Adelaide Rift Research Articles

Early-stage intracontinental rifting in the Neoproterozoic Centralian Superbasin: Systematic U–Pb and Lu–Hf isotopes in detrital and inherited zircons from the Yeneena Basin, northwest Australia

Detrital zircons from different lithostratigraphic units of Supersequence 1 across the Neoproterozoic northwest Officer and Yeneena basins were investigated using LA-ICPMS U−Pb and Lu − Hf isotope analyses to understand the source of detritus and potential links to regional tectono-magmatic events, as well as the crustal evolution of the sedimentary sources terranes. All data show that Proterozoic detritus dominate the age spectra of samples from the Yeneena Basin and northwest Officer Basin, with signatures varying in the heights of peaks rather than the presence or absence of age groups in different formations. The Archean and Proterozoic ages overlap with known magmatic events in the Pilbara Craton, Fortescue Basin, Rudall Province, Capricorn Orogen, Musgrave Province, Madura Province, and the Coompana Province suggesting these terranes are possible sources of the detritus. The detrital zircon population of the samples have main age peaks in the range 1300 to 1000 Ma, suggesting magmatic rocks of the Rudall and Musgrave provinces are the main sources of detrital zircons, which can be differentiated by Hf signature. The distal Musgrave Province persistently dominates the provenance of detritus in the northwestern Officer and Yeneena basins, while detritus from the proximal Rudall Province contributes to a lesser extent. This suggests that the distal Musgrave Province remained emergent during the evolution of the northwestern Officer Basin and the Yeneena Basin, whereas the proximal Rudall Province potentially became buried by the accumulating sedimentary rocks and contributed less detrital zircons. The 833.6 ± 1.4 Ma maximum depositional age obtained in this study for the sedimentary rocks of the northwestern Officer and Yeneena basins point to the approximate time of the start of basins development. The extensional event in this age period is related to emplacement and extrusion of the ca. 830 Ma Willouran Large Igneous Province (LIP). The Duke gabbro sills observed in the Yeneena Basin are part of this LIP, which extends from the Adelaide Rift Complex, through the Musgrave Province, central–northwest Officer Basin and into the Yeneena Basin. The sediment source terranes suggest a southeast–northwest oriented depositional system between central and Western Australia, possibly because of intracontinental rifting related to widespread mafic–ultramafic magmatic events that could include the earlier Ngaanyatjarra Rift formed by the 1090–1040 Ma Warakurna LIP as well as the ca. 830 Ma Willouran LIP, synchronous with the breakup of Rodinia in the mid-Neoproterozoic.

Stratigraphy and sequence stratigraphy of the Neoproterozoic (Cryogenian–Ediacaran) Stuart Shelf, South Australia

The Stuart Shelf is part of the Adelaide Superbasin overlying the Gawler Craton and Cariewerloo Basin in South Australia. The basin is of interest for sediment-hosted copper mineralisation known to be hosted in numerous stratigraphic intervals across the region. Therefore, facies analysis and understanding of spatial distribution of host units are essential for exploration targeting and mineral systems research. Our study presents improved and new definitions of Cryogenian and Ediacaran Stuart Shelf stratigraphy, and a detailed, regional-scale sequence stratigraphic analysis. The Cryogenian non-glacial interlude was of particular interest, as it includes the Tapley Hill Formation, a known host for copper mineralisation. The succession of Tapley Hill Formation (including Sturtian cap carbonates), Brighton Limestone and Angepena Formation represents a third-order depositional cycle, an equivalent to the lowermost cycle of the Cryogenian non-glacial interlude in the adjacent Adelaide Rift Complex. The Ediacaran post-glacial succession includes the Nuccaleena Formation and Tent Hill Formation and is interpreted as a second-order depositional cycle. The Sturtian and Marinoan cap carbonates form regional stratigraphic marker horizons and were deposited in a transgressive systems tract. Variations in facies and thickness can be linked to development of localised depocentres and topographic highs, such as the Pernatty High. Modelling of 3D surfaces reveals a shift in the basin orientation between the Cryogenian and Ediacaran sedimentation from an NNW–SSE axis towards a N–S axis. The Stuart Shelf sequence stratigraphic framework provides new insights in the basin sedimentology and evolution, which aids sediment-hosted mineral systems analysis and improves the understanding of Cryogenian and Ediacaran basin evolution in Australia.

