Periglacial paleosols and Cryogenian paleoclimate near Adelaide, South Australia

• Sand wedge periglacial paleosols are described in Cryogenian Reynella Siltstone.
• Sand wedges show paleosol-diagnostic correlation of δ18O vs δ13C of carbonate.
• Paleosols are weakly differentiated chemically like periglacial soils.
• Periglacial paleosols represent ice-free Cryogenian coasts during Elatina glaciation.
• These paleosols were not methane seeps, nor evidence of global Snowball Earth.

Late Cryogenian deformation of the Reynella Siltstone near Hallett Cove, South Australia 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 analyses of these same outcrops confirms an earlier interpretation of this intrastratal deformation as periglacial paleosols, which are evidence that even at maximum extent, 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 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 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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