Geology and Mineral Resources of the Northern Territory

Geology and mineral resources of the Northern Territory

Ahmad M and Munson TJ (compilers)

Northern Territory Geological Survey Special Publication 5

Chapter 35: Arafura Basin

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.

Disclaimer While 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. Arafura Basin Current as of September 2012 Chapter 35: ARAFURA BASIN M Ahmad and TJ Munson

INTRODUCTION Creek Orogen. To the west, it is unconformably overlain by relatively undeformed Mesozoic and Cenozoic sedimentary The Neoproterozoic to Permian Arafura Basin extends rocks of the Money Shoal Basin, which are up to 4.5 km from the onshore Northern Territory into Indonesian waters thick (Figure 35.2). This succession is continuous with that (Figure 35.1) and covers an area of about 500 000 km2. of the Bonaparte Basin to the west, but thins rapidly to the Structurally, the basin consists of northern and southern east, so as to form the onlapping edge of a vast Mesozoic sections separated by the large deformed Goulburn Graben to Cenozoic depositional area that extends over much of (Bradshaw et al 1990; equivalent to Arafura Graben of offshore northwestern Australia (Bradshaw et al 1990, Petroconsultants 1989). The Goulburn Graben is a west- Struckmeyer 2006b). Mesozoic and Cenozoic sedimentary northwest-trending asymmetric feature, over 350 km long rocks of the Carpentaria Basin onlap the Arafura Basin and up to 70 km wide, that contains a sedimentary section to the east and southeast, and are up to 1760 m thick. The in excess of 10 km thick. The region to the north of the northern limits of the Arafura Basin are not well de¿ned, Goulburn Graben forms the basin’s main depocentre and although seismic data indicate that it extends towards contains a sedimentary succession up to 15 km thick the Aru Ridge and Merauke Rise to the south of Papua, (Figures 35.2, 35.3). South of the Goulburn Graben a north- Indonesia (Moss 2001). Palaeozoic sedimentary rocks are dipping relatively undeformed ramp that extends onshore also known from central Papua, indicating that the original contains up to 3 km of sedimentary rocks. The Arafura limits of the basin prior to Mesozoic tectonism may have Basin succession comprises sandstone, shale, limestone, been at least this far to the north (Fortey and Cocks 1986, dolostone, coal beds and glacial deposits and is summarised Nicoll and Bladon 1991). To the northwest, the poorly in Figure 35.4 and Table 35.1. Totterdell (2006) described explored Barakan Basin in Indonesian waters is of similar four main phases of deposition within the basin (Basin age and has a similar structure to that of the Arafura Basin phases 1–4) in the Neoproterozoic (Wessel Group), middle (Barber et al 2004). Cambrian–Early Ordovician (Goulburn Group), Late This chapter focuses on the onshore sedimentary Devonian (Arafura Group) and Late Carboniferous–Early succession of the Arafura Basin in the NT. A full discussion Permian (Kulshill Group equivalent). These basin phases of the other components of the basin is beyond the scope were separated by long, relatively tectonically quiescent of this volume, although brief summaries of the offshore periods of non-deposition and erosion. Neoproterozoic and successions are also included. Signi¿cant studies of the Cambrian sedimentary rocks, which outcrop on the northern Arafura Basin and in particular, the onshore succession, extremity of Arnhem Land, from east of the Cobourg include Plumb (1963,1965), Rix (1964a, 1965), Dunnet Peninsula to the Wessel Islands and extending inland up (1965), Petroconsultants (1989), Bradshaw et al (1990), to about 80 km, are the only onshore manifestation of the McLennan et al (1990), Plumb and Roberts (1992), Rawlings basin. et al (1997), Carson et al (1999), Struckmeyer (2006a, b), The Arafura Basin succession is underlain by Palaeo- Totterdell (2006), Geoscience Australia (2008, 2012) and to Mesoproterozoic rocks of the McArthur Basin and Pine Zhen et al (2011).

130° 131° 132° 133° 134° 135° 136° 137° Mesozoic–Cenozoic 9° Indonesia Neoproterozoic–Palaeozoic Australia Palaeo–Mesoproterozoic basins

(northern Arafura Basin) Palaeo–Mesoproterozoic orogens 10°

Money Shoal Basin Archaean onshore Arafura Basin Bonaparte Basin (Goulburn Graben) Money Shoal Basin (Arafura Basin) offshore Arafura Basin (under cover) 11° road rail Carpentaria international border Basin 12° Nhulunbuy Jabiru locality Darwin Arafura Basin Jabiru Daly Basin 13° McArthur Arnhem Pine Creek Basin Carpentaria Province Orogen Basin

0300 kmNORTHERN TERRITORY A12-192.ai

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 35:1 Special publication 5 Arafura Basin

NEOPROTEROZOIC TO ?EARLY CAMBRIAN: (Totterdell 2006, Figure 35.3, see Structure and tectonics). BASIN PHASE 1 The ¿ll of these half grabens and the overlying sag phase sedimentary rocks comprise the Wessel Group (Plumb et al Wessel Group 1976, Figure 35.2, 35.4), which is a succession of shallow marine, mostly quartz sandstone, mudstone and minor Deposition in the Arafura Basin commenced in the carbonate rocks. It is the only part of the basin, along with Neoproterozoic during a period of upper crustal extension the middle Cambrian Jigaimara Formation (basal Goulburn that resulted in the formation of a series of half grabens, Group), that is exposed onshore, where it reaches a composite which form an overall northeast-trending depocentre in thickness estimated to be about 2300 m (Rawlings et al the northern basin that continues into Indonesian waters 1997). Offshore, in the central part of the basin, it reaches A B Goulburn Graben northern Arafura Basin Kulka 1 W ESE 0

4 Two-way time (s) Two-way

0 25 km

7 A09-246.ai Money Shoal Basin Arafura Basin Cenozoic Late Carboniferous–Permian Neoproterozoic–?early Cambrian Woodbine Group equivalent Kulshill Group equivalent Wessel Group (sag phase) Cretaceous Late Devonian Wessel Group (rift phase) upper Bathurst Island Group Arafura Group Wessel Group and older Figure 35.2. Geoseismic cross- lower Bathurst Island Group Cambrian–Ordovician section through Arafura and Jurassic–Early Cretaceous Goulburn Group Basement Money Shoal basins (modi¿ed after Flamingo Group equivalent– Totterdell 2006, ¿gure 5). Location Troughton Group equivalent shown on Figure 35.7.

132°30' 133°30' 134°30' 135°30' 136°30' 137°30'

9°30'

INDONESIA AUSTRALIA

5000 10°30' 4000

3000

2000

11°30' 1000

0 1000

2000

3000 12°30' TWT (ms) TWT 4000

5000

6000 NORTHERN 6473 TERRITORY 0 25 50 km

A09-247.ai Figure 35.3. Arafura Basin Petroleum exploration well Normal fault sediment thicknesses (milliseconds dry, abandoned Thrust fault two-way time), showing signi¿cant oil show normal faults involved in graben oil/gas show Goulburn Graben development and location of oil indication drillholes (modi¿ed from Totterdell oil/gas indication 2006, ¿gure 6).