Multiscale resistivity mapping from an intracontinental hydrothermal mineral system, Adelaide Rift Complex, Australia

Porphyry mineral systems provide a significant source of critical minerals for modern society, that are predominantly formed in subduction related settings from magmatic hydrothermal fluids derived from an underlying reservoir. However, in post-subduction settings the processes that entrain and focus magmatic hydrothermal fluids throughout the crust, from source to deposit are often poorly constrained. In this paper, we have undertaken a magnetotelluric (MT) survey in an intracontinental tectonic setting across the Adelaide Superbasin, which is a Neoproterozoic passive margin formed during rifting of the supercontinent Rodinia. This region contains the Burra magmatic copper deposit, along with numerous sedimentary-hosted copper deposits. Systematic three-dimensional modelling was undertaken, initially at regional-scale, with 55 km gridded long-period AusLAMP data over a 500 × 500 km area, followed by infilled 10 km gridded broadband MT data, at the district-scale, centred over an area of copper deposits in the Adelaide Rift Complex. The modelling reveals an elongated zone of low electrical resistivity (∼100 Ω.m), in an otherwise resistive crust, at lower crustal depth (20–40 km) that is contiguous with the regional lineament of the Adelaide Rift Complex and copper and gold mineralisation. A narrow zone of low resistivity (∼100 Ω.m) branches through the upper crust (5–20 km) to the surface, that is remarkably aligned with the Burra magmatic mineral system, in an otherwise quite resistive crust (∼1000 Ω.m). Additionally, in the upper crust a zone of low resistivity (1–10 Ω.m at a depth of 5–8 km) extends from the Burra magmatic copper deposit southwards, constrained by anticlinal folding from the ∼500 Ma Delamerian Orogenic deformation of the Adelaide Rift Complex and spatially aligned with the Kapunda sedimentary-hosted copper deposit. We argue that the low electrical resistivity signature encapsulates a whole-of-lithosphere magmatically-hosted copper system, that is further mobilised by low-temperature fluids by mapping the footprint of ore-forming fluids from the source to associated mineralisation. Where later basin wide fluid recycling in a permeability enhanced basin, via neo-tectonism along pre-existing structures/trends, has likely remobilised copper, from a prior magmatic event, with copper being leached in chloride-rich saline fluids at low temperatures.

Link between Lesser Himalayan Neoproterozoic Krol succession and the Adelaide rift complex of Australia ‒ evidence from the mudstone deposits of Krol succession, Himachal Pradesh, India

The mudstone deposits, a major constituent of the Lesser Himalayan Krol succession, represents the terminal phase of Marinoan snowball melting, an important part of the most significant events of the Proterozoic time, the Cryogenian Snowball Earth. The Facies analysis suggests that the sediments were supplied to the shelf by meltwater flows, and were later dispersed to the deeper part of the shelf by current/wave-enhanced bottom currents. The upper part of the deposit bears the signature of gradual shallowing of the basin caused due to glacioisostatic rebound. Petrographic study of the samples, randomly collected from different parts of the deposit, reveals that the mud deposit is composed of silt-sized particles of quartz and dolomite, and argillaceous material. Besides the variation in proportion of these constituent components, no other material was found within this deposit. This compositional homogeneity of this huge deposit primarily suggest derivation from some preexisting sediments. Mineralogical similarity and perfect match in trace and Rare Earth elements composition suggest that this mud was derived from the periglacial loess deposits of the Nuccaleena Formation of South Australia. This is in agreement with the provenance of the underlying Krol Sandstone. The mudstones, along with the underlying Krol Sandstone, thus suggest that the Lesser Himalayan Krol succession represent an extended part of the Adelaide rift system of South Australia. The relative position of India and Australia during Neoproterozoic, as per the reconstructed configuration of the Rodinia supercontinent, corroborates this inference.

Th–U powered metamorphism: Thermal consequences of a chemical hotspot

AbstractThe Arkaroola region of the northern Flinders Ranges, South Australia, records high geothermal gradient mineral assemblages that are not spatially or temporally associated with intrusive magmatism. Cordierite‐bearing schists from the base of a ~12 km thick Neoproterozoic sedimentary sequence known as the Adelaide Rift Complex directly overlie Mesoproterozoic metasedimentary and granitic rocks with regional heat production values of ~7.9 µW/m3 at 580 Ma, two to three times greater than global average values for granitic rocks. We integrate in‐situ U–Pb monazite geochronology, Y+HREE‐in‐monazite thermometry and mineral equilibria modelling to show that rocks at the base of the sedimentary succession record amphibolite facies metamorphism at c. 580 Ma while the overlying sediments were still accumulating. Metamorphism took place under average geothermal gradient conditions in excess of 180°C/kbar (>60°C/km) that propagated to depths of at least 12 km. These thermal gradients persisted for upwards of 150 Ma, maintained by a lack of crustal erosion, and are documented by long‐lived crustal anatexis. This system may be the archetypal example of Th–U powered metamorphism, recording the interplay between chemically extreme basement and thermally insulating sedimentary cover.

