Journal of the Geological Society

Neoproterozoic loess and limits to snowball Earth

Gregory J. Retallack

Journal of the Geological Society 2011; v. 168; p. 289-308 doi: 10.1144/0016-76492010-051

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© The Geological Society of London Journal of the Geological Society, London, Vol. 168, 2011, pp. 289–307. doi: 10.1144/0016-76492010-051.

Neoproterozoic loess and limits to snowball Earth

GREGORY J. RETALLACK Department of Geological Sciences, University of Oregon, Eugene, OR 97403, USA (e-mail: [email protected])

Abstract: Neoproterozoic tillites overlain by limestones and dolostones (cap carbonates) have been interpreted as evidence of abrupt climate change from glaciers to tropical seas on the assumption that cap carbonates were marine. This assumption is here challenged by evidence for loess deposition of the Nuccaleena Formation of South Australia from granulometry (angular, randomly oriented, silt, modal diameter 7) and sedimentary structures (climbing-translatent cross-stratification, linear dunes). In addition, a variety of palaeosols in the Nuccaleena Formation have red clayey horizons, replacive micritization, expansion cracks, and thufur mounds, as well as low carbon and oxygen isotope values, high strontium isotope ratios, and geochemical evidence of leaching and clay formation. This cap carbonate has no definitively marine features, and is here interpreted as periglacial loess overlying the fluvioglacial–intertidal Elatina Formation, so it is not a record of abrupt global warming from snowball Earth. Other cap carbonates around the world differ in various ways, and may have formed in different depositional environments, but could profitably be reconsidered from the new perspective of the loess depositional model proposed here.

Supplementary material: Tabular comparison of marine and non-marine interpretations of cap carbonates, petrographic textures and composition, and new isotopic and chemical analyses of Nuccaleena Formation are available at http:/www.geolsoc.org.uk/SUP18443.

The Nuccaleena Formation in South Australia is a regional Plummer 1978; Williams 1979; Link 1983; Walter & Bauld stratigraphic marker from its type area near Mt. Saturday (308S) 1983; von der Borch et al. 1988; Hegenberger 1987). Clastic to south of Hallett Cove (358S), near Adelaide (Fig. 1). The deposition of cap carbonates in deep marine rifts was proposed formation has international significance, because its base in by Eyles & Eyles (1983). These interpretations are all at variance Enorama Creek is the global boundary stratotype section and with the current paradigm that cap carbonates are marine point for the Ediacaran Period (Knoll et al. 2006). The Nuccalee- precipitates from a post-glacial global alkalization event, as a na Formation, like other post-glacial cap carbonates around the result of continental weathering (Hoffman et al. 1998; Hoffman world, has been interpreted to represent extreme and abrupt & Schrag 2002), methane clathrate release with sulphate reduc- global palaeoclimatic shift from frigid to tropical tion (Kennedy et al. 2001, 2008; Jiang et al. 2003) or oceanic (Williams 1979; Tucker 1986; Fairchild 1993; Hoffman & Schrag freshening (Shields 2005). This marine depositional model is 2002; Shields 2005; Jiang et al. 2006; McCay et al. 2006). Very critical to global geochemical modelling of the ‘hard snowball’ low carbon isotopic values from Nuccaleena dolostones and hypothesis of Neoproterozoic global change (Hoffman & Schrag other glacial cap carbonates have been taken as evidence of a 2002; Rose & Maloof 2010). Reinterpretation here of one cap uniquely Neoproterozoic geochemical perturbation on deglacia- carbonate as periglacial loess removes it as evidence for global tion from snowball Earth, on the assumption that cap carbonates Neoproterozoic ice (Hoffman & Li 2009). Prior assumptions record geochemical composition of the global ocean (Hoffman & about cap carbonates elsewhere also may need re-evaluation. Schrag 2002; Ridgwell et al. 2003; Peltier et al. 2007; Rose & Maloof 2010). Global correlation of the Nuccaleena Formation Materials and methods was undermined by radiometric dating of the Croles Creek Diamictite in Tasmania at 582–575 Ma (Calver et al. 2004), Exposures of the Nuccaleena Formation examined for this study much younger than ‘cap carbonates’ in China originally envi- are at the aboriginal art site in Chambers Gorge (30.948728S, saged as basal Ediacaran and dated at 635.23 0.57 Ma (Con- 139.230278E), the basal Ediacaran stratotype monument on don et al. 2005). This problem has now been resolved by Enorama Creek (31.331508S, 138.633398E), an abandoned rail- recognition of the Cottons Breccia in Tasmania as tillite road cutting 6.2 km south of Kulpara (34.121578S, 138.033848E), correlative with the Elatina and Chinese glaciation (Hoffman et and on the coastal rock platform 1 km NE of Hallett Cove al. 2009). railway station (35.68648S, 138.497748E), South Australia (Fig. This study presents a new and testable hypothesis that 1). Periglacial palaeosols of the Whyalla Sandstone and Cattle distinctive sedimentary, stratigraphic and geochemical features of Grid Breccia (Williams et al. 2007) were examined in the Mt. the Nuccaleena Formation formed as dolomitic aeolian silt Gunson Mine (31.442788S, 137.1353508E) workings on 5 July (loess) with periglacial palaeosols, comparable with Late Pleisto- 2007. Drillcore of Nuccaleena and Elatina Formation from 159.2 cene, carbonate-rich, Peoria Silt of the North American Midwest to 329.0 m in Wokurna 6 (33.71344638S, 138.14117388E) was (Bettis et al. 2003). Neoproterozoic periglacial loess has been examined at the Core Library of Primary Industries and described before (Edwards 1979; Deynoux 1982; Eyles & Eyles Resources South Australia (PIRSA), Glenside. 1983; Fairchild & Hambrey 1984; Fairchild & Spiro 1987). Some Rock specimens for analysis are from stratigraphic sections cap carbonates have been interpreted as supratidal, intertidal and measured using level and tape. Petrographic thin sections were shallow lacustrine (Spencer 1971; Spencer & Spencer 1972; point counted for grain size fractions and mineral content with

289 290 G. RETALLACK

Fig. 1. Thickness (contoured) and distribution of the Elatina and equivalent Formations (a) and overlying Nuccaleena and Seacliff Formations (b), and their lateral facies variation in a vertically exaggerated meridional cross-section (c) in South Australia. Also shown are localities examined as part of this study (a) and mentioned in the text (b). Stratigraphic subdivisions apply to the closest sector of outcrop shown (data from Preiss 1987).

error of 2% (Murphy 1983). In addition, grain size distribution ined in the following paragraphs is description, and then was determined by measuring the long axes of 1000 grains in interpretation of sedimentological features, and whether they are petrographic thin sections with a calibrated microscope eyepiece compatible with a marine or non-marine palaeoenvironment. scale. New analyses of carbonate ä13C and ä18O were made by K. Watts and I. Bindemann using a Finnegan MAT 253 mass Stratigraphic relationships spectrometer in the Department of Geological Sciences, Univer- sity of Oregon, against Vienna PDB standard. Specimens were The Nuccaleena Formation is thin (,69 m, but mostly ,5m) also analysed for major elements uisng X-ray fluorescence and laterally extensive (Fig. 1b). It overlies diamictites of the (XRF) and for ferrous iron using Pratt titration by ALS Chemex Elatina Formation in the central Flinders Ranges (Fig. 2) of Vancouver, BC. gradationally to sharply (Raub et al. 2007; Rose & Maloof 2010). Sharp contacts may be locally disconformable, consider- Sedimentology ing onlap on the Elatina Formation evident in Chambers Gorge (Williams et al. 2007; Schmidt et al. 2009). Near Blackfellow Ripple marks, lamination, silicate grains, and pink to red colour Creek, Nuccaleena Formation mantles an erosional surface (Figs 2 and 3) are the most striking field differences between through sandstones of the Elatina Formation and forms an Nuccaleena dolostone and grey massive carbonates of the angular unconformity with the underlying Trezona Formation Wonoka, Etina, Enorama and Trezona Formations in the same (Leeson 1970). In the type section at Mount Saturday to the region. Distinctive colour and sedimentary structures have made north, 9 m of yellow-weathering dolostone overlies Balparana impure dolostones of the Nuccaleena Formation a regional Sandstone, but the Nuccaleena Formation also includes 60 m of stratigraphic marker bed (Preiss 1987). The key question exam- overlying red siltstone with calcareous nodules and stringers. To NEOPROTEROZOIC LOESS 291

Fig. 2. Field photographs of the Nuccaleena Formation (a–d), Whyalla Sandstone (e) and Elatina Formation (a,f) at Enorama Creek (a–c), Hallett Cove (d, f), and Mt. Gunson Mine (e), showing dolostone overlying red tillite (a), red nodular siltstone facies (b), tent structure with axial growth fault (c) tent structures without faulting (e,f), interbedded dolostone and red siltstone facies (d). Hammer for scale (b–d, f) has a handle 25 cm long; sign beneath red gum (a) is 1.2 m tall; and trees atop mine face (e) are 3 m tall.