Geology and mineral resources of the Northern Territory 35:2 Special publication 5 Arafura Basin a maximum thickness of about 10 000 m (Totterdell 2006). of 790 and 770 Ma, respectively, were determined for The group outcrops in an arcuate belt along the northwestern a single glauconite from the Elcho Island Formation at coastline of the Territory, from WESSEL ISLANDS- the top of the group (McDougall et al 1965). Plumb et al TRUANT ISLAND1, through northern and western (1976) reinterpreted the age of the entire Wessel Group as ARNHEM BAY, to eastern and northern MILINGIMBI Cambrian, based on the purported presence of Skolithos and JUNCTION BAY (Figure 35.5). It unconformably trace fossils in the Buckingham Bay Sandstone (Plumb overlies various formations of the McArthur Basin and 1963, Dunnet 1965), and the discovery of a middle is overlain, probably disconformably, by the Jigaimara Cambrian metazoan fauna in what was then considered Formation. The onshore Wessel Group comprises, in to be the Elcho Island Formation. However, Rawlings ascending order, the Buckingham Bay Sandstone, Raiwalla et al (1997) reinterpreted the Skolithos trace fossils as Shale, Marchinbar Sandstone and Elcho Island Formation abiogenic dewatering structures and assigned the metazoan (Table 35.1). These generally form an arcuate to linear fauna to the Jigaimara Formation. The discovery of the outcrop tract parallel to the preserved margins of the basin carbonaceous fossil Chuaria in the Raiwalla Shale (Haines with the younging direction northward towards the basin’s 1998) subsequently reaf¿rmed a Neoproterozoic age for the offshore depocentre. Seismic data indicate that the basal, group, although an early Cambrian age for the top of the offshore rift-¿ll succession of the group is not represented group cannot be discounted. in onshore areas (Totterdell 2006). The Wessel Group is probably equivalent in age to The age of the Wessel Group is poorly constrained Supersequence 3 and 4 rocks of the Centralian A Superbasin between underlying Mesoproterozoic basement rocks to the south (see &HQWUDOLDQ6XSHUEDVLQ¿JXUH). and the overlying middle Cambrian Jigaimara Formation (Goulburn Group). It was originally considered to be Buckingham Bay Sandstone Neoproterozoic after Rb-Sr and K-Ar minimum dates The Buckingham Bay Sandstone (Plumb and Roberts 1992) unconformably overlies various units of the McArthur Basin 1 Names of 1:250 000 mapsheets are in large capital letters, eg and is overlain conformably by the Raiwalla Shale. The MILINGIMBI. formation outcrops in a broad gently dipping arc around

Series/ Group Formation Group Series/ Stage Stage

Period

Kulka-1

Cretaceous

Group

Bathurst Island

Cretaceous 1802

Money Shoal Basin Shoal Money

equivalent/

Cretaceous

Flamingo Gp Flamingo

Troughton Gp Troughton

Jurassic–early Tasman-1 2540 Torres-1

653 Arafura-1 Cisuralian 3095 400.5 Cisuralian equivalent Permian 1162 783 Kulshill Group Pennsylvanian

Car equivalent 694.5 1295 Kulshill Group Arafura Basin 1188 Darbilla Fm (Strunian) Pennsylvanian 1074 Goulburn-1 Car 1450 183 TD 3998 1590 Yabooma Fm Fammennian 1409 455

Group

1895 Arafura Djabura Fm ? 2275 2058 1704 776 Frasnian? 1835 977 Mooroongga Fm Floian 1998 2157 1146 Milingimbi Fm Ord Devonian Permian Claystone Tremadocian TD 2720 TD 1300 late TD 2758 Naningbura Siltstone Dolomite middle

Goulburn Group Cambrian 3126

Shale ? ? Arafura Basin Onshore Jigaimara Outcrop Formation Sandstone Depth (m) 3596 0 TD 3635 Elcho Limestone Island Fm

Marchinbar Dolostone Sandstone 1000

Wessel Group Wessel

Raiwalla Neoproterozoic Unconformity Shale

Buckingham Bay A12-184.ai Sandstone 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.

Geology and mineral resources of the Northern Territory 35:3 Special publication 5 Arafura Basin the margin of the onshore Arafura Basin in southwestern area on the northwestern side of Flinders Peninsula around the JUNCTION BAY, MILINGIMBI, ARNHEM BAY and mouth of the Kurala River. The formation is estimated to be southeastern WESSEL ISLANDS (Figure 35.5), and about 350 m thick near its type locality (Rawlings et al 1997). is best exposed near the coast of Flinders Peninsula and The basal unit consists of massive to Àat-bedded, cross- adjacent islands, with exposures becoming more broken and bedded, and occasionally rippled, medium- to coarse- disintegrating into sand inland (Rawlings et al 1997, Carson grained and sometimes pebbly, medium- to very thickly et al 1999). In the absence of a complete section through the bedded, white to pale pink and yellow (with local red formation, Plumb and Roberts (1992) nominated a reference iron-oxide staining) sandstone. A local basal breccia or

Unit, max thickness, Depositional Lithology Stratigraphic relationships (distribution) environment Carboniferous–Permian KULSHILL GROUP EQUIVALENT 5000 m (offshore) Interbedded sandstone, siltstone and claystone, Fluvial to marginal Unconformable on Arafura Group with minor coal, and dolomitic rocks; palynoÀora. marine to shallow succession. Unconformably overlain by marine. Jurassic–Cenozoic Money Shoal Basin succession. Late Devonian ARAFURA GROUP Darbilla Formation, Mudstone, sandy siltstone and lesser interbedded Non-marine, Apparently conformable on Yabooma 380 m (offshore) sandstone; includes ¿ning-upward intervals; possibly sabkha or Formation. Unconformably overlain by palynoÀora. tidal Àat, and Àuvial. Jurassic Money Shoal Basin succession. Yabooma Formation, Interbedded siltstone with dolomitic intervals, Nearshore shallow Unconformable on Djabura Formation. 335 m (offshore) occasional thin sandstone beds; sparse fossil fauna marine. Apparently conformably overlain by Darbilla of conodonts, ¿sh and bryozoans. Formation, or unconformably overlain by Jurassic Money Shoal Basin succession. Djabura Formation, Interbedded, mudstone, siltstone, sandstone and Nearshore shallow Unconformable on Goulburn Group 466 m (offshore) minor carbonate rocks; diverse fossil fauna, marine. succession. Unconformably overlain by including conodonts, ostracods, phosphatic Yabooma Formation, or by Kulshill Group brachiopods, conulariids and ¿sh fossils; equivalent. palynoÀora. middle Cambrian to Early Ordovician GOULBURN GROUP Mooroongga Formation, Shale, limestone, sandstone, glauconitic sandstone, Shallow marine. Probably conformable on Milingimbi 201 m (offshore) minor chert, dolomitic in part; becomes more Formation. calcareous up-section; limited conodont fauna. Milingimbi Formation, Dolostone, limestone, glauconitic sandstone, shale; Shallow marine. Conformable on Naningbura Dolomite. 169 m (offshore) becomes more siliciclastic up-section; conodont Probably conformably overlain by faunas. Mooroongga Formation, or unconformably overlain by Arafura Group. Naningbura Dolomite Dolostone with silty dolostone intervals; conodont Shallow marine. Apparently conformable on Jigaimara 1128 m (offshore) fauna near top. Formation. Jigaimara Formation White to grey interbedded limestone, shale and Low-energy, shallow Disconformable or unconformable on Elcho 470 m (offshore and dolostone, silici¿ed to chert and brecciated; marine, probably Island Formation. onshore) possible microbial laminations; rich fossil fauna subtidal. of trilobites, bradoriids, hyoliths, lingulate brachiopods and sponge spicules. Neoproterozoic to ?early Cambrian WESSEL GROUP Elcho Island Formation Fine- to coarse-grained, thinly to medium bedded Shallow marine, Locally disconformable or possibly 650–700 m (onshore) sandstone, often calcareous or dolomitic and occasional exposed; conformable on Marchinbar Sandstone. locally glauconitic, with cross-beds, ripples, periodic evaporitic current lineations and load casts; minor mudstone conditions. interbeds; occasional carbonate intervals, locally strongly leached or silici¿ed to chert breccia Marchinbar Sandstone White, quartz-rich, ¿ne- to medium-grained Relatively high- Conformable and gradational on Raiwalla 300 m (onshore) sandstone, mostly medium bedded, with horizontal energy very shallow Shale. laminations, trough cross-beds, wave and current marine. ripples, rare desiccation cracks. Raiwalla Shale Grey and green micaceous mudstone, red-brown Subtidal marine Conformable with sharp contact on 1000 m (onshore) when weathered, interbedded with ¿ne- to shelf, gradual Buckingham Bay Sandstone. medium-grained tabular sandstone. upward shallowing with increasing storm inÀuence. Buckingham Bay White, grey, pale pink, yellow and red, ¿ne- High-energy Unconformable on McArthur Basin Sandstone to coarse-grained, mostly medium to thickly shallow marine. succession. 350 m (onshore) bedded sandstone, with common cross-beds and occasionally ripples; rare mudstone interbeds; local basal breccia and conglomerate. Table 35.1. Summary of Palaeozoic stratigraphic succession of the Arafura Basin.

