palaeodrainage and its significance to mineral

ROSLYN
CHAN
PALAEODRAINAGE AND ITS SIGNIFICANCE TO MINERAL EXPLORATION IN THE BATHURST REGION, NSW
R.A . CHAN
Cooperative Research Centre for Lanascape Evolution ana Mineral Exploration, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601 ABSTRACT
The complex drainage evolution in the Bathurst region has been influenced by both regional and local factors .
Regional factors are tectonics, eustacy and cli mate. Local factors are bedrock lithology and structu re, and Tertiary
volcanism. Drainage evolution is traced back in time to the northeast-trending palaeodrainage to the Sydney
Basin in Earl y Triassic times. Northerly drainage to the Coonamble Emba yment of the Surat Basin in the Jurassic
was overprinted by northwesterly drainage across the Surat Basin In the Late Cretaceous. Incision of Surat Basin
sed iments was initiated in the Late Cretaceous. Warping associated with initial stages of rifting in the Late
Cretaceous initiated these latter events, wh ich resulted in the formation of two maj or drainage divides Gorge
erosion along rivers flowing to the Murray Basin accelerated in early Tertiary times . The onset of three periods of
Tertiary volcanism and the formation of erosion bowls due to erosion of incompetant lithol ogies have also been
significant factors in the drainage evolution of the Bathurst region . The sedimentation and erosion of drainage
systems have been variously affected by sea level fluctuations, climatic change and tectonics .
Drainage evolution is the key to understanding landform evolution and associated regolith evolution , which have
important ramifications for land use, the en vi ronment and mineral exploration. The framework for drainage
evolution presented in this paper may assist mineral exploration in locating placer deposits and deep leads as It
can be extrapolated to neighbouring regions of the highly prospective Lachlan Fold Belt where much of the
palaeodrainage is buried .
INTRODUCTION
Bathurst (Chan and Kamprad, 199 5), Orange (Chan and
The Bathurst region is in the northeast of the Lachlan Fold
Fleming, 199 5a), Molong (Chan and Fleming, 199 5b),
Belt, bordering on the Sydney Basin (Figure 1). The
Cowra (Chan and Goldrick, 1995), Bla yney (Chan, 1995)
Australian Geological Survey Organisation and the NSW
and Oberon (Hazell and Chan, 1995 ) sheets . Figure 2
Geologica l Survey carried out regional regolith -landfo rm
shows the location of the six 1: 100000 sheets, as well as
and geological mapping over the Bathurst 1250 000
the main cultural and mining features .
quadrangle (BATHURST) in the mid 1990s (Chan , 1998).
Eight regolith- landform maps were produced, two at 1:250
Figure 3 shows the main regolith types and erosional
000 scale (Chan et ai, 1995a, b), and six at 100 000 scale :
scarps identifi ed from this mapping. A central northwest­
_
_
Drainage outsIde
Lachlan Fold Selt
Dr alrlage parallel
10 stoke of bed·
rock In Lachlan
Fold Sell
Drainage oblique
to sluke of bed­
rock in Lachlan
Fold Sell
_ .. _ .. - Great
D!VIde
••• • • _-- Ca nobolas DivIde
- T - Great
I
Figure 1: Regiona l setting
38
Escarpmenl
t Lac hlan Fold Sell
ASPECTS
STEPHENSON & BROWN, eM, 1989 . The ancient Murray
River System BMR Journal of Australian Geology and
Geophysics, 11. 387-395.
sUPPEL, D.W & GILLIGAN , LB , 1993. Nymagee 1250
000 metallogenic map 51/55-2 . Metallogenic stud y
and mineral deposit data sheets . Geological Survey
of New South Wales, Sydney
WELLMAN, P , 1986. In trusio ns be neath large intraplate
volca noes . Exp loration Geophysics, 17 , 135 - 139.
WELLMAN, P & McDOUGALL, I , 1974. Potassium-Argon
ages of the Cainozoic volcanic rocks of New South
Wales. Journal of the Geological SOCiety of Australia,
21. 247-272 .
WILLIAMSON , WH, 1969. Cainozoic rocks outside the
Murray basin - 3 . The Lachlan Valley. In Packham, G.H.
( Ed ), The Geology of New South Wales . Journal of the
Geological Society of Australia, 16 (1 ), 545-549.
WILLIAMsOI'l, WH,
groundwater
1986.
resources
Investigatio n
of
the
La chlan
of
the
Valle y
alluvium . Part 1: Cowra to Jemalong Weir. Water
Resources
Commission
of New Sou th
Wales,
Hydrogeologica l Report 1986/12 .
WILSON, IR. & McNALLY, GH , 1996 . A geological
appraisal of Tertiary deep leads in the Parkes-Forbes
area . Abstracts, The Geological Evolution of Eastern
Australia . Sydney Universities Consortium of Geology
and Geophysics, 71-73
WOOLLEY, D.R. & WILLIAMS, DR,
1994. Cargelligo
Hydrogeological Map ( 1250 000 scale) . Aus tralian
Geological Survey Organisation, Canberra.
37
OF
PALAEODRAINAGE
PAlAEODR
A
IN
GA
E
SIGNIFIC
~~~$I1I.
.
A
NCE
0& Hill
Bald
..
E~d
(Au) ,
Moleng
THUR
1i!iJ®1l!ID~® REG
T
ION
• Turon River (Au)
orJnge
.Mt~~o~~s ..
Hill, moun!ain
0
0 I
Bulga
lit Canobol'$ iLucknow (Cu) •
(Au)
Town
Localily
Cargo, Ridgeway (Au) Mine
..
Oe po sit
G:l
Bathurst Granite
§
Glenella Basalt
Cowra
Dalbys
• Falls
30km
~o
10
1
ChaJ/acombs.
Crossing S
... Spring Gully (Au)
Manildra
• Toogong
B A
THE
N
(Au)
Hawkin~ Hill
Hili
I
1
Abercrombie
Caves.
• wrangala
LuIky Draw (Au) .Burraga (Cu,Au)
Figure 2 : BATHURST locality m ap
trending highly weathered plateau dominates the area
occur on the western slopes associated with the Lachlan
and transgresses the northerly trend of the underlying
and Belubula Rivers and Mandagery Creek . High level
bedrock. Multiple weathering profiles and residual clay
alluvial sediments occur on the plateau west of Mo long,
are associated with extensive Tertiary lava flows; residual
and terraced alluvial sediments occur along the Macquarie
and transported sand and gravel occur near and on the
River, especially in the highly weathered and eroded
Canobolas Divide. Rugged hilly to mountainous terrain
with moderately to unweathered bedrock predominate to
Bathurst Granite, and along the Lachlan River. Aeolian
the northeast and south. Large areas of transported
sed iments are probabl y scattered over much of the area
colluvial and alluvial deposits overlying weathered bedrock
(Scott, 1999) but have yet to be defined regionally
Transponed Reooilh
---.J i.rcsU.it1uaJmtn'S
:=J '''''''­
D c./llwllu­
Ifl -Si llJ Recolith
. J
R~f l'JI~' IPTllrttiif
D t...
itP), hlQMf ,,·~ltJIet~ d bl'!u\(lCt
n
Hr:.;hT)' ".IiolJ'htrtr.tWock
~ ((UJdeJ'litrlf~.rtr.!rsdbltrJ:·
:=
Sll r)lKlt'Mur"-tdu r:/rcu
D v-....~'"
\~
EROSIONAL SCARPS
~ ~1I"~·$I;U~G:I$UlD
(o ,,~ ~.~.'II/""QA*~
-
-
'-
~ljrl.~~Ils".::.MI
VOLCANIC LANDfORMS
~ A"..,,(,. (~1J.1IpWfu
~
W f*f;fV
( ~I!
' ¥ ----,"~,
""--_. ti
~ , . .,.
. 1­
,~."
."",
Figure 3 : BA77-JURST regolith types an d erosional sca,ps
39
~ ;:=:~&~~Qnl1
~,~(rdUr.1I
b..
Mgj(Il1c.a.~/a.(
----... IIIJIfflI:f~.,..
