Cambro-Ordovician Delamerian Orogeny Research Articles

Geological setting of the Moorowie Formation, lower Cambrian Hawker Group, Mt Chambers Gorge, eastern Flinders Ranges, South Australia

Geological mapping in 1970 of lower Cambrian outcrop in the eastern Flinders Ranges of South Australia included the description and naming of the Moorowie Formation, representing the uppermost Hawker Group. The mapping is supported by 885 m of measured sections. An early Cambrian regressive-marine shelf-margin succession is described, from the massive Wilkawillina Limestone (base), the grey laminated limestones, syndepositional slump-induced intraformational folds and breccias, collapse talus, and graded sediment gravity flow deposits of the Mernmerna Formation, terrace-edge attrition megabreccias and reefs of the Moorowie Formation, passing up to the red beds of the Billy Creek Formation (top). Rapid changes in sedimentary facies are attributed to basement block movements and diapiric influence on sedimentation with abrupt vertical relief and paleoslope indicated by syndepositional slumping and platform margin collapse. Tuffs in the Mernmerna Formation record contemporaneous volcanism. Massive archaeocyathan limestones, ooid grainstones, peloid limestones, reef-sourced megabreccias and red shales define five members in the overlying Moorowie Formation, signalling shallow-marine regressive conditions and the development of a biologically diverse carbonate platform seaward of evaporitic lagoons and supratidal sabkhas. The megabreccias of the Moorowie Formation formed as thin semi-autochthonous debris aprons or shallow tidal-channel infills resulting from gradual and persistent wave attrition and repeated collapse of a carbonate terrace that was vigorously reworked by tidal currents. Shale interbeds within the Moorowie Formation represent lightly channelised shallow-marine ramp deposits, with adjacent mud flats, as easterly equivalents of the Oraparinna Shale, the earliest of which formed the substrate to the attrition megabreccias. Emergent evaporite diapirs near Mt John and Mt Frome are the probable sources of coarse siliciclastics within the carbonates. Some of the siliciclastics were likely transported onto the carbonate platform by sandstorms or migrating dunes. Late in the Cambro-Ordovician Delamerian Orogeny, earlier salt diapirs at depth were compressed and reactivated as highly mobile evaporite-rubble breccias and intruded as small plugs and dykes into fissures in a lithified and folded cover.The intrusive breccias include metasediment and metabasic xenoclasts attributable to the Callanna Group, while diapir-related faults also host minor copper, lead and barite mineralisation. Documentation of this unique record contributes to wider investigations of the Ediacaran-to-early-Cambrian succession of the Flinders Ranges sector of the Arrowie Basin, adding to its global heritage values, effective management and appreciation by the wider community.

Recognition of c. 1780 Ma magmatism and metamorphism in the buried northeastern Gawler Craton: Correlations with events of the Aileron Province

The far northeastern Gawler Craton, South Australia, lies at the northern margin of one of the major building blocks of the Australian continent and is a region that is important in models for the Proterozoic tectonic evolution of Australia. However, this region is overlain by Phanerozoic sedimentary cover and consequently has had no previous geological study. We report the lithotypes, geochronology, geochemistry and isotopic composition of rocks recovered in a mineral exploration drill hole in the far northeastern Gawler Craton, the sole drill hole to sample crystalline basement in this region. Lithologies within the drill hole are garnet- and pyrite-bearing metasedimentary gneiss, pyroxenite and gabbroic intrusions, along with granitic bodies. Metasedimentary gneiss has a maximum depositional age of 1841±4Ma and a provenance pattern more similar to rocks of the Aileron Province of the Arunta Region, central Australia, in particular the Lander Rock Formation, than to other metasedimentary rocks of the Gawler Craton. Amphibolite facies metamorphism occurred at c. 1780Ma and was synchronous with emplacement of high Mg, crustally contaminated mafic rocks, along with several types of felsic intrusion. This metamorphic event is very similar in age and style to the Yambah Event of the Aileron Province, and has not been documented previously in the Gawler Craton. The overall geological features of this portion of the northern Gawler Craton support models that link it with the Aileron Province of the Arunta Region. The rocks of drill hole TB02 underwent thermal resetting recorded by a biotite 40Ar/39Ar age of c. 480Ma, likely asa result of the Cambro-Ordovician Delamerian Orogeny along the eastern margin of Gondwana.

