Tectonic evolution of the Pine Creek Inlier, Northern Territory

Precambrian Research, 40/41 (1988) 543-564 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

543

TECTONIC EVOLUTION OF THE PINE CREEK INLIER, NORTHERN TERRITORY R.S. N E E D H A M , P.G. S T U A R T - S M I T H a n d R.W. P A G E

Bureau of Mineral Resources, P.O. Box 378, Canberra, A.C. 71. 2601 (Australia) (Received November 18, 1986; revision accepted December 24, 1987)

Abstract Needham, R.S., Stuart-Smith, P.G. and Page, R.W., 1988. Tectonic evolution of the Pine Creek Inlier, Northern Territory. Precambrian Res., 40/41: 543-564. The Pine Creek Inlier (c. 66 000 km 2) contains Early Proterozoic sediments and volcanics deposited ~ 1900 Ma ago in a basin formed by crustal extension of c. 2500 Ma granitic basement. About 10 km of supracrustals accumulated in < 20 Ma in shallow marine to continental environments during sagging related to post-extensional subsidence. Deposition in this 'Pine Creek Geosyncline' was succeeded by an orogenic period lasting for 180 Ma (1870-1690 Ma, the 'Top End Orogeny'). Felsic and mafic intrusives were emplaced before and after the main deformation, which ranged in style from open to tight upright folds in low-grade areas in the centre of the region to reclined multiple isoclinal folds in areas of medium- to high-grade metamorphism in the northeast and west. During late orogenesis, rift-related felsic volcanics and continental sediments were deposited on the metamorphosed sequence to form the 'Katherine Volcanic Sequence', representing a second period of crustal extension. The lower part of this sequence was itself mildly deformed by the last significant perturbation of the Top End Orogeny. The hiatus between these Early Proterozoic events and Middle Proterozoic deposition is marked by a saprolitic weathered profile, indicating a stable subaerial period perhaps as long as 150 Ma. The cover rocks are the basal part of the McArthur Basin sequence, which extends eastward into Queensland. In the Pine Creek region they are plateauforming sandstone generally ~ 600 m thick, thickening to 2000 m in basins near Katherine formed by further movement of the late Early Proterozoic rift systems. The region has remained stable since Proterozoic time with only minor reactivation of rift structures to allow accumulation of locally thicker Mesozoic and Eocene sequences and intrusion of minor mafic to intermediate dykes in the Late Proterozoic and Palaeozoic. Later eustatic movement led to incursion of shallow Palaeozoic and Mesozoic seas.

Introduction T h e P i n e C r e e k Inlier c o n s i s t s o f E a r l y P r o terozoic m e t a s e d i m e n t s , v o l c a n i c s a n d igneous r o c k s c o v e r i n g ~ 66 000 k m 2 east a n d s o u t h o f Darwin, N o r t h e r n Territory, Australia (Fig. 1 ). It r e p r e s e n t s p a r t of a m u c h larger area o f E a r l y Proterozoic deposition and magmatism, which e x t e n d e d m a i n l y to t h e s o u t h a n d east, n o w c o n c e a l e d b y y o u n g e r rocks.

T h e regional geology is described b y N e e d h a m et al. (1980). A d e p o s i t i o n a l d o m a i n ( P i n e C r e e k G e o s y n c l i n a l S e q u e n c e ) of a b o u t 2 2 0 0 1880 M a rests on c. 2500 M a granitic b a s e m e n t . T h i s sequence is succeeded b y a p e r i o d of comp r e s s i o n a l a n d e x t e n s i o n a l t e c t o n i s m , with related igneous a c t i v i t y b e t w e e n 1870 a n d 1690 Ma, here n a m e d t h e T o p E n d Orogeny. Riftrelated felsic volcanics a n d v o l c a n i c l a s t i c s f o r m t h e K a t h e r i n e Volcanic S e q u e n c e developed

544

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F i g . 1. L o c a t i o n a n d s t r u c t u r a l s e t t i n g o f t h e P i n e C r e e k I n l i e r ( m o d i f i e d a f t e r N e e d h a m

during a period of extension in the latter part of the orogeny. Platform Cover sandstones of ~ 1650 Ma rest on these rocks with marked unconformity. Most compression in the Top E n d Orogeny took place in the Nimbuwah Event at ~ 1870 Ma, which correlates with the Barramundi Orogeny recognized through most of northern Australia by Etheridge et al. (1987). Owing to the apparent continuum of igneous activity and a complex and possibly overlapping series of tectonic events in the Pine Creek Inlier between 1870 and 1690 Ma, we have chosen to include all of this time period into one orogeny. This paper examines the tectonic control of

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these domains. The 'geosynclinal sequence' and 'orogenic sequence' developed in the same overall tectonic setting at different stages of one Early Proterozoic orogenic cycle• The boundary between these rocks and Middle Proterozoic 'platform cover' is dated at between 1688+ 13 and 1648___29 Ma (Page et al., 1980), which is roughly coincident with the 1700 + 30 Ma age recommended by Plumb et al. (1981) for the Early to Middle Proterozoic boundary. Tectonic

elements

The region is underlain by granitic (with minor metasedimentary rocks) late Archaean

545

terozoic rocks during the Top End Orogeny was concentrated in two areas; southwest of Darwin (Litchfield domain) and northeast of Oenpelli (Nimbuwah domain). Areas between underwent relatively mild two-phase folding and lowgrade metamorphism. The stratigraphy, geological history and structure of the Litchfield medium-grade terrain suggest that this part of the Top End Orogen represents an extension of the Halls Creek Mobile Belt (Hammond et al., 1984). No similar linearity or association is readily apparent in the Nimbuwah medium- to high-grade terrain, although the apparent shape of isograds,

basement exposed as three small domes (Fig. 2 ). Similar rocks have been intersected by drilling beneath thin Mesozoic cover in the Woolner area 65 km east of Darwin. These rocks, and Proterozoic granites, give rise to a relatively lowdensity geophysical basement of specific gravity (SG) = 2.6 below up to 5 km of supracrustals (SG=2.75-2.9), which in places are intruded by dolerite (SG=3.0) (Tucker et al., 1980). During deposition of the supracrustals, the basin subsided as a number of rigid shelves or platforms separated by faults (Figs. 3 and 4). Complex multiple folding and medium-grade metamorphism of the Archaean and Early Pro-

I .

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Phanerozoic cover

~+~+~+~+

Felstc volcanlcs Granltoids

Middle- Upper Proterozoic cover

syn- to postorogenic

Geosyncbnal sediments Archaean basement

Fig. 2. Generalized geology of the Pine Creek Inlier and surrounding areas. Archaean complexes: (1) Nanambu, (2) Rum Jungle, (3) Waterhouse. Proterozoic granites: (4) Burnside, (5) Prices Springs, (6) Margaret, (7) Jim Jim, (8) Malone Creek.

546

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DOMAIN

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Major fault zone Limit of Stuart-Smith et al's (1980) "South Alligator Trough"

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Fig. 3. Main tectonic elements. The Nimbuwah Event metamorphosed and deformed the entire region. Distribution of the Maud Event is not known but effects are most marked in the southeast. The Shoobridge Event is confined to the centre of the region.