Geology and mineral resources of the Northern Territory 35:4 Special publication 5 Arafura Basin conglomerate, up to several metres thick, contains poorly Raiwalla Shale sorted angular clasts up to boulder size in a sandstone The Raiwalla Shale (Plumb and Roberts 1992) outcrops matrix (Rawlings et al 1997). Higher in the succession, poorly in a broad arcuate belt through MILINGIMBI and pale grey, medium-grained, thickly bedded, massive to ARNHEM BAY (Figure 35.5). It overlies the Buckingham weakly Àat-bedded sandstone is interbedded with recessive, Bay Sandstone with a sharp concordant contact and is ferruginous and micaceous, thinly bedded, ¿ne-grained overlain conformably and gradationally by the Marchinbar sandstone and mudstone. Near the top of the formation, the Sandstone. The formation comprises mudstone with very lithology tends to be more uniform, comprising medium- ¿ne- to medium-grained tabular sandstone interbeds grained, medium- to thickly bedded sandstone, varying (Rawlings et al 1997). The lower mudstone-rich part of from white to red and yellow with weathering. No metazoan the formation is very recessive and is poorly exposed. fossils have been found in the Buckingham Bay Sandstone Sandstone scree dominates most surface exposures, so that and purported Skolithos trace fossils recorded by Plumb it is dif¿cult to determine the ratio of sandstone to shale. (1963) and Dunnet (1965), and used to suggest a Cambrian Better exposures in the upper half of the formation probably age for the entire Wessel Group by Plumb et al (1976), were reÀect an increasing proportion of sandstone interbeds up- subsequently interpreted as having been caused by the section. An accurate thickness cannot be determined for dewatering of Àuidised sand, and are therefore abiogenic the formation due to very shallow dips and the poor nature (Rawlings et al 1997). of outcrop, but it is estimated to be of the order of 1000 m The Buckingham Bay Sandstone is interpreted to (Rawlings et al 1997). Plumb and Roberts (1992) nominated have been deposited in a high-energy shallow marine a reference area for the formation around the Woolen River environment. The formation probably correlates with in ARNHEM BAY. the similar Bukalara Sandstone of the central northern Mudstone is micaceous, Àat- to wavy-laminated and Georgina Basin, which unconformably overlies the southern ¿ssile (shaly). Sandstone varies from quartz-rich to lithic McArthur Basin succession (Pietsch et al 1991). to micaceous, is ¿ne- (dominant) to medium-grained, and

Sphinx Head Baker Dog 050100 km Red Cliff Able Fox Easy

JUNCTION BAY WESSEL ISLANDS TRUANT ISLAND

Raiwalla Shale Truant Island

Elcho North Elcho South Elcho Maningrida Jigaimara Fm Galiwinku Probable Island Milingimbi

River

Milingimbi

r ve

r Woolen Ri

ve River

Goomadeer Ri Glyde MILINGIMBI ARNHEM BAY GOVE Rive r y River

River

River Cadell Gulbuwanga

River

Liverpool Mann

Goyder

Blyth

Mesozoic–Cenozoic Neoproterozoic–?early Cambrian Maningrida locality Jigaimara Fm Neoproterozoic– Milingimbi bauxite occurrence Palaeozoic Neoproterozoic Palaeo–Mesoproterozoic Elcho iron ore occurrence basins Elcho Island Fm Palaeo–Mesoproterozoic Wessel Group ARNHEM BAY 250k mapsheet NORTHERN orogens Marchinbar Sst TERRITORY

Raiwalla Shale A12-193.ai Buckingham Bay Sst

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.

