Setting the Scene – Syngenetic Karsts in the Southwest of Western Australia
Abstract
Western and Southern Australia have many karst areas developed on soft sandy limestones (calcarenites) which are quite differing to the traditional ‘hard-rock’ limestones found elsewhere.
These are the syngenetic karsts of the youthful Quaternary dune limestones and related soft-rock karsts of the Tertiary limestones (which also show some syngenetic features).
In syngenetic karst speleogenesis and lithogenesis are concurrent: caves and karst features are forming at the same time as the loose sediment is being cemented into a soft, porous rock.
The distinctive features of syngenetic karst are: shallow horizontal cave systems; a general lack of directed conduits (low irregular chambers occur instead); clustering of caves at the margins of topographic highs or along the coast; paleosoil horizons; vertical solution pipes which locally form dense fields; extensive breakdown and subsidence to form collapse-dominated cave systems; a variety of surface and subsurface breccias and locally large collapse dolines and cenotes; and limited surface sculpturing (karren).
In the southwest of WA syngenetic karst occurs in the coastal dune calcarenites of the Tamala Limestone.
Many features are similar to those seen in Eastern Australia and elsewhere, however there are some interesting differences also.
North from Perth, there is a long belt of Quaternary dune limestones that continues all the way to Cape Range (which is mainly Tertiary limestone). Within this the most interesting karst areas are the Nambung Pinnacles and the Yanchep area.
At Yanchep dune limestone overlies a quartz sand aquifer and aggressive water enters from below to dissolve caves at the base of the limestone.
South from Perth there are a few caves and springs where the Swan and other rivers cut through the dune ridges. In the Leeuwin-Naturaliste region a belt of dune limestone up to 6 km wide contains numerous caves (see conference Field Guide for details). Early work by speleologists (eg Bain and Bastian) in this area contributed to the concept of syngenetic karst. The special aspect of this area is the shallow impermeable basement, which can guide the water flow through the dune sand above. The caves are best developed in the older more-cemented dunes and are of three types: linear caves formed by cave streams which follow buried valleys above the impermeable basement; breakdown systems, including the ‘inclined fissure’ type, which modify and can completely replace, earlier solutional caves; and the horizontal watertable maze caves of the Augusta area – the last are relatively rare in the Leeuwin-Naturaliste region but common elsewhere. Water tracing has mapped some conduit flows from stream sinks at the inland margin of the dunes, through intermediate through-flow caves to springs on the coast, but much of the underground water flow seems to be lost offshore.