LITCHFIELD DOMAIN

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MARRAKAI

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Fig. 4. Possible disposition of tilt blocks controlling sedimentation in the Pine Creek Geosyncline. Names in double quotes are terms defined by Stuart-Smith et al., 1980.

where not concealed by cover strata, suggests a possible northeastern elongation. Similar rocks exposed in the Gunbatgarri and Mirarrmina

Complexes, respectively 120 and 220 km east of Oenpelli, may indicate continuation of this orogen to the west coast of the Gulf of C arpentaria.

547 Late orogenic normal faults developed in parts of the South Alligator Trough and Western Fault Zone show evidence of movement in Middle Proterozoic, Lower Palaeozoic, Mesozoic and Eocene time. However, the Middle Proterozoic and younger mainly shallow epeirogenic sedimentary basins which overlap the Early Proterozoic rocks of the region (Middle Proterozoic McArthur Basin sediments and volcanics of the Arnhem Land Plateau and sediments of the Victoria River Basin, Permian sediments of the Bonaparte Gulf Basin, Cambro-Ordovician sediments and basalt of the Daly River Basin, and Mesozoic sediments of the Bathurst Terrace) are mostly undeformed. Faulting involving several hundred metres displacement of McArthur Basin rocks probably took place ~ 1610 Ma ago, and represents the youngest significant faulting episode.

Basement ages and lithotypes Page et al. ( 1980 ) reported Rb-Sr and U - P b zircon ages of crystallization of ~ 2500 Ma for the Archaean component of the Nanambu Complex (a mantled gneiss dome also containing accreted Early Proterozoic metasediments). The youngest granite phase in the Rum Jungle Complex gives an age of ~2400 Ma (Richards et al., 1977). Evidence of older terrains is restricted to the Rum Jungle and Waterhouse Complexes, which contain minor enclaves of metasediments and banded iron formation. McAndrew et al. (1985) report U Pb zircon ion microprobe measurements on granitoid subcrop in the Woolner Granite. These provide the oldest basement ages yet measured in the Pine Creek Geosyncline, of 2675 + 15 Ma. Ferguson et al. (1980) interpreted chemical data from the complexes to indicate crystallization from mixed magmas derived from igneous and sedimentary sources. Foliations are mainly oriented N - S or N W - S E , and developed in the Top End Orogeny unrelated to Archaean crustal stress. Modelling of Bouger

anomalies indicates a geophysical basement of predominantly granitic composition (Tucker et al., 1980), in which it is not possible to distinguish the proportion of Archaean and Proterozoic components.

Basin development Basin development by extension of an Archaean granitic crust took place probably ~ 2000 Ma ago. Age determination on extrusives in the later subsidence phase indicates that initial extension was complete by ~ 1880 Ma. Maturity of the sedimentary infill indicates a subdued landscape during basin development, in which country rock similar to the Archaean granites had been largely eroded. From variations in thickness of the sedimentary pile, Stuart-Smith et al. (1980) interpreted a central trough bounded to the east by the South Alligator Hinge Zone (the South Alligator Trough), forming a rapidly subsiding N N W SSE trending structure flanked by shallow shelves. Recent structural work (Johnston, 1984), however, has shown that much of the greater thickness in some parts of the sequence was caused by early bedding-parallel thrusting and/or folding. The form of the initial structure was probably a broad half graben (Fig. 4).

Early basin sequence About 10 km of clastic, organic and chemical sediments with minor volcanics, constituting the Pine Creek Geosyncline Sequence, filled the basin (Fig. 5; Needham and Stuart-Smith, 1984). The depositional environments ranged from neritic to intertidal to fluviatile for the most part, but flysch-like sediments indicate that subsidence rates increased towards the end of sedimentation (Fig. 6). Island shorelines existed around Archaean domes in the west and east early in the depositional history, and a major fluviatile wedge indicates a nearby northern shoreline in the middle of the sequence. Regional grain size variation and distribution of

548

RUM JUNGLE AREA

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CENTRAL AREA

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SOUTH ALLIGATOR HINGE Z O N E

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ALLIGATOR RIVERS AREA

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:::::::::::::::::::::::::::::::::::::::::::::::::::::::::: (

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iss00Ms++++++++ =~ + + + + + + + + + + + + =~a~+ + + + + + + + + + + + $ ~ 1 + + + + Granitoids + + + -u~++ ++++++ ++++

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+ + + + Grunitoids + + + + + + + + + + + + + + + 4 " + +

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NANAMBU COMPLEX

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~ 2500 Ms

Carbonate dolomite-magnesite Schist,gneiss

~

Granitoids 16/NT/351

Fig. 5. Diagrammatic stratigraphy of the Pine Creek Geosyncline Sequence and Katherine Volcanic Sequence showing tectonic stages and events, and age determinations.

Namoona Group conglomerate beds in the top of the sequence indicate a westerly provenance for the final stages of sedimentation. This sequence therefore appears to be the northwestern part of a more extensive depositional basin, probably with a N - S to N W - S E trending depocentre.

The oldest sediments of the basin are arkosic sandstone and conglomeratic basal fluviatile deposits of the Beestons Formation and Kakadu Group (Fig. 6a). The complete sequence is not exposed in the centre of the inlier, so their full extent is not known. They extend at least

549

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Ea~e~ev~atipoar/i't/°~i~m~a/ : Reelcarbonate

~: Centretelsviocclansisubaerial m°f

Later turbidite

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N a m o o n a Group time.

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Mineraloccurrences

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Mineraloccurrences

Mineraloccurrences

early M o u n t Partridge Group time

d

El Sherana and Edith River Group time

29/NT/5-4

Fig. 6. Palaeofacies reconstructions of four Early Proterozoic time slices in the Pine Creek Inlier (after Needham and StuartSmith, 1984).

60 km east of exposed basement in the northeast. They may be transitional into unexposed subtidal high-energy sediments distant from source. They are succeeded by subtidal low-en-

ergy facies (Masson Formation) dominated by carbonaceous pelite with interbeds of calcareous sandstone and carbonate, transitional into intertidal to supratidal evaporitic facies of the

550 Celia Dolomite (the evaporitic parentage of the metamorphic carbonate is indicated by pseudomorphs after gypsum and halite, Crick and Muir (1980)), indicating maturity of the deposystem. In the west the evaporites formed in thick continuous stromatolitic algal mats (Crick and Muir, 1980) but in the east formed lens-shaped masses up to 250 m thick within partly carbonaceous and calcareous pelites of the lower part of the Cahill Formation (Needham and Stuart-Smith, 1976).

Mount Partridge Group Rejuvenation of a northern provenance led to development of an extensive fluvial fan system across the centre of the inlier (Fig. 6b). These conglomerate, arkose, sandstone and minor siltstone beds fine and thin southwards (Mundogie Sandstone), but thicken and coarsen in the west (Crater Formation), indicating uplift of basement tilt blocks in that area to form highlands. Pebbles of chert after carbonate in the fluviatile conglomerates in the northern centre of the inlier, provide evidence for an extended period of subaerial weathering of older carbonate and subsequent erosion, attendant with rejuvenation. The fluvial fans grade distally and vertically to subtidal high-energy finely banded pelites (bands commonly !-10 mm of grey siltstone and black carbonaceous shale ) of the Wildman Siltstone, which are reasonably interpreted as varvites. In the west, near the Archaean highlands, marine transgression was slower; intertidal to supratidal carbonate (Coomalie Dolomite ), subtidal low-energy carbonaceous pelite (Whites Formation), and fluviatile deposits (quartzite lenses in the Wildman Siltstone) represent gradual marine onlap. Finally, subtidal conditions extended throughout all the known parts of the basin.

