Late Paleozoic middle-latitude Gondwana environment-stable isotope records from Western Australia

Abstract

One hundred and twelve Permian fossil brachiopod samples from two Western Australian basins (Carnarvon and Canning Basins) have been analyzed for oxygen and carbon isotopes to study the middle paleolatitude environment in the Permian. All samples were thin sectioned and examined using petrography and cathodoluminescence microscopy for evaluating shell preservation. A total of 249 isotopic analyses from the best preserved portion (non-luminescent; NL) of shells were found useful for inferring paleoenvironments.For the Carnarvon Basin, average δ13C values of NL shells were between 4.2 and 4.5‰ for the Callytharra Formation (N=123; late Sakmarian–early Artinskian), Jimba Jimba Calcarenite (N=7; early Artinskian), Coyrie/Madeline Formations (N=71; both late Artinskian–early Kungurian), and Wandagee Formation (N=18; late Kungurian). Average δ13C values of NL shells were 5.0±0.4‰ (N=9) for the Quinnanie Shale/Cundlego Formations (both Kungurian). For the Canning Basin, mean δ13C values of NL shells were 5.1±1.1‰ (N=11), 5.3±0.2‰ (N=4), and 4.8±0.5‰ (N=6) for the Noonkanbah Formation. (Artinskian–Kungurian), Lightjack Formation (Roadian–Capitanian), and Hardman Formation (Wuchiapingian), respectively. Within the uncertainty of the stratigraphical correlation, the carbon isotope values spanning Late Sakmarian to Kungurian in middle latitude Western Australia are comparable to those of coeval low latitude Urals.Average δ18O values of NL shells were about 0‰ (−0.1 to 0.1‰) for the Callytharra Formation, Jimba Jimba Calcarenite, Coyrie/Madeline Formations, Quinnanie Shale/Cundlego Formations, Noonkanbah Formation, and Hardman Formation; and were −0.5±0.7‰ and −0.3±0.7‰ for the Wandagee Formation and Lightjack Formation, respectively. These oxygen isotope values are overall higher than the coeval values reported for low latitude regions and for eastern Australia. The positive oxygen isotope values are here interpreted to indicate a cool temperature and/or a possibly moderately high salinity condition for the two studied basins in Western Australia during the Permian.