Folding of angular unconformable sequences and effects on early folds, Tabberabbera district, eastern Victoria, Australia

Tecronoph~sics,

93

158 (1989) 93-111

Elsevier Science Publishers

B.V.. Amsterdam

- Printed

in The Ne~erlands

folding of angular unconformable sequences and effects on early folds, Tabberabbera district, eastern Victoria, Australia CHRISTOPHER Department

L. FERGUSSON

* and DAVID R. GRAY

of Earth Sciences, Monash University, Clayton, (Received

May 51987;

accepted

November

Vie. 3168 (Australia) 4,1987)

Abstract Fergusson,

C.L.

and

Tabberabbera

Gray,

district,

D.R.,

1989. Folding

eastern Victoria,

of angular

Australia.

unconformable

In: A. Ord (Editor),

sequences

Deformation

and

effects

of Crustal

on early

folds,

Rocks. Tectonoph,ysics,

158: 93-111. The Tabberabbera produced

by

upright,

tight, easterly

a locally intense unconformably

overlying

the syncline

F,

is approached.

the reorientation

folds

are

trending

elastic syncline

upright,

with

Fold

sequences.

Belt has a good Ordovician

quartz

F, folds of Early to Middle

Silurian

deformation

Emsian

west of the Mitchell

Lachlan

unconformable

Middle Devonian

unconformity predicted

of the southeast

of angular

subhorizontal,

i~ed~ately

district

folding

that has formed

sequence

(Wentworth

have steep northwest shallow

A folded foam-layer

plunges, analogue

of early fold axes and axial surfaces

an upright Group). trending and

matching

produced

example flysch

syncline

contains

(Mitchell

syncline)

of the FI folds in the Hotham

axial surfaces

gradually

fold pattern

Group)

age. These have been reoriented

isoclinal Many

of a complex (Hotham

become

the geometry

and are steeply plunging. inclined

to recumbent

of the Mitchell

by the unconformity-folding

syncline

surfaces,

eliminated conformity

does not obliterate the angular an unconformity, the controls

mechanism is buckling. The area investigated

of obliquely

inclined

the angular and where

discordance across an unthe early folds have been

substantially reoriented by the younger deformation. We use a foam-layer analogue to model the complex fold pattern assuming that the main fold

discordance across on the orientation

is

within

of any early folds beneath the unconformity are: (a) initial orientation of the early folds, (b) orien-

berabbera belt of the Benambra terrane the Lachlan Fold Belt of southeastern

tation of the deformation unconformable sequences,

(cf. Fergusson

P.O.

Department Box

1144,

affecting the angular and (c) the fold mecha-

of Geology,

University

Wollongong,

N.S.W.

0 1989 Elsevier

of Wol2500

Science Publishers

the

Tab-

(Fig. 1) of Australia

et al. 1986, 1987). Underlying

the

high-angular unconformity (see sub-Devonian unconformity: Fig. 2) are the low-grade Ordovician Hotham Group which are a monotonous succes-

(Australia). 0040-1951/89/$03.50

to that

nism (Ramsay, 1967, pp. 491-517). Our aim is to give an example of the case where strain has not

In his analysis

longong,

as the

shows that

event is identical

Ramsay (1967) gave the example of the Caledonian fold belt of northwest Scotland where an angular discordance across an unconformity is progressively obliterated by increasing strain. Where strain

address:

Group East of

by the model.

Introduction

* Present

by

in the

B.V.

sion of quartz-rich turbidites with minor chert and graptolitic black shale. Above the unconformity is the Lower Devonian Wentworth Group which

Ji

1MELBOURNE kkk

(CAMBRIAN-DEVONIAN)

Fig. 1. Regional

map of the Lachlan

O-Ordovician; Wentworth

\

Ott-Ordovician Group;

Dsr-Devonian

Devonian

Fold Belt in eastern metasedimentary volcanics

red beds; D-C-Upper

contains a basal eastward-thinning wedge overlain by a fining upwards sion (Talent, 1963).

