Conodonts from the Permian - UNE

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Australian Journal of Earth Sciences (2008) 55, (365 – 377)

Conodonts from the Permian – Triassic transition in Australia and position of the Permian – Triassic boundary I. METCALFE1*, R. S. NICOLL2 AND R. J. WILLINK3 1

School of Environmental and Rural Science, Faculty of Arts and Sciences, University of New England, Armidale, NSW 2351, Australia. 2 Earth and Marine Sciences, Australian National University, Canberra, ACT 0200, Australia. 3 Origin Energy Ltd, Ground Floor, South Tower, John Oxley Centre, 339 Coronation Drive, Milton, Qld 4064, Australia. Late Permian (late Changhsingian), possible Early Triassic Induan (Dienerian), and early Olenekian (Smithian) conodonts have been recovered from the Hovea Member of the Kockatea Shale in the exploration well Corybas 1, northern Perth Basin, Western Australia. Placement of the biostratigraphic Permian – Triassic boundary is in the lower part of the Sapropelic Interval of the Hovea Member. The Australian endemic Protohaploxypinus microcorpus palynofloral Zone is confirmed to be of late (but not latest) Changhsingian age. The Permian – Triassic boundary, based on international calibration using conodonts, carbon-isotope stratigraphy and new radio-isotopic dating, is placed in the lower part of the Kraeuselisporites saeptatus and Lunatisporites pellucidus Zones of western and eastern Australia, respectively, which corresponds approximately to the basal part of the Rewan Group and equivalents in eastern Australia. KEY WORDS: Australia, conodonts, international calibration, megaspores, Permian – Triassic boundary.

INTRODUCTION Recognition of the Permian – Triassic (P – T) boundary in Australia and Gondwana is limited by the paucity of marine index fossils (in particular conodonts and ammonoids) precluding correlation with global and northern hemisphere marine boundary sequences and the formal GSSP section in China. Interpretation of non-biostratigraphic proxies for the P – T boundary in Australia has also proved difficult. The P – T boundary and mass extinction levels fall within the magnetostratigraphic P – T Mixed Polarity Superzone above the Illawarra Reversal, but their positions with respect to detailed magnetic polarity are equivocal, with some authors placing the P – T boundary within a normal polarity zone and others placing it precisely coincident with a reversed to normal transition. However, it does appear that a reverse to normal transition occurs just below the mass extinction and P – T boundary levels globally (Szurlies et al. 2003). Unfortunately, a lack of reliable calibrated magnetostratigraphy in Australia precludes the use of this proxy to date. Changes in carbon isotope composition in organic matter in terrestrial sediments have been used as a proxy for the P – T boundary in Australia (Morante et al. 1994; Morante & Herbert 1994; Morante 1996; Hansen et al. 2000). However, isotopic negative excursions in these sediments are more related to the end-Permian mass extinction than to the currently defined biostratigraphic P – T boundary. They may also to some degree reflect the

origin of the organic matter, biofacies and depositional history (Foster et al. 1997), and further evaluation as a precise proxy for the P – T boundary and relationship to the mass extinction level is required. The important Origin Energy/Arc Energy Hovea 3 petroleum well (Thomas et al. 2004) provided cores through an apparently continuous marine Permian – Triassic sequence in the Perth Basin of Western Australia. This borehole provided spectacular data on this important level, including geochemical and biomarker studies (Thomas et al. 2004; Grice et al. 2005a, b) that have major implications for end-Permian mass extinction causative mechanism scenarios. The interpreted or inferred completeness of the succession is fundamental to underpinning interpretations of these geochemical and biomarker studies. The lack of marine fossils of global biostratigraphic utility, especially conodonts (used to define the P – T boundary and for precise correlation of the Permian and Triassic globally), and the lack of volcanic ashes that could be precisely dated for comparison with GSSP isotopic ages (Mundil et al. 2004) meant that international calibration of the Hovea 3 stratigraphy and endemic biostratigraphy was lacking. Demonstration of a complete stratigraphic record through the P – T transition was also problematic. The importance of the P – T boundary and mass extinction in Australia/eastern Gondwana has recently been emphasised by new data from Antarctica (McLoughlin

*Corresponding author: [email protected] ISSN 0812-0099 print/ISSN 1440-0952 online ! 2008 Geological Society of Australia DOI: 10.1080/08120090701769480


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et al. 1997; Retallack et al. 1998; Retallack & Krull 1999), the identification of a P – T marine boundary section of economic importance in the Perth Basin, Western Australia (Thomas et al. 2004), and reports of extraterrestrial impact tracers at the boundary in Antarctica (Basu et al. 2003; Poreda & Becker 2003) and Australia, and identification of a possible P – T impact crater (Bedout structure) offshore Western Australia (Becker et al. 2004). The precise locations of P – T boundary and mass extinction levels, demonstrated completeness of P – T successions, and the nature of extinction in Australia, despite 30 years of effort, are still poorly constrained. We report here, for the first time, conodonts from the P – T transition sequence in Australia. Our faunas come from the petroleum-exploration well Corybas 1 located in the northern Perth Basin *14 km east-northeast of Dongara at 298120 3.16900 S, 1158030 11.28400 E, and 13 km from the Hovea 3 well in the Perth Basin of Western Australia (Figure 1). This provides some important international calibration for P – T sequences in Western Australia and has a bearing on the completeness of the P – T succession in the Perth Basin.