Pf..,rtd~II1'~""lts
-
.... rnutt~;1 fJ ~. sl~ w~Ir>J~'
_._ - . IIItwt,e:l QttiJm rc/Qo:w:;
AddtI.t'~e
10 15km
IROSLYN
C
HAN
Figure 4 displays the regional relief of BATHURST and the
over a digital terrain model on which some major
main present-day dra inage and drainage divides . The
regolith-landforms are indicated
Great Divide separates coas tal from inland drainage, and
The geology dominantly consists of northerly trending
the northwest-trending Canobolas Divide separates the
belts of Ordovician to Devonian felsic to mafic volcanics
Darling River drainage from the more southerly Lachlan
and volcaniclastics, and metasediments of the Lachlan
River drainage (Figures 1 and 4) The tabl elands of the
Fold Belt. Th e Silurian Wyangala Granite and
the
Carboniferous Bathurst Granite are the largest of a number
Eastern Highlands grade to the western slopes in the
of granite intrusions. Sydney Basin sedimen ts onlap to the
western third of BATHURST. Figure 5 shows a perspective
east. Tertiary volcanics, noteably the Canobolas complex
view of airborne gamma-ray spectrometric da ta draped
shield volcano, overlie the older rocks.
-..­
-­-.­
lOO!r,-.~
ooorm-rQ.)!t
j-~,-
-­ -
;4 '0'_-'1"-:',11'*
_t...._
-t--
~~I G'r~ ...iI"
dII .. """,.."".
llc"~f!'rllaf·"boe
10
Figure 4 : BATHURST regional reiie/and drainage
[} 5£ lld ed Il!'Ilab boNI w Abf!fCfombie
La~lI n River C1I: Ot'l.;TM Il\
E!.C&lpm4r.: (, ~
WyangaJarsS6n'Clj'
Plaleil.UO!l Qtarut e
Figure 5: BATHURST digital lelTain model with 3 -bcmd composite gamma-ray
spectrometriC drape (potassium in red, thorium in green, uranium in blue)
40
15 kJlr
PALAEODRAINGAE
SIGNIFI
CA
N
C
E
IN
THE
B
A
THUR
S
T
RE
G
I
O
N
The oldest recognised drainage is across strike draining
RECOGNITIOl'l OF DRAINAGE TRENDS
Evidence for drainage evolution is in the form of Tertiary
towards the northeast; younger drainage parallels strike
lava flows (Figures 6 and 7), remnant high level alluvial
in a northerly direction; still younger drainage across
materials (Figure 8), landforms such as windgaps (Figure
strike drains a sector spanning from the northwest
9), ponding and drainage patterns . Interpretations are of
through to the south\'vest. Overprinting and disrupting
variable certainty. Sectors of present-day drainage align
these
to form trends which may be remnants of older drainage
systems. Figure 10 shows the dominant trends in
drainage
relationships
indicate
Tertiary
m ore full y understand the many complexities highlighted
such as cross-bedding,
by this regiona l mapping.
indicates these directions rather than their inverse .
Cross-cutting
with
dating of lava fl ows and high level all uvium is required to
Sub-dominant trends are to the northeast, west and
Field evidence,
trends associated
induced radial drainage. More detailed mapping and
BATHURST are in a northerly and northwesterly direction.
southwest.
trends are
volcanism which may relate to pre volcanic doming­
that
drainage directions young in an anticlockwise direction .
Figure 7: View to west from inverted basalt in Figure 6 across incised
J1IIacquarie River valley
Figure 6: Weathered 12 J1Ila C'cmobolas
inverted basalt flow over alluvial gravels
Figure 8: La te j urassic conglomerate will? rounded lithic and qu.artz
pebbles over vely coarse sandstone at Killonbutta Forest
41
I
ROSLYN
CHAN
(Figure 8) indicates a northerly drainage direction and
initial palynological dating of carbonaceous mudstones
indicates a Late Jurassic age (time equivalent to Surat
Basin Pilliga Sandstone) and a non-marine deposition
environment (Gibson & Chan, 1999). Rounded quartz
and lithic pebbles and cobbles are present. Lag from
similar sediments is preserved 9 km to the east in a wind
gap on the Canobolas Divide near the headwaters of
Mandagery Creek. Cuttings from drilling aSSOCiated with
reflection seismic profiling by AGSO in 1997 have been
interpreted to show a minimum original thickness of 70
m for sediments at Killonbutta and at least 100 m
Figure 9: Wind gap on Great Divide between Fish River flowing
to the n011h-northwest andJenolan River flowing to tbe south ­
southeast
maximum
original
thickness
in
the
area.
These sediments are on variably weathered Gumble
Granite and indicate a pre-depositional Jurassic terrain
with more relief than today's Sediments to the west of a
bedrock granite high noted from seismic drilling are
partly
ferruginised
and
probably
reflect
palaeogroundwater controls. Clearly these sediments
indicate a major drainage line to the Surat Basin in
the north during the Jurassic prior to the existence of the
Canobolas
further
Divide.
west
in
This
high
scenario
level
is
parallelled
sediments
around
Gunningbland National Forest (Gibson & Chan, 1999).
1 Inferred reconstruction
-+
-+
N trends (parallel to stnke)
_
NW-SW trends (across strike)
_
Radial drainage off Mt Canobolas volcano, Inferred from Java flows NE trends
High level quartz cobbles are also found north of
f of palaeodrainage
Canobolas Divide in the Oberon area beneath north­
trending Tertiary lava flows near Beatties Hill and on strath
terraces flanking the upper Fish and Duckmaloi Rivers
Figure 10: BAlliURST drainage trends
(Figures 2 and 4) . Well rounded quartz cobbles, up to 15
cm diameter, underlie a topographically inverted 12 Ma
(Wellman and McDougall, 1974) basalt, 60 m above the
Regional drainage patterns were analysed over much of
bedrock incised Lachlan River, at Glenella below Wyangala
the Lachlan Fold Belt in NSW to see if BATHURST trends
Dam ( Figure 2). Glen Logan Gravel deposits with well
were reflected regionally. This was largely the case as
rounded rounded quartz pebbles, mostly up to 3 cm In
shown in Figure 1. Oilier and Pain (1994) previously
noted
the
northwesterly
trends
over
much
diameter, near Cowra form terraces up to 40 m above the
of
present Lachlan River floodplain. Quartz gravels are found
southeastern Australia .
at various levels in the landscape and are buried further
downstream in the Lachlan (see below) and Macquarie
Rivers and further afield (Martin, 1991). The rounded
REGOLITH FEATURES RELEVANT TO
DRAINAGE
quartz boulders, cobbles and gravels are likely to have
HIGH LEVEL ALLUVIAL SEDIMENTS
redistributed and accumulated lower in the terrain over a
The sediments on
remnants astride
the
widespread front. Ultimately, the original source of these
northwest of Molong near
the
quartz clasts is from erosion of a very highly and deeply
Killonbutta State Forest (Figures 3 and 4) are alluvial .
weathered old regolith with only the most resistant
Canobolas Divide
plateau
been eroded from older high level sediments and
Cross-bedding in quartz sandstone and conglomerate
minerals such as vein quartz left unweathered.
42
PALAEODRAIN
GA
S
E
IGNifi
CANC
E
IN
THE
BA
THU
RST
REGION
LACHLAN VALLEY DEPOSITS
Based on interpretation of water bore logs of the Lachlan
The New South Wales Department of Water Resources
Valley by NSW
has identified a buried palaeovalley beneath the broad
Williamson (1986) reports that basal quartz gravels
floodplain of the present Lachlan River. This palaeovalley
occur within grey sands and gravels of the Late Miocene
Department of Water Resources ,
is up to 8 km wide and 100 m deep in BATHURST and
to Pl iocene Lachlan Formation deposited in a reduced
has an irregular cross section profile (Figure 11 )
swampy environment, and to a lesser extent in the
Williamson (1986) shows that the pa laeovalley deepens
brown gravels to clay in the more oxidising environment
downstream from 80m near Cowra to
of the Pliocene to Pleistocene Cowra Formation in the
140 m at
Jemalong Gap west of BATHURST. Upstream of Cowra
pa laeo Lachlan Valley. Gravels in the Cowra Formation are
there appears to be a buried nickpoint of the palaeo
uniformly distributed across
Lachlan River. The increasing depth of the palaeovalley
constant reworking by braided stream channels and
towards the west, and the age of the sediments within it,
reflect the local catchment lithologies (Williamson,
the va lley indicating
correlate with influences of the Murray Basin (Brown &
1986), as well as containing quartz pebbles from an
Stephenson , 1991 ). The palaeovalley continues up Back
older regolith mantle.
