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Periglacial paleosols and Cryogenian paleoclimate near Adelaide, South Australia
Accepted Manuscript Title: Periglacial paleosols and Cryogenian paleoclimate near Adelaide, South Australia Author: Gregory J. Retallack Brooklyn N. Gose Jeffrey T. Osterhout PII: DOI: Reference:
S0301-9268(15)00095-9 http://dx.doi.org/doi:10.1016/j.precamres.2015.03.002 PRECAM 4216
To appear in:
Precambrian Research
Received date: Revised date: Accepted date:
12-9-2014 13-2-2015 11-3-2015
Please cite this article as: Retallack, G.J., Gose, B.N., Osterhout, J.T.,Periglacial paleosols and Cryogenian paleoclimate near Adelaide, South Australia, Precambrian Research (2015), http://dx.doi.org/10.1016/j.precamres.2015.03.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights Sand wedge periglacial paleosols are described in Cryogenian Reynella
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Siltstone. Sand wedges show paleosol-diagnostic correlation of δ18O vs δ13C of
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carbonate.
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Paleosols are weakly differentiated chemically like periglacial soils. Periglacial paleosols represent ice-free Cryogenian coasts during
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Elatina Glaciation.
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These paleosols were not methane seeps, nor evidence of global
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Snowball Earth.
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Periglacial paleosols and Cryogenian paleoclimate near Adelaide, South
Gregory J. Retallack*, Brooklyn N. Gose, Jeffrey T. Osterhout
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Australia.
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Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272
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ABSTRACT
Late Cryogenian deformation of the Reynella Siltstone near Hallett Cove, South Australia
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has been interpreted as shallow marine methane seeps, which were furthermore proposed to have played a role in deglaciation from “Snowball Earth”. Re-examination and new
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analyses of these same outcrops confirms an earlier interpretation of this intrastratal
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deformation as periglacial paleosols, which are evidence that even at maximum extent,
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late Cryogenian glaciers did not cover all continental lowlands, let alone the world ocean. Known Miocene and Pliocene methane seeps of California examined for this study are
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very different in their gray color, rounded nodules and clayey host rock, compared with red, highly deformed, and silty to pebbly Reynella Siltstone of Hallett Cove. Stable isotopic composition of nodular carbonate in the Reynella Siltstone fails to show extreme carbon isotopic depletion or the meteoric oxygen lines characteristic of known methane seeps and sphaerosideritic waterlogged paleosols, but instead shows covariance of carbon and oxygen isotopic composition comparable with that in pedogenic and subaerial carbonate of well drained soils. Mass balance geochemical analyses show modest weathering compatible with sand wedge structures and evaporitic pseudomorphs
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Corresponding author. Tel.: +15413464558; fax:+15413464692.E-mail address:[email protected] (G.J.Retallack)
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3 indicative of arid and frigid paleoclimate. Outcrops of Reynella Siltstone near Hallett Cove represent frigid low latitude floodplains, not submarine methane seeps nor global glaciation.
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Key words: Cryogenian, periglacial, paleosol, South Australia, Snowball Earth.
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1. Introduction
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Coastal cliff exposures of the Reynella Siltstone Member of the Elatina Formation from Marino Rocks south to Hallett Cove, near Adelaide, South Australia (Fig. 1), played
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an important role in Neoproterozoic stratigraphy (Segnit, 1940), and remain important to understanding the last widespread Snowball Earth episode of the Neoproterozoic
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(Kennedy et al., 2008; Shields, 2008). There are beautifully preserved Permian tillites and
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striated pavements near Hallett Cove (Segnit, 1940), but other sea cliffs nearby have been
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disappointing as evidence of Cryogenian glaciation because they lack convincing Precambrian tillites like those known from the Flinders Ranges to the north (Mawson,
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1940). The term Marinoan, from nearby Marino Rocks is used in a chronostratigraphic sense to include late Cryogenian and Ediacaran (Fig. 2; Mawson and Sprigg, 1950), but the terminal Cryogenian glacial advance during the Marinoan time-rock interval is best called the Elatina Glaciation (Mawson, 1939; Williams et al., 2008), and not “Marinoan glaciation” (Kennedy et al., 1998; Hoffmann et al., 2004). For many years the Elatina Formation near Hallett Cove has been interpreted as evaporitic, oxidizing, supratidal and intertidal paleoenvironments (Dyson and von der Borch, 1983; Alexander, 1984), including calcareous nodular and ice-deformed paleosols (Dyson and von der Borch, 1986; Williams, 1991), but more recently Kennedy et al. (2008) have interpreted nodules
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4 and deformation as submarine methane seeps The paleosol hypothesis did not consider methane, and the methane hypothesis did not consider paleosols, so our study provides comprehensive stratigraphic, petrographic and geochemical data from multiple localities
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to test the alternatives of methane seeps or paleosols in the Reynella Siltstone Member of the Elatina Formation.
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Testing of these hypotheses has wider implications, because methane seeps have
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been proposed as a source of greenhouse warming responsible for termination of the Elatina Glaciation and the Cryogenian Period (Kennedy et al., 2008; Shields, 2008). A
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soil rather than seep interpretation of these strata negates the idea of methane-driven dramatic climatic warming after Snowball Earth, and confirms lack of evidence for
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glacial moraines at Hallett Cove (Mawson, 1940), thus limiting the seaward extent of
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Elatina Glaciation from their mountain hinterland in the current Peake and Denison
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Range of South Australia (Lemon and Gostin, 1990; Retallack, 2011). Extensive Cryogenian equatorial oceans have been predicted by some global circulation computer
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models (Hyde et al., 2000) and biomarker evidence for continued photosynthesis (Olcott et al., 2005). Equatorial seaways would have been important for the survival of life (Runnegar, 2000) compared with total freezing of lowlands and sea in snowball Earth scenarios of Hoffman et al. (1998; Hoffman and Schrag, 2002). Our study reconsiders these classical outcrops for evidence of methane seeps, glacial facies and paleosols, and implications for the extent and termination of the last Cryogenian glaciation.
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5 2. Materials and methods The Reynella Siltstone Member of the Elatina Formation was observed in two outcrops (Fig. 1): sea cliffs near Hallett Cove (S35.06317o E138.50136o), and
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the railway cuttings south of Kulpara (S34.12157o E138.033844o). Also
examined was drill core of the Reynella Siltstone Member from Wokurna DDH 6
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(S33.7134463o E138.1411738o), stored at Primary Industries Research of South
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Australia (PIRSA), in the Adelaide suburb of Glenside. At all locations paleosols were characterized, and measurements taken of the depth to salts, thickness of salt
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accumulation horizons and size of nodules (Supplementary Information Table S1). Other correlative sections examined for this work include the Elatina
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Formation below basal Ediacaran stratotype monument on Enorama Creek
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(S31.33150o E138.63339o), and the Whyalla Sandstone and Cattle Grid Breccia
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(Williams and Tonkin, 1985; Williams et al., 2008) in Mt Gunson Mine (S31.44278o E137.135350o) workings on July 5, 2007 (Figs. 1-2).
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For comparison with putative methane seeps of Hallett Cove (Kennedy et al.,
2008), some undisputed Neogene methane seeps were examined from California (Aiello et al., 2001; Aiello, 2005): near Santa Cruz (N36.949961o W122.04539o), Jalama Beach (N34.51831o W120.50828o), and Santa Barbara, California (N34.408948o W119.856324o).
Samples were collected for a variety of petrographic and geochemical studies, which aimed to characterize mineral composition, grain size, major element and stable isotopic composition of these rocks. Petrographic thin sections were point counted (500 points) for grain-size and mineral content, using a Swift
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6 ® automatic point counter and stage (Supplementary Information Tables S2-3). The same specimens were analyzed for major elements using XRF, and for ferrous iron using Pratt titration (Supplementary Information Table S4), by ALS
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Chemex of Vancouver, BC against standard SY-4 (diorite gneiss from Bancroft, Ontario). Analyses of δ13C and δ18O in carbonate were made using a Finnegan
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MAT 253 mass spectrometer in Department of Geological Sciences, University of
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Oregon, against Vienna PDB standard (Supplementary Information Table S5). Inline
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Inline Supplementary Tables S1-S5 can be found online at http://dx.doi.org/xx.xxxx/j.precamres.xxxx.xx.xx
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Hydrolytic chemical reactions characteristic of soil formation were investigated by calculating gains and losses (mass transfer of Brimhall et al., 1992) of elements in a soil
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at a given horizon (τw,j in moles) from the bulk density of the soil (ρw in g.cm-3) and parent material (ρp in g.cm-3) and from the chemical concentration of the element in soils
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(Cj,w in weight %) and parent material (Cp,w in weight %). Also needed is changes in volume of soil during weathering (strain of Brimhall et al., (1992), and estimated from an immobile element in soil (such as Ti used here) compared with parent material (εi,w as a fraction). The relevant equations 1 and 2 (below) are the basis for calculating divergence from parent material composition. w C j ,w j ,w i , w 1 1 p C j , p p C j, p i ,w 1 w C j , w
— equation 1
— equation 2
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3. Geological background The Elatina Formation is Cryogenian in age by definition, because the stratotype
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basal Ediacaran (GSSP of Fig. 2) has been placed at the base of the overlying Nuccaleena Formation in Enorama Creek, South Australia (Knoll et al., 2006). Radiometric dating in
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Namibia (Hoffmann et al., 2004), China (Condon et al., 2005), and Tasmania (Calver et
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al., 2013) places the Ediacaran-Cryogenian boundary at about 635 Ma. The Nuccaleena Formation is present as three lenses within the Seacliff Sandstone, which overlies the
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Reynella Siltstone Member of the Elatina Formation at Hallett Cove. Another constraint on minimum age comes from the correlation by Gostin et al. (2010) of glacial dropstones
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in the Bunyeroo Formation with glaciation dated by U-Pb at 582 ± 4 Ma below the Croles
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Diamictite of Tasmania (Calver et al., 2004), and at 583.7 ± 0.5 Ma in the Gaskiers
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Formation of Newfoundland (van Kranendonk et al., 2008). The Reynella Siltstone Member in turn overlies the Marino Arkose Member of
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the Wilmington Formation, which contains a detrital zircon dated by U-Pb at 657 ± 17 Ma (Ireland et al., 1998). A younger age for the base of the Elatina Formation comes from a Re-Os age of 643 ± 2.4 Ma for the Tindelphina Shale Member of the lowermost Tapley Hill Formation (Kendall et al., 2009), which is unconformably below both the Wilmington and Angepena Formations (Fig. 2). This date is incompatible with authigenic monazite cement in the sandstone of the Enorama Formation dated using Th-U-Pb at 680 ± 23 Ma (Mahan et al., 2010), and also with a U-Pb date of 659 ± 6 Ma for a tuff low in the Wilyerpa Formation (Fanning and Link, 2008). These inconsistencies are troubling,
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8 but allow the late Cryogenian (~635-640 Ma) age for the Elatina Glaciation proposed by Williams et al. (2008). Paleolatitude of deposition from chemical remnant magnetism of ultrafine
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hematite in the Elatina Formation, supported by a fold test in the Flinders Ranges, was
6.5 ± 2.2o (Schmidt and Williams, 1995; Williams et al., 2008). These measurements are
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the equator at the time of deposition of the Elatina Formation.
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currently 4o of latitude north of Hallett Cove, which was presumably also within 12o of
The rocks of Hallett Cove are gently folded in the Marino Anticline, a Delamerian
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deformation dated by Rb-Sr dating of sericite in cleavage at 531 ± 32 Ma (Early Cambrian) in the overlying Brachina Formation (Turner et al., 1994; Preiss et al., 1995).
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Some 2300 m of sedimentary cover culminating in the Bunyeroo Formation overlies the
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Elatina Formation in Hallett Cove (Dyson, 2002). This is compatible with high heat flow
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and lower greenschist facies metamorphism observed (Turner et al., 1994). The onset of brittle deformation near the Precambrian-Cambrian boundary (Preiss et al., 1995) makes
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it unlikely that Hallett Cove included an additional 1650 m of Ediacaran and 4852 m of Cambrian-Ordovician overburden found in the Flinders Ranges to the north (Retallack, 2011).
4. New observations
4.1. Field observations The upper Reynella Siltstone has pronounced intrastratal deformation in the cliff and rock platform below the Peera Street staircase (Fig. 3A-B), and Martin Kennedy generously provided us with Google Earth locations and numerous other images to guide
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9 our field observations, in addition to images published by Kennedy et al. (2008) and Shields (2008). Of particular interest were features identified as “branching pipes” and as a red “vertical cemented chimney” (Kennedy et al., 2008; Sheilds, 2008). The chimney is
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illustrated here in side view (Fig. 3B), showing that it can be traced 3 m laterally as a
three dimensional wedge-shaped structure 30 cm above the hammer. Supposed “pipes”
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present circular cross sections illustrated here (Fig. 3F), but on sampling proved to be
to ellipsoidal sections, but lack an internal conduit.
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yellow irregular to ellipsoidal nodules or clasts rather than pipes. They may have rounded
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Kennedy et al. (2008, fig. 1b) also illustrate wedge shaped structures filled with sand and breccia, and these were confirmed as common polygonal patterns on the rock
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platform (Fig. 3F) and wedges in the cliff (Fig. 3A). The fill of wedge-shaped cracks is
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not passive and horizontal, but vertical, and in places discontinuous and deformed, as
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better illustrated in our scale mapping of the cliff (Fig. 4). Small examples of these structures were regarded as desiccation-cracks by Dyson and von der Borch (1983), but
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their matrix is not clayey and their fill also vertical like that of the large examples. The sand-granule fill of the cracks is identical to that of beds which cap the red siltstone, and which are also folded upwards into small sharp ridges, best seen in plan on the rock platform (Fig. 3E). Not all parts of the measured section shows such deformation, which is common at 4-11 m, 29 m, 90-91 m, and especially prominent from 40-63 m (Fig. 5). Peloids in the sandy caps include clasts of micritic carbonate identical with those of irregular shape and diffuse boundaries lower within the deformed beds (Figs. 6, 7B-C). Thus carbonate nodules were already formed in the pink siltstones before cracking and intrusion by the deformed cover sands (as also indicated by Kennedy et al., 2008).
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10 Folding of sandy horizons within red silty horizons is not consistent from one bed to the next, as would be expected if this were a heterolithic, siltstone-sandstone sequence thrown into kink folds. These observations and continuity of cover sands down into the
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wedges are evidence that wedging was intrastratal and occurred between intervals of red silt deposition and alteration.
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Several sandstone beds have eroded margins like shallow paleochannels (between
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arrows in Fig.3G; 9, 17, 25, 28, 36 m levels of Fig. 5: see also Kennedy et al., 2008, fig.1b). These are filled with trough cross bedding, and low angle heterolithic cross
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bedding, and oriented toward the south and southwest, like paleocurrents measured from trough cross-bedding and ripple marks here by Alexander (1984). Undeformed red
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siltstones show both asymmetric and oscillation ripples, both as isolated bedforms (Fig.
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3I) and in ripple drift sequences (Fig. 3H). There also are common varvelike graded beds,
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each 2-12 mm thick and graded from silt to shale (Fig. 7A-C; Kennedy et al., 2008, fig. 1e), like those known elsewhere in the Elatina Formation (Williams et al., 2008). No
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hummocky bedding was found in the Elatina Formation, although it has been reported from the overlying Seacliff Sandstone and Brachina Formation at Hallett Cove (Dyson, 1983, 2002).
