The Sugarloaf Dam Sandstone: a new Cenozoic sandstone unit in the northern Eyre Peninsula
Geological mapping within the Six Mile Hill 1:75,000 map sheet area between Port Augusta and Iron Knob (Fig. 1), undertaken in association with the Mineral Systems Drilling Program (MSDP), identified a number of outcrops of a previously unrecognised Cenozoic lithostratigraphic unit, informally named the ‘Sugarloaf Dam Sandstone’ (Krapf et al. 2016; McAvaney et al. 2016). In this article we formally define the Sugarloaf Dam Sandstone, with descriptions of its distribution, lithology, thickness and contact relationships. We also provide interpretations of depositional environment and sediment provenance based on our sedimentological investigations and recent detrital zircon geochronology. Potential links to nearby Cenozoic sedimentary basins and paleodrainage systems are also discussed.
The Sugarloaf Dam Sandstone occurs in the northeastern corner of Six Mile Hill, where it is exposed in the dissected hilly terrain surrounding Cariewerloo Woolshed, about 25 km west-northwest of Port Augusta (Fig. 2).
The Six Mile Hill area on northern Eyre Peninsula forms part of the southern Gawler Ranges margin, one of South Australia’s emerging mineral provinces. The area partly overlaps with a number of geological provinces, including the Mesoarchean to Mesoproterozoic Gawler Craton, the Mesoproterozoic Cariewerloo Basin and the Neoproterozoic Stuart Shelf, and preserves an extensive history of weathering and erosion during the Phanerozoic and Cenozoic deposition.
The oldest rocks exposed on Six Mile Hill are the c. 1790 Ma Broadview Schist and the c. 1750 Ma Moonabie Formation, which were deposited in volcano-sedimentary basins that developed upon older Archean to Paleoproterozoic crystalline basement of the Gawler Craton. Igneous intrusive and extrusive rocks were emplaced at c. 1775 Ma and c. 1750 Ma. Paleoproterozoic deposition was widely terminated by the transpressional Kimban Orogeny, c. 1740–1690 Ma, which also caused movement along the Roopena Fault (McAvaney et al. 2016).
The earliest Mesoproterozoic history of Six Mile Hill is characterised by the c. 1590 Ma extrusion of felsic to mafic Gawler Range Volcanics, which was associated with the intrusion of Hiltaba Suite granites and volcano-sedimentary deposition in small extensional basins (Curtis, Wade and Reid 2018). This tectono-magmatic event contributes to the prospectivity of the region as it was responsible for the formation of iron oxide – copper–gold (IOCG) deposits in the eastern Gawler Craton and shear-hosted gold in the central Gawler Craton as well as epithermal-style base and precious metal mineralisation along the southern Gawler Ranges margin (Ferris and Schwarz 2003; Fraser, Skirrow and Holm 2007; Reid, Jourdan and Jagodzinski 2017). The Gawler Range Volcanics – Hiltaba Suite event was followed by deposition of fluviolacustrine redbed sediments of the Pandurra Formation in the Cariewerloo Basin at c. 1500 Ma (Cowley 1991a; Beyer et al. 2018).
There is no subsequent geological rock record preserved in the Six Mile Hill area until the Neoproterozoic, when deposition of sediments and minor volcanics occurred within the rift and sag basin complex of the Adelaide Rift Complex and the Stuart Shelf (Preiss 2000). Six Mile Hill forms part of the Stuart Shelf area, where deposition in half-grabens commenced in the early Neoproterozoic with coarse clastic fluvial sediments of the Backy Point Formation (Callanna Group), accompanied by the extrusion of the Beda Basalt and the intrusion of the contemporaneous c. 825 Ma Gairdner Dolerite (Mason, Thomas and Tonkin 1978; Cowley 1991b; Wingate et al. 1998; Wade and McAvaney 2014). After a hiatus of about 180 million years and following the Sturtian glaciation, a major transgression at c. 650 Ma flooded the Stuart Shelf and led to the widespread deposition of marine carbonaceous–dolomitic siltstone and carbonate rocks, the Tapley Hill Formation and the Brighton Limestone (lower Umberatana Group). This was followed by deposition of the silty to sandy Angepena and Wilmington formations in shallow marine to tidal flat environments. The overlying Whyalla Sandstone of the upper Umberatana Group was deposited in the Six Mile Hill area in fluvial to marginal marine – deltaic environments during the Marinoan glaciation. Rapid marine transgression following melting of the ice and climatic warming at c. 635 Ma led to the deposition of the dolomitic Nuccaleena Formation. It is overlain by the shaly to sandy coarsening-upwards succession of the Tent Hill Formation (Wilpena Group), which was deposited in marine environments, initially under low-energy conditions in greater water depths (Tregolana Shale Member) but gradually changing with time to higher energy conditions in shallow waters (Simmens Quartzite Member; McAvaney et al. 2016). The Simmens Quartzite Member is the youngest preserved sedimentary rock of the Adelaidean system in the Six Mile Hill area. In other places in South Australia, the rock record documents sedimentation into Cambrian times up to the onset of the Delamerian Orogeny at about 510 Ma (Preiss 2000).
During the Late Carboniferous to Early Permian a large area of South Australia was exposed to glacial erosion and deposition (Alley et al. 1995). However, there is no evidence for late Paleozoic glacial erosion or deposition preserved on Six Mile Hill. In the Jurassic between about 200 and 170 Ma, kimberlitic magma intruded into shallow crustal levels as dykes and sills or formed explosive diatremes. The ‘Sugarloaf Dam Kimberlite’ on Six Mile Hill occurs in the shallow subsurface at Sugarloaf Dam, where it forms a thin sill within the Neoproterozoic Tregolana Shale Member. It is related to similar kimberlite occurrences at El Alamein 10 km southwest of Port Augusta (Cooper and Morris 2012). Deposition of sediments similar to those found in the Eromanga Basin may have occurred during the Cretaceous in the Six Mile Hill area; however, no relicts are known.