Re-assessment of the depth to the base of magnetic sources (DBMS) in Australia from aeromagnetic data using the defractal method

SUMMARY Australia and North America are the only two continent-wide regions, currently, with the complete wavelength spectrum of magnetic anomalies that can address the much-debated issue of the existence and understanding the magnetic state of the upper mantle. The depth to the base of magnetic sources (DBMS) has been extensively investigated in Australia in previous studies, yielding differing DBMS estimates across the continent. In this contribution, we re-assess the DBMS in Australia from aeromagnetic data using the defractal method, which simultaneously estimates the DBMS and the fractal parameter of the magnetic field. We test the effect of window size on DBMS estimates for the defractal method and conclude that a window size 10 times the expected DBMS is required for the defractal method. We also demonstrate the dependence of derived spectral slopes (and thereby depths) on the size of window by comparing spectral slopes obtained from window sizes up to 1000 km. Our analysis yields DBMS estimates between 25 and 60 km over Australia and are complexly related to temperature structure, magnetic mineralogy and petrology and geological history. Deeper DBMS estimates (∼55–60 km) are obtained for the Yilgarn Craton (West Australian Craton) and the Gawler Craton (South Australian Craton). These estimates are significantly deeper than the Moho, suggesting the uppermost mantle is ferromagnetic for these cold and old cratonic regions. Shallower DBMSs of 25–35 km are obtained in the Pilbara Craton, the eastern and southern part of the Delamerian Orogen, the northwestern part of the Canning Basin, Officer, Cooper and Georgina Basins and the Tasmanides. The areas of shallower DBMS are positively correlated with high heat flow in the regions. On the other hand, regions of large concentrations of radiogenic elements associated with the major metallogenic deposits have high heat flow arising from the radiogenic heat, but deeper DBMSs, for example the eastern margin of the Gawler Craton covered by Neoproterozoic Adelaide Rift Complex, the McArthur Basin, the Pine Creek Inlier.

Age and geochemistry of the Boucaut Volcanics in the Neoproterozoic Adelaide Rift Complex, South Australia

The Adelaide Rift Complex in South Australia records the break-up of Rodinia at a time of great climatic and biological evolution. The Boucaut Volcanics within the Neoproterozoic Adelaide Rift Complex of the Adelaide Superbasin lie at the base of the Burra Group, marking the boundary between the Burra Group and underlying Callanna Group. Despite their significance as one of the few volcanic units within the rift complex, there has been no robust age determination published for the Boucaut Volcanics. We use U–Pb zircon LA-ICP-MS data to determine an age of 788 ± 6 Ma for the eruption of the bimodal Boucaut Volcanics. This has important implications for constraining the timing of stratigraphy within the Adelaide Superbasin. This also has far-reaching implications for plate tectonic reconstructions of Australia and Laurentia, and for correlating global isotope anomalies for the Neoproterozoic. KEY POINTS New U–Pb zircon data provide a revised age for the Boucaut Volcanics of 788 ± 6 Ma. Whole-rock geochemistry data highlight the bimodality of the Boucaut Volcanics, with both mafic and felsic components present. Boucaut Volcanics potentially correlate with units in the southwest United States, which would support an AUSWUS plate tectonic configuration during the Neoproterozoic.

Dating the Acraman asteroid impact, South Australia: the case for deep drilling the ‘hot shock’ zone of the central uplift

The deeply eroded Acraman structure in the Mesoproterozoic Gawler Range Volcanics, Gawler Craton, South Australia, ranks among the world’s largest known impact structures, with a transient crater ∼40 km in diameter and a ∼90 km-diameter collapse crater. An impact age of ca 590 Ma is suggested by Rb–Sr shale dating of Ediacaran strata hosting a related ejecta band of volcanic fragments in the Adelaide Rift Complex and Officer Basin, and ca 580 Ma by chemostratigraphy. Ascertaining an accurate and precise impact age would greatly benefit Ediacaran biostratigraphic, biogeochemical and magnetostratigraphic global correlations. However, numerous attempts to date the impact, using material from exposures at Acraman and the ejecta band, have been unsuccessful. U–Pb dating of zircons from shattered dacite basement, melt rock and ejecta by three laboratories gave ages related to the 1592 Ma Yardea Dacite. K/Ar and 40Ar/39Ar dating of end-member K-feldspar in melt rock by two laboratories provided early Paleozoic ages, indicating later fluid movement. Although 80% of terrestrial impact structures ≥40 km in diameter have been drilled, Acraman has not. Deep drilling of the ‘hot shock’ zone of the unexposed central uplift is required to obtain zircons with U–Pb ratios that were likely reset by the impact. High-resolution aeromagnetic data for Acraman reveal an elliptical magnetic low measuring 19.5 × 18.3 km, with NNE–SSW-trending major axis and a high-amplitude dipolar anomaly of the central uplift that is offset to the SSW. The magnetic signature suggests oblique impact from the SSW. The source of the dipolar magnetic anomaly, centred ∼1 km below the Lake Acraman salina and with a remanence direction concordant with that of the melt rock, marks the ‘hot shock’ zone and a potential target for drilling. The scientific rewards of such drilling could be substantial. KEY POINTS Acraman in the Mesoproterozoic Gawler Range Volcanics, South Australia, is one of the world’s largest known impact structures, with a related ejecta band of volcanic fragments hosted by Ediacaran (ca 590–580 Ma) strata in the Adelaide Rift Complex and Officer Basin at unfolded distances from 250 km up to 550 km from the impact site. Numerous attempts at U–Pb dating of zircons from shattered volcanic basement and melt rock at Acraman, and zircons from the ejecta, gave un-reset basement ages, and K/Ar and 40Ar/39Ar dating of melt rock gave early Paleozoic ages related to later fluid movement. An accurate and precise age for the impact would provide a reliable age for the ejecta-hosting Ediacaran strata and greatly benefit Ediacaran biostratigraphic, biogeochemical and magnetostratigraphic global correlations. Deep drilling of the ‘hot shock’ zone of Acraman’s central uplift, inferred to be the source of a high-amplitude dipolar aeromagnetic anomaly centred ∼1 km below the Lake Acraman salina, is required to obtain zircons with U–Pb ratios most likely to have been reset by the impact.