Acicular crystal casts are common at several stratigraphic levels (1-9 m, 14 m, 21-
24 m, and 27 m in Fig. 5). The crystals are in radiating arrays up to 2 cm in diameter (Fig. 3C) and full of chalcedony or clayey silt rather than the original mineral (Fig. 7D). Some are stout with interfacial angles and fishtail twins of gypsum (CaSO4.nH2O: Dyson and von der Borch, 1983), but others are highly acicular and may have been other evaporite minerals such as thenardite (Na2SO4) or mirabilite (Na2SO4·10H2O). The latter soda salts
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11 are suggested by high soda in chemical analyses of Viku and Vinarku paleosols, in which acicular crystals are common (Fig. 6). The form of the crystals is not orthorhombic like barite (BaSO4: Retallack and Kirby, 2007) and aragonite (CaCO3: Sumner, 2002). Water
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soluble evaporite minerals were likely precursors to the silicified crystal casts.
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4.2. Petrography
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In thin section, fine-grained red rocks of the Reynella Siltstone are surprisingly silty (Fig. 6). Clay is confined to thin seams that form the tops of varve-like silty graded
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beds (Fig. 7A, E), like those described for the Elatina Formation by Williams et al., (2008). The clays are metamorphosed to lower greenschist facies like those of the
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overlying Brachina Formation (Turner et al., 1994; Preiss et al., 1995), an observation
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incompatible with the identification by Kennedy et al. (2006) of common (40 %)
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dioctahedral smectite with structural disorder (SR 0.378 ± 0.14) within clay drapes to tidal laminae in the Reynella Siltstone.
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Also surprising in thin section is lack of bedding, not only on the cover sands
(Fig. 7B-C), but also in some parts of the red siltstones (Fig. 7A). The cover sands and wedge-shaped deformations are a very poorly sorted jumble of quartz, feldspar, micrite clasts, and siltstone peloids. The massive siltstones, in contrast, are very even grained and show abundant, irregular, subvertical disruptions of bedding.
4.3. Stable isotopes The isotopic composition of carbonate in the Reynella Siltstone at Hallett Cove shows an extreme range of compositions (Fig. 8): δ18OPDB of –24.5 to +12.1 ‰ and
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12 δ13CPDB. of –8.6 to +7.3 ‰ (Kennedy et al., 2008). Furthermore, some phases of the carbonate show highly significant correlation of δ18OPDB and CPDB from the same samples, as demonstrated here for replacive-micritic nodular carbonate (Fig. 8, R2=0.54)
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and by Kennedy et al., (2008) for early calcite spar cements (R2=0.84) and pore-filling
microcrystalline dolomite (R2=0.71). In contrast, various undifferentiated carbonates of
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Kennedy et al., (2008) from this locality, but unknown stratigraphic level, formed a
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cluster of mainly positive δ13CPDB but highly negative δ18OPDB (Fig. 8).
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4.4. Major element geochemistry
Major element chemical composition of Reynella Siltstone at Hallett Cove shows
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evidence of salinization (high Na2O/K2O) and calcification (high CaO+MgO/Al2O3), but
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only modest chemical weathering (low alumina/bases and Ba/Sr in Fig. 6). Some beds
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(Vani cap, Viku and Wadni of Fig. 9) show lower titania than lower in the same bed as if they gained volume during formation, but other beds have enriched titania, as if volume
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was lost during formation. The volume losses evidently included silica, alumina, iron, phosphorus, magnesium and soda, but conservation of potash (in Itala, Vani, and Vinarku beds of Fig. 9). The gains in volume were primarily in cap sands and beds with abundant relict bedding, whereas silty parts of other beds lost volume (Itala, Vani and Vinarku beds of Fig. 10).
4.5. Comparison with methane seeps For comparison, three seeps for natural gas of the late Miocene Santa Cruz Mudstone (Fig. 11A-B) and Pliocene Pico (Fig. 11C-D) and Sisquoc Formations (Fig.
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13 11E-F) were examined in coastal cliffs of California (Aiello et al., 2001; Aiello, 2005). These well documented methane seeps have nodules with tubular conduits, rimmed with bitumen. The nodules also are smooth and light gray in color, and contrast with their dark
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gray shaley host rock, which shows fine laminations and ripple marks. Nodules also
include local calcite veins. No breccias or wedge-shaped deformation was observed at
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any of these sites. We were unable to find an example of an intertidal methane seep, and
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documented methane seeps are all subtidal marine (Aiello et al., 2001; Aiello, 2005; Formolo et al. 2004; Matsumoto, 1989)
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Stable isotopic data on carbonate cements of these methane seeps are provided by Aiello et al. (2001): they are invariant and near zero for δ18OPDB and highly variable and
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negative for δ13CPDB. Four different sites within clathrate fields below the sea floor in the
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Gulf of Mexico were studied by Formolo et al. (2004) and another four sea floor sites off
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South Carolina by Matsumoto (1989), with similar results, little within-site variation in δ18OSMOW. Converted to the PDB scale of Fig. 8 the Gulf of Mexico clathrate field
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carbonate of Formolo et al (2004) varied from -7.46 to +4.40 ‰ in δ18OPDB and from 1.37 to -38.91 in δ13CPDB. Three of the sites studied by Matsumoto (1989) were similar and varied from +6.1 to +0.3 ‰ in δ18OPDB and from +3.5 to -31.8 in δ13CPDB, but a third site (533) was unusually heavy for both isotopes: from +8.2 to +3.4 ‰ in δ18OPDB and from +12.8 to -19.2 in δ13CPDB.
5. Alternative interpretations 5.1. Methane seeps
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14 Carbonate nodules, soft sediment deformation and brecciation in the Elatina Formation at Hallett Cove has been attributed to methane seeps (Kennedy et al., 2008) on the grounds of field relationships and stable isotopic composition, and this idea is further
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tested here with petrographic and geochemical data.
Our panel mapping (Fig. 4) and section measurements (Fig. 5) confirm that the
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deformation and nodules are strata concordant, but we failed to find pipe-like or bitumen-
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lined tubular features common in Neogene methane seeps documented here (Fig. 11) and elsewhere (Matsumoto, 1989; Peckmann et al., 2002, 2007; Formolo et al., 2004; Reitner
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et al., 2005). Known submarine methane seeps are all gray, unoxidized, nodular to veined shales and siltstones, lacking intraformational breccias (Aiello et al. 2001; Aiello, 2005;
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Formolo et al., 2004). Instead we found polygonal patterns of intraformational breccia on
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the rock platform (Fig. 3F) and wedge-shaped sandstone and breccia dikes in the cliff
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(Fig. 4). Perhaps the Elatina Formation was originally gray and reddened by Neogene lateritization, as has been considered for other Neoproterozoic rocks of the Flinders
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ranges (Retallack, 2013), but we observed the same red colors and wedge-shaped dikes in the Elatina Formation at a depth of 296-339 m in Wokurna 6 core (Fig. 3H-I), below 136 m of unweathered gray Seacliff Sandstone. Furthermore our observations (Fig. 3F) confirm those of Kennedy et al. (2008, fig. 1c), that red breccia clasts are found in gray matrix. Finally, paleomagnetic fold tests of the Elatina Formation in the Flinders Ranges (Schmidt and Williams, 1995) demonstrate oxidation of matrix hematite or iron hydroxides between original sedimentation and before burial diagenesis. The extreme range in both δ13C and δ18O isotopic composition and clear mixing lines of carbonates have been the main argument for methane seep interpretation of the
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15 Reynella Siltstone Member at Hallett Cove (Kennedy et al., 2008), and these are compared with mixing lines from a variety of other environments in Fig. 8. Neogene methane seep carbonates show vertical arrays of near-constant δ18O but highly variable
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δ13C (Fig. 8F), and modern marine limestones show near zero values for both δ13C and δ18O, displaced toward more negative values of δ18O in Cambrian and Neoproterozoic
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limestones (Fig. 8E). In some ancient methane seeps, two parallel vertical arrays are seen:
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one from the active seep and its associated fossils, and another in a petrographically distinct cement formed during shallow burial (Peckmann et al., 2002). Different vertical
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arrays are seen from one methane seep to another on the seafloor (Matsumoto, 1989; Formolo et al., 2004). Vertical arrays in crossplots of δ13C and δ18O are also well known
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from carbonate of methanogenic wetland paleosols (Ludvigson et al., 1998), Neither
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marine limestone, nor methane seeps show significant mixing lines (R2<0.2 in Fig. 8), but
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highly significant mixing lines are found in soils and soil crusts in which evaporative and other processes select for mass of both isotopes simultaneously within CO2 or pedogenic
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carbonate is formed on pre-existing carbonate (Knauth et al., 2003; Ufnar et al., 2008). Such a mixing line from pre-existing presumably marine carbonate clasts to methanogenic carbonate is proposed by Kennedy et al. (2008), but does not fit the local data as closely as a mixing line from marine carbonate clasts to soil crust (Fig. 8A). The range of values and covariance of stable isotopic compositions observed in the Reynella Siltstone at Hallett Cove are unknown in any marine rocks or sediments (Kennedy et al., 2008), but common in soils and paleosols (Fig. 8B). Unusually heavy values of +1.3 to +12.1 ‰ δ18OPDB and +2.4 to +7.3 ‰ δ13CPDB from “dolomite cemented siltstone (authigenic microcrystalline pore-filling dolomite,
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16 often iron rich)” of Kennedy et al. (2008), were not replicated in our own analyses (Fig.8A). Perhaps, as noted by Kennedy et al. (2008), such unusually positive values were produced during burial alteration several hundred meters below the sea floor by
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fluids generated within the zone of methane fermentation, as proposed by Matsumoto
(1989). This may have been within the source rocks for dolomite grains, which match
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marine isotopic composition (Figs. 8A,E). Metamorphic alteration to low in the
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greenschist facies (Turner et al., 1994; Preiss et al., 1995) typically creates lighter values of δ18OPDB and δ13CPDB (Algeo et al., 1992), so does not explain unusually heavy values
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at Hallett Cove, although perhaps contributing to the remarkable spread of isotopic values (Fig. 8A). Alternatively, Knauth et al. (2003) have shown that comparable isotopic
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enrichment is created by surface evaporation, and calcite soil coatings in the Atacama
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Desert of Chile reach values of +7.29 ‰ for δ18OPDB and +7.42 for δ13CPDB (Quade et
analyzed at micron scale.
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al., 2007). These unusually heavy stable isotopic values will remain a puzzle until
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Finally the methane-seep-cemented marine beds should show invariant mineral
composition and chemical composition, and strain-tau values within the dilate-and-gain field. Some beds (Viku, Vani and Wadni of Fig 6) show little mineral or chemical differentiation, but others (Vinarku and Itala) show surficial gains of clay and oxidized iron expected of Neoproterozoic paleosols (Retallack, 2013). Dilate-and-gain of sedimentary additions were seen in capping sands of some beds, but most of the beds lost mass and weatherable components in the collapse-and-loss field of soils and paleosols (Fig. 9). Titanium used as a stable constituent is mainly in ilmenite and other heavy minerals which fall to the base of sedimentary beds, but are enriched at the surface during
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17 soil formation which depletes the bed in weatherable components. The analyzed beds of the Elatina Formation are unlike marine event beds in this respect, but also are not deeply weathered. These petrographic and geochemical data can be added to stable isotopic and
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field observations which falsify the methane seep hypothesis of Kennedy et al. (2008).
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5.2. Paleosols
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Intrastratal deformation of the Reynella Siltstone at Hallett Cove (Fig. 4) is similar to periglacial sand wedge paleosols of the correlative Whyalla Sandstone and
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Cattle Grid Breccia (Fig. 2) described by Williams and Tonkin (1985), Williams (1986), and Williams et al. (2008). Polygonal networks of fissures (Fig. 3F) distinguish these
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features from seismically induced sand volcanoes (Galli, 2000). Sand wedges have
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vertical bedding, unlike passively filled ice wedges (Williams, 1986). The largest sand
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wedges are in beds with the largest calcareous nodules and the most disturbed bedding (Fig. 2A), so that variation in size may be related to varied durations of formation. The
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variation in size of the Reynella Siltstone sand wedges matches both ice and sand wedges (Fig. 12B-C) of Holocene (Leffingwell, 1915; Black, 1976a,b; Gell, 1978; Washburn, 1980; Davis, 2001) and Pleistocene age (Black, 1976a; Washburn, 1980; Wayne, 1991; Dylik, 1994; French, 1996; Owen et al., 1998; Davis, 2001; Van Vliet-Lanoë, 2010; Raffi and Stenni, 2011).
The fill and capping sands to layers include mildly deformed soft-sediment clasts (Fig. 7B) similar to frozen granules transported in periglacial outwash (van Vliet-Lanoë, 1985, 1998, 2010). These cap-sands and wedge fills are less geochemically weathered (base depleted) than associated red siltstones (Figs. 9-10). Cap-sands may represent
Page 17 of 66
18 episodes of fluvioglacial outwash terminating the formation of the sand wedges, but also mildly deformed by subsequent sand wedges at higher stratigraphic levels (Fig. 4). Periglacial sand wedges are one form of paleosol, but so are other features of the
ip t
Reynella Siltstone at Hallett Cove. Replacive spherulitic sand crystals (Fig. 7D) and
micritic nodules (Fig. 6) organized into horizons a fixed distance below the ancient land
cr
surface are characteristic of soils and paleosols (Gile et al., 1981; Dan and Yaalon, 1982;
us
Monger et al., 1991; Almohandis, 2002). Both are replacive because they contain preexisting grains of their matrix, and not displacive clear crystals of gypsum or calcite
an
common in marine nodules and evaporites (Shearman, 1978; El Khoriby, 2005; Ziegenbalg et al., 2010). Crystal casts extend beyond the spherulitic core of the aggregate
M
(Figs. 3D, 7D), and the micrite nodules are laterally linked into irregular aggregations
te
formation elsewhere.
d
(Fig. 3F) as evidence that they formed in place, rather than as clasts transported from
Periglacial sand wedges and replacive evaporites and micrites are comparable
Ac ce p
with soil features which destroy original fine lamination of the Reynella Siltstone, with destruction of bedding proportional to the size and abundance of the wedges, sand crystals and nodules. But there are other finer scale disruptions of lamination giving rise to massive beds, and best appreciated on the scale of thin sections. Some of this homogenization of bedding is a system of cracks and brecciation at the tops of massive beds (Fig. 7C), as noted for the Hallett Cove locality by Dyson and von der Borch (1983). Other bed tops have a system of filamentous disruptions of bedding spreading downward (Fig. 7A). The small diameter, irregularity and ferruginization of these filamentous structures is most like structures made by soil microbes (Poelt and Baumgärtner, 1964;
Page 18 of 66
19 Mihail and Bruhn, 2005; Garcia-Pichel and Wojciechowski, 2009). They are much thinner and irregular than fluid-escape structures or mud volcanoes, which tap into organic or fine grained lower layers not seen at Hallett Cove (Dionne, 1976; Nichols et
ip t
al., 1994).
