Contrasting depositional histories, detrital zircon ...

Did the Larapintine Seaway link the Canning and Amadeus basins?

Contrasting depositional histories, detrital zircon provenance and hydrocarbon systems: Did the Larapintine Seaway link the Canning and Amadeus basins during the Ordovician? PW Haines1, 2 and MTD Wingate1 The concept of an open east–west seaway across Australia during the Ordovician (Larapintine Seaway) has had a long history, and is now a ubiquitous feature on Ordovician palaeogeographic reconstructions. The Canning and Amadeus basins are both seen as major components of this hypothetical seaway, although the Ordovician of these respective basins show significant differences in stratigraphy, depositional style, and petroleum systems. In addition, fossil faunas show a high degree of endemism, with the few faunal species that are reported from both basins being mostly restricted to cosmopolitan taxa. The Ordovician of the Amadeus Basin is dominated by high-energy sandstone facies, whereas the Canning Basin was largely starved of such material and equivalent aged rocks are dominated by calcareous and fine-grained clastic sediment. Published detrital zircon age data from the Early Ordovician of the Amadeus Basin show a much closer match to early Palaeozoic sandstones of the eastern palaeo-Pacific continental margin than to local basement sources, supporting derivation of most of the sand via marine currents from the east before being cycled around the basin. Detrital zircon age spectra from four sandstone samples of similar age in the Canning Basin are unlike that of the Amadeus Basin, but show significant similarity to adjacent basement, including older sedimentary basins. The inference is that, at least during the Early Ordovician, there is no evidence of sand interchange between these basins; rather, they are more likely to have formed as separate embayments open to the east and west of the continent, respectively. We cannot rule out either brief or very restricted connections, but there seems to be no compelling reasons for hypothesizing such links. The alternate palaeogeographic models have implications for hydrocarbon prospectivity, particularly with regards to the distribution of source and reservoir facies. The lack of an open seaway, if this was indeed the case, may actually increase the source rock potential towards basin extremities, due to reduced circulation in these areas. In the case of the Canning Basin, increasing content of shoreline sands around the eastern end of the basin, as appears to be the case from limited well and outcrop data, may provide suitable reservoirs, which are generally elusive further west.

Introduction Palaeogeographic constraints are important to consider when developing basin-scale hydrocarbon system models and assessing exploration risks. This is particularly true in underexplored frontier provinces, such as the Ordovician portions of a number of onshore Australian basins. Ordovician sedimentation was widespread in Australia, including comparatively deep-water continental margin clastic systems across eastern Australia, and shallow-marine to paralic and periodically non-marine deposition in a number of inland and northern Australian basins. The thick and predominantly shallow-marine successions of the Amadeus and Canning basins (Figure 1) are particularly well developed, but in the case of the Canning Basin, not well exposed. Although now preserved as separate basins, it is widely believed that the Amadeus and Canning basins were linked as part of a transcontinental seaway, the Larapintine (or Larapinta) Seaway, during at least part of the Ordovician. This paper summarises the early Palaeozoic depositional histories of these basins and explores the evidence for and against the former existence of such a seaway, outlining avenues for further testing. Such debate is not only of academic significance, but has potential implications for hydrocarbon exploration in both basins. The early Palaeozoic of the Canning and Amadeus basins belongs to the Larapintine 2 petroleum system of Bradshaw (1993). The Amadeus Basin has produced significant oil and gas from Ordovician plays, and whereas the Canning Basin is yet to see production from the Ordovician, it contains some excellent source rocks and numerous hydrocarbon shows. Both remain ‘frontier basins’, significantly underexplored for hydrocarbon resources. The southeastern and western extremities of the Canning and Amadeus basins, respectively, are particularly poorly explored and their prospectivity is largely a matter of speculation. Assumptions regarding the distribution of source, reservoir, and seal facies – essential elements in any exploration model – will be dependant on palaeogeographic assumptions, including the presence or absence, nature, and duration of any open seaway linking the provinces. Larapintine seaway: History of a concept The hypothetical concept of a shallow east–west transcontinental seaway across Australia during the Ordovician is not new. Early illustrations of such a seaway can be found in Browne (1947) and David (1950). In 1947, Ordovician rocks were only known with certainty from the central Amadeus Basin (then “Amadeus trough”), but it was assumed that a sedimentary succession, known to outcrop in the western Amadeus Basin in Western Australia, would be shown to contain Ordovician rocks, and that these could then extend further west beneath the Great Sandy Desert. This prediction came partly true with the discovery of marine

Keywords: Northern Territory, Western Australia, Larapintine Seaway, Amadeus Basin, Canning Basin, Ordovician, palaeogeography, reconstruction, palaeogeographic maps, shorelines, stratigraphy, biostratigraphy, zircon, dates, absolute age, petroleum exploration, petroleum potential. 1

Geological Survey of Western Australia, Department of Industries and Resources, 100 Plain St, East Perth, WA 6004. 2 Email: [email protected]

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Figure 1. Present-day configuration of Neoproterozoic–Palaeozoic sedimentary basins in central and northwestern Australia with the location of preserved Ordovician sediments highlighted. Metamorphosed equivalents of Ordovician sedimentary rocks are inferred to be integrated with basement between the Amadeus and Georgina basins. Producing (Mereenie and Palm Valley) and significant non-producing (Pictor) Ordovician hydrocarbon fields are also indicated.

