Cenozoic fault control on ‘deep lead’ palaeoriver systems, Central Highlands, Victoria

Abstract

Basaltic eruptions across the Central Highlands of Victoria have sealed in-place Early to middle Cenozoic palaeodrainage systems (also known as deep leads). The basal gravels of the deep leads have been mined extensively in the past for their rich placer-gold deposits. Detailed mapping of the distribution of all palaeorivers has been carried out using drilling results and modern aeromagnetic/ radiometric surveys. The palaeochannel isopachs (including basalt and sediment) do not thicken in a modern downvalley direction. Instead, deeper depressions alternate with shallower areas. The variations in thickness, and parts of the palaeochannel courses, are controlled by a series of eastnortheast-trending basement highs. The basement highs are caused by a set of east-northeast-trending (Otway Basin-style) faults visible on radar shuttle imagery in the Central Highlands. They have not previously been recognised in regional geological mapping. Most published fault trends are northsouth oriented, parallel to the strike of the Palaeozoic basement rocks. Exceptions occur at Ballarat where there is an orthogonal east-northeast set mapped in underground quartz reef workings that show right-lateral strike-slip movements. The east-northeast faults show half-graben block-style rotational movement on basement, creating north-and south-facing fault scarps along the horst ridges. Where palaeochannels overlie the grabens, valleys broadened, infill thickens, and locally drainage directions may change. When the drainage cuts through the horsts, steeper incised valleys result, and this is where, in the historical past, some gold leads were 'lost'. The initial timing of the block movement pre-dates at least the Early Oligocene to Late Miocene ages of the basal palaeovalley sediments, as shown by revised palynological dating. In places, the modern drainage divide coincides with east-northeast-trending faults. In the Ballarat area, an earlier divide accentuated by the aeromagnetic palaeodrainage mapping occurs up to 25 km south and appears to pre-date the earliest basalt flows at around 7.0 Ma. This evidence suggests the divide can change position through time by differential movements along east-northeast faults and transferral of maximum uplift to adjacent blocks.

No mountains to snow on: major post-Eocene uplift of the East Victoria Highlands; evidence from Cenozoic deposits

“…A series of lineaments and faults with east-northeast and northeast trends are present in these data although these are not the only trends that can be discerned. Some are extensions of faults with significant Cenozoic movement in the Central Victoria Highlands (Holdgate et al 2006). Some are previously mapped faults on GeoScience Victoria geological maps, yet many are newly revealed in the high-resolution data.…”

“…This paper uses the methods used by Holdgate et al (2006) for the Central Highlands to chart the uplift history of the East Victoria Highlands. These include Shuttle Radar Terrain Model (SRTM) imagery interpretation, sub-basalt stream mapping from aeromagnetic data, sediment analysis and paleocurrent measurement using outcrop and bores, and analyses of deep lead subvolcanic ages from palynology.…”

Use of a conceptual hydrogeological model and a time variant water budget analysis to determine controls on salinity in Lake Burrumbeet in southeast Australia

“…Underlying the basalts are the Early to Middle Miocene and Pliocene sands, ligneous clays and gravels of the , representing the pre-existing stream system incised deeply into a highly weathered early Cainozoic palaeo-surface (Holdgate et al 2006;Raiber and Webb 2008). Deep Leads occur throughout Victoria; they are often important aquifers and are famous for their placer gold deposits (Raiber and Webb 2008).…”

Palaeovalley, Palaeodrainage, and Palaeochannel–what’s the Difference and why does it Matter?

“…the Tamar & Derwent Vallies, Baillie 1989) in origin. Palaeovalley fill include glacial, fluvial (both channel and non-channel facies), lacustrine (Clarke 1994a), marine (Clarke et al (1996), volcanic (Holdgate et al 2006), and aeolian in nature, and some or all of these (such as in the Lefroy and Cowan palaeovalleys, Clarke 1994a, b, Clarke et al 1996 in combination.…”

“…Representative examples of Australian nomenclature include: deep leads (used by the miners and some to the present day, e.g. Holdgate et al 2006), palaeochannels (Mazzucchelli 2005), palaeodrainage (van de Graaff et al 1977), inset valleys (De Broekert & Sandiford 2005), palaeodrainage channel (Clarke 1994a, b), and palaeorivers (Beard 1999). Examples of diverse usage for the same features include 'palaeodrainage channel' and 'palaeodrainage' for features on the Yilgarn Craton (Clarke 1994a, b), 'palaeovalley' (Hou 2003a, b) and 'palaeochannel' (Hou et al (2007a) for features on the Gawler Craton, 'palaeochannel' to describe both incised valleys and channels within a sedimentary succession (Anand and Paine (2002), and 'palaeochannel' and 'palaeodrainage' (Alley et al 1999).…”