CORYBAS 1 WELL STRATIGRAPHY AND SAMPLING The petroleum-exploration well Corybas 1 was drilled by Arc Energy/Origin Energy in January 2005. The well penetrated the Lower Triassic Kockatea Shale, the Upper Permian Dongara Sandstone/Wagina Formation and Lower Permian Carynginia Formation (Figure 2).

Kockatea Shale (1994 – 2271 m) The Kockatea Shale, up to 1060 m thick, comprises dark shale, micaceous siltstone and minor sandstone and

limestone in petroleum-exploration wells and is the main source rock of, and seal to, most hydrocarbon accumulations in the Perth Basin (Mory & Iasky 1996). The depositional environment is interpreted as shallowmarine, and sandstone bodies in the middle and lower part of the formation have been ascribed as strandlines and offshore bars. The lower 54 – 90 m of the Kockatea Shale is referred to as the Hovea Member and comprises a lower Inertinitic Interval (named for the high level of inert kerogen), a Sapropelic Interval (named for its dominantly algal-type kerogen) and an upper calcareous horizon termed the ‘Limestone Marker’ (Thomas et al. 2004). The Sapropelic Interval is a sequence of alternating dark-brownish-grey, finely laminated shale and thin limestone bands, some of which have a stromatolitic fabric. The shale is commonly pyritic. A low diversity fauna of ammonoids, fish remains and abundant thinshelled bivalves of the genus Claraia encrusted with spirorbids has been recorded in cores taken in the well Hovea 3. Immediately underlying the Sapropelic Interval, seeming conformably, is a more massively bedded, but likewise organic-rich sequence of dark-grey shale and minor calcareous bands referred to the Inertinitic Interval. A more diverse marine fauna of brachiopods, bryozoans and sponges has been observed in this unit in the Hovea 3 core. Geochemical studies indicate that the Sapropelic Interval was deposited under highly anoxic conditions, whereas the Inertinitic Interval was deposited in a more normally oxygenated environment (Thomas & Barber 2004). The Sapropelic Interval is an excellent oil-prone source rock that generated most of the oil that has been discovered in the North Perth Basin to date. The Kockatea Shale overlies massive shallow-marine sandstones of either the Dongara Sandstone/Wagina Formation (conformably or unconformably) or the Carynginia Formation (unconformably).

Figure 1 (Left) Location of Corybas 1 and Hovea 3 wells in the Perth Basin of Western Australia. (Right) Location of Corybas 1 and Hovea 3 and the Meishan P – T Boundary GSSP on an end-Permian paleogeographic reconstruction from Metcalfe (2005). SC, South China; T, Tarim; I, Indochina; Em, East Malaya; WS, West Sumatra; NC, North China; SI, Simao; S, Sibumasu; WB, West Burma; QI, Qiangtang; L, Lhasa; WC, Western Cimmerian Continent.

Figure 2 Stratigraphy and correlation of Corybas 1 and Hovea 3 ST1 and occurrence of important conodont elements and megaspores. Red bar indicates cored zone in Hovea 3. Depths indicated on Hovea 3 ST1 plot are true vertical sub-sea in this slightly deviated sidetrack (ST) hole. Corybas 1 depths are measured depth below kelly bushing. For discussion on placement of stage boundaries and isotopic ages see text.


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Dongara Sandstone/Wagina Formation (2271 – 2355 m) The Dongara Sandstone/Wagina Formation comprises predominantly bioturbated medium to coarse-grained sandstone with minor thin pebble bands, carbonaceous streaks and carbonaceous siltstone between 60 and 336 m thick. Depositional environments indicated are beach to shallow-marine including proximal fan to coarse-grained deltaic deposits. A marine influence is suggested by the presence of acritarchs and coeval marine sediments to the south, and one of us (RSN) has recovered a limited number of conodonts (new species of Vjalovognathus) from the coeval and interfingering Beekeeper Limestone to the south.

Carynginia Formation [2355 – 2627 (TD) m] The Carynginia Formation, 236 – 328 m thick, comprises dark micaceous and carbonaceous siltstone, and fine- to coarse-grained quartz sandstone with thin beds of conglomerate. Sandstone and conglomerate are lenticular, exhibit cross-laminae and symmetrical ripples and hummocky cross-stratification produced by wave and storm action. Erratic boulders up to 1.5 m in diameter are found in the lower part and are attributed to proglacial ice rafting into this shallow-marine environment.