Geochemical differentiation within individual beds reflects typical soil
cr
weathering: incongruent dissolution of feldspar and other weatherable minerals by
us
carbonic acid to form clay and release nutrient cations (Ca2+, Mg2+, Na+, K+). Modest progress of this hydrolysis reaction is revealed by increased clay and molar ratios of
an
alumina/bases towards the tops of profiles (Fig. 6). A more precise demonstration of hydrolysis is revealed by calculating chemical losses and gains, as well as volume loss
M
and gain to a stable constituent (Equations 1 and 2 above after Brimhall et al., 1992). The
d
field of collapse of volume and loss of cations characteristic of soils is demonstrated by
te
chemical analyses of paleosols in the Reynella Formation, but the cap sandstones show subdued change or plot in the dilate and gain field characteristic of sediments (Fig. 9).
Ac ce p
Within-bed differences in red color comparable with those of ferruginization of well drained paleosols and limited burial gelization (Retallack, 1997) are revealed by variation in ferrous to ferric iron ratio (Fig. 6). This red color was acquired during the Cryogenian as revealed by redeposited clasts of red siltstone (Fig. 7B), a paleomagnetic fold test (Schmidt and Williams, 1995), and similar color of paleosols in the Elatina Formation at depths of 296-329 m in Wokurna DDH6 drillcore (Fig. 3H-I). . Significant covariance (R2=0.54) between δ18O and δ13C is characteristic of paleosols, and its slope is low (0.13), perhaps reflecting low rates of paleoevaporation in a cold climate (Ufnar et al., 2008). Extremely enriched δ18OPDB values of +6.96 ‰ at
Page 19 of 66
20 Hallett Cove are found in highly evaporative soil crusts (Knauth et al., 2003; Quade et al., 2007). Very low δ18OPDB values (down to –22.5 ‰) are evidence of glacial meltwater or cold rain (Kennedy et al., 2008), but near zero δ18O values may represent Proterozoic
ip t
marine limestone fragments within the paleosols as silt-size grains of the parent loess
cr
(Fig. 7A,E).
us
6. Sedimentary interpretations
Sedimentary paleoenvironments of alluvial to tidal coastal flats are indicated for
an
the Reynella Siltstone by primary sedimentary structures including bimodal paleocurrents (Alexander 1984), herringbone and flaser bedding and bundled lamination (Kennedy et
M
al., 2008, fig. 1e; Williams et al., 2008, fig 7a-b,d). Compared with more complex
d
bundling in Reynella Siltstone of the Flinders Ranges (Williams, 1991), bundling near
te
Hallett Cove includes proximal neap-spring tidal cycles representing 13.1 ± 0.1 lunar months/year, 400 ± 7 solar days/year and 21.9 ± 0.4 hours/solar day (Williams et al.,
Ac ce p
2008). Intertidal facies, shallow paleochannels with heterolithic fill of sandstone and siltstone (Fig. 3G between arrows) and limestones are common in the lower 37 m of the measured section (Fig.5), north along the seacliffs toward Marino Rocks. These paleochannels not only contain heterolithic lateral accretion sets, but have steep cut banks (right hand arrow of Fig. 3G). Lateral accretion and cut banks are hallmarks of meandering streams, which Davies and Gibling (2010) argue are unknown before the Silurian. This interval of the Reynella Siltstone is also where evaporite pseudomorphs are most common and indicate largely dry supratidal coastal flats (Dyson and von der Borch, 1983). Meandering intertidal channels are known from Proterozoic (Bengtson et al.,
Page 20 of 66
21 2007; Dehler et al., 2012) and Cambrian (Hereford, 1977; Cloyd et al., 1990) intertidal facies, and remain common on unvegetated, low-relief, muddy, tidal flats today (Kleinhans et al., 2009).
ip t
Tidal or lagoonal varve-like lamination (Fig. 7A,E) is less common within the
interval 37-65 m, where periglacial sand wedges are common, and paleochannels are still
cr
of trough cross-bedded sand of a contrasting gray color to the intervening red siltstones.
us
From 65-92 m red color is found in three distinct facies: (1) siltstone paleosols, (2) siltstones with ripple drift cross-lamination, and (3) trough cross-bedded paleochannels.
an
These parts of the sequence are similar to alluvial floodplain sequences: the drab paleochannel facies is comparable with the Ediacaran, Ediacara Member of South
M
Australia (Retallack, 2013) and numerous Phanerozoic floodplain facies (Retallack,
d
2008; Morrison and Straight Cliffs Formations of Retallack, 2009). Higher in the
te
sequence both paleochannels and paleosols are red, comparable with the Ediacaran Bonney Sandstone of South Australia (Drexel et al., 1993) and other Phanerozoic
Ac ce p
floodplain facies (Retallack, 2001; Halgaito and Summerville Formations of Retallack, 2009).
7. Paleosol interpretations 7.1. Identification
Individual beds of the Reynella Siltstone Member with deformational, petrographic and geochemical characteristics of paleosols fall into five repeated types, here given non-genetic pedotype names from the Adnamatna aboriginal language
Page 21 of 66
22 (McEntee and McKenzie, 1992). These include Itala (crack in rock), Vani (flesh), Viku (eyebrow), Vinarku (gypsic alluvium), and Wadni (small). The various pedotypes of the Reynella Siltstone at Hallett Cove can be identified
ip t
within modern soil classifications (Table 1), such as those of the United States (Soil
Survey Staff, 2010) and the Food and Agriculture Organization (1975, 1978), but the
cr
Australian classification does not seem to have closely comparable soils at present
us
(Isbell, 1996). The most similar Russian soils (units Bx6-3b of Food and Agriculture Organization, 1978), are Gelic Cambisols associated with Calcic Cambisols and Calcaric
an
Regosols. Map unit Bx6-3b covers 10,816,000 hectares in floodplain and headwaters of the Olenek River, near Jerbogačon, Siberia, where climate is frigid and vegetation is pine
M
(Pinus sibirica)-larch (Larix dahurica) taiga forest. Comparable Canadian soils (units
d
Bx1-1a and Bx 2-1b of Food and Agriculture Organization, 1975) cover 6,449,000
te
hectares of marine sediments and glacial outwash west of Hudson Bay and north of Churchill, where climate is frigid and vegetation is mixed spruce (Picea mariana) taiga
Ac ce p
and tundra. Jerbogačon has a mean annual temperature of –6.7oC and mean annual precipitation of 323 mm, and for Churchill these values are –7.2oC and 435 mm (Müller, 1982). Such climate may be comparable with that of these Cryogenian paleosols, but such vegetation cannot be inferred for paleosols from before the advent of land plants.
7.2. Paleotopography Tidal laminae (Fig. 7A, E) persist at least to 63 m in the measured section (Fig. 5) and are evidence of a flat coastal plain to supratidal flats for the Reynella Siltstone. At these stratigraphic levels the sandstone paleochannels and lower levels of the paleosols
Page 22 of 66
23 are gray (Fig. 5), and ferrous iron is common (Fig. 6), but other parts of the paleosols are red and ferruginized (Fig. 6). Such alternation of redox character can be attributed to persistently high water table in a coastal plain, as for other comparable sequences of
ip t
coastal paleosols (Retallack, 2008, 2013). These floodplain could not have been
permanently waterlogged, because brittle, frozen soil is required for the formation of sand
cr
wedge polygons (Maloof et al., 2002). Red paleochannels above 65 m (Fig. 5) reflect
us
oxidation during low water stages in the original streams (Retallack, 2001, 2008). Local relief was thus no more than 3 m seen on some paleochannel cut banks. Periglacial
an
polygons (Fig. 4) and locally deformed outwash sands (Fig. 3E) would have given
M
ground roughness with an amplitude of about 20 cm.
d
7.3. Former parent material
te
Parent materials to the paleosols were quartzofeldspathic and dolomitic silts and sands (Stevenson, 1972). The high proportion of dolomitic silt seen in some levels (Fig.
Ac ce p
6) may be eolian and derived from extensive marine dolostones exposed in the Peake and Denison Ranges (Retallack, 2011). Quartzofeldspathic silt grains also are angular and randomly arranged, and may be loess distal to dune fields represented by the Whyalla Sandstone (Williams et al., 2008). These silty parent materials contrast with sandy to gravelly fluvioglacial outwash that forms caps to many of the beds (Fig. 6).
7.4. Time for formation Rates of ice and sand wedge growth in Antarctica ranged from 0.79 mm/yr in the 1960s (Black, 1973) to 0.04 mm/yr in the 1970s (Black, 1982). Such wide disparity is
Page 23 of 66
24 problematic for understanding rates and extrapolating age, though comparable with rates of growth of such features in the northern hemisphere (Bockheim, 1995). From their width in paleosols of the Reynella Siltstone (Fig. 12B) the sand wedges would have
ip t
formed in 10-72 years by the fast rate and 200-1425 years by the slow rate.
Longer estimates of time come from the formation of carbonate nodules in the
cr
paleosols. In desert soils of New Mexico (Retallack, 2005) diameter of carbonate nodules
us
(S in cm) is related to soil age (A in kyrs) by equation 3 (R2=0.57; S.E.± 1.8 kyrs). A = 3.92S0.34
— equation 3
an
This transfer function gives 16.5 ± 1.8 kyr for the type Itala paleosol and 6.6 ± 1.8 yr for the type Vani paleosol. An average duration with one standard deviation for 9 Itala
M
paleosols is 16.2 ± 5.5 kyr and for 20 Vani paleosols is 5.7 ± 3.0 kyr (Supplementary
d
Information Table S1). These paleosols thus formed on Milankovitch time scales, with
te
long episodes of weathering followed by shorter episodes of frigid deformation. Sandy caps to the beds show little chemical weathering (Figs 9-10), compared with the silty
Ac ce p
paleosols below with their carbonate nodules (Kennedy et al., 2008). Milankovitch precesssion cycles have been computed to have been 19,462 years, and obliquity cycles 31,243 years at 640 million years ago (by interpolation from table of Berger and Loutre, 1994). Wadni and Viku paleosols show little soil formation, and comparable Inceptisols and Entisols today represent millenia to centuries of soil formation, even in Antarctica (Campbell and Claridge, 1987; Retallack et al., 2001).
7.5. Paleoclimate
Page 24 of 66
25 Sand wedge polygons form today under a well constrained set of paleoclimatic controls: mean annual air temperatures of less than -12 to -20oC, mean coldest month temperature <-35oC, mean warmest month temperature of +4oC, and mean annual
ip t
precipitation <100 mm (Williams, 1986). Comparable paleoclimate has been inferred for fossil sand wedges of the Cattle Grid Breccia and lower Whyalla Sandstone of the Mount
cr
Gunson Mine on the Stuart Shelf, and like those periglacial paleosols, the comparable
us
features at Hallett Cove may record the coldest part of the Elatina Glaciation (Williams et al., 2008). Warmer, though still frigid, conditions are also evident from thufur mounds
an
and shadow dunes in the overlying Nuccaleena Formation (Retallack, 2011). Under warmer frigid conditions, a variety of structures are formed including ice wedge
M
polygons, periglacial involutions and thufur mounds (Young and Long, 1976; Krull,
d
1999; Retallack, 1999a,b, 2011, 2013). Ice wedges are best known from Greenland and
te
Arctic Canada, which are maritime glacial climates, as opposed to the continental glacial climate of modern Antarctica with sand wedges (Black, 1976a,b; Washburn, 1980). Sand
Ac ce p
wedge polygons have also been reported from the Lillfjället Formation of early Cryogenian age (Kumpulainen, 2011) and ice wedge polygons from Paleoproterozoic glacial deposits (Young and Long, 1976). Frigid paleotemperatures are supported by the pedogenic paleothermometer of
Óskarsson et al.,(2012), based on frigid modern soils under tundra vegetation of Iceland. This linear regression between mean annual temperature (T in oC) and chemical index of weathering (I = 100mAl2O3/(mAl2O3+mCaO+mNa2O), in molar proportions is given in equation 4 (R2 = 0.81; S.E. = ± 0.4oC) T = 0.21I –8.93
— equation 4
Page 25 of 66
26 Results for the analyzed type profiles are mean annual temperatures of –7.1 ± 0.4 o
C for the type Viku paleosol, –3.3 ± 0.4 oC for the type Vinarku paleosol, –6.6 ± 0.4 oC
for the type Vani paleosol, and –4.8 ± 0.4 oC for the type Itala paleosol. Other chemical
ip t
paleothermometers of Gallagher and Sheldon (2013) and of Sheldon et al.,(2002) have a training set of modern temperate forested soils, and yielded mean annual
cr
paleotemperatures in the range 7.2-10.7 oC, incompatible with evidence of sand wedge
us
polygons (Fig. 4).
Arid paleoclimate for the paleosols of the Reynella Siltstone is supported by three
an
distinct paleohyetometers. A chemical transfer function of Sheldon et al. (2002) based on temperate soils of North America uses chemical index of alteration without potash (R =
M
100mAl2O3/(mAl2O3+mCaO+mNa2O) in moles), which increases with mean annual precipitation (P in mm) in modern soils (R2 = 0.72; S.E. = ± 182 mm), as follows. — equation 5
te
d
P = 221e0.0197R
This formulation is based on the hydrolysis equation of weathering, which enriches
Ac ce p
alumina at the expense of lime, magnesia, potash and soda. Magnesia is ignored because not significant for most sedimentary rocks, and potash is excluded because it can be enriched during deep burial alteration of sediments (Maynard, 1992). The other two other paleohyetometers come from depth to salts in paleosols.
Depth (Dy in cm) to gypsic (By) horizon in modern soils (Retallack and Huang, 2010) has found the relationship of equation 5 with mean annual precipitation (P in mm: R2= 0.63, standard error ± 129 mm). Mean annual precipitation (P in mm) is also related to depth to carbonate nodules (Dk in cm) according to equation 6 (R2 = 0.52; S.E. = ± 147 mm). Both
Page 26 of 66
27 training sets include many sparsely vegetated soils paleosols, including polar soils with no vegetation visible to the naked eye (Retallack, 2005; Retallack and Huang, 2010). P = 87.593e0.0209·Dy
R = 137.24 + 6.45Dk - 0.013Dk2
— equation 7
ip t
— equation 6
Both these proxies (equations 6 and 7) for paleoprecipitation require decompaction of the
cr
paleosols, and in this case physical constants for Aridisols (equation 7 after Sheldon and
us
Retallack, 2001) were used giving depth to salts in the in original soil (Ds) from depth in the current paleosol (Dp) for a particular depth of burial (K in km). 0.38 -1)] e 0.17 K
an
Ds = Dp/[-0.62/(
— equation 8
M
Arid paleoclimate was calculated by all three methods: mean annual precipitation of 357 ± 182 mm from chemical composition and 158 ± 129 mm from depth to gypsum
d
for type Viku paleosol, 500 ± 182 mm from chemical composition and 162 ± 129 mm
te
from depth to gypsum for the type Vinarku paleosol, 275 ± 182 mm from chemical
Ac ce p
composition and 330 ± 149 mm from depth to micrite for the type Vani paleosol, and 325 ± 182 mm from chemical composition and 314 ± 149 mm from depth to micrite for the type Itala paleosol.