South Alligator Group The next group of rocks rests unconformably on the Wildman Siltstone and related units. A

bedding-parallel foliation is evident above the contact in places, which Johnston (1984) interpreted as a decollement; however, low-angle unconformity at the contact is apparent in many places, and basal ferruginous sandy quartz breccia with rare quartz pebbles at the contact in places, may be a regolith (Stuart-Smith et al., 1980). The angular discontinuity at the contact is consistent with tilting of the underlying units within the half graben structures formed during crustal extension, and can be likened to 'break-up unconformities' identified in many more recent sedimentary basins. The mature character of the sediments of the Koolpin Formation (pyritic carbonaceous shale with bands of chert, carbonaceous siltstone, dolomite ) is characteristic of sediments deposited during a thermal subsidence phase following cessation of major extension and normal faulting. Generally there is very little facies or thickness variation over the basin; however, massive biohermal dolomite (Crick and Muir, 1980) immediately west of the Waterfall Creek Fault (Figs. 2 and 6c) in the southeast may indicate some continued syn-sedimentary movement. Associated siliceous breccia and pyritic carbonaceous rocks including shungite, and possible gypsum casts in the dolomite, suggest a bioherm, with related environments similar to Phanerozoic fringing reef/back-reef facies, attached to a fault-bound easterly landmass. Little terrigenous material indicates low or no relief to the landmass. Koolpin-type sedimentation was interrupted by felsic volcanism, represented mainly by extensive tuff and argillite of the Gerowie Tuff and Mount Bonnie Formation (the Gerowie volcanic suite). Ripple drift in some finer beds indicates subaqueous deposition. Many pulses of volcanism are indicated by the interbedding of tuff and argillite with chert-banded shale; the shale interbeds become thinner ( < 10 cm) and less frequent upwards. Massive medium to coarse greywacke interbeds < 1 m thick in the upper part of the volcanic sequence are harbingers of marked rejuvenation of the deposystem

551 concomitant with volcanism. The lowermost greywacke is taken as the base of the Mount Bonnie Formation. East of the Waterfall Creek Fault, tuff and argillite of the Gerowie suite are absent and, owing to poor exposure, the lowermost greywacke cannot be recognized. Subdivision of the group in the same manner as elsewhere is therefore not possible, and the name Kapalga Formation is used to embrace this volcanicdeficient sequence (Fig. 5 ). The absence ofvolcanics in this area suggests continued displacement along the Waterfall Creek Fault and subsequent erosion of a short-lived land area to the east.

Finniss River Group The youngest sedimentary unit known in the basin is the Burrell Creek Formation, which is a thick monotonous sequence of interbedded, commonly structureless, greywacke and siltstone. The regular alternation and continuity of these two lithologies is remarkable. They represent part of the Bouma cycle where only divisions a and e are developed, and consequently may represent deep-water, high-energy deposits (Nelson and Nilsen, 1974). Walker (1967) invoked the formation of 'traction-carpets' whereby ungraded, sharp-topped beds could form by 'freezing' of a high density, high shear stress, high current velocity water-sediment mix (up to 50% grains by volume) as shear stress falls below a critical value and dispersive pressures are no longer maintained. The Mount Bonnie Formation represents a transitional period where interbedded carbonaceous pelite, turbidite and argillite/tuff represent gradual change from shallow low-energy facies typical of the Koolpin Formation to deeper, higher-energy facies typical of the Burrell Creek Formation, during the waning stages of Gerowie volcanism. The Burrell Creek Formation is recognized by the absence of argillite/tuff and carbonaceous pelite. The only exceptions to the greywacke/siltstone assemblage are rare ar-

kose and volcanolithic conglomerates which suggest minor instability related to remote terrestrial volcanism (Fig. 6c). Dickinson and Suczeck (1979) have shown that clast compositions in Phanerozoic sandstones vary in relation to provenance tectonic characteristics in plate tectonic environments. Burrell Creek greywacke data (Table I) are compared with Phanerozoic data in Fig. 7. Significant differences suggest that environments related to plate tectonic processes were not evident during the history of the Pine Creek Geosyncline. The older greywackes are relatively low in feldspar and quartz clasts, and their distribution in the QFL and QmFLt plots (which acccording to Dickinson and Suczeck reflect weathering, provenance relief, transport mechanism, and parent rock grain size ) may indicate a source area of comparatively high relief, little weathering, and generally fine grain size; the dominance of unstable lithic fragments supports a mass-flow transport method such as the 'traction carpets' of Walker (1967). They plot close to the more mature sediments of Phanerozoic recycled orogens. The QpLvLs and QmPK plots reveal the character of the polycrystalline and monocrystalline clast components, and are particularly useful in differentiating certain plate tectonic settings. The Burrell Creek greywackes define fields largely distinctive from the Phanerozoic ones, and the QpLvLs field reflects a low proportion of unstable sedimentary rock fragments, probably related to large distance ( > 150 km) from a mixed volcanic-sedimentary provenance. The QmPK plot approximates an index of provenance maturity, and as expected, suggests the Burrell Creek greywacke provenance was relatively mature.

Syn-depositional igneous activity Two mafic volcanic episodes are associated with the early extensional history of the basin

552 TABLE I Modal analyses of clasts in Pine Creek Geosyncline greywackes

Burrell Creek Formation Tollis Formation

No.

Q

F

L

Qm

F

Lt

Qp

Lv

Ls

Qm

P

K

7

41

7

52

25

7

68

23

71

6

79

7

14

6

13

8

79

11

8

81

3

97

<1

36

17

47

1000 point counts per sample. Results expressed as means calculated to a percentage of each plot group (refer to Fig. 7). Q, total quartzose grains; F, total feldspar grains; L, total unstable lithic fragments; Qm, monocrystalline quartz grains; Qp, polycrystalline quartz grains; Lt, total polycrystalline lithic fragments including Qp; Lv, total volcanic-metavolcanic rock fragments; Ls, unstable sedimentary-metasedimentary rock fragments; P, plagioclase grains; K, K-feldspar grains.

Shaded helds from Dickinson and Suczeck (1979) Q

F

=I®

Field, mean, Burrell Creek FF°~;t'n°:agr, eYTo~la:kF:Srmatlon

L

F

Qp

Lv

Qm

Lt Qm

Ls

P

K 16/NT,353

Fig. 7. Ternary plots showing field and mean of modal clast compositions for greywackes of the Burrell Creek (late Geosynclinal Sequence) and Tollis (Katherine Volcanic Sequence) Formations. Q, Total quartzose clasts; Qm, monocrystalline quartz; Qp, polycrystalline quartz; Lt, total lithic fragments including polycrystalline quartz; Lv, volcanic clasts; Ls, sedimentary clasts; P, plagioclase; K, K-feldspar.

