Geology and mineral resources of the Northern Territory · 35:1 Arafura Basin Creek Orogen. To the west, it is unconformably overlain by relatively undeformed Mesozoic and Cenozoic

Ahmad M and Munson TJ (compilers)

Northern Territory Geological SurveySpecial Publication 5

Chapter 35: Arafura Basin

Geology and mineral resourcesof the Northern Territory

BIBLIOGRAPHIC REFERENCE: Ahmad M and Munson TJ, 2013. Chapter 35: Arafura Basin: in Ahmad M and Munson TJ (compilers). ‘Geology and mineral resources of the Northern Territory’. Northern Territory Geological Survey, Special Publication 5.

DisclaimerWhile all care has been taken to ensure that information contained in this publication is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of its information. The Northern Territory of Australia gives no warranty or assurance, and makes no representation as to the accuracy of any information or advice contained in this publication, or that it is suitable for your intended use. You should not rely upon information in this publication for the purpose of making any serious business or investment decisions without obtaining independent and/or professional advice in relation to your particular situation. The Northern Territory of Australia disclaims any liability or responsibility or duty of care towards any person for loss or damage caused by any use of, or reliance on the information contained in this publication.

35:1

Arafura Basin

Creek Orogen. To the west, it is unconformably overlain by relatively undeformed Mesozoic and Cenozoic sedimentary rocks of the Money Shoal Basin, which are up to 4.5 km thick (Figure 35.2). This succession is continuous with that of the Bonaparte Basin to the west, but thins rapidly to the east, so as to form the onlapping edge of a vast Mesozoic to Cenozoic depositional area that extends over much of offshore northwestern Australia (Bradshaw et al 1990, Struckmeyer 2006b). Mesozoic and Cenozoic sedimentary rocks of the Carpentaria Basin onlap the Arafura Basin to the east and southeast, and are up to 1760 m thick. The northern limits of the Arafura Basin are not well de ned, although seismic data indicate that it extends towards the Aru Ridge and Merauke Rise to the south of Papua, Indonesia (Moss 2001). Palaeozoic sedimentary rocks are also known from central Papua, indicating that the original limits of the basin prior to Mesozoic tectonism may have been at least this far to the north (Fortey and Cocks 1986, Nicoll and Bladon 1991). To the northwest, the poorly explored Barakan Basin in Indonesian waters is of similar age and has a similar structure to that of the Arafura Basin (Barber et al 2004).

This chapter focuses on the onshore sedimentary succession of the Arafura Basin in the NT. A full discussion of the other components of the basin is beyond the scope of this volume, although brief summaries of the offshore successions are also included. Signi cant studies of the Arafura Basin and in particular, the onshore succession, include Plumb (1963,1965), Rix (1964a, 1965), Dunnet (1965), Petroconsultants (1989), Bradshaw et al (1990), McLennan et al (1990), Plumb and Roberts (1992), Rawlings et al (1997), Carson et al (1999), Struckmeyer (2006a, b), Totterdell (2006), Geoscience Australia (2008, 2012) and Zhen et al (2011).

Chapter 35: ARAFURA BASIN M Ahmad and TJ Munson

INTRODUCTION

The Neoproterozoic to Permian Arafura Basin extends from the onshore Northern Territory into Indonesian waters (Figure 35.1) and covers an area of about 500 000 km2. Structurally, the basin consists of northern and southern sections separated by the large deformed Goulburn Graben (Bradshaw et al 1990; equivalent to Arafura Graben of Petroconsultants 1989). The Goulburn Graben is a west-northwest-trending asymmetric feature, over 350 km long and up to 70 km wide, that contains a sedimentary section in excess of 10 km thick. The region to the north of the Goulburn Graben forms the basin’s main depocentre and contains a sedimentary succession up to 15 km thick (Figures 35.2, 35.3). South of the Goulburn Graben a north-dipping relatively undeformed ramp that extends onshore contains up to 3 km of sedimentary rocks. The Arafura Basin succession comprises sandstone, shale, limestone, dolostone, coal beds and glacial deposits and is summarised in Figure 35.4 and Table 35.1. Totterdell (2006) described four main phases of deposition within the basin (Basin phases 1–4) in the Neoproterozoic (Wessel Group), middle Cambrian–Early Ordovician (Goulburn Group), Late Devonian (Arafura Group) and Late Carboniferous–Early Permian (Kulshill Group equivalent). These basin phases were separated by long, relatively tectonically quiescent periods of non-deposition and erosion. Neoproterozoic and Cambrian sedimentary rocks, which outcrop on the northern extremity of Arnhem Land, from east of the Cobourg Peninsula to the Wessel Islands and extending inland up to about 80 km, are the only onshore manifestation of the basin.

The Arafura Basin succession is underlain by Palaeo- to Mesoproterozoic rocks of the McArthur Basin and Pine

Current as of September 2012

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Figure 35.1. Regional geological setting of Arafura Basin (modi ed from Totterdell 2006, gure 4). NT geological regions slightly modi ed from NTGS 1:2.5M geological regions GIS dataset. Offshore margins of basin after Petroconsultants (1989) and Totterdell (2006).

Geology and mineral resources of the Northern Territory Special publication 5

Arafura Basin

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NEOPROTEROZOIC TO ?EARLY CAMBRIAN: BASIN PHASE 1

Wessel Group

Deposition in the Arafura Basin commenced in the Neoproterozoic during a period of upper crustal extension that resulted in the formation of a series of half grabens, which form an overall northeast-trending depocentre in the northern basin that continues into Indonesian waters

(Totterdell 2006, Figure 35.3, see Structure and tectonics). The ll of these half grabens and the overlying sag phase sedimentary rocks comprise the Wessel Group (Plumb et al 1976, Figure 35.2, 35.4), which is a succession of shallow marine, mostly quartz sandstone, mudstone and minor carbonate rocks. It is the only part of the basin, along with the middle Cambrian Jigaimara Formation (basal Goulburn Group), that is exposed onshore, where it reaches a composite thickness estimated to be about 2300 m (Rawlings et al 1997). Offshore, in the central part of the basin, it reaches

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Figure 35.2. Geoseismic cross-section through Arafura and Money Shoal basins (modi ed after Totterdell 2006, gure 5). Location shown on Figure 35.7.

Figure 35.3. Arafura Basin sediment thicknesses (milliseconds two-way time), showing signi cant normal faults involved in graben development and location of drillholes (modi ed from Totterdell 2006, gure 6).


35:3

Arafura Basin

a maximum thickness of about 10 000 m (Totterdell 2006). The group outcrops in an arcuate belt along the northwestern coastline of the Territory, from WESSEL ISLANDS-TRUANT ISLAND1, through northern and western ARNHEM BAY, to eastern and northern MILINGIMBI and JUNCTION BAY (Figure 35.5). It unconformably overlies various formations of the McArthur Basin and is overlain, probably disconformably, by the Jigaimara Formation. The onshore Wessel Group comprises, in ascending order, the Buckingham Bay Sandstone, Raiwalla Shale, Marchinbar Sandstone and Elcho Island Formation (Table 35.1). These generally form an arcuate to linear outcrop tract parallel to the preserved margins of the basin with the younging direction northward towards the basin’s offshore depocentre. Seismic data indicate that the basal, offshore rift- ll succession of the group is not represented in onshore areas (Totterdell 2006).

