The Plains Karsts of the Smithton Basin
Email: chris.sharples@utas.edu.au
Abstract
The Togari Group of younger Precambrian rocks in the Smithton Basin of far northwest Tasmania includes two thick horizons of karstic dolomite, namely the upper Smithton Dolomite and the lower Black River Dolomite, separated by volcanics and clastic sediments. These were folded into a broad north-south oriented synclinorium, and subsequently reduced to a broad coastal plain by an unusually long (for Tasmania) period of continuous sub- aerial exposure and erosion lasting over 400 million years from Late Cambrian through to Early Tertiary times. Lateral karstic corrosion at the water table has created very broad, flat and (formerly) swampy karst plains on the Smithton Dolomite in the Duck and Montagu River valleys, with characteristic short steep marginal slopes. This extensive karst plain is one of the largest distinctively karstic landscapes in Tasmania. Mound springs in the Duck Valley are a notable feature of the plains karst; however the high water tables and flat relief mean that only a few caves have developed on isolated residual outcrops of dolomite rising above the plains surface; nonetheless some of these contain notable marsupial megafauna remains as at Montagu Caves. The mostly poorly-drained conditions resulted in accumulation of peat swamps during the Pleistocene, from which numerous megafauna bones have also been recovered near Smithton. Nonetheless much of the plains karst has been cleared and developed for agriculture, which historically required development of possibly the most extensive complex of artificial drainage channels in any part of Tasmania. The underlying Black River Dolomite is relatively siliceous, and so outcrops in a fringe of hillier country surrounding the plains karst, where sinkhole lakes such as Lake Chisholm and the Trowutta Arch cenote have developed along with some cave development as at Julius River Caves. Despite this, Tasmania’s best example of an enclosed plains karst or polje has also developed in carbonate rocks assumed to correlate with the Black River Dolomite at Dismal Swamp. Although some logging has occurred in the past, Dismal Swamp remains undrained and much of the polje has been protected as an important remnant Blackwood swamp forest habitat.