(Fig. 5). The first (Stag Creek Volcanics, Fig. 6a) immediately underlies an unconformity separating lower mainly pelitic sediments from

higher coarse fluvial fan deposits, suggesting a possible connection between volcanism, and major fault movements which accompanied

553

provenance rejuvenation. The volcanics include lavas of both oceanic and continental tholeiitic affinity consistent with deep fracturing of a thin continental crust beneath the basin (Stuart-Smith et al., 1980). Further minor mafic volcanics in the Wildman Siltstone at about the same stratigraphic level as the quartzite lenses in the west and north indicate a similar, less intense, phase of associated volcanism and rejuvenation limited to those areas (Fig. 6b). Widespread tuffs and restricted felsic flows within the South Alligator Group mark the transition from quiescent low-energy pelites to high-energy, psammo-pelitic sedimentation characterized by the Finniss River Group. Volcanism in Burrell Creek time is indicated by rare felsic flows and volcanolithic conglom-

erate containing well-rounded clasts of rhyolite and porphyry, and is suggested by the volcanolithic character of the greywacke. Alternatively, the conglomerate and greywacke may have formed following uplift and erosion of earlier volcanic suites, particularly the Gerowie suite, which contains lithologies identical to those in the conglomerate clasts.

Geochronology of volcaniclastic rocks in the South Alligator Group

U-Pb zircon analyses were conducted on tuff from two localities of low metamorphic grade using the methodology of Krogh (1973) and Steiger and Jager (1977). A 25 m thick por-

TABLE II

U-Pb analytical data for zircons from the South Alligator Group and Pul Pul Rhyolite Size ( # m ) and magnetic susceptibility

Fraction

Weight

(Fig. 8)

(rag)

Concentration (ppm)

2°6pb/2°4pb

Radiogenic ratios

(measured) Radiogenic

Total

207pb/20¢pb

20Spbff3sU

207pb/235U

0.11599 0.11440 0.11464 0.11584 0.11470 0.11426 0.11461 0.11392 0.11205

0.32638 0.32004 0.31937 0.31899 0.31913 0.31831 0.32922 0.31354 0.27669

5.2195 5.0479 5.0479 5.0948 5.0471 5.0148 5.2025 4.9249 4.2749

Pb Pb

U

Gerowie Tuff, dacitic tuff 7912.5015 -215NM5 - 150NM1 -150M1 -150M1 - 150M2 - 75NM1 -75NM1 -75M1 -45M2

1 2 3 4 5 6 7 8 9

0.81 1.18 2.17 2.27 4.98 1.09 0.58 0.74 0.82

120.05 117.7 125.6 103.4 139.4 109.2 141.6 142.0 127.1

Mount Bonnie Formation crystal tuff 7912.5001 - 150NM1 10 1.59 113.9 - 150M1 - 150M1 -150M2 -150M2 - 75NM1 -75M1 - 75M2 -45NM2

123.1 119.2 126.5 103.9 140.9 110.4 142.4 142.7 131.6

346.9 339.0 361.9 297.5 401.4 312.5 391.7 412.9 413.9

2208 3317 5834 8440 4820 4065 5540 6377 1373

1.35 1.01 1.04 1.13 1.49 0.17 0.83 0.66

123.9 141.6 133.6 122.2 124.2 154.4 151.1 131.5

133.3 146.1 158.4 158.2 139.2 140.4 178.9 168.4 149.9

358.8 380.5 394.1 427.5 336.7 390.7 467.4 477.7 425.4

317.1 301.8 435.0 293.4 380.0 405.9 313.6 452.0 369.1

0.11484 0.11413 0.11413 0.11390 0.11484 0.11494 0.11396 0.11420 0.11408

0.28510 0.29279 0.32355 0.28101 0.32454 0.28465 0.29411 0.28149 0.27284

4.5141 4.6073 5.1123 4.4133 5.1389 4.5111 4.6214 4.4323 4.2916

0.09 0.86 0.31

124.4 112.2 158.2

133.1 141.1 171.7

1304.0 1367.2 1771.2

345.6 214.4 428.8

0.09172 0.08997 0.08977

0.08219 0.06853 0.07603

1.0394 0.8501 0.9410

11 12 13 14 15 16 17 18

PulPulRhyolite8312.6040 -80NM5 -80M5 -45

554 ~9oo12

1 8 8 5 + 2 M a -......~/

0.34

0.32

% #_ C'4

0.30

~

..S '''''''''''8~'"~'~ ~2,3,5 ..,, .,,.,.f

18

°.28l,7, 9 ......3.,7 '"

~

Gerowie

...... ~

Mount

T u f f - gocitic tuff Bonnie Formation-

crystal tuff

18

I

I

5.0

4.5

207pb/235U

5.5 16/NT/352

Fig. 8. U-Pb concordiadiagramfor zircons fromSouth AlligatorGroup tufts. Numbersreferto Table II. phyritic quartz-feldspar-clinopyroxene dacitic tuff in the Gerowie Tuff near the Waterfall Creek Fault, and a fine crystal tuff from the Mount Bonnie Formation near Mount Douglas (Fig. 2), lie stratigraphically apart by ~ 500 m and both contain euhedral zircon crystals of apparent igneous origin. The U content of the zircons is moderate (297-414 ppm, Table II) and Pb varies from negligible in the dacitic tuff, to 179 ppm in the crystal tuff where it appears to form minute black inclusions. Following air abrasion to reduce discordance, the 2°TPb-2°6Pb ages of six fractions of the dacitic tuff group between 1863 and 1875 Ma (mean 1871 + 2 Ma), and when aligned with the more discordant no. 9 fraction on a model 1 trajectory (Fig. 8) indicate an igneous crystallization age of 1884 + 3 Ma (lower intercept 400 + 42 Ma). Two fractions (nos. 1 and 4) away from this chord exhibit older ages (1895 and 1893 Ma), reflecting pre-magmatic Pb inheritance. Seven of the nine data points from the crystal tuff cluster on a poorly defined discordia line with an upper intercept of 1873 +4s° Ma. The two separate and more concordant points (after

abrasion with pyrite ) plot on the discordia trajectory of the dacitic tuff, which narrows the age to 1877 + 11 Ma, and suggests no discernible difference between the ages of the two tuffs. The obliqueness of the two chords reflects differing Pb-loss histories. The lower intercept for the crystal tuff zircons (90 + 130 Ma) suggests present-day loss, superimposed on Palaeozoic loss, from grain margins. This explains the non-linear fit of the data (MSWD=12.0) and implies that the apparent 1877 Ma age is probably slightly young. Selected combined regression of data lowers the MSWD to 1.8 and improves age precision to 1885 _+2 Ma, which is the best estimate for the stratigraphic age of the South Alligator Group.

Faulting during sedimentation At least two major N-S to N W - S E faults, formed during extension of the Archaean basement, influenced sedimentation. Distribution of the Stag Creek Volcanics, and felsic flows in the South Alligator Group near and to the west of the Waterfall Creek Fault, suggest venting

555 on that structure and uplift east of it. The same sense of movement is implied by the development of reef carbonate west of the fault in the Koolpin Formation and the absence of the Gerowie volcanic suite to the east. Variations in sediment thickness indicate periodic subsidence in the half graben west of the fault throughout sedimentation. At times the upthrown block east of the fault may have been eroded, contributing detritus to areas west of the fault, particularly during South Alligator Group times. The sequence thins markedly in the Rum Jungle region, which is separated from the Litchfield Complex further west by the 'Western Fault Zone' (Fig. 3; Walpole et al., 1968). Therefore a broad half graben is indicated ~ 120 km wide in the centre of the basin, thickening towards and bounded by the Waterfall Creek Fault in the east (Figs. 3 and 4). Differences in preserved stratigraphic thickness across the basin suggest a differential subsidence of at least 5000 m across the graben. The evidence for tectonic thickening of the Kapalga Formation in the 'South Alligator Trough', proposed by Johnston (1984), need not preclude a shallow trough in this area where perhaps about twice the regional thickness of the unit's correlatives accumulated. The Litchfield Complex is a zone of thick sediment accumulation and may represent the thicker part of another similar west-thinning half graben bounded by the Western Fault Zone (Fig. 4).

