oepikodus, Paracordylodus gracilis, Prioniodus oepiki and Stiptognathus borealis (Fig. 2). Four Periodon species form an evolving lineage and are particularly useful for age determination and regional correlation (Zhen et al., 2020). In the upper Tremadocian to Floian (Lower Ordovician) samples, Paracordylodus gracilis dominates, followed by Periodon flabellum and Oepikodus evae in the upper Floian. In the Middle Ordovician samples Periodon hankensis (first occurrence in the upper Dapingian) and P. aculeatus (first occurrence in the lower Darriwilian) are the dominant forms. In the Upper Ordovician samples, Pygodus anserinus and Periodon aculeatus dominate in the lowest Sandbian whereas Periodon grandis and Scabbardella sp. indet. are the predominant taxa in younger Sandbian to middle Katian strata. Other taxa recognized are relatively rare in all samples examined.

    Based on the occurrences of conodonts, all 740 samples were biostratigraphically coded from 1 to 20, reflecting their age dating and varied confidence levels (Fig. 2). Codes with high confidence levels are those bearing age-diagnostic species that can constrain these samples within a narrow stratigraphic interval (e.g., within one biozone or superbiozone). Codes with a moderate confidence level are those with age determinations that may be affected by sampling bias. In most cases, their minimum ages are less well constrained. For instance, in samples coded 13, their maximum age is late Dapingian (well supported by the appearance of Periodon hankensis) but their minimum age (early Darriwilian) is determined in most cases by the absence of Periodon aculeatus which may be due to fewer sample numbers rather than a true absence. Those samples that lack age-diagnostic species are coded with low confidence level and only a broad age can be determined.

    Ordovician conodonts from the Open-Sea Realm can be readily subdivided into faunas inhabiting two adjacent but distinct depositional systems, namely slope and basin settings. Ordovician turbidites of deep-water basinal settings are widely distributed in the Lachlan Orogen in eastern Australia, but remnants of slope facies are rare in the region. Ordovician conodonts from in situ argillaceous limestone nodules, lenses and interlayers and calcareous siltstones, and from allochthonous limestones derived from shelf margins and redeposited within the slope sequences, are reported from several stratigraphic units of the Lachlan Orogen. These previous studies are discussed and incorporated herein to establish a composite biostratigraphical framework for the shelf margin and slope to basinal sequences in the Lachlan Orogen (Fig. 3). In comparison with the basinal conodont faunas those of the slope facies exhibit remarkably higher diversity and incorporate typical species inhabiting shelf settings (Zhen et al., 2015).

  • Among the samples examined, 96 are confirmed to be from the Lower Ordovician, including three from the Prioniodus oepiki Biozone in the middle part of the Paracordylodus gracilis Superbiozone, 16 from the undifferentiated P. gracilis Superbiozone and 74 samples from the Oepikodus evae Biozone in the lower part of the Periodon flabellum Superbiozone. Three additional samples are considered to be of broadly Early Ordovician age, based on co-occurrence of P. gracilis and Oepikodus sp. indet..

    The P. gracilis Superbiozone representing a stratigraphic interval extending from the uppermost Tremadocian to middle Floian (Fig. 2), is characterized by the appearance of the eponymous species. Conodonts recognized in sixteen samples are confirmed from this superbiozone. They include Acodus sp. indet., Ansella sp. indet., Oistodus sp. indet., Paracordylodus gracilis (Figs. 4j4l), Paroistodus proteus (Lindström, 1954), Periodon sp. indet. and Protopanderodus sp. indet.. Both Paracordylodus gracilis and Paroistodus proteus are morphologically distinctive and constrained to this superbiozone. The Prioniodus oepiki Biozone represents the middle part of the P. gracilis Superbiozone and is confined to the lower part of the Floian (Fig. 1). It is characterized by the presence of Prioniodus oepiki (Figs. 4a4g) and Stiptognathus borealis, co-occurring with Paracordylodus gracilis (Fig. 4h), Paroistodus proteus (Fig. 4i) andBergstroemognathus extensus.