Permian – Triassic boundary The P – T boundary is located within the basal Hovea Member of the Kockatea Shale and has been previously studied utilising core material from Hovea 3 (Figure 2). The P – T boundary was placed at the base of the Sapropelic Interval based on palynological correlation with the Salt Range section of Pakistan (base of Kraeuselisporites saeptatus Zone) and on brachiopod and bivalve macrofossils broadly indicating a Griesbachian age for the Sapropelic Interval and an early to middle Changhsingian age for the Inertinitic Interval consistent with an interpreted Changhsingian age for the upper part of the Inertinitic Interval represented by the Protohaploxypinus microcorpus palynofloral assemblage (Thomas et al. 2004). Attempts to extract conodonts from limited material of the Hovea 3 core were unsuccessful. This study undertook extraction of conodonts and other microfauna and flora using thirty-one 1 kg drill cutting samples from Corybas 1. These samples were taken over successive 3 m intervals over the entire Hovea Member at the base of the Kockatea Shale and just into the top of the Dongara Sandstone between 2180 m and 2273 m (measured depth below kelly bushing). The Hovea Member has distinctive lithological and petrophysical properties that allow it to be confidently correlated between wells in the northern Perth Basin (Figure 2) and hence mapped regionally in the subsurface (Thomas & Barber 2004). This unit is not exposed anywhere in the basin in surface outcrop.

MICROFAUNA AND FLORA Samples were digested in buffered 5% formic acid initially, with follow-up digestion in a variety of chemicals

to break down non-calcareous black shale. Insoluble residues contained conodonts, micro-bivalves, microgastropods,ichthyoliths,ostracods,smallforaminifera,spines of echinoderms/brachiopods and megaspores (Table 1).

Conodont fauna and age implications A total of only nine conodont elements were recovered from 31 kg of samples giving an average yield of approximately one conodont element per 3 kg of rock. This low conodont yield is consistent with other recorded low conodont yields in rocks of this age in Australia (Nicoll & Metcalfe 1998). Conodont preservation is moderately good, but some specimens are fragmentary due to harsh extraction techniques required to break down these black shales. The conodont colour alteration index (CAI) of recovered conodonts is a relatively low 1 – 1.5, indicating that the Kockatea Shale has been heated to between 50 and 908C and falls within the incipient maturation to low liquid window in terms of organic metamorphic facies and thermal maturation (Epstein et al. 1977). Despite recovering only nine conodont elements, seven of these are valuable age-diagnostic Pa (P1) elements. Of particular importance is the occurrence of a well-preserved (apart from a broken anterior outer platform margin) Pa (P1) element of Clarkina jolfensis Kozur (Figure 3, 13 – 17) in the 2225 – 2228 m depth sample. C. jolfensis is, according to current information, restricted to the late Dorashamian (late Changhsingian) C. yini – C. zhangi Zone of Iran and Transcaucasia (Kozur 2004, 2005). It is therefore of latest Permian (late Changhsingian) age, but still four local conodont zones and 3.5 m below the P – T boundary in the Abadeh section (Kozur 2005). Its occurrence in the basal part of the Sapropelic Interval of the Hovea Member suggests a late Changhsingian age for the basal part of this interval. This also confirms the late Changhsingian age of the Protohaploxypinus microcorpus and the basal part of the Kraeuselisporites saeptatus (equivalent to Lunatisporites pellucidus) palynofloral Zones (by correlation with Hovea 3) which has wider implications for international correlation (see below). The sample at 2213 – 2216 m (upper part of Sapropelic Interval) yielded a Pa (P1) element of Neospathodus dieneri Sweet. This species ranges from early Dienerian to early late Smithian (Orchard 2007) suggesting an early Dienerian or Smithian age for the upper Sapropelic Interval. The sample at 2207 – 2210 m (upper Sapropelic Interval) contained two fragmentary Pa (P1) elements, one of Clarkina sp. and one of Neospathodus sp. Neither of these specimens can be identified to specific level. The Clarkina has a somewhat different basal cavity architecture (more flared and with a pronounced lip) and lacks an ancillary denticle posterior to the cusp compared with the C. jolfensis at 2228 m. An early Triassic age is indicated by the occurrence of Neospathodus. Sample 2204 – 2207 m (mid Limestone Marker Horizon) yielded two Pa (P1) elements of Neospathodus and one fragmentary Sc/Sd (bipennate) element. One of the Neospathodus Pa (P1) elements can be confidently assigned to Neospathodus pakistanenis Sweet (Figure 3, 6 – 8). The other is too fragmentary and, while similar to N. pakistanensis, we here refer this to Neospathodus cf. pakistanensis. The Sc/Sd (bipennate) element (Figure 3,