Neoproterozoic geochronology and provenance of the Adelaide Superbasin

The Adelaide Superbasin (Adelaide Rift Complex, Stuart Shelf, Torrens Hinge Zone, Coombalarnie Platform, and Cambrian Stansbury and Arrowie Basins) is a vast sedimentary basin in southern Australia that initiated due to the break-up of central Rodinia and, evolved into the Australian passive margin on edge of the Pacific Basin. Rocks within it contain evidence for the evolving earth system through the Neoproterozoic, including type sections of the Ediacaran fauna, Sturtian and Marinoan glaciations, and the GSSP for the base of the Ediacaran period. Much research over the last century has unravelled the lithostratigraphy and sedimentology of the basin. Despite this, the rocks are poorly dated, and their sedimentary provenance and link with tectonic geography is poorly known. This poor chronology hampers global and local efforts to gain a detailed understanding and chronological framework of the interplay between tectonics and momentous changes to the earth system during this time. This paper presents a comprehensive database of detrital zircon geochronology and review of geochronology for the Neoproterozoic of the Adelaide Superbasin, highlighting the stratigraphic, and spatial locations of available data.In the north of the basin, zircons were sourced locally in the initial stages of rifting, ca. 830 Ma—from the adjacent Gawler Craton and Curnamona Province. During the late Tonian, detritus was transported along graben from the north-west, from the Musgrave Orogen, as the rift basin developed during the opening of the nascent Pacific Ocean. Cryogenian icesheets punctuate the detrital record with an ephemeral return to more localised rift shoulder sources. In the Ediacaran, there is an increasing influence of younger (<740 Ma) detrital zircon from an enigmatic source that we interpret to be from southern (i.e. Antarctic) sources, with a corresponding shift in the late Mesoproterozoic age peaks, from ca. 1180 Ma to ca. 1090 Ma, and corresponding decrease in older, ca. 1600 Ma, detritus. These changes in sediment source reflect the changing tectonic geography and large-scale environmental influence of the Cryogenian glaciations as the basin evolved from a local rift, to a larger rift basin and finally to a continental margin, with sedimentary input becoming increasingly restricted over time.

New U[sbnd]Pb, Hf and O isotope constraints on the provenance of sediments from the Adelaide Rift Complex – Documenting the key Neoproterozoic to early Cambrian succession

The Adelaide Rift Complex is arguably one of the most complete and best studied Neoproterozoic to early Cambrian successions worldwide, preserving evidence of the breakup of Rodinia, two Cryogenian glaciations and the interglacial phase, and one of the best documented Ediacaran to early Cambrian biotic transitions. However, the complex and protracted tectono-sedimentary history of this 0.8–0.5 Ga province is still being debated. We present new and published UPb ages and Hf and O isotope data for detrital zircons from the Adelaide Rift Complex, representing the most complete assembly of such data for this succession. Deposition during initial mid-Tonia extension was largely sourced locally from rift shoulders. As the basin evolved from rift- to sag-phase following continental breakup in the Cryogenian the provenance regions extended to more distal late Mesoproterozoic terranes to west and northwest. New data from Sturtian Glacial Epoch deposits are consistent with termination of this event at 0.66 Ga, with most deposition during deglaciation. Uplift of the Musgrave region during the Ediacaran to early Cambrian Petermann Orogeny led to dominant sediment supply from that terrane at that time in the north. In the south, earliest Cambrian deposition followed local tectonism, initially revitalising local proximal basement sources. An abrupt change in provenance occurred at the base of the Cambrian Kanmantoo Group, the youngest sediment package in the south. Paleocurrent data indicates transports from the south, probably from formerly contiguous Antarctica, possibly reflecting the onset of convergent tectonics and deposition in a foreland basin, consistent with the near depositional age of the dominant detrital zircon population. Whilst several episodes of significant crustal reworking are identified in the Hf and O isotope data, many of the zircon TDM ages lie within 0.5 Ga of the UPb ages indicating that new additions from the mantle were common in the provenance regions.

