Eocene and younger biostratigraphy and lithofacies of the Cascade Seamount, East Tasman Plateau, southwest Pacific Ocean

Six dredgings, five taken during fishing operations, have recovered sedimentary rocks from the volcanic framework of Cascade Seamount, which is built on thinned continental crust on the East Tasman Plateau off eastern Tasmania. The top of the original volcano is now about 600 m below sea‐level. Planktonic foraminifers and supplementary calcareous nannofossils yielded Late Eocene‐Early Oligocene, mid‐Late Oligocene, Early/Middle Miocene, approximately Late Miocene and Quaternary ages. The history of the seamount is more complex than for most seamounts built on oceanic crust and may involve several intervals of volcanism, burrowing or dissolution and cavity infilling. Some of the volcanism was subaerial and some submarine. Older rocks include conglomerate, volcaniclastic sandstone, and a variety of interstitial sparry calcite, formed under a shallow‐water, high‐energy regime, whereas later sediments are mainly oozes. The oldest sedimentary rocks are Late Eocene, shallow water, fully marine with calcareous algae and abundant echinoid debris. Two phases, Chiloguembelina‐dominated and Globigerinatheka‐dominated, can be differentiated. Warm waters prevailed in the Late Eocene and in the Middle Miocene, cooler conditions between and since, consistent with other indications around the southern Australian margin. Post‐depositional alteration consists of phosphatisation and development of ferromanganese crusts.

Key words:

• biostratigraphy
• Cascade Seamount
• Cenozoic
• East Tasman Plateau
• southwest Pacific Ocean