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| | | | Geological Survey of Western Australia |
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| | | | HM Howard, R Quentin de Gromard, PW Haines, and RH Smithies | | | | | Type | Basin | Lithology | sedimentary and volcanic rocks | Parent unit | | Child units | | Constituent lithostratigraphic units | | Affected by events | | Tectonic setting | |
| | | Summary | The Bentley Basin formed and evolved during the 1085–1030 Ma Giles Event, in concert with development of the long-lived, failed intracontinental Ngaanyatjarra Rift. The basin lies on the western side of the Musgrave Province, and contains the largest and best-preserved exposure of bimodal volcanic sequences in the Musgrave Region. The main components of the Bentley Basin are the Talbot Sub-basin and Blackstone Sub-basin, and the Tjauwata Group, for which no discrete sub-basin is defined. The volcano-sedimentary successions in Bentley Basin are assigned to the widespread Bentley Supergroup, the older parts of which are mainly preserved in the Blackstone Sub-basin and lower Tjauwata Group, and the younger parts of which are represented in the Talbot Sub-basin and upper Tjauwata Group. | | | Distribution | The deposits of the Bentley Basin are preserved over much of the west Musgrave Province. The most voluminous sequences lie in three main areas, two of which are defined sub-basins, the Blackstone Sub-basin and Talbot Sub-basin, and a third area within the footwall of the Woodroffe Thrust where the Tjauwata Group outcrops.
The Blackstone Sub-basin, defined by the outcrop of the Tollu Group is almost entirely restricted to the east-trending, Blackstone Syncline, to the south of the Blackstone Range. Small outliers are also found around Skirmish Hill, MacDougall Bluff and Mummawarrawarra Hill (BLACKSTONE), south of the Bell Rock Range (BELL ROCK) and to the north of the Jameson–Finlayson intrusion (FINLAYSON).
The Talbot Sub-basin lies on the western side of the west Musgrave Province, to the west of Jameson Community and extends westwards beyond Warburton Community and under the Gunbarrel Basin. The northern margin overlies the older units of the Bentley Supergroup and Musgrave Province. The southern margin extends to the Townsend Ridges where the sub-basin is in contact with units of the Officer Basin. South of the Musgrave Province, a drillhole (GSWA Empress 1) intersected basalt beneath the basal sedimentary rock unit of the Officer Basin, which gave a K–Ar date of 1058 ± 13 Ma (Stevens and Apak, 1999). Assuming this basalt is reliably correlated with basaltic rocks in the upper part of the Bentley Supergroup, this occurrence confirms that the Bentley Basin extends south beneath this Neoproterozoic basin.
The Tjauwata Group is distributed across the northern part of the region, within the footwall of the Woodroffe Thrust (between the Petermann and Bloods Ranges in both Western Australia and the Northern Territory). | | | Description | The Bentley Basin formed during the development of the Ngaanyatjarra Rift (Evins et al., 2010). The basin is host to the Bentley Supergroup, the deposits of which are preserved in two well-defined depocentres, the Blackstone and Talbot Sub-basins, and in the widely distributed Tjauwata Group. Deposits in the sub-basins overlap in age, with older parts of the Bentley Supergroup preferentially preserved in the Blackstone Sub-basin and lower Tjauwata Group. The well-preserved volcano-sedimentary deposits reveal the rift/basin structure at a variety of crustal levels and times in its evolution. An early rift stage (1085–1074 Ma) is characterized by voluminous magmatism (Warakurna Supersuite) within the upper crust and relatively little tectonic deformation, and a late rift stage (1074–1040 Ma) is characterized by tectonic deformation, synchronous with the deposition of a thick pile of volcanic and sedimentary rocks (Bentley Supergroup). This rift is unusual in that the crust was thickened by ~15 km and overall extension was very limited (Aitken et al., 2013). It is likely that magmatism was the dominant process, and that the extension observed was a response to magmatism-induced crustal thickening and the gravitational collapse of the crustal column, characteristics similar to other Proterozoic rifts (Aitken et al., 2013). | | | | | | Geochronology | | | Bentley Basin | Maximum age | Minimum age | Age (Ma) | 1085 | 1026 | Age | Mesoproterozoic | Mesoproterozoic |
| Maximum and minimum ages of the Bentley Basin are constrained by: 1) its tectonic setting and relationship to the 1085–1030 Ma tectono-magmatic Giles Event; 2) direct dating of volcano-sedimentary basin deposits; 3) direct dating of rocks that intrude basin deposits.
