The End-Cryogenian Glaciation of South Australia
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The Elatina Fm. records the younger Cryogenian ice age in the Adelaide Rift Complex (ARC) of South Australia, which has long-held the position as the type region for this low-latitude glaciation. Building upon a legacy of work, we document the pre- and syn-glacial sedimentary rocks to characterize the dynamics of the glaciation across the ARC. The Elatina Fm. records an array of well-preserved glacial facies at many different water depths across the basin, including ice contact tillites, fluvioglacial sandstones, dropstone intervals, tidal rhythmites with combined-flow ripples, and turbidites. The underlying Yaltipena Fm. records the pro-glacial influx of sediment from encroaching land-based ice sheets. The onset of the glaciation is heralded by the major element ratios (Chemical Index of Alteration) of the pre-glacial facies across the platform that show a reduction in chemical weathering and a deterioration in climate towards the base of the Elatina Fm. The advancing ice sheets caused soft-sediment deformation of the beds below the glacial diamictite, including sub-glacial push structures, as well as sub-glacial erosion of the carbonate unit beneath. Measured stratigraphic sections across the basin show glacial erosion up to 130 m into the carbonate platform. However, 13C measurements of carbonate clasts within the glacial diamictite units were used to assess provenance and relative timing of 13C acquisition, and suggest that at least 500 m of erosion occurred somewhere in the basin. Detrital zircon provenance data from the Elatina Fm. suggest that glacial sediment may have been partially sourced from the cratons of Western Australia and that the Whyalla Sandstone, even if stratigraphically correlative, was not a sediment source. The remainder of the Elatina Fm. stratigraphy mostly records the deglaciation and can be divided into three facies: a slumped sandstone, dropstone diamictite, and current-reworked diamictite. The relative sea level fall within the upper Elatina Fm. requires that regional deglaciation occurred on the timescale of ice sheet ocean gravitational interactions (instant) and/or isostatic rebound (~104 years). Structures previously interpreted as soft-sediment folds within the rhythmite facies that were used to constrain the low-latitude position of South Australia at the time of the Elatina glaciation are re-interpreted as stoss-depositional transverse ripples with superimposed oscillatory wave ripples. These combined-flow ripples across the ARC attest to open seas with significant fetch during the initial retreat of local glaciers. In addition, this interpretation no longer requires that the magnetization be syn-depositional, although we have no reason to believe that the low-latitude direction is a result of remagnetization, and positive reversal tests and tectonic fold tests are at least consistent with syn-depositional magnetization. Together, these paired sedimentological and chemostratigraphic observations reveal the onset of the glaciation and advance of the ice sheet from land to create a heavily glaciated terrain that was incised down to at least the base of the pre-glacial Trezona Fm.SOMMAIRELa Formation dElatina reprsente la phase prcoce de lge glaciaire du Cryognien de lAdelaide Rift Complex (ARC) dans le sud de lAustralie, rgion qui a longtemps t la rgion type de cette glaciation de basse latitude. partir dun legs de travaux, nous nous sommes appuys sur ltude des roches sdimentaires prglaciaires et synglaciaires pour caractriser la dynamique de la glaciation travers lARC. La Formation dElatina renferme une gamme de facis glaciaires bien prservs correspondant diffrentes profondeurs deau travers le bassin, dont des tillites de contact glaciaire, des grs fluvioglaciaires, des intervalles galets de dlestage, des rythmites tidales avec des combinaisons de rides dcoulement, et des turbidites. La Formation sous-jacente de Yaltipena est constitue de sdiments proglaciaires provenant de lentilles de glace en progression. Le dbut de la glaciation est reflt dans les ratios des lments majeurs (indice daltration chimique) des facis prglaciaires de la plateforme qui montre une rduction de laltration chimique et une dtrioration du climat lapproche de la base de la Formation dElatina. La progression des nappes de glace a entran une dformation des lits de sdiments meubles sous la diamictite glaciaire, montrant entre autres des structures de pousse sous-glaciaires ainsi que de lrosion sous-glaciaire de lunit de carbonate sous-jacente. Les mesures de coupes stratigraphiques travers le bassin montrent que lrosion glaciaire a enlev jusqu 130 m du carbonate de la plateforme. Toutefois, les signatures isotopiques 13C de fragments de carbonate dans les units de diamictites glaciaires utilises pour tablir la provenance et la chronologie dacquisition relative de la signature 13C des fragments, permet de penser quil y a eu au moins 500 m d'rosion quelque part dans le bassin. Les donnes de provenance sur zircons dtritiques de la Formation dElatina permettent de penser que les sdiments glaciaires provenaient partiellement des cratons de l'Australie occidentale et que le grs de la Formation de Whyalla, bien que stratigraphiquement corrl, n'a pas t une source de sdiments. Ce qui reste de la stratigraphie de la Formation dElatina reprsente principalement la dglaciation, laquelle peut tre divise en trois facis : un grs pliss, une diamictite galets de dlestage, et une diamictite remanie par des courants. La baisse du niveau relatif de la mer dans la partie suprieure de la Formation dElatina suppose une dglaciation rgionale sur une chelle de temps de lordre de celle de la nappe de glace interactions gravitationnelles de locan (instantanes) et/ou rebond isostatique (~104 ans). Des structures dcrites prcdemment comme des plis de sdiments mous dans des facis de rhythmites qui impliquait une position de basse latitude pour l'Australie du Sud l'poque de la glaciation Elatina,
