AGSO Research Newsletter 24

Coward, M.P. & Ries, A.C. (editors), Geo•logical Society of London, Special Publica•tion 95,155-172).

Breccia-type lodes are a possibility in the Gerry Well greenstone belt in the northwest, where chlorite in mafic rocks could indicate a very low-grade metamorphic environment suitable for deposits of the Wi luna and Mount Pleasant types. No faults where breccias could be sought have been recognised in this poorly exposed belt, but a major magnetic lineament strikes north through it, and a minor magnetic lineament strikes northwest through the southern part of it. The linea•ments are probably faults, and warrant investigation for breccias (site 1, Fig. 3).

Shear-zone deposits in stockworks or par•allel sets of quartz veins accompanied by al•teration zones are possible in competent rocks in the area. Sinusoidal bending of the two parts of the Deleta greenstone belt demonstrates the effect of considerable defonnation along the shear zone separating them. Folding of moder•ately competent rock types such as dolerite and banded iron fonnation during the shearing could have favoured the emplacement of quartz-vein stockworks (sites 2 and 3, Fig. 3) as found at Mount Charlotte, Lancefield, Victory Defiance, Sons ofGwalia, Hill 50, and

Mount Morgan. Sites of low mean stress favouring fluid

flow and hence gold deposition can be predicted near the margins of rigid rock bodies subjected to the far-field west-northwest•oriented compressive stress inferred in this area (Ridley 1993 : Ore Geology Reviews, 8, 23-37; Groves et al. 1995: op. cit.). Possible sites in northeast Duketon are:

the granitoid juxtaposed against the concave regions of the Deleta greenstone belt (both parts; sites 4 and 5, Fig. 3) and the unnamed greenstone belt (site 6) in the west; the greenstone belts are folded, and the more rigid granitoid in the hinge regions might have deflected the stress field and produced regions ofheterogene•ous stress and low mean stress, as at Granny Smith; and the north-northwesterly striking dyke•filled fracture in the southwest (site 7) that appears to be sinistrally offset by several kilometres along a minor linea•ment; the dilational jog so fonned could have dilated during later movement along the minor lineament, and focused ore-fluid flow, as at the Wiluna and Mount Pleasant deposits.

May 1996

Relevant maps and data New 1: 1 00 OOO-scale geological maps (Tate, Urarey, and De La Poer) of the northern half of the Duketon 1:250 000 Sheet area are available from AGSO in digital form and as coloured on-demand paper prints, together with an accompanying report (see announce•ment in AUS.GEO News 32, for February 1996, p. 6). Geophysical data for the Duketon Sheet area have been previously released+

I AGSO Division of Regional Geology & Minerals, GPO Box 378, Canberra, ACT 260 I; tel. +6 1 62499666, fax +6 1 62499983.

• Here named the Deleta greenstone belt after Deleta homestead, which is located at the margin of the northern portion of the belt.

+ Total magnetic intensity (TMI) contours and profiles, to•tal-count contours, and flight lines; magnetic and gamma-ray spectrometric digital data (point-located and gridded); and pixel-image maps (TMI in colour and grey•scale), all at 1:250000 (see AUS.GEO News 23, for Au•gust 1994, p. 10, and 25, for December 1994, p. 6). Dig•ital-elevation-model (DEM) data (point-located and gridded) and contours were released in February 1996 (see AUS.GEO News 32, for February 1996, p. 10). The Duketon Sheet area also f0I1115 part of gradient enhanced colour and grey-scale I: I 000 000 pixel-image TMI maps reduced to the pole for the northem Eastern Gold•fields (see AUS.GEO News 30, for October 1995, p.14).

How ancient is the Bungle Bungle Range of the East Kimberley, Western Australia?

The Bungle Bungle Range rises up to 640 m above sea level and 380 m above a surrounding sand plain to form the spectacular scenic attraction of Purnululu

David H. Blake1

National Park. An illustrated information booklet on this park ('Bungle Bungle Range', by Hoatson et a\.) is currently be•ing prepared by AGSO in collaboration

with the Geological Survey of Western Australia (GSWA) and the Department of Conservation & Land Management of Western Australia.

Fig. 4(a) Incised summit plateau ofthe Bungle Bungle Range, looking SE; (b) bevelled summit of the Osmond Range northwest ofPurnululu, showing steeply dipping Proterozoic sandstone truncated by the ancient (Early Miocene) land surface. Photographs by Ian Oswald-Jacobs.

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May 1996

The summit area of the Bungle Bungle Range is a gently undulating plateau, the rem•nant of an ancient mature land surface which has been incised by creeks of the present-day Ord River drainage system (Fig. 4a). The in•cision has led to the development of the deep gorges and clusters of steep-sided beehive•shaped hills that characterise the range, and the high cliffs along its western and north•western sides. These landforms have been carved out of flat-lying to moderately dipping sandstone and conglomerate of the Devonian Mahony Group, the youngest rocks preserved in the Palaeozoic Ord Basin. The Mahony Group was covered by an unknown thickness, probably several kilometres, of younger Palaeozoic rocks (Mory & Beere 1988: GSWA Bulletin 134) before sedimentation in the Ord Basin ceased with the onset of tec•tonic activity involving uplift, faulting, local thrusting, and mainly open folding. The Bun•gle Bungle Range lies within the broad Hardman Syncline, which fonned during this tectonism. The tectonism may be correlated with the Alice Springs Orogeny of central Australia 300 Ma ago.