7.6. Ancient life on land
Phosphorus is depleted in some paleosols of the Reynella Siltstone (Figs. 9-10),
and experimental studies of modern soils have shown that only organic ligands can weather phosphorus to this extent from apatite in soils (Neaman et al., 2005). Another indication of microbial activity in these paleosols is molar depletion of alkali and alkaline
Page 27 of 66
28 earth oxides (CaO+MgO+Na2O+K2O) in the paleosols (Figs 9-10) by CO2 from soil respiration (Retallack, 1997). Microtubular disruption of lamination in paleosols of the Reynella Siltstone is
ip t
sparse in lower parts of the profiles (Fig. 7E), but totally disrupts lamination in the upper part of the profiles (Fig. 7A). These ptygmatically compressed, near-vertical tubules are
cr
10-50 μm in diameter. Similar features in modern soils are produced by bundles of
us
sheathed cyanobacteria, like modern Microcoleus vaginatus (Garcia-Pichel and
Wojciechowski, 2009), rhizomorphs of fungi, like those of honey mushroom, Armillaria
an
mellea (Mihail and Bruhn, 2005), or lichen rhizines like those of bruised lichen, Toninia sedifolia (Poelt and Baumgärtner, 1964). Although the exact nature of these microbes are
M
unknown, communities of paleosols in the Reynella Siltstone may have been microbial
te
and Lange, 2003).
d
earths (Retallack, 2012), like those as known today from biological soil crusts (Belnap
Life on land is known back at least 2200 million years (Retallack et al., 2013), so
Ac ce p
evidence of microbes in paleosols of the Reynella Siltstone is unremarkable. Nor were these paleosols too cold to support life: comparable frigid soils in Antarctica today also have a rich microbial community, as well as rotifers, nematodes, tardigrades, mites and springtails (Cary et al., 2010). No megafossils were found in the Reynella Siltstone, although they are known in other Cryogenian rocks. Compressions such as Aspidella, some 766 million years old have been found on surfaces with old-elephant skin textures (Meert et al., 2011), like those of microbial earths (Retallack, 2012). Sponge-like skeletal remains such as Otavia, are known from marine limestones up to 760 million years old (Maloof et al., 2010; Brain et al., 2012).
Page 28 of 66
29
8. Correlations and causes Correlation of the Reynella Siltstone and Nuccaleena Formations between Hallett
ip t
Cove and the central Flinders Ranges is secure because of identical lithologies in each area, connected by Kulpara railway cutting and Wokurna core (Williams et al., 2008).
cr
These formations are separated by a regional disconformity in the central Flinders
us
Ranges, but to the south by channel incision at the base of the Seacliff Sandstone which includes stringers of Nuccaleena Formation (Dyson and von der Borch, 1986; Dyson,
an
2002). These relationships are problematic for the idea that the Reynella Siltstone includes methane seeps which terminated the last Cryogenian glacial advance (Kennedy
M
et al., 2008). Instead of global warming and sea level rise predicted by that view, the
d
basal Seacliff sandstone paleochannel incision records a local relative fall of sea level
te
(Dyson and von der Borch, 1994). Observations along this disconformity are also not supportive of the idea of a glacial advance all the way to the sea in a global Snowball
Ac ce p
Earth (Hoffman and Schrag, 2002), because several generations of observers (Segnit, 1940; Mawson, 1940; Williams et al., 2008) have failed to find evidence of dropped pebbles or moraines within the Seacliff Sandstone. Trough cross bedding of the Seacliff Sandstone is evidence of fluvial incision and fill (Dyson and von der Borch, 1994), at a time of relative sea level fall during frigid conditions that created the sand wedge paleosols described here. Post-glacial marine transgression was not until much later, as indicated by gray siltstones and hummocky cross bedding of the Brachina Formation (Dyson, 1983). The disconformity between Seacliff Sandstone or Nuccaleena Formation and the Elatina Formation is regional (Williams et al., 2008), but has nowhere yielded
Page 29 of 66
30 thick paleosols as evidence of longer duration than other paleochannels within the Elatina Formation (Fig. 5).
ip t
9. Conclusions
The Reynella Siltstone Member of the Elatina Formation between Marino Rocks
cr
and Hallett Cove may have been deposited in coastal plains to intertidal flats (Fig. 13;
us
Table 2), as envisaged by Mawson (1940), Dyson (1983), Dyson and von der Borch (1983, 1986), Alexander (1984) and Williams (1991). This passive margin coast was well
an
south of moraines of the Elatina Formation of the Flinders Ranges to the north (Lemon and Gostin 1990; Retallack 2011). An array of supratidal to alluvial paleosols in the
M
Reynella Siltstone includes Gelisols with sand wedge polygons formed near Hallett Cove in an arid frigid continental paleoclimate like that of modern Antarctica (Fig. 13). Such
te
d
cold conditions within 12o of the equator during the late Cryogenian (~635-640 Ma) peak of the Elatina Glaciation (Williams et al., 2008) confirm the remarkable Neoproterozoic
Ac ce p
spread of glacial facies that has been termed “Snowball Earth” (Hoffman and Li, 2009). Our reconstruction of the Reynella Siltstone Member of the Elatina Formation
from Marino Rocks to Hallett Cove confirms the conclusion of Mawson (1940) that these rocks contain neither tillites nor fully marine deposits. Our reconstruction thus does not support extreme interpretations of a hard Snowball Earth, with the whole ocean and all lowlands completely enveloped in ice (Hoffman et al., 1998; Hoffman and Schrag, 2002). Hallett Cove at least was not covered by glacial ice or ocean during the last glacial advance of the Cryogenian, perhaps represented by sand-wedge polygons in the Reynella Siltstone (Fig. 4) and Cattlle Grid Breccia and Whyalla Sandstone (Williams et al., 2008).
Page 30 of 66
31 It could also be that the last Cryogenian glacial maximum was within the ca. 50 m palaeovalley incision of the Reynella Siltstone by the Seacliff Sandstone, but neither the Seacliff Sandstone nor Brachina Formation has dropstones or any other evidence of sea
ip t
ice or icebergs (Dyson, 1983, 2002; Dyson and von der Borch, 1994). Alternative global
circulation models of the late Cryogenian glaciation allow large areas of ice-free tropical
cr
ocean (Hyde et al., 2000) more congenial to survival of life on Earth (Runnegar, 2000).
us
Our failure to find subtidal marine facies or pipe-like vent structures also does not support the interpretation of Kennedy et al. (2008) that intrastratal deformation in the
an
Reynella Siltstone was created by marine methane seeps. Genuine marine methane seeps examined for comparison in this work are gray, silty, and have large smooth calcareous
M
nodules with a very different isotopic signature of invariant δ18O but varied δ13C (Aiello et al., 2001; Peckmann et al., 2002). The Reynella Siltstone carbonate in contrast is red
te
d
and brecciated and shows strong covariance of δ18O and δ13C as in mixing lines observed in modern soils and soil crusts (Knauth et al., 2003; Ufnar et al., 2008). Finally beds of
Ac ce p
Reynella Siltstone show loss of mass and volume by mass balance with chemical composition lower in the same bed, more like paleosols than marine beds (Retallack, 2013). Thus there is little structural, isotopic or geochemical support at Hallett Cove for the idea of Kennedy et al. (2008) that methane emissions hastened the end of the Elatina Glaciation. Furthermore, periglacial paleosols in the overlying Nuccaleena Formation (Retallack, 2011), and persistent glacial conditions throughout the succeeding Ediacaran period (Gostin et al., 2010; Jenkins, 2011) are evidence that deglaciation at the end of the Cryogenian was neither geologically rapid nor profound.
Page 31 of 66
32 Acknowledgments Erick Bestland and Matt Forbes first showed GJR these paleosols. Martin Kennedy, Nora Noffke, Greg Ludvigson, Richard Behl, and Nathan Sheldon offered useful discussion.
cr
analyses. Rob Whittecar and Randall Parrish offered helpful reviews.
ip t
Ilya Bindemann and Jim Palandri are thanked for donation of carbonate stable isotopic
us
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tectonic/structural controls on fluid flow in an active transform margin. Palaeogeography Palaeoclimatology Palaeoecology 227, 124-142.
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paleosol and (D) its acicular sand crystals (both at 19 m in Fig. 5); (E) oblique view of
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uplifted wedge of cover sand above Viku paleosol (19 m in Fig. 5); (F) plan view of drab polygons in Vani paleosol on rock platforms ( 43 m in Fig. 5); (G) shallow paleochannel
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of sandstone and siltstone with lateral accretion sets (between arrows: at 18 m in Fig. 5); (H) Vani paleosol with small yellow nodules at 327 m (right) and Itala paleosol with
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large yellow nodules at 326 m (left); (I) Wadni paleosol at 301.7 m . Hammers for scale
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(A-F), cliff 30 m tall (G) and core is 5 cm diameter (H-I).
Fig. 4. Scale diagram of intrastratal deformation, interpreted as successive periglacial
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sand wedges in the late Cryogenian Reynella Siltsone Member of the Elatina Formation at Hallett Cove (interval 44-49 m of Fig. 5).
Fig. 5. Measured section of paleosols in the late Cryogenian Reynella Siltstone Member of the Elatina Formation at Hallett Cove.
Fig. 6. Petrographic and geochemical data for paleosols in the late Cryogenian Reynella Siltstone Member of the Elatina Formation at Hallett Cove.
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50 Fig. 7. Photomicrographs of paleosols in the late Cryogenian Reynella Siltstone Member of the Elatina Formation at Hallett Cove: (A) filamentous bioturbation of lamination in upper part of type Viku paleosol (21 m in Fig. 5A); (B) peloids in cover sand to Vani
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paleosol (46 m in Fig. 5A); (C) peloids in cover sand to Itala paleosol (47 m in Fig. 5A); (D) acicular sand crystal disruption to bedding in Viku paleosol (m in Fig. 5A); (E)
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varvoid bedding in Itala paleosol (47 m in Fig. 5A). Specimen numbers in Condon
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Collection of the Museum of Natural and Cultural History, University of Oregon are (A)
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R3879, (B) R3903, (C) S3896, (D) R3880, and (E) R3899.
Fig. 8. Distinct mixing lines of carbon and oxygen isotopic composition of carbonate in
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(A) in the late Cryogenian Reynella Siltstone Member of the Elatina Formation at Hallett
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Cove derived for this study (closed circles), and by Kennedy et al. (2008: closed squares);
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(B) soil nodules (above Woodhouse lava flow, near Flagstaff, Arizona, from Knauth et al., 2003; and in Yuanmou Basin, Yunnan, China, from Huang et al. 2005), (C) soil crusts
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on basalt (Sentinel Volcanic Field, Arizona, from Knauth et al. 2003), (D) Quaternary marine limestone altered diagenetically by meteoric water (Key Largo, Florida, from Lohman 1988, and Clino Island, Bahamas, from Melim et al. 2004), (E) unweathered marine limestone of Holocene (from Veizer et al.,2000) and Early Cambrian age (Ajax Limestone, South Australia, from Surge et al. 1997), and (F) marine methane cold seep carbonate (Miocene, Santa Cruz Formation, Santa Cruz, California, from Aiello et al. 2001). Slope of linear regression (m) and coefficients of determination (R2) for mixing lines show that carbon and oxygen isotopic composition is significantly correlated in soils and paleosols, but not in marine limestones or methane seeps.
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Fig. 9. Mass balance geochemistry of paleosols in the Reynella Siltstone of South Australia, including estimates of strain from changes in an element assumed stable (Ti)
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and elemental mass transfer with respect to an element assumed stable (Ti, following
Brimhall et al.,, 1991). Zero strain and mass transfer is the parent material lower in the
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profile: higher horizons deviate from that point due to soil formation or sedimentation as
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indicated in key to lower right.
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Fig. 10. Depth functions of elemental mass transfer with respect to an element assumed stable (Ti, following Brimhall et al.,, 1991) for paleosols in the Reynella Silstone of
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South Australia.
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Fig. 11. Neogene methane seeps from California: Santa Cruz Formation (late Miocene) near Santa Cruz (A-B), Sisquoc Formation (Pliocene) at Jalama Beach (C-D) and Pico
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Formation (Pliocene) at Santa Barbara, California (E-F).
Fig. 12. Hypothetical development of periglacial paleosols from examples of varied development in the late Cryogenian Reynella Siltsone Member of the Elatina Formation at Hallett Cove (A) and comparable dimensions of modern sand wedges (B).
Fig. 13. Conjectural reconstruction of paleosols and sedimentary paleoenvironment of the Reynella Siltstone near Adelaide. Table 1. Pedotype definition and classification for the Elatina Formation Pedotype
Diagnosis
Soil Survey Staff (2010)
Food and Agriculture Organization
Australian (Isbell
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52
Vani
Vinarku Viku Wadni
Drab yellowish gray sandy surface with deep wedge like deformation(A) over red siltstone with large calcareous nodules (Bk) Drab yellowish gray sandy surface with deep wedge like deformation(A) over pink siltstone with small calcareous nodules (Bk) Drab yellowish gray sandy surface (A) over diffuse zone of calcified gypsum rosettes (By) Drab yellowish gray sandy surface (A) over dark red bedded silstone (Bw) Ferruginized sitstone with cracking (A) over bedded siltstone with wispy mottles (C)
Haploturbel
(1975,1978) Gelic Cambisol
1996) Lithocalcic Calcarosol
Haploturbel
Gelic Cambisol
Lithocalcic Calcarosol
Haplogypsid
Gypsic Yermosol
Haploxerept
Eutric Cambisol
Hypersalic Rudosol Red-Orthic Tenosol Stratic Rudosol
Fluvent
Fluvisol
Viku
Microbial soil crust Microbial soil crust
Not diagnostic of climate
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Microbial soil crust
Early successional microbial soil crust Early successional microbial soil crust
Parent material Dolomitic and quartzofeldspathic silt Dolomitic and quartzofeldspathic silt Dolomitic and quartzofeldspathic silt Dolomitic and quartzofeldspathic sand Dolomitic and quartzofeldspathic silt
loess plain
well drained supratidal flat
Time for formation 10,00020,000 years 3,000-9,000 years 100-1,000 years 100-1,000 years
10-100 years
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Wadni
Frigid arid(mean annual temperature –4.8 ±0.4oC; mean annual precipitation 314 ± 147 mm) Frigid arid (mean annual temperature –6.6 ±0.4oC; mean annual precipitation 330 ± 147 mm) Frigid arid (mean annual temperature –3.3 ± 0.4oC; mean annual precipitation 162 ± 129 mm) Frigid arid (mean annual temperature –7.1 ± 0.4oC; mean annual precipitation 158 ± 129 mm)
Topographic setting Well drained loess-alluvial terrace Well drained loess-alluvial terrace Well drained supratidal flat
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Vinarku
Organisms
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Vani
Climate
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Table 2. Pedotype interpretative summary for the Elatina Formation Pedotype Itala
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Itala
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S0301-9268(15)00095-9 http://dx.doi.org/doi:10.1016/j.precamres.2015.03.002 PRECAM 4216
To appear in:
Precambrian Research
Received date: Revised date: Accepted date:
12-9-2014 13-2-2015 11-3-2015
Please cite this article as: Retallack, G.J., Gose, B.N., Osterhout, J.T.,Periglacial paleosols and Cryogenian paleoclimate near Adelaide, South Australia, Precambrian Research (2015), http://dx.doi.org/10.1016/j.precamres.2015.03.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Highlights Sand wedge periglacial paleosols are described in Cryogenian Reynella
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Siltstone. Sand wedges show paleosol-diagnostic correlation of δ18O vs δ13C of
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carbonate.