Geology and mineral resources of the Northern Territory 35:5 Special publication 5 Arafura Basin is thinly to medium bedded, with a few packets containing Elcho Island Formation thicker beds. The sandstone typically displays Àat- to wavy- The Elcho Island Formation outcrops extensively in and some cross-lamination, and wave and current ripples southern WESSEL ISLANDS, northwestern ARNHEM are common on bed tops. Synaeresis cracks and mudclasts BAY and northeastern MILINGIMBI (Figure 35.5), along are also common, and soft-sediment deformation features the coasts of northern Arnhem Land and Elcho, Howard are present locally. Small (millimetre-sized) iron-oxide and Banyan islands, and it is also sparsely exposed inland inclusions, which are locally abundant, suggest that some above the slightly more resistant Marchinbar Sandstone. It intervals of the formation are pyritic in the subsurface. was de¿ned by Plumb and Roberts (1992), who nominated No metazoan or trace fossils have been recorded from the a reference section as cliff outcrops on Elcho Island, but Raiwalla Shale, but carbonaceous impressions assigned to was rede¿ned in Rawlings et al (1997), who nominated a Chuaria have been used to assign a Neoproterozoic age to type locality in western ARNHEM BAY. The formation is the unit (Haines 1998). at least locally disconformable, or possibly conformable on The Raiwalla Shale was probably deposited under the Marchinbar Sandstone and is probably disconformably subtidal, marine shelf conditions (Rawlings et al 1997). overlain by the Jigaimara Formation of the Goulburn Group. The basal contact is probably a marine Àooding surface and A thickness of 650–700 m is estimated for the Woolen River represents a rapid deepening from the very shallow water area (Rawlings et al 1997). conditions interpreted for the Buckingham Bay Sandstone. The Elcho Island Formation is a succession of ¿ne- to There is evidence for gradual upward shallowing with coarse-grained, locally glauconitic, thinly to medium-bedded increasing storm inÀuence through the succession sandstone, generally interbedded with minor mudstone and (Rawlings et al 1997). The Raiwalla Shale is correlated with chert. Sedimentary structures include trough and tabular the Cox Formation of the central northern Georgina Basin; cross-beds, wave and current ripples (Figure 35.6a), current this unit overlies the Bukalara Sandstone, an equivalent of lineations and load casts. The succession is sometimes the Buckingham Bay Sandstone (Pietsch et al 1991). calcareous or dolomitic, and chert breccia and leached rocks after carbonate are present locally. The age of the formation is Marchinbar Sandstone poorly constrained between the Neoproterozoic lower Wessel The Marchinbar Sandstone conformably and gradationally Group and the middle Cambrian Jigaimara Formation, but overlies the Raiwalla Shale and outcrops in a relatively linear a Neoproterozoic age is more likely from the absence of belt through western TRUANT ISLAND, southeastern metazoan or trace fossils, and from radiometrically dating of WESSEL ISLANDS, northwestern ARNHEM BAY and a single glauconite from low in the Elcho Island Formation eastern MILINGIMBI (Figure 35.5). It was de¿ned by (McDougall et al 1965) at about 770 Ma (K-Ar) and 790 Ma Plumb and Roberts (1992), who nominated a reference (Rb-Sr). The Elcho Island Formation was deposited under section on Marchinbar Island in WESSEL ISLANDS. shallow marine shelf conditions, which at times, reached the The formation generally outcrops poorly, with exposures point of exposure and desiccation (Figure 35.6b). Periodic commonly restricted to places where creeks have eroded evaporitic conditions are indicated by halite pseudomorphs through the regional laterite capping. It is an estimated and desiccation cracks near the base and top. 300 m thick in the vicinity of the Woolen River (ARNHEM BAY), where the most complete exposed section is located MIDDLE CAMBRIAN TO EARLY ORDOVICIAN: (Rawlings et al 1997). The upper contact with the Elcho BASIN PHASE 2 Island Formation is regionally concordant, but at the only locality where the actual point of contact can be seen, the Goulburn Group boundary is erosional and marked by a thin granule and pebble lag, suggesting the possibility of at least a local The early middle Cambrian–Early Ordovician Goulburn disconformity at this level (Rawlings et al 1997). Group (Petroconsultants 1989, McLennan et al 1990, The Marchinbar Sandstone is composed largely of clean, Bradshaw et al 1990, Nicoll et al 1996, Rawlings et al 1997) white quartz sandstone, which is dominantly medium- disconformably or unconformably overlies the Wessel grained, but which includes some ¿ne-grained beds, Group and is unconformably overlain by the Late Devonian mainly near the base. Thin red, ferruginous and matrix- Arafura Group. It has sag- to sheet-like geometry and is rich intervals are a minor component of the formation. structurally conformable with the upper, post-rift portion Mudclasts are very common near the base, but decrease in of the Wessel Group. The succession reaches a maximum abundance upwards. Most of the unit is medium-bedded, thickness of about 2000 m in the offshore central part of although more thinly and thickly bedded intervals are the northern Arafura Basin and contains, in ascending also present. Sedimentary structures include common order, the Jigaimara Formation, Naningbura Dolomite, horizontal lamination, trough cross-bedding, wave and Milingimbi Formation and Mooroongga Formation. current ripples, and rare desiccation cracks (Rawlings et al The basal part of the Jigaimara Formation is exposed in 1997). No metazoan or trace fossils have been found in the southern WESSEL ISLANDS, northwestern ARNHEM formation and its interpreted Neoproterozoic age is based BAY and northeastern MILINGIMBI (Figure 35.5), entirely on its stratigraphic position (Zhen et al 2011). A but the upper part of the unit and the other formations relatively high-energy very shallow marine environment is are only intersected in petroleum exploration drillholes interpreted for the unit and it probably represents the top in the Arafura Sea (Figure 35.4). The Goulburn Group of a shoaling cycle that began in the lower Raiwalla Shale represents prolonged deposition on a shallow marine shelf (Rawlings et al 1997). in a stable intraplate setting.

Geology and mineral resources of the Northern Territory 35:6 Special publication 5 Arafura Basin

The age of the Goulburn Group has been established intervals that was deposited in a predominantly shallow from the presence of a middle Cambrian marine fauna in marine environment. It is apparently conformable between the basal Jigaimara Formation and from Early Ordovician the Jigaimara Formation (below) and the Milingimbi conodont faunas in the Milingimbi and Mooroongga Formation, and is equivalent to units O1 to O7 of formations (Zhen et al 2011). In offshore drillhole Money Petroconsultants (1989). Nicoll et al (1996) originally named Shoal-1, unnamed and poorly dated ?Cambrian strata (from this unit the Naningbura Formation, but it was not de¿ned 2530–2575 m) contain interbedded andesitic volcanic rocks, and only brieÀy described. The µNaningbura Formation’ indurated ¿ne- to medium-grained arkosic sandstone, and was subsequently mentioned in Rawlings et al (1997), dark grey-green to black carbonaceous shale. The volcanic Carson et al (1999) and Struckmeyer (2006b), but none rocks might be stratigraphic equivalents of the Antrim of these publications provided enough detail to properly Plateau Volcanics (Brown et al 1968, Petroconsultants establish the unit with this name. Nicoll (2006a) renamed 1989, see Kalkarindji Province) and if so, then a late early the unit the Naningbura Dolomite, allocated a type section Cambrian age is possible for the base of the group. a Jigaimara Formation The 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 Àat-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. b 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 f rom sequence st ratigraphic st udies of middle Cambr ian st rata in the Georgina Basin (Shergold et al 1988, Southgate and Shergold 1991, Laurie 2006c, see Centralian Superbasin: ¿JXUH). The Jigaimara Formation was deposited in low- energy, shallow marine, probably subtidal settings, following a regional transgression (Rawlings et al 1997).

Naningbura Dolomite In offshore drillhole Arafura-1, the Naningbura Dolomite Figure 35.6. Elcho Island Formation. (a) Megaripples on wave-cut platform (near 561200mE 8671600mN, Galiwinku, Elcho Island, is a thick largely dolostone succession with silty dolomitic after Rawlings et al 1997, plate 34). (b) Desiccation cracks in sandstone at top of unit (522300mE 8647500mN, Banyan Island, 2 Corresponds to the Idamean–Datsonian Australian stages. after Rawlings et al 1997: plate 35).

