Karst of the Cainozoic Limestones of the Otway Basin, Southeastern Australia

Abstract

The Tertiary and Quaternary limestones of the Otway Basin in south-western Victoria and south-eastern South Australia include Miocene and Oligocene marine limestones and Pleistocene calcareous dune and beach facies. These highly variable limestones of high primary porosity and high permeability show diverse karst features both in surface expression and underground, presenting particular problems with regards to the usual concepts of speleogenesis. Although there have been some studies of karst development in the Cainozoic limestones (White 1994, Grimes 1994), these are generally of small areas (less than 100 km²) and show some interesting contrasts in features. The Otway Basin is a large karst area where there is the potential to understand the regional patterns of karst development, in particular the times taken for karstification in Pleistocene calcareous dunes. Karst development shows substantial variation across the Otway Basin and between the different limestone lithologies. The karst shows important differences in the number and type of cave present per volume of limestone, total passage length, passage orientation, passage size, and cave form. Substantial differences in other karst features are also evident, such as the presence of the distinctive cenotes and other surficial features in some areas and yet their complete absence in otherwise similar sites. Although environmental factors such as relief and climate do not vary greatly across the basin, there are significant changes in lithology, structure and underground water conditions. The variation in karst features can be explained partly by lithological variation; especially in such highly variable karst host rock. The overall regional view can add some important insights into concepts of speleogenesis.