Ordovician rocks in outcrop in what is now the northern Canning Basin in 1949 (Guppy and Öpik 1950). A similar Ordovician succession was subsequently intersected in the subsurface in the western parts of the Canning Basin during petroleum exploration drilling from the mid-1950s onward. However, mapping of the western Amadeus Basin revealed that, apart from very small outliers of Ordovician strata near the Northern Territory–Western Australian border, the rocks of this part of the basin are mostly of Neoproterozoic age (Wells et al 1965, 1970). The term Larapintine Sea, first used without definition by Keble and Benson (1939), was given to the hypothetical Ordovician seaway by Webby (1978). ‘Larapintine’ being derived from the Larapintine Series of Tate (1896), later renamed the Larapinta Group (Prichard and Quinlan 1962), encompassing Upper Cambrian to Ordovician strata in the Amadeus Basin (Larapinta is the Aboriginal name for the Finke River). Later authors have referred interchangeably to the Larapintine or Larapinta Seaway (or Sea). Despite the lack of preserved stratal continuity, the seaway concept has continued to grow in popularity and has appeared with various interpreted configurations (see illustrations in Clarke et al 1962, Brown et al 1968, Wells et al 1970, Webby 1978, Nicoll et al 1988, Nicoll et al 1989, BMR Palaeogeographic Group 1990, Cook and Totterdell 1990, 1991, Walley et al 1991). A rare exception was Veevers (1976) who interpreted the Ordovician palaeogeography of the region as separate embayments,

stating: “The marine connection between the Amadeus and Canning basins, postulated by (earlier) authors, is not supported by the facies trends of terrigenous sedimentation along the neighbouring margins of these basins and carbonate rock away from these margins”. Although many authors schematically figured a seaway linking straight through the western end of the present Amadeus Basin to the southeastern end of the Canning Basin, some later workers (eg Nicoll et al 1989, Walley and Cook 1991) realised that facies and thickness trends in the Ordovician of the Amadeus Basin curve northwest to be truncated by the present tectonic margin of this basin. This implies that any link to the west that may have existed probably lay to the north, perhaps via the Ngalia Basin. This is also demonstrated by the thin outliers of Upper Ordovician strata in the western Amadeus Basin that unconformably overlie deformed Neoproterozoic sediments. The duration of the hypothetical Larapintine Seaway varies somewhat between authors. At the earliest, it may have conceivably developed in the late Tremadoc, synchronously with the oldest sediments in the Canning Basin, or somewhat later in the Arenig, as sediments of the respective basins are speculated to have prograded east and west. The approximately synchronous development of highly restricted hypersaline conditions in the Caradoc in both basins is generally seen as the time of termination of the seaway (Walley and Cook 1991).

Did the Larapintine Seaway link the Canning and Amadeus basins? Basin depositional histories

onshore Canning Basin can be subdivided into elongate northern and southern depocentres by relatively thin successions on the contiguous Broome (northwest) and Crosslands (southeast) platforms. To the north, thick (up to 15 km) successions are preserved in the Fitzroy Trough (northwest) and Gregory Subbasin (southeast), whereas to the south, thick successions are preserved in the Willara (northwest) and Kidson (southeast) sub-basins. The Fitzroy Trough and Gregory Sub-basin are flanked to the south by the Jurgurra, Mowla, and Barbwire terraces, transitional domains to the Broome and Crosslands platforms, respectively. The present northern margin of the basin is defined by the Lennard Shelf, northeast of the Fitzroy Trough, and the Billiluna Shelf and transitional Balgo and Betty terraces, northeast of the Gregory Sub-basin. The eastern margin of the basin is defined by thin successions on the Ryan Shelf, flanking the Crosslands Platform and Kidson Subbasin, whereas the Tabletop and Anketell shelves likewise define the southwest margin. Because of very poor or no outcrop of the older parts of the basin, and very limited well and seismic control in this area, it remains unclear to what extent the modern boundary reflects original depositional basin margins. Based on the Larapintine Seaway concept, it is generally assumed that for early Palaeozoic strata, the present eastern margin is tectonic and erosional.

Canning Basin The northwest–southeast-oriented Canning Basin extends from close to the Northern Territory border to offshore northern Western Australia, covering a total area of approximately 640 000 km2 (530 000 km2 onshore). It preserves a long and complex multi-phase depositional history, ranging from Early Ordovician to Cretaceous (Forman and Wales 1981, Brown et al 1984, Kennard et al 1994). The basin is surrounded by Proterozoic and Archaean provinces, and underlain by deformed sedimentary, metasedimentary and igneous rocks of mostly uncertain age. At least some of the basement to the Canning Basin is likely to include Neoproterozoic strata of the former Centralian Superbasin (Walter et al 1995), which was deformed by the late Neoproterozoic–earliest Palaeozoic Petermann and Paterson orogenies, as such rocks are present along the southern and southeastern margins of the basin. The Canning Basin has been internally divided into a series of sub-basins, platforms, shelves, and terraces bounded by northwest–southeast-trending fault systems (Shaw et al 1994, Hocking et al 1994, Figure 2). At the broadest scale, the

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Figure 2. Canning Basin showing major tectonic elements and petroleum exploration wells. The location of detrital zircon samples from wells Looma-1 and Acacia-2, and from outcrop samples of the Gap Creek and Carranya formations are indicated.

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Haines and Wingate Early Palaeozoic stratigraphy

basin and Ryan Shelf, the Nambeet Formation is replaced by the Wilson Cliffs Sandstone, with equivalents of the Willara Formation apparently missing, based on conodont data. Outcropping Ordovician strata near the southeastern end of the Lennard Shelf have been assigned to the Prices Creek Group, comprising the Kunian Sandstone, Kudata Dolomite, and Emanuel and Gap Creek formations, in ascending order, ranging from late Tremadoc to mid Arenig age (Nicoll et al 1993). Based on biostratigraphy, the Prices Creek Group correlates with the Nambeet and Willara formations, with any younger Ordovician rocks once deposited in this area having been removed by Devonian erosion. Sandstone and conglomerate, containing rare Early Ordovician macrofossils, outcrop in limited areas on the Billiluna Shelf and have been assigned to the Carranya Formation (Blake et al 1977). The fossils in

The early Palaeozoic stratigraphy of the Canning Basin (Figure 3) is somewhat complicated by the fact that separate stratigraphic nomenclatures have been erected for the subsurface and for the limited outcropping Ordovician strata along the northeastern basin margin. In the subsurface, four fossiliferous marine units are recognised in most areas, the Nambeet, Willara, Goldwyer and Nita formations, in ascending order ranging from late Tremadoc (Early Ordovician) to Llanvirn (Middle Ordovician). The Nita Formation is overlain, in most areas conformably, by the marginal-marine and evaporitic Carribuddy Group, comprising six formations ranging in age from Middle Ordovician to Silurian. In the far southeast of the Kidson Sub-

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Figure 3. Early Palaeozoic stratigraphy of the Canning and Amadeus basins. Major hydrocarbon source and reservoir units are indicated. Time correlation between these basins is based largely on conodonts (after Jones et al 1998). Time slices (TS1–4) refer to Figure 4.

