AGSO Research Newsletter 26

There is no indication that major fault movements in the Broken Hill region have continued into the Holocene, a feature that contrasts it with the Flinders Ranges, which continue to be tectonically active.

The distribution of the tectonically influ•enced landscapes, and the degree to which a possible weathered mantle and cover se•quence has been stripped from uplifted areas, provide a useful framework for further re•golith studies in the region.

Acknowledgment My thanks go to George Gibson and Russell Shaw (AGSO) and Steve Hill (CRC LEME, ANU) for stimulating discussions on this subject.

1 Cooperative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME); Minerals Divi•sion, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACf 2601; tel +61 6 249 9748; fax +61 6 249 9983; e-mail dgibson@agso.gov.au.

Fig. 22. Distribution of the main outcrops of the Wollogorang Formation in the southern McArthur Basin, and locations of drillholes sampled for zircon dating_

20

• Woliogorang Formation outcrop

D Cretaceous and Cambrian cover

Fault

May 1997

Why sequence stratigraphy and not lithostratigraphy for exploration? M.J. (Jim) Jackson1, Rod Pagel, Peter Southgate], & Deb Scottl

Intriguing results from recent field research and SHRIMP zircon dating of rocks from the Palaeoproterozoic McArthur Basin, northern Australia, illustrate why a modern sequence stratigraphic approach to studying sedimentary successions is likely to provide better results than a more traditional lithostratigraphic approach. The sequence stratigraphic approach facilitates an improved understanding of stratal relationships by providing a chronostratigraphic framework for correlation and mapping. It also encourages the identification of subtle but possibly significant breaks in sedimentation. As the Proterozoic successions of northern Australia contain several world-class sediment-hosted base-metal deposits, a better under•standing of regional stratigraphic relationships and accommodation history is crucial to improving our ability to predict the distribution and lateral variability of potential host rocks, and the distribution of surfaces that control stratal fluid flow.

.. TASMANI"'\:1

Gulf of Carpentaria

16°

f-IO z -' ,0

11 /NT{75

100 km L-__________________ I

The Wollogorang Formation -- a viable unit for resource mapping? Applying lithostratigraphic mapping prac•tices in the McArthur Basin, Roberts et al. (1963: 'Calvert Hills, NT -- 1:250000 Geo•logical Series, SE/53-8' , BMR [AGSO], Canberra) defined a unit called the Wollo•gorang Formation. This unit comprises a suc•cession of dolostone, shale, and sandstone sandwiched stratigraphically between igne•ous rocks in the upper part of the Tawallah Group. The definition was extended and re•fined by Jackson et al. 1987 (BMR Bulletin 220), who mapped the unit through the Calvert Hills-Bauhinia Downs area. Because it contains a number of distinctive rock types and sedimentary structures, the Wollogorang Formation was used as a key stratigraphic marker during the mapping of the southern half of the basin in the 1980s (Fig. 22).

Later, during mapping of the northern part of the basin (Katherine to Arnhem Land region), a sedimentary unit (McCaw Forma•tion) near the top of the Tawallah Group (and its equivalents) with a similar shaly dolomitic facies was also identified as an important local marker. It has been tentatively correlated with the Wollogorang Formation (Rawlings et al. in press, 'Arnhem Bay-Gove -- 1:250 000 Geological Series, SD/53-3 and 4', Northern Territory Geological Survey and AGSO).

In a basin context, therefore, the Wollo•gorang Formation and its lateral equivalents are important stratigraphic markers.

During the initial field season of the NABRE (,North Australian basins resource evaluation') project in June 1995, the type section of the Wollogorang Formation, near Redbank (Figs. 22 and 23), was re-examined and reassessed from a sequence stratigraphic perspective. This revisit suggested that a prominent mid-unit sequence boundary (a marked break in sedimentation, or uncon•formity) separates the clastic facies (VIa) from the underlying dolostones and shales. Further, the upper and lower parts of the formation should perhaps be considered as parts of different basin phases and not parts of one genetic stratigraphic unit.

To provide an independent test for this hypothesis, primary igneous zircon in thin tuffaceous beds from drillcore samples above and below this possible chronostratigraphic surface have been dated. Preliminary U-Pb SHRIMP results evince a significant break.