Following the separation of Australia from Antarctica, a number of Cenozoic basins formed at the South Australian continental margin and within the continental interior. The Six Mile Hill area is located near the northern margin of the Pirie Basin and the southern margin of the Torrens Basin. Marine conditions prevailed during the Late Oligocene to Middle Miocene in the Pirie Basin, whereas the deposits of the Torrens Basin are entirely non-marine (Alley and Benbow 1995; Alley and Lindsay 1995). The time of deposition of the Sugarloaf Dam Sandstone on Six Mile Hill probably overlaps with the Eocene to Plio-Pleistocene time-range of sediment deposition in these two basins. Widespread strong chemical alteration and duricrust development also occurred throughout the Cenozoic.
In the Quaternary alluvial fans as well as large landslide deposits comprising blocks of the Simmens Quartzite Member (Thomson 1965) formed along the steep flanks of the silcrete-capped hills of the Simmons Plateau, Tent Hill and Horseshoe Range (Fig. 2; McAvaney et al. 2016). The lowlands are occupied by extensive sandplains dominated by alluvial processes. Clay, sand and gravels were deposited in the Pirie Paleochannel, which today is followed by Myall Creek.
The Sugarloaf Dam Sandstone
Outcrops of the Sugarloaf Dam Sandstone are restricted to an area in the northeastern corner of the Six Mile Hill map sheet, along the foothills of the mesas in the vicinity of Cariewerloo Woolshed and along creek exposures between drillhole MSDP02 (SA Geodata drillhole no. 287995) and Martin Catch Dam (Fig. 2; Krapf et al. 2016).
The type locality for the Sugarloaf Dam Sandstone is a small hill located about 450 m northeast of Sugarloaf Dam on Cariewerloo Station (GDA94, zone 53, 741540mE, 6410005mN; lat. –32.420902°, long. 137.568624°) (Figs 2, 3a; McAvaney et al. 2016). A reference locality lies within the southwestern foothills of South Tent Hill (GDA94, zone 53, 734334.2mE, 6412125.6mN; lat. –32.4033284°, long. 137.4915326°), from which rock sample 2079358 (SA Geodata) was collected for geochronological analysis. Excellent exposures are located within the north–south-oriented creek about 1 km west of drillhole MSDP02.
The Sugarloaf Dam Sandstone is typically a white to light greenish grey or pale brown to pale red, moderately to well-sorted, fine- to medium-grained, rarely coarse-grained, kaolinitic sandstone (Figs 3a, b). Large parts have a massive appearance but variably distinct planar bedding structures ranging from 5 mm thick laminae up to 10 cm thick beds occur in places (Figs 3c–e). The sandstone commonly lacks larger lithic clasts. However, rare isolated cobble-sized well-rounded quartzite clasts were observed in the sandstone (Fig. 3f), which locally also contains rare small isolated shale clasts. The sandstone is primarily composed of angular to subangular, as well as minor well-rounded, quartz grains enclosed in a kaolinitic matrix. In some places the sandstone shows mottling of variable intensity. Small nodular mottling and larger tubular structures within the massive facies of the Sugarloaf Dam Sandstone are interpreted to represent rhizocretions and rhizomorphs (Figs 4a, b). Well preserved root casts have been observed in the creek exposures north of Corraberra Hill (Fig. 4c). Locally, the Sugarloaf Dam Sandstone grades laterally into a finer grained, friable to powdery, clay-rich material with scattered quartz grains. Mild silcretisation has turned this material into a cream-coloured, massive, slightly porous porcellanitic to travertine-like material (Fig. 4d).
The upper part of the Sugarloaf Dam Sandstone is often variably indurated by silica, forming a silcrete crust with nodular and laminar pedogenic silcrete fabrics. Columnar silcrete is best developed in the up to 5 m thick sandstone outcrops within the creek located between drillhole MSDP02 and Martin Catch Dam (Fig. 2). A variety of silcrete morphologies have formed in this white to cream coloured or pale yellowish, moderately to well-sorted, medium-grained sandstone. The most impressive silcrete morphology is the formation of two in situ developed columnar silcrete horizons, each up to 70 cm thick and exposed over nearly 50 m along the cut bank of the creek (Fig. 5a). The two silcrete horizons are 50 cm apart and separated by slightly silicified, planar-bedded sandstone. Individual silcrete columns are 10–20 cm wide (Fig. 5b) with the space between them often occupied by smaller scale columns (Fig. 5c). These two pedogenic columnar silcrete horizons may have formed as a response to changes in soil moisture during their formation. Siliceous ‘candle wax drops’ associated with columnar and glaebular silcrete have also been observed within the Sugarloaf Dam Sandstone (Fig. 6a–f). Individual glaebules are 0.5–4 cm in diameter and have multiple silica- and titania-rich cutans of variable thickness ranging from microscopic up to several millimetres (Figs 6c–f). The core of the pedogenic glaebules consists of reworked silicified quartzose sediment or silcrete clasts (Figs 6d–f).
The Sugarloaf Dam Sandstone unconformably overlies intensively weathered shales and minor sandstones of the Tregolana Shale Member (Fig. 7a). In many places the top of the Sugarloaf Dam Sandstone is indurated by a silcrete cap (Figs 3a, d). The Sugarloaf Dam Sandstone is unconformably overlain by coarse-grained, commonly gravelly sediments. For example, along the creek exposures between drillhole MSDP02 and Martin Catch Dam, the Sugarloaf Dam Sandstone is erosively overlain by Quaternary fluvial conglomerates (Fig. 7b). Locally, paleolandslide masses characterised by accumulations of Simmens Quartzite Member blocks also directly overlie the Sugarloaf Dam Sandstone.