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Did the Larapintine Seaway link the Canning and Amadeus basins? the Carranya Formation do not allow precise correlation with formations elsewhere, but are generally assumed to correlate with the Nambeet Formation and equivalents.

packages ranging from Neoproterozoic to late Palaeozoic in age (Wells et al 1970, Lindsay and Korsch 1991). It has an east–west orientation and, although its western exposed extremity is directly adjacent to the Canning Basin, there is no continuity of similar-aged strata. Neoproterozoic rocks of the western Amadeus Basin presumably dip beneath the younger Canning Basin sedimentary rocks. The early Palaeozoic component of the Amadeus Basin is restricted to the central and eastern parts of the basin (Figure 1) and is generally separated from similar-aged rocks of the Canning Basin by approximately 250 km. Small thin outliers of Late Ordovician sedimentary rocks are present near the Western Australia–Northern Territory border, but even at that stratigraphic level, the separation today is still 180 km. The northern margin of the Amadeus Basin is of tectonic origin, dating from the late Palaeozoic Alice Springs Orogeny; isopachs and facies trends are clearly truncated along this boundary (Wells et al 1970, Gorter 1991, Lindsay and Korsch 1991).

Early Palaeozoic depositional history K-Ar dating indicates a period of fault activity along the northern margin of the Canning Basin from the latest Neoproterozoic to Late Cambrian (560–500 Ma; Shaw et al 1992). Deposition began shortly thereafter during a phase of subsidence (Samphire Marsh Movement), following the cessation of contractional tectonic activity. Transgressive clastic sediments spread across the basin, forming the Kunian Sandstone, Carranya Formation and basal parts of the Nambeet Formation and Wilson Cliffs Sandstone. The oldest known basal sediments, of Tremadoc age, are present in early-formed depocentres in the far west and in the lower Prices Creek Group of the eastern Lennard Shelf (Nicoll 1993, Nicoll et al 1993). Elsewhere, the basal sediments are generally of early Arenig age, suggesting progressive transgression onto higher areas and to the southeast. With continued transgression, the depositional environment deepened to allow accumulation of carbonate and mudstone, comprising the Emanuel and upper Nambeet formations, whereas shallower nearshore sands continued to be deposited around the eastern end of the basin (upper Wilson Cliffs Sandstone and possibly Carranya Formation, Figure 4a). During highstand, an extensive shallow carbonate platform, with only minor siliciclastic input, developed across the basin in the Gap Creek and lower Willara formations. Localised shallow- to marginal-marine sand bodies (Acacia Sandstone Member of the Willara Formation) and coincident non-deposition or erosion at the eastern end of the basin indicate relative sea level fall during the mid Arenig (Figure 4b). A renewed phase of transgression led to deepening, with platform carbonates of the upper Willara Formation giving way to basinal shales and minor carbonates of the Goldwyer Formation of Llanvirn age (Figure 4c). The Goldwyer Formation records two major transgressive– regressive cycles, expressed in basinal areas as an alternation between comparatively deep-water shales and thin-bedded carbonates, and shallow platformal carbonates. The upper cycle culminated in deposition of intertidal to supratidal carbonates of the Nita Formation. Above the Nita Formation, deposition of the Carribuddy Group took place mostly in supratidal environments, with periodic deposition of thick evaporite units in large hypersaline lakes, lagoons or embayments (Figure 4d). The group is predominantly composed of redbed mudstones, dolomitic in part, with halite dominating the evaporite units. Depositional thicknesses of halite locally exceed 700 m, suggesting a marine source of the brines, perhaps percolating through porous barriers or over a shallow sill separating the salt basin from the ocean to the west. Rare conodonts indicate that deposition of this package extended into at least the Early Silurian (Nicoll et al 1994).

Palaeozoic stratigraphy and depositional history Unlike the Canning Basin, which was initiated in the Early Ordovician, there is no fundamental boundary between sediments of this age and older successions in the Amadeus Basin. However, an increase in the rate of subsidence, indicated by a substantial increase in the thickness/time ratio, characterises the Upper Cambrian to Upper Ordovician Larapinta Group, in comparison with the underlying Cambrian Pertaoorrta Group. The Larapinta Group, up to 2300 m thick, comprises the Pacoota Sandstone, Horn Valley Siltstone, Stairway Sandstone, Stokes Siltstone and Carmichael Sandstone, in ascending order (Figure 3). The Larapinta Group is mostly of shallow-marine origin and is volumetrically dominated by sandstone. Detailed seismic and well log correlations allow the recognition of a number of depositional sequences composed of smallerscale shallowing-upward parasequence sets (Gorter 1991). Because of the progradation of sequence packages through time, most are separated by variable duration hiatuses, depending on location. The Pacoota Sandstone straddles the Cambrian–Ordovician boundary and is notably cyclic on several scales, with shallowing-upward cycles typically ranging from shallow subtidal to high-energy shoreface/intertidal and occasionally fluvial environments. Palaeocurrent data indicate currents mainly from the west (Wells et al 1970, Figure 4a). In general, palaeontological evidence of marine influence decreases from east to west. Gorter (1991) recognised a number of hiatuses within the interval, generally increasing in magnitude to the west. The generally deeper-water Horn Valley Siltstone, of early to middle Arenig age, comprises chemically reduced and in part organic-rich mudstone, fine sandstone with thin carbonate interbeds (Figure 4b). The contact with shallowmarine sandstone of the overlying late Arenig to Llanvirn Stairway Sandstone is a significant hiatus in some areas, with progressive removal of upper Horn Valley Siltstone in the east (Gorter 1991). Palaeocurrents in this unit are directed mainly towards the west-northwest (Wells et al 1970, Cook 1972, Figure 4c). Mudstones with carbonate interbeds characterise the Llanvirn to Caradoc Stokes Siltstone.