    The Periodon flabellum Superbiozone spans the stratigraphical interval from the upper Floian to lower Dapingian (Fig. 2). It can be further subdivided into the Oepikodus evae Biozone of late Floian age in its lower part and an unnamed interval of early Dapingian age in its upper part. Periodon flabellum is one of the most abundant species in the samples investigated herein and represents the oldest species in the evolutionary lineage of Periodon reconstructed based on the material from the Lachlan Orogen (Zhen et al., 2020). Among the 740 samples examined, P. flabellum was identified in 115 samples (Fig. 2) extending from the upper Floian to lower Darriwilian (Fig. 2). The Periodon flabellum Superbiozone is characterized by the occurrence of P. flabellum without association of the younger species, Periodon hankensis. Other conodont species occurring in this superbiozone include Ansella sp. indet., Cooperignathus nyinti, Cooperignathus aranda, Drepanoistodus sp. indet., Oepikodus sp. indet., Oistodus sp. indet., Microzarkodina sp. indet., Paroistodus proteus, Paroistodus sp. indet., Protopanderodus sp. indet., Protoprioniodus simplicissimus, Protoprioniodus yapu, Protoprioniodus sp. indet. and Spinodus spinatus.

    The Oepikodus evae Biozone of late Floian age occupies the lower part of the P. flabellum Superbiozone (Fig. 2). This biozone is represented by 74 samples characterized by the appearance of Oepikodus evae (Figs. 2, 4m4r). Other conodont species recovered from these samples include Acodus sp. indet., Bergstroemognathus extensus, Cooperignathus aranda, Drepanodus sp. indet., Paracordylodus gracilis (Fig. 4w), Paroistodus proteus (Fig. 4v), Periodon flabellum (Figs. 4s4u), Protopanderodus sp. indet. and Protoprioniodus yapu. Abundant specimens of Oepikodus evae preserved on black shale bedding planes in association with graptolites were also reported from the Castlemaine Group exposed at the Devilbend Quarry in Victoria (Stewart and Nicoll, 2004; see Section 4 for further discussion).

  • A diverse conodont fauna from slope facies of Early Ordovician age has previously been reported from an allochthonous limestone block and an in situ laminated calcareous siltstone bed within the Hensleigh Siltstone of central New South Wales (Zhen et al., 2004). The Hensleigh fauna is composed of cosmopolitan species notably lacking shallow water forms. Numerically it is dominated by Juanognathus variabilis Serpagli, 1974 making up 34% of the total fauna. Occurrence of Paracordylodus gracilis in the fauna implies correlation with the P. gracilis Superbiozone established from the basinal turbiditic successions. Zhen et al. (2004) suggested correlation of this fauna with the upper Prioniodus elegans Biozone of the Baltoscandian succession and the Oepikodus communis Biozone of the North American midcontinent succession, largely based on the rare occurrence of a species assignable to Prioniodus sp. indet. (originally identified by Zhen et al., 2004 as Oepikodus sp. cf. O. communis). Prioniodus elegans was reported from deep-water cherts of the 'Hotham Group' (now recognized as part of the Abercrombie Formation of the Adaminaby Group) in north-eastern Victoria (Stewart and Fergusson, 1988), but this occurrence is yet to be confirmed.

  • Of 740 samples examined from the turbiditic sequences in New South Wales, 181 are confirmed to be of Middle Ordovician age. These include 45 samples from the Periodon hankensis Biozone, four from the Histiodella labiosa Biozone, six from the H. holodentata Biozone, five from the H. kristinae Biozone, 88 from the Pygodus serra Biozone, nine from the lower subbiozone of the Pygodus anserinus Biozone and an additional 24 samples from the undifferentiated Middle Ordovician characterized by the occurrence of Paroistodus horridus, Histiodella sp. indet. and Dzikodus sp. indet. (Fig. 2).

    Periodon flabellum (Figs. 5a5e) has a long stratigraphic range extending from the upper Floian to lower Darriwilian, confirmed by the association of P. flabellum and basal Darriwilian graptolites on bedding planes from southern New South Wales (Zhen et al., 2020). Therefore, samples bearing P. flabellum but without association of O. evae cannot be assigned confidently to the upper part of this superbiozone. Elsewhere, attempts were made through detailed morphological analysis of P. flabellum to establish a new species (representing the advanced variant) confined to the lower Dapingian (Stouge, 2012), but our material from chert thin sections allows only a broad species definition of P. flabellum.

