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 f or m a t i ons , s om e of w hi c h h a ve onl y be e n r e c ogni s e d s ubs e que nt l y . H ow e ve r , i n t e r m s of a r e a of out c r op, t he bul k of t he f or m e r ce nt r a l mount st ua r t be ds r e m a i ns w i t hi n t he now formally defined (Offe 1978, amended by Walter 1980) ce nt r a l mount st ua r t For m a t i on. Por t i ons of t he f or m e r ce n t r a l mount st ua r t be ds i n Woodgreen a r e now m a ppe d a s t h e Boko For m a t i on, oor a br a ar kos e a nd el yua h, gr a nt Bl uf f , oc t y a nd c ha ba l ow e f or m a t i ons . the ce nt r a l mount st ua r t For m a t i on i s w i de s p r e a d i n s out h w e s t e r n BarroW creek, s out he a s t e r n mount Peake, nor t he a s t e r n naPPerBy a nd nor t hw e s t e r n alcoota. the t ype s e c t i on i s a t ce nt r a l mount st ua r t i n MOUNT PEAKE (Offe 1978). Haines et al (1991) subdivided t he f or m a t i on i n BarroW creek i n t o t he For s t e r , tops a nd adne r a m e m be r s , i n a s c e ndi ng or de r . al l t hr e e o f t he s e uni t s c a n be r e c ogni s e d i n nor t he r n Woodgreen, a l t hough i t i s now de m ons t r a t e d t ha t t he f or m e r For s t e r me m be r i s a n a t t e nua t e d e qui va l e nt of t he gr a nt Bl uf f For m a t i on (s e e above). Walter (1980) recognised the Elkera Formation be ne a t h t he ce nt r a l mount st u a r t For m a t i on i n di a m ond dr i l l h ol e cms1 , ba s e d on t he pr e s e nc e of a dol om i t i c a nd evaporitic interval between 380 m and 580 m depth. It is clear t ha t t hi s s t r a t i gr a phi c i nt e r va l i s a l s o pr e s e nt i n ne a r by s ur f a c e e xpo s ur e s a nd i s l i ke l y t o be e qu i va l e nt t o a n i nt e r va l l ow i n t he tops me m be r i n BarroW creek a nd a t t he ce nt r a l mou nt st ua r t t ype s e c t i on t ha t di s pl a ys m i nor dol om i t i c a nd c a l c a r e ous l i t hol ogi e s i n s ur f a c e e xpos ur e s . expo s ur e of t hi s i nt e r va l i s ge ne r a l l y ve r y poor i n Woodgreen a nd t he c a r bo na t e s do not out c r op (a pa r t f r om a n e xpr e s s i on a s pa t c hy s ur f a c e c a l c r e t e i n s om e a r e a s ). Whe r e e xpos e d, i t m a i nl y c om p r i s e s r e d- br ow n a nd pur pl e s a nds t one a nd m u ds t one , which are difficult to distinguish from younger parts of the s t r a t i gr a phi c s uc c e s s i on, a nd i t ha s pr o ve d i m pos s i bl e t o m a p a c ons i s t e nt upp e r bounda r y t o t he c a r bona t e i nt e r va l i n out c r op. For t he s e r e a s ons , t he c a r bona t e a nd e va por i t e uni t i s i nc l ude d i n t h e to ps me m be r , but i t i s c onc e de d t ha t t hi s i nt e r va l i s a l i ke l y l a t e r a l e qui va l e nt of t he el ke r a For m a t i on i n H uckitta. you nge r pa r t s of t he ce nt r a l mount st ua r t For m a t i on a r e pr oba bl y l a t e r a l e qui va l e n t s of t he t ype mount Ba l d w i n For m a t i on (a t l e a s t i n pa r t ), t he u ni t t ha t ove r l i e s t he el ke r a For m a t i on i n H uckitta. thi s c or r e l a t i on r e qui r e s some qualification, as the Mount Baldwin Formation