Fig. 3. Hand specimens of naturally weathered Nuccaleena Formation showing primary sedimentary structures from Enorama Creek (a–d) and Hallett Cove (e). Specimens are R3574A (a), R3616 (b), R3574B (c), R3617 (d), and R3606A (e). 292 G. RETALLACK the south (Wilmington to Hallett Cove) up to three thin (,5m) lower Brachina Formation. By this model, sea-level fall was lenses of red–green-banded Nuccaleena dolostone (Fig. 2) are correlative with the last Marinoan glacial advance, represented interbedded with grey Seacliff Sandstone (Dyson & von der by tillite of the Elatina Formation in Enorama Creek, and marine Borch 1986, 1994). transgression followed deposition of the Nuccaleena Formation, The upper contact of the Nuccaleena Formation is at the point rather than preceding it as proposed in the sequence-stratigraphic where nodules and lenses of dolostone and limestone pass scenario of Knoll et al. (2006) and Raub et al. (2007). upward into siltstone of the Brachina Formation or Ulupa The source of Nuccaleena dolomite grains by the loess Siltstone (Lemon & Gostin 1990). This red, silty, nodular facies interpretation would have been physical erosion by glaciers and of the upper Nuccaleena Formation overlies the bedded dolostone then sorting by wind of older dolostones, such as the Burra facies, and fills erosional channels that in Warren Gorge erode Group near the Peake and Denison Ranges (Preiss 1987, 1993; all the way through basal dolostones of the Nuccaleena Forma- Veevers 2000) and Bitter Springs Formation in the Officer and tion (Plummer 1978). Amadeus Basins of central Australia (Skotnicki et al. 2008). These stratigraphic relationships are problematic for interpre- Burra Group and Bitter Springs dolostones flanking the Muloor- tation of the Nuccaleena Formation as a chemical precipitate in ina Ridge and Musgrave Block to the north and west were an ocean several hundred metres deep following glacial retreat uplifted and eroded into glacial landscapes during deposition of (Knoll et al. 2006). A unique short-term precipitate should be Sturtian tillites (Veevers 2000), and remained uplands during thicker within canyons into the underlying Elatina Formation. Elatina Glaciation (Preiss 1987, 1993; Williams et al. 2008). A There are no coarse-grained submarine canyon facies, as seen in minor source of limestone clasts could have come from the other Neoproterozoic marine ravinement sequences (Levy et al. underlying Trezona Formation uplifted with local diapiric intru- 1994; Smith et al. 1994). Instead, onlapped palaeohighs at sion (Leeson 1970; Plummer 1978). Chambers Gorge (Plummer 1978), Mt. Saturday (Preiss 1987) and Breakfast Time Creek (Leeson 1970) are mantled by bedded dolostones similar in thickness to those on palaeolows (Fig. 1c). Fossil content Also questionable is the view of Knoll et al. (2006) and Raub et Nuccaleena dolostones are unusually barren of stromatolites and al. (2007) that diamictites of the underlying Elatina Formation acritarchs compared with other Proterozoic limestones of the were marine tillite. Unlike Sturtian Pualco Tillite, which is grey Flinders Ranges (Preiss 1993), and laterally equivalent stromato- and bouldery over all of its outcrop as expected for a marine litic dolostone in central Australia (Williams et al. 2007). Crude tillite (Mawson 1949; Preiss 1987; Crossing & Gostin 1994; domes in Nuccaleena dolostones have been considered stromato- Lottermoser & Ashley 2000), the Elatina Formation is red and lites (by Plummer 1978), but subsequent observers considered bouldery in limited areas of palaeorelief near diapiric uplifts, like these a stage in the development of tent structures (‘tepee a ground moraine (Fig. 1; Mawson 1949; Preiss 1987, 1993). structures’ of Gammon et al. 2005; ‘tepee-like structures’ of Silty parts of the Elatina Formation have ripple marks and varve- Williams et al. 2007; ‘pseudo-tepees’ of Rose & Maloof 2010). like banding comparable with lacustrine and intertidal environ- No acritarchs and few mat fragments were macerated from the ments (Williams et al. 2008), and sandstones of the Elatina and Nuccaleena Formation, but palynomorphs are diverse in other Balparana Formations have trough cross-bedding similar to Australian Ediacaran carbonates of the Wonoka Formation, fluvial facies (Preiss 1987, 1993). Karlaya Limestone and Wilari Dolomite (Grey 2005). No By the new depositional model proposed here, the Nuccaleena permineralized microfossils are known from the Nucculeena Formation was a loess blanket mantling and onlapping pre- Formation either, although these can be exquisitely preserved in existing topography of glacial moraines and supratidal flats silicified grey stromatolitic limestones (Schopf & Kudryavtsev flanking emergent uplands of local diapirs rimmed by talus 2009). Some poorly preserved tubular structures are intercon- (envisaged by Plummer 1978), western aeolian dunefields nected with dilational cracks in Nuccaleena dolostones and (Whyalla Sandstone; Williams et al. 2008) and southern deltaic– siltstones (Raub et al. 2007): diffuse ferruginized tubular struc- estuarine sands (Seacliff Sandstone; Dyson & von der Borch tures (Fig. 4d); nested tubular structures, with inner ferruginized 1986, 1994). Onlapping of bouldery Elatina Formation in tubes 1–2 mm in diameter and outer reduction haloes 3–4 mm Chambers Gorge (Plummer 1978; Schmidt et al. 2009) could be diameter (Fig. 4e); and simple dark clayey branching filaments depositional onlap of loess behind a fluvially eroded ground (Fig. 4a and i). moraine. This is a specific example of a general observation: The various tubular features observed are comparable with where Elatina Formation is thin, Nuccaleena–Seacliff Formations filament traces of possible biological soil crusts in Cambrian are thick, and vice versa (Fig. 1). These reciprocal thicknesses palaeosols of the Flinders Ranges (Retallack 2008). Comparable are not related to faulting or rifting, as assumed for other tubestones in cap carbonates elsewhere are controversial in Neoproterozoic sequences in which thick carbonates are thought origin, with gas escape structures considered most likely (Cloud to have accumulated in rift valleys adjacent to thick tillites et al. 1974; Corsetti & Grotzinger 2005; Font et al. 2006). These (Eyles & Eyles 1983; Miller 1985; Corsetti & Kaufman 2005), tubular features remain problematic. because growth faults owing to rifting are not evident in excellent exposures of the Flinders Ranges (Preiss 1987; Rose & Maloof 2010). Observed thickness variations in Nuccaleena and Rare cobbles Elatina Formations (Fig. 1) are generally similar to the relation- ship between Quaternary loess and ground moraines in the North Conglomerates and lonestones are rare within the Nuccaleena American Midwest (Bettis et al. 2003). Formation, and are found only in the central Flinders Ranges Interpretation of the Nuccaleena and Seacliff Formations as near their source in the Oraparinna, Enorama and Blinman loess and fluviodeltaic (respectively) supports the sequence- diapirs. A lonestone of red porphyry 11 cm 3 6 cm in size within stratigraphic view of Dyson & von der Borch (1994) in which lower Nuccaleena dolostones has been illustrated by Plummer ravinement at the base of the Seacliff Sandstone was glacio- (1978, fig. 4b). This example in Chambers Gorge is so close to eustatic and post-glacial marine highstand was in the overlying the base of the Nuccaleena Formation that it may be a buried NEOPROTEROZOIC LOESS 293

