Origin of tertiary inset-valleys and their fills, Kalgoorlie, Western Australia

• Main Author: De Broekert, Peter
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Subjects: Goldfields; Antarc*; Antarctica
• Online Access: http://hdl.handle.net/1885/10816; https://doi.org/10.25911/5d778508e7815; https://openresearch-repository.anu.edu.au/bitstream/1885/10816/3/DEBROEKERT%2cP.pdf.jpg

An extensive network of valleys filled with a distinctive sequence of Tertiary clastic sediments lies buried beneath a Quaternary cover in the Eastern Goldfields region of southwestern Australia. Apart from being weathered in their upper part, the valley-fill sediments are excellently preserved and constitute the most comprehensive record of Cenozoic depositional and erosional events in the region. Locally containing economic concentrations of gold and uranium and forming important groundwater aquifers, good access to the valleys and their fills is provided by open-cut mines and drill core. Formerly referred to as 'palaeochannels', the buried valleys are herein termed 'inset-valleys' to emphasise their subordinate and entrenched position within the bedrock surface of another system of much broader and subtly defined 'primary-valleys'. Within the Kalgoorlie study area, which encompasses the upper to middle reaches of the Roe inset-valley network, the inset-valleys form a fairly coarse textured, sub-dendritic pattern with up to eight orders of tributaries. The inset-valleys have a width-depth ratio of approximately 15, increasing in dimensions with tributary order to a maximum width and depth of about 1.4 km and 75 m, respectively. Most inset-valleys have a symmetric, open V -shaped transverse form with rare structural benches and unpaired terraces forming small steps in the side-walls. In longitudinal section, the insetvalley network displays a smooth, concave-up profile typical of having been reduced by a stream system graded to base-level. Contrary to most previous interpretations, the inset-valleys are temporally and genetically distinct from the primary-valleys in which they occur. The primary-valleys were largely formed by prolonged fluvial erosion of the Yilgarn Craton during the Mesozoic, which contributed to filling of the evolving Bight rift basin between Australia and Antarctica. The inset-valleys, in contrast, were formed by stream rejuvenation following epeirogenic uplift of the Yilgarn Craton ...