    The succeeding Periodon hankensis Biozone represents the stratigraphic interval from the upper Dapingian to lower Darriwilian (Zhen et al., 2020; Fig. 2). It is characterized by the occurrence of the eponymous species (Figs. 5f5j) without association of P. aculeatus. In our study, 44 samples are assigned to the P. hankensis Biozone (Fig. 2). Other conodont species occurring in these samples include Ansella sp. indet., Baltoniodus sp. indet., Bergstroemognathus sp. indet., Cornuodus longibasis, Cooperignathus aranda, Drepanodus sp. indet., Microzarkodina sp. indet. (Fig. 5aj), Oepikodus evae?, O. sp. indet., Oistodus sp. indet., Paroistodus horridus, Paroistodus originalis, Paroistodus sp. indet., Periodon flabellum, Protoprioniodus sp. indet. and Spinodusspinatus.

    The Periodon aculeatus Superbiozone represents a stratigraphical interval extending from the lower Darriwilian to basal Sandbian. Periodon aculeatus (Figs. 5k5o) is the most common species, represented in 232 samples. It first appears in the H. labiosa Biozone of early Darriwilian age and extends into the upper Sandbian within the Periodon grandis Biozone (Fig. 2). The P. aculeatus Superbiozone is subdivided into five successive conodont biozones, including the Histiodella labiosa, H. holodentata, H. kristinae, Pygodus serra and P. anserinus biozones (Fig. 2).

    The Histiodella labiosa Biozone (Figs. 2, 5p) represents a stratigraphical interval of early Darriwilian age between the underlying P. hankensis Biozone and the succeeding H. holodentata Biozone. Other conodont species occurring in the H. labiosa Biozone include Baltoniodus sp. indet., Paroistodus horridus, Periodon aculeatus, P. hankensis and Spinodus spinatus.

    The succeeding H. holodentata Biozone of middle Darriwilian age (Fig. 2) is characterized by the appearance of the eponymous species (Fig. 5q) and the absence of H. kristinae. Associated conodonts recovered from these samples include Baltoniodus sp. indet., Bergstroemognathus? keramis, Histiodella wuhaiensis, Dzikodus sp. indet., Paroistodus horridus, Periodon aculeatus, P. hankensisand Spinodus spinatus. The overlying Histiodella kristinae Biozone (Fig. 2) is represented by the appearance of the eponymous species (Fig. 5r). Other species recovered from these samples include Ansella sp. indet., Eoplacognathus sp. indet., Histiodella holodentata, H. sp. cf. H. kristinae Stouge, 1984 (Fig. 5s), Paroistodus horridus, Periodon aculeatus, P. hankensis, Pygodus anitae and Spinodus spinatus.

    The Pygodus serra Biozone was previously reported from the shale bedding planes of the 'Pittman Formation' (now incorporated into the Abercrombie Formation) of the Canberra District (Nicoll, 1980), the 'Hotham Group' in Victoria (Stewart and Fergusson, 1988), and from the Nattery Chert Member of the Abercrombie Formation (Percival et al., 2011). We now recognize the Pygodus serra Biozone as the most widely distributed biostratigraphic unit in the Ordovician turbiditic sequences in central and southern New South Wales. Apart from the eponymous species (Figs. 5t5z), other species recovered from these samples include Ansella sp. indet., Baltoniodus sp. indet., Belodina sp. A (Fig. 5ai), Panderodus sp. indet., Paroistodus sp. indet., Periodon aculeatus, P. hankensis, Protopanderodus cooperi, P. sp. indet., Pseudobelodina sp. indet. (Fig. 5ah), Pygodus anitae, and Spinodus spinatus.

    The lower P. anserinus Biozone is characterized by the occurrence of the eponymous species (Figs. 5aa5ad) in association with P. serra (Figs. 5ae5ag). Other species recovered from these samples include Ansella sp. indet., Periodon aculeatus, P. hankensis and P. grandis.

  • A conodont fauna of early Darriwilian age from the lower slope facies was recovered from calcareous siltstones in the basal Weemalla Formation of central New South Wales (Zhen and Percival, 2004a). The abundance of Periodon aculeatus (slightly over half of the total number of specimens recovered) and rare occurrence of Histiodella holodentata in this fauna confirm its correlation with the H. holodentata Biozone of the P. aculeatus Superbiozone (Fig. 3). Coeval faunas which inhabited proximal island shelves and distal shelf edge settings have also been reported from central New South Wales, documenting conodont biofacies distribution in a transition from nearshore to basinal settings. For instance, typical shallow water fauna from a limestone lens within the Goonumbla Volcanics of central New South Wales (Zhen and Pickett, 2008) is the nearshore equivalent of a contemporaneous conodont fauna characterized by the co-occurrence of Paroistodus horridus with the advanced form of H. holodentata that was reported from allochthonous limestones derived from the outer shelf edge within the Oakdale Formation (Zhen and Percival, 2004b). Both are coeval with the lower slope conodont fauna described from the basal Weemalla Formation.