Fig. 4. Photomicrographs of Nuccaleena Formation in Enorama Creek: (a) surface (A horizon) of Adla palaeosol with truncated filaments (sample R3519); (b) caliche of subsurface (Bk) horizon of Adla palaeosol (R3518); (c) dolomite loess and peloids (sample R3513); (d) dolomitic siltstone with diffuse tubular structures (R3510); (e) dolomitic siltstone with nested tubular structures (sample R3514); (f) dolomitic siltstone with iron–manganese stain of bedding and joints (R3509); (g–i) mixed dolomite, quartz, feldspar silt with disrupted bedding (from ice heave?), desiccation cracks, and branching filaments (sample R3501). All thin sections were oriented vertical to ancient land surfaces and bedding (for sample location see Fig. 9).

pinnacle of underlying diamictite of the Elatina Formation dolostone specimen R3514). Also found were silt to sand size, (Mawson 1949). pelletoidal micrite grains (Fig. 4c). Sparry dolostone fills subhor- Other pebbles and boulders in the Nuccaleena Formation have izontal and vertical cracks and vugs, especially in Chambers and been interpreted as littoral talus around emergent land or glacial Parachilna Gorges (Gammon et al. 2005). Large grains of erratics dropped from icebergs (Plummer 1978). They could syndepositional carbonate fans, needles and shrubs, common in equally be hill-bottom talus and fluvioglacial outwash in the other Neoproterozoic cap carbonates (Frasier & Corsetti 2003; periglacial loess-plain palaeoenvironment envisaged here. Lone- Nogueira et al. 2003; Lorentz et al. 2004; de Alvarenga et al. stones would also be rare in hundreds of metres of post-glacial 2008), are not yet known from the Nuccaleena Formation. ocean envisaged by Knoll et al. (2006), and are rare in As products of a post-snowball-Earth marine transgression, Quaternary loess blankets (Bettis et al. 2003) considered here a grain size of the Nuccaleena Formation would reflect abiotically more suitable analogue for the Nuccaleena Formation. precipitated calcite in an ocean of exceptionally high alkalinity (Ridgwell et al. 2003; le Hir et al. 2009). Uniformity of grain size would be unusual during such a process, which is today Silt grain size limited to small lake, pond or cave travertine and tufa, and Most silicate and dolomite grains of the Nuccaleena Formation results in flamboyant grain-size variation, radial fabrics, and vugs are silt size, with a clear mode at 7 (Fig. 5). Silicate and (Julia 1983; Chafetz & Guidry 1999). Alternatively, the grain dolomite grains of the Nuccaleena Formation are highly angular, size could be due to dolomitization or neomorphism after burial moderately sorted, and randomly oriented (Fig. 4). Remarkably (Knoll et al. 2006). Neomorphism leads to grain sizes larger than few of them are rhombs or laths of euhedral crystals. Silicate silt and dolomitization creates loosely packed sand-size rhombs grains within dolostones include quartz (6.4%), rock fragments (saccharoidal dolomite; Boggs 1992): both very different from (2.0%), feldspar (12.6%) and mica (1.8% by volume for observed granulometry of the Nuccaleena Formation. 294 G. RETALLACK

were a primary precipitate, but traction current transportation of silicate silt grains offshore should have created some grain- supported stringers, as in the basal Nuccaleena Formation. Pelletoid grains in this (Fig. 4c) and other cap carbonates (James et al. 2001) are superficially similar to faecal pellets of marine animals, but are less regularly ellipsoidal in shape and predate commonly accepted evidence of metazoans (Xiao 2004; Fedon- kin et al. 2008). Mawson & Segnit (1949) were first to interpret Ediacaran red siltstones as loess based on dominantly silt grains (Fig. 5) and fabrics of randomly oriented, angular grains (Fig. 4). These key observations are true of dolostones of the Nuccaleena Formation as well as its upper red bed facies, and are characteristic of periglacial loess (Swineford & Frye 1951; Wang et al. 2006). Furthermore, the fine grain size of Nuccaleena Formation dolostones is evidence of a distant source terrane, more like Chinese than North American loess (Pye & Sherwin 1999). Both dolomite and silicate grains by this model would have been derived by glacial erosion of pre-existing dolostones, such as the Burra Group and Bitter Springs Formation, which were widely exposed to the north and west in the Muloorina Ridge and Musgrave Block of central Australia (Veevers 2000; Skotnicki et al. 2008). Pelletoidal grains are explained by a loess model as well, because they are characteristic of lake-marginal clay-dunes (lunettes) in periglacial and desert regions (Stone 2006). Much loess is quartzofeldspathic (Pye & Sherwin 1999), but high contents of carbonate in North American Quaternary loess of 42% (Fisk 1951), 32% (Ruhe & Olson 1980) and 31% (Grimley et al. 1998) are comparable with point-counted volumes of carbonate grains in dolostones of the Nuccaleena Formation.

Colour and oxidation Red colour is more intense in fresh Nuccaleena dolostone than in yellow-weathered surfaces, and derives from clay–hematite- rimmed grains of dolomite, quartz, feldspar, rock fragments and ilmenite (Fig. 4b–d). Other carbonates of the Trezona, Enorama, Etina and Wonoka Formation in the Flinders Ranges are bluish grey in colour and unoxidized, with thin, red, brecciated palaeokarsts from occasional subaerial exposure (McKirdy et al. 2001). Red colours in outcrop are identical to those in the Elatina and Nuccaleena Formation dolostones and siltstones at 159– 329 m in Wokurna 6 borehole. The nodular-siltstone facies of the upper Nuccaleena Formation is more intensely red than the dolostones (Fig. 2b), and dolostone stringers interbedded with grey Seacliff Sandstone have dark red siltstone laminae (Fig. 2d). One interpretation of red colour of Proterozoic rocks of the Flinders Ranges is oxidation during Mesozoic and Tertiary lateritic weathering (Gehling 2000). For both the Nuccaleena and Elatina Formations, red colour beneath grey beds in drill core, Fig. 5. Grain-size distribution of calcareous and weakly calcareous and fold tests and low-inclination palaeomagnetic directions rule Nuccaleena Formation. Frequency is from measurement under the petrographic microscope of 1000 long axes of grains. out oxidation during the Mesozoic and Cenozoic when Australia was at high latitude attached to Antarctica (Raub et al. 2007; Schmidt et al. 2009). A final line of evidence for synsedimentary oxidation is the existence of finely interbedded red and green Yet another explanation for cap carbonates is precipitation in laminae (Mawson & Segnit 1949), most obvious within Nucca- offshore marine water of normal chemical composition (Allen et leena dolostone stringers in the Seacliff Sandstone at Hallett al. 2004). This is unlikely for the Nuccaleena Formation because Cove (Fig. 2d). The green bands are comparable with burial of its low clay content and abundant silicate silt grains. Silicate gleization of organic matter common in palaeosols (Retallack silt grains dominate the basal 20 cm of the Nuccaleena Forma- 2008). tion in Enorama Creek (Fig 4g–i), but in the dolostone portion Unusually high oxidation of Ediacaran deep oceans has been of the Nuccaleena Formation, as much as 34% by volume of proposed by Canfield et al. (2007) to explain purple–red silicate grains are scattered among dolomite grains (Fig. 4c–f). siltstones in the Mistaken Point Formation of Newfoundland. Delivery by wind over the ocean could explain this if carbonate Ammonitico rosso (Mamet & Preat 2006), abyssal red clays NEOPROTEROZOIC LOESS 295

(Gleason et al. 2002), oolitic iron ores (Brett et al. 1998) or structures are aligned in rows, indicating that these are ripple banded iron formations (Pecoits et al. 2009) are well-known marks dissected by wind (‘setulfs’ as illustrated and interpreted models for deep-marine red beds, but these have oolites and by Hocking 1991). Comparable structures have been described extremely rare silicate grains, unlike Nuccaleena dolostones, from the upper cap carbonate of the Amadeus Basin by Kennedy which lack oolites and have 29–54% by volume terrestrial (1996). silicate grains. Abundant ripple marks and other primary sedi- Inverse grading of the Nuccaleena Formation is most like mentary structures in the Nuccaleena Formation (Fig. 3) attest to climbing-translatent cross-lamination, an aeolian sedimentary more rapid clastic sedimentation than found in deep-marine red structure (Hunter 1977). In the silty dolostones of the Nuccaleena beds. Formation inverse grading is found on planar surfaces, small A final explanation for red colour of Ediacaran siltstones in dune-like features and tent structures, not on the low-angle South Australia advanced by Mawson & Segnit (1949) is upwind side of large aeolian dunes where climbing-translatent oxidization on land during chemical weathering of loess. Their cross-stratification is best known in sandstones. Such dune-hosted supporting evidence includes gradational ferruginization down climbing-translatent cross-stratification is known from the from bedding planes, finely interbedded red and green–grey Whyalla Sandstone (Williams et al. 2007), laterally equivalent to layers, and red clasts redeposited in grey sandstones and lime- the Elatina Formation (Fig. 1a). Rather than large barchan dunes, stones: all true of the Nuccaleena Formation. Salt crystal casts in the Nuccaleena Formation inverse grading and ripples have the red beds support the idea of exposure of red tillite of the upper character of a deflation plain maintained by high groundwater or Elatina Formation near Beltana as a ground moraine (Coats freezing. Comparable ground-parallel inversely graded strata 1973). Rinds of hematite around grains (Raub et al. 2007) and a associated with ripple marks have been reported from Dutch palaeomagnetic fold test (Schmidt et al. 2009) are also evidence drift-sand areas (Riksen & Goossens 2007) and eastern Washing- of synsedimentary oxidation of the Nuccaleena and Elatina ton (USA) deflation plains (Sweeney et al. 2007). Formations.