Lithostratigraphy v. sequence stratigraphy According to the Australian Stratigraphic Code (Geological Society of Australia 1964), , .. . the fundamental procedure in lithostra•tigraphy is to observe, describe and correlate

May 1997

FACIES

GOLD CREEK VOLCANICS

150 Clastic facies

(Vlb)

Red shale & dololutite facies (mixed) (I and III)

ROCK STRUCT-TYPE URES DESCRIPTION

Medium to thick-bedded medium-grained quartz arenite with planar cross-stratification.

Thin parallel and ripple-laminated dolo/utite, biostromes of small columnar stromatolites, hopper halite casts

UNITS

AGSO Research Newsletter 26

In their monograph, Emery & Myers (1996: 'Sequence stratigraphy', Blackwell Science) provide the following simple defi•nition for sequence stratigraphy: ' ... the sub•division of sedimentary basin fills into ge•netic packages bounded by unconformities and their correlative conformities.', and 'It is used to provide a chronostratigraphic framework for the correlation and mapping of sedimentary facies.' (our italics).

In our particular example, therefore, se•quence stratigraphy would aim to break the Proterozoic succession into genetic associa•tions of units that would have been laid down together in contiguous facies, and also to look for marked changes in the vertical stack-

4 ing of facies that could represent significant breaks in sedimentation.

Clastic facies 100 (VIa)

Grey dolosiltstone facies M

Black shale facies (IV)

50

Crystalline dolomite facies (II)

Red shale facies (I)

SETTLEMENT CREEK

VOLCANICS

Thick-bedded coarse quartz and dolomitic sandstone with cross-stratification (tough and planar) and gypsiferous level; thin dolo/utite intervals with domal stromatolites and collapse breccia.

POTENTIAL IMPORTANT SEQUENCE BOUNDARY

Laminated to thin-bedded grey dolomitic siltstone and silty dolostone with cut and fill, authigenic quartz nodules after evaporites, and intraclast breccia.

Rhythmically laminated organic and sulphide•rich black mudstone and dolomitic siltstone; thin tuffs, and diagenetic bituminous nodules.

Crystalline dolostone with biotherms and bio•stromes of columnar stromatolites, wave and current ripples, oolites, oncolites, tepees, and shallow channels.

Red siltstone and mudstone with intraformational breccias and hopper halite casts.

3

2

16/NT/179

Fig. 23. Composite stratigraphic section of the Wollogorang Formation in the type area, near Redbank mine, as published in Jackson et al. (1987: op. cit., fig. 27).

sequences.' * Further, ' ... it is convenient to describe local successions in terms of named and defined units which should be described in terms of readily observable lithological characteristics (petrological, mineralogical, palaeontological, etc.).' In addition, 'The boundaries of each unit should be placed at positions of lithological change' and 'Boundaries are not defined to imply iso•chronous surfaces' (our italics).

Even though the Australian Code of 1964 has been superseded (Staines 1985: Austra•lian Journal of Earth Sciences, 32, 83-106; Salvador 1994: 'International stratigraphic guide', 2nd edition, International Union of Geological Sciences and Geological Society

* In this sense, the term 'sequence ' does not adhere to the strict definition applied to it in sequence stratigraphy.

of America), these principles remain the same. Hence, the basic process of lithostra•tigraphy is to identify, group, describe, and give names to mappable units.

This approach was applied to group to•gether the sedimentary rocks - red shale, stromatolitic and oolitic dolostones, grey dolosiltstone, black shale with nodules, and dolomitic quartz sandstone - between the Settlement Creek Volcanics and Gold Creek Volcanics (Fig. 23), and define them as the Wollogorang Formation. The upper and lower contacts are sharp lithological changes from sedimentary to igneous rocks. The formation was then mapped throughout the basin, where it was seen essentially as a mixed but li•thologically distinctive sedimentary unit bracketed by very different igneous units and associated coarse-grained clastics.

A sequence stratigraphic perspective of the Wollogorang Formation In a sequence stratigraphic context, the sharp change in the middle of the Wollogorang Formation - from low-energy, laminated dolomitic siltstone and shale (deep deposi•tional environments) to high-energy, trough cross-bedded coarse-grained clastic rocks (fluvial deposits) - would appear as a likely sequence boundary (at -72 m, Fig. 23). Down-hole gamma-ray logs also indicate a sharp change in geophysical response at this level, corresponding to the marked facies change. These changes reflect a marked bas•inward shift in facies accompanied by prob•able subaerial exposure at this junction, which fulfils the key criteria for defining as a sequence boundary (Wilgus et a!. 1988: 'Sea-level changes: an integrated approach', SEPM Special Publication 42).