Late Cryogenian glaciation in South Australia: Fluctuating ice margin and no extreme or rapid post-glacial sea-level rise

Postulated extreme sea-level rise of up to 1–1.5 km with the late Cryogenian Ghaub deglaciation in Namibia is contentious, as is the great rapidity (<104 yr) of the sea-level rise. Such extreme glacioeustatic events, if real, would have been global and affected all continents. In South Australia, up to six glacial advances and retreats during the late Cryogenian Elatina glaciation indicate a fluctuating ice margin. The latter stage of the Elatina glaciation and the immediate post-glacial environment are examined here for evidence of extreme and rapid sea-level rise. In the central Adelaide Rift Complex, diamictite with faceted and striated clasts occurs at the top of the Elatina Formation <1–2 m beneath the early Ediacaran Nuccaleena Formation ‘cap carbonate’. One hundred kilometres to the south, ∼30 m of siltstone and sandstone followed by ∼6 m of clast-poor diamictite with clasts 10 + cm long occur between tidal rhythmites and the cap carbonate. Three hundred kilometres further south, ∼70 m of siltstone, dolomitic siltstone and minor dolomite separate tidal rhythmites and early Ediacaran strata. Hence the rhythmites were deposited during a high stand (interstadial or interglacial), not during post-glacial sea-level rise. Storm-generated erosional surfaces within tidal rhythmites at Warren Gorge indicate intermittent rhythmite deposition, and water depth and other palaeoenvironmental factors are uncertain, casting doubt on a published estimate of rapid sea-level rise during rhythmite deposition. The lack of late Cryogenian deeply incised valleys and thick valley-fill deposits in South Australia and central Australia argues against extreme sea-level variations. A hiatus occurred between Elatina deglaciation and deposition of the Nuccaleena cap carbonate, and three palaeomagnetic polarity chrons identified in the cap carbonate imply slow deposition spanning 105–106 yr. This is supported by independent evidence from magnetic chronostratigraphy for Ediacaran strata in South Australia and California, and by stratigraphic and sedimentological arguments for condensed deposition of cap carbonates. It is concluded that neither extreme nor rapid sea-level rise was associated with late Cryogenian deglaciation in South Australia.

Lateral Facies Variability Along the Margin of an Outcropping Salt-Withdrawal Minibasin, South Australia

Many aspects of depositional processes adjacent to exposed evaporite diapirs arc not well understood, yet these processes are key to understanding sediments that are part of economically important hydrocarbon systems around the world. In the Adelaide Rift Complex of South Australia, diapirs intersected the seafloor and land surface during the Ediacaran-Cambrian at the same time as sediment was being deposited. Excellent exposures of these diapirs and their associated minibasins allow the character and distribution of these deposits to be studied in detail; this study examines the interaction between a diapir body and minibasin sediments from a sedimentological perspective. Numerous sections were measured along the minibasin margin, allowing the sedimentary facies, lateral sediment variability, and depositional processes to be determined. Deposition took place in a variety of environments, ranging from carbonate shelves to subaerially exposed tidal flats and alluvial fans. Minibasin sedimentation adjacent to the diapir is characterized by an abundance of gravity-flow deposits, including turbidites and debrites. These deposits often contain extraformational conglomerates brought to the surface by the diapir and redeposited into the minibasin depocenter. Within the minibasin fill, sedimentary facies are unevenly distributed, and sedimentary character is most affected by the diapir in areas where depositional thinning, onlap, and growth faulting are most common. Comparison of our observations with other localities where salt bodies intersect the surface show that syndepositional salt-sediment interaction results in a recurring set of features that can be useful in predicting the sedimentary character of these deposits. This study is one of relatively few to examine the sedimentology of an outcropping minibasin in detail and to describe lateral variability of sedimentary facies along an outcropping minibasin margin. The deposits discussed here therefore provide a valuable analogue for subsurface and seafloor deposits elsewhere where salt-sediment interactions cannot be studied in detail.

Neoproterozoic hydrothermal activity in the West Australian Craton related to Rodinia assembly or breakup?