  • Only 28 of the 740 samples analysed in this study are confirmed from the Upper Ordovician, a relatively insignificant proportion in comparison with those from either the Middle or Lower Ordovician (Fig. 2). They include 13 samples from the upper subbiozone of the Pygodus anserinus Biozone and 15 samples from the succeeding Periodon grandis Biozone. The upper subbiozone of the Pygodus anserinus Biozone is characterized by the occurrence of P. anserinus (Figs. 6a6g) with Periodon aculeatus (Figs. 6h, 6i) and the absence of Pygodus serra (Fig. 2). The Periodon grandis Biozone represents the highest biostratigraphical interval recognized among the studied samples. It is characterized by the appearance of the eponymous species (Figs. 6j6l), often in association with Belodina sp. B (Figs. 6m6o), Oistodus sp. indet., Periodon aculeatus, Protopanderodus sp. (Fig. 6p) andScabbardella sp. indet. (Fig. 6q).

  • The Lachlan Orogen of eastern Australia is a Middle Paleozoic accretionary orogen that forms the dominant structural domain in New South Wales and Victoria, and further extends into Queensland and Tasmania. Ordovician cherts and siliceous siltstones that are the focus of this study are widespread in distal parts of the turbiditic back-arc basins, particularly in the Albury-Bega (Adaminaby Group) and Hermidale (Girilambone Group) terranes of the Lachlan Orogen in New South Wales (Deyssing et al., 2018; Percival, 2012; Percival and Glen, 2007; Figs. 1, 7). Among the 740 samples examined in this study, 411 are from the Adaminaby Group, including 53 from the Mummel Chert Member, 28 from the Peach Tree Chert Member, 153 from the Nattery Chert Member and 177 from undifferentiated Abercrombie Formation (Figs. 1, 7). Our conodont data demonstrate that the Mummel Chert Member extends from the upper Floian (O. evae Biozone) to lower Dapingian (P. hankensis Biozone), the Peach Tree Chert Member from the upper Dapingian (P. hankensis Biozone) to middle Darriwilian (H. kristinae Biozone) and confines the Nattery Chert Member to the upper Darriwilian (P. serra Biozone) to lowest Sandbian (upper part of the P. anserinus Biozone, Fig. 7). Occurrence of O. evae and P. flabellum in the Milby Chert Member confirms its late Floian age (O. evae Biozone). Appearance of Periodon aculeatus, P. hankensis and Pygodus sp. in the Doongala Chert Member suggests a correlation with the upper part of the P. aculeatus Superbiozone; furthermore, the Numeralla Chert Member is coeval with deposition extending through the P. serra Biozone into the lower part of the P. anserinus Biozone (Fig. 7). Co-occurrence of Periodon aculeatus, P. hankensis, Pygodus anserinus, and P. serra confirms that the base of the Bumballa Formation extends into the topmost Darriwilian (lower part of the P. anserinus Biozone, Fig. 7).

    Figure 7.  Simplified Ordovician stratigraphy for the Adaminaby, Bendoc and Margules groups of the Lachlan Orogen in New South Wales and Victoria (modified after Deyssing et al., 2018) and the correlation between the newly recognized conodont biozones in the turbiditic sequences in New South Wales and well-established Victorian graptolite biozones (after VandenBerg and Cooper, 1992).

