Palaeobathymetry of ostracod associations before and after the Chinaman Gully regression (‘Eocene/Oligocene boundary’) in South Australia

Abstract

The Chinaman Gully Formation in South Australia is a regressive non-marine to marginal marine unit previously correlated with the ‘Eocene/Oligocene boundary’ type 1 Sequence Boundary Ta 4.3/4.4. It is barren of ostracods. Cytherellid, krithiid, pontocypridid, paracypridid and trachyleberidid ostracods are abundant in the underlying Blanche Point Formation (BPF), whereas bairdiids, xestoleberidids, hemicytherids and loxoconchids are more abundant in the Port Willunga Formation (PWF) which overlies the Chinaman Gully Formation. A major contrast in trachyleberidid, loxoconchid and hemicytherid species composition is evident between the BPF and PWF. A bootstrap variety of correspondence analysis identified Paracypris, Kuiperiana, Krithe, Tasmanocypris?, Argilloecia, Trachyleberis, Cytherella and Bythocypris as significant genera (indicating a palaeodepth of 120–200 m) in the BPF and Quadracythere, Hornibrookella?, Loxoconcha, Microcytherura, bairdiids, Schizocythere, Cytheralison, Kangarina and Xestoleberis as significant genera (indicating a palaeodepth of ≤ 50 m) in the PWF. The generic diversity and the adult/juvenile ratio is higher in the PWF than in the BPF, reflecting a greater number of niches in a shallower environment with higher hydrodynamic stress. The many autochthonous cytherellids in the BPF probably signify low oxygen conditions during deposition.

Four new species are proposed: Quadracythere willungaensis sp. nov., Quadracythere aldingaensis sp. nov., Quadracythere? sudaustralis sp. nov. and Loxoconcha ruwarungensis sp. nov.

Keywords:

• Ostracoda
• Australia
• Eocene
• Oligocene
• palaeoecology
• palaeobathymetry