The timing of final assembly and initiation of subsequent rifting of Rodinia is disputed. New rutile ages (913 ± 9 Ma, 900 ± 8 Ma and 873 ± 3 Ma) and published zircon, monazite, titanite, biotite, muscovite and xenotime geochronology from the Capricorn Orogen (West Australian Craton) reveal a significant early Neoproterozoic event characterized by very low to low metamorphic grade, abundant metasomatism, minor leucogranitic and pegmatitic magmatism and NW–SE fault reactivation episodes between ca. 955 and 830 Ma. Collectively, these are termed the ca. 955–830 Ma Kuparr Tectonic Event. An age range of ca. 955–830 Ma is concomitant with the final stages of Rodinia assembly and the initial stages of its attempted breakup. Very low- to low-grade metamorphic and structural geological evidence favor a distal north–south compressional regime as the driver for hydrothermal activity during ca. 955–830 Ma. Nearby continental collision or accretion from the west (e.g., South China and/or Tarim) are ruled out. The cessation of metasomatism and magmatism in the West Australian Craton after ca. 830 Ma is concomitant with the emplacement of the Gairdner–Amata dyke swarm and associated magmatic activity in South China and Laurentia, the inception of the Adelaide Rift Complex and the deposition of the Centralian Superbasin. We posit that the cessation of hydrothermal activity in the Capricorn Orogen was caused by a tectonic switch from compressional to extensional at ca. 830 Ma. Magmatic and hydrothermal fluids were transferred away from the Capricorn Orogen to the incipient Adelaide Rift Complex, terminating metasomatism in the West Australian Craton. Ultimately, the Kuparr Tectonic Event marked the final stages of Rodinia assembly and its cessation marks the initial stages of its attempted breakup.

Thermochronological history of the northern Olympic Domain of the Gawler Craton; correlations between cooling ages and mineralising systems

The Olympic Domain of the Gawler Craton is home to the world class Olympic Dam Iron Oxide Copper Gold (IOCG) mineral deposit in addition to numerous other IOCG mineral deposits. The Olympic Domain preserves a complex geological history that began in the Palaeoproterozoic. However, most published work conducted on these IOCG deposits have focused on their initial formation, with only a few studies investigating the post-formation thermal history of the Olympic Domain. This study uses multi-method thermochronology by combining apatite U/Pb, muscovite and potassium feldspar 40Ar/39Ar, zircon and apatite (U-Th-Sm)/He, and apatite fission track (AFT) dating to provide insights into the thermal history of the northern Olympic Domain between ~550°C and surface temperatures. Apatite U/Pb and muscovite 40Ar/39Ar record post magmatic cooling of the ~1850 Ma Donington Suite, and ~1590 Ma Hiltaba Suite. Potassium feldspar 40Ar/39Ar analyses record a cooling signal that is likely related to rifting in the Neoproterozoic Adelaide Rift Complex. A combination of AFT, and zircon and apatite (U-Th-Sm)/He dating preserves three thermal periods, at ~1000 Ma, ~430-400 Ma and ~200 Ma. The older two thermal periods are interpreted to be regional cooling. However, the youngest ages are preserved closest to known IOCG deposits suggesting that they reflect cooling of this elevated geothermal-gradient crust in the Mesozoic. These results have been modelled to produce a thermal history map of the northern Olympic Domain.

Depositional age and correlation of the Oonah Formation: refining the timing of Neoproterozoic basin formation in Tasmania

ABSTRACTThe Proterozoic Oonah Formation comprises a thick sequence of turbidites and mafic rocks that is widely exposed in western and northern Tasmania. The regional significance of the Oonah Formation is poorly understood as its contacts are faulted and estimates of its depositional age range from 1450 Ma to 700 Ma. Here we refine the depositional age of the Oonah Formation and clarify its relationship to other Proterozoic sequences in Tasmania. Magmatic apatite from the syn-sedimentary Cooee Dolerite has a 207Pb-corrected 238U/206Pb age of 733 ± 9 Ma that provides a robust estimate of the depositional age of the Oonah Formation. The Oonah Formation contains abundant 750 Ma detrital monazite and has detrital zircon age populations at 1800–1700 Ma, 1590 Ma, and 1450 Ma. The new age and provenance data support the interpretation that the Oonah Formation is a lateral equivalent of the base of the Togari Group and correlatives in Tasmania. We demonstrate that the stratigraphy and provenance of Neoproterozoic strata in Tasmania differ from time-equivalent sequences in Adelaide Rift Complex but are similar to the Pahrump Group (Death Valley, California) and the Cobham Formation (Transantarctic Mountains), which supports a southwest Laurentia—Tasmania—East Antarctica connection within supercontinent Rodinia.