Preservation of a fragmented late Neoproterozoic–earliest Cambrian hyper-extended continental-margin sequence in the Australian Delamerian Orogen

Abstract Mafic and ultramafic rocks intercalated with metamorphosed deep-marine sediments in the Glenelg River Complex of SE Australia comprise variably tectonized fragments of an interpreted late Neoproterozoic–earliest Cambrian hyper-extended continental margin that was dismembered and thrust westwards over the adjacent continental margin during the Cambro-Ordovician Delamerian Orogeny. Ultramafic rocks include serpentinized harzburgite of inferred subcontinental lithospheric origin that had already been exhumed at the seafloor before sedimentation commenced, whereas mafic rocks exhibit mainly enriched- and normal-type mid-ocean ridge basalt (E- and N-MORB) compositions consistent with emplacement in an oceanic setting. These lithologies and their metasedimentary host rocks predate deposition of the Cambrian Kanmantoo Group and are more likely to represent temporal equivalents of the older Normanville Group or underlying Neoproterozoic Adelaide Supergroup. The Kanmantoo Group is host to basaltic rocks with higher degrees of crustal contamination and yields detrital zircon populations dominated by 600–500 Ma ages. Except for quartz greywacke confined to the uppermost part of the sequence, metasedimentary rocks in the Glenelg River Complex are devoid of detrital zircon, and are interstratified with subordinate amounts of metachert and carbonaceous dolomitic slate suggestive of deposition in a deep-marine environment far removed from any continental margin. Seismic reflection data support the idea that the Glenelg River Complex is underlain by mafic and ultramafic rocks, and preclude earlier interpretations based on aeromagnetic data that the continental margin incorporates a thick pile of seawards-dipping basaltic flows analogous to those of volcanic margins in the North Atlantic. Correlative hyper-extended continental rift margins to the Glenelg River Complex occur along strike in formerly contiguous parts of Antarctica. Supplementary material: Geochemical data for mafic and ultramafic rocks in the Glenelg River Complex and correlative terranes, and U–Th–Pb data for western Victoria gabbros are available at http://www.geolsoc.org.uk/SUP18821

New constraints on Phanerozoic magmatic and hydrothermal events in the Mt Painter Province, South Australia

Zircon and monazite U–Pb dating show a punctuated history of tectonic, magmatic and hydrothermal activity in the Mt Painter Province (South Australia) that spans most of the Palaeozoic. The Cambro-Ordovician Delamerian Orogeny was characterised by very limited intrusions of sodic pegmatitic leucogranites, for which the Hf isotopic data show influence of an isotopically more primitive source than the local Mesoproterozoic granitoids and metasediments. Another phase of magmatic activity occurred in an intraplate setting around 460–440Ma, with more extensive met- and peraluminous granitoids and another phase of sodic leucogranites. Both intracrustal reworking and potential mantle input are seen at this stage. Iron-rich uranium ores yielded a monazite U–Pb age of 355±5Ma, and whole rock Sm–Nd isotope systematics indicate that the ores were sourced from the local Mesoproterozoic granitoids. The Palaeozoic events in the Mt Painter Province span a period of 200My, and occurred during more than 10km of exhumation of the Mesoproterozoic basement rocks. This study shows that central South Australia was more strongly affected by large-scale regional tectonics (Alice Springs Orogeny and Lachlan Orogeny) than hitherto known.

Basin geometry and salt diapirs in the Flinders Ranges, South Australia: Insights gained from geologically-constrained modelling of potential field data