Creek (a lso known as Koorawatha Creek and Crowther
Inspection of Williamson 's seismic and bore sections
Creek), a north trending southern tributary of the Lachlan
indicates a complex history of sedimentation and
River, and here is up to 85 m deep (Williamson, 1986 ).
erosion. The simplest possible sequence of events is
shown in Figure 7. erosion of a deep narrow va lley; valley
widening, deepening and extension, and deposition of
\ Palaeocene Acceleraled gorge erosion with downwarp 01
............
................ . .
...
~~""
...... 1
.. ..
.
~
. .. .
.
. . . . . . . ..
.
the Lachlan Formation, erosion of the Lachlan Formation
.
and deposition of the Cowra Formation with interfingering
~
of colluvial sediments along valley edges; formation of
terraces due to fluctuations in sediment supply and
2. Eocene-Miocene: Valley WIdening and deepening. and head­
ward extension 01 gorges. Base 01 valley affected by relative
~
•
..
.................
erosion . Williamson 's ( 1986) seismic profiles show the
.. .
. .......... . . .... ........ ..
...........
...... ".....
.
..
.
. ........ . Lachlan palaeovalley deflects considerably from its
............. .. ................
present course in places downstream of Cowra .
The Belubula Formation in the lower Belubula River has a
3 Late Miocene·Pliocene: Deposition of Lachlan Formation wittl
maximum thickness of 24 m downstream of Canowindra
~
Sediments
4 Pliocene: Erosion of Lachlan Formation exhuming some
sedimentation in the Cowra and Belubula Formati ons are
(Williamson, 1986) and has been dated from pollen as
Pleistocene (Martin, 1973); it is time equivalent of the
upper part of the Cowra Formation (Williamson, 1986) .
r~ : associated
wi th
the
sequence
of
still being deposited and eroded toda y.
Colluvial slope sed iments are presently accumu lating on
wide low angle pediments, fans and rises adj acent to the
Lachlan River near Cowra, along Nyrang Creek and its
5. Pliocene-Pleistocene: Deposition 01 Cow'a Formation.
tributaries in the Belubula River catchment, and the
upper f'liandagery Creek catchment Alluvial floodplain
sediments have aggraded along the Lachlan River
downstream of Wyangala Dam . The fl oodplain widens
downstream of Challacombs Crossing with multiple
terraces and meande rs whose amplitude is less
6. Holocene: Terrace formation .
~
downstream of Cowra than upstream.
DEEP LEADS
A reappraisal of deep leads in the Parkes-Forbes area
west of BATHURST by Wilson and McNally (1996)
Sedimentation ojthe
Lachlan palaeovalley
Figure 11:
indicates narrow deep pa laeovalleys containing chaotic
43
l
'OSlYN
C
H
AN
deposits with fl oors lying at depths of 30-140 m below
the Boree Creek on the Western Slopes near Toogong
the present land surface . They note the similarity of
has on ly been a few metres in 12 Ma since lava fl owed
incision and channel fill to that of the Lachlan Formati on
from Mt Can obolas, ie 0.4 m/M y.
in th e base of the Lachlan palaeoval ley and suggest the
sed iments are part of the same Late Miocene to Pliocene
Valley widening in conju nction with headward erOSion
depositional event The valley noors of the deep leads are
results in scarps ( Figure 3) that are acti vity fronts
the proximal parts of an integrated incised valley system
separating more deeply and highl y wea th ered bedrock
graded to the base of the Lach lan River palaeoval ley.
plateaus above, from a less wea thered and sha llower
weathered mantle, often with a transported component,
A wind gap In the Belubula-Mandagery drainage divide at
below. Val le y heads from headward erosion coa lesce and
the head of the south flowing Ca rgo Creek, a few
form a scarp which then effec tively retreats headwards.
kilometres to the wes t of Cargo, indicates a possible
Scarp retreat occurs on bo th sides of the Great Divide
palaeodrainage line, which is substantiated by records of
and the Canobolas Div ide , the Great Escarpme nt being
A summary
to the eas t of the Great Div ide (O ilier, 1982). Figure 12
description of the deep leads by J.E. Carne from sha fts
is an examp le of valley development across the
sunk in 1878 is reproduced in Andrews and Morrison
Abercrombie
(1915) Carne states that "two sets of deep leads, the
downstream from Wyangala Dam: the val ley deepens
mining in the Cargo Goldfield from 187 0.
Escarpment
no rth
of
Darbys
Falls
upper one of which, with the black clays and fOSSi l
from the s tream ou twards by slope retreat as it crosses
leaves, will be found resting in places in the older
the scarp and approaches the Lachla n River to the south .
roun ded quartz pebble drift" and occu r at depths up to
Va lley widening along fl oodplains of the Lachlan and
120 m below the present surface at the headl"vaters of
Belubula Rivers, mainly downstream from higher scarp
Cargo Creek. The nor therl y trending buried channel has
dom inated
steep, almos t vertical sides with slope debris fan
am plitudes incising terraces.
sed iments. Also, a sha ft immediatel y to the north o f the
mainl y al luvial terraces south o f Bathurst to strath
terra in, results in
mea nde rs of various
The progression from
wind gap in the Belubula-Mandagery drainage divide, is
terraces north of Bathurst along the Macquarie River and
rep orted to s ti ll be in alluvium at 34 m depth . Th us the
its tributaries indicates progressi ve valley widening and
sedime nts cross and pre-date the Belubula-Mandagery
deepen ing as the meanders of the Macquarie River
divide and so may be as old as Mesozoic. Andrews and
entrench across bedrock lithology and structure towards
Morrison also rep or t "river wash an d gravels" on hil ls 90
a lower base level to the northwes t.
m to 120 m above Cargo vil lage.
DRAINAGE AS A CONTROL ON LANDFORM
EVOLUTION
EROSION
The mos t acti ve erosion in th e area is via incision along
th e Bel ubula
River, the upper La ch lan River and
f"landagery Creek south o f Ca nobo las Divide, and the
"'"
headwaters of the Macquarie River, Turon River, LewIs
----_~
YOUflg
_
_
__
OJ,,!l:Crd!or.-ns
(a) Diagra mmatic cross sections of valley development aCfOSS !he Abercrombie Escarpment
Ponds Creek and Bell River to the north of Canobolas
Divide. Headward erosion of the Belubula River into the
high ly weathered plateau to the east has been fast. Th e
incision rate of the Belubula River below its nick point
northwes t o f Junction Reefs is 15.4 m/My. Here the river
is 200 m below a basalt-edged pla tea u where the 13 Ma
precu rser o f the Belubula River nowed prior to the
/"
I
Canobolas
eruption.
The
Macquarie
River
/ /
I
Pr(J"'O~"'!I
h t<.. :fv.'l>l ~'
~i~'::::':·
just
downstream from its junc ti on with the Turon River has
\
,
incised over 210m since sub-basaltic sediments were
:
I
\
~ r>C'$C8tpde~~
~
depOSited 12 Ma ago , a rate of 18 m/My ( Figures 6 and
(b) ScalP development as valley heads widen and co alesce to form erosional scarp.
7). These are some of the highest incision rates
de termined in the area. In contrast the inci Sion rate o f
Figure 12: Example of valley
developmen t. Darbys Falls
44
PALAfODRAIN
GA
f
S I
GN
IFI
CANCE
IN
THE
8ATHU
RS
T
REG
I
ON
Drainage migration is apparent along the Macquarie River
Williamson (1986) reports that 73 m of sediments
at Bathurst. Basalt remnants overlying alluvial sediments
overlies 12 Ma basalt, which overlies a further 9 m of
on Mt Panorama and adjacent hills, dated at 16- 19 Ma
alluvium,
(We llman and McDouga ll, 1974), form a parallel arc to
BATHURST. The upstream limit of the buried palaeovalley
on the lower Mandagery Creek west of
the southwest of the present Macquarie River which
is at Cha llacombs Crossing on the Lachlan River where
suggests a former course of the Macquarie River. To the
broad floodpla ins over the palaeovalley narrow to an
northeast near Kelso, a parallel arc of highly weathered
incised bedrock va lley. Six kilometres upstream of
granite rises occurs above a southwest facing scarp.
Challacombs Crossing there has been 60 m of incision
Swampy clay depressions and rounded quartz pebbles in
in to bedrock since basal t erupted at 12 Ma (Wellman &
red soil In this area indica te a remnant palaeoplain of the
McDougall, 1974) at Glenella . Be tween Challacombs
Macquarle River. The associated terrace deposi ts have
Crossing and Cowra a series of terraces on Pl iocene to
since been eroded through towards the southwest as the
Pleis tocene palaeovalley sediments up to 40 m above
Macquarie River migrated to its present course. Near
the present Lach lan River floodplain ind ica te subs tantial
Cowra , Back Creek has migrated to the northwest at its
incision in the Quaternary Downstream of Cowra there
con fluence with the Lach lan River due to build-up of
has been less incision into the palaeovalley sediments .
sediment on the southem side of the Lachlan River.