Amadeus Basin The Amadeus Basin in central Australia occupies an area of about 170 000 km2 and contains several major depositional

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Figure 4. Schematic time-slices with current basin configurations; see Figure 3 for stratigraphic position. (a) Early Arenig during deposition of the upper Nambeet Formation, Wilson Cliffs Sandstone, Emanuel and Carranya formations in the Canning Basin and upper Pacoota Sandstone in the Amadeus Basin. (b) Late Arenig during deposition of the Willara and Gap Creek formations in the Canning Basin and Horn Valley Siltstone in the Amadeus Basin. (c) Llanvirn during deposition of Goldwyer Formation in the Canning Basin and Stairway Sandstone in the Amadeus Basin. (d) Caradoc–Ashgill during deposition of the Carribuddy Group in the Canning Basin and Carmichael Sandstone in the Amadeus Basin (modified after Cook and Toterdell 1990).

Did the Larapintine Seaway link the Canning and Amadeus basins? Isolated thin outliers near the Western Australia–Northern Territory border that have been correlated with the Stokes Siltstone on biostratigraphic grounds (R Nicoll pers comm), are the westernmost documented progradation of Ordovician sediments in the Amadeus Basin. Deposition was initially fully marine, but became shallower and more saline with time, suggesting the development of restricted conditions, and culminated in supratidal hypersaline deposits lacking marine fossils (Walley and Cook 1991). In the Amadeus Basin, the Caradoc records a change in tectonic style from extensional to compressional and a permanent change in the style of sedimentation (Lindsay and Korsch 1991). The compressional Rodingan Movement, accompanied by substantial uplift and erosion in the eastern Amadeus Basin and adjacent basement, was the first in a series of such events, collectively termed the Alice Springs Orogeny, using the extended sense of this term (Haines et al 2001a). In the southern, central and western Amadeus Basin, immature deltaic clastic sediments of the Carmichael Sandstone were deposited unconformably over the Stokes Siltstone (Figure 4d). The Larapinta Group is unconformably overlain, at least locally, by the Mereenie Sandstone. It is biostratigraphically poorly age constrained between Late Ordovician and Early Devonian, although palaeomagnetic evidence supports a mainly Silurian age (Li et al 1991, Haines et al 2001a). The formation is of aeolian, fluvial and locally marginal-marine origin, deposited in what was probably a foreland basin, adjacent to elevated topography located to the north. The final preserved depositional phase comprises thick wedges of generally coarsening-upward non-marine clastic sediment of the Devonian Pertnjara and Finke groups, deposited synchronously with the early part of the main phase of the Alice Springs Orogeny (Haines et al 2001a). Faunal links between Amadeus basins

Canning

documented. There have been few recent studies, with most work summarised in Shergold et al (1991) and Webby et al (2000). Recent Late Ordovician conodont studies were reported in Zhang et al (2003), while trilobites of the Pacoota Sandstone and Horn Valley Siltstone were reported in Shergold (1991) and Laurie (2006), respectively. The general conclusion that can be drawn from published species lists and from comments in Webby et al (2000) is that macrofossils display a high degree of endemism in both basins, and the rare species in common between the basins are generally those of cosmopolitan distribution. One example is the small epipelagic trilobite Carolinites genacinaca which is known from the Arenig of both basins (Laurie and Shergold 1996, Laurie 2006), but this is probably the most widely distributed of all known trilobite species, having apparently encircled the planet between palaeolatitudes of 30°N and 30°S (McCormick and Fortey 1999). Only one other trilobite, Lycophron freemani, is tentatively identified in both basins (Laurie 2006). This species, common in the middle Horn Valley Siltstone of the Amadeus Basin, is probably synonymous with a trilobite in the lower Goldwyer Formation, assigned by Legg (1976) to ?Aulacoparina sp (Laurie 2006). Of numerous known nautiloid species, many of which were nektonic and thus likely to be widely distributed in the absence of barriers, only two species, Lobendoceras emanuelense and Anthoceras warburtoni are documented in both basins, but the former is also known from Siberia (Webby et al 2000). Even at the generic level, the majority of taxa are restricted to one or the other basin. Conodonts show more species in common than do macrofossils, with the identification of the same zones allowing correlation of lithologically very different units in the respective basins for the pre-Carribuddy Group interval (Nicoll and Laurie 1997, Jones et al 1998); this is the basis of the correlation presented in Figure 3. However, of the 26 species listed in the Ordovician of the Canning Basin (Nicoll 1993), only six are also listed in the Amadeus Basin (Cooper 1981, Shergold et al 1991, Zhang et al 2003), and most of these are cosmopolitan species found on several continents. Possible reasons for the general dissimilarity of documented faunas between these basins will be discussed later.