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Paleosols are weakly differentiated chemically like periglacial soils. Periglacial paleosols represent ice-free Cryogenian coasts during
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Elatina Glaciation.
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These paleosols were not methane seeps, nor evidence of global
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Snowball Earth.
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2
Periglacial paleosols and Cryogenian paleoclimate near Adelaide, South
Gregory J. Retallack*, Brooklyn N. Gose, Jeffrey T. Osterhout
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Australia.
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Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272
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ABSTRACT
Late Cryogenian deformation of the Reynella Siltstone near Hallett Cove, South Australia
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has been interpreted as shallow marine methane seeps, which were furthermore proposed to have played a role in deglaciation from “Snowball Earth”. Re-examination and new
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analyses of these same outcrops confirms an earlier interpretation of this intrastratal
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deformation as periglacial paleosols, which are evidence that even at maximum extent,
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late Cryogenian glaciers did not cover all continental lowlands, let alone the world ocean. Known Miocene and Pliocene methane seeps of California examined for this study are
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very different in their gray color, rounded nodules and clayey host rock, compared with red, highly deformed, and silty to pebbly Reynella Siltstone of Hallett Cove. Stable isotopic composition of nodular carbonate in the Reynella Siltstone fails to show extreme carbon isotopic depletion or the meteoric oxygen lines characteristic of known methane seeps and sphaerosideritic waterlogged paleosols, but instead shows covariance of carbon and oxygen isotopic composition comparable with that in pedogenic and subaerial carbonate of well drained soils. Mass balance geochemical analyses show modest weathering compatible with sand wedge structures and evaporitic pseudomorphs
*
Corresponding author. Tel.: +15413464558; fax:+15413464692.E-mail address:[email protected] (G.J.Retallack)
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3 indicative of arid and frigid paleoclimate. Outcrops of Reynella Siltstone near Hallett Cove represent frigid low latitude floodplains, not submarine methane seeps nor global glaciation.
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Key words: Cryogenian, periglacial, paleosol, South Australia, Snowball Earth.
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1. Introduction
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Coastal cliff exposures of the Reynella Siltstone Member of the Elatina Formation from Marino Rocks south to Hallett Cove, near Adelaide, South Australia (Fig. 1), played
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an important role in Neoproterozoic stratigraphy (Segnit, 1940), and remain important to understanding the last widespread Snowball Earth episode of the Neoproterozoic
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(Kennedy et al., 2008; Shields, 2008). There are beautifully preserved Permian tillites and
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striated pavements near Hallett Cove (Segnit, 1940), but other sea cliffs nearby have been
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disappointing as evidence of Cryogenian glaciation because they lack convincing Precambrian tillites like those known from the Flinders Ranges to the north (Mawson,
Ac ce p
1940). The term Marinoan, from nearby Marino Rocks is used in a chronostratigraphic sense to include late Cryogenian and Ediacaran (Fig. 2; Mawson and Sprigg, 1950), but the terminal Cryogenian glacial advance during the Marinoan time-rock interval is best called the Elatina Glaciation (Mawson, 1939; Williams et al., 2008), and not “Marinoan glaciation” (Kennedy et al., 1998; Hoffmann et al., 2004). For many years the Elatina Formation near Hallett Cove has been interpreted as evaporitic, oxidizing, supratidal and intertidal paleoenvironments (Dyson and von der Borch, 1983; Alexander, 1984), including calcareous nodular and ice-deformed paleosols (Dyson and von der Borch, 1986; Williams, 1991), but more recently Kennedy et al. (2008) have interpreted nodules
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4 and deformation as submarine methane seeps The paleosol hypothesis did not consider methane, and the methane hypothesis did not consider paleosols, so our study provides comprehensive stratigraphic, petrographic and geochemical data from multiple localities
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to test the alternatives of methane seeps or paleosols in the Reynella Siltstone Member of the Elatina Formation.
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Testing of these hypotheses has wider implications, because methane seeps have
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been proposed as a source of greenhouse warming responsible for termination of the Elatina Glaciation and the Cryogenian Period (Kennedy et al., 2008; Shields, 2008). A
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soil rather than seep interpretation of these strata negates the idea of methane-driven dramatic climatic warming after Snowball Earth, and confirms lack of evidence for
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glacial moraines at Hallett Cove (Mawson, 1940), thus limiting the seaward extent of
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Elatina Glaciation from their mountain hinterland in the current Peake and Denison
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Range of South Australia (Lemon and Gostin, 1990; Retallack, 2011). Extensive Cryogenian equatorial oceans have been predicted by some global circulation computer
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models (Hyde et al., 2000) and biomarker evidence for continued photosynthesis (Olcott et al., 2005). Equatorial seaways would have been important for the survival of life (Runnegar, 2000) compared with total freezing of lowlands and sea in snowball Earth scenarios of Hoffman et al. (1998; Hoffman and Schrag, 2002). Our study reconsiders these classical outcrops for evidence of methane seeps, glacial facies and paleosols, and implications for the extent and termination of the last Cryogenian glaciation.
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5 2. Materials and methods The Reynella Siltstone Member of the Elatina Formation was observed in two outcrops (Fig. 1): sea cliffs near Hallett Cove (S35.06317o E138.50136o), and
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the railway cuttings south of Kulpara (S34.12157o E138.033844o). Also
examined was drill core of the Reynella Siltstone Member from Wokurna DDH 6
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(S33.7134463o E138.1411738o), stored at Primary Industries Research of South
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Australia (PIRSA), in the Adelaide suburb of Glenside. At all locations paleosols were characterized, and measurements taken of the depth to salts, thickness of salt
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accumulation horizons and size of nodules (Supplementary Information Table S1). Other correlative sections examined for this work include the Elatina
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Formation below basal Ediacaran stratotype monument on Enorama Creek
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(S31.33150o E138.63339o), and the Whyalla Sandstone and Cattle Grid Breccia
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(Williams and Tonkin, 1985; Williams et al., 2008) in Mt Gunson Mine (S31.44278o E137.135350o) workings on July 5, 2007 (Figs. 1-2).
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For comparison with putative methane seeps of Hallett Cove (Kennedy et al.,
2008), some undisputed Neogene methane seeps were examined from California (Aiello et al., 2001; Aiello, 2005): near Santa Cruz (N36.949961o W122.04539o), Jalama Beach (N34.51831o W120.50828o), and Santa Barbara, California (N34.408948o W119.856324o).
Samples were collected for a variety of petrographic and geochemical studies, which aimed to characterize mineral composition, grain size, major element and stable isotopic composition of these rocks. Petrographic thin sections were point counted (500 points) for grain-size and mineral content, using a Swift
Page 5 of 66
6 ® automatic point counter and stage (Supplementary Information Tables S2-3). The same specimens were analyzed for major elements using XRF, and for ferrous iron using Pratt titration (Supplementary Information Table S4), by ALS
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Chemex of Vancouver, BC against standard SY-4 (diorite gneiss from Bancroft, Ontario). Analyses of δ13C and δ18O in carbonate were made using a Finnegan
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MAT 253 mass spectrometer in Department of Geological Sciences, University of
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Oregon, against Vienna PDB standard (Supplementary Information Table S5). Inline
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Inline Supplementary Tables S1-S5 can be found online at http://dx.doi.org/xx.xxxx/j.precamres.xxxx.xx.xx
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Hydrolytic chemical reactions characteristic of soil formation were investigated by calculating gains and losses (mass transfer of Brimhall et al., 1992) of elements in a soil
te
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at a given horizon (τw,j in moles) from the bulk density of the soil (ρw in g.cm-3) and parent material (ρp in g.cm-3) and from the chemical concentration of the element in soils
Ac ce p
(Cj,w in weight %) and parent material (Cp,w in weight %). Also needed is changes in volume of soil during weathering (strain of Brimhall et al., (1992), and estimated from an immobile element in soil (such as Ti used here) compared with parent material (εi,w as a fraction). The relevant equations 1 and 2 (below) are the basis for calculating divergence from parent material composition. w C j ,w j ,w i , w 1 1 p C j , p p C j, p i ,w 1 w C j , w
— equation 1
— equation 2
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7
3. Geological background The Elatina Formation is Cryogenian in age by definition, because the stratotype
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basal Ediacaran (GSSP of Fig. 2) has been placed at the base of the overlying Nuccaleena Formation in Enorama Creek, South Australia (Knoll et al., 2006). Radiometric dating in
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Namibia (Hoffmann et al., 2004), China (Condon et al., 2005), and Tasmania (Calver et
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al., 2013) places the Ediacaran-Cryogenian boundary at about 635 Ma. The Nuccaleena Formation is present as three lenses within the Seacliff Sandstone, which overlies the
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Reynella Siltstone Member of the Elatina Formation at Hallett Cove. Another constraint on minimum age comes from the correlation by Gostin et al. (2010) of glacial dropstones
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in the Bunyeroo Formation with glaciation dated by U-Pb at 582 ± 4 Ma below the Croles
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Diamictite of Tasmania (Calver et al., 2004), and at 583.7 ± 0.5 Ma in the Gaskiers
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Formation of Newfoundland (van Kranendonk et al., 2008). The Reynella Siltstone Member in turn overlies the Marino Arkose Member of
Ac ce p
the Wilmington Formation, which contains a detrital zircon dated by U-Pb at 657 ± 17 Ma (Ireland et al., 1998). A younger age for the base of the Elatina Formation comes from a Re-Os age of 643 ± 2.4 Ma for the Tindelphina Shale Member of the lowermost Tapley Hill Formation (Kendall et al., 2009), which is unconformably below both the Wilmington and Angepena Formations (Fig. 2). This date is incompatible with authigenic monazite cement in the sandstone of the Enorama Formation dated using Th-U-Pb at 680 ± 23 Ma (Mahan et al., 2010), and also with a U-Pb date of 659 ± 6 Ma for a tuff low in the Wilyerpa Formation (Fanning and Link, 2008). These inconsistencies are troubling,
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8 but allow the late Cryogenian (~635-640 Ma) age for the Elatina Glaciation proposed by Williams et al. (2008). Paleolatitude of deposition from chemical remnant magnetism of ultrafine
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hematite in the Elatina Formation, supported by a fold test in the Flinders Ranges, was
6.5 ± 2.2o (Schmidt and Williams, 1995; Williams et al., 2008). These measurements are
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the equator at the time of deposition of the Elatina Formation.
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currently 4o of latitude north of Hallett Cove, which was presumably also within 12o of
The rocks of Hallett Cove are gently folded in the Marino Anticline, a Delamerian
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deformation dated by Rb-Sr dating of sericite in cleavage at 531 ± 32 Ma (Early Cambrian) in the overlying Brachina Formation (Turner et al., 1994; Preiss et al., 1995).
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Some 2300 m of sedimentary cover culminating in the Bunyeroo Formation overlies the
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Elatina Formation in Hallett Cove (Dyson, 2002). This is compatible with high heat flow
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and lower greenschist facies metamorphism observed (Turner et al., 1994). The onset of brittle deformation near the Precambrian-Cambrian boundary (Preiss et al., 1995) makes
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it unlikely that Hallett Cove included an additional 1650 m of Ediacaran and 4852 m of Cambrian-Ordovician overburden found in the Flinders Ranges to the north (Retallack, 2011).
4. New observations
4.1. Field observations The upper Reynella Siltstone has pronounced intrastratal deformation in the cliff and rock platform below the Peera Street staircase (Fig. 3A-B), and Martin Kennedy generously provided us with Google Earth locations and numerous other images to guide
Page 8 of 66
9 our field observations, in addition to images published by Kennedy et al. (2008) and Shields (2008). Of particular interest were features identified as “branching pipes” and as a red “vertical cemented chimney” (Kennedy et al., 2008; Sheilds, 2008). The chimney is
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illustrated here in side view (Fig. 3B), showing that it can be traced 3 m laterally as a
three dimensional wedge-shaped structure 30 cm above the hammer. Supposed “pipes”
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present circular cross sections illustrated here (Fig. 3F), but on sampling proved to be
to ellipsoidal sections, but lack an internal conduit.
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yellow irregular to ellipsoidal nodules or clasts rather than pipes. They may have rounded
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Kennedy et al. (2008, fig. 1b) also illustrate wedge shaped structures filled with sand and breccia, and these were confirmed as common polygonal patterns on the rock
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platform (Fig. 3F) and wedges in the cliff (Fig. 3A). The fill of wedge-shaped cracks is
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not passive and horizontal, but vertical, and in places discontinuous and deformed, as
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better illustrated in our scale mapping of the cliff (Fig. 4). Small examples of these structures were regarded as desiccation-cracks by Dyson and von der Borch (1983), but
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their matrix is not clayey and their fill also vertical like that of the large examples. The sand-granule fill of the cracks is identical to that of beds which cap the red siltstone, and which are also folded upwards into small sharp ridges, best seen in plan on the rock platform (Fig. 3E). Not all parts of the measured section shows such deformation, which is common at 4-11 m, 29 m, 90-91 m, and especially prominent from 40-63 m (Fig. 5). Peloids in the sandy caps include clasts of micritic carbonate identical with those of irregular shape and diffuse boundaries lower within the deformed beds (Figs. 6, 7B-C). Thus carbonate nodules were already formed in the pink siltstones before cracking and intrusion by the deformed cover sands (as also indicated by Kennedy et al., 2008).
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10 Folding of sandy horizons within red silty horizons is not consistent from one bed to the next, as would be expected if this were a heterolithic, siltstone-sandstone sequence thrown into kink folds. These observations and continuity of cover sands down into the
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wedges are evidence that wedging was intrastratal and occurred between intervals of red silt deposition and alteration.
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Several sandstone beds have eroded margins like shallow paleochannels (between
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arrows in Fig.3G; 9, 17, 25, 28, 36 m levels of Fig. 5: see also Kennedy et al., 2008, fig.1b). These are filled with trough cross bedding, and low angle heterolithic cross
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bedding, and oriented toward the south and southwest, like paleocurrents measured from trough cross-bedding and ripple marks here by Alexander (1984). Undeformed red
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siltstones show both asymmetric and oscillation ripples, both as isolated bedforms (Fig.
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3I) and in ripple drift sequences (Fig. 3H). There also are common varvelike graded beds,
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each 2-12 mm thick and graded from silt to shale (Fig. 7A-C; Kennedy et al., 2008, fig. 1e), like those known elsewhere in the Elatina Formation (Williams et al., 2008). No
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hummocky bedding was found in the Elatina Formation, although it has been reported from the overlying Seacliff Sandstone and Brachina Formation at Hallett Cove (Dyson, 1983, 2002).
Acicular crystal casts are common at several stratigraphic levels (1-9 m, 14 m, 21-
24 m, and 27 m in Fig. 5). The crystals are in radiating arrays up to 2 cm in diameter (Fig. 3C) and full of chalcedony or clayey silt rather than the original mineral (Fig. 7D). Some are stout with interfacial angles and fishtail twins of gypsum (CaSO4.nH2O: Dyson and von der Borch, 1983), but others are highly acicular and may have been other evaporite minerals such as thenardite (Na2SO4) or mirabilite (Na2SO4·10H2O). The latter soda salts
Page 10 of 66
11 are suggested by high soda in chemical analyses of Viku and Vinarku paleosols, in which acicular crystals are common (Fig. 6). The form of the crystals is not orthorhombic like barite (BaSO4: Retallack and Kirby, 2007) and aragonite (CaCO3: Sumner, 2002). Water
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soluble evaporite minerals were likely precursors to the silicified crystal casts.