A similar situation occurs along the Mandagery Creek and
Landsurface lowering from at least 3 successive waves of
incision upstream of Toogong and net accumulation
scarp retreat, and denudation from weathering and
downstream s ince eruption of basalt at 12 Ma from
removal of regolith by overland water, gravity, wind and
Canobolas Volcano (Gibson & Chan, 1998; Bishop &
solution is moderated by the rate of lowering of the
Brown, 1992)
its eastern tribu tary, Bourimbla Creek, where there is net
weathering front. The rate of landsurface lowering on
moderately weathered plateaus based on eleva ti"ons of
Alluvial terraces also occur further east in BATHURST in
40 Ma lava remnants varies from 3.4 m/My at Big
erosiona l terrains . Narrower terraces occur along the
Brother, southeast of Blayney, to 1. 2 m/l'''1y at Bald Hill
Lachla n Ri ver upstream to at least Wyangala Dam, in
near HIli End . As the rate of landsurface lowering
places along the Belubula River, such as north of
outstnps the lowering of the weathering front, drainage
Mandurama just above a nick point, and along the Fish
is superimposed from
the regolith mantle (insitu
and 1'1acquarie Rivers near Bathurst. Strath terraces
weathered bedrock or sedimen ts) onto bedrock. A
occur on the eroded interfluves of the upper Fish River.
drainage line may keep its former pattern by entrenching
Grossly oversized alluvial terraces and a wide floodplain
itse lf and
form ing gorges across more resistant
lithologies, or the drainage line may be deflected by
along Windburndale Rivulet pOSSibly ind ica te a former
cou rse of the Macquarie River.
bedrock structure and alter its course accordingly. As
only the eroded core and m inor ramparts remain of the
Cl imate
volcanic pile associa ted with the 11 to 13 Ma Mt
southeastern Austra lia , wh ich in turn has influenced
has
influenced
vegetation
and
runoff
in
Canobolas shield volcano complex, significant stripping
aggradation . Climate trended from extremely wet and
IS inferred .
cool in Late Miocene to Pliocene to drier and warm in th e
AGGRADATION
at present (Martin , 1991 ). Martin indicates that sea level
Pleistocene, but the ra infall was always much higher than
Lava and sediments have accumu la ted in a number of
falls change a coastal climate to a continental climate with
drainage lines. In the Oberon area multiple lava flows
lower precipitation . She considers the Early to Middle
partially infilled certain valleys and drainage lines, some
Oligocene low sea level to have had little impact on
of which are now inverted in relief. Around Mt Canobolas,
vegetation because fluctuations in a higher precipitation
lavas infilled lower terrain to form a protective lava plain .
time were insufficient to become limiting and cause
Just west of BATHURST al luvial sediments conceal a lava
disruption to the vege tation However, a global sea level
flow in the lower Mandagery Creek (Wil liamson , 1986) .
fall of at least 180 m (Pigram et ai, 1992 ), in the Late
Miocene (6-11 Ma) (Haq et aI. , 1988), in the drier
Sedimentation, interrupted by major erosion events, has
Miocene-Pliocene tipped the balance between ra inforest
buried
the Lachlan palaeova lley and some major
and wet sclerophyl l forest (Martin, 1991). The resulting
tributaries . Such tributaries are Back Creek south of
combination of less water flowing down the rivers, and so
Cowra, Bland Creek adjacent to Lake Cowal, the lower
less water power to transport sediments, and increased
Belubula
sedimen t available for transport due to decreased
River,
and
the
lower Mandagery Creek .
HAN
gave rise to increased
the rivers
In
off the Eastern
Murray Basin. The Miocene
BEDROCK LITHOLOGY AND STRUCTURE
into the
There
Pliocene Lachlan Formation
in the Lachlan
low and
of
sediments
Eastern
northeast of
area
Less resistant
more easily eroded,
The terraces aiong the Lachlan River may, in part reflect
further
southwest of
CoW[a to south
1991).
in N.S.W
fOllows
drainage to the
climate
simi:arlv trending
in the
complex. The Belubula River
LOCAL DRAINAGE MODIFIERS
of its nick pOint has some structural and
to the
TERTIARY VOLCAI'IlSM
volcanic provinces
eg Bocobra Creek. Fault delimited
1"1a; Airly; 41
of
have preserved some earlier
and
followed by
palaeo-north
to
north
northeasterl'y'
were more extensive in
past than now and so these
were in existence before
and
Canobolas
verified by rounded quartz
to the south of the Canoboas
occurring
Divide (rlc'rY\',nn
close proximity to rv'lt Canobolas may indicate radial
due to the
in the headwaters of the
Belubula River, and a wind
nows have coalesced to form lava pla'ns
thick
the Canobolas Divide
up to 2.5km wide with extremely underfit
flows
northwesterly trending drainage systems Lava flows In
at least 150 m
units to the
streams. This Indicates that their headwaters probablv
associated with the Mt Canobolas compound
shield volcano
and
have
lines and modif:ed or obliterated others
the
the northwest of
rv'laniidra, there appears to be some lineament control,
13 Ma; Abercrombie,
,"r="n~.no
of a fold
the west of Carcoar and
control on Its meanders.
Lava flows from
espeCially
assoCiated with north-south trending bedrock
identified in all of the major valleys of the Western
of
and structural contro' on
drainage in some areas in BATHURST,
correlates with the late
Miocene sea
strong
on
Canobolas Divide
between the Belubula River to the south and Lewis
198
Ponds Creek to the north
southeast and west of Mt Canobolas which have
compiete'y obliterated prior drainage. The
The Bathurst Granite and a few small intrusions of granite
east-west
about 30 km north of Bathurst also show lithological
indicate that an east-west
control on drainage. Due
ex;stence 11 to 13 Ma and
a
further
itself, across
west. The Boree Creek - Bourimbla Creek
low resistance
to
m) erOsion bowl
has formed In the Bathurst Granite, and smaller shallower
Warree
lava plain
deep
ones have formed further north. The 19 Ma
west into
capping Mt Panorama
IS
on strongl'y weathered granite,
and is about 100 m below
scarp
the Bathurst
Granite and 180 m above the floor of the erosion bowl,
lava flows are now inverted in
to the
3) due
a rate
the lavas to
inverted but
Some flows
surface
incision and migration of the
flow due to
within valle'y's
Macquarie River.
of 9.5 m/My since
The
local low base
has
seen close to the Canobolas Divide In the Oberon area
increased the
Here, partly dissected mult'ple basalt flows partlv fill 100
of the Macquarie River, as eVidenced by stripping and
m deep vallevs wh;ch are Incised into a weathered
Due to the
power of the
entre'lchment in this area
of palaeostreams post-lava
or possibly
tributaries
and has deflected,
this
streams have incised their new courses on one side or
both sides of the flows, the
lateral streams".
termed 'twin
the northwest
of twin lateral streams are
has resulted l:l scarp retreat towards the southwest east
Creek and Grove Creek In the vicinity of the
Abercrombie Caves, and
Accelerated headward erosion
of
River and Sewells
which has resu'ted in the migration of the
Canobolas Divide to the southwest and capture and
boathook tributaries
Creek west of Oberon. Some streams cut across the
southeasterly
Inverted relief flows, such as Lewis Ponds Creek and
boathook tribu tar'es of the
River in the vicinity of their confluence, and
River
Evans Plains Creek
no~theast
46
PALAEODRAINGAE
SIGNifiCANCE
IN
THE
BATHURST
REGION
A number of northwesterly trending lineaments traverse
Sydney Basin along this convergent margin caused
the mapping area and probably relate to the "Lachlan
upwarping
River Lineament" which Scheibner and Stevens (1974)
development of a peripheral forebulge (Figure 13a) which
of the
Lachlan
Fold
Belt due
to
the
define as a zone of linear features of a fossil fracture
resulted in tilting and erosion of the Permian and Early
zone 40 to 50 km wide. A major west-northwest trending
Triassic sediments (Herbert, 1970), and formation of a
lineament defines the southern edge of the Bathurst
northwest trending divide. The rapid denudation of this
Granite which when extrapolated to the west passes
uplifted mass may be reflected in a sudden major cooling
through Mt Canobolas, and aligns with the Lachlan River
event in the northeast Lachlan Fold Belt during the Early
west of Condobolin. To the south a northwest trending
Triassic (around 245 Ma) as predicted by apatite fission
lineament, which can be seen on airborne magnetic
track analysis of data collected over the Bathurst area
imagery, aligns with the Lachlan River through Cowra. To
(O'Sullivan et ai, 1996)
the north a west-northwest trending lineament aligns
with the Turon River through Sofala.