The age of the Wessel Group is poorly constrained between underlying Mesoproterozoic basement rocks and the overlying middle Cambrian Jigaimara Formation (Goulburn Group). It was originally considered to be Neoproterozoic after Rb-Sr and K-Ar minimum dates

1 Names of 1:250 000 mapsheets are in large capital letters, eg MILINGIMBI.

of 790 and 770 Ma, respectively, were determined for a single glauconite from the Elcho Island Formation at the top of the group (McDougall et al 1965). Plumb et al (1976) reinterpreted the age of the entire Wessel Group as Cambrian, based on the purported presence of Skolithos trace fossils in the Buckingham Bay Sandstone (Plumb 1963, Dunnet 1965), and the discovery of a middle Cambrian metazoan fauna in what was then considered to be the Elcho Island Formation. However, Rawlings et al (1997) reinterpreted the Skolithos trace fossils as abiogenic dewatering structures and assigned the metazoan fauna to the Jigaimara Formation. The discovery of the carbonaceous fossil Chuaria in the Raiwalla Shale (Haines 1998) subsequently reaf rmed a Neoproterozoic age for the group, although an early Cambrian age for the top of the group cannot be discounted.

The Wessel Group is probably equivalent in age to Supersequence 3 and 4 rocks of the Centralian A Superbasin to the south (see ).

Buckingham Bay SandstoneThe Buckingham Bay Sandstone (Plumb and Roberts 1992) unconformably overlies various units of the McArthur Basin and is overlain conformably by the Raiwalla Shale. The formation outcrops in a broad gently dipping arc around

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Figure 35.4. Arafura Basin stratigraphic succession, showing correlations from offshore wells and onshore outcrop in the Arafura Basin (modi ed from Bradshaw et al 1990, gure 10). Money Shoal Basin stratigraphic succession after Geoscience Australia (2012). Abbreviations: Car = Carboniferous; Fm = Formation; Gp = Group; Ord = Ordovician; TD = total depth.


Arafura Basin

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the margin of the onshore Arafura Basin in southwestern JUNCTION BAY, MILINGIMBI, ARNHEM BAY and southeastern WESSEL ISLANDS (Figure 35.5), and is best exposed near the coast of Flinders Peninsula and adjacent islands, with exposures becoming more broken and disintegrating into sand inland (Rawlings et al 1997, Carson et al 1999). In the absence of a complete section through the formation, Plumb and Roberts (1992) nominated a reference

area on the northwestern side of Flinders Peninsula around the mouth of the Kurala River. The formation is estimated to be about 350 m thick near its type locality (Rawlings et al 1997).

The basal unit consists of massive to at-bedded, cross-bedded, and occasionally rippled, medium- to coarse-grained and sometimes pebbly, medium- to very thickly bedded, white to pale pink and yellow (with local red iron-oxide staining) sandstone. A local basal breccia or

Unit, max thickness, (distribution) Lithology Depositional

environment Stratigraphic relationships

Carboniferous–PermianKULSHILL GROUP EQUIVALENT5000 m (offshore) Interbedded sandstone, siltstone and claystone,

with minor coal, and dolomitic rocks; palyno ora.Fluvial to marginal marine to shallow marine.

Unconformable on Arafura Group succession. Unconformably overlain by Jurassic–Cenozoic Money Shoal Basin succession.

Late DevonianARAFURA GROUPDarbilla Formation,380 m (offshore)

Mudstone, sandy siltstone and lesser interbedded sandstone; includes ning-upward intervals; palyno ora.

Non-marine, possibly sabkha or tidal at, and uvial.

Apparently conformable on Yabooma Formation. Unconformably overlain by Jurassic Money Shoal Basin succession.

Yabooma Formation,335 m (offshore)

Interbedded siltstone with dolomitic intervals, occasional thin sandstone beds; sparse fossil fauna of conodonts, sh and bryozoans.

Nearshore shallow marine.

Unconformable on Djabura Formation. Apparently conformably overlain by Darbilla Formation, or unconformably overlain by Jurassic Money Shoal Basin succession.

Djabura Formation,466 m (offshore)

Interbedded, mudstone, siltstone, sandstone and minor carbonate rocks; diverse fossil fauna, including conodonts, ostracods, phosphatic brachiopods, conulariids and sh fossils; palyno ora.

Nearshore shallow marine.

Unconformable on Goulburn Group succession. Unconformably overlain by Yabooma Formation, or by Kulshill Group equivalent.

middle Cambrian to Early OrdovicianGOULBURN GROUPMooroongga Formation,201 m (offshore)

Shale, limestone, sandstone, glauconitic sandstone, minor chert, dolomitic in part; becomes more calcareous up-section; limited conodont fauna.

Shallow marine. Probably conformable on Milingimbi Formation.

Milingimbi Formation,169 m (offshore)

Dolostone, limestone, glauconitic sandstone, shale; becomes more siliciclastic up-section; conodont faunas.

Shallow marine. Conformable on Naningbura Dolomite. Probably conformably overlain by Mooroongga Formation, or unconformably overlain by Arafura Group.

Naningbura Dolomite1128 m (offshore)

Dolostone with silty dolostone intervals; conodont fauna near top.

Shallow marine. Apparently conformable on Jigaimara Formation.

Jigaimara Formation470 m (offshore and onshore)

White to grey interbedded limestone, shale and dolostone, silici ed to chert and brecciated; possible microbial laminations; rich fossil fauna of trilobites, bradoriids, hyoliths, lingulate brachiopods and sponge spicules.

Low-energy, shallow marine, probably subtidal.

Disconformable or unconformable on Elcho Island Formation.

Neoproterozoic to ?early CambrianWESSEL GROUPElcho Island Formation 650–700 m (onshore)

Fine- to coarse-grained, thinly to medium bedded sandstone, often calcareous or dolomitic and locally glauconitic, with cross-beds, ripples, current lineations and load casts; minor mudstone interbeds; occasional carbonate intervals, locally strongly leached or silici ed to chert breccia

Shallow marine, occasional exposed; periodic evaporitic conditions.

Locally disconformable or possibly conformable on Marchinbar Sandstone.

Marchinbar Sandstone300 m (onshore)

White, quartz-rich, ne- to medium-grained sandstone, mostly medium bedded, with horizontal laminations, trough cross-beds, wave and current ripples, rare desiccation cracks.

Relatively high-energy very shallow marine.

Conformable and gradational on Raiwalla Shale.

Raiwalla Shale1000 m (onshore)

Grey and green micaceous mudstone, red-brown when weathered, interbedded with ne- to medium-grained tabular sandstone.

Subtidal marine shelf, gradual upward shallowing with increasing storm in uence.

Conformable with sharp contact on Buckingham Bay Sandstone.

Buckingham Bay Sandstone 350 m (onshore)

White, grey, pale pink, yellow and red, ne-to coarse-grained, mostly medium to thickly bedded sandstone, with common cross-beds and occasionally ripples; rare mudstone interbeds; local basal breccia and conglomerate.

High-energy shallow marine.

Unconformable on McArthur Basin succession.

Table 35.1. Summary of Palaeozoic stratigraphic succession of the Arafura Basin.