and

The Ordovician successions of the Canning and the Amadeus basins contain an abundant marine macro- and microfauna, which is not yet fully documented. In the Canning Basin, studies have been carried out on trilobites (Legg 1976, 1978, Laurie and Shergold 1996, Laurie 1997), graptolites (McTavish and Legg 1972, Legg 1976, 1978, Skwarko 1967, 1974), brachiopods (Laurie 1997, Brock and Holmer 2004), nautiloids (Teichert and Glenister 1952), large organic walled microfossils (chitinozoans, scolecodonts, hydrozoans and foraminiferal linings; Foster et al 1999, Winchester-Seeto et al 2000) and conodonts (McTavish and Legg 1972, Nicoll 1984, Watson 1988, Nicoll 1993, Nicoll et al 1993, 1994). Collections have been documented from most intervals below the Carribuddy Group. The Carribuddy Group is generally devoid of age-diagnostic fossils, with the exception of conodonts (Nicoll et al 1994) and palynomorphs (Foster and Williams 1991) over very restricted intervals. Despite a much earlier beginning to investigations of the Amadeus Basin faunas, starting with collections from scientific expeditions in the 1880s and 1890s, the faunas are generally less well known than age equivalents in the Canning Basin. This is in part because preservation is typically poor in the thick sandstone intervals, but even the richly fossiliferous Horn Valley Siltstone is incompletely

Early Palaeozoic hydrocarbon systems Canning Basin Ordovician rocks in the Canning Basin are associated with oil and gas shows in numerous wells, but to date, an economic discovery at this stratigraphic level has remained elusive. The most significant discovery has been in Pictor-1 and -2 on the Mowla Terrace (Figure 1); both wells flowed oil to surface at sub-economic rates from a fractured dolostone reservoir in the Nita Formation. The Middle Ordovician Goldwyer Formation is recognised as containing the most regionally significant source rock units, correlated with periods of maximum flooding in the lower and upper parts of the formation, respectively (Foster et al 1986, Edwards et al 1997, Haines 2004). Source rocks within the Goldwyer Formation contain high concentrations of the marine microbe Gloeocapsomorpha prisca (Foster et al 1986, Edwards et al 1997). Similar G. prisca-bearing source rocks have been 42

Haines and Wingate Amadeus Basin

linked to commercial accumulations of hydrocarbons in low latitude Ordovician basins around the world (Foster et al 1986). Biomarker characteristics of most Canning Basin Ordovician oil samples have the signature of a G. priscabearing source (Edwards et al 1997), further supporting the link to the Goldwyer Formation. The geographic distribution of Goldwyer source rock quality remains poorly known, although richness appears to increase towards the east from the Broome Platform and adjacent terraces to the central Barbwire Terrace. Trends further east are poorly known, due to very limited well control and poor quality data. South of the central highs, the westernmost intersections of the Goldwyer Formation in the Willara Sub-basin appear to have low source potential (McCracken 1997, Edwards et al 1997), but whether there is an improvement in source properties to the east into the Kidson Sub-basin, as seen to the north, is not known, due to very limited well control and few reliable analyses. Elevated total organic carbon (TOC) values have been reported from other stratigraphic units, including the Nambeet, Willara, Nita and Bongabinni formations, but in most cases, these rocks have otherwise poor source rock properties, or have not been fully characterised. A local exception involves beds described as algal oil shale, with reported TOC values up to 62% and oil-prone properties, within the Bongabinni Formation along the Admiral Bay Fault Zone, separating the Broome Platform and Willara Subbasin (Edwards et al 1995, 1997, McCracken 1997, Ghori and Haines 2006, Haines and Ghori 2006). Some oil shows along the Admiral Bay Fault Zone can be geochemically linked to the Bongabinni source rocks (McCracken 1994, 1997, Edwards et al 1995). A lack of predictable reservoir rocks has been an ongoing problem for early Palaeozoic hydrocarbon exploration in the Canning Basin. Most targeted reservoirs are of secondary carbonate type, with intercrystalline, vuggy and fracture porosity often hard to predict. Vuggy dolomitised carbonate rocks of the Nita Formation are the most common target, because this interval overlies the known source rocks and is the stratigraphic interval most frequently associated with minor oil and gas shows. Although good reservoir properties are present locally (eg Karajas and Kernick 1984), the Nita dolostone reservoir is commonly tight and the diagenetic controls over reservoir quality are poorly understood. Regional seals are provided by mudstones of the Goldwyer Formation and Carribuddy Group, and by locally thick halite formations in the latter. Sandstone is comparatively uncommon in the central and western parts of the basin, where most exploration has taken place, apart from the basal clastic unit of the Nambeet Formation, and those present are commonly tight. A local, but important exception is the Acacia Sandstone Member of the upper Willara Formation, intersected in some wells on the Barbwire and Mowla terraces and eastern Broome Platform, with an equivalent possibly present in the Gap Creek Formation. In general, sand content increases towards the southeastern end of the basin, where the lower part of the stratigraphy is dominated by the Wilson Cliffs Sandstone, with the overlying Goldwyer Formation also having some fine sandy content. Reservoir properties in these sandy units are poorly documented.

