Erosion surfaces, river terraces, and river capture in the Launceston tertiary basin

Abstract

An erosion surface probably of Pliocene age, extends throughout the Launceston Tertiary. Basin at elevations between 550 and 1050 feet and IS here named the Woodstock Surface. It slopes gently eastwards from Deloraine and north-westwards from Tunbridge and is developed across Tertiary sediments, dolerite and basalt. Later~tic podzolic soils are typical of it on the sedIments, a!ld lateritic krasnozems with outcrops of laterIte (bauxite) on the dolerite and basalt. It is :l possible time marker, younger than the basalts at Deloraine-Westbury and at Campbell Town but of uncertain relationship to the basalt at Evandale. Several gently sloping surfaces can be recognised at levels below the Woodstock. The Cataract Gorge between Hadspen and Launceston separates these surfaces into two unrelated systems, the South Esk system above it and the Tamar system below. The former system cDmprises the present floodplains (Canola), and the Brumby and Brick-endon Terraces, respectively 8 to' 18, and 40 to 120 feet above the floDdplains. Local base levels for this system are set by the dolerite at the head Df the GDrge. The Canola soils are humic gleys; the Brumby meadDw podzolics or sDlods; and the Brickendon lateritic podzolics, distinguished from WODdstock sDils by the prevalence of siliceous gravels in them. In places windblown sands are superimposed upDn each terrace. Climatic fl.uctua-tions variatiDns in resistance of dolerIte to' corra:sion and renewed faulting are cDnsidered as PDssible causes of terrace formation. The BrickendDn Terraces are YDunger than the Evan-dale basalt and are thought to be PleistDcene. Their lateritizatiDn is ascribed to interglacial phases. The Tamar system which has not ~een fully investigated includes terraces at elevatIOns of 300-330 feet and 100-140 feet near Launceston and 130 feet near GeDrgetown, and a bench 1.t 450 feet in the valley of Rose Rivulet. Instances of river capture, involving the SDuth Esk River Liffey River, Brumby Creek and other streams a~e noted. A reconstruction of the late Cainozoic history of the Basin is attempted.