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Arafura Basin

conglomerate, up to several metres thick, contains poorly sorted angular clasts up to boulder size in a sandstone matrix (Rawlings et al 1997). Higher in the succession, pale grey, medium-grained, thickly bedded, massive to weakly at-bedded sandstone is interbedded with recessive, ferruginous and micaceous, thinly bedded, ne-grained sandstone and mudstone. Near the top of the formation, the lithology tends to be more uniform, comprising medium-grained, medium- to thickly bedded sandstone, varying from white to red and yellow with weathering. No metazoan fossils have been found in the Buckingham Bay Sandstone and purported Skolithos trace fossils recorded by Plumb (1963) and Dunnet (1965), and used to suggest a Cambrian age for the entire Wessel Group by Plumb et al (1976), were subsequently interpreted as having been caused by the dewatering of uidised sand, and are therefore abiogenic (Rawlings et al 1997).

The Buckingham Bay Sandstone is interpreted to have been deposited in a high-energy shallow marine environment. The formation probably correlates with the similar Bukalara Sandstone of the central northern Georgina Basin, which unconformably overlies the southern McArthur Basin succession (Pietsch et al 1991).

Raiwalla ShaleThe Raiwalla Shale (Plumb and Roberts 1992) outcrops poorly in a broad arcuate belt through MILINGIMBI and ARNHEM BAY (Figure 35.5). It overlies the Buckingham Bay Sandstone with a sharp concordant contact and is overlain conformably and gradationally by the Marchinbar Sandstone. The formation comprises mudstone with very

ne- to medium-grained tabular sandstone interbeds (Rawlings et al 1997). The lower mudstone-rich part of the formation is very recessive and is poorly exposed. Sandstone scree dominates most surface exposures, so that it is dif cult to determine the ratio of sandstone to shale. Better exposures in the upper half of the formation probably re ect an increasing proportion of sandstone interbeds up-section. An accurate thickness cannot be determined for the formation due to very shallow dips and the poor nature of outcrop, but it is estimated to be of the order of 1000 m (Rawlings et al 1997). Plumb and Roberts (1992) nominated a reference area for the formation around the Woolen River in ARNHEM BAY.

Mudstone is micaceous, at- to wavy-laminated and ssile (shaly). Sandstone varies from quartz-rich to lithic

to micaceous, is ne- (dominant) to medium-grained, and

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Figure 35.5. Onshore Arafura Basin, showing simpli ed geology of Wessel Group and Jigaimara Formation (basal Goulburn Group), derived from GA 1:1M geology and NTGS 1:2.5M geological regions GIS datasets. Locations of mineral occurrences are from NTGS Mineral Occurrence Database (MODAT). Fm = Formation; Sst = Sandstone.


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is thinly to medium bedded, with a few packets containing thicker beds. The sandstone typically displays at- to wavy- and some cross-lamination, and wave and current ripples are common on bed tops. Synaeresis cracks and mudclasts are also common, and soft-sediment deformation features are present locally. Small (millimetre-sized) iron-oxide inclusions, which are locally abundant, suggest that some intervals of the formation are pyritic in the subsurface. No metazoan or trace fossils have been recorded from the Raiwalla Shale, but carbonaceous impressions assigned to Chuaria have been used to assign a Neoproterozoic age to the unit (Haines 1998).

The Raiwalla Shale was probably deposited under subtidal, marine shelf conditions (Rawlings et al 1997). The basal contact is probably a marine ooding surface and represents a rapid deepening from the very shallow water conditions interpreted for the Buckingham Bay Sandstone. There is evidence for gradual upward shallowing with increasing storm in uence through the succession (Rawlings et al 1997). The Raiwalla Shale is correlated with the Cox Formation of the central northern Georgina Basin; this unit overlies the Bukalara Sandstone, an equivalent of the Buckingham Bay Sandstone (Pietsch et al 1991).

Marchinbar SandstoneThe Marchinbar Sandstone conformably and gradationally overlies the Raiwalla Shale and outcrops in a relatively linear belt through western TRUANT ISLAND, southeastern WESSEL ISLANDS, northwestern ARNHEM BAY and eastern MILINGIMBI (Figure 35.5). It was de ned by Plumb and Roberts (1992), who nominated a reference section on Marchinbar Island in WESSEL ISLANDS. The formation generally outcrops poorly, with exposures commonly restricted to places where creeks have eroded through the regional laterite capping. It is an estimated 300 m thick in the vicinity of the Woolen River (ARNHEM BAY), where the most complete exposed section is located (Rawlings et al 1997). The upper contact with the Elcho Island Formation is regionally concordant, but at the only locality where the actual point of contact can be seen, the boundary is erosional and marked by a thin granule and pebble lag, suggesting the possibility of at least a local disconformity at this level (Rawlings et al 1997).

The Marchinbar Sandstone is composed largely of clean, white quartz sandstone, which is dominantly medium-grained, but which includes some ne-grained beds, mainly near the base. Thin red, ferruginous and matrix-rich intervals are a minor component of the formation. Mudclasts are very common near the base, but decrease in abundance upwards. Most of the unit is medium-bedded, although more thinly and thickly bedded intervals are also present. Sedimentary structures include common horizontal lamination, trough cross-bedding, wave and current ripples, and rare desiccation cracks (Rawlings et al 1997). No metazoan or trace fossils have been found in the formation and its interpreted Neoproterozoic age is based entirely on its stratigraphic position (Zhen et al 2011). A relatively high-energy very shallow marine environment is interpreted for the unit and it probably represents the top of a shoaling cycle that began in the lower Raiwalla Shale (Rawlings et al 1997).

Elcho Island FormationThe Elcho Island Formation outcrops extensively in southern WESSEL ISLANDS, northwestern ARNHEM BAY and northeastern MILINGIMBI (Figure 35.5), along the coasts of northern Arnhem Land and Elcho, Howard and Banyan islands, and it is also sparsely exposed inland above the slightly more resistant Marchinbar Sandstone. It was de ned by Plumb and Roberts (1992), who nominated a reference section as cliff outcrops on Elcho Island, but was rede ned in Rawlings et al (1997), who nominated a type locality in western ARNHEM BAY. The formation is at least locally disconformable, or possibly conformable on the Marchinbar Sandstone and is probably disconformably overlain by the Jigaimara Formation of the Goulburn Group. A thickness of 650–700 m is estimated for the Woolen River area (Rawlings et al 1997).

The Elcho Island Formation is a succession of ne- to coarse-grained, locally glauconitic, thinly to medium-bedded sandstone, generally interbedded with minor mudstone and chert. Sedimentary structures include trough and tabular cross-beds, wave and current ripples (Figure 35.6a), current lineations and load casts. The succession is sometimes calcareous or dolomitic, and chert breccia and leached rocks after carbonate are present locally. The age of the formation is poorly constrained between the Neoproterozoic lower Wessel Group and the middle Cambrian Jigaimara Formation, but a Neoproterozoic age is more likely from the absence of metazoan or trace fossils, and from radiometrically dating of a single glauconite from low in the Elcho Island Formation (McDougall et al 1965) at about 770 Ma (K-Ar) and 790 Ma (Rb-Sr). The Elcho Island Formation was deposited under shallow marine shelf conditions, which at times, reached the point of exposure and desiccation (Figure 35.6b). Periodic evaporitic conditions are indicated by halite pseudomorphs and desiccation cracks near the base and top.