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4.2. Petrography
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In thin section, fine-grained red rocks of the Reynella Siltstone are surprisingly silty (Fig. 6). Clay is confined to thin seams that form the tops of varve-like silty graded
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beds (Fig. 7A, E), like those described for the Elatina Formation by Williams et al., (2008). The clays are metamorphosed to lower greenschist facies like those of the
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overlying Brachina Formation (Turner et al., 1994; Preiss et al., 1995), an observation
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incompatible with the identification by Kennedy et al. (2006) of common (40 %)
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dioctahedral smectite with structural disorder (SR 0.378 ± 0.14) within clay drapes to tidal laminae in the Reynella Siltstone.
Ac ce p
Also surprising in thin section is lack of bedding, not only on the cover sands
(Fig. 7B-C), but also in some parts of the red siltstones (Fig. 7A). The cover sands and wedge-shaped deformations are a very poorly sorted jumble of quartz, feldspar, micrite clasts, and siltstone peloids. The massive siltstones, in contrast, are very even grained and show abundant, irregular, subvertical disruptions of bedding.
4.3. Stable isotopes The isotopic composition of carbonate in the Reynella Siltstone at Hallett Cove shows an extreme range of compositions (Fig. 8): δ18OPDB of –24.5 to +12.1 ‰ and
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12 δ13CPDB. of –8.6 to +7.3 ‰ (Kennedy et al., 2008). Furthermore, some phases of the carbonate show highly significant correlation of δ18OPDB and CPDB from the same samples, as demonstrated here for replacive-micritic nodular carbonate (Fig. 8, R2=0.54)
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and by Kennedy et al., (2008) for early calcite spar cements (R2=0.84) and pore-filling
microcrystalline dolomite (R2=0.71). In contrast, various undifferentiated carbonates of
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Kennedy et al., (2008) from this locality, but unknown stratigraphic level, formed a
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cluster of mainly positive δ13CPDB but highly negative δ18OPDB (Fig. 8).
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4.4. Major element geochemistry
Major element chemical composition of Reynella Siltstone at Hallett Cove shows
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evidence of salinization (high Na2O/K2O) and calcification (high CaO+MgO/Al2O3), but
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only modest chemical weathering (low alumina/bases and Ba/Sr in Fig. 6). Some beds
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(Vani cap, Viku and Wadni of Fig. 9) show lower titania than lower in the same bed as if they gained volume during formation, but other beds have enriched titania, as if volume
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was lost during formation. The volume losses evidently included silica, alumina, iron, phosphorus, magnesium and soda, but conservation of potash (in Itala, Vani, and Vinarku beds of Fig. 9). The gains in volume were primarily in cap sands and beds with abundant relict bedding, whereas silty parts of other beds lost volume (Itala, Vani and Vinarku beds of Fig. 10).
4.5. Comparison with methane seeps For comparison, three seeps for natural gas of the late Miocene Santa Cruz Mudstone (Fig. 11A-B) and Pliocene Pico (Fig. 11C-D) and Sisquoc Formations (Fig.
Page 12 of 66
13 11E-F) were examined in coastal cliffs of California (Aiello et al., 2001; Aiello, 2005). These well documented methane seeps have nodules with tubular conduits, rimmed with bitumen. The nodules also are smooth and light gray in color, and contrast with their dark
ip t
gray shaley host rock, which shows fine laminations and ripple marks. Nodules also
include local calcite veins. No breccias or wedge-shaped deformation was observed at
cr
any of these sites. We were unable to find an example of an intertidal methane seep, and
us
documented methane seeps are all subtidal marine (Aiello et al., 2001; Aiello, 2005; Formolo et al. 2004; Matsumoto, 1989)
an
Stable isotopic data on carbonate cements of these methane seeps are provided by Aiello et al. (2001): they are invariant and near zero for δ18OPDB and highly variable and
M
negative for δ13CPDB. Four different sites within clathrate fields below the sea floor in the
d
Gulf of Mexico were studied by Formolo et al. (2004) and another four sea floor sites off
te
South Carolina by Matsumoto (1989), with similar results, little within-site variation in δ18OSMOW. Converted to the PDB scale of Fig. 8 the Gulf of Mexico clathrate field
Ac ce p
carbonate of Formolo et al (2004) varied from -7.46 to +4.40 ‰ in δ18OPDB and from 1.37 to -38.91 in δ13CPDB. Three of the sites studied by Matsumoto (1989) were similar and varied from +6.1 to +0.3 ‰ in δ18OPDB and from +3.5 to -31.8 in δ13CPDB, but a third site (533) was unusually heavy for both isotopes: from +8.2 to +3.4 ‰ in δ18OPDB and from +12.8 to -19.2 in δ13CPDB.
5. Alternative interpretations 5.1. Methane seeps
Page 13 of 66
14 Carbonate nodules, soft sediment deformation and brecciation in the Elatina Formation at Hallett Cove has been attributed to methane seeps (Kennedy et al., 2008) on the grounds of field relationships and stable isotopic composition, and this idea is further
ip t
tested here with petrographic and geochemical data.
Our panel mapping (Fig. 4) and section measurements (Fig. 5) confirm that the
cr
deformation and nodules are strata concordant, but we failed to find pipe-like or bitumen-
us
lined tubular features common in Neogene methane seeps documented here (Fig. 11) and elsewhere (Matsumoto, 1989; Peckmann et al., 2002, 2007; Formolo et al., 2004; Reitner
an
et al., 2005). Known submarine methane seeps are all gray, unoxidized, nodular to veined shales and siltstones, lacking intraformational breccias (Aiello et al. 2001; Aiello, 2005;
M
Formolo et al., 2004). Instead we found polygonal patterns of intraformational breccia on
d
the rock platform (Fig. 3F) and wedge-shaped sandstone and breccia dikes in the cliff
te
(Fig. 4). Perhaps the Elatina Formation was originally gray and reddened by Neogene lateritization, as has been considered for other Neoproterozoic rocks of the Flinders
Ac ce p
ranges (Retallack, 2013), but we observed the same red colors and wedge-shaped dikes in the Elatina Formation at a depth of 296-339 m in Wokurna 6 core (Fig. 3H-I), below 136 m of unweathered gray Seacliff Sandstone. Furthermore our observations (Fig. 3F) confirm those of Kennedy et al. (2008, fig. 1c), that red breccia clasts are found in gray matrix. Finally, paleomagnetic fold tests of the Elatina Formation in the Flinders Ranges (Schmidt and Williams, 1995) demonstrate oxidation of matrix hematite or iron hydroxides between original sedimentation and before burial diagenesis. The extreme range in both δ13C and δ18O isotopic composition and clear mixing lines of carbonates have been the main argument for methane seep interpretation of the
Page 14 of 66
15 Reynella Siltstone Member at Hallett Cove (Kennedy et al., 2008), and these are compared with mixing lines from a variety of other environments in Fig. 8. Neogene methane seep carbonates show vertical arrays of near-constant δ18O but highly variable
ip t
δ13C (Fig. 8F), and modern marine limestones show near zero values for both δ13C and δ18O, displaced toward more negative values of δ18O in Cambrian and Neoproterozoic
cr
limestones (Fig. 8E). In some ancient methane seeps, two parallel vertical arrays are seen:
us
one from the active seep and its associated fossils, and another in a petrographically distinct cement formed during shallow burial (Peckmann et al., 2002). Different vertical
an
arrays are seen from one methane seep to another on the seafloor (Matsumoto, 1989; Formolo et al., 2004). Vertical arrays in crossplots of δ13C and δ18O are also well known
M
from carbonate of methanogenic wetland paleosols (Ludvigson et al., 1998), Neither
d
marine limestone, nor methane seeps show significant mixing lines (R2<0.2 in Fig. 8), but
te
highly significant mixing lines are found in soils and soil crusts in which evaporative and other processes select for mass of both isotopes simultaneously within CO2 or pedogenic
Ac ce p
carbonate is formed on pre-existing carbonate (Knauth et al., 2003; Ufnar et al., 2008). Such a mixing line from pre-existing presumably marine carbonate clasts to methanogenic carbonate is proposed by Kennedy et al. (2008), but does not fit the local data as closely as a mixing line from marine carbonate clasts to soil crust (Fig. 8A). The range of values and covariance of stable isotopic compositions observed in the Reynella Siltstone at Hallett Cove are unknown in any marine rocks or sediments (Kennedy et al., 2008), but common in soils and paleosols (Fig. 8B). Unusually heavy values of +1.3 to +12.1 ‰ δ18OPDB and +2.4 to +7.3 ‰ δ13CPDB from “dolomite cemented siltstone (authigenic microcrystalline pore-filling dolomite,
Page 15 of 66
16 often iron rich)” of Kennedy et al. (2008), were not replicated in our own analyses (Fig.8A). Perhaps, as noted by Kennedy et al. (2008), such unusually positive values were produced during burial alteration several hundred meters below the sea floor by
ip t
fluids generated within the zone of methane fermentation, as proposed by Matsumoto
(1989). This may have been within the source rocks for dolomite grains, which match
cr
marine isotopic composition (Figs. 8A,E). Metamorphic alteration to low in the
us
greenschist facies (Turner et al., 1994; Preiss et al., 1995) typically creates lighter values of δ18OPDB and δ13CPDB (Algeo et al., 1992), so does not explain unusually heavy values
an
at Hallett Cove, although perhaps contributing to the remarkable spread of isotopic values (Fig. 8A). Alternatively, Knauth et al. (2003) have shown that comparable isotopic
M
enrichment is created by surface evaporation, and calcite soil coatings in the Atacama
d
Desert of Chile reach values of +7.29 ‰ for δ18OPDB and +7.42 for δ13CPDB (Quade et
analyzed at micron scale.
te
al., 2007). These unusually heavy stable isotopic values will remain a puzzle until
Ac ce p
Finally the methane-seep-cemented marine beds should show invariant mineral
composition and chemical composition, and strain-tau values within the dilate-and-gain field. Some beds (Viku, Vani and Wadni of Fig 6) show little mineral or chemical differentiation, but others (Vinarku and Itala) show surficial gains of clay and oxidized iron expected of Neoproterozoic paleosols (Retallack, 2013). Dilate-and-gain of sedimentary additions were seen in capping sands of some beds, but most of the beds lost mass and weatherable components in the collapse-and-loss field of soils and paleosols (Fig. 9). Titanium used as a stable constituent is mainly in ilmenite and other heavy minerals which fall to the base of sedimentary beds, but are enriched at the surface during
Page 16 of 66
17 soil formation which depletes the bed in weatherable components. The analyzed beds of the Elatina Formation are unlike marine event beds in this respect, but also are not deeply weathered. These petrographic and geochemical data can be added to stable isotopic and
ip t
field observations which falsify the methane seep hypothesis of Kennedy et al. (2008).
cr
5.2. Paleosols
us
Intrastratal deformation of the Reynella Siltstone at Hallett Cove (Fig. 4) is similar to periglacial sand wedge paleosols of the correlative Whyalla Sandstone and
an
Cattle Grid Breccia (Fig. 2) described by Williams and Tonkin (1985), Williams (1986), and Williams et al. (2008). Polygonal networks of fissures (Fig. 3F) distinguish these
M
features from seismically induced sand volcanoes (Galli, 2000). Sand wedges have
d
vertical bedding, unlike passively filled ice wedges (Williams, 1986). The largest sand
te
wedges are in beds with the largest calcareous nodules and the most disturbed bedding (Fig. 2A), so that variation in size may be related to varied durations of formation. The
Ac ce p
variation in size of the Reynella Siltstone sand wedges matches both ice and sand wedges (Fig. 12B-C) of Holocene (Leffingwell, 1915; Black, 1976a,b; Gell, 1978; Washburn, 1980; Davis, 2001) and Pleistocene age (Black, 1976a; Washburn, 1980; Wayne, 1991; Dylik, 1994; French, 1996; Owen et al., 1998; Davis, 2001; Van Vliet-Lanoë, 2010; Raffi and Stenni, 2011).
The fill and capping sands to layers include mildly deformed soft-sediment clasts (Fig. 7B) similar to frozen granules transported in periglacial outwash (van Vliet-Lanoë, 1985, 1998, 2010). These cap-sands and wedge fills are less geochemically weathered (base depleted) than associated red siltstones (Figs. 9-10). Cap-sands may represent
Page 17 of 66
18 episodes of fluvioglacial outwash terminating the formation of the sand wedges, but also mildly deformed by subsequent sand wedges at higher stratigraphic levels (Fig. 4). Periglacial sand wedges are one form of paleosol, but so are other features of the
ip t
Reynella Siltstone at Hallett Cove. Replacive spherulitic sand crystals (Fig. 7D) and
micritic nodules (Fig. 6) organized into horizons a fixed distance below the ancient land
cr
surface are characteristic of soils and paleosols (Gile et al., 1981; Dan and Yaalon, 1982;
us
Monger et al., 1991; Almohandis, 2002). Both are replacive because they contain preexisting grains of their matrix, and not displacive clear crystals of gypsum or calcite
an
common in marine nodules and evaporites (Shearman, 1978; El Khoriby, 2005; Ziegenbalg et al., 2010). Crystal casts extend beyond the spherulitic core of the aggregate
M
(Figs. 3D, 7D), and the micrite nodules are laterally linked into irregular aggregations
te
formation elsewhere.
d
(Fig. 3F) as evidence that they formed in place, rather than as clasts transported from
Periglacial sand wedges and replacive evaporites and micrites are comparable
Ac ce p
with soil features which destroy original fine lamination of the Reynella Siltstone, with destruction of bedding proportional to the size and abundance of the wedges, sand crystals and nodules. But there are other finer scale disruptions of lamination giving rise to massive beds, and best appreciated on the scale of thin sections. Some of this homogenization of bedding is a system of cracks and brecciation at the tops of massive beds (Fig. 7C), as noted for the Hallett Cove locality by Dyson and von der Borch (1983). Other bed tops have a system of filamentous disruptions of bedding spreading downward (Fig. 7A). The small diameter, irregularity and ferruginization of these filamentous structures is most like structures made by soil microbes (Poelt and Baumgärtner, 1964;
Page 18 of 66
19 Mihail and Bruhn, 2005; Garcia-Pichel and Wojciechowski, 2009). They are much thinner and irregular than fluid-escape structures or mud volcanoes, which tap into organic or fine grained lower layers not seen at Hallett Cove (Dionne, 1976; Nichols et
ip t
al., 1994).
Geochemical differentiation within individual beds reflects typical soil
cr
weathering: incongruent dissolution of feldspar and other weatherable minerals by
us
carbonic acid to form clay and release nutrient cations (Ca2+, Mg2+, Na+, K+). Modest progress of this hydrolysis reaction is revealed by increased clay and molar ratios of
an
alumina/bases towards the tops of profiles (Fig. 6). A more precise demonstration of hydrolysis is revealed by calculating chemical losses and gains, as well as volume loss
M
and gain to a stable constituent (Equations 1 and 2 above after Brimhall et al., 1992). The
d
field of collapse of volume and loss of cations characteristic of soils is demonstrated by
te
chemical analyses of paleosols in the Reynella Formation, but the cap sandstones show subdued change or plot in the dilate and gain field characteristic of sediments (Fig. 9).
