Origin of tertiary inset-valleys and their fills, Kalgoorlie, Western Australia
Date
2002
Authors
De Broekert, Peter
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Abstract
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.
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