Northeast Aowing drainage off this northwest trending divide
eroded mainly upper Devonian quartzites of the northern
Structural control of drainage is evident to the west of
Lachlan Fold Belt and deposited this sediment as the l'1iddle
BATHURST in the Parkes area, both in deep leads and
present drainage (I \/Jilson, Uni. NSW, pers com 1996).
Triassic Hawkesbury Sandstone which has a
A V-shaped low lying sediment dominated zone, narrowing
palaeocurrent direction of 34 degrees in this area
to the south, between south of Parkes and West Wyalong
(Conaghan, 1980). North-east aligned segments of present­
seems to be bounded by faults which align with the east
day drainage along the upper Fish River and Duckmaloi River
and west edges of the Coonamble Embayment of the
mean
downstream from Beatties Hill south of Oberon and the
Surat Basin. To the northwest of BATHURST the Macquarie
Pipers Flat Creek south of Portland (Figure 10) may be
Marshes are controlled by Pleistocene block faulting and a
gentle tilt to the east has caused the Macquarie River to
incised traces of this Triassic palaeodrainage to the Sydney
migrate to the east (Watkins & Meekin, 1996). It should
Basin. Energetic braided streams, as determined from
be noted that even a small tectonic disruption on the
sedimentological studies (Branagan etal, 1979), with strong
flatter deeply weathered and depositional terrain to the
unimodal current directions imply a moderately steep
west and northwest of the Bathurst region can have major
northeasterly palaeogradient (Standard, 1964).
impact on drainage as streams have long low gradient
profiles. The same disruption in the higher relief erosional
terrain of the Bathurst region would have relatively little
Passive Margin
affect on drainage as the higher energy higher gradient
In contrast to the convergent margin tectonic regime,
streams may well keep pace with the displacement and
divergent or passive margin tectonics were initiated in the
become antecedent streams. Having said this, no definite
antecedent streams were found in the Bathurst region.
Late Cretaceous as a prelude to the rifting of Pacifica (ie. that
However, many superimposed and entrenched streams,
part of Gondwana to the east of the present coast) from
some with broadly dendritic patterns, are present These
southeastern Australia. Crustal underplating with associated
imply the denudation of a higher palaeosurface with a
northeast trending doming (Lister and Etheridge, 1989) is
relatively uniform weathered or sedimentary mantle onto
interpreted to have occurred to the west of a zone of thinning
bedrock with its localised structural and lithological control.
of the crust betl-veen 100 and 80 Ma at the start of tectonic
REGIONAL DRAINAGE MODIFIERS
extension prior to the initiation of rifting along a northeast
trending zone to the east of the present coastline of Australia
TECTONICS
(Raza et ai, 1995; Korsch and Totterdell, 1996) Tilting
Convergent Margin
perpendicular to this uplifted zone would have produced a
To the northeast of BATHURST west to southwest
palaeoslope to the northwest on the western side of this
directed thrusting in the New England Orogen, which
zone and initiation of northwest trending drainage. Apatite
began in the Permian with maximum effect in the Early
fission track data over the Bathurst area indicates a majOr
Triassic, caused subsidence due to foreland loading of
the lithosphere so forming the Sydney and Gunnedah
cooling event about this time, i.e. around 95 Ma (O'Sullivan
Basins (see eg Korsch & Totterdell, 1995) (Figure 1).
et ai, 1995, 1996) The data indicate rapid denudation,
Continued thrusting of the New England Orogen over the
decreasing in magnitude towards the northwest
47
RO
S
L
YN
CHAN
The proto-Great Divide is a tectonically induced divide
and extended across the Canobolas Divide to at least as
initiated by the pre-rift doming. The present Great Divide
far south as Molong in BATHURST and Gunningbland west
separates coastal dra inage from inland drainage along
of
Parkes.
Towards
the
close
of
Surat
Bas in
the east coast of Austral ia, and touches on the eastern
sedimentation, drainage would have flowed as dendritic
edge of BATHURST (Figure 4 ). It has been modified by
meandering or braided streams in a northerly direction
capture of headwater catchments, for example in the
across a low relief floodplain (Figure 13d).
Kowmung River area west of Jenolan Caves, which
caused the Great Divide to jump to the west. In most
It is feasible that Surat Basin sediments covered a much
areas the Great Divide is well inland of the Great
larger area of the Lachlan Fold Belt and have since been
Escarpment (O ilier,
1982) with evidence of prior
stripped away Indeed, they may wel l have also covered the
drainage crossing the divide, for example, Pipers Flat
southwestern part of the New England Fold Belt, and parts of
Creek and Jenolan River (Figure 9) are two reversed
the Sydney Basin, as indicated by anomalously high vi trinite
streams on the east of the Great Divide .
refiectance data in the Sydney Basin (Middleton, 1989;
Branagan,
1983 ).
Apatite fission
track and vitrin ite
If Surat Basin sediments extended across the proto­
refiectance data indicate a much thicker sediment sequence
Great Divide, (see below) signs of possible reversed
in the Surat Basin than presently exists (Raza et ai, 1999 in
drainage to the east of the divide would probably not
press). This sediment cover, along with a palaeoslope to the
survive to be superimposed onto the rocks below.
northwest resulting from doming associated with the onset
However, a possible mechanism for the reversal and
of continental extension at 80-100 Ma, would give a
capture in evidence
differential downwarp, increasing
mechanism for a shift in drainage direction via migration due
IS
to the east. and occurring after complete stripping of the
to evulsion, and perhaps capture, from the north to the
Surat Basin sediments. This downwarp may have
northwest. The upwarp from which this northwesterly
accompanied the ini tiation of rifting at about 65 Ma, and
palaeoslope is derived also gives a mechanism for increased
if so post-dates the initial upwarp associated with pre-rift
incision to strip the Surat Basin sediments.
doming. Alternatively, the proto Great Divide may have
The 95 Ma apatite fission track date may indicate when
migrated wes twards with possible westward movement
much of the Surat Basin cover was stripped over
of the dissipating heat source from underplating.
BATHURST The denudation rate must have been at least
50 mlMy (ie 1 km denudation between 100 and 80 Ma),
BASINS
over most of BATHURST, which is 3 times higher than the
highest incision rate over BATHURST since 12 Ma . This
Surat Basin
extremely high denudation rate is unlikely unless there was
The Surat Basin unconformably overlies parts of the
a great thickness of unconsolida ted sediment, for example,
Permo-Triassic Bowen-Gunnedah-Sydney Basin system
volcanically derived mUdstone from an island arc to the
and the northern perimeter of the Lachlan Fold Belt. The
east of Australia (Jones & Veevers, 1983). A large volume
Surat Basin contains a l"liddle Jurassic to Early Cretaceous
of Late Cretaceous sediments up to 4 .5 km thick in a
transgressive sequence of fluvial and fluviolacustrine
southwesterly prograding delta forms the Ceduna Terrace
sediments through to marine muds. The Coonamble
in the eastern Great Australian Bight (Fraser & Tilbury,
1979). These sediments may well include the Surat Basin
Embayment is the southern lobe of the Surat Basin
sediments that were eroded off the highlands in the
onlapping the Lachlan Fold Belt. Pre-Surat Basin drainage
Lachlan Fold Belt and elsewhere.
would have been largely within strike controlled north
trending valleys continuing northwards beneath the Surat
Two major new northwest trending trunk streams, the proto­
Basin (Figure 13b) and then draining to the east via the
Macquarie and proto-Lachlan Rivers developed on this
Clarence-Morton Basin (Struckmeyer and Totterde ll,
northwest palaeoslope, separated by the proto Canobolas
1992 ). Surat Basin sediments infilled and probably buried
Divide (Figure 13e) The Canobolas Divide thus predates
( Figure 13c) northerly trending strike controlled valleys,
the initiation of the Murray Basin . As these rivers eroded
48
PALAEODR
A
INGAE
SIGN
IFI
CANCE
IN
THE
8
A
THUR
S
T
REGION
through the Surat Basin cover they superimposed their
occurs in the Lach lan palaeovalley from just upstream of
courses onto the bedrock below, for example,
the
Cowra, and passes laterally into other time equivalent
Macquarie River meanders across bedrock trends northwest
Murray Basin units downstream of BATHURST (Brown and
of Bathurst Some major tributaries, for example, the proto­
Stephenson, 1991). Erosion of the Lachlan Formation at
Turon River also superimposed their courses, but others
the end of the Tertiary resulted in valley widen ing followed
preferentially exhumed and re-established their courses on
by deposition of the Pleistocene to Pliocene (W illiamson,
the pre-Surat Basin topography in north trending strike
1986)
controlled va lleys. However, now the old north Aowing
Cowra
Formation
( Figure
13h).