The oldest volcano-sedimentary rocks of the Bentley Basin are the MacDougall Formation and the Mummawarrawarra Basalt of the Kunmarnara Group, towards the base of the Blackstone Sub-basin. The maximum depositional age of the MacDougall Formation is constrained by the youngest detrital zircon age component of c. 1179 Ma (Quentin de Gromard et al., 2016). A more realistic maximum age is the c. 1085 Ma regional maximum age of magmatism associated with the Giles Event (Edgoose et al., 2004; Evins et al., 2010), which must be nearly coeval with the onset of the Ngaanyatjarra Rift and the deposition of the Kunmarnara Group. The Mummawarrawarra Basalt is considered to be the lateral equivalent of the c. 1085 Ma Mount Harris Basalt, and is, therefore, assumed to have the same age of c. 1085 Ma.
Two rhyolite samples from the Smoke Hill Volcanics (Tollu Group), taken from around Mount Jane in the east of the Blackstone Syncline (i.e. eastern Blackstone Sub-basin), have crystallization ages of 1071 ± 8 Ma (GSWA 191728, Coleman, 2010b) and 1073 ± 7 Ma (GSWA 191706, Coleman, 2010a). The overlying Hogarth Formation yielded a date of 1068 ± 7 Ma (GSWA 185518, Kirkland et al., 2013), consistent with the geochemically related c. 1067 Ma Alcurra Dolerite (Seat, 2008; Howard et al., 2009) that intruded the Smoke Hill Volcanics in the western part of the Blackstone Syncline.
In the Talbot Sub-basin, the age of the lower part of the depositional sequence (the Mount Palgrave Group) is constrained by a crystallization age of 1077 ± 6 Ma (GSWA 174662, Kirkland et al., 2010) for a weakly to moderately foliated granitic unit of the Winburn granite pluton in the Northern Territory. However, this pluton includes both pre- and synvolcanic granite intrusions. Dating of many felsic igneous rocks of the Talbot Sub-basin indicates a maximum possible age range for the sub-basin of 1116 ± 28 to 1010 ± 20 Ma, although these rocks include a large proportion of recycled cognate material (i.e. zircon antecrysts). This means that previously interpreted crystallization ages have likely been variably over-estimated (i.e. true ages of volcanic deposition or intrusive crystallization could be younger). Smithies et al. (2013) suggested that the most conservative age range for magmatic activity within the Talbot Sub-basin is probably c. 1077 to 1047 Ma.
Deposition of the Tjauwata Group spanned the entire Giles Event from c. 1085 to 1030 Ma (Quentin de Gromard et al., 2017). The Wankari Volcanics (middle to upper Tjauwata Group) forms the main part of the Tjauwata Group in Western Australia with an age range of 1051–1039 Ma (Scrimgeour et al., 1999; GSWA 208489, Lu et al., 2017). With the exception of the Karukali Quartzite and Mount Harris Basalt in Western Australia, ages obtained from units of the Bentley Supergroup in the Rawlinson area are notably younger than those of the Talbot and Blackstone Sub-basins.
| | | | The Bentley Basin overlies the Musgrave Province and rocks within the basin, i.e. the Bentley Supergroup, have either faulted or unconformable contacts with their basement. | | | Tectonic setting | The Bentley Basin formed in response to development of the long-lived, intracontinental Ngaanyatjarra Rift of central Australia (Evins et al., 2010). An intracontinental setting is indicated for basement rocks of the Giles Event for at least 150 Ma prior to this event (Smithies et al., 2009). Rifting coincided with voluminous magmatism, and the earliest volcano-sedimentary deposits are represented by the Kunmarnara Group, a typical intracontinental rift sequence of basal conglomerate and sandstone (the MacDougall Formation) followed by basalt flows (the Mummawarrawarra Basalt). A similar sequence is found at the base of the Tjauwata Group (the Karukali Quartzite and Mount Harris Basalt). The Kunmarnara Group and basal sequences of the Tjauwata Group trace the rift basin boundaries.
This early rift phase was followed by prolonged and voluminous magmatism. Initial studies related the Giles Event to the effects of a deep-mantle plume (Zhao and McCulloch, 1993; Wingate et al., 2004; Pirajno, 2007). Similarly, the unusually large volumes of magmas and the relatively limited extension led Aitken et al. (2013) to argue in favour of magma-driven rifting rather than extension-driven rifting. However, the duration of related mantle-derived magmatism exceeds 50 Ma, and the entire history is recorded within the restricted area of the west Musgrave region, with no time-progressive spatial magmatic trend. It appears that the controls on this magmatism were long-lived and specifically attached to the lithosphere. It has recently been proposed that the reasons for the Giles Event are entirely tectonic, due primarily to a coincidence of two major factors — an extreme thermal pre-history of the region and mainly sinistral strike-slip movement along translithospheric faults (Smithies et al., 2015).