Despite fewer exploration wells, compared to the Canning Basin, the Ordovician of the Amadeus Basin has a proven economic track record, with the early discovery of the large producing Palm Valley and Mereenie gas and oil fields. The main source rock, euxinic black shale intervals in the Lower Ordovician Horn Valley Siltstone, contains significant concentrations of G. prisca (Gorter 1984, Summons and Powell 1991), similar to the Goldwyer Formation, but is significantly older. Gorter (1984) indicated that, based on average total organic carbon content and quality of kerogens, the source potential of the Horn Valley Siltstone improves from east to west and also becomes more oil-prone in that direction. Reservoirs are provided by thick sandstone units both underlying (Pacoota Sandstone) and overlying (Stairway Sandstone) the Horn Valley Siltstone, but are typically of poor quality due to the degree of burial diagenesis. Fracture porosity plays an important role, particularly at Palm Valley (Berry et al 1996). The Horn Valley Siltstone also acts as a seal, along with younger mudstone-dominated units, and the traps thus far discovered consist of large exposed anticlines that formed in the late Palaeozoic during the Alice Springs Orogeny. Carbonate reservoirs are unknown and carbonates do not tend to form significant accumulations in the Ordovician of the Amadeus Basin. Detrital zircon geochronology Previous sections highlight significant differences between the geological histories and depositional facies of the Ordovician successions of the Canning and Amadeus basins, which could have several explanations, and which by themselves do not disprove the seaway hypothesis. If the basins were linked by an open seaway, it is likely that some sediment would have passed through the link, either in one or both directions, leading to sediment of mixed provenance in one or both basins. Such provenance mixing is potentially detectable with detrital zircon geochronology, which in recent years, has proven to be an important tool in identifying sediment provenance and in tracking sediment distribution pathways. From the point of view of sand-sized sediment, the sand-rich verses generally sand-poor nature of the Amadeus and Canning Ordovician successions, respectively, and particularly the increase in sandstone towards the eastern end of the Canning Basin, would imply partial, but restricted sand transfer, mainly in a westerly direction from the Amadeus to the Canning Basin, if a seaway did exist. Several previous studies have documented detrital zircon age spectra at a number of stratigraphic levels within the Amadeus Basin, including the Ordovician succession, but no previous studies of this type have been reported from the Canning Basin. In order to test the sand transfer hypothesis, we dated a suite of detrital zircons from four widely spaced Early Ordovician sandstone samples from the Canning Basin, to establish their likely provenance and to compare with the published Amadeus Basin dataset. Canning Basin samples Detrital zircons were dated from four widely spaced sandstone samples (Figure 2): two from drill core of 43

Did the Larapintine Seaway link the Canning and Amadeus basins? the Acacia Sandstone Member (Looma-1: 19°7'24.86"S, 123°59'39.42"E, GSWA sample 136069, 2026.40–2028.37 m; Acacia-2: 19°19'46.95"S, 124°59'43.66"E, GSWA sample 136057, 1174.56–1175.30 m), one from outcrop of the upper Gap Creek Formation (18°37'45.29"S, 125°55'12.14"E, GSWA sample 184268), and one from outcrop of Carranya Formation (19°9'52.98"S, 127°39'3.12"E, GSWA sample 184269). Clean sandstone units, such as the Acacia Sandstone Member, which do become coarser towards the east, appear to be ideal candidates for sand washed through the seaway, if such existed. The sampled sandstone unit at the top of the Gap Creek Formation is a likely correlative of the Acacia Sandstone Member. Although now close to the preserved Ordovician basin margin on the southeastern Lennard Shelf, this margin is considered to have resulted from Devonian tectonic activity and erosion, and original deposition was in an offshore setting. The Carranya Formation does appear to be close to the original basin margin and shows interbedding of probable fluvial with shallow-marine fossiliferous facies. All samples consist of relatively clean quartz-rich sandstone. Limited palaeocurrent data can be inferred from dipmeter logs over the sampled intervals in the wells. In Looma-1, the pattern has a bipolar orientation with NWand SE-directed modes (Phipps et al 1998, appendix 7), suggestive of tidal channel deposition. In Acacia-2, inferred palaeocurrents form a bimodal pattern with transport directions towards the SSW and SSE over the sampled interval (Watson and Derrington 1982, appendix VII). However, the inferred palaeocurrent distribution throughout the entire member is far more complex, particularly in the case of Acacia-2, which has currents directed towards all quadrants, with an overall average towards the west. In any case, the complex nature of currents in shallowmarine environments makes it unlikely that there will be any simple relationship between current direction at a particular location, time and the ultimate source of the sand. No palaeocurrent data are available for the Gap Creek Formation, and limited field data for the Carranya Formation show significant variability, but average transport direction is from the north.

were avoided, and a few very high-uranium crystals (>1000 ppm 238U) were not dated, owing to the likelihood that they would yield discordant (and unusable) results. In addition, inherent biases in sampling and mineral separation (Sircombe and Stern 2002, Black et al 2004) are unavoidable. Full details of analytical methods are available in Wingate (2007a, b, c, d). Results Data tables of all analyses are available in Wingate (2007a, b, c, d), which can be obtained online from the Geological Survey of Western Australia (see references for links). The results are summarised here as histograms and probability density plots (Figure 5). Plotted ages >1000 Ma are based on 207Pb/206Pb ratios; those 10% discordant are not plotted. Sample 136069, Acacia Sandstone Member, Looma-1 Sixty-eight zircons were dated, of which 65 are less than 10% discordant. The youngest zircon has a near-concordant 238 U/206Pb age of 498 Ma, whereas the discordant 207Pb/206Pb age of 2814 Ma is a minimum age for the oldest zircon in this sample. The age spectrum (Figure 5a) is dominated (65%) by a single peak, which yields a weighted mean age of 1850 ± 4 Ma (95% confidence, MSWD = 1.1). Almost all of these crystals are light brown in colour with well grouped Th/U ratios between 0.1 and 0.9. It is likely that these zircons were derived from one or more closely related igneous sources. Two much smaller age components occur at ca 2530 and 1080 Ma. Sample 136057, Acacia Sandstone Member, Acacia-2 Sixty-five zircons were dated, of which 57 are less than 10% discordant. The youngest zircons are latest Neoproterozoic, whereas the oldest crystal has a near-concordant 207Pb/206Pb age of 2767 Ma. The age spectrum (Figure 5b) shows a much broader spread of significant peaks than in the Looma-1 sample. Major age components are identified at ca 852, 1155, 1192, 1792, 1865 and 1940 Ma. Minor peaks are present at 550–650, ca 1600 and 2500–2600 Ma. Sample 184268, Gap Creek Formation Sixty zircons were dated, of which 57 are less than 10% discordant. Ages range from 491 ± 7 Ma (238U/206Pb) to 2714 ± 8 Ma (207Pb/206Pb). The age spectrum is very similar to the Looma-1 sample, being strongly dominated by a single peak, which is slightly older at ca 1875 Ma (Figure 5c). Much smaller age components at ca 2538 and ca 1075 Ma are likewise similar to the Looma-1 sample.