Orogenesis and related igneous activity The period between Early Proterozoic geosynclinal sedimentation and Middle Proterozoic platform cover sedimentation was a time of relatively intense tectonic and igneous activity, the 'Top E n d Orogeny'. The first event was intrusion of dolerite sills, followed by mediumto high-grade metamorphism and complex folding in the northeast. Granitoids were emplaced during this and later, mild, events,

throughout the region. Igneous activity was concentrated in rifts which developed mainly in the south. Erosion was marked, with ~ 5 km of denudation during the period. The last event before platform deposition was intrusion of dolerite lopoliths in the east. Intrusion of dolerite sills At or near the end of sedimentation, the Zamu Dolerite, a suite of continental tholeiites with a composite thickness of ~ 1.5 km, intruded the sediments mainly as sills. These sills were folded and metamorphosed with the host rocks by the 1870 Ma regional event (Ferguson and Needham, 1978). In the medium- to high-grade metamorphic terrain and in the contact aureoles of the granitoids the rocks are amphibolite, but elsewhere the original textures and mineralogy are partly or wholly preserved and the adjacent sediments may be hornfelsed. The Zamu Dolerite ranges from quartz dolerite to granophyre, and is most abundant in the South Alligator Group. The Nimbuwah Event Rejuvenation of provenance areas and associated felsic volcanism at the close of Early Proterozoic sedimentation was closely followed by intrusion of granodiorite and tonalite of the Nimbuwah Complex, in the extreme northeast of the Geosyncline, at ~ 1870 Ma (Page et al., 1980). This 'Nimbuwah Event' marked the beginning and peak of deformation and metamorphism in the Top End Orogeny. The tonalites and granodiorites of the Nimbuwah Complex underwent partial migmatization in this period, as did the Early Proterozoic sediments in the same area. Resisters (i.e., rock types which when metamorphosed retain their essential precursor mineralogy) of quartzite, carbonaceous schist, calc-silicate gneiss and marble, indicate that some of these rocks were originally of the Kakadu Group or Cahill Formation; those metamorphic rocks to which no sedimentary

556

parentage can be objectively determined are ascribed to the Nourlangie Schist, or the partly metamorphically differentiated Myra Falls Metamorphics (Fig. 5; Needham, 1982). Five deformations are evident in the Top End Orogeny (Johnston, 1984). Bedding-parallel foliation (D~), recumbent mainly west-verging folds (D2), upright N N W - S S E trending folds (D3), and easterly tight folds (D4) are evident in the Nimbuwah domain and decrease in intensity westwards; in the central terrain, D1 and D2 are absent, but a late (Ds) kink foliation (Ds) is related to basement block movement. The fold and fault styles are consistent with outward overfolding and overthrusting from an orogen in the northeast, in the present position of the Nimbuwah Complex (Figs. 2 and 4). H a m m o n d et al. ( 1984 ) outlined a similar fiveelement deformation in the Litchfield domain but interpreted D1 and De as earlier, possibly Archaean events. Their models invoke overthrusting and overfolding both eastwards and westwards.

Graben formation and deposition of the E1 Sherana Group Following uplift and erosion related to the Nimbuwah Event, the Waterfall Creek Fault and subparallel fractures were reactivated as normal faults to form a wide, N W - S E trending, shallow graben within which a valley and ridge topography developed in the area of the present South Alligator Valley (Figs. 3 and 6d; Needham and Stuart-Smith, 1985). The graben was probably surrounded by a much larger area of incipiently rifted terrain which lay in the outer part of the foreland to the Nimbuwah Orogen, near the outer margin of overfolding, and which was related to a period of post-orogenic extension. The graben was the locus for extrusion of the Katherine Volcanic Sequence, which comprises mainly felsic volcanics, and related volcaniclastic sediments, of the E1 Sherana and Edith River Groups (Fig. 5). At ~ 1860 Ma, after a period of weathering

during which the Scinto Breccia developed on carbonate rocks as a siliceous saprolite, the valleys were partly filled with coarse fluviatile sediments (predominantlypolymictic conglomerate, sandstone and minor shale derived by the erosion of adjacent metasediments) and contemporaneous, mainly felsic, volcanics, of the Coronation Sandstone. Volcanism culminated with the extrusion of massive columnar-jointed ignimbrite sheets and rhyolite flows of the Pul Pul Rhyolite, which filled the graben and extended marginally eastwards on to the surrounding upland area of geosynclinal metasediments. Volcaniclastic flyschoid sediments and interbedded volcanics of the Big Sunday and Tollis Formations later spread across the surrounding uplands. The higher proportion of volcanics in the area of the main graben (rhyolite, basalt and tuff) than in the surrounding uplands suggests that most volcanic activity continued to be in the South Alligator Valley area. Absence of the Big Sunday and Tollis Formations above the Pul Pul Rhyolite east of the South Alligator Valley suggests that this area was again uplifted following extrusion of the Pul Pul Rhyolite, to form a provenance for the flyschoid sediments. Abrupt thickness variations indicate uplift by faulting along the eastern side of the Mount Callanan Basin (Fig. 2). Effusive centres for the volcanic rocks of the Big Sunday Formation were probably along this fault line. Clast compositions for Tollis Formation greywackes are compared with Burrell Creek Formation greywackes and Phanerozoic greywackes in Table I and Fig. 7. Along with the pre-orogenic basin fill greywackes of the Pine Creek Geosyncline, the greywackes of the Katherine Volcanic Sequence are relatively low in quartz and feldspar clasts, and appear to have had a provenance of considerable relief, little weathering, and generally fine grain size; the 'traction carpet' method of mass flow transport is again indicated by the dominance of unstable lithic fragments. The greater proportion of unstable fragments relative to the Burrell Creek

557 greywackes, and the tight Lv-rich field in the QpLvLs plot, indicate a Tollis Formation deposystem dominated by a volcanic-rich, juvenile (rifted) and nearby provenance. Most significantly, the greywackes from the Katherine Volcanic Sequence plot well off the trend defined by all the other groups, resulting from relative enrichment in K. Thus the K-rich constitution of Top-Endian felsic magmatism, demonstrated by Wyborn (1988) to be a characteristic of all late Early Proterozoic tectonism in Australia, is admirably shown by this plot to influence the modal chemistry of derived sediments.

El Sherana Group geochronology Sparse populations of very discordant highU zircons (Table II) from the Pul Pul Rhyolite quartz-feldspar porphyry, collected from Pul Pul Hill 10 km southeast of E1 Sherana township (Fig. 1 ) have 2°TPb-2°6pb ages of between 1460 and 1420 Ma, and 2°6pb-23sU ages from 509 to 427 Ma. The Palaeozoic Pb loss a n d / o r U gain may result from the same disturbance seen in uraninite/pitchblende U - P b data from the E1 Sherana and other U deposits in the South Alligator Valley (Greenhalgh and Jeffrey, 1959; Hills and Richards, 1976). This disturbance precludes any age estimate for the Pul Pul Rhyolite other than a minimum of 1460 Ma. A sparse yield of zircon from a clinopyroxene-olivine andesitic ignimbrite in the Tollis Formation was analysed using the S H R I M P ion microprobe at the Australian National University (Page and Williams, 1988). Preliminary data indicate an inherited component, but most grains give a crystallization age of between 1900 and 1870 Ma. The 1885___2 Ma age of the unconformably underlying Gerowie Tuff infers that intervening tectonism of the Nimbuwah Event, including several periods of folding, uplift and erosion, lasted for < 15 Ma. The timing of this tectonic event is now well con-

strained, as the above data confirm previously derived ages of 1886_+ 5 to 1866 + 8 Ma by UPb zircon dating of Nimbuwah Complex granitoids (Page et al., 1980).