Geology and mineral resources of the Northern Territory 35:7 Special publication 5 Arafura Basin in Arafura-1, and provided a more detailed description formation is considered to be of early Floian (late Early of its lithologies, distribution and conodont fauna. This Ordovician) age, based on the limited, but diagnostic name was also used by Zhen et al (2011), who provided conodont fauna (Zhen et al 2011), and is about the same age detailed descriptions of the conodont palaeontology and as the late Tremadocian to Floian Florina Formation of the biostratigraphic succession. These publications ¿rmly Daly Basin. establish the name of the unit as Naningbura Dolomite and this nomenclature is followed herein. The Naningbura LATE DEVONIAN: BASIN PHASE 3 Dolomite is 1128 m thick in the type section in Arafura-1, the only drillhole to penetrate the entire unit (Figure 35.4). Arafura Group Incomplete thicknesses intersected in other drillholes are in the range 154–601 m. A conodont fauna recovered from the The Upper Devonian Arafura Group (Petroconsultants top of the unit is late Furongian to early Tremadocian in age 1989, Bradshaw et al 1990, McLennan et al 1990) (Nicoll 2006a, Zhen et al 2011), but the undated base of the unconformably overlies units of the Goulburn Group. unit may be as old as middle Cambrian. A hiatus of about 100 million years separates the two groups which are generally structurally conformable. The Milingimbi Formation Arafura Group consists of shallow marine to non-marine The Milingimbi Formation (Bradshaw et al 1990, Nicoll interbedded mudstone, siltstone, sandstone and minor 2006a) corresponds to units O8 and O9 of Petroconsultants carbonate rocks. It has sag to sheet-like geometry in the (1989). It is conformable on the Naningbura Dolomite northern Arafura Basin, where it is about 1500 m thick, and is probably conformably overlain by the Mooroongga but the geometry of the group is more complex within the Formation, or is unconformably overlain presumably Goulburn Graben (Totterdell 2006). Bradshaw et al (1990) by the Late Devonian Arafura Group (Zhen et al 2011). divided the Arafura Group into the Djabura, Yabooma and The formation is of mixed lithology and comprises silty Darbilla formations. It is unconformably overlain by strata dolostone to limestone, glauconitic sandstone and shale, equivalent to the Upper Carboniferous–Lower Permian deposited predominantly in a shallow marine environment Kulshill Group of the Bonaparte Basin, or where these are (Nicoll 2006a, Zhen et al 2011). The lower part of the absent, by Jurassic strata of the Money Shoal Basin. Milingimbi Formation is dolomitic, but it becomes more siliciclastic up-section, where thin glauconitic sandstone is Djabura Formation interbedded with dolostone, limestone and shale (Bradshaw The Djabura Formation (Bradshaw et al 1990) has been et al 1990). In the type section in drillhole Arafura-1, the intersected in Tasman-1, Torres-1, Arafura-1 and Goulburn-1 Milingimbi Formation is 163 m thick and in Goulburn-1, the (Figure 35.4) and is equivalent to units D1௘–௘D4 of unit is 169 m thick. Pre-Devonian erosion has truncated the Petroconsultants (1989). It unconformably overlies various formation in Torres-1, where it is only 95 m thick, and has Cambrian and Ordovician units of the Goulburn Group and completely removed the unit in Tasman-1 (Figure 35.4). is unconformably overlain by the Yabooma Formation, or by Conodont faunas of Tremadocian (Early Ordovician) age younger (Upper Carboniferous) Kulshill Group equivalent have been described from the unit (Bradshaw et al 1990, sedimentary rocks (Nicoll 2006b). It ranges in thickness Zhen et al 2011). from 295 m to 466 m, and consists of interbedded, mudstone, siltstone, sandstone and minor carbonate rocks, which were Mooroongga Formation deposited in a nearshore shallow marine environment (Nicoll The Mooroongga Formation (Bradshaw et al 1990, Nicoll 2006b, Totterdell 2006). Diverse marine fossils, including 2006a) corresponds to units O10 to O133 of Petroconsultants conodonts, ostracods, phosphatic brachiopods, conulariids (1989). It is probably conformable on the Milingimbi and ¿sh fossils, are found throughout the unit. The conodonts Formation, but a major unconformity separates this unit indicate an early Famennian age for the formation (Nicoll from the overlying Upper Devonian Djabura Formation 2006b), but palynological dating suggests it is slightly older (Arafura Group). The Mooroongga Formation comprises (Frasnian; Purcell 2006). shale and interbedded limestone with some thin sandstone interbeds and minor chert, and becomes more calcareous Yabooma Formation upward. Glauconite is common and parts of the formation The Yabooma Formation (Bradshaw et al 1990) is are dolomitic (Zhen et al 2011). The depositional setting equivalent to the interval from unit D5 to the lower part was predominantly shallow marine (Zhen et al 2011). The of unit D7 of Petroconsultants (1989). It unconformably Mooroongga Formation is 131 m thick in Arafura-1, 201 m overlies the Djabura Formation and is intersected thick in Goulburn-1 and has been completely removed by in drillholes Torres-1, Arafura-1 and Goulburn-1 erosion in Tasman-1 and Torres-1 (Nicoll 2006a, Zhen et al (Figure 35.4). It is apparently conformably overlain by 2011, Figure 35.4). Petroconsultants (1989) reported the the Darbilla Formation, or is unconformably overlain by presence of conodonts, ostracods, ¿sh remains, conulariids, Jurassic sediments of the Money Shoal Basin (Bradshaw echinoderms, inarticulate brachiopods, gastropods, et al 1990, Nicoll 2006b). It ranges in thickness from 140 ?tentaculitids and sponge spicules from this unit. The to 335 m, and is predominantly composed of interbedded siltstone with dolomitic intervals and occasional thin 3 Petroconsultants (1989) did not describe Unit O13, but did sandstone beds. A relatively sparse fossil fauna includes include it in µGeological cross-section A–A1 Arafura Basin’. It conodonts, ¿sh and bryozoan fragments recovered in only occurs in Goulburn-1. cuttings from Goulburn-1. The conodont fauna is from

Geology and mineral resources of the Northern Territory 35:8 Special publication 5 Arafura Basin the base of the formation and indicates a late Famennian 1989). Palynological studies by Helby (2006) show that age (Nicoll 2006b). The Yabooma Formation is interpreted that this interval spans the Pennsylvanian–mid-Cisuralian to represent predominantly nearshore shallow marine APP11 to APP122 palynoÀoral zones of Price (1997), but deposition (Bradshaw et al 1990, Totterdell 2006). most of the succession is Early Permian in age and only the basal 100 m corresponds to the Late Carboniferous. Darbilla Formation Petroconsultants (1989) divided the succession in several The Darbilla Formation (Bradshaw et al 1990) is equivalent drillholes into four unnamed units and correlated these with to the upper part of unit D7 and unit D8 of Petroconsultants the Tanmurra Formation, Point Spring Sandstone, Kuriyippi (1989). It has only been intersected in Arafura-1 and Formation and Treachery Formation of the Bonaparte Torres-1 (Figure 35.4), where it is 380 m and 262 m thick, Basin. However, improved dating of the succession shows respectively. The unit is apparently conformable on the that a better correlation is with units from the younger Yabooma Formation and is overlain unconformably by interval Kuriyippi Formation–lower Keyling Formation Jurassic strata of the Money Shoal Basin. The formation is (see 3HGLUND%DVLQ¿JXUH). composed mostly of mudstone, sandy siltstone and lesser The Kulshill Group equivalent succession is separated interbedded sandstone, and includes ¿ning-upward intervals. by a major unconformity from overlying strata of the It does not contain marine fossils and is interpreted to Jurassic to Cenozoic Money Shoal Basin (Figure 35.2). represent a largely non-marine regression (Petroconsultants In contrast to Arafura Basin strata, which are complexly 1989, Bradshaw et al 1990, Nicoll 2006b, Totterdell 2006). faulted and folded, the Money Shoal Basin succession is Possible sabkha or tidal Àat and Àuvial depositional settings generally undisturbed. were suggested by Petroconsultants (1989). A palynoÀora from the base of the Darbilla Formation indicates a latest STRUCTURE AND TECTONICS Famennian (uppermost µStrunian’ sub-stage) age for the unit (Nicoll 2006b). The Arafura Basin was initiated in the Neoproterozoic as a result of northwest–southeast-directed upper crustal LATE CARBONIFEROUS–EARLY PERMIAN: extension that produced a series of northeast–southwest- BASIN PHASE 4 trending half grabens across the basin. The subsidence history was episodic, limited to four periods of basin- Kulshill Group equivalent wide subsidence (Basin phases 1–4) separated by long, relatively tectonically quiescent periods of non-deposition The Arafura Group is unconformably overlain by a Late and erosion. Minor localised deformation in the Devonian Carboniferous–Early Permian sedimentary succession that and Carboniferous was probably due to the effect of far- is approximately equivalent in age to the Kulshill Group ¿eld stresses associated with the Alice Springs Orogeny of the Bonaparte Basin (Totterdell 2006). Kulshill Group (Totterdell 2006). The WNW–ESE-trending Goulburn equivalent rocks reach a maximum thickness of about Graben (Figures 35.2, 35.3, 35.7) was formed in the Late 5000 m in the Goulburn Graben, which was formed at this Carboniferous to Early Permian, in response to oblique time, but the original thickness of the group was probably extension, and underwent oblique inversion in the Triassic much greater, as it is interpreted that up to 3000 m of during a phase of regional contractional deformation (Basin section has been eroded following deformation and uplift phase 5 of Totterdell 2006). The main deformations events in the Triassic (Struckmeyer et al 2006). The lower part of that have affected the basin are discussed below, in ascending the group thickens into the bounding planar normal faults date order. of the graben (Figure 35.2), indicating that it was a part of the rift succession. However, the upper part does not exhibit Neoproterozoic extensional faulting any noticeable divergence into the faults and is therefore considered to represent post-rift deposition. Kulshill Group Neoproterozoic half grabens occur over much of the equivalent rocks to the north of the Goulburn Graben northern basin (Figures 35.3, 35.7) and are in¿lled have a sag to sheet-like geometry and a relatively uniform by Wessel Group sediments of Basin phase 1. They thickness (maximum 3000 m), except where eroded around are bounded by simple planar normal faults that have a the margins of the basin. They are structurally conformable generally NE–SW strike, and dip to either the northwest with the underlying rocks and are also interpreted to be or southeast, suggesting approximately NW–SE part of the post-rift succession. Seismic and magnetic data extension. Towards the centre of the basin, a displacement indicate that there was some magmatic activity in the basin along these faults of up to 7000 m has been estimated during the rifting phase that resulted in the emplacement (Totterdell 2006). In the western part of the northern of sills and dykes, and a large magmatic body within the Arafura Basin are WNW–ESE-oriented accommodation Goulburn Graben (Totterdell 2006). A dolerite intersected zones across which the polarity of the faults switches from in drillhole Kulka-1 has been dated by K-Ar method at northwesterly directed throw in the south to southeasterly 293 ± 3 Ma (Bradshaw et al 1990). directed throw to the north. A series of small extensional The Kulshill Group equivalent succession comprises faults on the western margin of the basin has a NNW–SSE interbedded sandstone, siltstone and claystone, with minor orientation, sub-parallel to the interpreted direction of coal, and dolomitic rocks (Totterdell 2006). These were extension. Totterdell (2006) suggested that the orientation deposited in a variety of environments ranging from Àuvial of these cross faults may have been inÀuenced by the pre- to marginal marine to shallow marine (Petroconsultants existing structural fabric of the underlying Pine Creek