Methods Zircons were separated using conventional density and magnetic techniques. A representative fraction of several hundred grains from each sample was selected and cast, together with zircon reference standards, in an epoxy resin mount, then polished to expose the interiors of the crystals. Crystals were fully documented prior to U-Pb dating using the Sensitive High Resolution Ion Microprobe (SHRIMP) at Curtin University of Technology, Perth. Between 58 and 68 grains were dated per sample. With approximately 60 grains dated per sample (and assuming a random sample), no fraction of a population comprising more than 0.087 of the total should be missed at the 95% confidence level (Vermeesch 2004). Although it was intended that the dated crystals be as representative as possible of the true population from each sample, some bias is inevitable, because crystals with abundant cracks and inclusions

Sample GSWA184269, Carranya Formation Fifty-eight zircons were dated, of which 56 are less than 10% discordant. Ages range from 546 ± 7 Ma (238U/206Pb) to 3006 ± 8 Ma (207Pb/206Pb). The age spectrum shows a broad spread of peaks and, although not identical to the Acacia-2 sample, displays much more in common with this sample than that from Looma-1. A cluster of young peaks is present between 550 and 650 Ma, followed by age components at ca 871, 1169, 1566, 1640, 1742, 1859 and 2435 Ma (Figure 5d).

d

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Haines and Wingate

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Figure 5. Probability density and histogram plots (50 my bins) of zircon U-Pb ages for samples from (a) Looma-1; (b) Acacia-2; (c) Gap Creek Formation; and d) Carranya Formation. Ages >1000 Ma are based on 207Pb/206Pb ratios, those 10% discordant are not plotted. Mean ages of major probability peaks are indicated.

Amadeus Basin data

are dominated by age peaks in the range 1800–1400 Ma and these zircons are likely derived mainly from the adjacent Arunta Region, with similarly aged igneous and metamorphic events (Zhao et al 1992, Maidment et al in press). Younger Neoproterozoic to Early Cambrian sandstone samples often show the development of strong 1050–1200 Ma peaks, which are likely derived from the Musgrave Province and contiguous orogenic zones exposed to the south (Camacho et al 2002, Buick et al 2005, Maidment et al in press). Similar detrital zircon age spectra are found to the west in Neoproterozoic rocks of the western Officer Basin (Bagas 2003, Bagas et al 2001, Geological Survey of Western Australia 2005, Haines and Wingate 2007). A significant change in provenance is indicated in the data from the Upper Cambrian to Middle Ordovician (Goyder Formation, Pacoota Sandstone, Stairway Sandstone) presented in Buick et al (2005) and Maidment et al (in press). Samples from these formations show a dominant peak of young zircons around 500–600 Ma, a broader complex of smaller peaks in the 900–1200 Ma range, with minor peaks back to about 3500 Ma (Figure 6). The same pattern is found in the Lower Ordovician Tomahawk Formation of the Georgina Basin (Maidment et al in press), and in probable metamorphic equivalents of the Larapinta Group, now incorporated within the Arunta Region between these basins (Buick et al 2005). The provenance of these sediments is difficult to explain using known local sources, requiring long-distance transport, as discussed below.

Several previous studies have documented detrital zircon age spectra at a number of stratigraphic levels within the Amadeus Basin (Zhao et al 1992, Camacho et al 2002, Buick et al 2005, Maidment et al in press). Samples from the earliest phase of basin fill (Heavitree Quartzite) Acacia Sandstone Member, Acacia-2 Acacia Sandstone Member, Looma-1 upper Pacoota Sandstone (Buick et al 2005)

500 PWH40

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2500

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Figure 6. Probability density plots comparing zircon isotopic ages for the Acacia Sandstone Member from Looma-1 and Acacia-2, with data for the upper Pacoota Sandstone in the Amadeus Basin taken from Buick et al (2005). Ages >1000 Ma are based on 207 Pb/206Pb ratios, those 10% discordant are not plotted.

45

Did the Larapintine Seaway link the Canning and Amadeus basins? Discussion of zircon data

small contribution from local Arunta Region basement. We consider that the majority of zircons in this sample (and other samples of Cambro–Ordovician age in the Northern Territory, as described above) are derived from the same source as the lower Palaeozoic sands of the palaeo-Pacific margin of Australia. The ultimate source of the “Gondwana” zircon pattern is not known with certainty, but given that this signature is found beyond Australia, eg in Antarctica, Africa, Arabia, Mozambique, India, and New Zealand (Gibson and Ireland 1996, Jacobs et al 1998, Rickers et al 2001, Avigad et al 2003, Maidment et al in press), it requires a large orogenic source not necessarily close to sites of accumulation. The most likely source area lies in the laterally continuous, latest Neoproterozoic to early Palaeozoic Pan-African orogenic belts and the proposed suture between the East Antarctic craton, India and southwest Australia (eg Boger et al 2001). These extensive belts contain, or are adjacent to terrains containing source rocks of appropriate age. One possible explanation for the appearance of the “Gondwana” zircon age spectra in the Amadeus Basin is that the sand was washed in from the east via marine currents. Alternatively, it is possible that rivers crossing the continent from the presumed source to the southwest delivered sand directly to the Amadeus Basin. The generally west to east palaeocurrents reported for the Pacoota Sandstone (Wells et al 1970) might be more consistent with the latter, but could also reflect the cycling of sand-carrying currents around a closed embayment, the westerly directed part of which has been removed by tectonic activity. A source from the west via the Canning Basin is not supported by the lesser abundance of sand in similarly aged strata of that basin (and finer average grainsize) and our very different detrital zircon age data from the Canning Basin.

