Buchan Caves

PALAEOMAGNETIC ANALYSIS OF SEDIMENTS IN THE BUCHAN CAVES, SOUTHEASTERN AUSTRALIA, PROVIDES A PRE LATE PLEISTOCENE DATE FOR LANDSCAPE AND CLIMATE EVOLUTION... 

The topographically lower caves in the Buchan area are predominantly horizontal systems, usually developed sub-parallel to the strike of bedding, and are present only within the Buchan Caves Limestone. The main passages in these caves are less than 30 m above the nearest surface stream or river bed, and many contain active streams in their lowest levels. Thus these caves are of more recent origin than the Murrindal Limestone caves. 

Table 1. Rock magnetic parameters for representative samples from the Buchan cave sediments.

Figure 10. Day plot showing representative s ples from the Buchan cave sediments. Theoretical frelds for single-domain (SD), pseudo-single-somain (PSD), and multidomain and/or superpar agnetic (MD + SP) assemblages of magnetite are indicated.

The presence of a reversed polarity component can be demonstrated in one Bone Chamber sample, and there is evidence for reversed polarity components in several of the samples from Reed Chamber. At some stage between deposition of the Buchan cave sediments and the present, they have been exposed to a reversed field. Although brief intervals of intermediate and reversed polarity are known from the Brunhes Chron (e.g., the Blake and Emperor events), these total only about 5% of its 780,000 year extent (Jacobs, 1984 Cande and Kent, 1995). Consequently, it is most likely that the cave sediments have been in place since at least 780 ka. Understanding how the age of acquisition of the reversed polarity component relates to the age of cave sedimentation depends on an appreciation of the magnetisation process involved. 

Sedimentation and Diagenesis of the Magnetic Mineralogy of the Buchan Cave Sediments... 

Grotaers, L. D., 1995, The Buchan Caves Gravels implications for past depositional environment, B.A. (Honours) thesis, Deakin University, Geelong, Australia (unpublished). 

Limestone cave levels within the Dukes system in the Buchan district of East Gippsland in Victoria, southeastern Australia (Fig. 1) can be correlated with a series of three river terraces in the valley of the Buchan River (Webb et al., 1992). Both caves and terraces are thought to have developed in the Pleistocene, because uranium series dating of speleothems in this system gives ages of more than 350,000 years, and perhaps greater... 

Figure 1. Generalised geology of die Buchan Synclinorium, showing the location of die Dukes Cave system.

Thus the development of the lowest epiphreatic cave level may have been contemporaneous with or pre-date the sediment deposition. This indicates that even the lowest cave level (and the river terrace with which it is correlated) formed in the mid-Pleistocene, or earlier. The sea level and climatic fluctuations of the late Pleistocene, responsible for the bulk of Northern Hemisphere karst geomorphology, have left little discernible trace in the Buchan area (Webb et al., 1992). 

Substantial transport of wind-blown dust has occurred during the arid phases that have periodically characterised the climate of southern Australia in the Quaternary (Bowler, 1976, 1982 Colhoun, 1991). The palaeomagnetic minimum age for the "red earths" in the cave at Buchan suggests that these wind-blown sediments record a period of aridity older than 780 ka. Although the most intense periods of aridity in southern Australia occurred during the last 700 000 years (Bowler, 1982), seasonal aridity became established at about 6 Ma, and progressive desiccation was well advanced by 2.5 Ma (Bowler, 1976). In central Australia the onset of arid-zone aeolian facies may have been at around 1.1 Ma (Chen and Barton, 1991). Thus the "red earths" at Buchan appear to record the early imprint of these climatic changes on southern Australia, probably in the early Pleistocene. 

Sweeting, M. M., 1960, The caves of the Buchan area, Victoria, Australia, Zeitschrift Geomorph., Supplementband 2 81-91. 

Webb, J. A., Fabel, D., Finlayson, B. L., Ellaway, M., Li Shu, and Spiertz, H.-P., 1992, Denudation chronology from cave and river terrace levels the case of the Buchan Karst, southeastern Australia, Geol. Mag. 129 307-317. 

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