Thermal history of the northern Olympic Domain, Gawler Craton; correlations between thermochronometric data and mineralising systems

Multi-method thermochronology applied to the northern Olympic Domain of South Australia reveals a complex thermal evolution with multiple thermal events. Apatite U/Pb (closure temperature ~550–350 °C) and muscovite 40Ar/39Ar ages (~400–350 °C) record post magmatic cooling from 1850 to 750 Ma for the ~1850 Ma Donington Suite, and from ~1560–1500 Ma for the ~1590 Ma Hiltaba Suite and the Gawler Range Volcanics. Potassium feldspar 40Ar/39Ar ages (~350–150 °C) record disturbed spectra that are likely related to hydrothermal alteration at ~1000–650 Ma in the Neoproterozoic Adelaide Rift Complex. Apatite fission track (~60–120 °C), zircon (U-Th-Sm)/He (~180–200 °C), and apatite (U-Th-Sm)/He (~45–75 °C) ages reveal regional low temperature thermal events at ~1000 Ma, ~430–400 Ma, 350–330 Ma, and ~200 Ma, in addition to localised thermal events during the Cretaceous. The ~1000 Ma apatite fission track ages are amongst the oldest recorded for South Australia and are only preserved near the margins of the Olympic Domain. The ~430–400 Ma and ~350–330 Ma cooling events are interpreted to be driven by the Alice Springs Orogeny. The Mesozoic thermochronometric ages are interpreted to record localised thermal perturbations, possibly related with hydrothermal activity within the northern Olympic Domain. The presence of abundant IOCG deposits near these young thermal anomalies may indicate that these zones record more favourable thermal conditions and/or exposure levels for IOCG discoveries in the study area.

Neoproterozoic carbon isotope stratigraphy of the Amadeus Basin, central Australia

Neoproterozoic shallow-marine and carbonate- rich strata of the northern Amadeus Basin, central Australia, comprise a geochemical record of Neoproterozoic seawater that facilitates correlation with other basins in Australia and worldwide. This study includes new carbon and oxygen isotope measurements of Cryogenian and Ediacaran carbonate in the northern Amadeus Basin that, in combination with previous data from Tonian strata of the Bitter Springs Formation, form a composite Amadeus Basin Neoproterozoic δC record. Key features of the Cryogenian-Ediacaran portion of this record include (1) negative δC excursions in cap carbonates overlying Cryogenian glacigenic deposits of the Areyonga and Olympic Formations (i.e., the Rasthof and Maieberg excursions, respectively); (2) a mid-Cryogenian negative excursion in the Ringwood Member of the Aralka Formation (seemingly correlative with the Taishir excursion of SW Mongolia); (3) a subsequent mid-Cryogenian positive excursion (the Keele Peak excursion) in the Limbla Member of the Aralka Formation; and (4) relatively low-magnitude excursions in the Ediacaran Julie Formation. In comparison with other Neoproterozoic stratigraphic successions, key portions of the overall Neoproterozoic δC record that appear to be absent from the Amadeus Basin based on existing data include large-magnitude negative excursions (Islay and Trezona excursions) that immediately preceded basaland terminal-Cryogenian (i.e., Sturtian and Marinoan, respectively) glaciations. These missing δC excursions are associated with significant unconformities in the Amadeus Basin. δC and δO from carbonate clasts in diamictite of the Areyonga and Olympic Formations suggest that deposition of Islay- and Trezona-equivalent strata was limited or nonexistent in the Amadeus Basin, as opposed to the possibility that these strata were deposited but subsequently removed by glacial erosion. Our new Amadeus Basin results refine Cryogenian and early Ediacaran correlations among the Amadeus Basin, Adelaide rift complex, and Namibia. The lower part of the Ringwood Member, which was deposited during early stages of the Cryogenian nonglacial interlude, includes two negative δC excursions that are also evident in previous results from the Tapley Hill Formation of the Adelaide rift complex. The upper part of the Ringwood Member contains abundant stromatolites and seems to correlate with Cryogenian stromatolite reefs of the Balcanoona Formation in the Adelaide rift complex. The overlying Limbla Member in the Amadeus Basin correlates with the Yankaninna, Etina, Amberoona, and Enorama Formations of the Adelaide rift complex, as well as carbonate from other continents that record the Keele Peak excursion. δC from the early Ediacaran Olympic cap of the Amadeus Basin is (1) comparable to previous results from the Maieberg and Elandshoek Formations in Namibia; and (2) consistent with diachronous deposition of Marinoan cap dolostone in the Amadeus Basin. Additionally, new δC from the Julie Formation of the Amadeus Basin is similar to previous results from the late Ediacaran Kuibis Subgroup in southern Namibia. These correlations underscore that Ediacaran strata of Namibia, as well as the Cryogenian nonglacial interlude strata of the Adelaide rift complex, are roughly an order of magnitude thicker than stratigraphic counterparts in the Amadeus Basin. The wide disparity in sedimentation rates implied by this finding is likely attributable to fundamental differences in the Neoproterozoic tectonic settings of the three locations.