MIDDLE CAMBRIAN TO EARLY ORDOVICIAN: BASIN PHASE 2

Goulburn Group

The early middle Cambrian–Early Ordovician Goulburn Group (Petroconsultants 1989, McLennan et al 1990, Bradshaw et al 1990, Nicoll et al 1996, Rawlings et al 1997) disconformably or unconformably overlies the Wessel Group and is unconformably overlain by the Late Devonian Arafura Group. It has sag- to sheet-like geometry and is structurally conformable with the upper, post-rift portion of the Wessel Group. The succession reaches a maximum thickness of about 2000 m in the offshore central part of the northern Arafura Basin and contains, in ascending order, the Jigaimara Formation, Naningbura Dolomite, Milingimbi Formation and Mooroongga Formation. The basal part of the Jigaimara Formation is exposed in southern WESSEL ISLANDS, northwestern ARNHEM BAY and northeastern MILINGIMBI (Figure 35.5), but the upper part of the unit and the other formations are only intersected in petroleum exploration drillholes in the Arafura Sea (Figure 35.4). The Goulburn Group represents prolonged deposition on a shallow marine shelf in a stable intraplate setting.


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Arafura Basin

The age of the Goulburn Group has been established from the presence of a middle Cambrian marine fauna in the basal Jigaimara Formation and from Early Ordovician conodont faunas in the Milingimbi and Mooroongga formations (Zhen et al 2011). In offshore drillhole Money Shoal-1, unnamed and poorly dated ?Cambrian strata (from 2530–2575 m) contain interbedded andesitic volcanic rocks, indurated ne- to medium-grained arkosic sandstone, and dark grey-green to black carbonaceous shale. The volcanic rocks might be stratigraphic equivalents of the Antrim Plateau Volcanics (Brown et al 1968, Petroconsultants 1989, see Kalkarindji Province) and if so, then a late early Cambrian age is possible for the base of the group.

Jigaimara FormationThe Jigaimara Formation (Haines in Rawlings et al 1997) disconformably or unconformably overlies the Elcho Island Formation and is apparently conformably overlain by the Naningbura Dolomite (Rawlings et al 1997, Zhen et al 2011). It is a succession of interbedded limestone, shale and dolostone that is exposed at Warnga Point on Elcho Island and on several small islands north and northeast of Milingimbi township (Figure 35.5). Exposures are scattered and nearly

at-lying, and individual sections are only a few metres thick. The rocks are silici ed and consist of white to grey-brown chert (presumably after limestone and calcareous siltstone). They are invariably brecciated to various degrees (jigsaw t to totally chaotic) and have a siliceous matrix. Individual clasts are commonly well laminated and possible microbial laminations are also present, as are enigmatic doughnut-shaped ?algal structures, about 20 cm in diameter (Rawlings et al 1997, Carson et al 1999). The formation reaches a maximum thickness of 470 m in offshore drillhole Arafura-1 (Zhen et al 2011).

The Jigaimara Formation is very fossiliferous and contains a fauna of trilobites, bradoriids, hyoliths, lingulate brachiopods and sponge spicules at its base; this fauna is most likely to be middle to late Templetonian (early middle Cambrian) in age (Shergold in Plumb et al 1976, Laurie 2006a, b, Zhen et al 2011). The age of the top of the formation is constrained by the apparently conformably overlying Naningbura Dolomite, which is Furongian2 (late Cambrian) to early Tremadocian (Early Ordovician). The Jigaimara Formation is therefore Templetonian–?Mindyallan in age and can be correlated with sequence 2 (latest Ordian–early Mindyallan) of the Centralian B Superbasin. This is the second of two successive widespread sedimentary successions, characterised by distinctive invertebrate faunas, that have been recognised in central and northern Australia from sequence stratigraphic studies of middle Cambrian strata in the Georgina Basin (Shergold et al 1988, Southgate and Shergold 1991, Laurie 2006c, see Centralian Superbasin:

). The Jigaimara Formation was deposited in low-energy, shallow marine, probably subtidal settings, following a regional transgression (Rawlings et al 1997).

Naningbura DolomiteIn offshore drillhole Arafura-1, the Naningbura Dolomite is a thick largely dolostone succession with silty dolomitic

2 Corresponds to the Idamean–Datsonian Australian stages.

intervals that was deposited in a predominantly shallow marine environment. It is apparently conformable between the Jigaimara Formation (below) and the Milingimbi Formation, and is equivalent to units O1 to O7 of Petroconsultants (1989). Nicoll et al (1996) originally named this unit the Naningbura Formation, but it was not de ned and only brie y described. The Naningbura Formation’ was subsequently mentioned in Rawlings et al (1997), Carson et al (1999) and Struckmeyer (2006b), but none of these publications provided enough detail to properly establish the unit with this name. Nicoll (2006a) renamed the unit the Naningbura Dolomite, allocated a type section

Figure 35.6. Elcho Island Formation. (a) Megaripples on wave-cut platform (near 561200mE 8671600mN, Galiwinku, Elcho Island, after Rawlings et al 1997, plate 34). (b) Desiccation cracks in sandstone at top of unit (522300mE 8647500mN, Banyan Island, after Rawlings et al 1997: plate 35).

a

b


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in Arafura-1, and provided a more detailed description of its lithologies, distribution and conodont fauna. This name was also used by Zhen et al (2011), who provided detailed descriptions of the conodont palaeontology and biostratigraphic succession. These publications rmly establish the name of the unit as Naningbura Dolomite and this nomenclature is followed herein. The Naningbura Dolomite is 1128 m thick in the type section in Arafura-1, the only drillhole to penetrate the entire unit (Figure 35.4). Incomplete thicknesses intersected in other drillholes are in the range 154–601 m. A conodont fauna recovered from the top of the unit is late Furongian to early Tremadocian in age (Nicoll 2006a, Zhen et al 2011), but the undated base of the unit may be as old as middle Cambrian.

Milingimbi FormationThe Milingimbi Formation (Bradshaw et al 1990, Nicoll 2006a) corresponds to units O8 and O9 of Petroconsultants (1989). It is conformable on the Naningbura Dolomite and is probably conformably overlain by the Mooroongga Formation, or is unconformably overlain presumably by the Late Devonian Arafura Group (Zhen et al 2011). The formation is of mixed lithology and comprises silty dolostone to limestone, glauconitic sandstone and shale, deposited predominantly in a shallow marine environment (Nicoll 2006a, Zhen et al 2011). The lower part of the Milingimbi Formation is dolomitic, but it becomes more siliciclastic up-section, where thin glauconitic sandstone is interbedded with dolostone, limestone and shale (Bradshaw et al 1990). In the type section in drillhole Arafura-1, the Milingimbi Formation is 163 m thick and in Goulburn-1, the unit is 169 m thick. Pre-Devonian erosion has truncated the formation in Torres-1, where it is only 95 m thick, and has completely removed the unit in Tasman-1 (Figure 35.4). Conodont faunas of Tremadocian (Early Ordovician) age have been described from the unit (Bradshaw et al 1990, Zhen et al 2011).