Geology and mineral resources of the Northern Territory 35:9 Special publication 5 Arafura Basin

Orogen. In the eastern part of the basin, there appears to Minor Palaeozoic deformation be a change in architecture to large-displacement, widely- spaced faults. This change in structural style could reÀect No known signi¿cant deformation events occurred between variations in the underlying basement fabric from west to deposition of Basin phases 2 (middle Cambrian–Early east, from the complex deformation and strong structural Ordovician), 3 (early–middle Palaeozoic) and 4 (Late fabric of the Pine Creek Orogen to the mildly deformed Carboniferous–Early Permian). Despite the presence and eastward-thickening succession of the McArthur of lengthy hiatuses between the Wessel, Goulburn and Basin (Totterdell 2006). Arafura groups, the Palaeozoic basin succession is

132° 133° 134° 135°

9°

inferred Indonesia Neoproterozoic extension Australia direction

(northern Arafura Basin)

Tuatara-1

Cobra-1A 0° A Poor seismic imaging: large, widely spaced faults B Kulka-1

Money Shoal-1 Chameleon-1

Money Shoal Torres-1 Arafura-1 Basin Tasman-1 Goulburn-1 inferred Carb–Permian extension inferred ° direction Triassic compression direction

(Arafura Basin)

Money Shoal Basin Pine Creek Orogen

2°

Arafura Basin

McArthur Basin

0 50 km Pine Creek Orogen A12-183.ai

Mesozoic–Cenozoic Goulburn Graben Triassic thrust fault Petroleum exploration well Palaeo–Mesoproterozoic Offshore Arafura Basin Late Carboniferous dry, abandoned basins (under cover) normal fault oil show Palaeo–Mesoproterozoic A B orogens section in Figure 35.2 Neoproterozoic oil/gas show normal fault Archaean oil indication oil/gas indication 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.

Geology and mineral resources of the Northern Territory 35:10 Special publication 5 Arafura Basin relatively structurally conformable (Totterdell 2006). followed by erosion and the formation of a peneplain across The only indication of structural movement in the early– the basin and adjacent basement areas. During this period middle Palaeozoic succession is the absence of parts of the of erosion, the basin appears to have been affected by a Goulburn and Arafura groups in some of the wells drilled minor extensional episode that involved relatively small- in the Goulburn Graben, suggesting that there was some displacement, planar normal faulting within the upper part localised uplift and erosion prior to deposition of the Arafura of the Arafura Basin succession. On the western margin Group. The timing of this minor disturbance coincides with of the basin, some older faults were reactivated and the Middle Devonian Pertnjara-Brewer events of the Alice Triassic inversion anticlines were offset (Totterdell 2006). Springs Orogeny in central Australia (see Aileron Province) This faulting appears to predate the unconformity at the and suggests it may be related to the far-¿eld effects of these base of the Money Shoal Basin and is therefore probably events. The hiatus of approximately 45 million years between older than later Jurassic extensional episodes that partly the Arafura Group and the overlying Kulshill Group correlates controlled deposition of the Money Shoal Basin succession with the ¿nal, Early Carboniferous phase (Eclipse Event) of (Struckmeyer 2006c). the Alice Springs Orogeny. Although there is no seismic evidence of any widespread contractional deformation of the MINERAL RESOURCES Arafura Basin at that time, there is evidence of signi¿cant localised uplift and erosion, with at least 1000 m of Arafura The offshore Arafura Basin is very prospective for Group missing at Tasman-1 (Totterdell 2006). petroleum, but to date, there have been no commercial hydrocarbon discoveries. In the onshore Arafura Basin, Late Carboniferous–Early Permian extensional faulting known mineral occurrences include bauxite on Marchinbar and Elcho islands, and a small iron ore occurrence near The Goulburn Graben formed during a phase of Late Galiwinku (Figure 35.5). The following summary of these Carboniferous–Early Permian northeast–southwest occurrences is derived from Ferenczi (2001). extension. It is a narrow, highly structured zone that has a west-northwest–east-southeast trend in the east and a Bauxite northwest–southeast trend in the west (Figures 35.3, 35.7). This orientation might be reÀecting the underlying structural Lateritic bauxite has developed on Neoproterozoic rocks of grain of basement rocks (Totterdell 2006). Along much of its the Wessel Group at Marchinbar and Elcho islands. length, the Goulburn Graben has the morphology of a half graben, with master detachment faults de¿ning the northern Marchinbar Island margin and the southern marginal faults only intermittently developed. The bounding fault system to the north dips at Bauxite deposits were ¿rst reported from Marchinbar an angle of about 50º to the south-southwest or southwest. Island ()LJXUH ) by Owen (1949), after he received Carboniferous–Permian extensional faulting appears to samples, collected by the Northern Territory Coastal have been con¿ned to the Goulburn Graben, as there is Patrol Service, that assayed up to 40.8% Av.Al2O3. The little seismic evidence for extensional faulting of this age main lateritic bauxite deposits lie on the east coast of the elsewhere (Totterdell 2006). island and were investigated by the Australian Aluminium Commission in the early 1950s. Ore resources for the seven Mid–Late Triassic contraction tested deposits total 9.94 Mt and average 46.0% available

