A Paleozoic perspective of Western Australia A.J. Mory1,2 & P.W. Haines1 Keywords: Paleozoic, Western Australia, biostratigraphy, isopachs, paleogeography Abstract Introduction Inter-basinal correlations for the Cambrian to Carboniferous successions of Western Australia are mostly poorly constrained, largely due to unfavourable biogeographic factors, but also because biostratigraphic studies have been skewed to certain basins and intervals. By comparison, Permian inter-basinal correlations have benefited from numerous, mostly unpublished, spore-pollen studies, but correlations to the international timescale are poorly constrained because calibration of the latter is based largely on marine species that are rare in Australia. Nevertheless, a moderately robust correlation is possible for intervals of 10–30 m.y. in the Paleozoic, and reveals broad similarities between basins implying overriding far-field tectonic influences across west Australia. Devonian– Carboniferous events in central Australia—grouped together as the Alice Springs Orogeny—have the most obvious control, at least on the northern basins, but the underlying mechanisms, especially for the initiation of the intracratonic basins, remain obscure. The juxtaposition of west Australia against Greater India and other continental blocks, now dispersed throughout southeastern Asia, and the enormous thickness of Mesozoic successions along the North West Shelf, make unravelling Paleozoic structural history especially difficult. Isopach images reveal repeated reactivation of the major basin elements throughout the Paleozoic, implying continued propagation of Precambrian basement structures. Even so, the orientations of the mostly intracratonic northern and central basins (west-northwest) appear to have been the product of significantly different stress regimes than the basins on the western margin of the West Australian Craton (north to northnorthwest). Westerly extension along the western margin of this craton appears to have commenced in the Devonian, whereas roughly northeasterly extension associated with events in central Australia controlled the development of the central and northern basins throughout the Paleozoic. Paleozoic strata are preserved over approximately 30% of onshore Western Australia (Fig. 1), but rocks of this era have yielded a significantly smaller proportion of the State’s petroleum and mineral resources. Nevertheless, the resource status of the Paleozoic is likely to improve given the growing interest in shale and tight gas, and CO2 sequestration. This review of the Paleozoic depositional and structural history of Western Australia utilises a series of state-wide paleogeographic reconstructions and isopach images derived mostly from outcrop and well sections, with the correlation between basins (Fig. 2) constrained by paleontologicalbiostratigraphic studies (summarised in Fig. 3). The review is mainly concerned with the onshore successions, as the Paleozoic is mostly deeply buried below Mesozoic and younger strata offshore. The first general observations on Paleozoic strata in Western Australia were published by Gregory (1849) and von Sommer (1849), who reported on ‘Carboniferous’ fossils (now known to be Permian in age) from exposures along the Irwin (Perth Basin) and Lyons (Southern Carnarvon Basin) rivers. The first paleontological study was Foord’s (1890) descriptions of material collected by Hardman (1885) from the Cambrian of the Ord Basin. Systematic paleontological studies on Paleozoic fossils began in the 1930s, and were largely based on outcrop studies by university researchers. From the 1950s to 1980s, further paleontological and biostratigraphic studies were conducted, following mapping projects by the Bureau of Mineral Resources (now Geoscience Australia) and Geological Survey of Western Australia (GSWA), in part to encourage petroleum exploration. Early onshore exploration results were disappointing and this, combined with significant offshore discoveries, inhibited additional studies onshore, especially of the Paleozoic. Apart from the Perth Basin, where work since the 1960s has identified significant gas reserves in Upper Permian rocks, onshore activities declined until the 1980s, when interest in the Canning Basin increased due to the Blina discovery and studies of the Devonian reef complex. Even so, parts of the Paleozoic have had little company activity since the 1960s to 1970s. 1 Geological Survey of Western Australia, 100 Plain St, East Perth, Western Australia 6004, arthur.mory@dmp.wa.gov.au 2 University of Western Australia, Perth, Australia Perth, WA, 18–21 August 2013 West Australian Basins Symposium 2013 1 A.J. MORY & P.W. HAINES 115° Timor 120° 125° 10° 130° NORTHERN BONAPARTE BASIN 500 km 135° MONEY SHOAL BASIN MONEY SHOAL ARAFURA BASINS Goulbourn Graben Petrel S-b SBB BROWSE BASIN LF E SH 15° ROEBUCK BASIN Bro T ES W NORTHERN CARNARVON BASIN TH R O 20° NORTH AUSTRALIAN CRATON Le nn ar ro d y– S ome Gr eg helf Pla o t r f orm yT Wil lara ro ug S-b h CANNING BASIN Fit z N Ki DALY BASIN ORD BASIN WISO BASIN GEORGINA BASIN ds on S- b OFFICER BASIN Ga Pla sco tfo yne rm SOUTHERN CARNARVON BASIN -b hS leig 25° rlin Me r ie rn orm Be latf P AMADEUS BASIN CANNING OFFICER BASINS Byro S-b Musgrave Province EROMANGA BASIN ugh Darling n Tro araga Dand Coolcalalaya S-b WEST AUSTRALIAN CRATON PERTH BASIN 30° OFFICER BASIN Fault GAWLER CRATON Collie S-b BIGHT BASIN Bunbury Trough 35° AJM900 04.06.13 Cenozoic–Mesozoic dominant Trough–sub-basin–graben Shelf – outlier – interior basin NT WA Platform–ridge Terrace SA Kalkarindji LIP Milliwindi Dyke Neoproterozoic–Paleozoic basins Precambrian terranes 1000 km Figure 1. Simplified tectonic elements map of Western Australia emphasising Paleozoic structure. SBB = Southern Bonaparte Basin, S-b = Sub-basin. 2 West Australian Basins Symposium 2013 Perth, WA, 18–21 August 2013 A PALEOZOIC PERSPECTIVE OF WA S N Lesueur Ss Early Woodada Fm Sabina Ss Chinty Fm ? Rosabrook Coal Measures Artinskian Woodynook Ss Sakmarian Mosswood Fm ? Asselian �Cullens D� Kennedy Gp Byro Gp Carynginia Fm Irwin River CM High Cliff Ss Holmwood Sh Noonkanbah Formation Wooramel Gp Lyons Gp Kulshill Group Reeves Formation Bashkirian Visean OFFICER3 Moogooree Lst Hermannsberg Ss Mimili Formation Gneudna Fm Mellinjerie Fm Nanyarra Ss Clapp Ridge Fm C Givetian Parke Slt Eifelian Emsian Buttons Fm Ningbing Group C Munabia Ss Sweeny Mia Fm Cockatoo Group C Tandalgoo Fm Pragian Kopke Ss Mereenie Ss Wenlock Carmichael Ss Faure Fm Coburn Fm Yaringa Fm Late red beds Stokes Slt Middle Early Stairway Ss Larapinta Group Munda Group Llandovery Mallowa Salt Tumblagooda Ss Minjoo Salt Indulkana Sh inferred ? Nita Fm Goldwyer Fm Mt Chandler Ss Willara Fm Wanna Fm Horn Valley Slt Pacoota Ss 485 Ajana Fm Blue Hills Ss Worral Fm Wilson Cliffs Ss Nambeet Fm unnamed Lennis Ss ? Prices Creek Gp Skewthorpe Fm Clark Ss Furongian Pretlove Ss Elder S-g Hart Sp Ss Epoch 3 ? C ? Precambrian Wirrildar beds in SA McFadden Fm/Lungkarta Ss, Vines Fm and Durba Ss in WA Terreneuvian Pertaoorrta Group Marla Group Epoch 2 Negri S-g Tararra Fm Table Hill Volcanics 541 Pander Gs Carranya Fm unnamed inferred Goose Hole Group Pridoli Ludlow 444 Antrim Plateau Volcanics Antrim Plateau Volcanics Albert Edward and Louisa Downs Groups AJM898 17.04.13 Dominant lithotype Sandstone (conglomerate) Volcanics Mudstone Halite, evaporite Carbonate�mudstone Disconformity Korsch & Kennard (1991), Nicoll & Laurie (1997); (2003 and references therein); 5 C Conglomerate Ck Creek CM Coal Measures D Diamictite Carbonate Mixed sandstone, mudstone and (Permian only) coal Figure 2. Correlation of the Paleozoic in west Australia. Principal references: 2 Mahony Group Boll C reef complexes Carlton Group SILURIAN Langfield Group Fairfield Group Willaraddie Fm Bonaparte Fm Late M DEVONIAN Early Brewer C Lochkovian CAMBRIAN Weaber Group Williambury Fm Tournaisian ORDOVICIAN Anderson Formation Yindagindy Fm AMADEUS2 Frasnian Wadeye Group Harris Ss Quail Fm 359 419 Poole Ss Grant Group Moscovian Famennian Fossil Head Formation Callytharra Fm Nangetty Formation Late Hyland Bay Subgroup Liveringa Gp Carribuddy Group PERMIAN Cisuralian Pennsylvanian Wagina�Dongara Ss Serphukhovian Mississippian CARBONIFEROUS Kungurian Osprey Fm Mount Goodwin Subgroup Millyit Ss Redgate CM Beekeeper Fm Ashbrook Ss ORD7 Sahul Group Blina Sh ? Willespie Fm Guadalupian SOUTHERN BONAPARTE6 Erskine Ss Locker Sh Kockatea Sh Lopingian 299 N CANNING5 Mungaroo Fm Lesueur Ss Middle S Dirk Hartog Group TRIASSIC 252 SOUTHERN CARNARVON4 PERTH1 AGE 3 Fm Gp 1 Formation Group Gs Greensand Lst Limestone S-g Subgroup Sh Sp Shale Spring Ss Sandstone Crostella & Backhouse (2000), Mory & Iasky (1996); Haines et al. (2008), Jackson & van de Graaff (1981), Morton (1997); Haines (2004, 2009), Allen (pers. comm., 2012), Playford et al. (2009), Mory (2010); 6 4 Mory et al. Mory & Beere (1988), Gorter et al. (1998, 2005, 2008); 7 Mory & Beere (1988). Perth, WA, 18–21 August 2013 West Australian Basins Symposium 2013 3 A.J. MORY & P.W. HAINES S 252 TRIASSIC