The Adelaide Basin in Australia is a complex of late Neoproterozoic to Early Cambrian rift and sag basins which was inverted during the Cambro–Ordovician Delamerian Orogeny. The deposition of evaporitic sediments during the earliest stage of basin development in the late Neoproterozoic (Willouran age) played a major role in the subsequent tectonic evolution of the basin. Previous studies have shown that early mobilization, vertical transport and withdrawal of the evaporites influenced the sedimentation during the late Neoproterozoic and Early Cambrian. The evaporites also influenced deformation during the inversion of the basin and the development of the Delamerian fold and thrust belt. However, the control exerted by basement structures in the deposition of the evaporitic beds and the role of these tectonic structures in the later inversion of the basin have been poorly constrained. In this work, we use a combination of published and original geological observations along with the interpretation of potential field datasets (total magnetic intensity and Bouguer anomaly data) to better constrain the geology of the basin at depth. We construct two and a half dimensional forward models of the potential field data along selected profiles across the Adelaide Basin. These models are constrained by the geology at the surface, drill hole data and measured petrophysical properties (specific gravity and magnetic susceptibility). We achieved the best fit between observed and modelled potential fields with a model favouring thick-skinned deformation, where the diapirs are not randomly distributed, but located directly above basement-penetrating normal faults. Furthermore, these normal faults were probably active during the sedimentation of the late Neoproterozoic and Early Cambrian sediments, and underwent partial or total inversion during the Cambro–Ordovician Delamerian Orogeny. Mobilisation and withdrawal of the evaporites was therefore initiated and facilitated by the extension coeval with the opening of the basin, and continued during the subsequent Delamerian Orogeny.

Early Palaeozoic foreland thrusting and basin reactivation at the Palaeo-Pacific margin of the southeastern Australian Precambrian Craton: a reappraisal of the structural evolution of the Southern Adelaide Fold-Thrust Belt

Regional and detailed structural mapping, kinematic analysis and balancing and restoration of cross sections has lead to a re-interpretation of the Adelaide Fold Belt in South Australia. In this belt sedimentary rocks of the Late Proterozoic Adelaidean and Early Cambrian Normanville and Kanmantoo sequences were deposited during at least two episodes of subsidence related to major crustal attenuation. Contraction and crustal thickening accompanied by granitoid intrusions are related to the Cambro-Ordovician Delamerian orogeny. During this event both basins were reactivated (“inverted”) and the sedimentary rocks are now incorporated in a WNW-verging foreland fold and thrust belt at the margin of the Proterozoic southeast Australian craton. Maximal shortening of the orogen was around 55%. Within the Adelaidean basin, shortening is dominantly accommodated by major mylonitic shear zones and reverse faults. In the Cambrian Kanmantoo basin, several thrusts are demonstrably reactivated growth faults, across which thickness changes of Cambrian sedimentary rocks are revealed by balancing and restoration of cross sections. Owing to the steep easterly dips of these faults, in the western part of the Kanmantoo basin lateral shortening of up to 58% is largely accommodated by intense folding and fold axial-planar flattening strain. Further east, strain magnitudes wane and the overall shortening of around 30% is accommodated by both, discrete thrust zones and regional folds. The basin reactivation along the Australian part of Gondwana's palaeo-Pacific craton margin reflects the rapid transition from passive to active tectonism, which possibly is a consequence of the disintegration of the formerly conjugate margins of Gondwana and Laurentia.

Sm-Nd isotopic evidence for the provenance of sediments from the Adelaide Fold Belt and southeastern Australia with implications for episodic crustal addition

In South Australia, Late Proterozoic and Cambrian sediments were deposited in basins formed within Archaean to Early Mid-Proterozoic basement. These sequences were subsequently deformed and uplifted by the Cambro-Ordovician Delamerian Orogeny to form the Adelaide Fold Belt. In using the Adelaide Fold Belt to address models for lithospheric evolution, it is necessary to understand the mechanisms of basin formation and whether the sequences merely reflect reworking of existing cratonic material or if there were new crustal additions. This study presents geochemical and isotopic data on the sediments and basement rocks to complement existing data on the Delamerian igneous rocks as a means of investigating these questions. The data reveal that the Archaean-Mid Proterozoic basement rocks of the Gawler Craton have Nd depleted mantle model ages which average about 2.6 Ga and ϵNd values at 800 Ma in the range −9 to −24 or at 500 Ma from −11 to −28. The Late Proterozoic Adelaidean sediments, however, have an average model age of 1.9 Ga and initial ϵNd values in the range −5 to −11. The sequence can be divided into lowermost rift-phase sediments derived from erosion of local Gawler Craton material, and a subsequent, widely distributed, thermal-sag-phase sedimentation whose isotopic signature is not compatible with the basement. This implies that a more primitive or juvenile source was accessed via broadening of the provenance region. The Cambrian sediments have slightly older model ages (2.1 Ga) and lower ϵNd (−9 to −13) and can be interpreted as mixtures of basement and reworked Adelaidean detritus. The Early Palaeozoic sedimentary sequences of the Lachlan Fold Belt to the east have isotopic compositions very like the South Australian Late Proterozoic and Cambrian sequences. They have an average model age of 1.85 Ma but much higher K 2O/Na 2O. This indicates that the widespread flysch of eastern Australia is the result of recycling of the Late Proterozoic and Cambrian sediments by erosion of the Adelaide Fold Belt, with some modification by the mixture of the Cambro-Ordovician mafic and felsic magmatic rocks in this orogen. Our data suggest regional, intracontinental, crustal evolution combined with recycling of older crustal material with new additions from the mantle during orogenesis and, to a lesser extent, episodes of extension and sedimentation.