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2183 – 2186

2186 – 2189

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2195 – 2198

2198 – 2201

2201 – 2204

2204 – 2207

2207 – 2210

2210 – 2213

2213 – 2216

2216 – 2219

2219 – 2222

2222 – 2225

2225 – 2228

2228 – 2231

2231 – 2234

2234 – 2237

2237 – 2240

2240 – 2243

2243 – 2246

2246 – 2249

2249 – 2252

2252 – 2255

2255 – 2258

2258 – 2261

2261 – 2264

2264 – 2267

2267 – 2270

2270 – 2273

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sp indet

indet

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(pyritized)

(pyritized)

element

element

(Sweet)

element

element

Gen et sp

element

Pa (P1) element

pakistanensis

(Sweet) Pa (P1)

Sweet Pa (P1)

element

Kozur

Pa (P1)

Microgastropods

Microbivalves

M

sp. Pa (P1)

bicuspidatus

cf.

(Digyrate)

Sc/Sd (Bipennate)

Neospathodus

Neospathodus

Neospathodus

dieneri

jolfensis

pakistanensis

Conodont elements Neospathodus

Neospathodus

Clarkina

sp. Pa (P1)

Clarkina

2180 – 2183

Depth (m)

Table 1 Numbers of conodont elements and occurrence of other microfauna and microflora in Corybas 1 well-cutting samples.

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Ichthyoliths

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(pyritized)

Ostracods

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forams

Small

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Bryozoa?

Other microfauna and microflora

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brachiopod)

(echinoderm,

Spines

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Megaspores


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Figure 3 Conodont elements recovered from Corybas 1. Specimens (CPC numbers) are reposited at Geoscience Australia, Canberra. Scale bars represent 100 mm unless otherwise indicated. (1) M (digyrate) element (Gen. et sp. indet.), inner lateral view, 2195 – 2198 m depth sample. CPC 39486. (2) Sc/Sd (Bipennate) element (Gen. et sp. indet.), inner lateral view, 2204 – 2207 m depth sample. CPC 39487. (3 – 5) Neospathodus bicuspidatus (s.l.) Muller, Pa (P1) element (early Smithian), 2201 – 2204 m depth sample. CPC 39488: (3) inner lateral view; (4) oral view; (5) basal view. (6 – 8) Neospathodus pakistanenis Sweet, Pa (P1) element, 2204 – 2207 m depth sample. CPC 39489: (6) oral view; (7) outer lateral view; (8) basal view. (9 – 10) Neospathodus dieneri Sweet (early Dienerian – late Smithian), Pa (P1) element, 2213 – 2216 m depth sample. CPC 39490: (9) basal view; (10) lateral view. (11 – 12) Clarkina sp., Pa (P1) element, 2207 – 2210 m depth sample. CPC 39491: (11) basal view; (12) oral view. (13 – 17) Clarkina jolfensis Kozur (late Changhsingian), Pa (P1) element, 2225 – 2228 m depth sample. CPC 39492: (13) inner lateral view; (14) oral view; (15) basal view; (16) oral view detail showing platform ornamentation, deflected two posteriormost carinal denticles and fused carina; (17) detail of basal view showing basal cavity and pit.


Conodonts from the P – T transition, Australia 2) is similar to Sc/Sd (bipennate) elements that form part of the Neospathodus apparatus (Orchard 2005). Neospathodus pakistanensis ranges from late Dienerian to mid-Smithian (Orchard 2007). Sample 2201 – 2204 m (upper Limestone Marker Horizon) yielded a well-preserved Pa (P1) element of Neospathodus bicuspidatus Muller (Figure 3, 3 – 5). This species is restricted to the early Smithian (Orchard 2007) and indicates an early Smithian age for the Limestone Marker Horizon. This age would be consistent with the occurrence of Neospathodus pakistanenis 3 m lower in the sample from 2207 m. Sample 2195 – 2198 m yielded only one well-preserved M (digyrate) element (Figure 3, 1). This element is typical of M (digyrate) elements of Neospathodus (Orchard 2005) that occur in the early Triassic.

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the early Triassic (Late Dienerian or Smithian) of the Kockatea Shale in the Dongara 4 well of the Perth Basin by McTavish (1973). McTavish (1973) also reported this species from Smithian and possibly Dienerian of the Locker Shale of Cunaloo 1, Fortescue 1 and Sholl Island 1 wells of the Carnarvon Basin. Neospathodus pakistanenis Sweet (Figure 3, 6 – 8) This specimen is confidently assigned to Neospathodus pakistanenis. This species has been previously reported from the early Triassic (Smithian) of the Locker Shale in Fortescue 1 well of the Carnarvon Basin (McTavish 1973).