 PERTH N SOUTHERN CARNARVON S CANNING N SOUTHERN ORD INTERVALS IN THIS STUDY BONAPARTE Middle ~250 Early Lopingian 249±2 ? 243±5 middle�upper Permian (Fig. 11b) ~265 ? lower Permian (Fig. 11a) Artinskian Sakmarian Asselian ? Pennsylvanian lowermost Permian (Fig. 10b) z 297±7 Late z 306±8 Moscovian upper Carboniferous (Fig. 10a) Bashkirian Serphukhovian Mississippian CARBONIFEROUS 299 Kungurian 269±0.09 270±0.4 269±8 Alice Springs Orogeny PERMIAN Cisuralian Guadalupian Visean Tournaisian AMADEUS� OFFICER z 352±8 359 Late Famennian M DEVONIAN Givetian Eifelian Emsian Early uppermost Devonian� Carboniferous (Fig. 9a) middle� upper Devonian (Fig. 8b) Frasnian lower Devonian (Fig. 8a) Pragian z 412±6 Lochkovian 419 lower Carboniferous (Fig. 9b) Pridoli SILURIAN Ludlow Silurian (Fig. 7b) Wenlock Llandovery 444 ? ORDOVICIAN Late middle�upper Ordovician (Fig. 7a) 467±6 Middle ? lower Ordovician (Fig. 6b) Early 485 Furongian upper Cambrian (Fig. 6a) CAMBRIAN Epoch 3 ~500� 510 509±2.5 Epoch 2 Terreneuvian 541 Ediacaran Petermann Orogeny 509.1±2.2 middle Cambrian (Fig. 5b) lower Cambrian (Fig. 5a) King Leopold Orogeny ~530� 560 ~530�580 AJM899 Span of local zones Biostratigraphic resolution International zone Radiometric ages (Ma) Period Extrusion or tuff Substage No internal evidence Intrusion Stage�substage Disconformity Basement heating event z Stage 21.02.13 Youngest zircon date from provenance study Epoch Figure 3. Summary of biostratigraphic resolution for west Australia (based on Fig. 2), also showing the intervals for the paleogeographic maps and isopach images, and available radiometric dates. 4 West Australian Basins Symposium 2013 Perth, WA, 18–21 August 2013 A PALEOZOIC PERSPECTIVE OF WA Regional structure Before exploration extended offshore, differentiation of Western Australia’s basins (Fig. 1) initially depended largely on their separation by Precambrian terranes. Extrapolating basin nomenclature offshore through the Mesozoic succession is unsatisfactory due to the lack of clear structural features along which the basins can be partitioned. Offshore Paleozoic basins are effectively masked by the thick Mesozoic section. Despite the relatively little information on offshore Paleozoic successions, the BMR Palaeogeographic Group (1990) inferred that they onlap the shelves on the margins of these basins (especially along the North West Shelf where Cretaceous strata usually are unconformable on Precambrian basement). This inference is followed here, with the proviso that the areas shown as non-deposition or erosion (see key in Fig. 4) on the paleogeographic maps and isopach images (Figs 5–11) are not necessarily continental. An implication of such onlap is that the Paleozoic had a relatively low thermal maturity along these margins prior to Mesozoic deposition, as is typical onshore where Paleozoic strata onlap Precambrian basement. Onshore basin subdivisions of equivalent status are amalgamated where they adjoin in Figure 1. Some of these boundaries are arbitrary, and most likely did not represent discrete structural domains during the Paleozoic. The Upper Carboniferous to Lower Permian succession formerly included in the Gunbarrel Basin, for example, is now considered a southern extension of the Canning Basin on sedimentological grounds. The underlying lower Paleozoic section is returned to the Officer Basin (as in Iasky, 1990), and the Gunbarrel Basin is confined to the Mesozoic section between 23°S and 30°S (R. Hocking, pers. comm., 2012). The major Ordovician–Silurian depocentres in Western Australia were the Kidson-Willara sub-basins (Canning Basin) and Gascoyne-Bernier platforms (Southern Carnarvon Basin). By contrast, the Merlinleigh-Byro sub-basins and Coolcalalaya Sub-basin (Southern Carnarvon and northern Perth basins), plus the Fitzroy Trough and Gregory Sub-basin (Canning Basin) were largely active in the Devonian to Early Permian. To the north, the Petrel Sub-basin (Southern Bonaparte Basin) appears to have been active throughout most of the Paleozoic, although the succession pre-dating the Upper Devonian is poorly understood. In the south of the State, the Dandaragan– Bunbury troughs of the Perth Basin represent a southern extension of the Permian depocentre formed along the eastern margin of the Southern Carnarvon Basin. Further afield, the Arafura Basin (Fig. 1) contains up to 7 km of Paleozoic strata within the Goulburn Graben, a trough with a similar westnorthwesterly orientation to the Fitzroy Trough (Struckmeyer, 2006). To date, relatively little data is available, as just seven wells have intersected Paleozoic strata within this graben. Hocking (1994) cautioned that onshore basin boundaries reflect the limits of preservation rather than inferred limits of deposition, and that currently-used basin subdivisions refer to “presently expressed tectonic elements”. This implies Perth, WA, 18–21 August 2013 that subdivision descriptors, such as ‘platforms’, ‘shelves’ and ‘troughs’ “do not necessarily have a paleogeographic connotation” (Mory & Hocking, 2011, p. 5) and ‘present structural configuration … should not be taken to imply paleogeography’ (Mory & Haig, 2011, p. 6). Nevertheless, thermal maturity studies lend credence to the longevity of such features: for example, low maturities across the Gascoyne Platform (Southern Carnarvon Basin) imply it remained at a relatively high level at the same time as a Permian succession, up to 5 km thick, was deposited in the adjoining sub-basins to the east (Mory et al., 1998). Similarly, the low maturity of the Ordovician section across the Broome Platform and Willara Sub-basin in the Canning Basin (Nicoll, 1993) indicates significant segregation from the deposition of up to 5 km of post-Ordovician strata in the Fitzroy Trough. Typically, ‘shelf ’ and ‘platform’ basin subdivisions have low organic maturities, thereby implying some longevity to their ‘present structural configuration’. Although these areas have significance in terms of limited sediment accumulation, they do not necessarily coincide with the distribution of paleoenvironments. Note also that the outlines of the structural subdivisions on the paleogeographic maps and isopach images (Figs 5–11) are meant as a guide to location, and do not always signify that these sub-basins were tectonically active at those times. Biostratigraphic controls Faunal and floral studies of diverse Paleozoic groups from Western Australia, of which there have been several hundred, underpin biostratigraphic correlations: without such systematic studies, age determinations and correlations are inherently unreliable. However, not all fossil groups are useful for detailed biostratigraphy, and this review briefly assesses the groups most useful for regional correlations. The biostratigraphic resolution of successions in each basin is ranked from high (where international zones can be recognised) to none (for units with no internal age evidence; Fig. 3). Overall, interbasin correlation of the mid-Devonian to Permian is the most robust, as marine facies are more prevalent during this time than in the lower Paleozoic. A useful summary of Australian Paleozoic biostratigraphy is given by Young & Laurie (1996, especially chapters 2.1–2.6). Despite being overshadowed by changes to the international timescale (Gradstein et al., 2012 is followed here), the notes in Young & Laurie (1996) provide a good summary of each period. Geoscience Australia’s biozonation and stratigraphy charts (e.g. Nicoll et al., 