New perspectives on the Cenozoic history of the Tamar Valley and Launceston Gorge

The Tamar Rift Valley carries evidence of an often-turbulent history during the Cenozoic period. For this research the principal rock units: dolerite, sediments, basalts (vents and flows) and conglomerates, interacting over the past 70 million years were computer modelled. Reconstructions showed that the Longford Basin was integral to the evolution of today’s Tamar Valley, so were extended over the north of that basin. The main outcomes were:

• The Tamar Graben and Longford Basin filled with sediments to 140 m elevation in a lake (Lake Tamar–Longford) upstream of a wall-to-wall volcanic dam across the Tamar Rift Valley. This dam coincided approximately with the current Batman Highway.
• Sediments of the Longford Basin sector of the lake were subsequently raised from 140 m to 190 m elevation, upstream of a second volcanic dam based on the 140-m surface near Evandale in the Tamar Valley. This sector was separated from the Tamar by a five-km-wide dolerite horst, minimum height 190 m above sea level.
• The lake (Lake Longford) overflowed along fractures in the dolerite, commencing near future Hadspen at 190 m, across the Trevallyn Fault near Launceston. The resulting high energy waterfall, draining the new Lake Longford, retreated approximately 15 km back through columnar dolerite to Hadspen, forming the Cataract Gorge.

Key Words: Tamar, Cenozoic, Longford, volcanic dam, Cataract, waterfall, retreat.