Victoria

succession;

and sedimentary Devonian

the location

S-Silurian

conglomerate elastic succes-

rocks of the Buchan

The folded unconformity is only preserved in the tight to isoclinal, shallowly south-plunging Mitchell syncline, which contains the Wentworth Group (Fig. 2). The Mitchell syncline has a steep axial surface and changes geometry along its trend and folding

In the north the structure is a simple tight syncline, with a steep east-dipping axial surface, that has been intruded by several silicic dykes (Fig. 3a). To the south the syncline is faulted along its core and is slightly tighter than isoclinal (Fig. 3b). Further south the structure is W-shaped with a central anticline and two bounding synclines (Fig. 3~). The western syncline contains interbedded sandstones and mudstones with abundant mesoscopic isoclinal folds. A weak slaty cleavage axial planar to these folds occurs in all fine-grained lithologies in the Wentworth Group. Southwards the syncline rapidly widens out and there are abundant isoclinal folds and in places, boundinage and scaly fabric in mudstones char-

acteristic ations

of the Tabberabbera

volcanics

to Lower Carboniferous

Folds within the sequence above the unconformity

due to the transition between faulting along the hinge zone (Fig. 3).

showing

and

sedimentary

Rift; De-Lower sedimentary

of broken

Key to symbols:

rocks;

Dwg-Devonian

Devonian

rocks;

Du-Upper

rocks and volcanics.

formations,

of faults (Fergusson

Structural relationships formity On a regional

district.

and several gener-

et al., 1987, fig. 39).

in rocks below the uncon-

scale the structure

of the Hotharr

Group is dominated by E-W trending shallowl!, plunging Fi folds with steep to vertical axial surfaces (Fig. 4). These folds are close to tight with narrow, planar limbs.

mainly angular, Bedding planes

hinges and long along limbs com-

monly contain slickensides at a high angle to Fi fold axes, and faults are locally developed in hinges. These faults comprise narrow zones of tectonic breccia with lenticular sandstone fragments aligned stone matrix.

parallel

to a scaly fabric in a mud-

In the Tabberabbera district the Hotham Group contains a well-developed bedding-parallel slaty cleavage in mudstone layers (cf. Wilson and De Hedouville,

1985). This fabric is everywhere

folded

around Fi fold closures and therefore predates the F, folding. Fi folds may have a weak to moderate S, axial planar crenulation cleavage in mudstones and in some areas a stripy cleavage is developed in sandstone. Many F, hinges do not contain axial planar fabrics.

95

piiJ

Upper Devonian sedimentary rocks and volcanics

_

Tabberabberan Unconformity Lower

w

Wonnangatta

/>’

Bedding

Devonian

~-/~~_

Upright-younging determined )?

Overturned 17

fl

Minor

with

plunge

Syncline

% ,

folds

4

Tectonic

melange

Fig. 2. Regional geology of the Ordovician Hotham Group and the folded angular unconformable sequence of the Mitchell syncline. Tabberabbera belt of the Benambra terrane (see Fig. 1 for location).

At map-scale the F, folds consist of several anticlinoria and synclinoria with long planar limbs. Hinge regions are either narrow with few parasitic folds or, as in the area west of Tabberabbera, wide with abundant parasitic folds and many orders of folds.

In many areas these F, folds are post-dated by younger deformations. The most significant of these is developed in the Hotham Group to the east of Tabberabbera. This deformation has formed north-trending, close to open, F3 folds with low amplitude-to-wavelength ratios, steep

96

a y 400

-I

300 200 100 0 m

/vv

Fades

change

0 00

Conglomerate

-

Uncontormlty

)

Fault H.S.=V.S.

b

W 200

100

100

a

0 m

rl

H.S. = V.S.

cw

E 100 0

m

Fig. 3. Profile sections

of the folded unconformity-overlying

axial surfaces, and well developed crenulation cleavage. To the west berabbera district this deformation

axial-planar of the Tabis weak or

Wentworth

Group

in the Mitchell

syncline

(see Fig. 2 for location).

difficult to detect in weakly crenulated rocks. The adoption of F3 chronology for this generation of N-S folds is based on regional overprinting criteria

97

,2m Fig. 4. Outcrop syncline

X F,

. sketch

showing

typical

(see Fig. 2 for location). cleavage.

of map-scale

structures

structures

of the Hotham

These Fi folds are upward

Section is - 40 m in length.

(Fergusson,

deformation

1987) rather

therefore

and kinks. These structures only affect layers with no sign of accompanying in interbedded represent

sandstone

layers

deare

separately for the areas west (area A) (area B) of Tabberabbera respectively

Area immediately

west of the Mitchell

plunging

structures

Syncline

(Area A)

unconformity

of the Mitchell

with an axial surface

equal area stereographic

S, crenulation

projection.

Mitchell River area and hinges are commonly spaced at 10 m or less. The abundance of F; folds and changing orientations of enveloping surfaces indicates that several major F, hinges are probably present, although their geometry cannot be precisely

determined

due

to lack of continuous

exposure. 1;; folds are scattered

district the Fi folds show that are spatially related to

the north-trending regional Mitchell syncline scribed above. These structural relationships discussed and east (Fig. 2).

and

only low levels of shortening.

