No abstract

One of the best-developed carbonate successions of the Lower Permian in eastern Gondwana is the Callytharra Formation, deposited above glacially influenced sediments in a narrow elongate rift basin far from the open ocean. This formation and the underlying Carrandibby Formation provide evidence for faunal changes that accompanied the melting of the Sakmarian Gondwanan ice sheets. Diverse smaller benthic foraminifera including organic-cemented agglutinated, hyaline, and porcelaneous calcitic forms, accompany a heterozoan macrofauna (mainly bryozoans, crinoids and brachiopods). Fusulinid foraminifera and calcareous algae are absent. Seven foraminiferal assemblage zones subdivide the succession and parallel changes in associated macrofauna and lithofacies. Environmental changes determined from the succession include (1) a transition from nearshore, lowenergy, low-salinity, cool waters to higher-energy, normal-marine waters supporting bryozoan, crinoidal and non-skeletal-macroalgal meadows; and (2) a transition from well-oxygenated, shallow-water, sandy substrates to deeper-water, less-oxygenated, muddy substrates. Faunal comparisons suggest that at paleolatitudes higher than 45؇S, in widely separated interior basins in eastern Gondwana, a change from low-diversity, siliceous-agglutinated to high-diversity, Calcitornella-rich, foraminiferal assemblages took place during the Sakmarian. This change may have been associated with a reduction in melt-water influx into the interior basins from retreating Gondwanan ice sheets. The warming was insufficient to allow colonization of the shallow seas by fusulinids and calcareous algae, which only ranged as far south as 45؇S paleolatitude.

Jurassic–Cretaceous ammonites are particularly robust fossil tools in global stratigraphy and correlation. The successive evolution and extinction of these cephalopod mollusks was so rapid that many ammonite zones are no more than one million years in duration. A well-preserved ammonite specimen from the Fortissimo-1 core, Browse Basin, NW Australia is assignable to the widespread latest Jurassic dimorphic berriaselline genus, Blanfordiceras Cossmann, recorded previously from the Spiti area, Nepal, Tibet, Madagascar, Papua-New Guinea, Antarctica, and southern South America. This is the first report of ammonites of this age in the Australian region. The evolute shell of an estimated 90-100 mm diameter (when extrapolated) and pronounced ornamentation of variably bifurcating, curvilinear and flexuous ribs, intercalated with simple, non-bifurcating ribs, is consistent with Blanfordiceras wallichi (Gray, 1832), which has traditionally been restricted to the uppermost Tithonian Stage, ca. 146.5-145.5 Ma, but may well have survived into the earliest part of the Berriasian. The first recorded occurrence of this ammonite in Australia fills an anomalous absence in the paleobiogeographic distribution of Jurassic–Cretaceous boundary ammonites in the Indo-SW Pacific Subrealm with important implications for the calibration of offshore rocks and wells in Australia.