Howeve r,
sedimentation of the Lachlan va lley wes t of the highlands
streams are fragmented and in part reversed, as they are
seems to have more or less continued into the Holocene
diverted into the northwest trending trunk streams.
and still continues presently, but at a slower rate
Th is is reflected in some north trend ing segments of the
DRAINAGE HISTORY
Canobolas Divide which are exhumed pre-Surat Basi n
The previous sections have presented relevant regolith ,
local divides.
landform and bedrock geology features of the study area
in the context of sediment deposition and eros ion
M.urray Basin
The Murray Basin to the west of the Lachlan Fold Belt wa s
history. These ideas are drawn together in this section as
initiated at the beginning of the Tertiary by structural
a history of dra inage development
downwarping from very late thermal subsidence over an
Mid Triassic (Figure 13a)
older reactivated rift basin (P. O'Brien, AGSO, pers com
• Northeasterly drainage eroding Devonian quartzites on
1996). The sed iment derived from erosion of the
north Lachlan Fold Belt and depositing sediments as
northwest pa laeoslope would have probabl y been
Hawkesbury Sandstone in the Sydney Basin .
transported to the southern continental margin prior to
Early Jurassic (Figure 13b)
inception of Murray Basin sedimentation. As the vo lume
• North to north-northeasterly trending and flowing
of clastic sediment in the Murray Basin requires onl y a
drainage in str ike controlled valleys within the Lachlan
small amount of stripping from its catchment (D. Gibson,
Fold Belt perhaps ultimately draining to the eastern
CRC LEME / AGSO, pers com 1998 ), it is inferred that
seaboa rd via the Clarence-Morton Basin .
most stripping occurred during the Late Cretaceous,
prior to Murray Basin sedimentation. The erosion of the
Late Jurassic to Early Cretaceous (Figure 13c)
Surat Basin sediments in Late Cretaceous is not due to
• In-filling of structurally con trolled val leys by fluvial
changes in base levels associated wi th the Murray Basin ,
sedimen ts of mixed provenance transported from the
but to the northwest palaeoslope resulting from upwarp
south and deposited as Surat Basin sedime nts .
which preceeded co ntinental breakup .
Interbedded lacustrine sediments deposited .
Early Cretaceous (Figure 13d)
• Dendritic meandering or braided dra inage flows to
Fluctuations in base level in the Murray Basin and climate
across
Surat
Basin
floodpla ins
into
a
resulted In periods of accelera ted erosion and valley
north
infilling in the river systems draining to the Murray Basin .
fluviolacustrine environment as a prelude to the
Isostatic readjustment (for example, Bishop and Brown,
inundation by Cretaceous seas from the Gulf of
1992 ) to the stripping off of the Surat Basin cover and
Carpentaria . Marine cond itions may have then been
subtle movements on baseme nt faults in the Murray
established in BATHURST, but no record remains. Sea
Basin ma y well have been factors contributing to the
regresses northwards, leaving north-flowing drainage
Ini tiation of gorge formation on the northwesterly tilted
across a low gradient deposi tional plain.
slope (Figure 13f). Gorges up to at least 140m deep
Late Cretaceous (Figure 13e)
along the palaeo Lachlan River developed prior to the
• Commencement of crustal thinning (between 100 and
Eocene sedimentation in the Murray Basin which
80 Ma) to the east of the present coastline and
extended upstream to Hillston on the Lachlan Ri ver and
associated doming across BATHURST as a prelude to
Narranderra on th e Murrumbidgee Ri ver (Martin, 1991 )
rifting of south-east Austral ia from Pac ifica. Proto Great
A climate change to one with lower precipi tation due to
Div ide formed along axis of doming.
a global sea level fall in the Late Miocene resulted in the
• Shift of northerly drainage across Surat Basin Aoodplains
depositi on of the Lachlan Formation (Figure 13g), which
towards the northwest on wes tern Aank of upwarp .
49
ROSLYN
C H A N
~01'1.:1~
Sa."IO~:IX'.e
U:7<\o".PO<I
~1'IIa..
_...
Fclc:Dor.
hE IIov.~
,
<"""Go
olt lllCiti;)n
(a) MID TRIASSIC
Foreland loading on convergent margin
(b) EARLY JURASSIC
Erosion prior to Surat Basin sedimentation
(c) LATE JURRASIC
Early Surat Basin sedimentation
(d) EARLY CRETACEOUS
Close of Surat Basin sedimentation
C.1nobol' .... Divfdo
T r ~OIwc:mcr--~
J o~nCOf!~
t'Q1 IJ:n . ptCCO ·.'J.. '·;]Y
F.:..et' s~:em
(e) LATE CRETACEOUS - 80-100 Ma
Upwarp, and Proto Murray River system on
NW palaeoslope
(I) PALAEOCENE - OLIGOCENE
Accelerated gorge erosion w~h downwarp
01 Murray Basin
(g) MIOCENE
Volcanism and sedimentation in gorges
(h) QUATERNARY
Figure 13: Stages of Drainage Evolution in BAlliURST
50
.. Canobolas Province
• Initiation of incision of Surat Basin northwest
(11-13
to the
southern ocean
consequent development of
erosional divides, eg Canobolas Divide, Prior to
initiation of the
consequent radial lava flows. Initiation of twin lateral
Basin.
.. Superimposition and entrenchment of
exhumed
for example, Macquarie
River. Some streams defeated
and structure and ( nnc;,~n'
streams .
onto
.. Lower rainfall Induces Increased sediment
due to reduced
bedrock
diverted.
sea level fall in the Late Miocene. Widespread
retreat
of sediments in the Lachlan
increased
headward erosion via nick point retreat, and valley
>Jaiacvv,~,,,:y
in the Late
via slope retreat, on both sides of the proto
,"Iirv-a,,,,,,
sediments in other palaeo
Great Divide.
Basin from
.. Downwarp to
east results in
migration of
to the Great Divide
Great
east of the divide forms as heads
lead valleys at Parkes and Forbes, aiso infilled.
Pliocene
..
Continued deposition of
of valleys coalesce.
.. Redistribution
levels and possible deposition
Gravel and time
from
..
cover.
Basin at the
of the Pliocene,
Continued
and erOSion, and deepening of
..
Relief inversion o· lava flows continued .
Bathurst Granite erosion bowl,
Murray Basin by structural
sediments derived from waning
erosion in
.. Accelerated gorge erosion
to the
Basin frOM the new lower base level associated
.
Deposition
Cowra and Belubula Formations In
Lac'llan and palaeo Belubula
..
or
entrenchme.'It
continue with denudation
Eocene to Oligocene (figure 13f)
.. Continued erosion of
13h)
Quaternary
Murray Basin.
nnlA!r1H,;o,
bedrocks with
Erosion of Lachlan Formation and valley Widening
..
Palaeocene (Figure 13f)
and trapping
valleys
assOCiated With hiatus in sedimentation in eastern
Glen
May be derived from
stripping of Surat
.. Initiation of
Formation and tiMe
sediments in
older rounded quartz
..
Basin cover and
Continued
retreat frOM both sides of
Canobolas and Great Divides. The
incision of the
Basin
River catchment basin
and Lachlan Rivers
northwards Into
IS
on both sides of Canobolas Divide. Narrow deep
Macquar'e River catchment is
western rim of the Bathurst Granite
into the palaeosurface
order tributaries, for example,
low
lead valleys
exhumed
..
and establishment of
present LaC'llan River course.
..
Province volcaniSM in Eocene (41
.. Initial
on the
Formation of multiple terraces and meandering river
on Lachlan
surface beneath
Surat Basin sediments.
..