Ac ce p
Within-bed differences in red color comparable with those of ferruginization of well drained paleosols and limited burial gelization (Retallack, 1997) are revealed by variation in ferrous to ferric iron ratio (Fig. 6). This red color was acquired during the Cryogenian as revealed by redeposited clasts of red siltstone (Fig. 7B), a paleomagnetic fold test (Schmidt and Williams, 1995), and similar color of paleosols in the Elatina Formation at depths of 296-329 m in Wokurna DDH6 drillcore (Fig. 3H-I). . Significant covariance (R2=0.54) between δ18O and δ13C is characteristic of paleosols, and its slope is low (0.13), perhaps reflecting low rates of paleoevaporation in a cold climate (Ufnar et al., 2008). Extremely enriched δ18OPDB values of +6.96 ‰ at
Page 19 of 66
20 Hallett Cove are found in highly evaporative soil crusts (Knauth et al., 2003; Quade et al., 2007). Very low δ18OPDB values (down to –22.5 ‰) are evidence of glacial meltwater or cold rain (Kennedy et al., 2008), but near zero δ18O values may represent Proterozoic
ip t
marine limestone fragments within the paleosols as silt-size grains of the parent loess
cr
(Fig. 7A,E).
us
6. Sedimentary interpretations
Sedimentary paleoenvironments of alluvial to tidal coastal flats are indicated for
an
the Reynella Siltstone by primary sedimentary structures including bimodal paleocurrents (Alexander 1984), herringbone and flaser bedding and bundled lamination (Kennedy et
M
al., 2008, fig. 1e; Williams et al., 2008, fig 7a-b,d). Compared with more complex
d
bundling in Reynella Siltstone of the Flinders Ranges (Williams, 1991), bundling near
te
Hallett Cove includes proximal neap-spring tidal cycles representing 13.1 ± 0.1 lunar months/year, 400 ± 7 solar days/year and 21.9 ± 0.4 hours/solar day (Williams et al.,
Ac ce p
2008). Intertidal facies, shallow paleochannels with heterolithic fill of sandstone and siltstone (Fig. 3G between arrows) and limestones are common in the lower 37 m of the measured section (Fig.5), north along the seacliffs toward Marino Rocks. These paleochannels not only contain heterolithic lateral accretion sets, but have steep cut banks (right hand arrow of Fig. 3G). Lateral accretion and cut banks are hallmarks of meandering streams, which Davies and Gibling (2010) argue are unknown before the Silurian. This interval of the Reynella Siltstone is also where evaporite pseudomorphs are most common and indicate largely dry supratidal coastal flats (Dyson and von der Borch, 1983). Meandering intertidal channels are known from Proterozoic (Bengtson et al.,
Page 20 of 66
21 2007; Dehler et al., 2012) and Cambrian (Hereford, 1977; Cloyd et al., 1990) intertidal facies, and remain common on unvegetated, low-relief, muddy, tidal flats today (Kleinhans et al., 2009).
ip t
Tidal or lagoonal varve-like lamination (Fig. 7A,E) is less common within the
interval 37-65 m, where periglacial sand wedges are common, and paleochannels are still
cr
of trough cross-bedded sand of a contrasting gray color to the intervening red siltstones.
us
From 65-92 m red color is found in three distinct facies: (1) siltstone paleosols, (2) siltstones with ripple drift cross-lamination, and (3) trough cross-bedded paleochannels.
an
These parts of the sequence are similar to alluvial floodplain sequences: the drab paleochannel facies is comparable with the Ediacaran, Ediacara Member of South
M
Australia (Retallack, 2013) and numerous Phanerozoic floodplain facies (Retallack,
d
2008; Morrison and Straight Cliffs Formations of Retallack, 2009). Higher in the
te
sequence both paleochannels and paleosols are red, comparable with the Ediacaran Bonney Sandstone of South Australia (Drexel et al., 1993) and other Phanerozoic
Ac ce p
floodplain facies (Retallack, 2001; Halgaito and Summerville Formations of Retallack, 2009).
7. Paleosol interpretations 7.1. Identification
Individual beds of the Reynella Siltstone Member with deformational, petrographic and geochemical characteristics of paleosols fall into five repeated types, here given non-genetic pedotype names from the Adnamatna aboriginal language
Page 21 of 66
22 (McEntee and McKenzie, 1992). These include Itala (crack in rock), Vani (flesh), Viku (eyebrow), Vinarku (gypsic alluvium), and Wadni (small). The various pedotypes of the Reynella Siltstone at Hallett Cove can be identified
ip t
within modern soil classifications (Table 1), such as those of the United States (Soil
Survey Staff, 2010) and the Food and Agriculture Organization (1975, 1978), but the
cr
Australian classification does not seem to have closely comparable soils at present
us
(Isbell, 1996). The most similar Russian soils (units Bx6-3b of Food and Agriculture Organization, 1978), are Gelic Cambisols associated with Calcic Cambisols and Calcaric
an
Regosols. Map unit Bx6-3b covers 10,816,000 hectares in floodplain and headwaters of the Olenek River, near Jerbogačon, Siberia, where climate is frigid and vegetation is pine
M
(Pinus sibirica)-larch (Larix dahurica) taiga forest. Comparable Canadian soils (units
d
Bx1-1a and Bx 2-1b of Food and Agriculture Organization, 1975) cover 6,449,000
te
hectares of marine sediments and glacial outwash west of Hudson Bay and north of Churchill, where climate is frigid and vegetation is mixed spruce (Picea mariana) taiga
Ac ce p
and tundra. Jerbogačon has a mean annual temperature of –6.7oC and mean annual precipitation of 323 mm, and for Churchill these values are –7.2oC and 435 mm (Müller, 1982). Such climate may be comparable with that of these Cryogenian paleosols, but such vegetation cannot be inferred for paleosols from before the advent of land plants.
7.2. Paleotopography Tidal laminae (Fig. 7A, E) persist at least to 63 m in the measured section (Fig. 5) and are evidence of a flat coastal plain to supratidal flats for the Reynella Siltstone. At these stratigraphic levels the sandstone paleochannels and lower levels of the paleosols
Page 22 of 66
23 are gray (Fig. 5), and ferrous iron is common (Fig. 6), but other parts of the paleosols are red and ferruginized (Fig. 6). Such alternation of redox character can be attributed to persistently high water table in a coastal plain, as for other comparable sequences of
ip t
coastal paleosols (Retallack, 2008, 2013). These floodplain could not have been
permanently waterlogged, because brittle, frozen soil is required for the formation of sand
cr
wedge polygons (Maloof et al., 2002). Red paleochannels above 65 m (Fig. 5) reflect
us
oxidation during low water stages in the original streams (Retallack, 2001, 2008). Local relief was thus no more than 3 m seen on some paleochannel cut banks. Periglacial
an
polygons (Fig. 4) and locally deformed outwash sands (Fig. 3E) would have given
M
ground roughness with an amplitude of about 20 cm.
d
7.3. Former parent material
te
Parent materials to the paleosols were quartzofeldspathic and dolomitic silts and sands (Stevenson, 1972). The high proportion of dolomitic silt seen in some levels (Fig.
Ac ce p
6) may be eolian and derived from extensive marine dolostones exposed in the Peake and Denison Ranges (Retallack, 2011). Quartzofeldspathic silt grains also are angular and randomly arranged, and may be loess distal to dune fields represented by the Whyalla Sandstone (Williams et al., 2008). These silty parent materials contrast with sandy to gravelly fluvioglacial outwash that forms caps to many of the beds (Fig. 6).
7.4. Time for formation Rates of ice and sand wedge growth in Antarctica ranged from 0.79 mm/yr in the 1960s (Black, 1973) to 0.04 mm/yr in the 1970s (Black, 1982). Such wide disparity is
Page 23 of 66
24 problematic for understanding rates and extrapolating age, though comparable with rates of growth of such features in the northern hemisphere (Bockheim, 1995). From their width in paleosols of the Reynella Siltstone (Fig. 12B) the sand wedges would have
ip t
formed in 10-72 years by the fast rate and 200-1425 years by the slow rate.
Longer estimates of time come from the formation of carbonate nodules in the
cr
paleosols. In desert soils of New Mexico (Retallack, 2005) diameter of carbonate nodules
us
(S in cm) is related to soil age (A in kyrs) by equation 3 (R2=0.57; S.E.± 1.8 kyrs). A = 3.92S0.34
— equation 3
an
This transfer function gives 16.5 ± 1.8 kyr for the type Itala paleosol and 6.6 ± 1.8 yr for the type Vani paleosol. An average duration with one standard deviation for 9 Itala
M
paleosols is 16.2 ± 5.5 kyr and for 20 Vani paleosols is 5.7 ± 3.0 kyr (Supplementary
d
Information Table S1). These paleosols thus formed on Milankovitch time scales, with
te
long episodes of weathering followed by shorter episodes of frigid deformation. Sandy caps to the beds show little chemical weathering (Figs 9-10), compared with the silty
Ac ce p
paleosols below with their carbonate nodules (Kennedy et al., 2008). Milankovitch precesssion cycles have been computed to have been 19,462 years, and obliquity cycles 31,243 years at 640 million years ago (by interpolation from table of Berger and Loutre, 1994). Wadni and Viku paleosols show little soil formation, and comparable Inceptisols and Entisols today represent millenia to centuries of soil formation, even in Antarctica (Campbell and Claridge, 1987; Retallack et al., 2001).
7.5. Paleoclimate
Page 24 of 66
25 Sand wedge polygons form today under a well constrained set of paleoclimatic controls: mean annual air temperatures of less than -12 to -20oC, mean coldest month temperature <-35oC, mean warmest month temperature of +4oC, and mean annual
ip t
precipitation <100 mm (Williams, 1986). Comparable paleoclimate has been inferred for fossil sand wedges of the Cattle Grid Breccia and lower Whyalla Sandstone of the Mount
cr
Gunson Mine on the Stuart Shelf, and like those periglacial paleosols, the comparable
us
features at Hallett Cove may record the coldest part of the Elatina Glaciation (Williams et al., 2008). Warmer, though still frigid, conditions are also evident from thufur mounds
an
and shadow dunes in the overlying Nuccaleena Formation (Retallack, 2011). Under warmer frigid conditions, a variety of structures are formed including ice wedge
M
polygons, periglacial involutions and thufur mounds (Young and Long, 1976; Krull,
d
1999; Retallack, 1999a,b, 2011, 2013). Ice wedges are best known from Greenland and
te
Arctic Canada, which are maritime glacial climates, as opposed to the continental glacial climate of modern Antarctica with sand wedges (Black, 1976a,b; Washburn, 1980). Sand
Ac ce p
wedge polygons have also been reported from the Lillfjället Formation of early Cryogenian age (Kumpulainen, 2011) and ice wedge polygons from Paleoproterozoic glacial deposits (Young and Long, 1976). Frigid paleotemperatures are supported by the pedogenic paleothermometer of
Óskarsson et al.,(2012), based on frigid modern soils under tundra vegetation of Iceland. This linear regression between mean annual temperature (T in oC) and chemical index of weathering (I = 100mAl2O3/(mAl2O3+mCaO+mNa2O), in molar proportions is given in equation 4 (R2 = 0.81; S.E. = ± 0.4oC) T = 0.21I –8.93
— equation 4
Page 25 of 66
26 Results for the analyzed type profiles are mean annual temperatures of –7.1 ± 0.4 o
C for the type Viku paleosol, –3.3 ± 0.4 oC for the type Vinarku paleosol, –6.6 ± 0.4 oC
for the type Vani paleosol, and –4.8 ± 0.4 oC for the type Itala paleosol. Other chemical
ip t
paleothermometers of Gallagher and Sheldon (2013) and of Sheldon et al.,(2002) have a training set of modern temperate forested soils, and yielded mean annual
cr
paleotemperatures in the range 7.2-10.7 oC, incompatible with evidence of sand wedge
us
polygons (Fig. 4).
Arid paleoclimate for the paleosols of the Reynella Siltstone is supported by three
an
distinct paleohyetometers. A chemical transfer function of Sheldon et al. (2002) based on temperate soils of North America uses chemical index of alteration without potash (R =
M
100mAl2O3/(mAl2O3+mCaO+mNa2O) in moles), which increases with mean annual precipitation (P in mm) in modern soils (R2 = 0.72; S.E. = ± 182 mm), as follows. — equation 5
te
d
P = 221e0.0197R
This formulation is based on the hydrolysis equation of weathering, which enriches
Ac ce p
alumina at the expense of lime, magnesia, potash and soda. Magnesia is ignored because not significant for most sedimentary rocks, and potash is excluded because it can be enriched during deep burial alteration of sediments (Maynard, 1992). The other two other paleohyetometers come from depth to salts in paleosols.
Depth (Dy in cm) to gypsic (By) horizon in modern soils (Retallack and Huang, 2010) has found the relationship of equation 5 with mean annual precipitation (P in mm: R2= 0.63, standard error ± 129 mm). Mean annual precipitation (P in mm) is also related to depth to carbonate nodules (Dk in cm) according to equation 6 (R2 = 0.52; S.E. = ± 147 mm). Both
Page 26 of 66
27 training sets include many sparsely vegetated soils paleosols, including polar soils with no vegetation visible to the naked eye (Retallack, 2005; Retallack and Huang, 2010). P = 87.593e0.0209·Dy
R = 137.24 + 6.45Dk - 0.013Dk2
— equation 7
ip t
— equation 6
Both these proxies (equations 6 and 7) for paleoprecipitation require decompaction of the
cr
paleosols, and in this case physical constants for Aridisols (equation 7 after Sheldon and
us
Retallack, 2001) were used giving depth to salts in the in original soil (Ds) from depth in the current paleosol (Dp) for a particular depth of burial (K in km). 0.38 -1)] e 0.17 K
an
Ds = Dp/[-0.62/(
— equation 8
M
Arid paleoclimate was calculated by all three methods: mean annual precipitation of 357 ± 182 mm from chemical composition and 158 ± 129 mm from depth to gypsum
d
for type Viku paleosol, 500 ± 182 mm from chemical composition and 162 ± 129 mm
te
from depth to gypsum for the type Vinarku paleosol, 275 ± 182 mm from chemical
Ac ce p
composition and 330 ± 149 mm from depth to micrite for the type Vani paleosol, and 325 ± 182 mm from chemical composition and 314 ± 149 mm from depth to micrite for the type Itala paleosol.