Observations on the tectonic evolution of the southern Adelaide Fold Belt

The Mount Lofty Ranges, Fleurieu Peninsula and Kangaroo Island regions of South Australia expose a section across the west and northwestern margin of the southern Adelaide Fold Belt that was deformed and metamorphosed during the Cambro-Ordovician Delamerian Orogeny. In the external parts of the belt, thrust complexes involving basement and a late Proterozoic platformal succession show a characteristic asymmetry reflecting west to northwest vergence towards the Australian craton. In the more internal zones, convergent deformation involving large-scale upright folding and high- T, low- P metamorphism of mainly Cambrian sediments, which were in part turbiditic, occurred between c. 516 and c. 490 Ma. In this part of the belt the distribution of heat and, possibly, strain reflects intimately the advective movement of magmas within the orogenic belt. Convergent deformation in the southern Adelaide Fold Belt was immediately preceded by rapid subsidence initiated during the deposition of the upper parts of the Normanville Group and which continued during deposition of the Kanmantoo Group. Mafic alkaline volcanism attendant with Normanville Group deposition is indicative of lithospheric thinning at c. 526 Ma.

Basement-cover interaction in the Adelaide Foldbelt, South Australia: the development of an arcuate foldbelt

The upper Proterozoic- to Cambrian-aged sedimentary and volcanic rocks comprising the Adelaide Foldbelt were deformed and, in places, metamorphosed during the Cambro-Ordovician Delamerian Orogeny. Tectonic fabrics developed in the central portion of the foldbelt (Mt. Lofty Ranges) demonstrate westward transport during the orogeny. The sigmoidal shape outlined by the Kangaroo Is., Mt. Lofty Ranges, Olary portion of the foldbelt is interpreted to have been the result of dextral wrench faulting in the lower- to mid-Proterozoic basement. Thus, cover rocks overlying such basement wrench fault zones would have suffered a transpressional stress regime, giving rise to the observed fold axis oriented at an oblique angle to the thrust boundary. In the northern portion of the foldbelt (Northern Flinders Ranges), wrench faulting is interpreted to have accommodated considerable basement shortening which initiated a basement-cover décollement and resulted in thrust-bound pop-up structures in the cover.

A carbonatitic diatreme from Umberatana, South Australia

The breccia body at Umberatana, previously mapped as a diapir, intrudes Late Proterozoic metasediments and envelops Palaeozoic alkaline to peralkaline granites. Lineament-controlled emplacement led to limited structural disruption of the adjacent country rock and resulted in a phlogopite-rich breccia which contains albitite fragments, metasomatized Late Proterozoic clasts, altered alkaline basalts and dismembered metasomatized pelites of the granite thermal aureole. Breccia formation occurred after peralkaline granite emplacement (418 ± 2 Ma) and is thus unrelated to the Cambro-Ordovician Delamerian Orogeny. The breccia body is re-interpreted as a diatreme which resulted from multiple, possibly CO 2 -rich, halogen-bearing pulses of mantle-derived fluids rich in Ca, Mg,Ti,Fe 3+ ,P, Sr, Na and Ce. Fluid contributions from a sedimentary source cannot be excluded, because the fluids traversed Late Proterozoic metasediments. The close association of alkaline to peralkaline igneous lithologies and mantle-derived fluids is related to the deep fracturing of the lithosphere and mantle-degassing.