The Maud Creek Event The E1 Sherana Group rocks were folded (here named the 'Maud Creek Event'), eroded and subjected to further faulting before deposition of the Edith River Group (Fig. 5; Needham and Stuart-Smith, 1985) at ~ 1850 Ma. The Maud Creek Event is characterized by tight and mainly horizontal folds, with vertical axial surfaces. Steeper limb dips and axial plunges are common east of the Edith Falls Basin.

Further rifting, and deposition of the Edith River Group Following uplift and erosion, the Mount Callanan, Edith Falls and Birdie Creek Basins were initiated by deepening of the graben through further rifting, and by downwarping along N W SE and N N E - S S W axes and some reactivation of the normal faults developed during deposition of the E1 Sherana Group. The eastern margin of the Mount Callanan Basin is clearly defined by several major faults near E1 Sherana (Figs. 2 and 3). These probably formed a steep active scarp during Edith River Group deposition. The more gentle western margin is defined by a number of faults with moderate to small displacements, giving the basin an asymmetric profile. The Birdie Creek and Edith Falls Basins are symmetrical and less clearly defined by faults. The basal coarse clastic sediments of the Edith River Group (Kurrundie, Phillips Creek and Hindrance Creek Sandstones) were the first deposits in the basins, possibly forming coalescing fans flanking marginal fault scarps. The Kurrundie Sandstone formed a southwesterly thinning wedge into the Mount Callanan

558 Basin. Conglomerate clasts in the northeast consist mainly of locally derived rocks from the E1 Sherana Group, whereas clasts in the southwest are mainly of Pine Creek Geosyncline Sequence, indicating derivation from a western upland provenance which received no El Sherana Group deposition. Two areas of thick conglomerate development (up to 80% of formation thickness) and correspondingly thicker sequences (up to 350 m) along the northeastern margin at E1 Sherana and at Big Sunday 20 km to the southeast, indicate possible entry points of major fluvial systems from the northeast, although there is no significant variation in clast size. Limited current direction measurements in the Mount Callanan Basin, indicated by lenticular and tabular cross-beds, range from west to north, suggesting an overall northwestern gradient on the valley floor parallel to the basin margins. Evidence of current directions in the Phillips Creek and Hindrance Creek Sandstones is too sparse to allow construction of drainage directions for the Birdie Creek and Edith Falls Basins. The Plum Tree Creek Volcanics were extruded ~ 1850 Ma ago as an extensive ignimbrite sheet. Minor interbeds of conglomerate, sandstone and siltstone along the northeast margin of the Mount Callanan Basin indicate a probable continuation of the earlier fluvial systems. The Grace Creek Granite is, apart from groundmass grain size and the presence of corroded feldspar and quartz xenocrysts, petrologically very similar to the Plum Tree Creek ignimbrite. The granite is almost surrounded by the ignimbrite (Fig. 2) and occupies a position coincident with the intersection of the N W - S E and E N E - W S W fault sets which roughly control the shape of the Mount Callanan, Edith Falls and Birdie Creek Basins (Fig. 6d). This position may coincide with the effusive centre of Plum Tree Creek volcanism, and the Grace Creek Granite is interpreted to be the intrusive remnants of a magma chamber from which an estimated 2400 km 3 of ignimbrite was extruded.

Age of the Edith River Group and Maud Creek Event The total-rock ages of ~ 1750 Ma for 'Edith River Volcanics' summarized by Compston and Arriens (1968) and Walpole et al. (1969) are, owing to open-system behaviour, minimum ages. Several rhyodacitic rocks from the comagmatic Plum Tree Creek Volcanics and Grace Creek Granite have igneous crystallization and stratigraphic U - P b zircon ages of between 1870 and 1860 Ma (unpublished data). Zircon fractions from other specimens contain inherited components, as seen in the older E1 Sherana and South Alligator Groups. The minimum age of 1870 Ma demonstrates the relative brevity ( < 10 Ma) of the Maud Creek Event between the E1 Sherana and Edith River Groups.

Granitoid intrusion With the exception of granites in the Archaean basement, all the granitoids of the region (dominantly granite, ranging from granodiorite to leucogranite) are considered to have been emplaced during the Top End Orogeny. The earliest known granitoid intrusion took place ~ 1870 Ma (the early syn-orogenic or pre-orogenic granitoid precursor to the Nimbuwah Complex). Those in the Litchfield domain were emplaced at about 1850-1840 Ma (Page et al., 1985) at or before major granitoid intrusion in the low-grade central domain (1850-1800 Ma). Contact hornfelsing of basal sediments in the Edith River Group indicates intrusion during, or possibly after, Edith River Group time for the Grace Creek Granite. Contact between granitoids and volcanics is rare and generally equivocal; recrystallization of matrix in ignimbrite close to granite in the south of the Cullen Batholith (Fig. 2) indicates that this granite was emplaced after Edith River Group ignimbrite extrusion. Such a relationship is consistent with the 1870-1860 Ma age for the Edith River Group (see above) and a prelimi-

559 nary age of 1848-1800 Ma for different phases in the Cullen Batholith.

The Shoobridge Event The 'Shoobridge Event' is the last regional metamorphic and deformational event of the Top End Orogeny (Fig. 5). Deformation accompanying this event is characterized by the development or reactivation of linear shear zones parallel to the regional N-S to N W - S E strike within both granitoids and Early Proterozoic metasediments. The deformation was accompanied by widespread retrogressive metamorphism, which is particularly noticeable in the granitoids, and localized prograde, low-grade regional metamorphism adjacent to some of the major shear zones. The age of the event is probably reflected by ubiquitous Rb-Sr total-rock ages of 1770 to 1780 Ma for granitoids in the central part of the inlier, and possibly by the 1800 + 24 Ma ages recorded in the K-Ar and Rb-Sr systematics of the metamorphic rocks of the Alligator Rivers region in the northeast (Page et al., 1980). The two youngest granitoids in the region (the Burnside and Prices Springs Granites, Fig. 2) are approximately of this age (1780-1765 Ma) and have anomalously high U contents both in zircon and in the total rock.

Dolerite lopoliths, weathering and erosion The last known Early Proterozoic igneous event was intrusion of a series of continental tholeiitic dolerite lopoliths and minor dykes (Oenpelli Dolerite) in the east, mainly in the medium-high-grade metamorphic terrain but extending into low-grade rocks in the South Alligator Valley area. The lopoliths, measuring up to 140 × 55 km, are roughly symmetrically differentiated layered sheets grading inwards from porphyritic to equigranular olivine dolerite, quartz dolerite, granophyric dolerite and granophyre up to 250 m thick. Their intrusion at about 1-2 km depth (Stuart-Smith and Fergu-

son, 1978) indicates significant post-orogenic denudation before lopolith emplacement. Local faulting is indicated in the South Alligator Valley area during this period or during the Shoobridge Event, by tilting of Edith River Group rocks.