Megaspores and age implications SYSTEMATIC NOTES

Clarkina jolfensis Kozur 2004 (Figure 3, 13 – 17) For synonymy, diagnosis and description see Kozur (2004). Remarks This species has its type locality at Jolfa (northwest Iran) in the Kuh-e-Ali Bashi Locality 1, Section III. Sample J101, Upper Paratirolites Limestone, 10 cm below the so-called ‘Boundary Clay’ (Kozur 2004). The species is recorded as common in the C. yini – C. zhangi Zone of Iran and Transcaucasia. Our specimen from Corybas 1 has a fused carina typical of some Permian Clarkina species but less typical of Triassic forms. The erect cusp and posterior deflection is similar to that seen in Clarkina deflecta (Wang & Wang) but only the cusp and posteriormost carinal denticle are deflected (as in other specimens of C. jolfensis) rather than the deflection occurring more anteriorly and involving more carinal denticles. Another similar form is C. praetaylorae Kozur 2004, also a late Changhsingian species, but this has discreet carinal denticles, and its cusp extends beyond the posterior margin. Our specimen is extremely close in morphology to the specimens illustrated by Kozur (2004 plate 4, figures 10 – 13). Neospathodus bicuspidatus (s. l.) Muller (Figure 3, 3 – 5) Our specimen belongs to the group of Neospathodids that include Neospathodus bicuspidatus, Neospathodus bransoni, Neospathodus concavus and two new as yet unnamed species of Neospathodus (Orchard 2007) and their transitional forms. All these species are indicative of the early Smithian. We here classify our specimen as Neospathodus bicuspidatus (sensu lato) and more specific taxonomic classification awaits additional material. Some of this Neospathodid material may be better assigned to Guangxidella Zhang (Zhang & Yang 1991) but again further work on multi-element apparatuses of these forms is required. Neospathodus dieneri Sweet (Figure 3, 9 – 10) We confidently assign this specimen to Neospathodus dieneri. This species has been previously reported from

Megaspores (Figure 4) appear in sample residues below the Sapropelic – Inertinitic Intervals boundary from 2228 m. Preservation is not good, and surface details are often abraded. However, some specimens are sufficiently well preserved to allow specific or generic identification. The flora includes Banksisporites endosporitiferus (Singh) (from 2249 – 2252 m: Figure 4, 9 – 10), Banksisporites rotundus (Singh) (from 2258 – 2261 m: Figure 4, 8), Banksisporites sp. (from 2231 m: Figure 4, 1) and Singhisporites? spp. (from 2231 – 2234 m, 2234 – 2237 m, 2240 – 2243 m and 2243 – 2246 m: Figure 4, 2 – 7). Glasspool (2003) has recently reviewed the taxonomy, occurrence and distribution of Permian Gondwana megaspores and provides a useful identification key. Banksisporites endosporitiferus was previously known from the Permian and Triassic of India and the Permian of Brazil and Central and South Africa. Banksisporites rotundus is known previously only from the Permian of India (Glasspool 2003 p. 241). The genus Singhisporites ranges from Permian to Triassic. The recovered megaspores therefore suggest a Permian age and are consistent with the interpreted late Wuchiapingian – Changhsingian age of the Inertinitic Interval of the Kockatea Shale from which they have been recovered. SYSTEMATIC NOTES

Banksisporites endosporitiferus (Singh) (Figure 4, 9 – 10) For synonymy and description, see Glasspool (2003 pp. 234 – 236). Our illustrated specimen from 2249 – 2252 m is placed in this species as it exhibits the typical peripherally expanded trilete rays and smooth to granulate ornament that distinguish this species from others. Banksisporites rotundus (Singh) (Figure 4, 8) For synonymy and description, see Glasspool (2003 pp. 241 – 242). Our specimen from 2258 – 2261 m is included in this species because it is unornamented and has distinct contact faces, and its size is consistent with this species.


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Conodonts from the P – T transition, Australia PERMIAN – TRIASSIC BOUNDARY AND POSITION OF STAGE BOUNDARIES IN THE HOVEA MEMBER Based on recovered conodonts and correlation of the Corybas 1 section with that in Hovea 3, we place the P – T boundary in the lower part of the Sapropelic Interval (Figure 5). This placement is above the first downhole appearance of Permian megaspores, and above the level at which a marked negative excursion in d13C, and major change in palynofacies is observed in Hovea 3. In the absence of basal Triassic conodont faunas (e.g. Hindeodus parvus, Isarcicella spp.) and other evidence for the presence of the lower Griesbachian both in Corybas 1, Hovea 3 and other P – T boundary sequences in Western Australia, the completeness of the boundary sequence remains an outstanding question to be resolved. Further sampling of, and conodont recovery from, the Sapropelic Interval would resolve this, as would high-precision and accurate radio-isotopic dating of suitable material from this level. The hypothesis of Wignall & Twitchett (2002), based on global paleoenvironmental data, suggests that in Western Australia the late Changhsingian should be anoxic and the early Griesbachian free from benthic anoxia. If the anoxic lower Sapropelic Interval is late Changhsingian in age, then this would support the hypothesis as this level does indeed suffer from benthic anoxia (R. J. Twitchett 2003 unpubl. Hovea 3 Final Report to Origin Energy). The upper part of the Inertinitic Interval of the Hovea Member is well oxygenated, contrasting with the latest Changhsingian (above the mass extinction level) globally, which is largely oxygen-restricted. The occurrence of algal mats and small stromatolites in the Sapropelic Interval also correlates well with the global occurrence of microbiolites in the P – T interval immediately following the late Changhsingian mass extinction (Baud et al. 2005; Pruss & Bottjer 2005). This correlates well with similar stratigraphies elsewhere at the P – T boundary. Identification of stage and sub-stage boundaries in the P – T sequence of the Perth Basin is also problematic. Based on our interpretations of the new and previous biostratigraphic information, we place the base of the Induan (P – T boundary) in the lower part of the Sapropelic Interval of the Hovea Member of the Kockatea Shale. The base of the Changhsingian stage is placed in the middle of the Inertinitic Interval of the Hovea Member of the Kockatea Shale, above the recorded Dulhuntyispora parvithola palynology zone (regarded as Wordian – Wuchiapingian) and before the Protohaploxypinus microcorpus palynofloral zone of late Changhsingian age. Placement of other stage boundaries follows Archbold (2002).