The preserved thickness of the Sugarloaf Dam Sandstone is mainly between 1 to 2 m. Sandstone exposures along the creek between drillhole MSDP02 and Martin Catch Dam locally reach up to 5 m in thickness.
The detrital grains of the Sugarloaf Dam Sandstone are almost entirely composed of quartz implying a high compositional maturity. However, most detrital quartz grains are highly angular as seen in thin section. Furthermore, the sandstone matrix contains a significant amount of kaolinitic clays. This kaolin was probably not formed by in situ weathering of the sandstone as no altered detrital feldspar grains were observed in thin section. This indicates that the kaolinitic clays are largely of detrital origin and were derived from a source that contained intensively weathered rocks. The angularity of most detrital quartz grains, the significant matrix content of the sandstone and the resulting moderate sorting thus record a rather low textural maturity for the Sugarloaf Dam Sandstone. This suggests short transport distance from source to sink.
The Sugarloaf Dam Sandstone has mainly a massive to minor vaguely bedded character. The sparseness of bedding can be attributed to some degree to local destratification by plant bioturbation but is probably mostly a primary sedimentary feature. The rare bedding structures in the Sugarloaf Dam Sandstone usually have a planar geometry. These bedding characteristics, the rare occurrence of isolated, typically oversized quartzite clasts, and the matrix-rich nature of the sandstone suggest sediment transport in, and deposition from, sheet-like mass flows under high-energy conditions in an alluvial–fluvial environment. These mass flows were probably triggered by occasional high rain falls, which episodically redistributed kaolinitic clays and resistive detrital quartz grains mainly derived from the underlying intensively weathered shales and sandstones of the Tregolana Shale Member. Occasionally these flows also transported some rare larger quartzite clasts (Fig. 3f), which were most probably derived from the Simmens Quartzite Member that caps the hills surrounding the depositional area of the Sugarloaf Dam Sandstone.
The currently known outcrop distribution of the Sugarloaf Dam Sandstone implies that it was deposited within an intermontane basin or sheltered embayment surrounded by the high tablelands composed of Neoproterozoic rocks of the Tent Hill Formation (Wilpena Group), e.g. the Horseshoe Range in the northwest, South Tent Hill in the northeast, as well as Corraberra Hill and Nutt Knob in the southwest (Fig. 2). This landscape formed in the Cenozoic (McAvaney et al. 2016).
The presence of root casts, rhizocretions and plant bioturbation structures in the Sugarloaf Dam Sandstone rules out a Neoproterozoic depositional age. Its distribution is restricted to a Cenozoic intermontane basin in the South Tent Hill area (Fig. 2), which formed by incision and dissection of a paleosurface developed within the Neoproterozoic Simmens Quartzite Member. This paleosurface was named ‘Tent Hill surface’ by Twidale, Shepherd and Thomson (1970) and is preserved in the study area as silcreted mesa tops of South Tent Hill, the Horseshoe Range and Corraberra Hill (Figs 2, 8), and in the wider region as the Simmens–Arcoona Plateau. This topographically high-lying silcrete surface is correlated with the Cordillo Silcrete surface (Benbow 1982; Webb 2006; Greenfield, Gilmore and Mills 2010).
Following the deposition of the Sugarloaf Dam Sandstone within this erosional basin, it was affected by a silicification event that resulted in the formation of a topographically low-lying silcrete surface within the study area. This lower silcrete surface was named ‘Corraberra surface’ by Twidale, Shepherd and Thomson (1970) and can be correlated to the ‘Beda surface’ of Hutton et al. (1972), which lies below the higher Arcoona Plateau. Consequently, the deposition of the Sugarloaf Dam Sandstone must have occurred between the formation of the older and higher lying silcreted paleosurface of the Simmens Plateau (Cordillo Silcrete) and the formation of the younger and lower lying silcrete (Corraberra and Beda surfaces) that developed at the top, and locally within, the Sugarloaf Dam Sandstone (Fig. 8).
Evidence from other areas in South Australia, based on host rock ages and basinal stratigraphy, indicates that there were at least three major episodes of silcrete formation. The oldest one occurred in the Late Cretaceous, followed by two younger Cenozoic phases, one in the mid to late Paleogene (Cordillo Surface of Late Eocene to Early Oligocene age according to Wopfner 1978; Alley 1998; Croke, Magee and Price 1998; Webb 2006) and another one in the mid- to late-Neogene (Lintern and Sheard 1998; Sheard and Callen 2000). The silcrete surface at the top of the Simmens Plateau is attributed to the Paleogene silcretisation episode (Hutton et al. 1972). This implies that the silcretisation of the Sugarloaf Dam Sandstone occurred during the younger silicification phase in the Neogene, which places the deposition of the Sugarloaf Dam Sandstone between two major Cenozoic silcretisation events. Consequently, the depositional age of the Sugarloaf Dam Sandstone probably lies within the Oligocene–Miocene time period.
The silcretised top of the Sugarloaf Dam Sandstone is at its type locality near Sugarloaf Dam unconformably overlain by paleolandslide blocks of the Simmens Quartzite Member (McAvaney et al. 2016). Thomson (1965) has reported that in the South Tent Hill area these slumped quartzite blocks show pronounced secondary silicification, i.e. this silcrete developed after deposition of the landslide blocks. However, we are uncertain if the silcretisation of the Sugarloaf Dam Sandstone and the silcretisation of the landslide deposits are two consecutive, temporally distinct silcrete formations at lower topographic levels or if the Sugarloaf Dam Sandstone and the overlying quartzite blocks were silcretised together at the same time after the landslides happened.
To further constrain the age of the Sugarloaf Dam Sandstone, and to characterise more thoroughly its provenance, geochronological dating of detrital zircons was undertaken.