    Graptolites have also been recovered from various horizons in several stratigraphic units of the turbiditic sequences in New South Wales (Percival et al., 2015). Documenting additional associations of age-diagnostic graptolite and conodont species in the siliciclastic sequences will be crucial to precisely correlate the graptolite biozonation well-established from shale beds in Victoria with the newly recognised conodont biozonation recognised in cherts and siliceous siltstones. For instance, occurrence of the graptolite Didymograptus (Expansograptus) elongatus from the upper Hensleigh Siltstone exposed south of Wellington in central New South Wales suggests correlation with the graptolite Pendeograptus fruticosus Biozone, consistent with the age determination obtained from the conodont fauna (the Prioniodus elegans Biozone, Figs. 3, 7) from allochthonous limestones within underlying beds (Percival et al., 2015; Zhen et al., 2004; VandenBerg and Cooper, 1992). At the Devilbend Quarry in Victoria, Stewart and Nicoll (2004) reported the bedding plane association of O. evae and graptolites of uppermost Bendigonian to Chewtonian age, confirming direct correlation between the O. evae conodont Biozone and graptolite faunas extending from the Pendeograptus fruticosus Biozone to the Isograptus primulus-Didymograptus protobifidus Biozone (Fig. 7). Stewart and Fergusson (1988, fig. 4) suggested that the O. evae Biozone might extend into the basal Castlemainian (the Isograptus victoriae lunatus Biozone) based on the association of conodonts from cherts and siliceous mudstones with graptolites recovered from the turbiditic sequence of the 'Hotham Group' in eastern Victoria. Zhen et al. (2020) reported a bedding plane assemblage of basal Darriwilian age from the Abercrombie Formation in southern New South Wales that included the graptolites Levisograptus austrodentatus and Isograptus divergens in association with Periodon flabellum and Paroistodus sp.. Further investigations of sites with occurrences of both fossil groups will lead to an increased precision in the regional correlation of the turbiditic sequences of the Lachlan Orogen (Fig. 7).

  • Zhen and Percival (2003) proposed the Open-Sea Realm (OSR) as a paleobiogeographically distinctive entity characterized by conodont faunas inhabiting deep-water, slope to basinal settings in the Ordovician. They argued that OSR faunas, being dominated by cosmopolitan and widespread taxa, were crucial to achieving precise regional and global correlations of Ordovician rocks but were inadequate in establishing the paleobiogeographic relationships of Ordovician continents and terranes. The conodont biozonation scheme established herein spans the upper Tremadocian to middle Katian interval, representing one of the most detailed conodont biozonal classifications for OSR faunas in Ordovician siliciclastic rocks (Fig. 3). Its integration with conodont faunas described from the slope-basinal sequences in Kazakhstan, and slope (mixed carbonates and siliciclastics) successions established on the Jiangnan Slope of the South China Plate and in western Newfoundland of North America, provides the basis for global correlation of Ordovician strata developed in this widely distributed deep marine depositional system (Fig. 3).

  • Ordovician strata characterized by mixed siliciclastic rocks, thinly bedded argillaceous marl and nodular limestones of the slope facies are widely exposed in a narrow belt referred to as the Jiangnan Slope, which separates the Yangtze Platform and the deep-water Zhujiang Basin (Zhang et al., 2019; Zhen et al., 2015). Seventeen conodont biozones recognized from this region form a nearly continuous biozone succession extending from the base of the Tremadocian to the lower Katian (Wang et al., 2019, fig. 2; Fig. 3), which provides the best resolution globally available using conodonts to subdivide and correlate Ordovician rocks of slope facies. Noticeably, conodont biozones from the upper Floian to the Darriwilian of the Jiangnan Slope are nearly identical with those established for the deep- water turbiditic sequences of New South Wales and so can be directly correlated (Fig. 3). Some zonal index species in the uppermost Tremadocian to Floian and in the Upper Ordovician of the Jiangnan Slope are also widely distributed on the Yangtze Platform. For instance, Triangulodus bifidus Zhen in Zhen et al. (2006) and Serratognathus diversus An, 1981 are typical shallow water forms that abundantly occur in the Honghuayuan Formation (latest Tremadocian to early Floian), which is a distinctive Lower Ordovician reefal carbonate unit on the Yangtze Platform (Zhen et al., 2009). These taxa are rare or very rare in the slope deposits, but are biostratigraphically crucial to establish direct correlation with the shallow-water conodont successions recognized in the platform carbonates.

  • A conodont biozonal succession consisting of 11 biozones and 5 subbiozones extending the Lower and Middle (middle Darriwilian) Ordovician is recognized from slope facies represented by the Cow Head Group and the overlying Lower Head Formation and the Table Head Group of western Newfoundland (Stouge et al., 2017; Stouge, 2012, 1984; Cooper et al., 2001; Pohler, 1994; Stouge and Bagnoli, 1988; Fig. 3). The Cow Head Group comprises a 500 m thick sequence of coarse conglomerate, calcarenite, bedded limestone, shale, chert, dolomitic siltstone and quartz-rich calcarenite, deposited in various slope facies off Laurentia. Six conodont biozones recognized in the Lower Ordovician Cow Head Group can be readily correlated with those established in the slope-basinal sequences of New South Wales and Kazakhstan and those from the slope facies in the Jiangnan Slope of South China (Fig. 3). Conodonts of Dapingian to middle Darriwilian age were reported from the Upper Cow Head Group, Lower Head Formation and the Table Head Group. The Periodon macrodentatus Biozone and the succeeding Periodon zgierzensis Biozone defined by Stouge (2012) approximately correlate with the lower part (Histiodella holodentata and Histiodella kristinae biozones) of the Periodon aculeatus Superbiozone defined from New South Wales. The base of the Periodon aculeatus Superbiozone lies within the H. labiosa Biozone, slightly higher than the level depicted in the Cow Head Group (Stouge, 2012, fig. 7; Fig. 3).