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INTERNATIONAL CALIBRATION OF THE AUSTRALIAN PERMIAN – TRIASSIC TRANSITIONAL INTERVAL Biostratigraphy, chemostratigraphy and magnetostratigraphy International correlation and calibration of the Australian P – T transitional interval is an ongoing major problem. This is due to the endemic nature of the Australian P – T fauna and flora and the absence of reliable proxies such as precise radio-isotopic ages, equivocal chemostratigraphy and a lack of usable magnetostratigraphy. The previous lack of internationally useful biostratigraphic fossils from the Australian P – T boundary interval, including the boundary-defining conodonts and important ammonoids, has made recognition and calibration of the P – T boundary in Australia extremely difficult despite more than 30 years of effort. The problems of recognition and correlation of the P – T boundary in Australia have been addressed by Foster et al. (1997, 1998), who concluded that despite 25 years of investigation, the position of the P – T boundary in Australia remained elusive. The P – T boundary has been generally placed at major facies changes, for example from coal-bearing strata to braided river fluvial sandstone deposits in the Bowen Basin (Michaelsen 2002), coinciding with major faunal and floral changes and extinctions (e.g. extinction of Glossopteris). Paleontologically, the boundary has been suggested to occur at the base of the Protohaploxypinus microcorpus Zone or possibly at a higher level at the base of the Lunatisporites pellucidus Zone in eastern Australia or at the base of the Kraeuselisporites saeptatus palynological Zone of western Australia or coincide with the first appearance of Aratrisporites (Foster et al. 1998; Thomas et al. 2004), but no international calibration or correlation of these palynozones with the P – T GSSP has been available, apart from tenuous correlations with Pakistan. Based on carbon-isotope studies, Morante et al. (1994) and Morante (1996) have suggested that the boundary should be placed at the base of the Protohaploxypinus microcorpus Zone in both western and eastern Australia but also indicated it could be at the base of the Lunatisporites pellucidus Zone in eastern Australia (Paradise Boreholes: Morante 1996 figure 4) or at the base of the Playfordia crenulata Zone within the top part of the Rangal Coal Measures in the Bowen Basin of Eastern Australia. Carbon-isotope data from Newlands Open Cut Coal Mine in the northern Bowen Basin (Hansen et al. 2000) suggest placement of the P – T boundary at the base of the Sagittarius Sandstone which overlies the Rangal Coal Measures. Interestingly, the major d13Corg negative excursion in both western and eastern Australia occurs

3 Figure 4 Megaspores recovered from Corybas 1. Scale bars represent 100 mm for (1 – 4, 6, 8, 9) and 20 mm for (5, 7, 10). Specimens (CPC numbers) are reposited at Geoscience Australia, Canberra. (1) Banksisporites sp., 2228 – 2231 m depth sample (Changhsingian), CPC 39495. (2 – 7) Singhisporites? spp. (Changhsingian): (2) from 2231 – 2234 m depth sample, CPC 39512; (3) from 2234 – 2237 m depth sample, CPC 39506; (4 – 5) from 2240 – 2243 m depth sample, CPC 39497; (6 – 7) from 2243 – 2246 m depth sample, CPC 39500. (8) Banksisporites rotundus (Singh), 2258 – 2261 m depth sample (late Wuchiapingian), CPC 39512. (9 – 10) Banksisporites endosporitiferus (Singh), from 2249 – 2252 m, depth sample (Changhsingian), CPC 39508.