Since then, the Kimberley region appears to have been tectonically stable, apart from some local minor faulting, and has been sub•jected to subaerial denudation. During this time, vast amounts of rock have been stripped from the region and deposited elsewhere. Units of the Ord Basin succession that over•lay Proterozoic rocks to the west and north of the present basin outcrop have been removed by erosion, as has the kilometres-thick se•quence of Palaeozoic rocks that was depos•ited on top of the Mahony Group.

The land surface represented by the sum•mit plateau of the Bungle Bungle Range is the same as that fonning the bevelled sununits of high ranges nearby - including those along the Osmond (Fig. 4b) and Matsu Ranges to the northwest and north, Mount Ranford and Nicks Bite to the west, and the Albert Edward Range to the southwest - all of which are fonned of moderately to steeply dipping Proterozoic sedimentary rocks. The same land surface is also preserved to the south on Dixon Range, which is fonned of flat-lying Devonian Mahony Group, like the Bungle Bungle Range (Fig. 4a). The present-day Sturt Plateau, to the south and southeast (Warren 1994: AGSO Re•search Newsletter 20, 14- 16), is part of the same land surface, and shows what the ancient land surface was probably like in the Bungle Bungle Range area before subsequent dissec•tion: a subdued landscape consisting of flat to gently undulating terrain with scattered low residual hills and ridges fonned of resistant sandstone. Remnants of this land surface in the East Kimberley - plateaus and bevelled ridge crests - are shown in Figure 5.

To the east of the Bungle Bungle Range, at White Mountain (Fig. 5), the ancient land surface - here formed on Late Cambrian sandstone - is overlapped by the White Mountain Fonnation. This is a unit of fluvial

128'

Erosional land surface preserved as D plateaus&

bevelled ridge crests

o Remnant of ancient depositional surface

I Halls Creek

/

B A Nicks Bite Ord River I

600 I Mt Ranford Bungle Bungle Range

Miocene sea level

AGSO Research Newsletter 24

C Mt Buchanan

I

o Agate Hills

Miocene land surface

129'

17'

o

18'

o WhiteMt

Ord River I I ~ 400

~ 200 OJ-_~P~r~es~en~t~se~a~/e~v~el ________________________ ~

30km Y.. =12 ~---_____ ~I H

Fig. 5. Remnants of the ancient (Early Miocene) land surface in the Purnululu region of the East Kimberley, and Early Miocene and present-day profiles (along line A-B-C-D).

sandstone and siltstone up to 110m thick capped by up to 3 m of chert containing non•marine gastropods (Syrioplanorbis hardmani) and marine foraminifera (Ammonia beccarii) which indicate an age probably no older than Early Miocene (Lloyd 1968; McMichael 1968 - both in BMR Bulletin 80) - that is, younger than 23.8 Ma (JonesI995: AGSO Phanerozoic Time Scale 1995). The chert was probably deposited in coastal lagoons at a time when sea level rose during a marine transgression to around 300- 350 m higher here than it is now. Sometime after deposi•tion, the White Mountain Formation was

tilted to dip 25° to the northeast, along a lo•cal fault. Chert about 45 m thick (and 360 m above sea level) that occurs 40 km to the southwest - at Agate Hills, where it overlies Cambrian basalt (Antrim Plateau Volcanics) and limestone (Headleys Lime•stone) - may also be part of this fonnation (Mory & Beere 1988: op.cit.).

After the deposition of the White Moun•tain Fonnation, a relative lowering of sea level by more than 300 m resulted in a new phase of erosion. The Ord River and its tributaries cut down into the old land surface, eventually to fonn the present-day landscape. The onset of

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AGSO Research Newsletter 24

renewed erosion on land probably coincided with a sudden increase in sedimentation at the beginning of the Miocene offshore in the Joseph Bonaparte Gulf, where sedimentation rates have remained high to the present day (Petrel Sub-basin stratigraphic chart 1996: AGSO). The spectacular landforms of the Bungle Bungle Range are the results of this Miocene to Recent erosion, as are most of the steep-sided ridges and plateaus in the Kim•berley region. A profile of the land surface immediately before incision, the sea level at that time, a profile of the present-day land surface, and the present-day sea level are shown in Figure 5.

The amount of erosion that has taken place in the Purnululu National Park of the East Kimberley during the last 23.8 Ma is

comparable to that in the Warrumbungles National Park of New South Wales, where the erosional remnants of a large central volcano that was active 19 Ma ago provide another•although very different - scenic attraction (Duggan & Knutson 1993: 'The Warrum•bungle volcano'; AGSO booklet).