2009a, b; Mantle et al., 2010) link biostratigraphic zones to regional stratigraphy, but do not discuss the distribution of biozones within rock units, in spite of providing extensive reference lists. Although the main emphasis on Australian faunas and floras in Wright et al. (2000) is paleobiogeographic, they also include useful insights into biostratigraphy. The most recent summaries of described Western Australian Paleozoic faunas and floras are West Australian Basins Symposium 2013 5 A.J. MORY & P.W. HAINES Skwarko’s (1987a; 1987b; 1988a; 1988b; 1993) compilations on the fossils of the Cambrian, Ordovician, Devonian, Carboniferous and Permian respectively. Only the Permian volume was published, but the unpublished reports can be consulted through the Department of Mines and Petroleum’s library in Mineral House, East Perth. Cambrian Lower Cambrian sections in Western Australia can only be inferred by extrapolation from eastern Amadeus and Officer basin successions of the Northern Territory and South Australia, respectively. Both of those areas contain predominantly clastic facies between Ediacaran and Epoch 2 ages, which has been dated using archaeocyathids. The base of the Cambrian and Fortunian (Early Terreneuvian) in the central and eastern Amadeus Basin is well established based on the presence of the Phycodes pedum ichnozone (Walter et al., 1989). This ichnozone (now referred to by various authors as Trichophycus pedum, Treptichnus pedum, or Manykodes pedum) also defines the base of the Cambrian at the GSSP (Global Boundary Stratotype Section and Point) in Newfoundland (Gradstein et al., 2012, fig. 19.4). Equivalent lower Cambrian strata possibly extend into the western Amadeus and Officer basins in Western Australia, but outcrop there is poor and needs to be re-evaluated. Possible lower Cambrian strata are also present in the east Kimberley region, but no fossils have been reported from these rocks. Fossiliferous upper Cambrian strata in Western Australia are only known in the Southern Bonaparte and Ord basins, and these assemblages show strong links to the Wiso and Georgina basins to the east, in the Northern Territory. The last work on the paleontology of the Southern Bonaparte Basin was that of Shergold et al. (2007) who documented the trilobite faunas previously only listed by Öpik (1969). In the report on the Ord Basin by Kruse et al. (2004) the older ‘Ordian’ faunas are regarded as ‘earliest Middle Cambrian’ (close to the Epoch 2/Epoch 3 boundary). Argon-argon dating of the underlying Kalkarindji LIP (Large Igneous Province; including the Antrim Plateau and Table Hill Volcanics) by Evins et al. (2009) has been recalculated as c. 509 Ma, close to the present estimate for the base of Epoch 3, thereby pointing to the ‘Ordian’ lying mostly within Epoch 3 if the 509 Ma age for the base of that epoch is verified. Nevertheless, the relative ages of these successions remain unchanged. The suggestion by Retallack (2009) of a possible Cambrian age for the lower part of the Tumblagooda Sandstone in the Carnarvon Basin is uncertain as it relies on the correlation of soil profiles. Conodonts from a similar lithofacies in Wandagee 1 (core 5), 400 km north of the outcrop belt, indicate an Early Ordovician age (Mory et al., 1998), but it is unclear if this age applies to the Tumblagooda Sandstone as a whole or if it points to the presence of an older succession equivalent to that inferred from seismic data offshore from Kalbarri (Iasky et al., 2003). 6 Ordovician The most comprehensive paleontological record for the Lower to lower Middle Ordovician is from the Canning Basin, for which Nicoll (1993) identifies conodont assemblages based on descriptions by McTavish (1973) and Watson (1988). Other groups, such as graptolites and trilobites (Legg, 1978; Laurie & Shergold, 1996), are also useful for this interval, but overall play a much smaller role in Ordovician age determinations. A minor, lowermost Ordovician section in the Southern Bonaparte Basin has an abundant Tremadocian conodont assemblage (Jones, 1971). Conodonts of this age have been reworked into the Upper Devonian, and Darriwillian (upper Middle Ordovician) fossils have been recovered from Visean (Early Carboniferous) strata, indicating that the preserved Ordovician section is incomplete (Nicoll, 1995). A study of Middle Ordovician conodonts from an isolated outcrop in the western Amadeus Basin is in progress (R. Nicoll, written comm., 2012). Paralic facies in the upper Middle and Upper Ordovician (Carribuddy Group) of the Canning Basin are poorly constrained biostratigraphically. Acritarch assemblages are long-ranging, and the rare conodonts recovered, some of which may be reworked, have not been studied in detail. The Mallowa Salt has been assigned a Late Ordovician to earliest Silurian age based on the occurrence of the land-plant crytospore Tetrahedraletes medinensis (Foster & Williams, 1991). The only other significant section deduced to be Ordovician from its stratigraphic position is the Tumblagooda Sandstone (Southern Carnarvon Basin; Mory et al., 1998); we consider the internal evidence for the age of this unit (Trewin & McNamara, 1995; Retallack, 2009) to be ambiguous. Possible Ordovician strata in the western Officer Basin are only loosely constrained by their stratigraphic position to the middle Cambrian to Carboniferous; a tentative Ordovician age is suggested for these rocks based on detrital zircon provenance data (Haines et al., this volume). Silurian In Western Australia only two areas with Silurian ages have been identified, in the Southern Carnarvon and Canning basins. The age determinations for both successions depend on limited conodont assemblages. Mory et al. (1998) list four assemblages (#2–5) spanning the mid-Llandovery to Pridoli in the former, whereas Nicoll et al. (1994) describes an Early–Middle Llandovery fauna from the Canning Basin based on five samples from Acacia 1 and Boab 1. The only other potentially useful Silurian fossils in Western Australia are thelodont scales from Kemp Field 1 (Canning Basin; Turner, 1993) but they need to be verified. Devonian Early Devonian fossils are rare. In the Carnarvon Basin, a single element of Ozarkodina pandora, a conodont species West Australian Basins Symposium 2013 Perth, WA, 18–21 August 2013 A PALEOZOIC PERSPECTIVE OF WA restricted to the lower Pedavis pedavis Zone, indicates an early Lochkovian age for the base of the Kopke Sandstone (Mory et al., 1998). In the Canning Basin, the thelodont Turinia australiensis, a widely distributed species of the Lower Devonian of eastern Australia, was described from the Tandalgoo Formation in core 5 of Wilson Cliffs 1 by Gross (1971), although it is unclear if the fossils are from this core or core 8 (Worral Formation). Middle Devonian age determinations are uncertain as they depend largely on low diversity acritarch assemblages with long ranging species (such as Geminospora lemurata) that extend into the Frasnian (Young, 1996). Consequently some formations assigned Middle Devonian ages in the southerncentral Canning Basin may be partially equivalent to Upper Devonian units further north in that basin. Late Devonian ages are best known from the Canning Basin reef complexes, in which the Frasnian is divided into 13 Palmatolepis conodont assemblages similar to those from the Montagne Noir in southern France (Klapper, 2009). In addition, this section contains abundant, diverse goniatite assemblages that have been divided into 19 zones (Becker & House, 2009)—implying a biostratigraphic resolution of less than one million years. However, these detailed biostratigraphic subdivisions, devised from outcrop of deeper water facies along the northern basin margin, have only been sporadically applied to the subsurface due to unsuitable facies and irregular sampling. Frasnian conodonts are also known from the Gneudna Formation in the Southern Carnarvon Basin (Seddon, 1969) and the Cockatoo Group in the Southern Bonaparte Basin (Druce, 1969), but these assemblages were insufficient to assign to international zones. Famennian conodont assemblages in the Canning Basin are dominated by taxa of the genus Polygnathus, but the lack of agreement on species-level taxonomy and the prevalence of endemic forms limits their