The Tasman Fold Belt System in Victoria

To provide a background to a model of the evolution of the Tasman Fold Belt System in Victoria, brief descriptions are given of the various tectonic elements, arranged in eight structural zones and of the structural deformation affecting them. The evolution extended over four main depositional episodes, punctuated by orogenic activity. 1. (1) The Cambrian to Late Ordovician depositional episode Involved the diachronous formation of four large troughs (Stawell, Ballarat, Wagga, Melbourne Troughs). Early Cambrian mafic and ultramafic igneous activity was followed by thick diachronous flysch sedimentation. The Cambro-Ordovician Delamerian Orogeny stabilized part of the Stawell Trough and caused eastward shift in sedimentation into the newly formed Ballarat Trough. Late Middle Ordovician diastrophism led to cessation of sedimentation in the Ballarat Trough and to the formation of the Wagga and Melbourne Troughs. The episode was terminated by the Benambran Orogeny, which folded and stabilized the Ballarat and Wagga Troughs and caused onset of rapid subsidence of the Melbourne and newly formed Buchan and Grampians Troughs. 2. (2) Silurian deposition was confined to the Grampians, Melbourne and Buchan Troughs. The Melbourne Trough received a thick, relatively deep marine sequence, while in the other two troughs, acid volcanicity was followed by non-marine and shallow-marine sedimentation in the Grampians and flysch and shallow-marine sedimentation in the Buchan Trough. Widespread diastrophism and granite intrusion marks the Bowning Orogeny, which stabilized the Grampians Trough and changed the Buchan Trough from orogenic to transitional character. The Melbourne Trough remained relatively unaffected. 3. (3) Deposition during the Early Devonian episode was mainly confined to the Melbourne and Buchan Troughs. Eastward shift of the Melbourne Trough axis was followed by a gradual change from flysch to non-marine sedimentation. In eastern Victoria, formation of a large volcanic arch, superimposed on the Buchan Trough, was followed by widespread shallow-marine sedimentation. The Middle Devonian Tabberabberan Orogeny stabilized the Melbourne and Buchan Troughs and caused major translocation along the Indi—Long Plain Fault System in eastern Victoria. 4. (4) The Late Devonian to Early Carboniferous episode includes intrusion of granite in the Ballarat and Melbourne Troughs; extrusion of volcanics in large cauldron subsidences in the area of the former Melbourne Trough; volcanicity and “redbed” sedimentation in the newly formed Howitt Trough; and some “redbed” sedimentation in eastern Victoria. Mild deformation of these rocks in the Kanimblan Orogeny marked the close of the evolution of the Tasman Fold Belt System in Victoria.

Orogenic evolution of Australia

Three main basement provinces can be recognized in Australia, viz. Tasman (Late Precambrian-Phanerozoic), Arunta-Gawler (Early and Middle Proterozoic), Pilbara-Yilgarn (Archaean). The Tasman Province records a history of mobile-belt evolution, east of a craton represented by pre-Adelaidean basement rocks. Orogenic activity shifted episodically away from the craton, causing the stabilization (‘cratonization’) of successive sub-provinces, in the principal phases. (1) The Late Precambrian Penguin orogeny in the orthotectonic mobile belt and the associated Cambro-Ordovician Delamerian orogeny in the paratectonic zone adjacent to the craton. (2) The Devonian ‘Tabberabberan’ orogeny (Lachlan sub-province). (3) The Permian Hunter-Bowen orogeny (New England sub-province). The regressive orogeny represented in these three sub-provinces does not necessarily indicate a corresponding lateral continental accretion. The three sub-provinces are quite different in sedimentary facies. A major volcanic chain of Andeantype, presumably subduction controlled, was apparently virtually fixed in position, during the Devonian and Carboniferous, between the Lachlan and New England sub-provinces. It was probably of less importance, but not greatly different in position, during the Lower Palaeozoic. Thus the main deformation and plutonism of the Lachlan sub-province took place behind the volcanic chain while that of the New England sub-province took place in front. In neither case, therefore, does the plutonism represent the roots of the pre-existing volcanic arc and alternative interpretations are discussed. The stabilization of the New England Province caused a restriction of tectonic activity to more easterly areas (e.g. New Zealand and New Caledonia) which have since been rifted from the Australian continent. If large-scale accretion has occurred, it must be essentially by ensimatic back-arc accretion in former marginal basins, but the evidence for this is not compelling. The northern (Arunta) and southern (Gawler) sub-provinces of the Arunta-Gawler Province are separated in central Australia by the Amadeus Transverse Zone. A remarkable feature of this intracontinental zone is the presence of conjugate dislocations of the crust which have produced thrusting towards the Amadeus Basin from both sides. These were initiated as major mylonitic zones, which have brought up granulite facies rocks, prior to 1100 m.y., and they were re-juvenated in the late Precambrian (Petermann Ranges orogeny) and Upper Palaeozoic (Alice Springs orogeny). The Arunta sub-province, like the Lachlan sub-province, is largely non-volcanogenic. One major plutonic episode (ca. 1700–1850 m.y.) and associated compressional deformation can be recognized right across the province. However, no major calc-alkaline volcanic chain has been recognized and it must be inferred that any active continental margin lay east of the province as at present exposed. In the Gawler sub-province the existence of extensive areas of high-grade metamorphism in regions of relatively thin and mature earlier Proterozoic sedimentation are not yet satisfactorily explained. Neither the absolute, nor relative ages of the Archaean greenstone sequences in the Pilbara and Yilgarn sub-provinces are firmly established but the presence of older ‘granites’ (ca. 3100 m.y.) and a different tectonic style suggest that the Pilbara sub-province is older. Actualistic island-arc or ocean-floor models are unsatisfactory in explaining development of the Yilgarn sequences in shallow water, simultaneously over wide areas. Development on a proto-continental crust, not necessarily ‘granitic’, with a high geothermal gradient appears to be required. As in the Arunta and Lachlan sub-provinces the main plutonic episode (ca. 2650 m.y.) is closely associated with the relatively short-lived terminal compressional deformation.