According to sequence stratigraphic con•cepts, the rocks above and below this 'un•conformity' were laid down as parts of very different genetic packages, and should there•fore be treated separately. Hence, the poten•tial for lithostratigraphy and sequence stra•tigraphy to see the same package of rocks from a very different viewpoint. Instead of laterally tracing a sedimentary unit con•strained between two largely igneous units, the sequence stratigraphic approach would encourage tracing the sequence boundary lat•erally to assess its character and see how much of the underlying package has been removed. It would also emphasise looking lat•erally, but separately, for associations between the sedimentary facies and igneous facies above and below the sequence boundary.

A geochronological test of the sequence stratigraphic perspective of the Wollogorang Formation Black shale in the lower Wollogorang For•mation contains thin green mudstones inter•preted as tuffaceous beds (Donnelly & Jack•son 1988: Sedimentary Geology, 58, 145-169). Samples of the tuffaceous material were collected from Mount Young 2 and three other drillholes at widely spaced localities (Fig. 22). One of the drillholes, CRA HC1

21

AGSO Research Newsletter 26

contains tuffaceous material above and below the proposed sequence boundary.

The tuff layers in the black shale facies of the lower Wollogorang Formation are com•monly around 20 cm thick. Their vitroclastic texture is evident from fine shard-like out•lines now altered to a felted mass of fine white mica and chlorite. These features are consistent with the pristine igneous morphol•ogy of contained zircons, which should pro•vide ages for the contemporary volcanism and, by implication, for the deposition of the enclosing black shale.

Two of the tuffs in Mount Young 2 are only a few metres apart stratigraphically. They have inseparable U-Pb SHRIMP zircon crystallisation ages of 1730 ± 3 Ma and 1729 ± 4 Ma. Fifty zircon analyses reveal no ex•traneous results, and, together with their iso•topic concordance, suggest that these are stra•tigraphically meaningful ages.

A thicker tuff (36 cm) in the black shale facies of the lower Wollogorang Formation in CRA HC1 contains concordant, zoned igneous zircons and some older Palaeoproterozoic de•trital grains. The main igneous population has an age of 1730 ± 3 Ma - identical with that of the shaly facies in Mount Young 2.

Some 62 m above this tuff in CRA HCl, a tuffaceous layer in the clastic facies of the upper part of the formation has been analysed in order to estimate the time break represented by the proposed sequence boundary. This

Continued/rom/acing page.

An attempt to correlate the loops and cusps with significant geological events in the Phanerozoic evolution of the North West Shelf is described in AGSO Record 1996/52.

'" "C

"

40

20

~ 0-20 '" '" 0;

CL

-40

-60

·80

I

I

-500 -400

tuff contains fewer zircons, of which several per cent are detrital grains up to 2000 Ma old. The igneous zircon population in this tuff is somewhat altered, possibly because of higher U content. Treatment of the most concordant U-Pb data gives an igneous crys•tallisation age of 1723 ± 4 Ma from 14 zircon analyses.

Despite the overlap in the 95% error lim•its, this result is statistically younger than the 1730-Ma results (at the 1 % level using student's t test). We regard it as a useful stratigraphic age for the upper Wollogorang Formation. At face value, the results suggest a time break of several million years between the lower and upper parts of the Wollogorang Formation. The geological consistency of these results is being further evaluated by SHRlMP zircon analyses from other tuffs in the sequence near Wollogorang and Borroloola.

We note that the Hobblechain Rhyolite, about 200 m higher up in the Tawallah Group, has effectively the same age (1725 ± 2 Ma) as the upper Wollogorang tuff. This suggests that there is essentially no time break between the two, and that the intervening Gold Creek Volcanics were deposited quite rapidly. Alternatively, one or both of these ages is influenced by the presence of inherited zircons crystallised a few million years before the deposition of the tuff. These uncertainties are currently being addressed.