Al2O3 and 4.0% reactive SiO2 (Owen 1953). During the Middle–Late Triassic, the Arafura Basin, and in The deposits are developed over sedimentary rocks particular the Goulburn Graben, experienced a major phase of the Marchinbar Sandstone. The bauxite ore consists of contractional deformation (Basin Phase 5 of Totterdell predominately of cemented pisoliths of gibbsite, that have 2006). The effects of this deformation varied markedly across light brown and red-brown cores (Owen 1954). A tubular the basin. In the Goulburn Graben, it was relatively intense bauxite bed underlies pisolitic ore in several of the deposits and was characterised by folding, inversion on pre-existing (eg Able, Sphinx Head, Dog and Easy). Tubular ore reaches a faults, the formation of new thrust faults, uplift and erosion. maximum thickness of about 2 m and lenses out to the west, In the northern Arafura Basin, the affects of the deformation where the westerly deposits are all pisolitic (Ferenczi 2001). were less intense; limited contractional reactivation of The underlying laterite is up to 10 m thick and largely consists Neoproterozoic half grabens resulted in the inversion of of nodular ferricrete. The largest known deposit on the island some Neoproterozoic extensional faults and the formation of is Able, which occupies an area of about 880 000 m2. One inversion anticlines. The direction of regional compression hundred and forty-two sampling pits were excavated by the is interpreted to have been NNW௘–௘SSE and was highly Australian Aluminium Commission on a 61 x 122 m grid. oblique to the dominant fault trends of the Goulburn Graben, A non-JORC Resource is given at 4.7 Mt at 47.1% Al2O3 resulting an element of dextral strike-slip or transpressional (Ferenczi 2001). Ore thickness varies from 0.76 m (cut-off) to movement on parts of the fault system (Totterdell 2006). 5 m and averages 2.4 m (Owen 1953). Pisolitic bauxite forms the bulk of the resource (97%); the remaining 3% consists of Minor latest Triassic/Early Jurassic extensional faulting massive and tubular bauxite, which underlies the pisolitic ore in the eastern section of the deposit. Bauxite quality usually After the Triassic deformation event, the margins of the varies with depth and lower grades are often found in the Arafura Basin were uplifted, resulting in a basinward tilt, upper and lower portions of the pro¿le. The upper 0.5–1 m

Geology and mineral resources of the Northern Territory 35:11 Special publication 5 Arafura Basin of the bauxite bed in the eastern area of the deposit contains These occurrences consist of thin loose intervals of pisolitic 10–25% quartz sand and ¿ne detrital material. Dry screening and tubular bauxite that unconformably overlie unaltered of samples from this area gave an average recovery factor of Marchinbar Sandstone (Plumb and Gostin 1973). The 96% silica is in the form of free quartz (1.1%) and reactive pisolitic layer is about 2 m thick and forms a series of

(mostly kaolinite) silica (3.0%). Iron oxides (Fe2O3), which discontinuous exposures over a 3 km strike length. Sand are mainly in the form of goethite, average 15.7% and TiO2 dunes cover the bauxite, adjacent to and away from the averages 3.3% (Ferenczi 2001). coast. A single sample from one of the occurrences assayed

45.7% Al2O3 and 25% SiO2 (Plumb 1965). The high silica Elcho Island value may indicate contamination by quartz grains derived from nearby sand dunes. A laterite sample obtained during At the eastern side of Elcho Island, two bauxite occurrences, reconnaissance work by BHP (1964) near the southernmost separated by about 3 km, were recorded by Plumb (1965). occurrence assayed 25.7% Al2O3, 28.0% total SiO2 and 23.3% Fe O . This area is essentially untested and may host 136°30' 2 3 11°00' bauxite deposits comparable to those on Marchinbar Island (Ferenczi 2001).

Arafura Sea Marchinbar Island Iron ore Wessel Islands Elcho Island iron ore deposit

12°00' Gove 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 Arnhem Land haematite layer and an upper haematitic sandstone bed are 0 150 km present in the upper part of the laterite pro¿le. The massive

13°00' 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 Bauxite deposit and 0.054% P), as estimated by Rix (1964b). Most of the ore Sand dunes lies at or near the surface, with the overburden gradually Laterite increasing to the north where it reaches a maximum of 6 m. Marchinbar Sandstone The overlying haematitic sandstone is up to 1.2 m thick and averages 40.4% Fe and 0.57% P (Rix 1964b). Raiwalla Shale 3 Strike and dip of strata Petroleum 0 5 10 km BAKER 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 SPHINX HEAD are known from a number of stratigraphic levels. Nine ABLE exploration wells have been drilled, all within the Goulburn DOG 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 RED CLIFF and Petroleum Company, which drilled several unsuccessful FOX holes in the 1920s on Elcho Island (Bell 1923). In the 1960s and early 1970s, stratigraphic drilling was carried out on EASY Bathurst and Melville islands (McLennan et al 1990). In

3 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

3 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 A12-194.ai 1980s involved a number of operators, including Diamond )LJXUH. Geology and location of bauxite deposits on Marchinbar Shamrock Corporation, Esso Australia Pty Ltd, Petro¿na Island (after Ferenczi 2001, modi¿ed from Plumb 1965). Exploration Australia SA and Sion Resources Ltd. A

Geology and mineral resources of the Northern Territory 35:12 Special publication 5 Arafura Basin number of wells were drilled at this time to test the Arafura Basin, it is conceivable that hydrocarbon generation and Basin succession and Arafura-1 recorded oil shows over a expulsion from these rocks may have occurred and may 425 m interval in the Devonian and Ordovician sections have charged younger reservoir units (Struckmeyer and (Struckmeyer and Earl 2006a). Earl 2006b). In the late 1980s and early 1990s, BHP Petroleum Pty Drillhole data and regional correlations indicate that Ltd targeted Mesozoic plays in the Goulburn Graben. The a number of potential source rock intervals occur within exploration program included an extensive 17 000 line km the Arafura Basin succession (Bradshaw et al 1990, seismic survey, a regional aeromagnetic survey, and the Edwards et al 1997). The Neoproterozoic Wessel Group drilling of three exploration wells. During the early 1990s, contains promising source rocks (eg Raiwalla Shale and Geoscience Australia acquired a total of 5342 line km of Elcho Island Formation), but no geochemical or organic regional deep seismic data across the Arafura Basin. In the petrological data are available for this interval and it is yet 2000s, a number of exploration activities have contributed to to be properly evaluated. Samples from the Cambrian– the available datasets and have improved the prospectivity Ordovician Goulburn Group have returned TOC values of the region. These include non-exclusive regional 2D of up to 8.6%, but the higher values represent migrated oil seismic datasets by TGS Nopec Geophysical Company Pty and solid bitumen rather than dispersed organic matter. Ltd in 1998 and Veritas DGC Inc in 2002, and Synthetic However, the presence of both abundant bitumen and oil Aperture Radar acquisition and interpretation across the stains in early Palaeozoic samples is indicative of a multi- region by INFOTERRA Ltd in 2003 (Struckmeyer 2006d). charge history from a proli¿c nearby source (Sherwood et al 2006). Oil stains in samples of Early Palaeozoic rocks Source rocks from drillholes Arafura-1 and Goulburn-1 have similar geochemical and isotopic characteristics to the early middle Potential source rock intervals occur at a number of levels Cambrian Thorntonia(!) petroleum system of the Georgina in the stacked McArthur, Arafura and Money Shoal basins Basin (Boreham and Ambrose 2007).This suggests that the (Figure 35.9). The Palaeo- to Mesoproterozoic McArthur effective source rock within the Goulburn Group is most Basin, which is interpreted to underlie much of the eastern likely to occur in the Jigaimara Formation, which is also Arafura Basin, contains at least ¿ve potential source rock middle Cambrian in age (Sherwood et al 2006). Limited intervals, de¿ned as having total organic carbon (TOC) data from the Upper Devonian Arafura Group suggest a greater than 0.5%. Of these, the Barney Creek Formation generally poor source potential for this interval, although (McArthur Group) and Velkerri Formation (Roper Group) potentially fair source rocks may be present within marine have the highest TOC values, which range up to 8% and calcareous mudstones. Good to very good source rocks 12%, respectively (Crick et al 1988, Jackson et al 988). If with Type II/III kerogen are present in the Permian– these potentially excellent source rocks underlie the Arafura Carboniferous Kulshill Group equivalent succession. The

