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Abstract:

Isaacs Creek Caves are situated in the Hunter Valley of New South Wales and form a distinct unit within the Timor karst region. The larger caves such as Man, Helictite and Belfry all show evidence of early development under sluggish phreatic conditions. Nevertheless later phases of dynamic phreatic and vadose development occurred in Belfry and Helictite caves. In the case of Helictite Cave sluggish phreatic, dynamic phreatic and vadose action may have operated simultaneously in different parts of the same cave. After each cave was drained through further valley incision by Isaacs Creek, extensive clay fills derived from surface soil were deposited in it. There has been considerable re-excavation of the fills; in Main Cave younger clay loams have partially filled the resulting cavities and thus underlie the older clays. The earliest speleogenesis took place in Main Cave which pre-dates the valley of Isaacs Creek. This cave now lies in the summit of Caves Ridge about 100m above the modern valley floor. Helictite and Shaft Caves formed when the valley had been cut down to within 30m of its present level and some early phreatic development also took place in the Belfry Cave at this time. Later phases of dynamic phreatic and vadose development in Belfry Cave occurred when the valley floor lay about 12m above its present level and can be correlated with river terraces at this height. Evidence from cave morphology, isotopic basalt dates and surfaces geomorphology indicates that Main Cave formed in the Cretaceous and that Helictite Cave, Shaft Cave and the early development in Belfry Cave date from the Palaeogene. Although the dynamic phreatic and vadose action in Belfry Cave is more recent, it may still range back into the Miocene. This is a much more ancient and extended chronology than has hitherto been proposed for limestone caves and is in conflict with widely accepted ideas about cave longevity. Nevertheless evidence from Isaacs Creek and other parts of the Hunter Valley indicates that the caves and landforms are ancient features and thus notions of cave longevity developed in younger geological environments of the northern hemisphere do not apply in the present context.