The review has highlighted an urgent need to improve Australia 's Mesozoic polepath for better control in time and space on the plate-tectonic processes that shaped the main source and reservoir rocks of the North West Shelf.

1 -300 -200

May 1997

Conclusions Preliminary zircon SHRIMP dating supports a significant age difference between the sedi•mentary rocks that constitute the lower and upper parts of the Wollogorang Formation, and has thus confirmed what was hypothesised mainly on sequence stratigraphic grounds.

Sequence stratigraphic concepts have the potential to significantly revise our ideas on how sedimentary successions are genetically linked, and, hence, how the mineral deposit•hosting Proterozoic basins in north Australia evolved (cf. Bradshaw et al. 1996: AGSO Research Newsletter 25 , 21-22).

Acknowledgments Dave Rawlings (CODES, University of Tas•mania) kindly supplied unpublished data that helped us to refine some of the ideas pre•sented here, but he does not necessarily agree with all the interpretations presented. We thank him and AGSO colleagues Ian Sweet, Albert Brakel, Mart Idnurm, Andrew Kras•say, and Barry Bradshaw for their critical reviews of the manuscript.

t NABRE Research Team, Minera ls Division , Australian Geological Survey Organisation , GPO Box 378, Can•berra , ACT 260 I ; tel. +61 6 249 9205 (MJJ), +6J 6 249 4261 (tel. & fax for RP), +61 6 249 9206 (PS), +61 62499446 (OS), [ax +61 62499956 (MJJ , PS, & OS); e-mail jjackson@agso.gov.au.

I Minerals Division and Petroleum & Marine Division , Australian Geological Survey Organisation , GPO Box 37S, Canberra, ACT 2601 ; te l. +61 62499324, fax +61 62499986, e-mail cklootwi@agso.gov.au.

1 HI -100 OMa

20-7/323

Fig. 24. Australian late Palaeozoic-Cenozoic polepath with approximate ages for loops and cusps shown.

22

May 1997

COlllil/lIedjrol1l back page.

The review demonstrated that several prevalent geodynamic models are depreci•ated by palaeomagnetic interpretations which are outdated owing to more recent palaeo•magnetic developments or newly acquired geological information. Accordingly, Phanerozoic palaeomagnetic data of north•eastern Gondwanaland have been re-exam•ined for the main dispersive cratons (Aus•tralia and India), the main accretionary craton (Siberian Platform), and the continental frag•ments of established or presumed Gond•wanan origin which became accreted onto Asia to the east of the Urals and onto the North American Cordillera.

Reconstructions The re-examination of palaeomagnetic con•straints focused particularly on the former location of the main Cathaysian landmass (the North China, South China, Indochina, and Tarim blocks) in northeastern Gond•wanaland. Among other things, it revealed that the Cambrian to Late Devonian polepath trajectories for the North China block and Australia are superposed by rotation around an Eulerian pole that relocates North China off northwestern Australia and to the west of Irian Jaya (Fig.26A). This outcome is in good agreement with an independently de•rived reconstruction (Fig.26B) based on bio•stratigraphic and lithostratigraphic indicators (Metcalfe & Nicoll 1994: "Proceedings of the Symposium on 'Gondwana Dispersion and Asian Accretion " ', I GCP Project 32]). Previous attempts to relocate North China with respect to Australia were based on a comparison of individual pole positions of similar ages. Such a procedure leaves the palaeolongitudes indeterminate.

The unique lock on the relative Palaeozoic positions of North China and Australia pro•vides a cornerstone for relocating other Cathaysian continents. Lacking more suitable data sets, such relocations have to be based on matching individual pole positions rather than on polepath fitting. The resulting palaeo•magnetic reconstruction (Fig.26C), based on matching pole positions of about Middle Dev•onian age, is in good agreement with Metcalfe & Nicoll's reconstruction.

The re-examination also demonstrated some palaeomagnetic support for hypotheses that terranes now residing in the North Ameri•can Cordillera (e.g., the Alexander terrane; Fig.26D) originated from the northeastern

Fig. 25. Phanerozoic palaeo latitudinal evolution of Australia (Armidale reference location) according to data from cratonic Australia and the New England Orogen.

and eastern margins of Gondwanaland (e.g., N ur & Ben-Avraham 1983: in 'Accretion tectonics in the circum-Pacific regions', Ter•rapub, 3-18; Gehrels & Saleeby 1987: Tec•tonics, 6, 151-173).