Lachlan
the f"',acquarie River catchment baSin but the
gorges erode
..
Eastern
order tributaries, for example,
the Great Divide and beheading, reversal and capture
with the
as Lachlan Formation
of some drainage lines towards the
southwest for example, the
of palaeo Lachlan Valley sediments
River at
downstream in the Lachlan River as an exte'lsion of
Bathurst as it erodes its old floodplain Back Creek
Murray Basin sedimentation.
diverted in its confluence with
.. Initiation of relief inverSion of lava fiows
..
Local tectoniC movements in areas to the
..
Cont:nued weathering and denudation of
Miocene (figure 13g)
Lava
Miocene (18­
flows preserved
lines as
inverted relief, and twin lateral streams ·ormed
.. Erosion of the Bathurst Granite
and consequent rearrangement
local
of
BATHURST
.. Abercrombie Province volcanism in
23
Lachlan River due
to sediment accumulation .
which mostly
..
form an erosion bowl
espeCially
0:
and pre-
capture around its periMeter.
due to
astride
Continued exhumation of
topography,
astride
I
ROSLYN
CHA
N
MINERAL EXPLORATION IMPLICATIONS
Thus, the degree to which s tru ctu re (for example,
Results from the Bathurst regolith-landform mapping
mineralisation aSSOCiated with faults and lineamen ts),
have been fundamental in analysing the drainage
lithology and hydrothermal alteration (leading to mineral
evolution of this region and, in conjunction with resul ts of
enrichment) interact with the regolith evolution to effect
more regional work (for example, Gibson and Chan ,
mineralisation can be qualified
1999), related studies (for example, Martin,
1991;
Williamson, 1986) and historical mining records (for
Appa rentl y discrete and isola ted sediment deposits may
exam ple, And rews and Morrison, 1915), have elucidated
be related to each oth er within a palaeodra lnage
a complex history of drainage evolution . In turn, drainage
framework.
evo lution is the key to understanding landform evolution
extrapolated unde r cover, whe ther lava fl ows or sediment
and associated regolith evolution, in particular the
For example, northerly trending structurally controlled
Such
palaeodrainage
lines
may
be
processes involved, and their dimension, timing and
drainage east of Orange is probably in herited from Earl y
sequence . Th is knowled ge is highly relevant to land use,
Jurassic times, and can be inferred under the Canobolas
environmenta l issu es and mineral exploration, all of
lava plains to link with present drainage lines within m ore
which are pertinent to the Bathurst region .
weathe red terrain
to the south o f the lava
field
(Figures 3, 10).
Much of this area has a history from "gold rush" days of
allUVial gold and gemstones, such as diamonds (for
By incorporating an understanding of the processes o f
examp le, SoFala, Hill End, Oberon, Peak Hill, Dubbo,
placer deposition (for example, riffles ) with knowledge of
Forbes , Parkes, Young etc). In recent yea rs there has
physical attributes of potential trap sites (for example,
been an upsurge in bedrock exploration for gold and
terraces and constrictions du e to competan t lithologies)
copper and several maj or new mines have opened. The
and the location o f historical workings, prospective type
main mines and deposits are shown in Figure 2. By
understanding
the
linkage
between
areas for placer deposits may be delineated. In this
bedrock
context th e Macquarie and Turon Rivers would be good
mineralisation , deep leads and placer deposits (Figure
to study in terms of alluvial gOld. Since all o f the obv ious
14) a grea tl y expanded exploration potential is made
loca tions have already bee n worked through , thiS model
available. Palaeodrainage lines may not on ly define
could be extrapolated with more benefit under cover
m inera l deposits directly aSSOCiated with them, but the y
Similarly, place r deposits of diamonds may be located by
may also be vectors to bedrock m ineral isation as they
tying together a model for the sou rce of diamonds with
can give the provenance and geochemical Signature of
a model for the weathering and erosion history of these
potential bedrock mineralisation.
source areas (for example, Chan , 1998) and assOCiated
fluvial pathways and architecture . One such working
model that accounts for the known location o f diamond
sources in southeastern Australia is active subduction of
ocea nic crust and associated sediments under the
eastern margin of the Australian plate (Barron et ai ,
1994) . These diamonds have been brought to the
surfa ce in the Oberon area with Ca inozoic alkali basalts.
New deep lead s of th e style at Cargo on the tablelands
Figure 14: Relationships ofplacer gold deposits
may be discovered in otherwise un likely places due to an
understan ding of the palaeod rainage. Burled deep leads
may be discovered by tracing projected palaeodrainage
under cove r and veri fying its existence by focussed
A nu mber of Palaeozoic volcanic centres contai ning
studies, such as enha nced airborne magnetic Imagery to
hydro thermal alteration have recently been discovered
trace maghem itelmagnetite, seism ic surveys, selected
and are presently under investi gation for thei r mineral
potentia l.
By
distinguishing
transported
drilling for lithologica l logging, mineralogy, geochem istry
mate ria l
(including partial extraction methods), tranSient electro
associated with drainage lines from insitu regolith,
magnetics and hydrogeochemical studies. Geochem ical
geochemical and hyd rogeoche mical results can be
studi es would also be beneficial to check for anomal ies in
interpreted more effec tively. The inFluences of bedrock
exposed areas whe re the only trace of palaeodrainage IS
ca n then be better discerned from those of the regolith .
high ly wea thered inverted reli ef, for example, in the
52
PAlAEODRAING
A
E
S
IGNIFICANCE
IN
CHAN R. A.
vicin ity of the Abercrombie Caves south of Blayney, a
THE
BATHURST
REG
I
ON
1998 . Bathu rst regolith-landfo rms . In
sinuous ridge of highly weathered bedrock may have
Watkins J.J. and Pogson D. (Cmpls. ) Explanatory
been covered by a lava flow but has since been removed .
Notes for Bathurst 1250000 geological sheet SHISI
55-8. Geological Survey of New South Wales / AGSO .
CHAN RA AND FLEMING C. 1995a. Orange regolith­
ThiS study provides a framework for extrapolating
landforms, 1100000 scale map, sheet 8 731, NSW.
palaeodrainage further afield on the exposed Lachlan
AGSO / NSW Dept. Minera l Resources .
Fold Belt and under cover on and adjacent to the north
CHAN R.A. AND FLEMING C. 1995b. Molong regolith­
Lachlan Fold Belt. Additionally, it may well provide a
landforms, 1100 000 scale map, sheet 8631, NSW.
model for linking alluvia l and deep lead deposits much
AGSO / NSW Dept. Mineral Resources.
further afield, such as linking historic alluvial and deep
CHAN RA AND GOLDRICK G. 1995. Cowra rego lith­
lead finds to possible palaeovalleys throughout the
landforms, 1·100000 scale map, sheet 8630, NSW.
Lachlan Fold Belt and under the margins of the Surat and
AGSO / NSW Dept. Mineral Resources.
Murray Basins .
CHAN RA AND KAMPRAD J.K. 1995. Bathurst regolith­
landforms, 1: 100,000 scale map, sheet 8831, NSW.
AGSO / NSW Dept. I"lineral Resources.
ACKNOWLEDGEMENTS
CHAN RA , HAZELL M. S, KAMPRAD J L, FLEMING C,
I thank David Gibson for many thought provoking
GOLDRICK G. AND JURKOWSKI
I. 1995a. Bathurst
discussions on this topic and for his comments on earlier
regolith, 1250000 scale map, sheet SI55-8, NSW.
drafts. Thanks also to Neil Corby and Paul Brugman,
AGSO / NSW Dept. Minerai Resources.
AGSO, and Colin Steel, CRC LEME, for preparing the
CHAN RA, HAZELL M. S, KAMPRAD JL, FLEMING C,
illustrations . This paper is published with the permission
GOLDRICK G. AND JURKOWSKI I 1995b. Bath urst
of the Executive Director, Australian Geological Survey
landforms, 1250 000 scale map , sheet SI55-8 ,
Orga nisation, and the Director, Cooperative Research
NSW. AGSO / NSW Dept. Mineral Resources .
CONAGHAN P.J.
Centre for Landscape Evolution and Mineral Exploration.
1980. The Hawkesbury Sandstone :
gross characteristics and depositional environment.
In Herbert C and Hel by R. (Eds) . A guide to the
Sydney Basin . Geological Survey of New South
Wa les , Bulletin 26, 189-253 .
REFERENCES
FLEI"IING C.