Detrital zircon geochronology
Geochronology sample location
Sample 2079358 was collected for detrital zircon geochronology from the dissected hilly area between Sugarloaf Hill in the west and South Tent Hill in the east (Fig. 2). The sample locality lies within a small gully, near the gully head, which eroded into the northern slope of a low hill complex exposing the Sugarloaf Dam Sandstone along the gully walls (Figs 9a, b). The Sugarloaf Dam Sandstone at this site is overlain to the southwest by landslide masses composed of large Simmens Quartzite Member blocks. The underlying shales and sandstones of the Tregolana Shale Member are exposed in situ about 400 m north of the gully head.
Figure 9 Sugarloaf Dam Sandstone at collection site of geochronology sample 2079358.
Sample lithology and petrography
The Sugarloaf Dam Sandstone at the geochronology sample locality is a white to pale red, fine- to coarse-grained, moderately sorted, kaolinitic and variably silicified quartzose sandstone (Figs 9a–f). It has largely a massive appearance (Fig. 9b) but in places horizontal bedding can be observed (Figs 9c–d). The sparseness of bedding can probably be attributed to extensive destratification by plant bioturbation.
Geochronology sample 2079358 is a cream-coloured to pale buff, fine- to medium-grained sandstone consisting of clastic quartz grains enclosed in a kaolinitic clay matrix, with rare larger (~5 mm) rounded clasts of reddish brown shale. The sandstone sample shows no sedimentary bedding structures but exhibits an indistinct laminar mottling due to variably intense silcretisation (Fig. 9f).
In thin section the sandstone displays a moderately sorted, clast- to matrix-supported, arenaceous siliciclastic sedimentary fabric. Clastic grains are nearly entirely composed of quartz, mostly 0.1–0.5 mm in size and uniformly distributed throughout the rock (Fig. 10a). These quartz grains are mainly angular to subangular, with larger grains tending to show better rounding. Some of these larger grains are exceptionally well rounded and show a high degree of sphericity (Fig. 10b). However, many of the larger grains are actually angular composite detrital grains. These composite grains consist of a well-rounded quartz grain core that has a partly eroded relict quartz overgrowth attached (Figs 10c–d). This shows that the well-rounded quartz grains did not obtain their rounding during sedimentary transport that led to the deposition of the Sugarloaf Dam Sandstone but that these rounded grains are inherited from an older sedimentary rock and thus represent a recycled component. The quartz grains are mostly monocrystalline and many show shadowy strain effects suggesting derivation from a metamorphic source. Some quartz grains are clouded by abundant fluid inclusions and some contain acicular crystal inclusions indicating derivation from hydrothermal vein quartz. Very rare are chert-like quartz grains showing a microgranular internal grain texture. No fresh or altered feldspar or lithic grains were observed. Accessory detrital zircon and yellowish brown titanium oxide crystals occur in trace amounts. Detrital opaque iron–titanium oxide grains are extremely rare and usually strongly altered and corroded.
The fine-grained matrix under plane-polarised light varies from translucent to almost opaque, depending on the intensity of silcretisation. Translucent portions of the matrix with a lower intensity of silcretisation consist of a microgranular mosaic of quartz, clay minerals (mainly kaolinite) and titania (probably anatase; Fig. 10e). In contrast, in intensely silcretised portions of the matrix, this microgranular mosaic appears to be impregnated by a micro- to cryptocrystalline silica–titania cement. This cement appears black in plane-polarised light under normal light illumination (Fig. 10a) but displays a honey-like yellowish brown colour under high power illumination with interposed condenser lens (Fig. 10f). Under reflected light, small (<5 µm) highly reflective crystals of secondary iron–titanium oxides can be seen scattered throughout the fine-grained matrix.
Figure 10 Sugarloaf Dam Sandstone, geochronology sample 2079358.
Zircons from a sample of the Sugarloaf Dam Sandstone (sample 2079358; Fig. 2) were separated via standard crushing, density and magnetic separation methods at a commercial laboratory, Geotrack, Melbourne. The zircons were then mounted in epoxy resin and polished to expose the interior of the grains. Zircons were imaged using transmitted and reflected light microscopy along with cathodoluminescence imaging techniques using a Quanta 600 scanning electron microscope.
U–Pb isotopes in zircons were analysed at Adelaide Microscopy, Adelaide University, with an ASI RESOlution ArF excimer laser ablation system with a S150 large format sample chamber. The laser was coupled to an Agilent 7900x inductively coupled plasma - mass spectrometry (ICP-MS). The spot size utilised was 30 µm with a fluence at the sample of ~2 J/cm2 and a repetition rate of 5 Hz. Samples were ablated in a He atmosphere (flow rate 0.35 L/min) and the aerosol mixed with Ar carrier gas (flow rate 1.01 L/min) for transport to the ICP-MS. An analysis consisted of 30 seconds of gas background collected while the laser was not firing followed by 30 seconds of ablated signal. The following isotopes were measured with counting times in brackets: 204Pb (10 ms), 206Pb (15 ms), 207Pb (30 ms), 208Pb (10 ms), 232Th (10 ms), and 238U (15 ms), giving a total sweep time of ~0.1 seconds. The GJ zircon (Jackson et al. 2004) was used as the primary standard to correct for downhole fractionation and instrument drift and mass bias. Four analyses of GJ–1 were analysed every 10 unknown zircons.
In addition, analysis of reference zircons of known age was also undertaken to check instrument performance. During the October 2016 session reference zircons 91500 (1065.4 ± 0.3 Ma; Wiedenbeck et al. 1995), 206Pb/238U standard Plešovice (337.13 ± 0.37 Ma; Sláma et al. 2007) and 207Pb/206Pb standard OGC-1 (3465.4 ± 0.6 Ma; Stern et al. 2009) were measured concurrently with the unknown zircons. Results of analysis of these reference zircons are detailed in Appendix Table A1.