Bedding and ripple marks Tent (‘tepee’) structures Especially enigmatic are large synsedimentary dilational– Nuccaleena dolostones have an extraordinary array of primary constructional features here designated by the descriptive term sedimentary structures: flaggy bedding, straight crested and ‘tent structure’. Tent structures have been called ‘tepee struc- linguoid ripple marks, trough cross-bedding, scour-and-fill, cres- tures’ (Allen & Hoffman 2005) and ‘tepee-like structures’ centic marks and inverse grading (Plummer 1978; Lemon & (Williams et al. 2007), but this is a misnomer because these Gostin 1990; Rose & Maloof 2010). Inverse graded beds 2–4 cm features show some low-angle branching, but not fully connected thick may be accentuated by sheet cracks with isopachous polygonal connections of playa tepees (Gammon et al. 2005; cement or within tent structures (Rose & Maloof 2010, figs 3d Schmidt et al. 2009). Rose & Maloof’s (2010) term ‘pseudo- and 4f). These small-scale sedimentary structures are especially tepee’ is accurate, but perpetuates the misnomer; a new non- clear on weathered exposures of dolostone (Fig. 3), and in genetic term is needed. Nuccaleena tent structures are linear to interbedded siltstones and dolostones (Fig. 2d). Large-scale dune undulose features (Fig. 2f). Crests can be traced in plan for up to cross-bedding is not found in the Nuccaleena Formation, 80 m on dip slopes at Enorama Creek (Plummer 1978) and are although it is conspicuous in the underlying Whyalla Sandstone aligned NE–SW (Fig. 6: see also Hoffman & Li 2009; Rose & to the west (Fig. 2e). Other Neoproterozoic cap carbonates Maloof 2010). In addition to a ruptured and dilational base, elsewhere in the world also preserve comparably clear sedimen- many tent structures are capped by distinctive aggrading beds tary structures (James et al. 2001; Katsuta et al. 2007). forming asymmetric chevrons at their crests (Fig. 3a and b). The Deep-sea oozes and chemically precipitated limestones and basal layers of tent structures were brecciated with upended evaporites are massive to poorly bedded (Boggs 1992), like other flagstones, and dilation continued to propagate in some cases as Neoproterozoic limestones of the Flinders Ranges such as the growth faults (Fig. 2c), through as much as 15 m of overlying lower Skillogalee Dolomite and upper Wonoka Formation (Preiss accreting dolomitic silt (Gammon et al. 2005; Schmidt et al. 1987). Flaggy bedding, ripple marks, scour-and-fill structures 2009). Continued dilation is also indicated by a palaeomagnetic and inverse grading can form in submarine fans and contourites fold test of growth-faulted examples (Schmidt et al. 2009). (Mulder et al. 2001; Sylvester & Lowe 2004; Howe 2008), and are abundant in intertidal facies (Reineck & Singh 1973). In deep marine to intertidal settings these structures are accentuated by clay drapes and grain size ranging from clay to gravel, and associated with many normally graded beds. Inverse grading of gravel to shale in deep marine settings ranges from 9 to 450 cm thick and commonly is the base of a bed that continues to fine upward (Naylor 1980; Hand 1997; Mulder et al. 2001; Sylvester & Lowe 2004). This is distinct from the thin (,2 cm), inverse- graded beds of silt in the Nuccaleena Formation (Fig. 3a and e; Plummer 1978, fig. 2e). Coarse-grained layers in the Nuccaleena Formation are ungraded breccia, associated with cores of tent structures and other laterally impersistent deformation (Rose & Maloof 2010). Crescentic markings illustrated from the Nuccaleena Forma- tion (Plummer 1978, fig. 2f) are not flute casts protruding down from the soles of turbidites (Potter & Pettijohn 1963), but Fig. 6. Compass orientation of tent structures on dip slopes of upstanding ellipses in the top of the bed. The highest parts of the Nuccaleena dolostone facies in Enorama Creek. 296 G. RETALLACK

Different tent structures vary in the relative thickness of the two bedding matches excavation of linear (seif) dunes in Libya distinct components: (1) basal dilational breccia and growth (McKee & Tibbits 1964). faulting; (2) upper accretionary chevrons. Accretionary chevrons are thicker than basal breccias in tent structures of the Nuccalee- Palaeopedology na Formation (Fig. 2c), but basal dilational uparching is more prominent than accretional chevrons in the Reynella Siltstone Non-marine palaeoenvironments of the upper Elatina and Nucca- Member of the Elatina Formation at Hallett Cove (Fig. 2f; see leena Formations in Enorama Creek are confirmed by beds with also Dyson & von der Borch 1986), and in the Cattle Grid distinctive features of palaeosols, such as gradational soil Breccia (Preiss 1993, p. 190; Williams et al. 2007, fig. 12b) and horizons and dilational soil structures, and these beds were upper Whyalla Sandstone at Mt. Gunson (Fig. 2e). sampled closely to detect geochemical evidence of soil formation Dilational brecciation and growth faulting of tent structures (Figs 7 and 8). Traces of roots are used to distinguish palaeosols was attributed by Gammon et al. (2005) to recrystallization, in geologically younger rocks, but are not known or likely in cementation or dolomitization after burial and independent of Neoproterozoic palaeosols (Retallack 1997). Periglacial palaeo- sedimentary environment. This mechanism is inadequate to sols have been recognized in the Cattle Grid Breccia, Whyalla explain the observed magnitude and persistence of dilation Sandstone and Elatina Formation (Dyson & von der Borch 1986; because crystal patterns of Nuccaleena cements are ordinary Williams et al. 2007) and in Neoproterozoic glacial deposits cavity fills (Gammon et al. 2005), and because dolomite crystal- elsewhere in the world (Spencer 1971; Deynoux 1982), but not lization from calcite or aragonite results in volume decrease, not until now in the Nuccaleena Formation. increase (Boggs 1992). Another explanation for dilation of tent structures illustrated from the Elatina Formation at Hallett Cove Palaeosol horizons by Kennedy et al. (2008) is escape of methane gas from underlying layers. Unlike known methane seep deposits, which Enrichment in red clay and nodularization with micritic carbo- are pipes in chemically reduced, dark grey shales and diatomites nate in the Nuccaleena Formation show key features of palaeosol (Aiello et al. 2001), tent structures of the Nuccaleena, Whyalla horizons: gradations of intensity and diffuse contacts down from and Elatina Formations are unconnected to deeper layers and truncated surfaces. This reflects gradation of weathering intensity entirely within red, organic-lean, siltstones and sandstones (Wil- and processes below an ancient land surface (Retallack 1997). In liams et al. 2007). The anoxic methane seep model of Kennedy the nodular red siltstone facies of the Nuccaleena Formation et al. (2008) and subsurface crystal expansion model of Gammon (Fig. 2b) red colour and clayeyness are most intense below an et al. (2005) also fail to explain the petrographic observations of erosional surface (Fig. 8). A similar relationship can be seen Raub et al. (2007), who demonstrated that hematite of tent structures (including Fig. 2c) was magnetized in an oxidizing environment. An alternative mechanism proposed here for observed dilation of tent structures is ice crystallization within sediment or soil, which would create breccias and sheet cracks in red siltstone in the cores of tent structures, as well as later growth fault propagation into overlying sedimentary layers. Acquisition of magnetization during soil formation (as is common in palaeosols; von Suchodoletz et al. 2009) would be followed by cryogenic growth faulting to account for the palaeomagnetic fold test (Schmidt et al. 2009). Comparable structures in Quaternary periglacial loess are thufur mounds, and growth faults to accommodate continued growth of subsurface discontinuous permafrost (Schunke & Zoltai 1988). In addition to dilational breccia and faulting of tent-structure cores, the crests of some tent structures show an unusual form of accretionary bedding in which apical chevrons are alternately thinner on one side and then the other (Fig. 3). Lower microfaulted uplift of tent cores passes up-section into inverted- V-shaped accretionary beds. Allen & Hoffman (2005) interpreted accretionary parts of large tent structures as giant wave ripples, but had to postulate bimodal current velocities and fetch in ocean storms much more violent than any currently observed. The new interpretation of dolomitic siltstones as loess now allows inter- pretation as linear dunes, which have comparable alternating Fig. 7. Carbonate carbon and oxygen isotopic cross-plot, showing linear bedding at the apex as a result of downwind migration of relationship in red clayey dolostone (Ika) palaeosols and also other undulose crests (Bristow et al. 2000). The upper accretionary (Yaldati, Adla) carbonate palaeosols. In contrast, there is no relationship parts of tent structures are here envisaged as linear shadow dunes within pink bedded dolostone (loess) of the Nuccaleena Formation, downwind from periglacial thufur mounds and ridges, and some- which was derived largely from mechanical erosion and modest times later faulted by accretion of additional discontinuous weathering of isotopically similar Burra Group. Marine stromatolites of permafrost at depth. Linear shadow dunes are well known the Enorama Shale and intertidal siltstones of the Reynella Siltstone are downwind of rock outcrops (yardangs; Hesp et al. 1989) and isotopically distinct. This graph includes new analyses of palaeosols clumps of vegetation (nabkhas; Clemmensen 1986; Saqqa & combined with analyses from Williams (1979), Calver (2000), Hill & Atallah 2004; Riksen & Goossens 2007). Their distinctive crestal Walter (2000), McKirdy et al. (2001) and Kennedy et al. (2008). NEOPROTEROZOIC LOESS 297