Mooroongga FormationThe Mooroongga Formation (Bradshaw et al 1990, Nicoll 2006a) corresponds to units O10 to O133 of Petroconsultants (1989). It is probably conformable on the Milingimbi Formation, but a major unconformity separates this unit from the overlying Upper Devonian Djabura Formation (Arafura Group). The Mooroongga Formation comprises shale and interbedded limestone with some thin sandstone interbeds and minor chert, and becomes more calcareous upward. Glauconite is common and parts of the formation are dolomitic (Zhen et al 2011). The depositional setting was predominantly shallow marine (Zhen et al 2011). The Mooroongga Formation is 131 m thick in Arafura-1, 201 m thick in Goulburn-1 and has been completely removed by erosion in Tasman-1 and Torres-1 (Nicoll 2006a, Zhen et al 2011, Figure 35.4). Petroconsultants (1989) reported the presence of conodonts, ostracods, sh remains, conulariids, echinoderms, inarticulate brachiopods, gastropods, ?tentaculitids and sponge spicules from this unit. The

3 Petroconsultants (1989) did not describe Unit O13, but did include it in Geological cross-section A–A1 Arafura Basin’. It only occurs in Goulburn-1.

formation is considered to be of early Floian (late Early Ordovician) age, based on the limited, but diagnostic conodont fauna (Zhen et al 2011), and is about the same age as the late Tremadocian to Floian Florina Formation of the Daly Basin.

LATE DEVONIAN: BASIN PHASE 3

Arafura Group

The Upper Devonian Arafura Group (Petroconsultants 1989, Bradshaw et al 1990, McLennan et al 1990) unconformably overlies units of the Goulburn Group. A hiatus of about 100 million years separates the two groups which are generally structurally conformable. The Arafura Group consists of shallow marine to non-marine interbedded mudstone, siltstone, sandstone and minor carbonate rocks. It has sag to sheet-like geometry in the northern Arafura Basin, where it is about 1500 m thick, but the geometry of the group is more complex within the Goulburn Graben (Totterdell 2006). Bradshaw et al (1990) divided the Arafura Group into the Djabura, Yabooma and Darbilla formations. It is unconformably overlain by strata equivalent to the Upper Carboniferous–Lower Permian Kulshill Group of the Bonaparte Basin, or where these are absent, by Jurassic strata of the Money Shoal Basin.

Djabura Formation The Djabura Formation (Bradshaw et al 1990) has been intersected in Tasman-1, Torres-1, Arafura-1 and Goulburn-1 (Figure 35.4) and is equivalent to units D1–D4 of Petroconsultants (1989). It unconformably overlies various Cambrian and Ordovician units of the Goulburn Group and is unconformably overlain by the Yabooma Formation, or by younger (Upper Carboniferous) Kulshill Group equivalent sedimentary rocks (Nicoll 2006b). It ranges in thickness from 295 m to 466 m, and consists of interbedded, mudstone, siltstone, sandstone and minor carbonate rocks, which were deposited in a nearshore shallow marine environment (Nicoll 2006b, Totterdell 2006). Diverse marine fossils, including conodonts, ostracods, phosphatic brachiopods, conulariids and sh fossils, are found throughout the unit. The conodonts indicate an early Famennian age for the formation (Nicoll 2006b), but palynological dating suggests it is slightly older (Frasnian; Purcell 2006).

Yabooma FormationThe Yabooma Formation (Bradshaw et al 1990) is equivalent to the interval from unit D5 to the lower part of unit D7 of Petroconsultants (1989). It unconformably overlies the Djabura Formation and is intersected in drillholes Torres-1, Arafura-1 and Goulburn-1 (Figure 35.4). It is apparently conformably overlain by the Darbilla Formation, or is unconformably overlain by Jurassic sediments of the Money Shoal Basin (Bradshaw et al 1990, Nicoll 2006b). It ranges in thickness from 140 to 335 m, and is predominantly composed of interbedded siltstone with dolomitic intervals and occasional thin sandstone beds. A relatively sparse fossil fauna includes conodonts, sh and bryozoan fragments recovered in cuttings from Goulburn-1. The conodont fauna is from


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Arafura Basin

the base of the formation and indicates a late Famennian age (Nicoll 2006b). The Yabooma Formation is interpreted to represent predominantly nearshore shallow marine deposition (Bradshaw et al 1990, Totterdell 2006).

Darbilla FormationThe Darbilla Formation (Bradshaw et al 1990) is equivalent to the upper part of unit D7 and unit D8 of Petroconsultants (1989). It has only been intersected in Arafura-1 and Torres-1 (Figure 35.4), where it is 380 m and 262 m thick, respectively. The unit is apparently conformable on the Yabooma Formation and is overlain unconformably by Jurassic strata of the Money Shoal Basin. The formation is composed mostly of mudstone, sandy siltstone and lesser interbedded sandstone, and includes ning-upward intervals. It does not contain marine fossils and is interpreted to represent a largely non-marine regression (Petroconsultants 1989, Bradshaw et al 1990, Nicoll 2006b, Totterdell 2006). Possible sabkha or tidal at and uvial depositional settings were suggested by Petroconsultants (1989). A palyno ora from the base of the Darbilla Formation indicates a latest Famennian (uppermost Strunian’ sub-stage) age for the unit (Nicoll 2006b).

LATE CARBONIFEROUS–EARLY PERMIAN: BASIN PHASE 4

Kulshill Group equivalent

The Arafura Group is unconformably overlain by a Late Carboniferous–Early Permian sedimentary succession that is approximately equivalent in age to the Kulshill Group of the Bonaparte Basin (Totterdell 2006). Kulshill Group equivalent rocks reach a maximum thickness of about 5000 m in the Goulburn Graben, which was formed at this time, but the original thickness of the group was probably much greater, as it is interpreted that up to 3000 m of section has been eroded following deformation and uplift in the Triassic (Struckmeyer et al 2006). The lower part of the group thickens into the bounding planar normal faults of the graben (Figure 35.2), indicating that it was a part of the rift succession. However, the upper part does not exhibit any noticeable divergence into the faults and is therefore considered to represent post-rift deposition. Kulshill Group equivalent rocks to the north of the Goulburn Graben have a sag to sheet-like geometry and a relatively uniform thickness (maximum 3000 m), except where eroded around the margins of the basin. They are structurally conformable with the underlying rocks and are also interpreted to be part of the post-rift succession. Seismic and magnetic data indicate that there was some magmatic activity in the basin during the rifting phase that resulted in the emplacement of sills and dykes, and a large magmatic body within the Goulburn Graben (Totterdell 2006). A dolerite intersected in drillhole Kulka-1 has been dated by K-Ar method at 293 ± 3 Ma (Bradshaw et al 1990).

The Kulshill Group equivalent succession comprises interbedded sandstone, siltstone and claystone, with minor coal, and dolomitic rocks (Totterdell 2006). These were deposited in a variety of environments ranging from uvial to marginal marine to shallow marine (Petroconsultants

1989). Palynological studies by Helby (2006) show that that this interval spans the Pennsylvanian–mid-Cisuralian APP11 to APP122 palyno oral zones of Price (1997), but most of the succession is Early Permian in age and only the basal 100 m corresponds to the Late Carboniferous. Petroconsultants (1989) divided the succession in several drillholes into four unnamed units and correlated these with the Tanmurra Formation, Point Spring Sandstone, Kuriyippi Formation and Treachery Formation of the Bonaparte Basin. However, improved dating of the succession shows that a better correlation is with units from the younger interval Kuriyippi Formation–lower Keyling Formation (see ).

The Kulshill Group equivalent succession is separated by a major unconformity from overlying strata of the Jurassic to Cenozoic Money Shoal Basin (Figure 35.2). In contrast to Arafura Basin strata, which are complexly faulted and folded, the Money Shoal Basin succession is generally undisturbed.