The zircon age spectra shown by the Acacia Sandstone Member in Looma-1 and the sandstone unit in the upper Gap Creek Formation are very similar. The main peaks (1850–1875 Ma) are of similar age to widespread granites and felsic volcanics across the North Australia Craton and Haines and Wingate (2007) showed that very similar detrital zircon age spectra are present in post-orogenic sedimentary and metasedimentary rocks at widely spaced localities across this craton. In contrast, the sample of Acacia Sandstone Member from Acacia-2 and the sample from the Carranya Formation show significantly different spectra, containing a much high proportion of younger zircons. Although the zircon age spectra of these two samples are not identical to each other, there are much closer similarities between them than there are to the former samples. Haines and Wingate (2007) argued that each is broadly similar to detrital zircon age spectra from Neoproterozoic sediments in the Amadeus and western Officer basins. It appears likely that at least two different sand source terrains, presumably with different entry points to the basin, were operating in the Canning Basin during the Early Ordovician. One source is probably the North Australia Craton (or sedimentary rocks derived from it), which lies to the northeast of the Canning Basin. The other may involve a significant component of sediment reworked from uplifted Neoproterozoic rocks from parts of the Centralian Superbasin (or their source areas), which outcrop to the east, southeast and south of the Canning Basin. It is possible that the Acacia Sandstone Member comprises several geographically discrete sand bodies of similar age, or alternatively, that source switching has caused it to be vertically stratified with regard to sand source (Haines and Wingate 2007). The Amadeus Basin Late Cambrian to Middle Ordovician sandstone zircon age spectrum is not similar to any of our Early Ordovician Canning Basin samples (Figure 6), but does show a significant similarity to early Palaeozoic sandstone units from across the Ordovician, palaeo-Pacific, eastern continental margin of Australia. This so called “Gondwana pattern”, also found in other parts of Gondwana, is commonly associated with thick and often turbiditic clastic successions and is remarkable for its consistency over wide areas. This age spectra is characterised by a dominant ca 500–650 Ma “Pan-African” peak, a complex of ca 1000–1300 Ma “Grenvillian” peaks, and a scatter of minor peaks back to about 3500 Ma. It first appears in Australia in the predominantly turbiditic Cambrian Kanmantoo Group of southeastern South Australia (Ireland et al 1998, Haines et al 2001b, Haines et al 2004, Figure 7), on the then palaeo-Pacific margin. In the Ordovician, this pattern became widespread in turbiditic sandstone units of the Lachlan Foldbelt and equivalents to the north and south, including Tasmania (Veevers 2000, Ireland et al 1998). Figure 7 compares the Early Ordovician upper Pacoota Sandstone sample of Buick et al (2005) with data from the Kanmantoo Group and a sandstone sample from the Lachlan Foldbelt near Mount Kosciusko, NSW, both from Ireland et al (1998). The only significant difference is the presence of a moderate 1400–1500 Ma peak in the Pacoota Sandstone sample, which could be explained by a

Discussion The strongest argument in favour of a continuous seaway between the Canning and Amadeus basins is their juxtaposition and alignment along a major east–west- to

Sandstone, Mount Kosiuszko (Ireland et al 1998) Kanmantoo Group, 3 samples (Ireland et al 1998) upper Pacoota Sandstone (Buick et al 2005)

500 PWH39

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Figure 7. Probability density plots comparing zircon isotopic ages for the upper Pacoota Sandstone (Buick et al 2005), as plotted in Figure 6, with lower Palaeozoic sandstone units from the eastern palaeo-Pacific margin, specifically Kanmantoo Group, SA, and Lachlan Foldbelt near Mount Kosciusko, NSW (data are weighted mean ages from Ireland et al (1998).

46

Haines and Wingate west-northwest-trending structural corridor. This corridor, clearly visible on continent-scale gravity and magnetic images, is a fundamental feature of the Australian crust, presumably related to plate assembly during the Precambrian, but periodically reactivated during the Phanerozoic. The juxtaposition of the basins on maps is somewhat misleading as there is no continuity of similarly aged rocks between these basins. What was later to become the Amadeus Basin was initiated during the early Neoproterozoic as part of the Centralian Superbasin (Walter et al 1995), deposition of which extended beneath at least part of what subsequently became the Canning Basin. Deposition in at least part of this area was terminated by the contiguous Petermann and Paterson orogenies in the Ediacaran to Early Cambrian, with folding of Neoproterozoic strata in the far western Amadeus Basin largely or entirely dating from this event. In the northcentral to eastern Amadeus Basin, deposition continued through the Cambrian to the Ordovician and beyond. The Canning Basin was initiated by rifting (Samphire Marsh Movement), beginning either in the Late Cambrian or Early Ordovician [the oldest dated sediments are Early Ordovician (Tremadoc) in age]. If this extensional event was related to extension in the Amadeus Basin at the same time, the question remains whether rifting ever reached an extent where open exchange of seawater occurred between the basins. As indicated earlier, the Ordovician isopachs and facies belts of the Amadeus Basin curve northward toward their western extremity, to be truncated by the present tectonic northern margin, and the preserved Larapinta Group extended further west and south with time, unconformably onlapping Cambrian or variably deformed Neoproterozoic sediments at the basin margin (Wells et al 1965, 1970, Gorter 1991). The decreasing separation of preserved Ordovician strata between the basins with time, from approximately 250 km during the Early Ordovician to about 180 km by the Late Ordovician (Caradoc), suggests westward onlap of the Amadeus Ordovician marine package with time. However, by the Caradoc, open marine deposition had ceased in the Canning Basin. If a seaway existed before this time, the connection could only have been to the north of the present Amadeus Basin and any evidence of this would have been removed by uplift and erosion during the Alice Springs Orogeny, with the possible exception of Ordovician strata preserved in the Ngalia Basin. The Ngalia Basin lies about 70 km north of the central Amadeus Basin, and is likewise a tectonic relic that survived erosion during the Alice Springs Orogeny. The Djagamara Formation of the central Ngalia Basin is considered a likely correlative of the Larapinta Group (Wells and Moss 1983). This relatively thin (