biostratigraphic usefulness (Klapper, 2009). Despite this, 14 Famennian goniatite zones have been recognised covering the Famennian (duration ~1 m.y. each) through to the Middle Palmatolepis gracilis expansa conodont Zone (Becker & House, 2009). By comparison, Famennian conodonts from the Southern Bonaparte Basin only provide general ages due to reworking (Druce, 1969). Ammonoids are rare in the uppermost part of the Famennian, and conodont assemblages are from shallowwater biofacies, limiting their biostratigraphic value (Druce, 1974; Nicoll & Druce, 1979). However, the uppermost Famennian also contains a distinctive miospore assemblage characterised by Retispora lepidophyta, the upper limit of which is considered close to the Devonian/Carboniferous boundary (Playford, 2009). The lower age limit of this assemblage is poorly defined in Australia, but Young (1996) correlates it with the base of the Palmatolepis perlobata postera Zone. This is a zone earlier than Streel (2008) indicates for Europe, and implies its first appearance in the Canning Basin is from a level equivalent to the uppermost part of the reef complex. Perth, WA, 18–21 August 2013 Carboniferous Biostratigraphic studies of the Mississippian have been based on conodonts, ostracods, foraminifera, brachiopods and palynomorphs, especially in the Canning and Southern Bonaparte basins. Of these, the ostracods offer the best biostratigraphic resolution at present, with eight assemblages forming an ‘interim biostratigraphic scheme’ covering the Tournaisian almost to the end of the Visean (Jones, 1989; 2004). However, there have been no ostracod studies in the Southern Carnarvon Basin, largely due to unsuitable facies and the paucity of subsurface sections. Other groups are of limited biostratigraphic value, due to poor resolution, facies dependence, or the limited number of sections studied. In the Southern Bonaparte Basin, Jones’ (1989, 2004) ages depend on those determined by Mamet & Belford (1968) based on foraminifera. However, many ostracod taxa are cosmopolitan (e.g. Paraparchitidae; Jones, 2004), and their short-ranging species are useful for international correlations. After Mamet & Belford (1968), the only published study on Western Australian Carboniferous foraminifera was by Edgell (2004) on material from the Canning Basin. However, work is underway to describe calcareous algae and foraminifera from the Yindagindy Formation (Southern Carnarvon Basin), with the former group indicating a Holkerian (early late Visean) age (D. Vachard & D. Haig, written comm., 2013). Although calcareous algae have the potential for good biostratigraphic resolution, the existing studies on the Southern Bonaparte Basin are reconnaissance at best (Veevers, 1970; Mamet & Roux, 1983), limiting their use at present. The conodont studies of Druce (1969) and Nicoll & Druce (1979) from the Southern Bonaparte and Canning basins respectively, yielded few of the deeper-water Tournaisian forms (such as Siphonodella and Gnathodus) on which identification of the international zones depend. The dominance of shallow-water forms (such as Bispathodus, Clydagnathus, Polygnathus and Pseudognathodus) in these assemblages hinders correlation, even locally, as their stratigraphic ranges are not well established and are strongly facies influenced. Although Visean conodont assemblages are biostratigraphically more useful, they have been recovered from relatively few levels. Conodonts, including Declinognathus noduliferus inequalis from the Arco Formation in Lesueur 1 (Gorter et al., 2005), are the youngest marine Carboniferous fossils (no older than basal Pennsylvanian) recovered in west Australia, and show that the Spelaeotriletes ybertii spore-pollen zone ranges into the Bashkirian. The spore-pollen zonation of Kemp et al. (1977) for the Tournaisian and Visean has in part been refined by Playford (1976; 1985; 1991), but still remains of low resolution, as these Mississippian zones approximate stages in duration. The Pennsylvanian contains just two spore-pollen zones that appear to have significant temporal overlap (Mory, 2010). Largely due to the paucity of marine fossils, the Pennsylvanian is poorly constrained, and the position of the Carboniferous/Permian West Australian Basins Symposium 2013 7 A.J. MORY & P.W. HAINES boundary remains elusive in Australia. The only Carboniferous zone of apparently short duration—the Grandispora maculosa zone—is problematic as its late Visean to early Serpukhovian range in Western Australia (Gorter et al., 2005) barely overlaps with its early−late Visean range in New South Wales (Fielding et al., 2008, supplementary paper). Although there is a well-described brachiopod succession in the Southern Bonaparte Basin, with nine Tournaisian assemblages and one Visean (Roberts, 1971), there are few species in common with the other two basins containing strata of these ages (Canning and Southern Carnarvon basins; Skwarko, 1988b). In the Tournaisian, just one brachiopod species (Punctospirifer plicatosulcatus) is known from all three basins, with three other species present in two of the basins, limiting the usefulness of this group for interbasin correlation in Western Australia. In the Visean, none of the known brachiopod species are known to extend between basins. Permian Permian paleontological studies have included a large range of groups for which biostratigraphic schemes have been proposed, including brachiopods, ammonoids, foraminifera, bryozoa and spore-pollen. Of all these groups, ammonoids reputedly allow the best correlation to international stages (e.g. Glenister & Furnish, 1961), but their rarity is a significant limitation. Furthermore, due to the dominance of endemic ammonoid species, many age determinations are based on implied—and therefore questionable—phylogenetic affinities (Boiko et al., 2008; Leonova, 1998; 2011). On the whole, the spore-pollen zonation, first established at Collie (Fig. 1) by Backhouse (1991) and later extended to other basins (Backhouse, 1993; 1998; Mory & Backhouse, 1997), offers the best overall biostratigraphic control. This is largely because palynological studies have been carried out on most exploration wells, many of the deeper government water bores, and some mineral exploration bores, although much of this work is unpublished (e.g. see the summary of Canning Basin by Mory, 2010). This zonation is based on an evolutionary lineage, but suffers from relying on relatively few species and, apart from in the upper Kungurian to upper Capitanian, its resolution is limited, possibly due to the influence of cool climates in the Cisuralian and evolutionary conservatism in the mid-Capitanian to Lopingian. Unfortunately, palynomorph recovery from outcrop is rare due to oxidisation, so correlation to the subsurface, where identifiable macrofossils are scarce, is hindered by the difficulties in matching ages obtained from different fossil groups. Other difficulties include inconsistencies due to changing species concepts and the limited revisions of older work, especially material from petroleum exploration wells. In addition, comparing zircon dates from tuff beds with associated spore-pollen zones between eastern and Western Australia reveals some late Early–Middle Permian zones are either of short duration or diachronous (T. Kelly, written comm., 2013). Nevertheless, correlations based on palynology 8 and supported by macrofossils are likely to be moderately reliable as significant barriers to floral and faunal dispersal are unlikely, at least in the west of the continent based on its passive margin tectonic setting. Although brachiopod assemblages have been well studied, with 18 identified assemblage zones spanning the Permian (Archbold, 1993; 1998), these studies have been largely restricted to outcrop sections and the faunal succession is incomplete in each basin. Strong facies control is likely, especially for the eight brachiopod assemblages in the Byro Group (Southern Carnarvon Basin) most of which are confined to a few beds in one formation of the eight within the group (Mory & Haig, 2011, Fig. 7). Early studies of foraminifera by Crespin (1958) identified eight assemblages spanning the Permian, but more recent work in Western Australia has so far concentrated on paleoclimatic and