STRUCTURE AND TECTONICS

The Arafura Basin was initiated in the Neoproterozoic as a result of northwest–southeast-directed upper crustal extension that produced a series of northeast–southwest-trending half grabens across the basin. The subsidence history was episodic, limited to four periods of basin-wide subsidence (Basin phases 1–4) separated by long, relatively tectonically quiescent periods of non-deposition and erosion. Minor localised deformation in the Devonian and Carboniferous was probably due to the effect of far-

eld stresses associated with the Alice Springs Orogeny (Totterdell 2006). The WNW–ESE-trending Goulburn Graben (Figures 35.2, 35.3, 35.7) was formed in the Late Carboniferous to Early Permian, in response to oblique extension, and underwent oblique inversion in the Triassic during a phase of regional contractional deformation (Basin phase 5 of Totterdell 2006). The main deformations events that have affected the basin are discussed below, in ascending date order.

Neoproterozoic extensional faulting

Neoproterozoic half grabens occur over much of the northern basin (Figures 35.3, 35.7) and are in lled by Wessel Group sediments of Basin phase 1. They are bounded by simple planar normal faults that have a generally NE–SW strike, and dip to either the northwest or southeast, suggesting approximately NW–SE extension. Towards the centre of the basin, a displacement along these faults of up to 7000 m has been estimated (Totterdell 2006). In the western part of the northern Arafura Basin are WNW–ESE-oriented accommodation zones across which the polarity of the faults switches from northwesterly directed throw in the south to southeasterly directed throw to the north. A series of small extensional faults on the western margin of the basin has a NNW–SSE orientation, sub-parallel to the interpreted direction of extension. Totterdell (2006) suggested that the orientation of these cross faults may have been in uenced by the pre-existing structural fabric of the underlying Pine Creek


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(northern Arafura Basin)

Arafura Basin

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(Arafura Basin)

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Offshore Arafura Basin(under cover)

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dry, abandonedPetroleum exploration well

oil showoil/gas showoil indicationoil/gas indication

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Money Shoal-1 Chameleon-1

Torres-1

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Poor seismic imaging:large, widely spaced faults

Figure 35.7. Arafura Basin fault map (compiled from Totterdell 2006, gures 19, 21). Neoproterozoic extensional faults (purple) are mapped at base of Wessel Group. Dashed red lines show accommodation zones, across which the polarity of faults switches from northwesterly directed throw in south to southeasterly directed throw in north. Base Kulshill Group equivalent faults (blue) are mostly Late Carboniferous extensional faults, many of which experienced Middle–Late Triassic reverse reactivation. Thrust fault to south of Kulka-1 formed during Triassic deformation.

Orogen. In the eastern part of the basin, there appears to be a change in architecture to large-displacement, widely-spaced faults. This change in structural style could re ect variations in the underlying basement fabric from west to east, from the complex deformation and strong structural fabric of the Pine Creek Orogen to the mildly deformed and eastward-thickening succession of the McArthur Basin (Totterdell 2006).

Minor Palaeozoic deformation

No known signi cant deformation events occurred between deposition of Basin phases 2 (middle Cambrian–Early Ordovician), 3 (early–middle Palaeozoic) and 4 (Late Carboniferous–Early Permian). Despite the presence of lengthy hiatuses between the Wessel, Goulburn and Arafura groups, the Palaeozoic basin succession is


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Arafura Basin

relatively structurally conformable (Totterdell 2006). The only indication of structural movement in the early–middle Palaeozoic succession is the absence of parts of the Goulburn and Arafura groups in some of the wells drilled in the Goulburn Graben, suggesting that there was some localised uplift and erosion prior to deposition of the Arafura Group. The timing of this minor disturbance coincides with the Middle Devonian Pertnjara-Brewer events of the Alice Springs Orogeny in central Australia (see Aileron Province) and suggests it may be related to the far- eld effects of these events. The hiatus of approximately 45 million years between the Arafura Group and the overlying Kulshill Group correlates with the nal, Early Carboniferous phase (Eclipse Event) of the Alice Springs Orogeny. Although there is no seismic evidence of any widespread contractional deformation of the Arafura Basin at that time, there is evidence of signi cant localised uplift and erosion, with at least 1000 m of Arafura Group missing at Tasman-1 (Totterdell 2006).

Late Carboniferous–Early Permian extensional faulting

The Goulburn Graben formed during a phase of Late Carboniferous–Early Permian northeast–southwest extension. It is a narrow, highly structured zone that has a west-northwest–east-southeast trend in the east and a northwest–southeast trend in the west (Figures 35.3, 35.7). This orientation might be re ecting the underlying structural grain of basement rocks (Totterdell 2006). Along much of its length, the Goulburn Graben has the morphology of a half graben, with master detachment faults de ning the northern margin and the southern marginal faults only intermittently developed. The bounding fault system to the north dips at an angle of about 50º to the south-southwest or southwest. Carboniferous–Permian extensional faulting appears to have been con ned to the Goulburn Graben, as there is little seismic evidence for extensional faulting of this age elsewhere (Totterdell 2006).

Mid–Late Triassic contraction

During the Middle–Late Triassic, the Arafura Basin, and in particular the Goulburn Graben, experienced a major phase of contractional deformation (Basin Phase 5 of Totterdell 2006). The effects of this deformation varied markedly across the basin. In the Goulburn Graben, it was relatively intense and was characterised by folding, inversion on pre-existing faults, the formation of new thrust faults, uplift and erosion. In the northern Arafura Basin, the affects of the deformation were less intense; limited contractional reactivation of Neoproterozoic half grabens resulted in the inversion of some Neoproterozoic extensional faults and the formation of inversion anticlines. The direction of regional compression is interpreted to have been NNW–SSE and was highly oblique to the dominant fault trends of the Goulburn Graben, resulting an element of dextral strike-slip or transpressional movement on parts of the fault system (Totterdell 2006).

Minor latest Triassic/Early Jurassic extensional faulting

After the Triassic deformation event, the margins of the Arafura Basin were uplifted, resulting in a basinward tilt,

followed by erosion and the formation of a peneplain across the basin and adjacent basement areas. During this period of erosion, the basin appears to have been affected by a minor extensional episode that involved relatively small-displacement, planar normal faulting within the upper part of the Arafura Basin succession. On the western margin of the basin, some older faults were reactivated and Triassic inversion anticlines were offset (Totterdell 2006). This faulting appears to predate the unconformity at the base of the Money Shoal Basin and is therefore probably older than later Jurassic extensional episodes that partly controlled deposition of the Money Shoal Basin succession (Struckmeyer 2006c).

MINERAL RESOURCES

The offshore Arafura Basin is very prospective for petroleum, but to date, there have been no commercial hydrocarbon discoveries. In the onshore Arafura Basin, known mineral occurrences include bauxite on Marchinbar and Elcho islands, and a small iron ore occurrence near Galiwinku (Figure 35.5). The following summary of these occurrences is derived from Ferenczi (2001).

Bauxite

Lateritic bauxite has developed on Neoproterozoic rocks of the Wessel Group at Marchinbar and Elcho islands.