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Figure 5 Correlation of Western Australian Perth Basin and eastern Australian Bowen Basin stratigraphies with the Global Stratotype Section and Point (GSSP) at Meishan, China showing suggested placement of the P – T boundary in Australia. Conodont zones after Orchard & Tozer (1997), Mei et al. (1998) and Kozur (2005). Palynology zones after Helby et al. (1987), Backhouse (1998) and Foster et al. (1998). Meishan GSSP stratigraphic column from Metcalfe et al. (2001). d13C curves for the Bowen Basin, Meishan and Perth Basin are from Morante (1995), Jin et al. (2000) and Thomas et al. (2004), respectively. For discussion of U – Pb isotopic ages, see text.

in the Protohaploxypinus microcorpus Zone (Figure 5), here interpreted to be of late Changhsingian age. At the GSSP section at Meishan, China and other boundary sections around the world, this major d13C negative excursion (carbonate and organic) occurs in the upper Changhsingian, at or immediately following the endPermian mass extinction level but before the currently defined P – T chronostratigraphic boundary at the first appearance of Hindeodus parvus (Yin et al. 2001; Korte et al. 2004; Krull et al. 2004; Korte & Kozur 2005). This is consistent with our interpretation of the upper part of the Inertinitic Interval, lowest part of the Sapropelic Interval, and Protohaploxypinus microcorpus Zone and basal Kraeuselisporites saeptatus/Lunatisporites pellucidus Zones as late Changhsingian and our placement of the P – T boundary in the lower part of the Sapropelic Interval of the Hovea Member of the Kockatea Shale (Figure 5). d13Corg data on insoluble kerogen from the Inertinitic and Sapropelic Intervals of the Hovea Member of the Kockatea Shale in Hovea 3 (Thomas et al. 2004) (Figure 5) show an abrupt negative excursion from approximately 723.5% to 731% across the boundary between the Inertinitic Interval and the Sapropelic Interval, and this also coincides with a change from charcoal – wood palynofacies to algal palynofacies. However, the globally recognised Early Triassic positive d13C rebound is not evident in the Hovea data. The Inertinitic Interval— Sapropelic Interval boundary corresponds to the main P – T mass extinction level elsewhere.

U – Pb SHRIMP dating of volcanic ashes in eastern Australia (Roberts et al. 1996) suggested that the P – T boundary should be placed much lower than previously suggested by paleontological and chemostratigraphic studies, and even indicated that Glossopteris extended into the Lower Triassic! This result was challenged by stratigraphers and biostratigraphers (Draper & Fielding 1997) and previously unrecognised problems with the SHRIMP dating, including standards used, and lack of required precision (Black et al. 2003, 2004) now throw these SHRIMP dates into question (see below). New high-precision accurate U – Pb IDTIMS dating of single zircons from P – T volcanic ashes in Australia (using the latest annealing and etching preparation techniques) is now under way as a collaborative effort (part of a major international P – T research program) between IM, and R. Mundil and P. R. Renne of the Berkeley Geochronology Centre, California, USA. A preliminary result from the Bowen Basin places the P – T boundary near the base of the Sagittarius Sandstone of the Rewan Group in the Bowen Basin of eastern Australia (Mundil et al. 2006 and see below), consistent with previous biostratigraphic and facies indications. The recovery of conodonts from the P – T transition in western Australia provides some vital international calibration of this level. Unfortunately, the diagnostic boundary conodonts, including Hindeodus parvus and Isarcicella spp., have not been recovered. However, the calibration of the Protohaploxypinus microcorpus palynofloral assemblage as late (but not latest) Changhsingian age


Conodonts from the P – T transition, Australia (see above) indicates that the P – T boundary in Australia should be placed in the basal part of the Kraeuselisporites saeptatus palynomorph Zone of western Australia and in the basal part of the Lunatisporites pellucidus palynomorph zone of eastern Australia. In terms of magnetostratigraphy, a reverse to normal transition occurs just below the end-Permian mass extinction level (just below the P – T biostratigraphically defined boundary) internationally (Szurlies et al. 2003). Unfortunately, lack of reliable magnetostratigraphy in the P – T transition of Australia precludes the use of this proxy in Australia so far.