palaeosol. Similar relationships comparable with palaeosol hor- izons were observed in the Nuccaleena Formation in Wokurna 6 core.

Palaeosol structures Dilational cracking to create fitted breccias (peds) and modified surfaces (cutans) is a hallmark of soil formation (Retallack 1997). Uparched red breccia mounds are synsedimentary expan- sions at the base of tent structures and are associated with ‘sheet cracks’ and ‘crackle breccias’ cemented with sparry carbonate (Gammon et al. 2005; Raub et al. 2007; Rose & Maloof 2010). All outcrops of Nuccaleena dolostones show sparry veins of these dilational features, but they are especially marked in Chambers Gorge (Williams et al. 2008) and Lame Horse Gully (Rose & Maloof 2010). Also dilational, but on a microscopic scale, are bedding plane disruptions (Fig. 4g) comparable with soil disruption by needle ice and freeze–thaw banding (Wash- burn 1980). In addition to these clear disruptions, many palaeo- sol thin sections viewed in cross Nicols show wispy alignments of clay characteristic of various kinds of sepic plasmic fabrics (Fig. 4a), which are characteristic of soils. Surface weathering of the dilated fragments is slight in the Nuccaleena Formation, as expected in periglacial palaeosols, but some surfaces and V- shaped desiccation cracks show weathering rinds (Fig. 4h; diffusion ferrans in soil terminology; Retallack 1997). Other soil structures in the Nuccaleena Formation are micritic nodules up to 12 cm in diameter, and laterally linked within specific horizons in both red siltstones (Fig. 2b) and dolostones (Fig. 2d), in a fashion comparable with carbonate nodular (Bk) to bench (K horizon) development in Aridisols (Gile et al. 1980). Within the nodules, quartz and other silicate grains have etched margins and float within micritic matrix (Fig. 4b); these are common replacive fabrics of caliche nodules (Retallack 1997). At Hallett Cove, comparable nodules form clasts in interbedded conglomerates of the Elatina Formation (Dyson & von der Borch 1986; Kennedy et al. 2008) as evidence for penecontempora- neous formation of comparable palaeosols, rather than a deep burial origin. Calver (2000) reported both dolomite and anhydrite nodules in the red silty upper portion of the Nuccaleena Formation in SCWY1a bore.