Break-up phases Dispersion of Greater Australia fragments that are now embedded in southern and east•ern Asia has been mainly latitudinal, and obviously northward.

Fragments that now form part of the North American Cordillera and possibly also of northeastern Russia, however, probably dis•persed initially eastward across the Pantha•lassa Ocean (palaeo-Pacific), before large•scale northward movement along the Cor•dillera and possibly beyond, along northern Siberia . These northward movement patterns show up clearly in plots of palaeolatitudinal evolution. An analysis of the palaeolatitude plots for separate fragments shows four north•ward movement phases of potential global relevance:

Early Devonian (Kazakhstan); latest Devonian-middle Carboniferous (northeastern Gondwanaland); possibly middle to Late Permian (north•eastern Gondwanaland); and Triassic (Cathaysian continents, north•eastern Gondwanaland, Gondwanan frag•ments in southeast Asia, and terranes in the North American Cordillera).

These ages provide minima for the various phases of fragmentation of Gondwanaland 's northeastern margin and, by implication , of

AGSO Research Newslett er 26

the North West Shelf. The plots further iden•tify southward rebound of northeastern Gond•wanaland after each of the three later north•ward movement phases (Fig. 25). These three phases can be identified as geological events on the North West Shelf.

Loops and cusps on northeastern Gondwanaland polepaths The Australian Phanerozoic polepath (Figs. 26A and 24; also cf. fig. 24 in Kloot•wijk 1995: AGSO Research Newsletter, 22, 14-17), supplemented by the Indian polepath, shows considerable potential to relate loops and cusps on the polepath to fundamental tectonic events that affected northeastern Gondwanaland, and subsequently the Aus•tralian and Indian plates after final breakup of Gondwanaland. Identified loops and cusps and available age constraints are:

A IIslraiia India Loop/cusp (age; Ma) (age; Ma)

c /o loop DI (Ce) ClISp

Ce- I, 330- 335 loop Cl- Pe, 300 loop Pc-I, 270- 280 Ma loop -260 loop -245-215 -250 cusp - 185+ Ma 210-200 loop -140 130- 150 loop - LOO- 115 < Ll7>84 cusp >60 - 70 cusp

- 40- 45 cusp Tm- Tpl , >12<5 cusp

These loops and cusps on the polepaths probably reflect significant changes in global plate-movement patterns which might have resulted in tectonic phases, basin initiation, megashearing, etc. Far-field stresses origi•nating from such changes can be transmitted widely across the lithosphere. Proper deter•mination of the loops and cusps in space and time thus may provide important controls on, and - for periods before the oldest pre•served (mid-Jurassic) seafloor-spreading pat•terns - the only effective means of:

delineating changes in plate movements and resulting stress patterns; and predicting and dating the resulting tec•tonic events.

COl/til/lied 01/ jacil/g page.

Fig. 26A (from back cover). Reconstruction of the North China block's position with respect to Australia (and Gondwanaland) during the early Palaeozoic. The polepath trajectories and positions of these two Gondwanaland components were rotated 47.9° about an Euler pole at latitude 9.7°N and longitude 180.3°E. This Euler pole was deI'ived from a visual superposition of splines fitted to a Cambrian to Late Devonian polepath trajectory for Australia and a Middle Cambrian to Late Devonian trajectory for NOl'th China. The North China block and fitted polepath are shown before and after rotation about the Euler pole. B, Reconstruction of northeastern Gondwanaland during the Late Silurian according to biostratigraphic and lithostratigraphic evidence (after Metcalfe & Nicoll 1994, fig.3). C. Middle Devonian palaeomagnetic reconstruction of the South China, Indochina, and Tarim blocks adjacent to the North China-Australia (Gondwanaland) composite (Fig.26A) according to a comparison of individual pole positions. This leaves the relative palaeolongitudes indeterminate. D. Early Devonian relocation of the North China-Australia (Gondwanaland) composite according to a 'splined' pole position fOl' Australia (392.5 Ma). The shaded zone shows the palaeolatitudinal position and associated erl'Or for a refe.-ence locality on the Alexander ten'ane of the North American Cordillel'3. Note the agreement with the general palaeolatitude of the Lachlan Fold Belt.

23