Sedimentological interpretation of an Ediacaran delta: Bonney Sandstone, South Australia

ABSTRACTThe type section of the late Ediacaran (ca 565 Ma) Bonney Sandstone in South Australia provides an opportunity to interpret a succession of Precambrian clastic sediments using physical sedimentary structures, lithologies and stacking patterns. Facies models, sequence stratigraphic analysis, and process-based architectural classification of depositional elements were used to interpret depositional environments for a series of disconformity-bounded intervals. This study is the first detailed published work on the Bonney Sandstone, and provides additional context for other Wilpena Group sediments, including the overlying Rawnsley Quartzite and its early metazoan fossils. Results show that the ∼300 m-thick section studied here shows a progressive change from shallow marine to fluvially dominated sediments, having been deposited in storm-dominated shelf and lower shoreface environments, lower in the section, and consisting primarily of stacked channel sands, in a proximal deltaic environment near the top. Based on the degree of influence of wave, tidal or fluvial depositional processes, shallow marine sediments can be classified into beach, mouth bar, delta lobe and channel depositional elements, which can be used to assist in predicting sandbody geometries when only limited information is available. Sediments are contained within a hierarchical series of regressive, coarsening-upward sequences, which are in turn part of a larger basin-scale sequence that likely reflects normal regression and filling of accommodation throughout a highstand systems tract. Paleogeographic reconstructions suggest the area was part of a fluvially dominated clastic shoreline; this is consistent with previous reconstructions that indicate the area was on the western edge of the basin adjacent to the landward Gawler Craton. This research fills in a knowledge gap in the depositional history of a prominent unit in the Adelaide Rift Complex and is a case study in the interpretation of ancient deposits that are limited in extent or lacking diagnostic features.

Sedimentology, depositional environments and significance of an Ediacaran salt‐withdrawal minibasin, Billy Springs Formation, Flinders Ranges, South Australia

AbstractThe late Ediacaran Billy Springs Formation is a little‐studied, mudstone‐dominated unit deposited in the Adelaide Rift Complex of South Australia. Sediments are exposed in an approximately 11 km × 15 km synclinal structure interpreted as a salt‐withdrawal minibasin. The stratigraphic succession is characterized by convolute‐laminated slump deposits, rhythmically laminated silty mudstones, rare diamictites and fining‐upward turbidite lithofacies. Lithofacies are the product of deposition in a deepwater slope or shelf setting, representing one of the few such examples preserved within the larger basin. Although exact correlations with other formations are unclear, the Billy Springs Formation probably represents the distal portion of a highstand systems tract, and is overlain by coarser sediments of the upper Pound Subgroup. Diamictite intervals are interpreted to be the product of mass flow processes originating from nearby emergent diapirs, in contrast to previous studies that suggest a glacial origin for extrabasinal clasts. Within the spectrum of outcropping minibasins around the world, the sediments described here are unique in their dominantly fine‐grained nature and overall lithological homogeneity. Exposures such as these provide an opportunity to better understand the sedimentological processes that operate in these environments, and provide an analogue for similar settings in the subsurface that act as hydrocarbon reservoir‐trap systems.

Systematic description of Cryogenian Aralka Formation stromatolites, Amadeus Basin, Australia

Allen, H.-J., Grey, K. & Haines, P.W., November 2015. Systematic description of Cryogenian Aralka Formation stromatolites, Amadeus Basin, Australia. Alcheringa 40, xxx–xxx. ISSN 0311-5518Recognition of stratigraphically constrained stromatolite assemblages has been useful in Australia-wide correlations of Neoproterozoic successions, and in particular in recent Geological Survey of Western Australia and Northern Territory Geological Survey revisions to Neoproterozoic–Cambrian stratigraphy and correlations in the Amadeus Basin, Australia. The Aralka Formation, a proven hydrocarbon source in the Northern Territory, previously mapped in only the northeastern part of the Amadeus Basin, is now recognized across much of the basin. The discovery of new outcrop and drillhole intersections with stromatolite occurrences has prompted systematic revision of stromatolites in the Aralka Formation and analysis of their distribution. The stromatolites include a new Group and Form, Atilanya fennensis, which has a distinctive pillared microstructure giving rise to a characteristic wrinkled lamina pattern of bioherms. The previously defined Tungussia inna is emended to include observations from new localities; robust bridging, columns that rarely develop for more than a few centimetres without interruption as a result of bridges being so profuse, and a wall that ranges from continuous to patchy. In situ occurrences of Tungussia inna are now known from ten localities in the Amadeus Basin, stratigraphically constrained within the Aralka Formation. Likewise, in situ Atilanya fennensis is, thus far, unique to the Aralka Formation, although similar forms elsewhere in the Centralian Superbasin and Adelaide Rift Complex are yet to be investigated.Heidi-Jane Allen [heidi.allen@dmp.wa.gov.au], Kathleen Grey [kath.grey@dmp.wa.gov.au] and Peter Wyatt Haines [peter.haines@dmp.wa.gov.au], Geological Survey of Western Australia, 100 Plain Street, East Perth WA, 6004.