ANDREWS E. C. AND MORRISON M. 1915. Geological
1992 . A comparison between regolith
Survey of the Cargo Gold-field. Mineral Resources No.
landform mapping and gamma ra y spectrometry over
19 . Department of Mines, Geological Surve y.
Orange NSW. Honours thesis, Universi ty of Canberra.
AND
FRASER A.R. AND TILBURY LA 1979 . Structure and
BARRON B.J. 1994. Subduction diamonds in New
stratigraphy of the Ced una Terrace region, Great
BARRON
L.M , LlSHMUND SR,
OAKES G.M.
Australian
South Wales implications for exploration in eastern
Australia . Geologica l Survey of new South Wa les,
GIBSON
Quarterly Notes 94, 1-23 .
Bas in.
Australian
Petroleum
D L. AND
CHAN
RA
1999.
Aspects of
palaeodrainage of the north Lachlan Fold Belt region.
BISHOP P and BROWN R. 1992 . Denudational isosta tic
Proceedings
rebou nd of intraplate highlands : the Lachlan Ri ver
Valle y,
Bight
Exploration Association Journal, Vol 19(1),53-65
from
Regolith
1998
conference,
Kalgoorlie, May 1998 . CRC LEME, Perth.
Austral ia. Earth Surface Processes and
HAQ BU , HARDENBOL J. AND VAIL PR 1988. Mesozoic
Landforms, V 17, 345-360 .
and Cenozoic chronostratigraphy and eustatic cycles.
BRANAGAN D.F. 1983 . The Sydne y Basin and its
van ished sequence . Journal of Geological Society of
Australia , 30 , 75-84.
In Wilgus C.K,
Posamentier H,
Kendall
(Edts .)
CG.C
integrated
BRANAGAN D , HERBERT C. AND LANGFORD-SMITH 1.
approach .
Palaeontologists
1979 . An outline of the geology and geomorphology of
and
Ross CA and
Sea-level
Society
changes
of
Mineralogists .
an
Economic
Special
Publication 42, 71-108.
the Sydney Basin. University of Sydney, Science Press.
HAZELL M.S. AND CHAN RA 1995 . Oberon rego lith­
BROWN C. M. AND STEPHENSON A. E. 1991. Geology of the
landforms, 1.100,000 scale ma p, sheet 8830, NSW.
Murray Basin, southeastern Australia. BMR Bulletin 235.
AGSO / NSW Dept. Mineral Resources .
CHAN RA, 1995 . Blayney regolith-landforms, 1100
HERBERT C. 1970. A synthesis of Narrabeen Group
000 scale map, sheet 8730, NSW. AGSO / NSW
nomenclature . Notes, Geological Survey of New
South Wales, 1, 3-10 .
Dept. Mineral Resources.
53
R OS
L YN
CHAN
JONES G.J. AND VEEVERS J.J. 1983 . Mesozoic origin
RAZA A., HILL K.C AND KORSCH R.J.
and antecedents of Australia's eastern highlands.
1995. Mid­
Cretaceous regional uplift and denudation of the
Journal of the Geological Society of Australia, 30,
Bowen-Surat Basins, Queensland and its relation to
305-322.
Tasman Sea rifting. In supplemen t to Follington
I.L.,Beeston Jw. and HAMILTON L.H. (Edts), Bowen
KORSCH R.J. AND TOTTERDELL J.M . 1995 . Permian and
Mesozoic tectonic and structural events in the Bowen
Basin symposium 1995 - 150 years on. Proceed ings
and
of Geological Society of Australia, Coal Geology
Surat
Basins
and
new
England
Orogen,
Southwest Pacific Rim. PACRIM 1995.
Group, Brisbane, 1-8 .
LISTER G.S . AND ETHERIDGE MA 1989. detachment
RAZA A.,
HILL K.C AND KORSCH R.J. 1999. Mid­
model for the uplift and colcanism of the eastern
Cretaceous uplift and denudation of the Bowen-Surat
Highlands. In Johnson W. (Ed ). Interplare volcanism
Basins, Eastern Australia ; its relation to Tasman Sea
in eastern Australia and New Zealand . Cambridge Uni.
rifting, from apatite fission track and vitrinite refleca nce
Press, New York, 297 -312.
data. In Korsch, RJ and Totterdell , J. M. (Edts)
MARTIN H.A. 1991. Tertiary stratigraphic pal ynology and
Evolution and petroleum potential of the Bowen ,
palaeocl imate of the inland ri ver systems in New
Gunnedah and Surat Basins,
South Wales . In Williams M.A.J. , De Deckker P and
Austra lian Geological Survey Organisation Bulletin, in
press.
Kershaw AP ( Edts .) The Cainozoic in Australia a re­
appraisal of the evidence. Geological
eas tern Australia .
Society of
SCHEIBNER E AND STEVENS BPJ 1974. The Lach lan
MARTIN HA 1973. Upper Tertiary palynology in New
deposits. Geological Survey of New South Wales ,
River lineament and its re lationsh ip to metallic
Australia Inc., Special publica tio n 18.
Quarterly Notes, 14, 8-18.
South Wa les . Geological Society of Australia, Special
Publication 4, 35-54.
SCOTT K.M. 1999. Regol ith effects and Au exploration in
MIDDLEMOST EAK . 1981. The Canobolas complex,
the Blayney-Orange district, NSW. Proceedings from
NSW, an alkaline sh ield volcano. Journa l of the
Regolith 1998 conference, Kalgoorlie, May 1998.
Geological Society of Australia, 28; p 33-49 .
CRC LEME, Perth.
STANDARD J. C
MIDDLETON M.F. 1989. Coal ran k trends of eastern
1964 . Stratigraphy, structure and
petrology of the Hawkesbury Sandstone, PhD thesiS,
Australia in Permian coal basins . In Harrington H.J. et
University of Sydney.
al. Permian coals of eastern Australia , Bureau of
Mineral Resources, Australia, Bulletin 231, 333-351.
STRUCKMEYER H.I.M. AND TOTTERDELL J.M. (Coord )
MINERAL RESOURCES, 1994. Gold in New South Wales.
1992 . Australia : evolution of a continent. Bureau of
OLLIER CD . 1982. The Great Escarpment of Eastern
Mineral Resources, Canberra. AGPs .
WATKINS JJ AND MEAKIN N.S. 1996. Explanatory Notes
Australia : tectonic and geomorphic significance, Journal
for Nyngan and Walgett 1250000 geologica l sheets,
of the Geological Society of Australia, 29 13-23.
SH/ss-1s and SH/ss-11. Geological Survey of New
OLLIER CD. AND PAIN CF. 1994. Landscape evolution
and tectonics in southeastern Austral ia.
South Wales .
AGsO
WELLMAN P 1986. Intrusions beneath large intraplate
Journal of Australian Geology and Geophysics, 15(3),
volcanoes. Exploration Geophysics 17 , 135-139.
335-345 .
O'SULLIVAN P B , KOHN B. P , FOSTER D. A. AND
WELLMAN P AND MCDOUGALL I. 1974. Potassium-argon
GLEADOW J. W 1995. Fission track data from the
ages on the Cainozoic volcanic rocks of New South
Bathurst
Batholith :
evidence
for
rapid
Wales. Journal of the Geological Society of Austral ia,
mid­
21(3) 247.
Cretaceous uplift and erosion within the eastern
WILLIAMSON
highlands of Australia. Australian Journal of Earth
W.H .
groundwater
Sciences, 42, 597-607.
1986.
resources
Investigation
of
the
of
Lachlan
the
Valley
O'SULLIVAN P B., KOHN B. P, FOSTER D. A. AND GLEADOW
alluvium . Part 1: Cowra to Jemalong Weir. Water
J. W. 1996. Mesozoic and Cenozoic thermotectonic
Resources Commission. Hydrologica l Report No.
history of the Lachlan Fold Belt, Austra lia . Geological
1986/12 .
WILSON I.R . AND MCNALLY G.H.
Society of Australia., Abstracts No . 41. 13th Australian
1996. A geological
reappraisal of Tertiary deep leads in the Parkes­
Geological Convention, Canberra.
Forbes
PIGRAM CJ , DAVIES PJ, FEARY DA AND SYMONDS
area,
I'lSW. Abstracts:
The
Geological
PA 1992. Absolu te magnitude of the second-order
Evolution of Eastern Australia. Sydney University
Middle to Late Miocene sea-level fall , Marion Plateau,
Consortium of Geology and Geophysics, 71-73 .
northeast Australia . Geology, V20, 858-862.
54