An economically important effect of the latest erosional phase, following deposition of the White Mountain Formation, may be the uncovering of the Argyle diamond pipe and the subsequent deposition of diamond•bearing alluvial sediments downstream. The diamond pipe crops out in a small valley at an elevation more than 200 m lower than the old land surface represented by the bevelled top of the nearby Matsu Range. The pipe, which consists of two or more volcanic

May 1996

vents filled with pyroclastics similar in age to country rocks alongside (Boxer & Jaques, 1990: Australasian Institute of Mining & Metallurgy, Monograph 14, 697- 706), may not have been exposed before the Miocene. The alluvial gold deposits currently being worked in the Halls Creek area to the south•west of the Bungle Bungle Range also post•date the White Mountain Fonnation.

How ancient is the Bungle Bungle Range? Being a result of erosion that has taken place in the East Kimberley since the Early Miocene, the range, like most other ranges in the region, is no more than 23 .8 Ma old.

I AGSO Division of Regional Geology & Minerals, GPO Box 378, Canberra, ACT2601 ; tel. +61 62499667, fax +61 6 249 9983.

The Mount Isa geodynamic transect The deep seismic reflection profile south of Mount

Isa and Cloncurry Bruce Golebyl, Barry Drummond!, & Tyler MacCready2

Interpretations of the deep seismic reflec•tion and geological mapping data acquired along the Mount Isa geodynamic transect have contributed to the develop•ment of a new crustal-scale depth section of the Mount Isa Inlier. Key features of the section include a transitional crust•mantle boundary, a marked difference in the structure of the top 5-10 km between the Eastern Fold Belt and the Western Fold Belt, and the imaging of several ma•jor fault systems that cut through the crust. The structure imaged in the East•ern Fold Belt appears to reflect east-west shortening that developed after an epi•sode of rifting (Lake Mary Kathleen Zone). West-vergent thrusting and folding continued across the Pilgrim Fault and into the Kalkadoon-Leichhardt Belt, but in the Leichhardt River Fault Trough (Western Fold Belt) the sense of structur•ing flipped to east-vergent. The Mount Isa Fault system and the associated litho•logical sequences now all dip to the west.

The Australian Geodynamics Cooperative Research Centre (AGCRC) undertook the Mount Isa geodynamic transect as part of a multidisciplinary study of the structure and evolution of the Mount Isa Inlier. This transect is one of several AGCRC projects designed to provide a geodynamic synthesis of the inlier. Its objective is to map the depth extent and in•ternal fonn of the structures and geological blocks in the inlier, which in tum will improve our understanding of mineralisation at the lo•cal scale by placing it in a regional three•dimensional context.

6

AGCRC conducted a series of field ex•periments designed to define the crustal struc•ture of the inlier - primarily its boundaries and the geometry of its major domain bound•ary faults; and secondarily, the geometry of internal features of its tectonic units, and the fonn of its granite intrusions. On the regional scale, the crustal structure beneath the Mount Isa Inlier will be compared with the crustal structure of the crust to the east and west of the inlier. Understanding these features will provide constraints on the relative importance of compressional thrusting and folding, extensional tectonics and basin fonnation, and strike-slip and transfonn faulting during the evolution of the inlier.

A wide range of geological, geo•chemical, and geophysical studies supported the transect, which comprised two seismic reflection traverses (Fig. 6): one oriented east- west and 255 km long; the other subparallel and 30 km long in the Wonga Belt north of Duchess. The main traverse, about 20 km south of the townships of both Mount Isa and Cloncurry, extends through a region of excellent outcrop and covered by numerous detailed maps. The Duchess traverse crossed both the Pilgrim Fault and the Fountain Range Fault.

To provide constraints on the crustal structure, a seismic crustal-refraction survey 500 km long was carried out along the full length of the reflection profile (Fig. 6). Complementary seismic work - including low-fold 3D reflection surveys, a tomo•graphic survey, and high-resolution refrac•tion surveys - was undertaken over several

important structures to improve our under•standing of them. Potential-field studies along the seismic traverses included the acquisition of detailed gravity, ground mag•netic, susceptibility, and radiometric data. Magnetotelluric and SIROTEM soundings were made at several key sites . Detailed geological, structural , and metamorphic mapping was undertaken along the transect, including the logging of seismic shot-hole cuttings . Regolith data were recorded both from the seismic shot-holes and the sur•rounding area. Bottom-hole samples from the shot-holes were taken for whole-rock and trace-element geochemical analyses, and selected samples were used for fission•track analysis.

This wealth of new data, primarily coming from depth sections, and the vast amounts of existing Mount Isa data have necessitated the development of better visualisation method•ologies. The new data have been built into a Mount Isa geodynamic transect GIS, which uses, as its base, the early AGSO Mount Isa Mineral Province 1:500 000 GIS (Jagodzinski et al. 1993: AGSO Metallogenic Atlas Series 1, Vol. 1). The transect GIS is the subject of a companion paper (Wyborn et aI., p. 10 of this issue).

Large-scale crustal structure The seismic refraction results indicate that the crust is thick throughout the region. The crust- mantle boundary is not sharp but is a thick (up to 15 km) transitional zone. The most interesting feature is the presence of a high-velocity (7km s -1) lens-shaped body in