The Pliocene Epoch (1.78-5 Myr) was the last time when global average temperatures were warmer than the present (Crowley 1996). The period often is categorised as transitional between the mostly temperate climates of the Tertiary and more arid and large magnitude climate cycles of the Quaternary. A more realistic assessment is that the Pliocene, like the Quaternary, encompasses episodes of climatic variability imposed on the global cooling trend initiated some 40 Myr earlier in the Early Eocene. The nature of Pliocene floras, vegetation and climates in Australia has been difficult to determine because of minimal sediment accumulation onshore. Many sections are deeply weathered and any fossils are poorly preserved or have been destroyed (Kershaw et al., 1994; Macphail, 1997).

This situation changed dramatically when Drs. John Backhouse and Mike Macphail were alerted to the occurrence of a thick sequence of lake sediments in a 1990 hydrocarbon exploration well drilled by Ampol Exploration Ltd at Yallalie, on the southern boundary of the Watheroo National Park about 200 km north of Perth. This small (< 15 km diameter) basin, informally named the Yallalie Basin, appears to be the result of a meteor impact in the Dandaragan Trough during the Early Cretaceous (Dentith et al., 1992). Over 3 km of sediments have accumulated since that time and a preliminary pollen analysis (Macphail, 1994) confirmed that the lacustrine section was probably Pliocene (age range Late Miocene to Pliocene).

ARC funds permitted the lacustrine section, between 67-177 m depth, to be continuously cored in 1998. There was virtually 100% core recovery and the sediment features and plant fossils including diatoms, spores, pollen and charcoal are spectacularly well-preserved. The core has been described, dated via palaeomagnetism, and analysed for a selection of chemical properties as well as examined for the first stages of an analysis of diatoms, spores and pollen. This paper presents an overview of the sequence and preliminary results on the fossil analyses from the upper 30 m.

Key findings are: (1) The palaeomagnetic record shows a good match with the Gaussian Chron and includes the Kaena and Mammoth Subcrons, allowing the construction of an age-depth model for the lake sequence. (2) Two major lithostratigraphic units are present — an upper unit which accumulated between ~2.5-3.51 Myr (mid Pliocene) and which is characterised by fine organic laminae, and a lower relatively massive unit where sedimentary features are less well preserved. (3) Climates were generally wetter than at present but were interrupted by short intervals of semi-arid conditions between ~2.53-2.58 Myr. Other cycles may exist elsewhere in the record. (4) Conditions were particularly variable, and possibly seasonal between 2.87-3.0 Myr and 3.22-3.06 Myr.

Chemical analyses indicate that water depth was variable and that salinity levels and diatom abundance reflect climate rather than nutrient flux. During the wetter phases, the dryland vegetation was dominated by a mixture of sclerophyll and rainforest trees and shrubs, in particular Myrtaceae and Araucaria. This community superficially resembles dry rainforest, now confined to coastal Queensland and northern NSW. In fact it is a vanished vegetation type since less common elements are endemic to eastern Australia, Tasmania and/or islands in the South West Pacific, e.g. Agathis, Dacrycarpus, Phyllocladus and Nothofagus (Brassospora), or are wholly extinct. A possible relative of the recently discovered NSW Wollemi Pine (Wollemia) coexisted with a eucalypt (Eucalytus spathulata) now confined to semi-arid southwestern WA. Essentially the same rainforest type is found in other Pliocene sites along the southern margin, e.g. at Grange Burn, southwest Victoria (Macphail 1996).

During semi-arid episodes, chenopod (salt-bush) shrubland developed around the lake. Charcoal is abundant throughout the late Pliocene — evidence that natural wildfires were a perennial feature of the environment in southwestern Australia 3 Myr before humans arrived. In spite of fire, there is no pollen evidence for extensive grassland but pollen representing many of the herbs and sclerophyll shrubs that make the southwestern Australian flora the most diverse in Australia are preserved. Proteaceae examples are Banksia, Conospermum, Dryandra, Isopogon, Musgravieae, Stirlingia and Xylomelum. In some instances, more than one species are represented, e.g. Acacia and Banksia/Dryanda. There is a great richness in the Myrtaceae which dominates the uppermost pollen assemblage. There are several distinctive fossil pollen types that cannot be matched to living genera and there are many ferns that no longer occur in the region, e.g. Calochlaena/Culcita, Cyathea and Pteris.

The Yallalie record offers the best opportunity found so far to use fossils to study the development of high biodiversity of a region which contains half the number of flowering plant species of Australia. Yallalie is located within the midst of one of the richer areas in the Southwest. Much additional work is required to flesh-out this and other evidence preserved at Yallalie and to assess local climatic events against the regional and global record. For example, the interval of semi-arid conditions between ~2.53-2.58 Myr coincides with the development of large continental glaciers at ~2.54 Myr in the Northern Hemisphere. Adding to the challenge, and international significance of Yallallie, is that it should contribute to debates centred upon closure of the Indonesian Seaway in the early Pliocene (Srinivasan & Sinhas 1998, Wei 1998), events in the Southern Ocean including possible instability of the East Antarctic Ice Sheet during the mid Pliocene (Hodell et al. 1991, Burkle et al., 1996, Gersonde et al., 1997), and non-linear (124 Kyr) and orbital-scale (41 Kyr, 32-19 Kyr) climatic forcing recorded in the parallel (~3-2.6 Myr) high resolution continental record at Pula Maar in Hungary during the late Pliocene (Willis et al., 1999).