Early Ordovician conodont distribution from craton to basin and island terranes in East Gondwana
Ian G. Percival1, Yong–Yi Zhen2 and Barry D. Webby3
1 Geological Survey of New South Wales, P.O. Box 76, Lidcombe, N.S.W. 2141, Australia. E–mail: email@example.com
2 Division of Earth and Environmental Sciences, The Australian Museum, 6 College Street, Sydney, N.S.W., 2010, Australia. E–mail: firstname.lastname@example.org
3 Centre for Ecostratigraphy and Palaeobiology, Department of Earth and Planetary Sciences, Macquarie University, N.S.W. 2109, Australia. E–mail: email@example.com
Key words: Conodonts. Faunas. Early Ordovician. Biogeography. Australia.
Early Ordovician conodonts are documented from sporadic outcrops along a transect of more than 1000 kms across New South Wales (N.S.W.), extending from the cratonic margin of east Gondwana to oceanic islands and basins of the Proto–Pacific Ocean. Tectonic settings of these outcrops and their localities are discussed in an accompanying article (Percival and Glen, 2003). The restricted age range of these faunas (upper Prioniodus elegans to Oepikodus evae zones) permits analysis of conodont distribution across a variety of near–contemporaneous depth–controlled environments within the Tropical Domains of both the Shallow–Sea and Open–Sea Realms (Zhen and Percival, 2002, 2003).
Conodont distribution: biogeographic subdivisions
To replace the traditional two–fold subdivision of North American Midcontinent Province (NAMP) and North Atlantic Province (NAP), Zhen and Percival (2002, 2003) proposed a more detailed scheme of regional distribution for Ordovician conodonts in which two major biogeographical entities (separated by the shelf–slope break) are defined: the Shallow–Sea and Open–Sea Realms, both of which are further subdivided into Domains (Tropical, Temperate and Cold). Six conodont regional provinces, namely Australian, Laurentian, and North China (Tropical Domains), South China and Argentine Precordillera (Temperate Domains), and Balto–Scandian (Cold Domain), are recognised in the Early Ordovician Shallow–Sea Realm. The Provinces are based on the distribution of endemic conodont taxa, whereas the Open–Sea Realm is dominated by cosmopolitan and widespread taxa and formal subdivision at provincial level is yet to be recognised.
Early Ordovician conodonts from the Koonenberry Belt in far northwestern N.S.W. are referable to the Australian Province of the Tropical Domain. Offshore islands of the volcanic Macquarie Arc in central N.S.W., deepwater basins of the Proto–Pacific Ocean, and the exotic Narooma Terrane, have faunas assigned to the Tropical Domain of the Open–Sea Realm.
Figure 1. Representative Early Ordovician conodonts from New South Wales; 1-14 are SEM photographs of elements from limestone residues, magnified X40 (100 micron bar scale beneath photo 2); 15-29 are specimens from thick-sections of cherts, magnified X50 (100 micron bar scale below photo 15). Repositories: MC = Geological Survey of N.S.W. Microfossil Collection; AMF = Australian Museum, Sydney, Fossil Collection; CPC = Commonwealth Palaeontological Collection, Canberra. Stratigraphic horizons and localities: 1-6, Hensleigh Siltstone, between Wellington and Molong, central N.S.W. (Percival et al. 1999; Zhen et al. in press b); 7-14, Tabita Formation, Mount Arrowsmith, western N.S.W. (Zhen et al. in press a); 15-18, lower Adaminaby Group, Tullibigeal area, central N.S.W.; 19, 20, lower Adaminaby Group, Gunning area, southeast N.S.W.; 21, 22, lower Adaminaby Group, Bungonia area, southeast N.S.W.; 23-25, Narooma Chert, Narooma, south coast, N.S.W. (Stewart and Glen 1991); 26-29, Budhang Chert Member, Oberon area, south-central N.S.W. (Murray and Stewart 2001). 1. Paracordylodus gracilis, S element, MC2490, sample C1481. 2-4. Bergstroemognathus extensus, 2, Sa element, CPC35050, C1536; 3, Pb element, CPC35044, C1481; 4, Sb element, CPC35053, C1481. 5. Acodus comptus, Sa element, MC2397, C1536. 6. Drepanodus arcuatus, Sb? element, MC2439, C1481. 7. Bergstroemognathus kirki, Sa element, AMF120311, Y4-4. 8. Baltoniodus sp., P element, AMF120423, Y4-7. 9, 10. Erraticodon patu, 9, Sc element, AMF120380, Y4-6; 10, Pb element, AMF120373, Y4-5. 11. Protoprioniodus nyinti, Sb element, AMF120329, TAB1/8.1. 12. Oepikodus communis, Pb element, AMF120403, M/A12-2. 13. Jumudontus gananda, Pa element, AMF120395, M/A4. 14. Prioniodus amadeus, Sc element, AMF120414, TAB1/78.2. 15-17, 19, 22. Oepikodus evae, S elements; 15, 16, sample GCP238, 17, GPC236, 19, AYW155, 22, AJJ33. 18, 21. Oepikodus evae, M elements; 18, GPC236, 21, AJJ33. 20. Drepanodus arcuatus, Sb? element, AYW155. 23. Paracordylodus gracilis, P element, PL2924. 24, 27. Paracordylodus gracilis, S elements; 24, PL2924, 27, R15975. 25. Acodus cf. A. comptus, Sa? element, N96-3. 26, 29. Prioniodus sp., P and M elements, R15975. 28. Oepikodus sp., M element, R15975.