  • Ordovician conodonts in cherts and siliceous siltstones were widely reported from various parts of Kazakhstan (e.g., Tolmacheva, 2019, 2014; Tolmacheva et al., 2009, 2008, 2004; Tolmacheva and Purnell, 2002; Dubinina, 2000; Tolmacheva and Löfgren, 2000), which consists of a series of paleo-microcontinents, island arcs, basins and accreted terranes within the Central Asian Orogenic Belt. The biozonal succession established in primarily siliciclastic rocks (basinal facies) of the Ordovician System widely exposed in Kazakhstan consists of 12 biozones (Tolmacheva, 2019). These biozones are nearly identical with those recognized from New South Wales (Fig. 3); in particular, faunas recovered from cherts and other siliceous rocks that were assigned to the Periodon grandis Biozone (Tolmacheva et al., 2009), the Prioniodus oepiki Biozone (Tolmacheva, 2014), and to the stratigraphical interval coeval with the Paracordylodus gracilis Superbiozone of New South Wales.

  • In the Southern Uplands of Scotland, Armstrong et al. (2001) reported conodonts recovered from bedding planes in siliceous mudstone of the Crawford Group which is mainly composed of bedded cherts, mudstones and volcanic rocks deposited in basinal settings. These conodonts were assigned to two disparate biozones, namely the Oepikodus evae Biozone of late Floian age and the Pygodus anserinus Biozone of latest Darriwilian to earliest Sandbian age. Armstrong et al. (2001) suggested the existence of a significant stratigraphic gap between these two biozones within the Crawford Group. A chert sample from the Castle Hill locality (previously assigned to the O. evae Biozone) collected by one of the authors (IGP) in 2006 yielded Pygodus serra, indicating a late Darriwilian age and confirming widespread distribution of the late Darriwilian faunas in southern Scotland. Interestingly, ages of these faunas from Scotland match the two most widely distributed faunas developed during global eustatic sea level rises shown by our data from New South Wales.

  • Ordovician conodonts from the Open-Sea Realm can be readily subdivided into biofacies indicative of two separate depositional systems, namely the slope and basin settings. The new conodont biozonation scheme documented herein subdivides Ordovician deep-water siliciclastic sequences of the Lachlan Orogen into three superbiozones, nine biozones and two subbiozones, enabling them to be precisely dated and correlated. This scheme has been further enhanced by incorporating conodont faunas from slope settings previously known from New South Wales, into a biostratigraphical framework for Ordovician deep-water slope to basinal sequences of the Lachlan Orogen in eastern Australia.

    Ordovician conodonts from deep water settings (Open-Sea Realm) are dominated by cosmopolitan and widespread taxa, and are therefore crucial in achieving precise regional and global correlations. The new conodont biozonation scheme established herein from New South Wales significantly increases precision in age determination and correlation of Ordovician siliciclastic sequences regionally and in a global context.

    Highly diverse conodont faunas from the slope facies are typically preserved in abundance within argillaceous carbonate nodules, lenses and interlayers within siliciclastic sequences. Rare occurrence of endemic or other distinctive forms derived from the carbonate platforms in these faunas provide keys to achieving direct correlation with the conodont successions established from shallow water carbonate-dominated successions facilitating analysis of their paleobiogeographic affinities. Increased influxes of typical shallow water taxa into faunas of the slope settings were typically associated with major sea-level falls. Faunas from the basinal settings are lower in diversity and are often characterized by almost monospecific assemblages. Due to a lack of endemic forms, faunas from basinal settings provide little taxonomic data useful in establishing paleobiogeographic relationships of Ordovician continents and terranes, but their cosmopolitan composition ensures their biostratigraphic value in correlating throughout the Open-Sea Realm.

Reference (81)

Catalog

    /

    DownLoad:  Full-Size Img  PowerPoint
    Return
    Return