7.6. Ancient life on land
Phosphorus is depleted in some paleosols of the Reynella Siltstone (Figs. 9-10),
and experimental studies of modern soils have shown that only organic ligands can weather phosphorus to this extent from apatite in soils (Neaman et al., 2005). Another indication of microbial activity in these paleosols is molar depletion of alkali and alkaline
Page 27 of 66
28 earth oxides (CaO+MgO+Na2O+K2O) in the paleosols (Figs 9-10) by CO2 from soil respiration (Retallack, 1997). Microtubular disruption of lamination in paleosols of the Reynella Siltstone is
ip t
sparse in lower parts of the profiles (Fig. 7E), but totally disrupts lamination in the upper part of the profiles (Fig. 7A). These ptygmatically compressed, near-vertical tubules are
cr
10-50 μm in diameter. Similar features in modern soils are produced by bundles of
us
sheathed cyanobacteria, like modern Microcoleus vaginatus (Garcia-Pichel and
Wojciechowski, 2009), rhizomorphs of fungi, like those of honey mushroom, Armillaria
an
mellea (Mihail and Bruhn, 2005), or lichen rhizines like those of bruised lichen, Toninia sedifolia (Poelt and Baumgärtner, 1964). Although the exact nature of these microbes are
M
unknown, communities of paleosols in the Reynella Siltstone may have been microbial
te
and Lange, 2003).
d
earths (Retallack, 2012), like those as known today from biological soil crusts (Belnap
Life on land is known back at least 2200 million years (Retallack et al., 2013), so
Ac ce p
evidence of microbes in paleosols of the Reynella Siltstone is unremarkable. Nor were these paleosols too cold to support life: comparable frigid soils in Antarctica today also have a rich microbial community, as well as rotifers, nematodes, tardigrades, mites and springtails (Cary et al., 2010). No megafossils were found in the Reynella Siltstone, although they are known in other Cryogenian rocks. Compressions such as Aspidella, some 766 million years old have been found on surfaces with old-elephant skin textures (Meert et al., 2011), like those of microbial earths (Retallack, 2012). Sponge-like skeletal remains such as Otavia, are known from marine limestones up to 760 million years old (Maloof et al., 2010; Brain et al., 2012).
Page 28 of 66
29
8. Correlations and causes Correlation of the Reynella Siltstone and Nuccaleena Formations between Hallett
ip t
Cove and the central Flinders Ranges is secure because of identical lithologies in each area, connected by Kulpara railway cutting and Wokurna core (Williams et al., 2008).
cr
These formations are separated by a regional disconformity in the central Flinders
us
Ranges, but to the south by channel incision at the base of the Seacliff Sandstone which includes stringers of Nuccaleena Formation (Dyson and von der Borch, 1986; Dyson,
an
2002). These relationships are problematic for the idea that the Reynella Siltstone includes methane seeps which terminated the last Cryogenian glacial advance (Kennedy
M
et al., 2008). Instead of global warming and sea level rise predicted by that view, the
d
basal Seacliff sandstone paleochannel incision records a local relative fall of sea level
te
(Dyson and von der Borch, 1994). Observations along this disconformity are also not supportive of the idea of a glacial advance all the way to the sea in a global Snowball
Ac ce p
Earth (Hoffman and Schrag, 2002), because several generations of observers (Segnit, 1940; Mawson, 1940; Williams et al., 2008) have failed to find evidence of dropped pebbles or moraines within the Seacliff Sandstone. Trough cross bedding of the Seacliff Sandstone is evidence of fluvial incision and fill (Dyson and von der Borch, 1994), at a time of relative sea level fall during frigid conditions that created the sand wedge paleosols described here. Post-glacial marine transgression was not until much later, as indicated by gray siltstones and hummocky cross bedding of the Brachina Formation (Dyson, 1983). The disconformity between Seacliff Sandstone or Nuccaleena Formation and the Elatina Formation is regional (Williams et al., 2008), but has nowhere yielded
Page 29 of 66
30 thick paleosols as evidence of longer duration than other paleochannels within the Elatina Formation (Fig. 5).
ip t
9. Conclusions
The Reynella Siltstone Member of the Elatina Formation between Marino Rocks
cr
and Hallett Cove may have been deposited in coastal plains to intertidal flats (Fig. 13;
us
Table 2), as envisaged by Mawson (1940), Dyson (1983), Dyson and von der Borch (1983, 1986), Alexander (1984) and Williams (1991). This passive margin coast was well
an
south of moraines of the Elatina Formation of the Flinders Ranges to the north (Lemon and Gostin 1990; Retallack 2011). An array of supratidal to alluvial paleosols in the
M
Reynella Siltstone includes Gelisols with sand wedge polygons formed near Hallett Cove in an arid frigid continental paleoclimate like that of modern Antarctica (Fig. 13). Such
te
d
cold conditions within 12o of the equator during the late Cryogenian (~635-640 Ma) peak of the Elatina Glaciation (Williams et al., 2008) confirm the remarkable Neoproterozoic
Ac ce p
spread of glacial facies that has been termed “Snowball Earth” (Hoffman and Li, 2009). Our reconstruction of the Reynella Siltstone Member of the Elatina Formation
from Marino Rocks to Hallett Cove confirms the conclusion of Mawson (1940) that these rocks contain neither tillites nor fully marine deposits. Our reconstruction thus does not support extreme interpretations of a hard Snowball Earth, with the whole ocean and all lowlands completely enveloped in ice (Hoffman et al., 1998; Hoffman and Schrag, 2002). Hallett Cove at least was not covered by glacial ice or ocean during the last glacial advance of the Cryogenian, perhaps represented by sand-wedge polygons in the Reynella Siltstone (Fig. 4) and Cattlle Grid Breccia and Whyalla Sandstone (Williams et al., 2008).
Page 30 of 66
31 It could also be that the last Cryogenian glacial maximum was within the ca. 50 m palaeovalley incision of the Reynella Siltstone by the Seacliff Sandstone, but neither the Seacliff Sandstone nor Brachina Formation has dropstones or any other evidence of sea
ip t
ice or icebergs (Dyson, 1983, 2002; Dyson and von der Borch, 1994). Alternative global
circulation models of the late Cryogenian glaciation allow large areas of ice-free tropical
cr
ocean (Hyde et al., 2000) more congenial to survival of life on Earth (Runnegar, 2000).
us
Our failure to find subtidal marine facies or pipe-like vent structures also does not support the interpretation of Kennedy et al. (2008) that intrastratal deformation in the
an
Reynella Siltstone was created by marine methane seeps. Genuine marine methane seeps examined for comparison in this work are gray, silty, and have large smooth calcareous
M
nodules with a very different isotopic signature of invariant δ18O but varied δ13C (Aiello et al., 2001; Peckmann et al., 2002). The Reynella Siltstone carbonate in contrast is red
te
d
and brecciated and shows strong covariance of δ18O and δ13C as in mixing lines observed in modern soils and soil crusts (Knauth et al., 2003; Ufnar et al., 2008). Finally beds of
Ac ce p
Reynella Siltstone show loss of mass and volume by mass balance with chemical composition lower in the same bed, more like paleosols than marine beds (Retallack, 2013). Thus there is little structural, isotopic or geochemical support at Hallett Cove for the idea of Kennedy et al. (2008) that methane emissions hastened the end of the Elatina Glaciation. Furthermore, periglacial paleosols in the overlying Nuccaleena Formation (Retallack, 2011), and persistent glacial conditions throughout the succeeding Ediacaran period (Gostin et al., 2010; Jenkins, 2011) are evidence that deglaciation at the end of the Cryogenian was neither geologically rapid nor profound.
Page 31 of 66
32 Acknowledgments Erick Bestland and Matt Forbes first showed GJR these paleosols. Martin Kennedy, Nora Noffke, Greg Ludvigson, Richard Behl, and Nathan Sheldon offered useful discussion.
cr
analyses. Rob Whittecar and Randall Parrish offered helpful reviews.
ip t
Ilya Bindemann and Jim Palandri are thanked for donation of carbonate stable isotopic
us
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Fig. 1. Location of Hallett Cove (A), Wokurna 6 bore (B) and Kulpara (B) in South Australia.
Fig. 2. Cryogenian-Ediacaran stratigraphy and radiometric dating of Adelaide-Kulpara and neighbouring regions of South Australia (after Ireland et al.,, 1998; Calver et al.,, 2004, 2013; Kendall et al.,, 2006; van Kranendonk et al.,, 2008; Williams et al.,, 2008; Fanning and Link 2008; Mahan et al.,, 2010). Shaded units are red beds: others are bluish and greenish gray.
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49 Fig. 3. Outcrops (A-G) at Hallett Cove, near Adelaide, South Australia and core (H-I) at Wokurna DDH 153 km to the northwest: (A) intrastratal deformation zone of upper Reynella Siltstone Member (true-scale sketch is shown in Fig. 4); (B) oblique view of
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intrastratal deformation of upper Reynella Siltstone Member showing wedge shaped deformation (at 50 m in Fig. 5; also illustrated by Shields 2008, Fig. 1); (C) Viku
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paleosol and (D) its acicular sand crystals (both at 19 m in Fig. 5); (E) oblique view of
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uplifted wedge of cover sand above Viku paleosol (19 m in Fig. 5); (F) plan view of drab polygons in Vani paleosol on rock platforms ( 43 m in Fig. 5); (G) shallow paleochannel
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of sandstone and siltstone with lateral accretion sets (between arrows: at 18 m in Fig. 5); (H) Vani paleosol with small yellow nodules at 327 m (right) and Itala paleosol with
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large yellow nodules at 326 m (left); (I) Wadni paleosol at 301.7 m . Hammers for scale
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(A-F), cliff 30 m tall (G) and core is 5 cm diameter (H-I).
Fig. 4. Scale diagram of intrastratal deformation, interpreted as successive periglacial
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sand wedges in the late Cryogenian Reynella Siltsone Member of the Elatina Formation at Hallett Cove (interval 44-49 m of Fig. 5).
Fig. 5. Measured section of paleosols in the late Cryogenian Reynella Siltstone Member of the Elatina Formation at Hallett Cove.
Fig. 6. Petrographic and geochemical data for paleosols in the late Cryogenian Reynella Siltstone Member of the Elatina Formation at Hallett Cove.
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50 Fig. 7. Photomicrographs of paleosols in the late Cryogenian Reynella Siltstone Member of the Elatina Formation at Hallett Cove: (A) filamentous bioturbation of lamination in upper part of type Viku paleosol (21 m in Fig. 5A); (B) peloids in cover sand to Vani
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paleosol (46 m in Fig. 5A); (C) peloids in cover sand to Itala paleosol (47 m in Fig. 5A); (D) acicular sand crystal disruption to bedding in Viku paleosol (m in Fig. 5A); (E)
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varvoid bedding in Itala paleosol (47 m in Fig. 5A). Specimen numbers in Condon
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Collection of the Museum of Natural and Cultural History, University of Oregon are (A)
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R3879, (B) R3903, (C) S3896, (D) R3880, and (E) R3899.
Fig. 8. Distinct mixing lines of carbon and oxygen isotopic composition of carbonate in
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(A) in the late Cryogenian Reynella Siltstone Member of the Elatina Formation at Hallett
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Cove derived for this study (closed circles), and by Kennedy et al. (2008: closed squares);
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(B) soil nodules (above Woodhouse lava flow, near Flagstaff, Arizona, from Knauth et al., 2003; and in Yuanmou Basin, Yunnan, China, from Huang et al. 2005), (C) soil crusts
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on basalt (Sentinel Volcanic Field, Arizona, from Knauth et al. 2003), (D) Quaternary marine limestone altered diagenetically by meteoric water (Key Largo, Florida, from Lohman 1988, and Clino Island, Bahamas, from Melim et al. 2004), (E) unweathered marine limestone of Holocene (from Veizer et al.,2000) and Early Cambrian age (Ajax Limestone, South Australia, from Surge et al. 1997), and (F) marine methane cold seep carbonate (Miocene, Santa Cruz Formation, Santa Cruz, California, from Aiello et al. 2001). Slope of linear regression (m) and coefficients of determination (R2) for mixing lines show that carbon and oxygen isotopic composition is significantly correlated in soils and paleosols, but not in marine limestones or methane seeps.
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Fig. 9. Mass balance geochemistry of paleosols in the Reynella Siltstone of South Australia, including estimates of strain from changes in an element assumed stable (Ti)
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and elemental mass transfer with respect to an element assumed stable (Ti, following
Brimhall et al.,, 1991). Zero strain and mass transfer is the parent material lower in the
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profile: higher horizons deviate from that point due to soil formation or sedimentation as
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indicated in key to lower right.
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Fig. 10. Depth functions of elemental mass transfer with respect to an element assumed stable (Ti, following Brimhall et al.,, 1991) for paleosols in the Reynella Silstone of
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South Australia.
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Fig. 11. Neogene methane seeps from California: Santa Cruz Formation (late Miocene) near Santa Cruz (A-B), Sisquoc Formation (Pliocene) at Jalama Beach (C-D) and Pico
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Formation (Pliocene) at Santa Barbara, California (E-F).
Fig. 12. Hypothetical development of periglacial paleosols from examples of varied development in the late Cryogenian Reynella Siltsone Member of the Elatina Formation at Hallett Cove (A) and comparable dimensions of modern sand wedges (B).
Fig. 13. Conjectural reconstruction of paleosols and sedimentary paleoenvironment of the Reynella Siltstone near Adelaide. Table 1. Pedotype definition and classification for the Elatina Formation Pedotype
Diagnosis
Soil Survey Staff (2010)
Food and Agriculture Organization
Australian (Isbell
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Vani
Vinarku Viku Wadni
Drab yellowish gray sandy surface with deep wedge like deformation(A) over red siltstone with large calcareous nodules (Bk) Drab yellowish gray sandy surface with deep wedge like deformation(A) over pink siltstone with small calcareous nodules (Bk) Drab yellowish gray sandy surface (A) over diffuse zone of calcified gypsum rosettes (By) Drab yellowish gray sandy surface (A) over dark red bedded silstone (Bw) Ferruginized sitstone with cracking (A) over bedded siltstone with wispy mottles (C)
Haploturbel
(1975,1978) Gelic Cambisol
1996) Lithocalcic Calcarosol
Haploturbel
Gelic Cambisol
Lithocalcic Calcarosol
Haplogypsid
Gypsic Yermosol
Haploxerept
Eutric Cambisol
Hypersalic Rudosol Red-Orthic Tenosol Stratic Rudosol
Fluvent
Fluvisol
Viku
Microbial soil crust Microbial soil crust
Not diagnostic of climate
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Microbial soil crust
Early successional microbial soil crust Early successional microbial soil crust
Parent material Dolomitic and quartzofeldspathic silt Dolomitic and quartzofeldspathic silt Dolomitic and quartzofeldspathic silt Dolomitic and quartzofeldspathic sand Dolomitic and quartzofeldspathic silt
loess plain
well drained supratidal flat
Time for formation 10,00020,000 years 3,000-9,000 years 100-1,000 years 100-1,000 years
10-100 years
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Wadni
Frigid arid(mean annual temperature –4.8 ±0.4oC; mean annual precipitation 314 ± 147 mm) Frigid arid (mean annual temperature –6.6 ±0.4oC; mean annual precipitation 330 ± 147 mm) Frigid arid (mean annual temperature –3.3 ± 0.4oC; mean annual precipitation 162 ± 129 mm) Frigid arid (mean annual temperature –7.1 ± 0.4oC; mean annual precipitation 158 ± 129 mm)
Topographic setting Well drained loess-alluvial terrace Well drained loess-alluvial terrace Well drained supratidal flat
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Vinarku
Organisms
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Vani
Climate
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Table 2. Pedotype interpretative summary for the Elatina Formation Pedotype Itala
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