Data from the ICP-MS was processed using the Iolite v2.5 software (Paton et al. 2011). U and Th decay constants of Jaffey et al. (1971), as recommended by Steiger and Jäger (1977), have been utilised. All time-resolved isotope analyses were reviewed to check the integrity and consistency of the downhole analytical trace. Where zones of compositional change and/or increased 204Pb were encountered during the analysis, portions of the 30 second analysis were selected to integrate for age information for the individual zircon that excluded such compromised domains.
Note that although 204Pb was monitored during the analysis, isobaric interference by 204Hg present in the Ar–He carrier gas and the low overall 204Pb counts, mean that no correction for common (non-radiogenic) Pb has been made for this data. Weighted mean ages were calculated using Isoplot 3 (Ludwig 2003). Probability density distributions have been constructed using AgeDisplay (Sircombe 2004). A summary of the U–Th–Pb data and derived ages presented in this report are given in Appendix Table A2.
Zircons from this sample are variable in shape and colour (Fig. 11a). The majority of grains are transparent; however, some grains also have brown discolouration, possibly a result of iron oxide staining. The zircons are generally between ~100 and 300 µm in length and have rounded to subrounded morphology. The rounding of the grains is consistent with surficial transport.
Cathodoluminescence images reveal a variety of internal morphologies as expected for a detrital suite (Fig. 11b). Many grains have irregular or diffuse zonation suggesting modification by high-temperature processes (metamorphism). Most grains have some form of oscillatory zonation suggestive of derivation from igneous rock.
One hundred zircons were analysed from this sample. The data ranges from concordant to severely discordant and has a range of ages from as old as c. 3000 Ma to as young as c. 656 Ma (Fig. 12; Appendix Table A2). Analysis 358-54 is compromised by non-radiogenic Pb and is not considered further. Data that is >10% discordant has been affected by Pb loss, although for the majority of these analyses the corresponding 207Pb/206Pb age is broadly similar to those derived from more concordant analyses (Fig. 13).
The concordant to near concordant data (n = 83) yields a number of age populations, with two age ranges being the most volumetrically significant: c. 1350–1060 Ma and c. 690–656 Ma (Fig. 13). Forty-seven zircons yield ages in the range c. 1350–1060 Ma age. This range produces a broad curve on a probability density distribution with a maximum at c. 1180 Ma (Fig. 13). One outlier at the younger end of this range at c. 1012 Ma forms a single spike in the probability density curve.
Sixteen zircons have 206Pb/238U ages in the range c. 690–656 Ma. Within this range, smaller age peaks can be identified at 656 Ma, 668 Ma and 687 Ma. One grain yields a slightly younger age of 646 ± 4 Ma and lies slightly younger than the c. 656 Ma peak.
The remaining zircons yield a range of Mesoproterozoic and Paleoproterozoic ages from c. 2120 Ma to c. 1340 Ma (Fig. 13). The majority of these age ‘peaks’ in the probability density distribution are defined by one analysis, with the exception of six analyses with ages ranging from c. 1636 ± 26 (analysis 358-79) to 1605 ± 18 Ma (analysis 358-88) that form a minor age population centred at c. 1618 Ma.
Three grains yield Archean 207Pb/206Pb ages (358-53, 2700 ± 79 Ma; 358-7, 3033 ± 8 Ma; 358-46, 3340 ± 150 Ma); however, all are strongly discordant and the age significance of these zircons is uncertain.
In the discussion below we describe the potential primary igneous or metamorphic source rocks for the provenance populations of the Sugarloaf Dam Sandstone, as well as secondary (meta)sedimentary sequences that represent potential sources for recycled zircons.
The oldest zircons from the Sugarloaf Dam Sandstone sample have Archean ages between c. 3340 and 2700 Ma. These ages correspond with the age range of the oldest known basement of the Gawler Craton, including the Cooyerdoo Granite and granitic gneisses of the Middleback Range that crop out in the northeastern Eyre Peninsula to the west of Six Mile Hill. These granites and gneisses range from c. 3250 to 3000 Ma in age, contain inherited zircons as old as c. 3400 Ma, and document metamorphic zircon overgrowth at c. 2.6 Ga (Fraser et al. 2010; Jagodzinski, Reid and Farrell 2011; McAvaney 2012; Keyser et al. 2019).
Recycled zircons of this age range could be derived from volcano-sedimentary rocks of the Sleaford Complex on the western Eyre Peninsula or metasedimentary rocks of the Middleback Group on the eastern Eyre Peninsula, which were deposited before c. 2450 Ma (Teale, Schwarz and Fanning 2000; Szpunar et al. 2011).
The detrital zircon age spectrum of the Sugarloaf Dam Sandstone has a minor middle Paleoproterozoic component with single zircon ages at c. 2120 and 2030 Ma, which might be partly derived from the c. 2000 Ma Miltalie Gneiss (Fanning, Reid and Teale 2007). A more common later Paleoproterozoic component is represented by single zircons ages of c. 1860 Ma, c. 1790 Ma, and 1680 Ma, as well as a c. 1620 Ma zircon population represented by six grains. These four ages correspond well with a number of igneous rocks and suites of the Gawler Craton, namely the c. 1865 Ma felsic metavolcanics of the Bosanquet Formation (Fanning, Reid and Teale 2007), the c. 1850 Ma granitoids of the Donington Suite (Reid and Hand 2012), the c. 1790 Ma Myola Volcanics (Fanning et al. 1988; Fraser and Neumann 2010), the c. 1690–1670 Ma granitoids of the Tunkillia Suite (Ferris and Schwarz 2004; Payne et al. 2010), and the c. 1630–1610 Ma igneous rocks of the St Peter Suite (Reid et al. 2019). Most of these Paleoproterozoic igneous rocks are exposed today on the northeastern Eyre Peninsula, except for the Tunkillia and St Peter suites of the northwestern Eyre Peninsula.
