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Stephen McLoughlin, Swedish Museum of Natural History (Sweden)
Jean Galtier, CNRS (France)
Paul R. Blake, Geological Survey of Queensland (Australia)
Ruth Mawson, Macquarie University (Australia)
John Talent, Macquarie University (Australia)
Gregory E. Webb, Queensland University of Technology (Australia)
John Rolfe, Central Queensland University (Australia)
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The Late Devonian to Mississippian Drummond Basin of eastern Australia contains a predominantly non-marine succession with three major cycles of sedimentation. It preserves some of the best fossil evidence for the rise of arborescent vegetation in Gondwana. Furthermore, suites of unusual stromatolitic structures are exposed on the soil surface in at least three localities where they have apparently weathered out from lacustrine units within the Tournaisian-Visean Telemon and Ducabrook formations, spanning the basin's second and third cycles of sedimentation. The stromatolitic carbonates form columnar encrustations up to 23 cm thick around single lycophyte axes with diameters up to 18 cm or, in a few cases, develop around two closely spaced or branched axes, with the composite stromatolite up to 70 cm in diameter. The largest stromatolites reach 82 cm high with little reduction in diameter over their length. The tissues of the original encrusted lycophytes are not preserved but impressions of the stem's leaf scars are evident on the inner side of some fragments of the encrusting stromatolites. Leaf scar patterns vary from tightly spaced rhomboidal to widely spaced and spindle-shaped suggesting that at least three lycophyte taxa were encrusted. Most axes are surrounded by concentric stromatolitic laminae but in some cases lamina development has been reduced or curtailed on one side of the structure; the opposite side showing continued laminae accretion. In these cases, the original plant and its stromatolitic casing toppled permitting carbonate accretion only on the upper surface of the log. The central cavity of the stromatolite, representing the decayed plant stem, is typically filled with sparry calcite or, in some cases, with a geopetal fill of terrigenous mud or mud-clasts. In a few specimens, thinly developed stromatolites have shattered and are preserved as conglomeratic layers incorporating plates and shards of carbonate laminae. Rare Earth Element data is consistent with a non-marine setting for the deposits. Textures vary from relatively smooth, thin, concentric laminae (early development) to discontinuous, arched or cauliform laminae forming micro-columns and micro-ridges (late development) that are separated by narrow, irregular fenestrae filled with sparry calcite or micrite. The stromatolites are inferred to have developed following inundation of arborescent lycophyte communities during transgression of carbonate-charged lakes.
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