The Ngaanyatjarra Rift is the main crustal expression of the Giles Event, and was responsible for almost 50 Ma of near-continuous magmatism in the west Musgrave region (Evins et al., 2010). The Giles Event does not, therefore, reflect a simple single event, but rather a protracted and complex geodynamic setting. The event included the c. 1075 Ma Warakurna Large Igneous Province (LIP), associated granites of which have volcanic equivalents in the Smoke Hill Volcanics, and emplacement of the regional Alcurra Dolerite dyke swarm and its volcanic equivalent, the Hogarth Formation (Blackstone Sub-basin).
The long duration of mantle-derived mafic and felsic magmatism included the development of a silicic LIP over a period of >30 Ma, formed by a series of large rhyolite eruptions, including some of super-volcano size, interleaved with regional tholeiitic basalt flows (Smithies et al., 2013). These were deposited in the Talbot Sub-basin, which included to at least two caldera-like eruptive centres, and comprise the most extensive volcanic succession of the Bentley Basin. | BookMark | | | | | Child units | | | | Tectonic unit name | Unit code | Age (Ma) | Type | | PTBNT | 1085–1030 | Sub-basin | | PTBNB | 1085–1067 | Sub-basin |
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| BookMark | | | | | Constituent lithostratigraphic units | | | Unit name | Unit code | Rank | GSWA status | | | Supergroup | Formal | | | Formation | Formal | | | Formation | Formal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Formation | Formal | | | Formation | Informal | | | Group | Formal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Formation | Formal | | | Member | Informal | | | Formation | Formal | | | Formation | Formal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Formation | Formal | | | Group | Formal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Formation | Formal |
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| | | | | | References | Aitken, ARA, Smithies, RH, Dentith, MC, Joly, A, Evans, S and Howard, HM 2013, Magmatism-dominated intracontinental rifting in the Mesoproterozoic: The Ngaanyatjarra Rift, central Australia: Gondwana Research, v. 24, no. 3–4, p. 886–901, doi:10.1016/j.gr.2012.10.003. | Coleman, PM, Kirkland, CL, Wingate, MTD and Smithies, RH 2010a, 191706.1: mylonitic rhyolite, Mount Maria; Geochronology Record 915: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Coleman, PM, Kirkland, CL, Wingate, MTD and Smithies, RH 2010b, 191728.1: rhyolite, Mount Jane; Geochronology Record 917: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Edgoose, CJ, Scrimgeour, IR and Close, DF 2004, Geology of the Musgrave Block, Northern Territory: Northern Territory Geological Survey, Report 15, 46p. | Evins, PM, Smithies, RH, Howard, HM, Kirkland, CL, Wingate, MTD and Bodorkos, S 2010, Redefining the Giles Event within the setting of the 1120-1020 Ma Ngaanyatjarra Rift, west Musgrave Province, central Australia: Geological Survey of Western Australia, Record 2010/6, 36p. View Reference | Howard, HM, Smithies, RH, Kirkland, CL, Evins, PM and Wingate, MTD 2009, Age and geochemistry of the Alcurra Suite in the western Musgrave Province and implications for orthomagmatic Ni–Cu–PGE mineralization during the Giles Event: Geological Survey of Western Australia, Record 2009/16, 16p. View Reference | Kirkland, CL, Wingate, MTD and Howard, HM 2010, 174662.1: granophyre, Eliza Rocks; Geochronology Record 909: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Kirkland, CL, Wingate, MTD, Howard, HM and Smithies, RH 2013, 185518.1: rhyolite, Barnard Rocks; Geochronology Record 1128: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Lu, Y, Wingate, MTD, Quentin de Gromard, R, Howard, HM and Haines, PW 2017, 208489.1: metarhyolite, Mount Deering; Geochronology Record 1407: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Pirajno, F 2007, Ancient to modern Earth: The role of mantle plumes in the making of continental crust, in Earth's oldest rocks edited by Van Kranendonk, MJ, Bennett, VC and Smithies, RH: Elsevier B.V., Burlington, Massachusetts, USA, Developments in Precambrian Geology 15, p. 1037–1064. | Quentin de Gromard, R, Howard, HM, Smithies, RH, Wingate, MTD and Lu, Y 2017, The