Interestingly, igneous zircons from the c. 1775 Ma Tip Top Granite, the c. 1755 Ma McGregor Volcanics and the associated volcaniclastic Moonabie Formation, as well as the c. 1750–1740 Ma Moola Suite granites and felsic intrusives, which are exposed today within the Six Mile Hill map sheet area (McAvaney et al. 2016), appear not to be present in the detrital zircon age spectrum of the Sugarloaf Dam Sandstone.
Potential sources for the Paleoproterozoic zircon population could also be recycled zircons from metasedimentary sequences exposed on northeastern Eyre Peninsula, such as the Darke Peak Group, deposited between c. 2000 Ma and c. 1865 Ma (Szpunar et al. 2011). Slightly younger sequences include the Broadview Schist and the Cleve Group, deposited at c. 1795–1775 Ma and c. 1780–1730 Ma, respectively (Szpunar et al. 2011; McAvaney et al. 2016). Both sequences show prominent c. 1860 and c. 1790 Ma detrital zircon peaks. Another potential source for recycled Paleoproterozoic zircons is the c. 1650 Ma Corunna Conglomerate, which is exposed in the Baxter and Uno ranges to the west of Six Mile Hill. The Corunna Conglomerate from these ranges contains a prominent 1690–1660 Ma zircon population, but also a prominent 1750–1720 Ma population (Fraser and Neumann 2010); the latter not present in the Sugarloaf Dam Sandstone.
A more distal potential source for c. 1680 and c. 1620 Ma zircons may be meta-igneous and metasedimentary rocks of the Birksgate Complex in the eastern Musgrave Province, which have an age span of c. 1700–1550 Ma (Wade et al. 2006; Krapf et al. 2018).
The main age population of the Sugarloaf Dam Sandstone sample is late Mesoproterozoic and is represented by 47 zircon analyses. This population forms a broad curve on the probability density distribution between c. 1300 and c. 1060 Ma, with a maximum at c. 1180 Ma (Fig. 13). Zircons of this late Mesoproterozoic age range are not represented in the crystalline basement geology of the Gawler Craton. However, this age range overlaps with the 1220–1140 Ma age range of the Pitjantjatjara Supersuite granites of the Musgrave Province (Howard et al. 2015; Krapf et al. 2018). Furthermore, zircons from the younger margin of this provenance peak with ages between c. 1100–1060 Ma can be ascribed to c. 1090–1040 Ma igneous rocks of the Warakurna Supersuite (Giles Event) of the Musgrave Province (Howard et al. 2015; Smithies, Howard et al. 2015; Smithies, Kirkland et al. 2015). The slightly younger c. 1000 Ma age spike in the probability density curve is difficult to interpret as there are no known potential zircon sources of that age in South Australia.
The geochronology sample of the Sugarloaf Dam Sandstone contains a very subordinate population of early Mesoproterozoic zircons with ages around 1525, 1445 and 1340 Ma. Potential sources for such zircons are rare within the Gawler Craton but include the c. 1500 Ma Spilsby Suite (Fanning, Reid and Teale 2007) as well as c. 1460 Ma unnamed granites from the Nawa Domain (Morrissey et al. 2019). The c. 1525 Ma Bunburra Suite of the Coompana Province (Wise, Pawley and Dutch 2018) would represent a distal potential source. Another potential source for some of the early Mesoproterozoic zircons are c. 1400–1360 Ma metasediments of the eastern Musgrave Province (Wade et al. 2008).
Interestingly there is an absence of early Mesoproterozoic, c. 1595–1575 Ma zircons of the Hiltaba Suite – Gawler Range Volcanics event in the Sugarloaf Dam Sandstone. The Gawler Range Volcanics, Hiltaba Suite granites and the contemporaneous volcano-sedimentary Fresh Well Formation all occur within the Six Mile Hill area (McAvaney et al. 2016), with the Gawler Range Volcanics cropping out extensively in the northwestern part of the map sheet in fairly close proximity to the outcrop area of the Sugarloaf Dam Sandstone (Fig. 1). The slightly younger c. 1500 Ma redbed Pandurra Formation is also widely exposed on Six Mile Hill, with a fluvial sandstone sample from Red Rock Hill near Roopena dominated by c. 1575 Ma zircons (Fraser and Neumann 2010). The absence of c. 1.6 Ga early Mesoproterozoic zircons suggests the Gawler Range Volcanics and the Pandurra Formation are unlikely sources for the Sugarloaf Dam Sandstone despite their present-day proximity.
Neoproterozoic zircons form the second-most important age population of the Sugarloaf Dam Sandstone. Sixteen zircons yielded mid-Neoproterozoic (Cryogenian) ages in the range c. 690–645 Ma (Fig. 13). Within this range, smaller age peaks are identified at 687, 668 and 656 Ma. One grain yielded a slightly younger age of 646 ± 4 Ma.
Felsic igneous rocks that could form the source for these Cryogenian zircons are not known from South Australia. However, such zircons have been documented in Neoproterozoic sedimentary rocks of the Adelaide Rift Complex and the adjacent Stuart Shelf. A single detrital 657 ± 17 Ma zircon was reported from the late Cryogenian Marino Arkose Member (upper Umberatana Group; Ireland et al. 1998). A tuffaceous horizon within the glaciogenic Wilyerpa Formation (lower Umberatana Group) was originally dated at 659 ± 6 Ma (Fanning and Link 2008). More recently, a more precise CA-ID-TIMS U–Pb zircon age of 663.03 ± 0.11 Ma was determined for this volcaniclastic layer (Cox et al. 2018), dating the final phase of the Sturtian glaciation in South Australia. The underlying tillitic Bolla Bollana Tillite also contains a Cryogenian zircon population with an age range of c. 690–650 Ma (Cox et al. 2018), matching well with the age range of Cryogenian zircons in the Sugarloaf Dam Sandstone. Furthermore, Cryogenian zircons also occur in the Yaltipena Formation, the Elatina Formation and the Whyalla Sandstone of the upper Umberatana Group (Rose et al. 2013).