Marchinbar Island

Bauxite deposits were rst reported from Marchinbar Island ( ) by Owen (1949), after he received samples, collected by the Northern Territory Coastal Patrol Service, that assayed up to 40.8% Av.Al2O3. The main lateritic bauxite deposits lie on the east coast of the island and were investigated by the Australian Aluminium Commission in the early 1950s. Ore resources for the seven tested deposits total 9.94 Mt and average 46.0% available Al2O3 and 4.0% reactive SiO2 (Owen 1953).

The deposits are developed over sedimentary rocks of the Marchinbar Sandstone. The bauxite ore consists predominately of cemented pisoliths of gibbsite, that have light brown and red-brown cores (Owen 1954). A tubular bauxite bed underlies pisolitic ore in several of the deposits (eg Able, Sphinx Head, Dog and Easy). Tubular ore reaches a maximum thickness of about 2 m and lenses out to the west, where the westerly deposits are all pisolitic (Ferenczi 2001). The underlying laterite is up to 10 m thick and largely consists of nodular ferricrete. The largest known deposit on the island is Able, which occupies an area of about 880 000 m2. One hundred and forty-two sampling pits were excavated by the Australian Aluminium Commission on a 61 x 122 m grid. A non-JORC Resource is given at 4.7 Mt at 47.1% Al2O3 (Ferenczi 2001). Ore thickness varies from 0.76 m (cut-off) to 5 m and averages 2.4 m (Owen 1953). Pisolitic bauxite forms the bulk of the resource (97%); the remaining 3% consists of massive and tubular bauxite, which underlies the pisolitic ore in the eastern section of the deposit. Bauxite quality usually varies with depth and lower grades are often found in the upper and lower portions of the pro le. The upper 0.5–1 m


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of the bauxite bed in the eastern area of the deposit contains 10–25% quartz sand and ne detrital material. Dry screening of samples from this area gave an average recovery factor of 96% silica is in the form of free quartz (1.1%) and reactive (mostly kaolinite) silica (3.0%). Iron oxides (Fe2O3), which are mainly in the form of goethite, average 15.7% and TiO2

averages 3.3% (Ferenczi 2001).

Elcho Island

At the eastern side of Elcho Island, two bauxite occurrences, separated by about 3 km, were recorded by Plumb (1965).

These occurrences consist of thin loose intervals of pisolitic and tubular bauxite that unconformably overlie unaltered Marchinbar Sandstone (Plumb and Gostin 1973). The pisolitic layer is about 2 m thick and forms a series of discontinuous exposures over a 3 km strike length. Sand dunes cover the bauxite, adjacent to and away from the coast. A single sample from one of the occurrences assayed 45.7% Al2O3 and 25% SiO2 (Plumb 1965). The high silica value may indicate contamination by quartz grains derived from nearby sand dunes. A laterite sample obtained during reconnaissance work by BHP (1964) near the southernmost occurrence assayed 25.7% Al2O3, 28.0% total SiO2 and 23.3% Fe2O3. This area is essentially untested and may host bauxite deposits comparable to those on Marchinbar Island (Ferenczi 2001).

Iron ore

Elcho Island iron ore deposit

The Elcho Island iron ore deposit is a bauxitic lateritic pro le developed within the Elcho Island Formation. The deposit extends for about 2.5 km along the western coastline of the island, just to the north of Galiwinku. A lower sandy haematite layer and an upper haematitic sandstone bed are present in the upper part of the laterite pro le. The massive lower haematite bed is up to 0.45 m thick and contains the bulk of the iron ore resource (600 000 t grading 60.4% Fe and 0.054% P), as estimated by Rix (1964b). Most of the ore lies at or near the surface, with the overburden gradually increasing to the north where it reaches a maximum of 6 m. The overlying haematitic sandstone is up to 1.2 m thick and averages 40.4% Fe and 0.57% P (Rix 1964b).

Petroleum

The Arafura Basin is considered to have signi cant potential for petroleum, but so far there have been no commercial discoveries. Oil shows and in situ occurrences of bitumen are known from a number of stratigraphic levels. Nine exploration wells have been drilled, all within the Goulburn Graben, and four of these have recorded signi cant oil shows in Palaeozoic strata. The majority of the basin outside the Goulburn Graben remains underexplored.

In the early 1920s, bitumen was reported from Elcho Island, leading to the formation of the Elcho Island Naphtha and Petroleum Company, which drilled several unsuccessful holes in the 1920s on Elcho Island (Bell 1923). In the 1960s and early 1970s, stratigraphic drilling was carried out on Bathurst and Melville islands (McLennan et al 1990). In 1971, Shell Development (Australia) Pty Ltd drilled the

rst well in the offshore Arafura Basin (Money Shoal-1) to test the Mesozoic Money Shoal Basin succession. At about the same time, Elf Aquitaine Petroleum was operating in the central southern region of the Arafura Sea. These two operators carried out extensive mapping based on seismic data and de ned the Goulburn Graben as an important structural feature. The next phase of exploration in the early 1980s involved a number of operators, including Diamond Shamrock Corporation, Esso Australia Pty Ltd, Petro na Exploration Australia SA and Sion Resources Ltd. A

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. Geology and location of bauxite deposits on Marchinbar Island (after Ferenczi 2001, modi ed from Plumb 1965).


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Arafura Basin

number of wells were drilled at this time to test the Arafura Basin succession and Arafura-1 recorded oil shows over a 425 m interval in the Devonian and Ordovician sections (Struckmeyer and Earl 2006a).

In the late 1980s and early 1990s, BHP Petroleum Pty Ltd targeted Mesozoic plays in the Goulburn Graben. The exploration program included an extensive 17 000 line km seismic survey, a regional aeromagnetic survey, and the drilling of three exploration wells. During the early 1990s, Geoscience Australia acquired a total of 5342 line km of regional deep seismic data across the Arafura Basin. In the 2000s, a number of exploration activities have contributed to the available datasets and have improved the prospectivity of the region. These include non-exclusive regional 2D seismic datasets by TGS Nopec Geophysical Company Pty Ltd in 1998 and Veritas DGC Inc in 2002, and Synthetic Aperture Radar acquisition and interpretation across the region by INFOTERRA Ltd in 2003 (Struckmeyer 2006d).

Source rocks

Potential source rock intervals occur at a number of levels in the stacked McArthur, Arafura and Money Shoal basins (Figure 35.9). The Palaeo- to Mesoproterozoic McArthur Basin, which is interpreted to underlie much of the eastern Arafura Basin, contains at least ve potential source rock intervals, de ned as having total organic carbon (TOC) greater than 0.5%. Of these, the Barney Creek Formation (McArthur Group) and Velkerri Formation (Roper Group) have the highest TOC values, which range up to 8% and 12%, respectively (Crick et al 1988, Jackson et al 988). If these potentially excellent source rocks underlie the Arafura

Basin, it is conceivable that hydrocarbon generation and expulsion from these rocks may have occurred and may have charged younger reservoir units (Struckmeyer and Earl 2006b).