Organic, mineralogic and magnetic indications of metamorphism in the Tapley Hill Formation, Adelaide Geosyncline

The dispersed carbonaceous matter (kerogen), illite, and magnetic response of the Tindelpina Shale Member in the lower part of the thick, extensive Tapley Hill Formation provide three complementary methods for zoning the incipient metamorphic character of rocks comprising the late Precambrian Adelaide System where they crop out between Adelaide, Olary, and Marree in the Adelaide Geosyncline. The methods are based on the following parameters: kerogen structure (as determined by X-ray diffraction) and composition (percentage carbon, hydrogen to carbon atomic ratio, δ 13CPDB); illite crystallinity; and amplitude and type of aeromagnetic anomalies. Kerogen is the most definitive indicator of metamorphic change in the Tindelpina Shale. It has been used to delineate a western subgraphitic zone (85–91%C, H/C > 0.10) which is separated from an eastern graphitic zone (91–98%C, H/C < 0.10) by a north-trending line through Adelaide, Mintaro, Orroroo, and Baratta. A similar two-fold zonation appears to exist in the Mouth Painter—Copley—Marree area. Metamorphic adjustment of the stable carbon isotopic composition of the kerogen is also evident. Kerogen rank correlates well with illite crystallinity. Illites in the western zone have Weaver indices of less than six. Crystallinity increases to the east where 2M illite becomes the dominant illite polymorph. The eastern graphitic zone largely coincides in location and extent with a zone of linear aeromagnetic anomalies of amplitude exceeding 100 gammas. In the lower Tapley Hill Formation the anomaly is attributed to remanent magnetism, probably associted with metamorphic growth of magnetite. All three indicators suggest an increase in metamorphic grade from west to east across the geosyncline, in agreement with published observations based on conventional petrographic analysis of pelitic (and to a lesser extent, carbonate) rocks. Illite and chlorite, characteristic of the anchizone of burial diagenesis, are the dominant sheet silicates in the shales studied, although incipient metamorphic alteration of chlorite to biotite has occurred in some specimens from the graphitic zone. The subgraphitic and graphitic facies of the Tindelpina Shale correspond with the chlorite and biotite (and higher) zones, respectively, of low-pressure intermediate-type metamorphism previously established for the Mount Lofty and Flinders Ranges. Greater depth of burial of the lower Tapley Hill Formation in the eastern half of the geosyncline would account for the metamorphic trend observed in its organic matter and clay mineral content. Differential burial does not, however, adequately explain the magnetics, nor the absence of biotite-grade rocks in the central Flinders Ranges. Other magnetically anomalous beds are found throughout the Adelaide System and in overlying Cambrian strata. For the magnetism in these different stratigraphic intervals to be of metamorphic derivation, a regional thermal event, perhaps related to the Cambro-Ordovician Delamerian Orogeny, must be postulated.