has pr e v i ous l y be e n c ons i de r e d t o b e e nt i r e l y ca m br i a n i n a ge (Walter 1980, Freeman 1986). The type section of the Mount Ba l d w i n For m a t i on i n t he el ua ra nge i n H uckitta i s l i t ho l ogi c a l l y ve r y s i m i l a r t o t he uppe r t w o t hi r ds of t he tops me m be r a nd t he ove r l yi ng adn e r a me m be r of t he ce nt r a l mou nt st ua r t For m a t i on, a nd a s e a r c h t hr oughout t hi s mou nt Ba l dw i n For m a t i on s e c t i on r e ve a l e d no t r a c e f os s i l s . ea r l y ca m br i a n t r a c e f os s i l s ha ve be e n doc um e nt e d f r om a unit mapped as Mount Baldwin Formation in the Jervois ra ng e f u r t he r e a s t (Wa l t e r et al 1989), and Freeman (1986) a l s o doc um e nt e d a pr onounc e d ba s a l unc onf or m i t y i n t hi s a r e a , w hi c h i s a ppa r e nt l y m i s s i ng a t t h e t ype s e c t i on. the l i ke l y e xpl a na t i o n i s t ha t w ha t ha s be e n m a ppe d a s mount Ba l d w i n For m a t i on i n H uckitta c om pr i s e s a c om pos i t e of e qui v a l e nt s of pa r t of t he ce nt r a l mount st ua r t For m a t i on a nd o c t y F or m a t i on. H a i ne s et al (1991) suggested that the Central Mount st ua r t For m a t i on w a s de pos i t e d a t a t i m e of m i l d t e c t oni s m , br oa dl y c o i nc i de nt w i t h t he Pe t e r m a nn or oge ny . se d i m e nt s w e r e s he d f r om upl i f t e d r e gi ons , pr oba bl y t o t he w e s t , a nd w e r e de pos i t e d i n s out he a s t - t r e ndi ng f a ul t - c ont r ol l e d t r oughs . suc h a t r ough i s i nf e r r e d t o e xt e nd f r om t he mount ski nne r a r e a nor t hw e s t t o t he ce nt r a l mount st u a r t t ype s e c t i on. the t hi c ke s t de ve l opm e nt s of t he ce nt r a l mount st ua r t For m a t i on a r e w i t hi n t hi s t r ough. Tops M ember ( LPst) in o u t c r o p , t h e to p s me m b e r c o m p r i s e s m a i n l y r e d - b r o w n a n d p u r p l e , l i t h i c a n d f e l d s p a t h i c s a n d s t o n e a n d a r k o s e , i n t e r b e d d e d w i t h m u d s t o n e . th e l o w e r t h i r d o f t h e to p s me m b e r i s v e r y r e c e s s i v e . sa n d s t o n e r a n g e s f r o m m e d i u m g r a i n e d t o c o a r s e a n d g r a n u l a r . tr o u g h c r o s s b e d d e d s a n d s t o n e b e d s i n t h e u p p e r p a r t o f t h e m e m b e r r e a c h 4 m i n t h i c k n e s s a n d o f t e n f o r m p r o m i n e n t b e n c h e s . mu c h o f t h e to p s me m b e r i s c h a r a c t e r i s e d b y a m o d e r a t e k- d o m i n a t e d (around 1.3% equivalent K) radiometric signature. However, in some areas (such as around 425000mE 7556500mN and 429300mE 7557500mN), the uppermost section of the u n i t (s t r a t i g r a p h i c a l l y a b o v e t h e g r e e n b a n d s d i s c u s s e d b e l o w ) h a s a l m o s t n o r a d i o m e t r i c r e s p o n s e , i n t h i s r e s p e c t , a p p e a r i n g t o b e i d e n t i c a l t o t h e o v e r l y i n g ad n e r a me m b e r . th e r e a r e a l s o i n d i c a t i o n s o f s i m i l a r l y l o c a t e d v a r i a t i o n w i t h i n t h e u p p e r m o s t to p s me m b e r i n la