Drillhole data and regional correlations indicate that a number of potential source rock intervals occur within the Arafura Basin succession (Bradshaw et al 1990, Edwards et al 1997). The Neoproterozoic Wessel Group contains promising source rocks (eg Raiwalla Shale and Elcho Island Formation), but no geochemical or organic petrological data are available for this interval and it is yet to be properly evaluated. Samples from the Cambrian–Ordovician Goulburn Group have returned TOC values of up to 8.6%, but the higher values represent migrated oil and solid bitumen rather than dispersed organic matter. However, the presence of both abundant bitumen and oil stains in early Palaeozoic samples is indicative of a multi-charge history from a proli c nearby source (Sherwood et al 2006). Oil stains in samples of Early Palaeozoic rocks from drillholes Arafura-1 and Goulburn-1 have similar geochemical and isotopic characteristics to the early middle Cambrian Thorntonia(!) petroleum system of the Georgina Basin (Boreham and Ambrose 2007).This suggests that the effective source rock within the Goulburn Group is most likely to occur in the Jigaimara Formation, which is also middle Cambrian in age (Sherwood et al 2006). Limited data from the Upper Devonian Arafura Group suggest a generally poor source potential for this interval, although potentially fair source rocks may be present within marine calcareous mudstones. Good to very good source rocks with Type II/III kerogen are present in the Permian–Carboniferous Kulshill Group equivalent succession. The

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RM

IAN

CA

RB

ON

IF.

MIDDLE

LATE

EARLY

LATE

EARLY

LATE

EARLY

Shows PotentialSource

Stratigraphic unit Rock type / palaeoenvironment

A

DE

VO

NIA

N

Darbilla FmYabooma FmDjabura Fm

Mooroongga FmMilingimbi Fm

Naningbura Dol

Jigaimara Fm

?KulshillGroup

equivalent

SealRes250

300

350

400

450

500

?

Terrestrial

Fluvial–deltaic

Shallow marine

LimestoneShale

Unconformity

Sandstone Dolostone Petroleum exploration welloil showoil/gas showoil indication A12-195.ai

Figure 35.9. Stratigraphic succession and petroleum systems elements for the Arafura Basin (modi ed from Totterdell 2006: gure 9). Abbreviations: A = Arafura-1; Dol = Dolomite; Fm = Formation; G = Goulburn-1; K = Kulka-1; Res = Reservoir; Ta = Tasman-1.


Arafura Basin

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typical TOC range is <0.4 to 3% and several samples contain up to 9% TOC (Sherwood et al 2006).

Reservoirs and seals

Potential reservoir rocks occur in both the Arafura and overlying Money Shoal basins. In the Arafura Basin, they include shallow marine limestone and dolostone units of the Cambrian–Ordovician Goulburn Group, terrestrial to uvio-deltaic interbedded sandstone and shale units of the Devonian Arafura Group and parts of the Permian–Carboniferous Kulshill Group equivalent (Struckmeyer and Earl 2006b, Figure 35.9). Goulburn Group carbonate rocks have been shown to host oil and gas shows, and oil indications. They are generally ne grained, although some sandier intervals are present in the Ordovician section (Petroconsultants 1989). Original porosity is likely to have been mostly poor, but secondary porosity (vugs, fractures) has improved reservoir quality to 7.7% maximum porosity (Struckmeyer and Earl 2006b). Arafura Group sandstone and shale intervals also host oil shows. These rocks have a reported maximum porosity and permeability of 19% and 7.83 mD, respectively, but average 9.6% porosity. Diagenesis has destroyed a signi cant proportion of the primary porosity, but where the rocks have not been so deeply buried in the northern part of the basin and are less hydrothermally altered, reservoir quality is likely to be better (Struckmeyer and Earl 2006b). Kulshill Group equivalent rocks generally have poor reservoir quality, with porosities averaging 5.5%, except for the upper parts of this unit where a maximum porosity of 17.7% has been recorded. Multiple fracture sets could enhance the overall permeability and porosity of this interval.

Potential seal rocks are present throughout the Arafura Basin succession, and potential regional seals are present in the Devonian and Cretaceous successions. Shale interbeds could provide intraformational seals for carbonate reservoir rocks within the Goulburn Group, and a variety of diagenetic seals and traps could also be present in carbonate units. Relatively thick (up to 400 m) shale intervals within the upper part of the Arafura Group could form intraformational seals and possibly a regional seal. Potential seals within the Kulshill Group equivalent succession are likely to be intraformational. Fine-grained Cretaceous sedimentary rocks of the Bathurst Island Group (Money Shoal Basin) directly overlie Palaeozoic rocks of the Arafura Basin in the eastern part of the basin and could provide a regional seal (Struckmeyer 2006b).

Thermal maturity

Sherwood et al (2006) evaluated the thermal maturity of potential source rocks from a number of levels within the Arafura and Money Shoal basins using a combination of FAMM (Fluorescence Alteration of Multiple Macerals) and conventional organic petrological analyses. Boreham (2006) provided analyses of the organic geochemical maturity of a number of source rock samples from the same succession. Based on these datasets, sedimentary rocks of the Cambrian–Ordovician Goulburn Group have been assessed as being presently mature to overmature for oil

generation, Devonian Arafura Group rocks are early mature to mature for oil generation, and Carboniferous–Permian Kulshill Group equivalent rocks are immature to mature for oil generation, with maturity dependent on the thickness of Money Shoal Basin overburden (Struckmeyer and Earl 2006b). Maturation levels of potential source rocks from the underlying McArthur Basin range from marginally mature to overmature for oil generation (Crick et al 1988, Jackson et al 1988, Ambrose and Silverman 2006).

Prospectivity

A variety of possible structural and stratigraphic play types are present within the Arafura Basin, involving the juxtaposition of potential source, reservoir and seal rocks. Structural plays include large faulted anticlines, tilted fault blocks, and inversion anticlines formed during the Triassic deformation, whereas stratigraphic plays involve regional unconformities, intraformational and regional facies changes, and diagenesis (Petroconsultants 1989, Struckmeyer 2006d). The overlying Money Shoal Basin also contains a variety of stratigraphic and combined structural/stratigraphic plays that could have been charged by hydrocarbons sourced from underlying Palaeozoic and Mesozoic source rocks (McLennan et al 1990, Struckmeyer 2006d). The thick successions of the Arafura and Money Shoal basins therefore provide a diverse range of potential traps at a number of stratigraphic levels.

Evidence for hydrocarbon generation and expulsion in the Arafura Basin includes oil shows/indications and gas indications in most drillholes (Earl 2006), and the presence of interstitial solid bitumen in many samples (Sherwood et al 2006). However, in the Goulburn Graben, geohistory studies have indicated that oil generation from early Palaeozoic rocks may have been halted after a period of early migration and before signi cant structures that could trap the oil were generated (Moore et al 1996). Early-formed hydrocarbon accumulations might also have been breached by erosion following the Triassic deformation event (Higgins 2009). This might explain the failure to date to nd a commercial accumulation in this portion of the basin. Indirect hydrocarbon indications in the northern Arafura Basin include shallow gas interpreted on sub-bottom pro le data and conventional seismic data, degraded seismic data, which could represent hydrocarbons within the succession, and Synthetic Aperture Radar (SAR) slicks on the sea surface. These indicators provide evidence for active petroleum systems within this part of the basin (Struckmeyer 2006d). Hydrocarbon generation in the northern Arafura Basin could have occurred much later than in the Goulburn Graben, with migration postdating structuring, suggesting that this area could be more prospective for petroleum than previously considered (Moore et al 1996, Higgins 2009).

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Documents

Geology and mineral resources of the Northern Territory · 35:1 Arafura Basin Creek Orogen. To the west, it is unconformably overlain by relatively undeformed Mesozoic and Cenozoic