deep seismic reflection profile 11GA-YO1 in the west Musgrave Province: An updated view: Geological Survey of Western Australia, Record 2017/8, 20p. | Quentin de Gromard, R, Wingate, MTD, Kirkland, CL, Howard, HM and Smithies, RH 2016, Geology and U–Pb geochronology of the Warlawurru Supersuite and MacDougall Formation in the Mitika and Wanarn areas, west Musgrave Province: Geological Survey of Western Australia, Record 2016/4, 29p. View Reference | Scrimgeour, IR, Close, DF and Edgoose, CJ 1999, Petermann Ranges, Northern Territory: Northern Territory Geological Survey, 1:250 000 Geological Map Series Explanatory Notes SG52–07, 59p. | Seat, Z 2008, Geology, petrology, mineral and whole-rock chemistry, stable and radiogenic isotope systematics and Ni-Cu-PGE mineralisation of the Nebo-Babel intrusion, west Musgrave, Western Australia: The University of Western Australia, Perth, Western Australia, PhD thesis (unpublished). | Smithies, RH, Howard, HM, Evins, PM, Kirkland, CL, Bodorkos, S and Wingate, MTD 2009, West Musgrave Complex - new geological insights from recent mapping, geochronology, and geochemical studies: Geological Survey of Western Australia, Record 2008/19, 20p. View Reference | Smithies, RH, Howard, HM, Kirkland, CL, Werner, M, Medlin, CC, Wingate, MTD and Cliff, JB 2013, Geochemical evolution of rhyolites of the Talbot Sub-basin and associated felsic units of the Warakurna Supersuite: Geological Survey of Western Australia, Report 118, 74p. | Smithies, RH, Kirkland, CL, Korhonen, FJ, Aitken, ARA, Howard, HM, Maier, WD, Wingate, MTD, Quentin de Gromard, R and Gessner, K 2015, The Mesoproterozoic thermal evolution of the Musgrave Province in central Australia - plume vs. the geological record: Gondwana Research, v. 27, no. 4, p. 1419–1429, doi:10.1016/j.gr.2013.12.014. | Stevens, MK and Apak, SN 1999, GSWA Empress 1/1A composite well log, in GSWA Empress 1 and 1A well completion report, Yowalga Sub-basin, Officer Basin, Western Australia by MK Stevens and SN Apak: Geological Survey of Western Australia, Record 1999/4, Plate 1. View Reference | Wingate, MTD, Pirajno, F and Morris, PA 2004, Warakurna large igneous province: A new Mesoproterozoic large igneous province in west-central Australia: Geology, v. 32, no. 2, p. 105–108. | Zhao, J and McCulloch, MT 1993, Sm-Nd mineral isochron ages of Late Proterozoic dyke swarms in Australia: Evidence for two distinctive events of mafic magmatism and crustal extension: Chemical Geology, v. 109, no. 1–4, p. 341–354. |
| | | | Recommended reference for this publication | Howard, HM, Quentin de Gromard, R, Haines, PW and Smithies, RH 2020, Bentley Basin (PTBN): Geological Survey of Western Australia, WA Geology Online, Explanatory Notes extract, viewed 05 August 2025. <www.dmp.wa.gov.au/ens> |
| | | This page was last modified on 20 January 2020. | | | | | Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Locations mentioned in the text are referenced using Map Grid Australia (MGA) coordinates, Zones 49 to 52. All locations are quoted to at least the nearest 100 m. Capitalized names in text refer to standard 1:100 000 map sheets, unless otherwise indicated. WAROX is GSWA’s field observation and sample database. WAROX site IDs have the format ‘ABCXXXnnnnnnSS’, where ABC = geologist username, XXX = project or map code, nnnnnn = 6 digit site number, and SS = optional alphabetic suffix (maximum 2 characters). All isotopic dates are based on U–Pb analysis of zircon and quoted with 95% uncertainties, unless stated otherwise. U–Pb measurements of GSWA samples were conducted using a sensitive high-resolution ion microprobe (SHRIMP) in the John de Laeter Centre at Curtin University, Perth, Western Australia. Digital data related to WA Geology Online, including geochronology and digital geology, are available online at the Department’s Data and Software Centre and may be viewed in map context at GeoVIEW.WA. | | | Further details of geological publications and maps produced by the Geological Survey of Western Australia are available from: Information Centre Department of Mines, Industry Regulation and Safety 100 Plain Street EAST PERTH, WA 6004 Telephone: +61 8 9222 3459 Facsimile: +61 8 9222 3444 |
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