Isotopic ages and time-scale calibration The end-Permian main mass extinction level at the GSSP and other important paratype sections in China (e.g. Shangsi) is now accurately and precisely dated at 252.6 + 0.2 Ma (Mundil et al. 2004) using closed-system zircons. The biostratigraphically defined P – T boundary (base Induan Stage) is dated by interpolation between robustly dated closely spaced volcanic ashes at the GSSP as 252.5 Ma. These dates for the mass extinction and P – T boundary are considered the currently most reliable, and the dates have now been accepted by the IUGS Subcommissions on Permian and Triassic stratigraphy and are also consistent with recent new isotopic dating in this interval (Crowley et al. 2006) using the latest single crystal U – Pb zircon dating techniques. The time-scale proposed by Bowring et al. (1998) and the boundary age proposed in the recently published international time-scale (Gradstein et al. 2004) are no longer considered reliable. It must be stressed here that a precise and accurate time-scale for P – T events and processes vitally underpins all studies of the mass extinction and recovery, and provides invaluable constraints on potential mass extinction causative mechanism(s). Numerical calibration of other Late Permian and Early Triassic stage and substage boundaries is poorly constrained due to limited robust isotopic numerical tie points. The Capitanian – Wuchiapingian boundary and Wuchiapingian – Changhsingian boundary are reasonably robustly constrained at 260 Ma and 256 Ma, respectively (Mundil et al. 2004). The Lower Triassic is now internationally divided into the Induan and Olenekian Stages. The Induan Stage is broadly equivalent to the previously used stages Griesbachian and Dienerian, and the Olenekian Stage is broadly equivalent to the previously used Smithian and Spathian stages. The Early – Middle Triassic (base Anisian Stage) boundary is constrained at 248 Ma by recent U – Pb IDTIMS data from China (Ovtcharova et al. 2006). The duration of the entire Early Triassic is therefore of the order of 4.5 million years. The Smithian – Spathian boundary is placed at 251 Ma by Ovtcharova et al. (2006), who also estimated the duration of the Spathian as *3 million years. Unfortunately, no reliable tie points are available for the Griesbachian – Dienerian boundary or the Induan – Olenekian (Dienerian – Smithian) boundary. Placement of the Smithian – Spathian at 251 Ma suggests that the combined Griesbachian, Dienerian and Smithian are only of about 1.5 million years’ duration. This would

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suggest that the Griesbachian and Dienerian represent very short periods of geological time, perhaps of the order of 500 000 years each or less. Further reliable radio-isotopic tie points are required to confirm this. A preliminary result (Mundil et al. 2006) from collaborative U – Pb isotopic dating of Permian – Triassic ashes in the Bowen Basin, Queensland, eastern Australia, has yielded new U/Pb age data on single zircons from an 8 cm-thick volcanic tuff layer located in the uppermost Kianga Formation (equivalent to the Bandanna Formation elsewhere) 79 cm below the base of the conformable Sagittarius Sandstone Formation (Rewan Group). A preliminary mean 206Pb/238U age of 252.2 + 0.4 Ma is derived, and this is essentially equivalent to the P – T boundary age for the P – T boundary at the GSSP from the same technique and laboratory (Mundil et al. 2004). This indicates that the P – T boundary is very close to the Kianga Formation—Rewan Group boundary (Figure 5). The negative d13C isotopic shift recorded in the basal part of the Rewan Group (Morante 1995) would suggest that the P – T biostratigraphic boundary should be placed a short distance above the base of the Rewan Group. The tuff yielded few palynomorphs, but an assemblage 3.82 m above the base of the Rewan Formation belongs to the Lunatisporites pellucidus Zone and lacks Aratrisporites spp. (C. B. Foster in Mundil et al. 2006). This U – Pb isotopic result is consistent with our here suggested placement of the P – T boundary in Australia.

CONCLUSIONS Late Permian (late Changhsingian), possible Early Triassic Induan (Dienerian) and early Olenekian (Smithian) conodonts have been recovered from the P – T transition in the petroleum exploration well Corybas 1, northern Perth Basin, Western Australia. Based on the recovered conodonts, coupled with megaspore data and previously reported carbon-isotope, macrofossil and geochemical data, the P – T boundary is placed in the lower part of the Sapropelic Interval of the Hovea Member of the Kockatea Shale. The lack of basal Triassic boundary conodonts and other lower Griesbachian or uppermost Changhsingian fossils may suggest a stratigraphic break at the P – T boundary in the Hovea Member of the Kockatea Shale of Western Australia, but the logs and sedimentology could also indicate continuous deposition. Further work is required on the Sapropelic Interval of the Hovea Member to resolve this. The presence of Clarkina jolfensis Kozur internationally calibrates the Protohaploxypinus microcorpus palynofloral zone and the basal part of the Sapropelitic Interval of the Hovea Member as late (but not latest) Changhsingian age. The P – T boundary, based on international calibration using conodonts, carbon-isotope stratigraphy and new radio-isotopic dating, is placed in the lower part of the Lunatisporites pellucidus palynological Zone and at the base or in the lower part of the Kraeuselisporites saeptatus palynological Zone of western Australia. This level corresponds to a position low in the Saggitarius


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Sandstone of the Rewan Group and equivalents in eastern Australia.

ACKNOWLEDGEMENTS IM acknowledges support from the Australian Research Council for P – T boundary and mass extinction studies. We thank Mike Orchard, Heinz Kozur, Bruce Wardlaw and Charles Henderson for discussions on conodont taxonomy. Ian Glasspool and Clinton Foster are thanked for discussions on palynology and assistance with megaspore identifications. We thank Bruce Thomas for helpful comments on an early draft of the paper. Origin Energy and Arc Energy kindly provided the Corybas 1 drill cuttings samples, and support for sample preparation and digestion. Charles Henderson and an anonymous referee are thanked for their helpful reviews of the paper.

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Received 9 February 2007; accepted 16 October 2007