Territory Stories

Woodgreen, Northern Territory : explanatory notes

Details:

Title

Woodgreen, Northern Territory : explanatory notes

Issued by

Northern Territory Geological Survey

Collection

E-Publications; E-Books; PublicationNT; Australia 1:100 000 Geological Map Series

Date

2007

Location

Australia 1:100 000 Geological Map Wood 5458; Australia 1:250 000 Geological MapAlcoota SF5310; Australia 1:250 000 Geological MapAlcoota SF5310

Map scale

1:100 000

Notes

Made available via the Publications (Legal Deposit) Act 2004 (NT); Available from GEMIS - Geoscience Exploration and Mining Information System

Language

English

Subject

Geology; Georgina Basin; Arunta Region

Publisher name

Northern Territory Government; Northern Territory Government

Place of publication

Darwin

Edition

1st ed.

Series

Australia 1:100 000 Geological Map Series

File type

application/pdf

ISBN

9780724571321

ISSN

0811-6296

Use

Attribution International 4.0 (CC BY 4.0)

Copyright owner

Northern Territory Government

License

https://creativecommons.org/licenses/by/4.0/

Related links

https://geoscience.nt.gov.au/gemis/ntgsjspui/handle/1/81885 [GEMIS]

Parent handle

https://hdl.handle.net/10070/794795

Citation address

https://hdl.handle.net/10070/794807

Page content

16 formations, some of which have only been recognised subsequently. However, in terms of area of outcrop, the bulk of the former Central Mount Stuart beds remains within the now formally defined (Offe 1978, amended by Walter 1980) Central Mount Stuart Formation. Portions of the former Central Mount Stuart beds in Woodgreen are now mapped as the Boko Formation, Oorabra Arkose and Elyuah, Grant Bluff, Octy and Chabalowe formations. The Central Mount Stuart Formation is widespread in southwestern BARROW CREEK, southeastern MOUNT PEAKE, northeastern NAPPERBY and northwestern ALCOOTA. The type section is at Central Mount Stuart in MOUNT PEAKE (Offe 1978). Haines etal (1991) subdivided the formation in BARROW CREEK into the Forster, Tops and Adnera members, in ascending order. All three of these units can be recognised in northern Woodgreen, although it is now demonstrated that the former Forster Member is an attenuated equivalent of the Grant Bluff Formation (see above). Walter (1980) recognised the Elkera Formation beneath the Central Mount Stuart Formation in diamond drillhole CMS, based on the presence of a dolomitic and evaporitic interval between 380 m and 580 m depth. It is clear that this stratigraphic interval is also present in nearby surface exposures and is likely to be equivalent to an interval low in the Tops Member in BARROW CREEK and at the Central Mount Stuart type section that displays minor dolomitic and calcareous lithologies in surface exposures. Exposure of this interval is generally very poor in Woodgreen and the carbonates do not outcrop (apart from an expression as patchy surface calcrete in some areas). Where exposed, it mainly comprises redbrown and purple sandstone and mudstone, which are difficult to distinguish from younger parts of the stratigraphic succession, and it has proved impossible to map a consistent upper boundary to the carbonate interval in outcrop. For these reasons, the carbonate and evaporite unit is included in the Tops Member, but it is conceded that this interval is a likely lateral equivalent of the Elkera Formation in HUCKITTA. Younger parts of the Central Mount Stuart Formation are probably lateral equivalents of the type Mount Baldwin Formation (at least in part), the unit that overlies the Elkera Formation in HUCKITTA. This correlation requires some qualification, as the Mount Baldwin Formation has previously been considered to be entirely Cambrian in age (Walter 1980, Freeman 1986). The type section of the Mount Baldwin Formation in the Elua Range in HUCKITTA is lithologically very similar to the upper two thirds of the Tops Member and the overlying Adnera Member of the Central Mount Stuart Formation, and a search throughout this Mount Baldwin Formation section revealed no trace fossils. Early Cambrian trace fossils have been documented from a unit mapped as Mount Baldwin Formation in the Jervois Range further east (Walter etal 1989), and Freeman (1986) also documented a pronounced basal unconformity in this area, which is apparently missing at the type section. The likely explanation is that what has been mapped as Mount Baldwin Formation in HUCKITTA comprises a composite of equivalents of part of the Central Mount Stuart Formation and Octy Formation. Haines etal (1991) suggested that the Central Mount Stuart Formation was deposited at a time of mild tectonism, broadly coincident with the Petermann Orogeny. Sediments were shed from uplifted regions, probably to the west, and were deposited in southeasttrending faultcontrolled troughs. Such a trough is inferred to extend from the Mount Skinner area northwest to the Central Mount Stuart type section. The thickest developments of the Central Mount Stuart Formation are within this trough. TopsMember(LPst) In outcrop, the Tops Member comprises mainly redbrown and purple, lithic and feldspathic sandstone and arkose, interbedded with mudstone. The lower third of the Tops Member is very recessive. Sandstone ranges from medium grained to coarse and granular. Trough crossbedded sandstone beds in the upper part of the member reach 4 m in thickness and often form prominent benches. Much of the Tops Member is characterised by a moderate Kdominated (around 1.3% equivalent K) radiometric signature. However, in some areas (such as around 425000mE 7556500mN and 429300mE 7557500mN), the uppermost section of the unit (stratigraphically above the green bands discussed below) has almost no radiometric response, in this respect, appearing to be identical to the overlying Adnera Member. There are also indications of similarly located variation within the uppermost Tops Member in Landsat 7 imagery. It is thus arguable, on geophysical evidence alone, that the map boundary between the two units, in these instances, could more appropriately be shifted to correspond with the break between radiometric signatures. Diamond drillhole CMS provides the best section through the Tops Member and reveals that the lower recessive interval contains a major grey dolostone and dolomitic sandstone component, with horizons of anhydrite and chert nodules after anhydrite. Some dolostone beds are oolitic. These rock types have not been recognised in outcrop and the only indication of the carbonate component at depth is patchy surface calcrete and common folding and contortion of outcropping friable sandstone beds that is interpreted to have resulted from solution collapse. The depositional environment is inferred to range from marginal marine sabkha in the lower third to deltaic in the upper two thirds. The upper parts of the Tops Member contain a number of reduced greygreen horizons typically of about a metre, but up to several metres in thickness (Figure18). In outcrop, these green bands sometimes display sparse secondary copper mineralisation and have consequently been the subject of several mineral exploration programs (see Economic geology: copper, lead, zinc). The green bands represented on the mapface are based on the results of exploration mapping and on aerial photograph interpretation. Most actually denote several closely spaced metrescale green bands. With few exceptions, the green bands tend to be poorly exposed, except where cut by exploration costeans. The Tops Member is 580 m thick in CMS1 (the hole was spudded at approximately the base of the Adnera Member) and this is considered to be close to the maximum preserved thickness. The lower carbonatebearing interval in this hole, extending over 380580 m depth, is 200 m in thickness. The member thins to the north, and near the central northern boundary of Woodgreen, is estimated to be only about 00 m thick.


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