Tectonic influences during the Top End Orogeny The stratigraphic distribution of Zamu Dolerite mainly in the South Alligator Group implies a regionally undeformed, essentially horizontal disposition of the geosynclinal sequence at the time of emplacement, i.e., no significant differential uplift heralding orogeny took place. The most significant syn-depositional fault formed during sedimentation in the early basin (Waterfall Creek Fault) remained active during the orogenic period. The fault formed the major eastern bounding fault to the South Alligator Valley graben. A nearby parallel fault has along it several small feldspar porphyry plugs, indicating a major crustal fracture. A subsidiary E N E - W S W rift passing through Katherine is coincident with a gravity lineament, possibly representing another deep crustal fracture system. The oldest granitoids lie in the higher-grade Litchfield and Nimbuwah domains, reflecting the onset of elevated lower crustal temperatures in those areas first. Linearity of granitoids in the Litchfield area reflects the prominent meridional grain of the northern end of the Halls Creek belt. Elsewhere, evidence of structural control to granitoid emplacement is limited to a possible association of the Cullen Batholith, Margaret and Prices Springs Granites with the Pine Creek Shear Zone, and coincidence of the Jim Jim, Malone Creek and Grace Creek Granites with a prominent northerly gravity lineament (Needham and Stuart-Smith, 1985). Geographic distribution of the Zamu and Oenpelli Dolerites suggests that their intrusion

560 was in part controlled by the Waterfall Creek Fault or related structures before the Nimbuwah Event, and by the Bulman Fault or related structures after the Shoobridge Event. Tucker et al. (1980) describe an elongate northerly positive gravity anomaly near Oenpelli which they calculate to represent a vertical-sided dense block extending to 6 km depth; it may represent a feeder to Oenpelli Dolerite.

is developed in some places. A regolith is common below the Cretaceous, and lateritization took place during the Tertiary. Hays (1965) identified three Cenozoic land surfaces which are commonly superimposed, on which extensive deep weathering profiles (up to 60 m) are developed.

Peneplanation and platform cover sedimentation

The unconformity plane at the base of the Kombolgie Formation has a morphology similar to that of the undulating sand plains which cover much of the Pine Creek Inlier today. The unconformity undulates gently with an amplitude of ~ 20 m, although there are scattered basement hills and ridges up to 250 m. The Oenpelli Dolerite forms large arcuate basement ridges up to 100 m high and the granites in places form plateaux ~ 100 m above the general level of the unconformity. Development of this surface involved removal of 1-2 km of supracrustals after intrusion of Oenpelli Dolerite at ~1690 Ma. Subsequently the surface underwent chemical weathering to form a saprolitic weathering profile at least 50 m deep, during the c. 40 Ma before Kombolgie Formation deposition. Although the upper parts of the profile are commonly stripped, preservation of most of it over a very wide area (from at least 60 km east of Oenpelli, to Katherine, and at Rum Jungle) indicates this to be a markedly tectonically stable episode. No evidence remains of the type of process involved in removing the 1-2 km of rock. It was possibly fluvial, akin to the process active in provenance areas to the Kombolgie Formation.

Middle Proterozoic plateau-forming sandstone and minor interbedded, mainly intermediate, volcanics of the Kombolgie Formation (part of the McArthur Basin sequence) rest with marked unconformity on most older rocks. The Kombolgie Formation sandstone largely conceals the eastern extent of the Early Proterozoic metasediments and their gneissic equivalents, which are exposed only in a few inliers within the sandstone plateau. Minor dolerite intrusions of Late Proterozoic age (1370-1200 Ma) cut the sandstone in places; additional similar intrusions are represented by long linear magnetic patterns coincident with major fractures in the sandstone. These intrusions, and Late Proterozoic phonolite dykes ( ~ 1316 Ma) which intrude the Nanambu and Nimbuwah Complexes, represent the final known igneous events in the region (Page et al., 1980). Late Proterozoic sediments are exposed at the margins of extensive Cretaceous tablelands in the southwest. The age of the sandstone-dominant Tolmer Group which crops out in the same area is obscure, as no relationships are exposed between it and rocks of proven Late Proterozoic age; it is either basal Late Proterozoic, or Middle Proterozoic. The northern and southern margins of the Pine Creek Geosyncline Sequence are concealed by Mesozoic (Bathurst Terrace) and Palaeozoic (Daly River Basin) strata. A palaeoweathering profile is common below the Kombolgie Formation, and a rare regolith

Peneplanation

Platform Cover sedimentation The Kombolgie Formation of the McArthur Basin and Depot Creek Sandstone of the Victoria River Basin are both fluvial red-bed sandstone successions deposited as braided alluvial fans (Ojakangas, 1979) from a provenance beneath today's Van Diemen Gulf. Both se-

561 quences underlie mainly clastic sequences with minor carbonates. The Kombolgie Formation also contains flood basalt flows extruded at 1650 Ma in a saturated sand or subaqueous environment (Needham, 1978). Earliest deposition saw little reworking of the substrata, as clasts other than vein quartz or quartzite pebbles are rare. Intact fragments of feldspar and mica books, and rare granite pebbles are evident in places within 2 m of the base within labile basal conglomerate, distinct from mature conglomerate higher in the sequence. In places above ridges and hills in the peneplain surface large angular blocks of substrate form breccia conglomerate in palaeovalleys.

Tectonic influence during Platform Cover sedimentation The plateau sandstones show generally little thickness variation except in the E1 SheranaKatherine area, where the sequence thickens to twice the norm in three local basins (Fig. 2). The basins lie within or adjacent to the N W SE and E N E - W S W trending graben active during the latter part of the Top End Orogeny, reflecting continued movement along bounding faults. Basalt in the Kombolgie Formation is thickest near the Bulman Fault and in one of the basins, indicating fault-related fissure feeders. Tremors related to extrusion are indicated by shock-induced giant load casts formed between wet sand and basalt (Needham, 1978).

Late tectonism The age of the faulting of the cover rocks is unknown, being constrained by 1650 Ma maximum (basalt in the cover rocks) and c. 150 Ma minimum (Mesozoic) ages. Chlorite alteration dated at c. 1610 Ma in some U orebodies (Page et al., 1980) may be related to a faulting event. Joints and faults filled with 1379 and 1200 Ma dolerite represent periods of mainly N W - S E and N-S crustal extension in the Middle Pro-

terozoic. Thickened Mesozoic strata beside the South Alligator Fault and a small 200 m thick Eocene basin in the same area represent longlived periodic rejuvenation of the South Alligator fault system. The region was stable throughout the Palaeozoic. Periods of marine deposition in the various Palaeozoic basins were controlled by eustatic changes allowing incursions of seas from the northwest (Cambrian Arafura Basin ), south (Cambro-Ordovician Daly River Basin), west (Permian Bonaparte Gulf Basin), and north (Mesozoic Bathurst Terrace). The most extensive was probably the Mesozoic sea, to which the bounding escarpments of the Platform Cover plateau sandstone sequences, which dominate today's landscape, formed sea cliffs. Tectonic e v o l u t i o n