(Ma) Rock type / Era Age Stratigraphic unit Shows Potential palaeoenvironment Source Res Seal 250 LATE

EARLY

PERMIAN ? Kulshill A 300 Group K LATE equivalent Ta

EARLY

350 CARBONIF. Darbilla Fm LATE Arafura Yabooma Fm A Group Djabura Fm G MIDDLE

400 EARLY DEVONIAN

SILURIAN PALAEOZOIC Arafura Basin

450 ORDOVICIAN

Mooroongga Fm A Milingimbi Fm Goulburn G 500 Group Naningbura Dol Jigaimara Fm CAMBRIAN

? Wessel Figure 35.9. Stratigraphic succession Group and petroleum systems elements for Neoproterozoic the Arafura Basin (modi¿ed from Totterdell 2006: ¿gure 9). Abbreviations: A = Arafura-1; Dol = Dolomite; Shallow marine Sandstone Dolostone Petroleum exploration well oil show Fm = Formation; G = Goulburn-1; Fluvial–deltaic Shale Limestone K = Kulka-1; Res = Reservoir; Ta = oil/gas show Tasman-1. Terrestrial Unconformity oil indication A12-195.ai

Geology and mineral resources of the Northern Territory 35:13 Special publication 5 Arafura Basin typical TOC range is <0.4 to 3% and several samples generation, Devonian Arafura Group rocks are early mature contain up to 9% TOC (Sherwood et al 2006). to mature for oil generation, and Carboniferous–Permian Kulshill Group equivalent rocks are immature to mature Reservoirs and seals for oil generation, with maturity dependent on the thickness of Money Shoal Basin overburden (Struckmeyer and Earl Potential reservoir rocks occur in both the Arafura and 2006b). Maturation levels of potential source rocks from the overlying Money Shoal basins. In the Arafura Basin, they underlying McArthur Basin range from marginally mature include shallow marine limestone and dolostone units of to overmature for oil generation (Crick et al 1988, Jackson the Cambrian–Ordovician Goulburn Group, terrestrial et al 1988, Ambrose and Silverman 2006). to Àuvio-deltaic interbedded sandstone and shale units of the Devonian Arafura Group and parts of the Permian– Prospectivity Carboniferous Kulshill Group equivalent (Struckmeyer and Earl 2006b, Figure 35.9). Goulburn Group carbonate A variety of possible structural and stratigraphic play rocks have been shown to host oil and gas shows, and oil types are present within the Arafura Basin, involving the indications. They are generally ¿ne grained, although some juxtaposition of potential source, reservoir and seal rocks. sandier intervals are present in the Ordovician section Structural plays include large faulted anticlines, tilted (Petroconsultants 1989). Original porosity is likely to have fault blocks, and inversion anticlines formed during the been mostly poor, but secondary porosity (vugs, fractures) Triassic deformation, whereas stratigraphic plays involve has improved reservoir quality to 7.7% maximum porosity regional unconformities, intraformational and regional (Struckmeyer and Earl 2006b). Arafura Group sandstone facies changes, and diagenesis (Petroconsultants 1989, and shale intervals also host oil shows. These rocks have a Struckmeyer 2006d). The overlying Money Shoal Basin reported maximum porosity and permeability of 19% and also contains a variety of stratigraphic and combined 7.83 mD, respectively, but average 9.6% porosity. Diagenesis structural/stratigraphic plays that could have been charged has destroyed a signi¿cant proportion of the primary by hydrocarbons sourced from underlying Palaeozoic and porosity, but where the rocks have not been so deeply buried Mesozoic source rocks (McLennan et al 1990, Struckmeyer in the northern part of the basin and are less hydrothermally 2006d). The thick successions of the Arafura and Money altered, reservoir quality is likely to be better (Struckmeyer Shoal basins therefore provide a diverse range of potential and Earl 2006b). Kulshill Group equivalent rocks generally traps at a number of stratigraphic levels. have poor reservoir quality, with porosities averaging 5.5%, Evidence for hydrocarbon generation and expulsion except for the upper parts of this unit where a maximum in the Arafura Basin includes oil shows/indications and porosity of 17.7% has been recorded. Multiple fracture sets gas indications in most drillholes (Earl 2006), and the could enhance the overall permeability and porosity of this presence of interstitial solid bitumen in many samples interval. (Sherwood et al 2006). However, in the Goulburn Graben, Potential seal rocks are present throughout the geohistory studies have indicated that oil generation from Arafura Basin succession, and potential regional seals early Palaeozoic rocks may have been halted after a period are present in the Devonian and Cretaceous successions. of early migration and before signi¿cant structures that Shale interbeds could provide intraformational seals for could trap the oil were generated (Moore et al 1996). carbonate reservoir rocks within the Goulburn Group, Early-formed hydrocarbon accumulations might also and a variety of diagenetic seals and traps could also be have been breached by erosion following the Triassic present in carbonate units. Relatively thick (up to 400 m) deformation event (Higgins 2009). This might explain the shale intervals within the upper part of the Arafura Group failure to date to ¿nd a commercial accumulation in this could form intraformational seals and possibly a regional portion of the basin. Indirect hydrocarbon indications in seal. Potential seals within the Kulshill Group equivalent the northern Arafura Basin include shallow gas interpreted succession are likely to be intraformational. Fine-grained on sub-bottom pro¿le data and conventional seismic data, Cretaceous sedimentary rocks of the Bathurst Island Group degraded seismic data, which could represent hydrocarbons (Money Shoal Basin) directly overlie Palaeozoic rocks of within the succession, and Synthetic Aperture Radar the Arafura Basin in the eastern part of the basin and could (SAR) slicks on the sea surface. These indicators provide provide a regional seal (Struckmeyer 2006b). evidence for active petroleum systems within this part of the basin (Struckmeyer 2006d). Hydrocarbon generation Thermal maturity in the northern Arafura Basin could have occurred much later than in the Goulburn Graben, with migration Sherwood et al (2006) evaluated the thermal maturity of postdating structuring, suggesting that this area could be potential source rocks from a number of levels within the more prospective for petroleum than previously considered Arafura and Money Shoal basins using a combination of (Moore et al 1996, Higgins 2009). FAMM (Fluorescence Alteration of Multiple Macerals) and conventional organic petrological analyses. Boreham REFERENCES (2006) provided analyses of the organic geochemical maturity of a number of source rock samples from the same Ambrose GJ and Silverman M, 2006. Beetaloo Sub-basin: in succession. 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