Stable isotopic geochemistry Carbonate in the Elatina and Nuccaleena Formations has an Fig. 8. A measured section of the Nuccaleena Formation at the Ediacaran extreme range of carbon and oxygen isotope compositions, with stratotype location in Enorama Creek, South Australia. Palaeosol location is shown by height of grey box, and development by width of grey box values of both declining markedly toward the top of the (following Retallack 1997). Calcareousness was measured by field Nuccaleena Formation (Rose & Maloof 2010). Especially dis- ä13 ä13 application of dilute HCl, and colour is from a Munsell Chart. Also tinctive are very low Ccarb (1.3 to 9.5 C‰PDB) and ä18 ä18 shown is magnetostratigraphy (Raub et al. 2007), and carbon isotope Ocarb (1.8 to 24.5 O‰PDB; Veizer & van Hoefs 1976; values of dolostone (Knoll et al. 2006). Williams 1979; Knoll et al. 2006; Kennedy et al. 2008; Rose & Maloof 2010). There are four analyses each of Nuccaleena nodules and matrix from boreholes BWM1a-1 and SCYW1a by immediately below basal grey beds of the Nuccaleena Formation Calver (2000) indicating that nodules are isotopically lighter than extending down into a zone of small nodules in the reddened matrix in carbon (ä13C ¼2.87 0.13‰, mean and standard upper Elatina Formation. In soil terminology these are surface error for nodules, but matrix ä13C ¼2.41 0.20‰) but com- (A) horizons over calcic (Bk) horizons. Similarly, red brecciated parable in oxygen (nodule ä18O ¼7.89 0.22‰, matrix silty and clayey dolostone is sharply overlain by accretionary ä18O ¼7.95 0.12‰). Finally, despite wide scatter of aggre- layers of tent structures (cumulative or Ac) but passes gradation- gated isotopic analyses (Rose & Maloof 2010), there are local 18 13 ally downward into unbrecciated and planar bedded dolostone positive correlations of ä Ocarb v. ä Ccarb in both Nuccaleena (Fig. 2c), as in a cryoturbated surface (Af) horizon over a little (Fig. 7) and Elatina Formations (Kennedy et al. 2008). altered parent sediment (C horizon). Even thin interbeds of red This remarkable isotopic range is beyond the range known in siltstone (Fig. 2d) have a sharp top and gradational lower unweathered Phanerozoic marine rocks (Kennedy et al. 2008). boundary as in an A–C horizon sequence of a poorly developed Such low carbon isotope values are very distinct from those of 298 G. RETALLACK other Cambrian and Precambrian carbonates of South Australia surfaces with a shallow water table such as deflation plains. known to be marine from stromatolites and microfossils (Fig. 7). Silicate palaeosols, on the other hand, reveal more substantial Considered as marine rocks, such low carbon isotope values clay formation and desalinization than dolomitic palaeosols. require unusual conditions such as methane seeps (Kennedy et Other evidence of weathering of the Nuccaleena Formation al. 2008) or global oceanic freezing and cessation of photosynth- comes from analyses reported from Nuccaleena Formation esis with atmospheric reversion to mantle carbon isotope values dolostones by Calver (2000): elevated 87Sr/86Sr (0.71308), (Hoffman & Schrag 2002). The unusually low oxygen isotope unusually low Sr (133 ppm), and high molar Mn/Sr ratios (26) values reflect frigid or continental conditions (Kennedy et al. and Fe2O3 (1.76 wt%). These values are very different from 2008). For both carbon and oxygen, marine cements are similar those for genuine marine limestones of comparable geological 87 86 to or slightly isotopically heavier than carbonate grains (Mutti & age: Sr/ Sr ,0.70790, Sr .200 ppm, Mn/Sr ,1, and Fe2O3 Weissert 1995; Tobin et al. 2005). The significant covariance of ,0.1 wt% (Calver 2000). These differences are better explained carbon and oxygen isotope values requires improbably precise as increased terrestrial weathering and oxidation owing to sea- proportional mixing of methanogenic and photosynthetic carbon level fall and exposure, rather than abruptly diminished sea-floor (Kennedy et al. 2008), of rock flour and clastic grains (Fairchild spreading since deposition of preglacial carbonates (Veizer & et al. 1994), or of deep, cold, anoxic, and warm, shallow, aerated Compston 1976). Models of Marinoan and post-Marinoan weath- ocean waters (Shields et al. 1997; Calver 2000; Hurtgen et al. ering based on strontium isotopes should consider not only 2006; Valladares et al. 2006; Corkeron 2007). silicate weathering (Asmerom et al. 1991; Valladares et al. 2006; Such isotopic range, low values and covariance are distinctive Nogueira et al. 2007), but also the likely abundance of highly of palaeokarsts (McKirdy et al. 2001) and palaeosols in South reactive, calcareous loess and glacial flour (Fairchild 1982; Australia (Retallack 2008), and of modern soils and palaeosols Fairchild et al. 1994; le Hir et al. 2009). (Knauth et al. 2003; Retallack 2004) and meteoric weathering of Geochemical approaches not applied yet to interpretation of marine carbonate (Knauth & Kennedy 2009). Covariance of the Nuccaleena Formation include boron and calcium isotopes carbon and oxygen isotopic composition is universal for pedo- (Kasemann et al. 2005), iron isotopes (Halverson 2005), sulphur genic carbonate, produced by isotopic fractionation of CO2 in isotopes (Fike et al. 2006; Halverson & Hurtgen 2007), iridium 17 proportion to biogenic soil CO2 concentrations (Knauth et al. content (Bodiselitsch et al. 2005), ˜ O of sulphate (Bao et al. 2003; Ufnar et al. 2008). Carbon isotopically lighter in nodules 2008, 2009), and magnetic susceptibility (Wu et al. 2005). If than matrix is common in soils and palaeosols developed on applied to the Nuccaleena Formation, their interpretation for marine carbonate (Allan & Matthews 1982; Shapiro et al. 1995) non-marine environments envisaged here would be different from and meteoric cement in marine carbonate (‘inverted J alteration interpretation within a marine concept for cap carbonates. path’ of Rao 1993; McKirdy et al. 2001). The low oxygen values, and shallow slope and low correlation coefficient of the Pedotypes covariant trend for the Nuccaleena Formation (Fig. 7) are indications of low soil productivity and cold climate (Ufnar et al. Four distinct kinds of palaeosols in Enorama Creek (Figs 8 and 2008). In contrast, marine carbonates and loess derived from 9) represent as many different soil palaeoenvironments (Tables 1 them by mechanical erosion have scattered and uncorrelated and 2), and are named from descriptive features using the oxygen and carbon isotopic values (different kinds of crosses in Adnamatna aboriginal language (McEntee & McKenzie 1992). Fig. 7; see also Rose & Maloof 2010, fig. 10A). The overlap of In order of relative soil development, based on degree of values for Burra Group dolostones (Hill & Walter 2000) and disruption of primary bedding and chemical differentiation, these Trezona Formation palaeokarsts (McKirdy et al. 2001) with are Wadni (‘small’), Yaldati (‘red’), Ika (‘heaped up’), and Alpa Nuccaleena dolomite grains (Fig. 7) is compatible with the view (‘yellow fat’) pedotypes. The size of microbially precipitated that dolomitic silt grains came from underlying glacially eroded pedogenic nodules is proportional to the duration of their marine carbonates. Isotopically lighter cement than grains is formation in radiocarbon-dated Quaternary soils (Retallack especially useful for distinguishing carbonate aeolianites from 2005). Assuming that this was true for these Precambrian intertidal carbonates (Loope & Abegg 2001), and is evidence palaeosols with the light carbon isotopic composition of biologi- against Plummer’s (1978) intertidal intepretation of Nucaleena cally active soils (Knauth & Kennedy 2009), a minimal time for Formation dolostones. formation for Nuccaleena Formation palaeosols was 2.6 1.8 ka for Yaldati palaeosols, and 78.6 9.3 ka for Alpa palaeosols Chemical weathering (standard errors from transfer function and standard deviations of 11 palaeosols respectively). Ika palaeosols represent an inter- Whole-rock chemical and petrographic analysis demonstrates that mediate amount of time (2–5 ka), because comparable buried ferruginous and aluminous parts of the palaeosols are depleted in thufur mounds are known in post-glacial late Pleistocene loess alkali and alkaline-earth bases, and in rock fragments and (Washburn 1980). Wadni palaeosols have the least pedogenic feldspar (Fig. 9), as expected for hydrolytic weathering of well- differentiation, now formed within centuries of weathering drained palaeosols (Retallack 1997). Furthermore, these trends (Retallack 1997). The geomorphological setting of these various are within single profiles, and not produced solely by glacial pedotypes are interpreted as near-stream (Yaldati, Wadni), flood- transport from variably weathered source terrains, as proposed plain (Alpa) and loess plains (Ika) flanking the Blinman uplift, for other Neoproterozoic glacial deposits (by Dobrzinski et al. NW of the ocean, based on regional facies analysis (Fig. 1b; 2004). Dolomitic periglacial palaeosols are markedly salinized Preiss 1993, 1987). (high molar soda/potash), moderately gleyed (high ferrous/ferric Additional information about palaeoenviroments can be iron), low in clay production (low alumina/bases), and include a gleaned by comparison with modern soils implied by classifica- highly leached component (high molar Ba/Sr), compared with tion of pedotypes within modern soil taxonomies (Food & silicate palaeosols (Ika v. Yaldati and Adla profiles in Fig. 9). Agriculture Organization 1978; Soil Survey Staff 2000). The These geochemical differences support sedimentological evi- Nuccaleena Formation includes soils of geologically young, dence that the dolomitic palaeosols formed on geomorphological disturbed surfaces (Fluvents), soils of frigid climates with ice- NEOPROTEROZOIC LOESS 299

Fig. 9. Palaeosols of the Nuccaleena Formation in Enorama Creek. Grain size and mineral composition were determined by point-counting petrographic thin sections, and molecular weathering ratios from XRF chemical analysis.

Table 1. Pedotypes of the Nuccaleena and flanking formations

Pedotype Adnamatna Diagnosis Soil Survey Food & Agriculture meaning Staff (2000) Organization (1978) (McEntee & McKenzie 1992)

Alpa Yellow fat Red silty surface (A horizon) with fine green mottles over horizon of large Haplocalcid Calcic Xerosol (4–6 cm) yellow micritic nodules (Bk) Ika Heaped up Red micritized dolomitic siltstone surface (A horizon) brecciated and uparched Haploturbel Gelic Regosol above less deformed red dolomitic siltstone (Bw horizon) Wadni Small Red silty surface (A horizon) above sandy bedded layers (C horizon) Fluvent Calcaric Fluvisol Yaldati Red Red silty surface (A horizon) above horizon of small (4–8 mm) calcareous Haplocalcid Calcic Xerosol nodules and veins (Bk) deformation (Haploturbel), and of deserts with carbonate accu- ultimately from the Cascade Mountains (Bettis et al. 2003; mulation (Haplocalcid; Table 1). Gaylord et al. 2003), and southward from terminal moraines and large glacial lakes, which eroded widespread Palaeozoic dolo- stones and limestones (King 1977; Paull & Paull 1977; Nelson Quaternary analogue 1995; Patterson et al. 2003). The Nebraska Sand Hills is a local The Nuccaleena Formation was comparable with late Pleistocene region of metastable barchan dunes of sand (Feng et al. 1994; Peoria Silt, which mantles much of the North American Midwest Bettis et al. 2003), but over most of the region loess mantles (Fig. 10). Loess thins eastward from volcanic ash sources, topography and creates fertile soils, commonly preserved as 300 G. RETALLACK

Table 2. Pedotype interpretation of the Nuccaleena and flanking formations

Pedotype Climate Organisms Topographic setting Parent material Time for formation (ka)

Alpa Arid cold temperate: MAT 10 8.8 8C; MAP Microbial soil crust River floodplain Silicate loess 78.6 18.6 432 294 mm; MARP 31 44 mm Ika Arid frigid: MAT 9.6 8.8 8C Microbial soil crust Loess plain Dolomitic loess 2–5 Wadni Not diagnostic of climate Microbial soil crust Loess plain Silicate loess 0.01–0.1 Yaldati Arid cold temperate: MAT 8 8.8 8C; MAP Microbial soil crust Glacial moraine Calcareous till 2.6 3.6 342 294 mm; MARP 47 44 mm

MAT, mean annual temperature; MAP, mean annual precipitation; MARP, mean annual range of precipitation (difference between driest and wettest month).