The Early Proterozoic sediments of the Pine Creek Geosyncline Sequence were deposited in an intra-cratonic basin under alternating continental and shallow marine environments. The intracratonic setting is indicated by: (1) the prevalence of shallow-marine and continental deposits and the absence of undoubted deep-water deposits; (2) the presence of continental tholeiites (the Zamu Dolerite and Oenpelli Dolerite); (3) gravity patterns suggesting a homogeneous basement continuous with the exposed Archaean granitoids. The geosyncline was initiated by incipient rifting and marginal subsidence of the Archaean crust. The Early Proterozoic sequence of the Pine Creek Inlier may be divided into three parts related to three tectonic stages (Fig. 9). The quartz-rich lower part of the geosynclinal sequence with minor bimodal volcanics (Kakadu Group, Namoona Group, Mount Partridge Group ) represents mainly fluviatile processes developed during the initial phase of

562

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Fig. 9. Sequential Early Proterozoic development of the Pine Creek region by (a) crustal extension to form shallow half graben, which (b) subsequently deepened to form tilt blocks, followed by (c) sagging related to thermal subsidence. Uplift of distant provenances resulted in (c) rapid infill in the last stages of geosynclinal sequence sedimentation. Following (d) the Top End Orogeny, extension led to reactivation of major fault zones, forming the South Alligator graben over the South Alligator Trough and Hinge Zone. These reactivated fault zones were the loci of felsic and mafic magmatism. c r u s t a l e x t e n s i o n (Fig. 9a a n d b ) . T h e incursion of s h a l l o w seas of t h e S o u t h A l l i g a t o r G r o u p , a n d t r a n s i t i o n to m a i n l y distal pelitic sediments and mainly proximal carbonate pre-

c i p i t a t i o n , r e p r e s e n t s w i d e s p r e a d sagging of t h e b a s i n p r o b a b l y r e l a t e d to p o s t - e x t e n s i o n subsidence (Fig. 9c). T h e t u r b i d i t i c u p p e r sequence (upper South Alligator Group, Finniss

563

River Group) may mark provenance and intrabasinal uplift heralding the onset of the Top End Orogeny. The rift valley environment of the Katherine Volcanic Sequence indicates a second period of crustal extension (Fig. 9d). The N W - S E trending faults related to this event acted as transfer faults during the subsequent 1700-1680 Ma extension in the McArthur Basin. In both cases, the cratonized Pine Creek Geosyncline Sequence was a provenance area at least in the early depositional stage. The region has formed a stable part of the North Australian Craton since the late Proterozoic. Palaeozoic tectonism involved mainly epeirogenic movements, with minor movements along some of the N W - S E trending faults as recently as the Mesozoic and Tertiary.

Acknowledgements The manuscript was typed by Edna Monteiro and the figures drawn by Shane McMahon. The helpful comments of Drs. John Ferguson, Margorie Muir and Mike Etheridge are gratefully acknowledged. This paper is published with the permission of the Director, Bureau of Mineral Resources, Canberra, Australia.

References Crick, I.H. and Muir, M.D., 1980. Evaporites and uranium mineralization in the Pine Creek Geosyncline. In: J. Ferguson and A.B. Goleby (Editors), Uranium in the Pine Creek Geosyncline. International Atomic Energy Agency, Vienna, pp. 531-542. Dickinson, W.R. and Suczeck, C.A., 1979. Plate tectonics and sandstone compositions. Am. Assoc. Petrol. Geol., 63: 2164-2182. Etheridge, M.A., Rutland, R.W.R. and Wyborn, L.A.I., 1987. Orogenesis and tectonic process in the early to middle Proterozoic of Northern Australia. Geodynamic Series, 17, Am. Geophys. Union, Washington, DC, pp. 131-147. Ferguson, J. and Needham, R.S., 1978. The Zamu Dolerite. A Lower Proterozoic preorogenic continental tholeiitic suite from the Northern Territory, Australia. J. Geol. Soc. Aust., 25: 309-322. Ferguson, J., ChappeU, B.W. and Goleby, A.B., 1980. Granitoids in the Pine Creek Geosyncline. In: J. Ferguson

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564 synclinal Sedimentation. Soc. Econ. Palaeontol. Mineral. Spec. Publ., 19: 69-91. Ojakangas, R.W., 1979. Sedimentation of the basal Kombolgie Formation (Upper Precambrian-Carpentarian) Northern Territory, Australia: Possible significance in the genesis of the underlying Alligator Rivers Unconformity-type uranium deposits. U.S. Dept. of Energy, GJBX- 173 (79), 38 pp. Page, R.W. and Williams, I.S., 1988. Age of the Barramundi Orogeny in northern Australia by means of ion microprobe and conventional U - P b zircon studies. Precambrian Res., 40/41: 21-36. Page, R.W., Compston, W. and Needham, R.S., 1980. Geochronology and evolution of the late Archaean basement and Proterozoic rocks in the Alligator Rivers Uranium Field, Northern Territory, Australia. In: J. Ferguson and A.B. Goleby (Editors), Uranium in the Pine Creek Geosyncline. International Atomic Energy Agency, Vienna, pp. 39-68. Page, R.W., Bower, M.J. and Guy, D.B., 1985. An isotopic study of granitoids in the Litchfield Block, N.T. Bur. Miner. Resour. J. Aust. Geol. Geophys., 9: 219-223. Plumb, K.A., Derrick, G.M., Needham, R.S. and Shaw, R.D., 1981. The Proterozoic of northern Australia. In: D.R. Hunter (Editor), Precambrian of the Southern Hemisphere. Elsevier, Amsterdam, pp. 205-307. Richards, J.R., Ruxton, B.P. and Rhodes, J.M., 1977. Isotopic dating of the leucocratic granite, Rum Jungle, Australia. Proc. Australas. Inst. Min. Metall., 264: 33-34.

Steiger, R.H. and Jager, E., 1977. Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett., 36: 359-362. Stuart-Smith, P.G. and Ferguson, J., 1978. The Oenpelli Dolerite - - a Precambrian continental tholeiitic suite from the Northern Territory, Australia. Bur. Miner. Resour. J. Aust. Geol. Geophys., 3: 125-133. Stuart-Smith, P.G., Wills, K., Crick, I.H. and Needham, R.S., 1980. Evolution of the Pine Creek Geosyncline. In: J. Ferguson and A.B. Goleby {Editors), Uranium in the Pine Creek Geosyncline. International Atomic Energy Agency, Vienna, pp. 23-38. Tucker, D.H., Stuart, D.C., Hone, I.G. and Sampath, N., 1980. The characteristics and interpretation of regional gravity, magnetic and radiometric surveys in the Pine Creek Geosyncline. In: J. Ferguson and A.B. Goleby (Editors), Uranium in the Pine Creek Geosyncline. International Atomic Energy Agency, Vienna, pp. 101-140. Walker, R.G., 1967. Turbidite sedimentary structures and their relationship to proximal and distal depositional environments. J. Sediment. Petrol., 37: 25-43. Walpole, B.P., Crohn, P.W., Dunn, P.R. and Randal, M.A., 1968. Geology of the Katherine-Darwin region, Northern Territory. Aust., Bur. Miner. Resour., Bull. 82, 304 pp. Wyborn, L.A.I., 1988. Petrology, geochemistry and origin of a major Australian 1880-1840 Ma felsic volcano-plutonic suite: a model for intracontinental felsic magma generation. Precambrian Res., 40/41: 37-60.