Paleogene basalts prove early uplift of Victoria's Eastern Uplands

Abstract

Profiles of five Paleogene rivers reconstructed from basalts that flowed down them show a complex landscape. All five flow fields are in Victoria's Eastern Uplands, two in the Baw Baw region in the southwest and three in the Nunniong–Deddick region in the east. Three flow fields (Toorojil, Aberthomson, Timbuchan) originate on high plateaus (<1000 m), from which they descend steadily via deeply incised valleys to lowland settings of below 200 m. Local topography at the edge of the upland plateaus shows Paleogene basalt lying between 650 and 900 m below adjacent mountain peaks, proving that the Paleogene streams were deeply incised. Two additional Paleogene (Eocene, ∼38–42 Ma) basalt fields in the Nunniong–Deddick region (Butrindal and Bondick) indicate that the Eocene Snowy River valley's base was overlooked by mountains up to 800–1050 m higher. In the Baw Baw region, the Paleogene northern edge of the Gippsland Plains is marked by an upland front at which Paleogene basalts debouch onto and spread over pre-basaltic alluvial fans. Pliocene alluvial fans overlying the basalt debouch from the same positions, showing that there has been no significant Neogene shift of the upland's edge. The Paleogene landscape contained all major geomorphic elements visible today: high plateaus bordered by steep escarpments that were breached by deep valleys that descended to dissected lowlands, debouching onto alluvial fans of the Gippsland Plain. The Paleogene landscape is best viewed as a snapshot taken during the prolonged evolution of the landscape. The uplift must therefore be considerably older, and is tied to the onset of major sedimentation into the receiving basins of the Southern Rift System at 93 Ma (Cenomanian), also recorded as a regional cooling event in apatite fission-track ages.