paleoecological controls (e.g. Haig, 2003; Dixon & Haig, 2004) rather than refining the biostratigraphy. Nevertheless, studies underway show that foraminifera can provide new insights on the Permian: the Beekeeper Formation (northern Perth Basin), for example, contains Guadalupian foraminifera of Tethyan aspect (D. Haig, pers. comm., 2010). Most bryozoan studies were by Crockford (1957, and references therein), but this work requires updating due to significant changes in the phylum’s systematics. Ongoing studies support the biostratigraphic potential of this group, in part based on the resolution achieved in other parts of the Tethyan Realm (Ernst et al., 2008, and references therein) and for Tasmania (Reid, 2003). The utility of other groups, such as conodonts (Nicoll & Metcalfe, 1998), suffers because of rarity, both in number of specimens and stratigraphic levels from which they have been recovered, and the predominance of endemic forms. Radiometric dates Available radiometric dates are shown on Figure 3. Apart from the mid-Cambrian Kalkarindji LIP (dated as 509 ± 2.2 Ma from the Antrim Plateau Volcanics; recalculated by F. Jourdan pers. comm., 2012, based on Evins et al., 2009), Paleozoic extrusive volcanic rocks are rare in Western Australia. The Milliwindi Dyke in the west Kimberley, dated as 510.52 ± 0.32 Ma from U–Pb CA-TIMS, is considered comagmatic with the Kalkarindji LIP (Jourdan et al., 2012). In the Canning Basin, the lower Middle Ordovician Goldwyer Formation contains at least three tuff beds, with another one in the Willara Formation, but only one bed from the Goldwyer Formation has been dated (at 467 ± 6 Ma) using the SHRIMP U–Pb method on zircons (M. Wingate written comm., 2013). The only other radiometrically-dated tuffs are Kungurian– Wordian (269 ± 8 Ma & 273 9 Ma, late Early to Middle Permian) beds from the Binthalya Formation (Southern Carnarvon Basin; Lever & Fanning, 2004), and levels from the Lightjack Formation (269 ± 0.09 Ma and 270 ± 0.4 Ma, West Australian Basins Symposium 2013 Perth, WA, 18–21 August 2013 A PALEOZOIC PERSPECTIVE OF WA dated using ID-TIMS in the Canning Basin; Mory et al., 2012). Attempts to date tuffs from the Artinskian (mid-Early Permian) Irwin River Coal Measures (northern Perth Basin) have only yielded Precambrian ages to date (V. Davydov, pers. comm., 2011). Paleozoic intrusive rocks are known from the northwestern Canning Basin (Reeckmann & Mebberson, 1984) and in Edel 1 on the Edel Terrace, Southern Carnarvon Basin (Gorter & Deighton, 2002). The available K–Ar dating (265–250 Ma and 253–249 Ma, respectively) is open to interpretation, but at the very least indicates heating events in the Late Permian to Early Triassic (Fig. 3). Stratigraphic constraints are poor as all but two examples intrude pre-Permian strata, raising the possibility of several episodes of intrusion. Geochemical analysis and new dating are required to reveal the affinities of these intrusions and to better constrain their ages. Similarly, K–Ar dates of 510–500 Ma and 560–530 Ma obtained from the northern margin of the Canning Basin reveal that heating events continued into the Cambrian (Shaw et al., 1992), and Rb–Sr biotite ages of 500–430 Ma from the southwestern margin of the West Australian Craton are interpreted to represent early Paleozoic cooling during uplift (Libby & de Laeter, 1979; de Laeter & Libby, 1993). In the Musgrave Province, K–Ar, Rb–Sr and Sm–Nd metamorphism and cooling ages of 580–530 Ma are attributed to the Petermann Orogeny (Edgoose et al., 2004; Howard et al., 2011). A third source of radiometric dates is from zircon provenance studies in which the youngest zircons are within the stratigraphic age of the unit. At present, we know of just four such dates, all from the Canning Basin (Fig. 3): 412 ± 6 Ma from low in the Tandalgoo Formation, 352 ± 8 Ma from low in the Anderson Formation (Haines et al., this volume), 306 ± 8 Ma from high in the Reeves Formation and 297 ± 7 Ma low in the Grant Group (J. Martin, written comm., 2013). Although the errors in these analyses limit their value relative to the biostratigraphy, they imply igneous activity throughout the Paleozoic, most likely in eastern Australia, Cimmeria and/or Antarctica. Paleozoic evolution of Western Australia Approach Biostratigraphic resolution (discussed above) also imparts a measure of marine influence—albeit affected by paleoecological and paleoenvironmental controls, including currents and latitudinal changes—and connectivity to typical Tethyan faunas. Such faunas are generally mid-latitude to equatorial in aspect, but Tethyan elements are usually rare in west Australian Paleozoic assemblages, indicating restricted access to open marine circulation—hardly surprising given the dominantly intracratonic position of most west Australian basins and the continent’s distance from Tethys during this era (Metcalfe, 1996; 1998; Cocks & Torsvik, 2013). Perth, WA, 18–21 August 2013 Marked climatic changes or local tectonic events can limit detailed interbasin correlations: in Western Australia, climatic variations are not always obvious because faunal and facies changes may be equally attributed to oceanic circulation patterns or the nature of barriers along the Cimmerian continent between west Australia and Paleo-Tethys. For example, extensive Upper Ordovician evaporitic facies in the Canning Basin (once considered Devonian, e.g. Lehmann, 1984, or Silurian, e.g. Brennan & Lowenstein, 2002) imply near-equatorial conditions. Although consistent with paleogeographic reconstructions, this does not give a clear explanation of why evaporites are absent in this basin during the Silurian but present in the Southern Carnarvon Basin at the same time. To explain this apparent Silurian anomaly, it is necessary either to invoke a barrier that restricted oceanic waters accessing the Canning Basin but not the Southern Carnarvon Basin, or to seek a local tectonic explanation. Clearly the two explanations are not mutually exclusive and illustrate how ambiguous the geological record can be. A clearer example of a climatic difference is the abundance of hummocky cross-stratification in the Kungurian (late Early Permian) of the Southern Carnarvon Basin and the seeming absence of this facies in the Canning Basin. The difference can be interpreted as a latitudinally restricted storm belt akin to the ‘roaring forties’ (D. Haig, pers. comm., 2011), even allowing for possible differences in continental topography windward of these basins. Selection of intervals to depict the Paleozoic evolution of Western Australia was primarily based on biostratigraphic resolution (Fig. 3), but other considerations such as regional stratigraphic events (e.g., the end of glacial conditions in the late Sakmarian; Fig. 2) and interval duration were also given some weight. Thus, the Cambrian was divided around the c. 509 Ma Kalkarindji LIP, itself assigned an interval to isolate volcanic from sedimentary facies. The Ordovician was separated into early and middle–late (Fig. 3) based on significant facies change and local breaks close to that level in the Canning and eastern Amadeus basins, whereas the Silurian interval is approximate at best given the extremely poor biostratigraphic control (see above). The major Devonian succession with good age control is the Middle−Upper Devonian, with the reef complexes in the Canning and Southern Bonaparte basins ending late in the Famennian. Although there are local breaks in these successions, data are insufficient to show if any extend beyond one basin. Thus, the Lower Devonian interval was chosen more or less by default, and the uppermost part of the Devonian (i.e. the postreef complex succession) is included with the Tournaisian on sedimentological grounds. In Western Australia the Devonian/ Carboniferous boundary is not associated with a significant change in facies and has only been recognized with reasonable precision in one section (in the Canning Basin), but published accounts (Talent et al., 1993; Young, 1996, p. 107) are brief. In the Southern Bonaparte and Canning basins the upper part of the Tournaisian (Early Carboniferous) is absent due to West Australian Basins Symposium 2013 9 A.J. MORY & P.W. HAINES a disconformity, but the presence of this break in the