Origin of tertiary inset-valleys and their fills, Kalgoorlie, Western Australia
Creators: name: De Broekert, Peter
Publisher: name: The Australian National University
Year: 2002

description: 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 interior well after the primary-valleys were established. The cause for epeirogenic uplift of the Yilgarn Craton remains uncertain, but can be tentatively linked to a change in the horizontal stress field resulting from a major plate tectonic re-organisation in the late Middle Eocene (-43 Ma), Climate change and eustatic sea-level fall appear to be incapable of generating valleys of the dimensions and lateral extent exhibited by the inset-valley networks in the Easten Goldfields. In any case, both of these allogenic variables were poorly disposed to fluvial incision during the early Tertiary - the climate being sufficiently warm and humid to support a thick mesothermal rainforest cover, and the shoreline being restricted to the continental shelf, over 500 km distant from the upper reaches of the Roe inset-valley network. Following formation of the primary-valleys and preceding incision of the inset-valleys, there occurred a period of limited fluvial activity which facilitated widespread deep weathering of the Precambrian granitoid and greenstone basement rocks that constitute the bulk of the eastern Yilgarn Craton. This is evidenced by the weak influence of bedrock lithology and structure on the pattern of inset-valley incision and the chemical maturity of the inset-valley fills. In common to all sections of the inset-valley fills examined are three unconformities, the lowermost of which is the inset-valley form. These provide a convenient framework for subdividing the inset-valley fills into two alloformations and an overlying sequence of 'primary-valley fill'. The basal alloformation (AFl), comprises an assemblage of dominantly coarse-grained, fluvially deposited lithofacies whose composition and architecture depend strongly on position within the inset-valley network, and for 'tributary' (~5th-order) inset-valleys, also on the type of underlying bedrock. This allows for the definition of three AFl 'fill-styles'- fill styles I and 2 incorporating the fill of tributary inset-valleys developed over weathered granite and mafic-ultramafic rocks, respectively; and fill-style 3 comprising the fill of trunk inset-valleys. All three fill styles clearly reflect derivation from deeply weathered basement and were deposited in high gradient, low sinuosity bedload channels that occupied the full width of the inset-valley floor, or migrated repeatedly across it destroying all evidence of overbank sedimentation. Channel fills of regularly alternating coarse- and fine-grained strata indicate that stream discharge was strongly cyclic. Alloformation 2 (AF2) by contrast, is dominantly composed of clay-rich lithofacies deposited in a hydrologically fluctuating, wetland environment. Lenses of ferruginous gravel, sourced from both within and outside the 'basin' of deposition, reflect the episodic establishment of fluvial conditions. Weathering and diagenetic overprints are typically extensive, assuming the form of dolomite pods, opaline silica lenses, and abundant ferruginous nodule and mottle structures. Deposition of AFl occurred rapidly in response to an excess of sediment supply affected by a change to a markedly seasonal climate during the middle-late Middle Eocene. Subsequent marine transgressions during the late Middle-Late Eocene invaded the lower reaches of the Eastern Goldfields inset- and primary-valley networks, but did not penetrate for far inland because sediment supply was able to keep pace with the generation of accommodation space. Instead, within the middle,-upper reaches of the inset-valley networks, fluvial deposition continued with the influence of a rising baselevel being transferred upstream as changes in fluvial style. Closely following the Late Eocene sea~level maximum there occurred a major marine regression resulting from the first Cenozoic continental-scale glaciation of Antarctica (Terminal Eocene Event). This rapidly brought the shoreline back to the continental shelf causing widespread erosion within the inset-valleys and removal of any evidence of the marine transgressions from the middle-upper reaches of the Roe inset-valley network. Low sea-levels and a marked cooling of global climate following the Terminal Eocene Event saw the development of cool. dry conditions onshore, transforming the inset-valley floors into a series of wetlands. Deposition of AF2 commenced at a very slow rate in the Early Oligocene and may have continued through to the Early Miocene. Deposition of allofonnations 1 and 2 within the middle-upper reaches of the Roe insetvalley network was, therefore, dominantly controlled by climate and did not result from distant rises in sea-level, as is generally thought to be the case. A second phase of epeirogenic uplift of the Yilgarn Craton occurred in the Middle Miocene bringing the inset-valley fills to their current elevations. However, owing to further reductions in rainfall and fluvial activity, erosion associated with this phase of uplift was slight. Arid to semi-arid conditions prevailed during the Late Miocene-Holocene, resulting in deposition of the primary-valley fill and completion of the Cenozoic succession.