Fig. 10. Quaternary palaeoenvironments and isopachs of Peoria Silt in North America (a), with a simplified and vertically exaggerated meridional geological cross-section (b). Sand dune and loess distribution is modified from Bettis et al. (2003), and the cross-section compiled from data of Fisk & McFarlan (1955), Murray (1961), King (1977), Paull & Paull (1977), Autin et al. (1991), Nelson (1995) and Patterson et al. (2003). NEOPROTEROZOIC LOESS 301 palaeosols within loess (Fig. 11b and d). In southern Louisiana, Sangamon Geosols in Peoria Silt were completely decalcified by calcareous Peoria Silt is interbedded with, and dips beneath, humid forest interglacial pedogenesis (Ruhe & Olson 1980; Holocene sands of the Mississippi Delta for an unknown distance Grimley et al. 1998). Carbonate content of surface horizons of into the subsurface (Fisk & McFarlan 1955; Murray 1961; Autin Nuccaleena palaeosols are less completely decalcified (Fig. 9), et al. 1991). and their lower horizons include caliche nodules and dispersed Similarly, the Nuccaleena Formation in the Flinders Ranges is carbonate cement as found in palaeosols and soils (Fig. 11b) a widespread marker bed of silt-size dolomite and silicate grains, formed on loess in semiarid grasslands of Kansas and Nebraska which thickens southward from underlying ground moraines of (Feng et al. 1994; Feng 1997). Thus the Nuccaleena Formation the upper Elatina Formation and mantles northern depressions considered as loess had a glacial bedrock source like that of that may have been deep glacial lakes (Fig. 1b). Three thin beds Illinois–Indiana–Mississippi loess, but a soil-forming regime of Nuccaleena dolosiltstones intercalated with thick palaeochan- like that of Kansas–Nebraska loess. nel sandstones at Hallett Cove are comparable with Peoria Silt Other differences are also apparent. Palaeosols in Peoria Silt interdigitated with deposits of the Mississippi Delta. Compass were either Alfisols with large root traces or Mollisols with orientation of Nuccaleena tent structures (Fig. 6), here inter- earthworm pellets and fine root traces reflecting Phanerozoic preted as linear dunes, is evidence of persistent winds from local evolution of forests and grasslands, respectively (Retallack 2004). diapiric hills and the Muloorina Ridge to the NE. Diapiric Large amounts of rhyodacitic volcanic ash in Peoria Silt in Kansas synsedimentary uplifts and glacial scouring before deposition of derive from Cascade Mountain and other western US volcanoes Elatina Formation calcareous red tillites exposed large areas of (Swineford & Frye 1951), whereas volcanic ash is lacking from carbonates, including local Etina and Trezona Formations (Plum- the Nuccaleena Formation. Periglacial palaeosols (Gelisols) and mer 1978; Preiss 1993), and more widespread dolostones of the deflation plain fabrics like those of the Nuccaleena Formation are Burra Group and Bitter Springs Formation in central Australia uncommon in Peoria Loess, but known from Quaternary loess in (Hill & Walter 2000; Veevers 2000). The Nebraska Sand Hills New Zealand (Fig. 11c) and Washington State (Fig. 11a). These are comparable with cross-bedded sandstones of the Whyalla differences between Peoria Loess and Nuccaleena Formation may Formation in Mount Gunson mine (Fig. 2e). be due to more frigid palaeoclimate, more frequent flooding, and Grain size distribution and mineral composition of the Nucca- more mountainous glacial source terrain in the South Australian leena Formation is similar to Peoria Silt, which is silt sized, Marinoan than in the American Midwest, more like the Quatern- angular and calcareous. Peoria Silt is sandy in Nebraska but fine ary palaeogeography of Washington State (Gaylord et al. 2003) silty (6) in Mississippi (Pye & Sherwin 1999). Loess in and the South Island of New Zealand (Soons 1963). Illinois–Indiana–Mississippi effervesces very strongly with acid (Bettis et al. 2003), unlike weakly calcareous volcaniclastic loess Snowball Earth reconsidered of Kansas–Nebraska (Swineford & Frye 1951). Analyses by X- ray diffraction (XRD) reveal at least five times more dolostone A loess-palaeosol interpretation of the Nuccaleena Formation than limestone grains in Illinois–Indiana–Mississippi loess, developed here is a new and testable hypothesis. The rest of this derived from glacial scouring and wind sorting of local Palaeo- paper elaborates palaeoclimatic and other palaeoenvironmental zoic dolostones (Grimley et al. 1998). Peoria Silt has a total predictions of this new view (Tables 1 and 2), with special carbonate content of 42% at Vicksburg, Mississippi (Fisk 1951), reference to the snowball Earth hypothesis. Palaeoclimatic inter- 32% at Cumback, Indiana (Ruhe & Olson 1980) and 31% in core pretations outlined below set limits to snowball Earth, because G56 in Illinois 80 km north of St. Louis (Grimley et al. 1998), South Australian facies are here regarded as evidence of comparable with point-counted volumes of carbonate grains in incomplete cover by Marinoan glaciers (Fig. 1). The calcareous the Nuccaleena Formation. Palaeosols such as the Farmdale and loess model advocated here for the Nuccaleena Formation has