Southern Carnarvon Basin is uncertain as the Williambury Formation (previously shown as extending from the Tournaisian into the Visean; Hocking et al., 1987) has no internal age evidence. Similarly, the upper limit of the Visean coincides with a disconformity in the Southern Bonaparte and Canning basins, but it is unclear if the units of similar age within the Southern Carnarvon Basin (Harris Sandstone and Quail Formation) correlated with units above or below this break. The ‘upper’ Carboniferous (Serpukhovian–Pennsylvanian) interval presented is subjective as the position of the Carboniferous/ Permian boundary has not been identified clearly in Western Australia (discussed by Mory, 2010). However, a basal Permian disconformity is present on the margins of most basins, with upper Carboniferous strata mostly restricted to local depocentres. The Permian is divided into three intervals based on two widespread spore-pollen/sedimentological datums: the Pseudoreticulatispora confluens/P. pseudoreticulata boundary coinciding with the end of glacial deposition in the Sakmarian (Early Permian), and the Praecolpatites sinuosus/ Microbaculispora villosa boundary coinciding with a change from marine to deltaic facies near the end of the Kungurian (late Early Permian). Apart from the mid-Cambrian interval covering the Kalkarindji LIP (duration <5 m.y.), the duration of the other intervals averages 22 m.y. with a 10–30 m.y. range. The maximum sediment thickness preserved within each interval ranges from 1 to 3.5 km, and averages 2.1 km. Depocentre shifts through time are such that the maximum thickness in any basin is less than 15 km. �on-deposition � erosion � �land �laya�evaporitic includin� sabkha Thickness (m) 0 Alluvial Cambrian �luvial � red bed � eolian Lower delta-plain 1000 Shoreface�beach Shallow marine Shallow-marine carbonate dominated 2000 Glaciomarine Glaciofluvial 3000 Tectonism�intrusives Extrusive volcanics Dykes AJM901a 03.04.13 Figure 4. Key to paleogeographic maps and isopach images. 10 Figure 4 provides a key to the paleogeographic maps and isopach images. The former follow the style of maps produced by the BMR Palaeogeographic Group (1990), and are considerably idealized as they each cover a relatively long interval. Although the edge of depositional environments identified in the paleogeographic maps coincide with the preserved edge on the isopach images, there is little to indicate the original extent of various environments, especially shorelines and shallow-marine facies, across areas with limited or no sedimentary record. The isopach images have been generated mostly from well data with some input from outcrop and seismic sections. Note that the scale is the same for all isopach images (right-hand side of Figs 5–11), with the greatest thickness (3500 m) in the upper Ordovician interval of the Southern Carnarvon Basin, as well as in the middle−upper Devonian of the Fitzroy Trough and possibly the Petrel Sub-basin. The section in the centre of the Petrel Sub-basin attributed to the Carboniferous–Permian by Geoscience Australia (Nicoll et al., 2009a, line 100/03) is almost 7 seconds TWT (i.e. about 20 km thick) but has little to constrain its age. Although it is possible this section was deposited on a rapidly subsiding continental margin, it is anomalous compared to other Paleozoic basins in Western Australia, and the implied thickness is not accepted, at least for this study. Four main phases of basin evolution are evident from the series of paleogeographic maps and isopach images: (a) Cambrian intracratonic deposition, probably associated with the Centralian basins, followed by three extensional phases: (b) Ordovician to Lower Devonian rifting, possibly also intracratonic; (c) Middle Devonian to mid-Carboniferous renewed extension, apparently associated with the Alice Springs Orogeny in central Australia, and (d) mid-Carboniferous to Permian rifting, with a strong east–west component of extension opening a narrow seaway along the western margin of the continent. These phases are discussed below. The late Ediacaran to early Cambrian was a time of orogenesis in central and northwestern Australia, with the locus of uplift in the Musgrave Province separating the Amadeus and Officer basins. The Petermann Orogeny (580– 530 Ma) in this area was probably at least partly coeval with the Paterson Orogeny along the northern margin of the West Australian Craton and the King Leopold Orogeny just north of the Canning Basin (Tyler et al., 2012). Orogenesis probably extended through much of the intervening area now covered by the Canning Basin, although basement events have not been dated there. Petermann Orogeny uplift was accompanied by clastic sedimentation in deltaic, fluvial, alluvial and eolian settings in the adjacent Amadeus (Korsch & Kennard, 1991) and Officer (Morton, 1997) basins (Fig. 5a), but the general lack of fossils precludes differentiating the late Ediacaran from early West Australian Basins Symposium 2013 Perth, WA, 18–21 August 2013 A PALEOZOIC PERSPECTIVE OF WA a) 115° 120° 115° 125° 120° 125° 15° 15° ? ? 20° ? 25° 25° 30° 30° 35° 35° b) 115° 120° ? 20° 125° 115° 15° 15° 20° 20° 25° 25° 30° 30° 120° 125° Thickness (m) 0 1000 2000 35° 35° 500 km 3000 01.03.13 AJM901 Figure 5. Paleogeographic maps (left) and isopach images (right) for the: a) early Cambrian; and b) mid-Cambrian volcanics. Perth, WA, 18–21 August 2013 West Australian Basins Symposium 2013 11 A.J. MORY & P.W. HAINES Cambrian components, except in the more marine central and eastern Amadeus Basin. The thickest syn-Petermann strata of deltaic to alluvial facies are preserved in a foreland-basin setting in the western Amadeus Basin, immediately in front of the Petermann thrust zones (Haines et al., 2012a, b). Coeval eolian sedimentation in the western Officer Basin implies that the uplifted mountain range generated a climatic divide with a rain shadow to the southwest. In the East Kimberley, significant parts of the ~3.7 km thick Louisa Downs Group above the Ediacaran Egan Formation, and ~1.6 km of the Albert Edward Group above the Boonall Dolomite (partially equivalent to the Egan Formation), may be lower Cambrian in age (Figs 2, 5a) in spite of the lack of obvious ichnofauna and the apparent lack of shelly fossils (based on descriptions by Dow & Gemuts, 1969). The Kalkarindji LIP erupted in a brief event at about 509 Ma, approximately coeval with the Epoch 2/Epoch 3 boundary (Evins et al., 2009; Jourdan et al., 2012). Basalt flows in the Ord (Antrim Plateau Volcanics) and Officer (Table Hill Volcanics) basins reach a maximum thickness of about 1500 m in the former suggesting its proximity to a major eruptive centre (Fig. 5b). Geochemically related dykes are present well to the west of preserved flows and the event can be traced as far east as western Queensland, attesting to its continental significance. Flows were channeled along dune corridors in the southern Officer Basin, implying eruption during continuing arid environments of deposition there. Subsidence following the eruption of the Antrim Plateau Volcanics led to shallow-marine to tidal inundation in the Southern Bonaparte and Ord basins, with deposition of mostly marine siliciclastic sediments continuing to the end of the Cambrian in the north (Fig. 6a). The faunas imply a link to the Wiso and Georgina basins to the east, but a more northerly marine connection is also possible. A similar, though intermittently marine, succession was deposited in the eastern Amadeus Basin in the Northern Territory, but a lack of fossil assemblages and identifiable time markers such as the Kalkarindji LIP makes tracing this succession to the western Amadeus Basin equivocal, especially in Western Australia where apparently coeval sections are entirely non-marine. In the Southern Carnarvon Basin, deposition of up to 1500 m of sediment is estimated entirely from seismic data (Iasky et al., 2003). Ordovician to Lower Devonian The Ordovician to Lower Devonian is grouped together largely based on the high level of uncertainty in interbasin correlations. Apart from the Ordovician in the Canning Basin, much of this interval has little age control (Fig. 3); some parts depend on seismic interpretations with little, if any, well (or outcrop) data (e.g. the ?lower Ordovician in the Southern Carnarvon Basin, Iasky et al., 2003; the ?upper Ordovician in the Southern Bonaparte Basin, Mory, 1991). Fault control along the northeastern and southwestern margins of the West 12 Australian Craton is evident over this period (Figs 6b, 7a, b, 8a). For the Southern Carnarvon Basin, the inferred downto-the-east movement (from thicknesses along the western margin of the Merlinleigh Sub-basin) is opposite to that in the Permian (Figs 10b, 11). The Early Ordovician onset of deposition in the Canning Basin contrasts with the Southern Bonaparte Basin, where deposition continued without an obvious break from the Cambrian (Jones, 1971), as well as in the Arafura Basin (Struckmeyer, 2006; Zhen et al., 2012). A similar situation is inferred for the Southern Carnarvon Basin (Iasky et al., 2003). Whether the Nambeet Formation and Wilson Cliffs Sandstone (with thicknesses exceeding 775 m and 731 m, respectively) extend into the upper Cambrian is uncertain at present. The best-known Ordovician sections are from the Canning Basin, with the most marine interval corresponding to the Tremadocian−Darriwilian (Early−Middle Ordovician; e.g. Haines, 2004). Although these sections have clear correlatives in the Amadeus, Wiso and Georgina basins to the east, whether or not they were connected via the supposed Larapintine Seaway remains conjectural. Despite having a wide acceptance (e.g. Webby, 1978; Nicoll et al., 1988; Cook & Totterdell, 1990), Haines & Wingate (2007) indicate that the evidence for this seaway is weak, but concede it may have had a short-lived presence. In our view, evidence for the seaway requires a detailed comparison of non-pelagic shelly fossils between the Amadeus and Canning basins, but existing systematic studies of these Ordovician successions are far from complete. By comparison, an early Ordovician non-marine connection, via the Officer Basin into South Australia, seems more likely (Fig. 6b) assuming the Lennis Sandstone in Western Australia is of this age. By the late Ordovician, marine influence waned and deposition became paralic in the Canning Basin, but was widespread in the Southern Carnarvon Basin and probably across much of the Petrel Sub-basin of the Southern Bonaparte Basin (Fig. 7a). Evaporitic conditions were periodically established in the Canning Basin, forming thick halite accumulations (Haines, 2009); the salt-bearing succession in the Bonaparte Basin is inferred to be coeval (Fig. 7a; note that Mory, 1991, suggested a Silurian to Early Devonian age). Barriers allowing intermittent ingress of seawater presumably developed near the northwestern ends of these basins, or across the Cimmerian continent. The sandy red beds in the Southern Carnarvon Basin (Tumblagooda Sandstone) show elements of marine influence in the ichnofauna (Hocking, 1991; Trewin & McNamara, 1995) but, as with the other basins, connections to Tethys could only have been indirect at this time. Silurian deposition appears to have been restricted to the Southern Carnarvon and Canning basins (Fig. 7b). Mory et al. (1998) inferred long periods of isolation from Tethys at this time, based on limited conodont assemblages from the Dirk Hartog Group (Southern Carnarvon Basin). Even fewer ages are available from the apparently coeval Worral Formation in West Australian Basins Symposium 2013 Perth, WA, 18–21 August 2013 A PALEOZOIC PERSPECTIVE OF WA a) 115° 120° 125° 115° 15° 15° 20° 20° 120° 125° ? ? ? 25° ? 25° ? ? 30° 30° 35° 35° b) 115° 120° 115° 125° 15° 15° 20° 20° 25° 25° 30° 30° 120° 125° Thickness (m) 0 1000 2000 35° 35° 500 km 3000 01.03.13 AJM902 Figure 6. Paleogeographic maps (left) and isopach images (right) for the: a) late Cambrian; and b) early Ordovician. Perth, WA, 18–21 August 2013 West Australian Basins Symposium 2013 13 A.J. MORY & P.W. HAINES a) 115° 120° 125° 15° 15° 20° 20° 25° 25° 30° 30° 35° 35° b) 115° 120° 125° 115° 120° 125° 115° 120° 125° 15° ba rri er 15° 20° ? ba rri er 20° 25° 25° 30° 30° Thickness (m) 0 1000 2000 35° 35° 500 km 3000 06.03.13 AJM903 Figure 7. Paleogeographic maps (left) and isopach images (right) for the: a) middle–late Ordovician; and b) Silurian. 14 West Australian Basins Symposium 2013 Perth, WA, 18–21 August 2013 A PALEOZOIC PERSPECTIVE OF WA the Canning Basin, but this lack does not necessarily mean the basin was less-connected with Tethys than the Southern Carnarvon Basin at this time. The overall similarity in lithology (apart from the lack of salt in the Canning Basin), inferred restricted marine conditions for both basins, and low overall deposition rate (<50 m/m.y.), indicates comparable intracratonic controls, in many ways similar to those for the Early Devonian. In the Early Devonian, deposition fluctuated between very shallow-marine and sabkha, and fluvial–eolian conditions in the Southern Carnarvon Basin, whereas fluvial–eolian conditions dominate in the Canning Basin (Fig. 8a). In spite of the likelihood that both regions lay in the same climatic belt, no direct connections between the two can be demonstrated for this epoch. According to Cocks & Torsvik (2013), rifting between Sibumasu and South China-Annamia (Indochina) commenced in the Early Devonian approximately parallel to the present North West Shelf, whereas previous extension and subduction in the region is shown as virtually orthogonal to this direction. However, this change is not obvious in northwestern Australia, where northerly extension is indicated throughout the Paleozoic based on the west-northwesterly orientation of major depocentres within the Canning, Southern Bonaparte and Arafura basins. Middle Devonian to mid-Carboniferous The Middle−Late Devonian (Fig. 8b) saw the widespread development of carbonate platforms and reefs, albeit locally interrupted by thick clastic deposits along the faulted margins of the Southern Carnarvon, Canning and Bonaparte basins (e.g. Hocking et al., 1987; Playford et al., 2009; Mory & Beere, 1988; respectively). Evidence for interbasin connection is clear for the first time in the Paleozoic, with fluvial clastic deposition in the Ord Basin linked with deltaic deposition in the northeastern Canning Basin (Mory & Beere, 1988), and the likely extension of carbonate deposition around the margin of at least part of the North Australian Craton (Playford et al., 2009). In the Canning Basin, the Late Devonian probably had the most direct connection to Tethys oceanic circulation of any part of the Paleozoic, based on faunal similarity with that of Laurentia (see above). Although terranes such as Sibumasu, and other blocks within the Cimmerian continent, lay between Laurentia and west Australia (Baillie et al., 1994; Metcalfe, 1998, 2013; Cocks & Torsvik, 2013), they presumably did not hinder oceanic circulation into the Canning Basin at this time. Influxes of siliciclastic sediment were locally dominant throughout the Frasnian of the Southern Bonaparte Basin, intermittent for the entire Late Devonian in the Canning Basin, and dominant throughout the Famennian in the Carnarvon Basin. These deposits are inferred to be synorogenic in origin following Haines et al. (2001), and are likely related to the Alice Springs Orogeny in central Australia. In the case of the Southern Bonaparte and Ord basins, strike-slip movements Perth, WA, 18–21 August 2013 along the Halls Creek Mobile Zone were probably related to north-south shortening (Thorne & Tyler, 1996), but other coarse-grained clastic deposits, especially in the Southern Carnarvon Basin, are too remote from central Australia to be explained so easily. Compared to the basin margins, the thick mudstone sections in the depocentres have been inadequately investigated, partly as they are buried relatively deeply, and seismic data are ambiguous. Nevertheless, at least 3.5 km of sediment accumulated over relatively wide areas during this period, most notably in the Fitzroy Trough and possibly also in the Petrel Sub-basin (Fig. 8b), denoting an episode of significant extension. By the end of the Devonian, carbonate deposition declined, even though carbonate banks were still present locally, especially in the Canning Basin (Fig. 9a), and sedimentation contracted markedly into the Petrel, Fitzroy-Gregory and northern Merlinleigh sub-basins (Figs 9a, b). This period was