Cratonic margin faunas (Shallow–Sea Realm – Tropical Domain – Australian Province)
Marginal marine to shallow shelf carbonate sediments of the Tabita Formation, at Mount Arrowsmith in far northwestern N.S.W., yield 30 conodont species (Zhen et al., 2001; in press a) of O. evae Zone age (late Bendigonian–Chewtonian). Prioniodids such as Oepikodus, Prioniodus and Baltoniodus are present, though relatively uncommon. Four species apparently endemic, though widely distributed across cratonic Australia – Acodus sp. cf. emanuelensis, Bergstroemognathus kirki, Triangulodus larapintinensis, and Prioniodus sp. cf. amadeus – numerically constitute slightly more than half of the fauna. The Mount Arrowsmith assemblage closely resembles the shallow subtidal Diaphorodus biofacies of Johnston and Barnes (1999), identified in contemporaneous conodont faunas of western Newfoundland.
Island and basin faunas (Open–sea Realm – Tropical Domain)
In striking contrast to the significant endemic component of cratonic conodont assemblages, deeper water faunas inhabiting offshore parts of the Gondwanan margin are characterised by overwhelming dominance of cosmopolitan and widespread species.
1. In the northern Macquarie Arc, the Hensleigh Siltstone was deposited on the lower slopes of a volcanic island complex (Percival et al., 1999). The Hensleigh conodont fauna (16 species obtained from limestones) is of P. elegans Zone age (Figure 1), in agreement with associated early–middle Bendigonian graptolites in siltstones of this formation (Zhen et al., 2001, in press). Ten of the fourteen genera, including Acodus, Drepanodus, Drepanoistodus, Paltodus, Scolopodus, Oistodus, Oepikodus, Paracordylodus, Cornuodus and Protopanderodus, are cosmopolitan in distribution. Others (Reutterodus, Bergstroemognathus, Juanognathus and Jumudontus), although widespread, have a more restricted distribution, being common in deeper water facies on the margins of Laurentia, on the cooler (and deeper) South China Block, and in the Argentine Precordillera.
2. Another deepwater conodont assemblage, including Paracordylodus gracilis, Prioniodus sp. and Oepikodus sp. (Figure 1) – probably of late Lancefieldian to early Bendigonian age (Murray and Stewart, 2001) – occurs in the Budhang Chert Member near Oberon, about 140 kms to the south. From the regional context this area probably lies near the southern end of the Rockley–Gulgong Volcanic Belt (Glen et al., 1998) and hence also represents part of the intraoceanic volcanic island complex of the Macquarie Arc.
3. Conodont faunas identified from deep–water cherts in the extensive quartz–rich turbiditic successions of the Central and Eastern Lachlan Orogen are dominated by cosmopolitan genera (and probably also species), and lack the diversity of those assemblages preserved in the limestones. However, this latter difference may be more apparent than real, due to the difficulties in sampling and identifying elements entombed in thick–sections. Conodonts of Oepikodus evae Zone age have been recognised in cherts of the Wagga Group from the Tullibigeal area west of Condobolin, and from cherts within the lower Adaminaby Group in the Boorowa–Gunning–Goulburn region of southeast N.S.W. (Figure 1). Comparable Bendigonian–Chewtonian conodonts were identified previously (Stewart and Glen, 1991) in turbidite facies on the far south coast of N.S.W.
4. Similar Early Ordovician conodonts are known from deep–water graptolitic siltstones in two areas of Victoria (Stewart, 1988). The Tabberabbera district in the northeast of the state has Paracordylodus gracilis and Prioniodus elegans at one level, and Bergstroemognathus extensus with Oepikodus evae higher in the succession (Stewart and Fergusson, 1988). Another association of O. evae with B. extensus occurs in the Castlemaine Group of the Mornington Peninsula, south of Melbourne (Zhen et al., 2001).
5. Cherts of the oceanic Narooma Terrane, which migrated westwards onto the Proto–Pacific Plate before becoming accreted to East Gondwana, contain conodont faunas ranging from Late Cambrian to Darriwilian/early Late Ordovician (Gisbornian) (Stewart and Glen, 1991), although sedimentation may have been discontinuous through the interval. Occurrrences of Paracordylodus gracilis and Acodus cf. A. comptus (Figure 1), are indicative of a probable Bendigonian age.
Analysis of conodont faunas (elegans to evae zones) of middle to late Early Ordovician age (Bendigonian to Chewtonian stages), demonstrates the applicability of revised biogeographic concepts proposed by Zhen and Percival (2002, 2003). A 1000 km–long transect across N.S.W., extending from the East Gondwanan cratonic margin to deepwater basins and offshore arc of the Proto–Pacific Ocean, provides an ideal section to investigate conodont distributions in Tropical Domains of the two major biogeographic Realms within a restricted time frame. Significant faunal differences exist – the Tropical Domain of the Shallow–Sea Realm is characterised by a large number of endemic species, whereas the Open–Sea Realm is dominated by cosmopolitan or widespread taxa. Preliminary observations of conodont faunas preserved in deep–water cherts suggest that these may assist in distinguishing domains and provinces within the Open–Sea Realm.
We thank the following Geological Survey of N.S.W. staff: David Barnes for photography of conodonts in cherts, and Gary Dargan for preparation of chert sections. Ian Percival publishes with permission of the Director General, N.S.W. Department of Mineral Resources. Yong–yi Zhen’s research was supported by a fellowship from Sydney Grammar School.
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Received: February 15, 2003
Accepted: June 15, 2003