--> Abstract: Glendonite Formation in Assocation with Methane Oxidation in the Middle Permian Wandrawandian Siltstone, Southern Sydney Basin, Australia, by Stephanie G. Thomas; #90039 (2005)

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Glendonite Formation in Assocation with Methane Oxidation in the Middle Permian Wandrawandian Siltstone, Southern Sydney Basin, Australia

Stephanie G. Thomas
University of Nebraska-Lincoln, Lincoln, NE

Glendonites, enigmatic pseudomorphs after the mineral ikaite (CaCO3∙6H2O), are found throughout the middle Permian Wandrawandian Siltstone, a marine unit recording shelfal to lower shoreface depths.  Ikaite, a metastable mineral, forms authigenically near the sediment-water interface in near-freezing temperatures in association with the decomposition of organic matter.  Glendonites are present in all lithofacies excluding channel sands of the lower half of the formation and have three distinct morphologies: bladed forms, which reach up to 30cm long, are hosted within fine sands; rosettes, which comprise smaller radial clusters of glendonite crystals, are found in siltstones; and stellate forms are contained in intermediate lithologies.  Carbon and oxygen isotopic compositions of petrographic phases record evolving porewater compositions.  Phase 1, the earliest phase, comprises amber tabular crystals, with common zonation and abundant fluid inclusions, and has d13C values ranging from -27 to - 20‰ PDB and d18O values from -2 to 0‰ PDB.  Phase 2, clear spherical to tabular calcite crystals, Phase 3, medium brown spherical to tabular calcite crystals, and subsequent calcite cements have d13C values ranging from -23 to – 5‰ PDB and d18O values from -22 to -1‰ PDB.  Assuming a mid-Permian seawater composition of 0 ‰ PDB, oxygen isotope data provide unrealistic paleotemperature estimates.  However, the range of d13C values suggests that methane oxidation likely played a role in the formation of ikaite within the Wandrawandian Siltstone, consistent with recent documentation of modern examples.  Their association with abundant organic matter may link glendonites to hydrocarbon source rocks.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005