also characterised by rapid deposition (~240 m/m.y. in the latest Devonian to Tournaisian), implying continued tectonic control, and presumably is related to late phases of the Alice Springs Orogeny. Mid-Carboniferous to Permian The onset of glacial conditions in the mid-Carboniferous was also marked by high rates of sedimentation in elongate zones, such as the half-grabens along the western rift margin of the West Australian Craton, the Fitzroy Trough and Petrel Sub-basin. However, the elongate zone of deposition in the centre of Western Australia (Fig. 10a) is more likely to represent an intracratonic sag, as there is no clear structural control and deposits are relatively thin. The relationship between glaciation and tectonics has been explained as a by-product of the merging of Gondwana and Laurasia, and uplift along the present position of the Gamburtsev Subglacial Mountains in East Antarctica (Veevers, 2009). Widespread deposition across the centre of west Australia in the earliest Permian (Fig. 10b) implies a decrease in glacial conditions, freeing sediment caught up in, or trapped by, ice in the late Carboniferous. This episode produced the most widespread deposits of the Paleozoic, with significant onlap onto the West and North Australian cratons. The establishment of sedimentation along the entire length of the narrow western rift during the Late Carboniferous to Early Permian indicates strengthening of east-west extension, presumably associated with the rifting and separation of the Cimmerian continent from Gondwana (Baille et al., 1994; Metcalfe, 1998; 2013; Cocks & Torsvik, 2013). East-west extension continued to dominate throughout the remainder of the Permian in the western basins, whereas sedimentation in the northern basins contracted into long-lasting depocentres (Figs 10b, 11a, b). However, Guadalupian (Middle Permian) foraminifera of Tethyan aspect from the northern Perth Basin imply a more direct seaway around west Australia at this time, bypassing the earlier interior seaway (Fig. 11b). Although poorly West Australian Basins Symposium 2013 15 A.J. MORY & P.W. HAINES a) 115° 120° 125° 15° 15° 20° 20° 25° 25° 30° 30° 35° 35° b) 115° 120° 125° 15° 15° 20° 20° 25° 25° 30° 30° 115° 120° 125° 115° 120° 125° Thickness (m) 0 1000 2000 35° 35° 500 km 3000 01.03.13 AJM904 Figure 8. Paleogeographic maps (left) and isopach images (right) for the: a) early Devonian; and b) middle–late Devonian. 16 West Australian Basins Symposium 2013 Perth, WA, 18–21 August 2013 A PALEOZOIC PERSPECTIVE OF WA a) 115° 120° 125° 115° 15° 15° 20° 20° 25° 25° 30° 30° 35° 35° 120° 125° Western Australia b) 115° 120° 115° 125° 15° 15° 20° 20° 25° 25° 30° 30° 120° 125° Thickness (m) 0 1000 2000 35° AJM905 35° 500 km 3000 01.03.13 Western Australia Figure 9. Paleogeographic maps (left) and isopach images (right) for the: a) latest Devonian – earliest Carboniferous; and b) early Carboniferous. Perth, WA, 18–21 August 2013 West Australian Basins Symposium 2013 17 A.J. MORY & P.W. HAINES a) 115° 120° 125° 115° 120° 125° 115° 120° 125° 15° 15° Ice 20° 20° 25° 25° Ice Ice 30° 30° 35° 35° b) 115° 120° 125° 15° 15° Ice 20° 20° 25° 25° Ice 30° 30° Thickness (m) 0 1000 2000 35° 35° 500 km 3000 27.02.13 AJM906 Figure 10. Paleogeographic maps (left) and isopach images (right) for the: a) late Carboniferous; and b) earliest Permian. 18 West Australian Basins Symposium 2013 Perth, WA, 18–21 August 2013 A PALEOZOIC PERSPECTIVE OF WA a) 115° 120° 125° 15° 15° 20° 20° 25° 25° 30° 30° 35° 35° b) 115° 120° 125° 15° 15° 20° 20° 25° 25° 30° 30° 115° 120° 125° 115° 120° 125° Thickness (m) 0 1000 2000 35° 35° 500 km 3000 01.03.13 AJM907 Figure 11. Paleogeographic maps (left) and isopach images (right) for the: a) early Permian; and b) middle–late Permian. Perth, WA, 18–21 August 2013 West Australian Basins Symposium 2013 19 A.J. MORY & P.W. HAINES dated, intrusive volcanic rocks in the Canning and Southern Carnarvon basins were probably related to rifting along the western margin of the continent throughout the Permian, prior to the creation of Meso-Tethys (Metcalfe, 1998; 2013). Conclusions Interbasinal stratigraphic correlations underpinning explanations of the Paleozoic evolution of Western Australia are mostly reliant on biostratigraphic studies as the influence of many events evident in the stratigraphic record appears to have been diminished by the distance between basins, and their isolation between Precambrian terranes. In addition, volcanic facies that can be dated radiometrically are rare. However, biostratigraphic resolution is low largely due to the Cimmerian continent selectively hindering the spread of Tethyan biotas into Western Australian basins for much of the Paleozoic. In spite of some notable exceptions, such as parts of the Ordovician and deeper water facies of the Upper Devonian reef complexes in the Canning Basin, overall the Cambrian to Carboniferous successions are more poorly age constrained than the Permian. Consequently, only a relatively coarse subdivision of the Paleozoic into intervals of 10 to 30 m.y. is defensible to generate paleogeographic maps and isopach images covering the State. Evidence for tectonism and basement heating during the Ediacaran–Cambrian transition is scattered around Paleozoic basin margins, particularly in the Canning Basin, suggesting these events influenced later subsidence and deposition. Future dating of metamorphic overprints around or beneath the Canning and other basins may further refine the role of tectonism in basin development for the Paleozoic of Western Australia. Paleozoic deposition in Western Australia was largely intracratonic and appears to have had strong Centralian influences in the Cambrian, followed by roughly northsouth extension throughout most of the remainder of the Paleozoic. The main phases of basin evolution following the Cambrian are: Ordovician to Lower Devonian rifting, Middle Devonian to mid-Carboniferous renewed extension; and mid-Carboniferous to Permian rifting with a strong east-west component of extension. It is likely that the shallow Precambrian basement areas around the margins of the West and North Australian cratons, now largely covered by relatively thin Mesozoic strata, were also depositional edges throughout most of the Paleozoic, and that sedimentation continued northwest across the Cimmerian continent. Onshore, the influence of tectonic events associated with Cimmeria and other continental blocks along the western edge of the Australian continent do not become obvious until the mid-Devonian, coincident with the Alice Springs Orogeny in central Australia. The opening of a narrow seaway along the western margin of the West Australian Craton in the late Carboniferous to Permian was a precursor to Mesozoic rifting 20 along the North West Shelf, when stresses almost orthogonal to those in the Paleozoic became dominant. This, in combination with the deposition of thick Mesozoic sedimentary successions, effectively masks earlier events offshore. Braun et al. (1991) explains the partitioning of late Paleozoic compressive stresses in central Australia coeval with extension to the west as stress decoupling along the Lasseter Shear Zone, whereas Klootwijk (2013) suggests partitioning with strike-slip movement is limited to the Halls Creek Fault. However, the underlying mechanism for such partitioning of stresses remains ambiguous because of the sparse data available both to the south (Antarctica) and north (West Papua) of the continent. Given the lack of data from these regions, it is doubtful that more detailed modelling of the assembly and dispersal of continental blocks in the Neoproterozoic to Paleozoic would allow a more specific explanation. Another difficulty is that the Cambrian–Permian section in the Goulburn Graben (Arafura Basin) lies east of the Hall Creek Fault trend (and the putative Lasseter Shear Zone), but is an extensional feature, indicating Paleozoic partitioning of the continent must have involved more than a single structural feature. A possible explanation for this partitioning is the interplay of subduction along the northeastern margin of the Australian continent versus separation of various blocks within Cimmeria (Metcalfe, 1998, 2013). 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