This indicates that the c. 690–645 Ma Cryogenian zircons in the Sugarloaf Dam Sandstone are likely sourced from Neoproterozoic sedimentary rocks of the Stuart Shelf area and/or of the Adelaide Rift Complex, which contain these zircons as a syn-depositional primary pyroclastic component or as a secondary recycled component. A potential proximal source are the sedimentary deposits of the Tent Hill Formation (lower Wilpena Group) that immediately underlie and surround the outcrop area of the Sugarloaf Dam Sandstone. These rocks were deposited after 635 Ma in early Ediacaran times, based on the inferred depositional age of the underlying Nuccaleena Formation (Rose et al. 2013). Other Neoproterozoic sedimentary rocks that underlie the Nuccaleena Formation in the study area, such as the Whyalla Sandstone, also form potential proximal sources. Neoproterozoic sediments of the Adelaide Rift Complex, exposed today in the Flinders Ranges to the east and northeast of the study area, represent more distal potential sources.
As no local Cryogenian magmatism has been documented within South Australia, the c. 690–650 Ma ‘exotic’ zircons, preserved as primary pyroclastic or secondary detrital components within Neoproterozoic sedimentary rocks of the Adelaide Rift Complex and the Stuart Shelf, must be ultimately derived from a distal volcanic source. The nearest known volcanic source for these Cryogenian zircons is located in Antarctica (Veevers et al. 2006). The metasedimentary, partly volcaniclastic–tuffaceous Skelton Group of South Victoria Land contains clasts of synsedimentary rhyolitic volcanics dated at c. 650 Ma (Cooper et al. 2011). This Cryogenian volcanism is inferred geochemically to relate to extension in a continental rift within East Antarctica. These metasedimentary rocks of the Skelton Group also contain detrital zircons with an age range of c. 680–630 Ma, matching well with the age range of the Cryogenian zircon population in the Sugarloaf Dam Sandstone. Other metasedimentary sequences of South Victoria Land and adjacent Ross Orogen areas, whose sedimentation ages are inferred to be similar to the Skelton Group, contain a slightly larger detrital zircon age range between c. 710–600 Ma (Cooper et al. 2011). Cryogenian granitic magmatism is documented in Western Australia within the basement of the Canning Basin (c. 654 Ma; Haines et al. 2018) and for the northern Paterson Orogen in form of the c. 655–625 Ma O’Callaghans Supersuite (Johnson 2013; Maidment 2017).
Despite the interpreted Cenozoic age of the Sugarloaf Dam Sandstone the sample did not contain any Phanerozoic zircons.
The presence of mainly angular quartz grains in the Sugarloaf Dam Sandstone, including composite quartz grains with well-rounded core and relict quartz overgrowths, suggests derivation from a proximal local source and short transport distances from source to sink. The most likely source are the Neoproterozoic sedimentary rocks of the Tent Hill Formation (lower Wilpena Group), which immediately underlie and surround the outcrop area of the Sugarloaf Dam Sandstone. The well-rounded quartz grains with quartz overgrowths are probably derived from the Simmens Quartzite Member, which was deposited in a high-energy shallow marine environment and later silicified. The kaolinitic matrix of the Sugarloaf Dam Sandstone is probably mainly derived from the underlying intensively weathered Tregolana Shale Member.
Detrital zircon geochronology of the Sugarloaf Dam Sandstone shows that other locally outcropping rock units, including the Mesoproterozoic Pandurra Formation, the felsic Gawler Range Volcanics and the Paleoproterozoic felsic volcaniclastic Moonabie Formation, are unlikely to be a source due to the absence of c. 1590 Ma and c. 1750 Ma zircons, respectively.
The two most significant provenance populations of the Sugarloaf Dam Sandstone are represented by a group of late Mesoproterozoic zircons with an age range of c. 1350–1060 Ma (47 grains) and by a group of mid-Neoproterozoic, Cryogenian zircons of c. 690–656 Ma (16 grains). A less prominent population of latest Paleoproterozoic zircons with an age of about 1620 Ma is represented by six grains.
The presence of Neoproterozoic–Cryogenian zircons supports the interpretation that sedimentary rocks of the Umberatana and Wilpena groups are the main detritus source for the Sugarloaf Dam Sandstone. The Cryogenian zircons in these sedimentary rocks of Ediacaran age represent a recycled component derived from reworking of slightly older, Cryogenian sedimentary rocks of the Adelaide Rift Complex such as the Wilyerpa Formation (lower Umberatana Group), which contains volcanic zircons of pyroclastic ash fall origin (Cox et al. 2018). The volcanic source of these Cryogenian pyroclastic zircons is most probably located in eastern Antarctica, where rhyolitic rift volcanics are preserved within the Skelton Group (Cooper et al. 2011).