Fig. 11. Sedimentary structures (a–c) and thin section (d) of Quaternary loess for comparison with the Nuccaleena Formation: (a) ripple drift cross-lamination and inverse grading in Palouse loess at the Starbuck site, eastern Washington, USA (Gaylord et al. (2003); (b) red palaeosols with conspicuous yellow calcite nodules (Aridisols) in Peoria Loess, 3 km south of Pratt, Kansas (Feng et al. 1994); (c) small tent structures in Otiran loess south of Rakaia Gorge bridge, New Zealand (Soons 1963); (d) petrographic thin section showing loess texture and earthworm faecal pellet in Sangamon Geosol, in ash quarry 3 km SW of Eustis, Nebraska (Feng et al. 1994). 302 G. RETALLACK not been considered for other cap carbonates around the world, marine, because of post-glacial sea-level rise (Williams 1979; but if demonstrated elsewhere would further limit the snowball Kennedy 1996; Hoffman & Schrag 2002). Loess has been Earth hypothesis. The Elatina style glacial deposits proposed identified in Neoproterozoic glacial deposits in Norway (Edwards here can also be used to re-evaluate a contrasting Sturt style of 1979) and Spitsbergen (Fairchild et al. 1989), and central Neoproterozoic glaciation (Kennedy et al. 1998). Australian cap carbonates were compared with evaporitic lakes in the Dry Valleys of Antarctica by Walter & Bauld (1983). To these few cases can now be added interpretation here of the Nuccaleena Palaeoclimate of the Nuccaleena Formation and Elatina Formations as deposits of a fluvioglacial and loess Thufur mounds like those of Ika palaeosols now form in cold plain with estuaries and tidal flats toward the SE (Fig. 12). Many temperate to frigid climates (mean annual temperature less than parts of this facies tract, particularly in the south and west (Fig. 1), 6 8C; Washburn 1980). Evidence for cold temperate palaeocli- appear to have been both non-marine and unglaciated. Sedimento- mate also comes from chemical analysis of the palaeosols logical evidence for winds (Figs 2c and 6) and geochemical compared with molar alkali ratios of Quaternary soils of known evidence of salinization and gleization (Fig. 9) are indications that temperature (Sheldon et al. 2002): mean annual palaeotempera- the Nuccaleena Formation may have been a deflation plain. ture 8 4.4 8C for Yaldati, 9.6 4.4 8C for Ika, and 10 4.4 8C Palaeocurrents (Fig. 6) inferred from interpretation of accretion- for Alpa (standard errors are from regression of data for ary upper portions of tent structures as linear shadow dunes would comparable Quaternary soils). These palaeotemperatures were then be evidence of katabatic winds from high mountains to the calculated from alkali ratio (S ¼ (mK2O + mNa2O)/mAl2O3,in north. This landscape was not entirely covered by ice, so as to be a moles), which is related to mean annual temperature (T in 8C) in crisis for photosynthetic life (Hoffman & Schrag 2002). Whether soils, using the ;equation T ¼18.5S + 17.3 (R2 ¼ 0.37; this was the only refuge from ice for life on Earth may be S.E. ¼4.4 8C). These estimates are risky because they are evaluated by comparable re-examination of cap carbonates and based on very different modern soils of non-frigid climates glacial successions elsewhere in the world. supporting vascular land plants (Sheldon et al. 2002). Neverthe- Many features of cap carbonates can be interpreted either as less, low palaeotemperatures also explain the very low oxygen marine or non-marine, although with very different palaeoenvir- 18 isotopic values (2.6 to 12.7‰ ä OPDB) of presumed pedo- onmental implications, but marine interpretations strain credulity genic micrite in the Reynella, Elatina, Nuccaleena and Brachina the most. For example, suites of features here interpreted as Formations (Calver 2000; Kennedy et al. 2008), comparable with palaeosols are evidence of prolonged (2–79 ka) non-marine oxygen isotope values in pedogenic carbonate and ice at high conditions at 14 distinct stratigraphic levels within the 15 m altitude and latitude today (Kennedy et al. 2008). The very measured (Fig. 8). As many glacioeustatic sea-level changes different palaeotemperature of the Nuccaleena Formation of (250 m) would be needed for marine deposition of palaeosol 40 8C calculated by Williams (1979) using a transfer function for parent material without leaving any traces in the form of littoral Quaternary marine carbonates is undermined not only by meta- talus, ravinement, stromatolites, acritarchs or coastal terraces. morphic–meteoric alteration and drift of oxygen isotopic values Similarly, tent structures interpreted as giant ripples require though geological time (Donnelly 1981), but also by reinterpreta- storms of unparalleled violence (wave periods of 21–30 s and tion here of Nuccaleena Formation carbonate as non-marine. wind velocities of 20 m s1: Allen & Hoffman 2005), but as Subhumid to semiarid palaeoclimates, driest in Ika palaeosols, linear shadow dunes of thufur mounds are unremarkably small. are revealed by depth to carbonate nodules compared with modern Stable isotope compositions in cap carbonates that are unremark- soils, including polar soils with no vegetation visible to the naked able for soils (Knauth et al. 2003) require extraordinary explana- eye (Retallack 2005). Mean annual precipitation (R in mm) is tions for marine environments, such as catastrophic methane related to depth to carbonate nodules (Do in cm) in alluvial post- outbursts (Kennedy et al. 2008) or global cessation of photo- glacial soils according to the equation R ¼ 137.24 + 6.45Do synthesis by glaciation with reversion to mantle carbon values 2 2 13 0.013Do (R ¼ 0.52; S.E. ¼147 mm). Thickness of the soils (which is still inadequate at about 5‰ ä C; Hoffman & with carbonate (Ho in cm) also increases with mean annual range Schrag 2002). Global change predictions arising from the of precipitation, defined as the difference of means between assumption of marine environments for cap carbonates are wettest and driest months (M in mm) using the equation extreme enough to be threatening to life on the planet, dwarfing M ¼ 0.79H + 13.71 (R2 ¼ 0.58; S.E. ¼22 mm). Original depth comparable geochemical changes at the largest known mass to carbonate nodules (Do) and thickness of carbonate soil (Ho) extinction at the Permian–Triassic boundary (Meyer et al. 2008; before burial compaction were derived (Sheldon & Retallack Retallack & Jahren 2008). From study of Australian acritarchs 2001) from palaeosol measurements (Dp or Hp) corrected for an through the Elatina glaciation, ‘there does not seem to be any estimated 7.4 km depth of overburden (K in km) using the change in the nature of the populations at this time other than a 0:37K equation Do ¼ Dp/[0.51/(0.49/e ) 1]. Such measurements decline in the number of species. There is no corresponding rapid give mean annual precipitation of 342 147 mm for the Yaldati recovery and recolonization, just a slow and steady increase in palaeosol and 432 147 mm for Alpa palaeosols (standard population numbers, without introduction of new species. Rapid errors from transfer function), and seasonal precipitation differ- diversification of phytoplanktonic acanthomorphs did not happen ences typical of cool to cold temperate climates: 47 22 mm for until about 20 Ma after the glacial episode’ (Grey 2005, p. 158), Yaldati and 31 22 mm for Alpa palaeosols (standard errors of some 500 m higher stratigraphically, at the level of Acraman transfer function). Thus palaeosols of the Nuccaleena Formation ejecta in the Bunyeroo Formation. Acritarch diversification indicate seasonally frozen, cold temperate to frigid, arid climate. continued with appearance and adaptive radiation of Ediacaran biota (Runnegar 2000; Knoll et al. 2006; Fedonkin et al. 2008). Limits to snowball Earth Sturt and Elatina glacial styles? In the large literature on Neoproterozoic cap carbonates there has been remarkably little discussion of non-marine environments: Sir Douglas Mawson (1949) decided upon a glacial interpretation tillites are widely considered marine and cap carbonates deep for rocks near the junction of Elatina and Enorama Creeks and NEOPROTEROZOIC LOESS 303

Fig. 12. A reconstruction of landscape and soil during deposition of the Nuccaleena Formation. Palaeogeography was adapted from Preiss (1987), and Hallett Cove stratigraphy (cross-sections) is from Dyson & von der Borch (1986).

also at Marino Rocks well after recognizing tillites along the chronological (Corsetti & Lorentz 2006), and facies-mapping Sturt River. He emphasized differences between successive studies (Hoffman et al. 2007; Rose & Maloof 2010), which Neoproterozoic Sturtian and Marinoan glacial episodes. Sturtian demonstrate that glaciations were not synchronous between and Marinoan Stages or Epochs were correlated across Australia continents or within regions. Reinterpretation here of some of by Preiss et al. (1978), Kennedy (1996) and Walter et al. (2000). the most negative carbon and oxygen values as palaeosols (Fig. Differences between rocks of these glacial episodes were listed 7) now suggests that global isotopic chemostratigraphic correla- by Kennedy et al. (1998), who proposed global ‘Sturtian and tion of ice ages needs to be reassessed to determine carefully Marinoan glaciogenic deposit groups’. Recently, Williams et al. which samples are marine and which are non-marine. Marine (2008) have proposed that Marinoan be used only as a time unit, and non-marine environments have very different, often antitheti- and ‘Marinoan Glaciation’ be replaced by the Elatina Glaciation cal, effects on fractionation of oxygen, carbon and strontium for glaciogenic deposits including and associated with those of isotopes. the Elatina Formation. Similarly, Preiss (2011) has proposed It is here proposed that Sturt and Elatina style cap carbonates Sturt Glaciation based on the Sturt Tillite and associated rocks. in their type areas represent marine and non-marine glacial Sturt and Elatina glacial styles (or ‘glaciogenic deposit groups’ deposits, respectively, supporting recent palaeoenvironmental sensu Kennedy et al. 1998) were recognized in Canada (Aitken interpretations of correlative glacial deposits in the Officer Basin, 1991) and California (Prave 1999). Neoproterozoic glacial Western Australia (Haines et al. 2008). My observations of episodes were correlated by Halverson et al. (2005) and MacDo- Elatina glaciation rocks do not extend to thick sequences nald et al. (2010) using chemostratigraphic data from Australia, including the Penuarta Tillite to the east, which may well be California, Canada, Svalbard, Scotland, Norway and Namibia. marine, but apply only to the rocks observed from Mt. Gunson Use of such criteria to date glacial deposits around the world has and Chambers Gorge south to Hallett Cove (Fig. 1). From my been undermined by palaeomagnetic (Raub et al. 2007), geo- field inspection of two well-known localities for Sturtian tillites 304 G. RETALLACK in the central Flinders Ranges (Pichi Richi Pass: 32.473538S, M. Kennedy, M. Preiss, K. Grey, M. Walter, N. Noffke, P. Allen and 137.918548E; Booralina Creek: 31.421608S, 138.778518E), most J. 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Received 29 March 2010; revised typescript accepted 27 September 2010. Scientific editing by James Hendry. From the Geological Society Publishing House

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