n d s a t 7 i m a g e r y . it i s t h u s a r g u a b l e , o n g e o p h y s i c a l e v i d e n c e a l o n e , t h a t t h e m a p b o u n d a r y b e t w e e n t h e t w o u n i t s , i n t h e s e i n s t a n c e s , c o u l d m o r e a p p r o p r i a t e l y b e s h i f t e d t o c o r r e s p o n d w i t h t h e b r e a k b e t w e e n r a d i o m e t r i c s i g n a t u r e s . di a m o n d d r i l l h o l e cms1 p r o v i d e s t h e b e s t s e c t i o n t h r o u g h t h e to p s me m b e r a n d r e v e a l s t h a t t h e l o w e r r e c e s s i v e i n t e r v a l c o n t a i n s a m a j o r g r e y d o l o s t o n e a n d d o l o m i t i c s a n d s t o n e c o m p o n e n t , w i t h h o r i z o n s o f a n h y d r i t e a n d c h e r t n o d u l e s a f t e r a n h y d r i t e . so m e d o l o s t o n e b e d s a r e o o l i t i c . th e s e r o c k t y p e s h a v e n o t b e e n r e c o g n i s e d i n o u t c r o p a n d t h e o n l y i n d i c a t i o n o f t h e c a r b o n a t e c o m p o n e n t a t d e p t h i s p a t c h y s u r f a c e c a l c r e t e a n d c o m m o n f o l d i n g a n d c o n t o r t i o n o f o u t c r o p p i n g f r i a b l e s a n d s t o n e b e d s t h a t i s i n t e r p r e t e d t o h a v e r e s u l t e d f r o m s o l u t i o n c o l l a p s e . th e d e p o s i t i o n a l e n v i r o n m e n t i s i n f e r r e d t o r a n g e f r o m m a r g i n a l m a r i n e s a b k h a i n t h e l o w e r t h i r d t o d e l t a i c i n t h e u p p e r t w o t h i r d s . th e u p p e r p a r t s o f t h e to p s me m b e r c o n t a i n a n u m b e r o f r e d u c e d g r e y - g r e e n h o r i z o n s t y p i c a l l y o f a b o u t a m e t r e , b u t u p t o s e v e r a l m e t r e s i n t h i c k n e s s (Figure18). in o u t c r o p , t h e s e g r e e n b a n d s s o m e t i m e s d i s p l a y s p a r s e s e c o n d a r y c o p p e r m i n e r a l i s a t i o n a n d h a v e c o n s e q u e n t l y b e e n t h e s u b j e c t o f s e v e r a l m i n e r a l e x p l o r a t i o n p r o g r a m s (s e e Economic geology : copper, lead, zinc). th e g r e e n b a n d s r e p r e s e n t e d o n t h e m a p f a c e a r e b a s e d o n t h e r e s u l t s o f e x p l o r a t i o n m a p p i n g a n d o n a e r i a l p h o t o g r a p h i n t e r p r e t a t i o n . mo s t a c t u a l l y d e n o t e s e v e r a l c l o s e l y s p a c e d m e t r e - s c a l e g r e e n b a n d s . Wi t h f e w e x c e p t i o n s , t h e g r e e n b a n d s t e n d t o b e p o o r l y e x p o s e d , e x c e p t w h e r e c u t b y e x p l o r a t i o n c o s t e a n s . The Tops Member is 580 m thick in CMS1 (the hole w a s s p u d d e d a t a p p r o x i m a t e l y t h e b a s e o f t h e ad n e r a me m b e r ) a n d t h i s i s c o n s i d e r e d t o b e c l o s e t o t h e m a x i m u m p r e s e r v e d t h i c k n e s s . th e l o w e r c a r b o n a t e - b e a r i n g i n t e r v a l in this hole, extending over 380580 m depth, is 200 m i n t h i c k n e s s . th e m e m b e r t h i n s t o t h e n o r t h , a n d n e a r t h e c e n t r a l n o r t h e r n b o u n d a r y o f Woodgreen, i s e s t i m a t e d t o b e o n l y a b o u t 1 0 0 m t h i c k .