The latest Paleoproterozoic to late Mesoproterozoic zircons most probably originate from the Musgrave Province in central Australia. However, it is unlikely that they have been transported from their source region directly to the Cenozoic sedimentary basin where the Sugarloaf Dam Sandstone was deposited. Rather, it is much more likely that these zircons were re-eroded from a secondary source, in this case likely Neoproterozoic rocks of the Stuart Shelf and/or the Adelaide Rift Complex. Zircons from the Musgrave Province are known to be present in abundance throughout the Neoproterozoic successions of the Adelaide Rift Complex and the Stuart Shelf (Gehrels, Butler and Bazard 1996; Ireland et al. 1998; Preiss 2000; Reid et al. 2013; Rose et al. 2013;). In a study on Neoproterozoic sediments deposited during the Marinoan glaciation, Rose et al. (2013) have documented that the Whyalla Sandstone of the Stuart Shelf and the age-equivalent Elatina Formation of the Adelaide Rift Complex show distinct differences in their zircon populations, with the former having a Gawler Craton dominated provenance and the latter containing a large proportion of zircons derived from the Musgrave Province. Consequently, the Musgrave Province dominated detrital zircon spectrum of the Sugarloaf Dam Sandstone suggests also significant detrital input from a more distal source within the Adelaide Rift Complex area such as the uplifted Flinders Ranges to the east of the study area.
Furthermore, the Musgrave Province also forms a major source for Ordovician sandstones of the Lachlan Orogen and the adjacent Gnalta Shelf in southeastern Australia (Glen et al. 2017), located to the east of the Adelaide Rift Complex. Interestingly, these lower Paleozoic sedimentary rocks contain not only Mesoproterozoic zircons derived from the Musgrave Province but also Cryogenian zircons derived from c. 650 Ma intracontinental rift volcanism in East Antarctica (Glen et al. 2017; Cooper et al. 2011).
The subordinate group of Archean to Paleoproterozoic zircons in the Sugarloaf Dam Sandstone with an age range of >3300–1780 Ma can be ascribed to the Gawler Craton basement (Reid and Hand 2012). However, grains of this age range probably also present a minor detrital component within Neoproterozoic sedimentary rocks, which have been recycled into the Sugarloaf Dam Sandstone.
The absence of Phanerozoic zircons in the Tertiary Sugarloaf Dam Sandstone virtually excludes the Mesozoic sediments of the Eromanga Basin as a potential source.
The Sugarloaf Dam Sandstone – possible link to the Pirie and Torrens basins
The recent identification and mapping of the Sugarloaf Dam Sandstone on Six Mile Hill (Krapf et al. 2016; Fig. 2) raises the question of its possible relationship to the nearby Cenozoic Pirie and Torrens basins. The northern part of the Pirie Basin consists of two parallel narrow north–south-oriented depocentres, the so-called ‘Pirie Paleodrainage’ (Hou et al. 2012) in the west and the upper Spencer Gulf area in the east (Fig. 14). Outcrops of the Sugarloaf Dam Sandstone are located just a few kilometres north of the Pirie Paleodrainage and about 30 km southwest of the Pirie–Torrens basin boundary (Fig. 14; Alley and Lindsay 1995; Alley and Benbow 1995).
The Pirie Paleodrainage is filled with up to 50 m thick sandy clays and clayey, locally gritty or gravelly sands of Cenozoic age. Drainage in the study area during deposition of the Sugarloaf Dam Sandstone was likely to the south following the Pirie Paleodrainage. Modern drainage in the South Tent Hill area is dominantly directed to the east via two creeks flowing towards Lake Dempsey west of Port Augusta. Correlation between the paleovalley fill and the Sugarloaf Dam Sandstone is problematic due to the uncertain age of the paleodrainage sediments.
The Cenozoic sediments in the Pirie Basin were interpreted to be continuous with those of the Lake Torrens lowland, and the two areas were originally referred to as the Pirie–Torrens Basin. However, sediments beneath the Torrens Basin are exclusively non-marine, with deposition commencing in the Early Eocene, whereas those of the Pirie Basin contain marine facies, with deposition commencing in the Late Eocene; hence the two are considered as two discrete basins (Alley and Lindsay 1995). Nevertheless, the nature of the boundary between these two basins is still unclear.
Based on our inferred Oligocene to Miocene age of the Sugarloaf Dam Sandstone, possible correlatives would be the broadly age-equivalent Neuroodla Formation of the Torrens Basin, which was deposited in terrestrial playa lake and floodplain environments at the same time as the marine Melton Limestone was deposited in the Pirie Basin (Alley and Lindsay 1995). In the Wilkatana Salt 1 drillhole (DH 148497; Sprigg and Papalia 1964), 40 km north-northeast of the outcrops of the Sugarloaf Dam Sandstone, a 7 m thick, very soft, red-brown fine- to medium-grained clayey sandstone was intersected between 7.3 and 14.3 m within a clayey sandstone interval in the uppermost 24.4 m of the drillhole. This sandstone interval is part of the non-marine sedimentary succession of the interior Torrens Basin. It forms part of the Neuroodla Formation, which is typically composed of black argillaceous and white calcareous–dolomitic mudstones. These fine-grained sediments are interpreted as deposits within a floodplain and playa lake environment and are correlated with the Early to Middle Miocene Etadunna Formation of the Lake Eyre Basin (Johns 1968; Johns et al. 1981). The suggested age of the Neuroodla Formation is broadly equivalent to our age estimate for the Sugarloaf Dam Sandstone.
The outcrop area of the Sugarloaf Dam Sandstone can be considered as a western outlier of the Torrens and Pirie basins, which experienced subsidence and sediment deposition in the Cenozoic while the Flinders Ranges were rising to the east (Alley and Benbow 1995; WV Preiss, Geological Survey of South Australia, GSSA, pers. comm. 2019). Consequently, distal detrital input from the Flinders Ranges in addition to a more proximal local sediment source seems highly plausible for the Sugarloaf Dam Sandstone.
Appendix: Zircon geochronology data
A summary of zircon geochronology data and derived ages for Sugarloaf Dam Sandstone sample 2079358 is attached to this article (XLSX 32 KB).
We would like to thank Wolfgang Preiss (GSSA) and Jarred Lloyd (University of Adelaide) for their very helpful and constructive reviews, which significantly improved the manuscript.
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