In the Tabberabbera changes in orientation

14 km west of the folded

Lower hemisphere

than overprinting of local crenulations. Locally the mudstones in the Hotham Group may contain several or more crenulation cleavages, crenulation lineations mudstone

Group

facing shallowly

axes

throughout

the area and

have a style and orientation similar to those east of the Mitchell syncline (compare Figs. 6 and 8). They are commonly shallowly plunging which contrasts with the steep to moderate plunges of the F, folds (Fig. 6). The F3 folds are both upward and do~ward facing whereas the Fl folds face to the south and southeast (Fig. 6); “facing” is used to denote the direction of younging in the fold axial surface,

after Borradaile

(1976). Overall

the

F3 folds have

sistent

an S vergence looking north, conwith their location on the western limb of

Immediately west of the Wentworth Group F1 structures within the Hotham Group trend north-

the Mitchell syncline worth Group.

westerly whereas farther west they are easterly (Fig. 2). This change in trend is accompanied by an increase in plunge of most IT;; folds so that in the Mitchell River area they are mainly steeply to moderately plunging to the southeast (Figs. 5 and 6). These F, folds generally lack axial plane foliations (Fig. 7). Axial surfaces of the Fi folds are steep to vertical (Fig. 6, stereonet insets) with folds reclined to vertically plunging, such that the fold profile can only be seen on formline maps (Fig. 6, insets 1 and 2). F, folds are very abundant in the

Clearly the structure of the Mitchell River area is anomalous compared to the regional structure of the Hotham

Group

which

preserves

as shallowly

the Went-

plunging,

vari-

ably facing, F3 folds are associated with steeply to moderately plunging F, folds, which consistently face to the south and southeast. Area east of the Mitchell Syncline (Area B) The Sandy Creek area to the southeast of Tabberabbera (Fig. 2) has F, folds with a variable orientation

(Fig.

8). In the east the F, folds are

98

shallowly west

plunging,

trends,

upward

have northwest

near vertical

facing

where

determined

Fl folds have orientation

to west-north-

axial surfaces (Fig.

structures.

and are all

erate

8). These

Farther

northwest

typical

plunges,

are moderately

. Poles to axial surface

+/

of the regional

F,

west the F, folds have modto shal-

(F1)

+ F, fold axes

Fig. 5. Lower hemisphere

equal area stereographic River and area north

projections

for the Tabberabbera

of the granitoid

at Dargo

district

showing

(Fig. 2); (b) Mitchell

F, fold axes and poles to axial

surfaces:

(a) Wonnangatta

River area (4

contours,

AS1 with 4% and 16% contours); (c) Sandy Creek area domains 1, 2 and 4 (see Fig. 8); (d) Sandy Creek area. southwestern half of domain 3; and (e) Sandy Creek area, northeastern half of domain 3.

with 4% and 10%

01-66

‘dd

,,

,,

,,

11,

,,/,

“.

103

Fossils Creek

are

scattered

area (Fig.

with

this

throughout

8) and

proposed

the

their ages are consistent

synclinorial

structure.

oldest rocks occur in the east and become sively younger

to the southwest.

development mudstones and

stratigraphic

levels

(Fig. 8). The major preservation Sandy

mudstones

from

F, fold in interbedded

the Mitchell

lack of axial surface

sandstones

River area (Area foliation

A). Note

and

part

of the Sandy Creek area lies on an overturned southwest-younging limb of an F, syncline (Fig. flat-lying and folds. Farther and die out surfaces (Fig.

ABCL) (Fig. lob) the limb is near folded by close downward-facing F3 east these F3 folds are less extensive rapidly up and down their axial lob).

In the southern half of the area the F, folds are more abundant and include W-shaped folds that indicate the presence of an Fi synclinorial hinge (Fig. lOa). From

east to west the axial surface

the synclinorium changes from recumbent as the unconformity Parts

of the synclinorium

of

near-vertical to is approached.

are refolded

structural

Merrijig

Creek

also explains

Ordovician

strata

within

the

in lower

Creek (Fig. 8), since this stratigraphic

level

the Hotham

Group. Structural relationships

lowly inclined and are even locally reclined (Figs. 5, 8 and 9) but there are instances where the Fi folds have the regional orientation. Axial surface foliations are well developed in the east but are

IOa). In section

in upper

lowest

and

the

in the pelite.

progressively harder to find farther west. At map scale the structure of the northern

the

encountered

the

in sandstones

F3 warping

of Upper

is not normally Fig. 7. Steeply plunging

with

The

progres-

Furthermore,

of S, cleavage coincides

Sandy

by F3 so

across the unconformity

The Fl folding clearly predates the unconformity and from regional relationships is known to have occurred in the Early to Middle Siiurian (Bolger, 1982; Fergusson et al., 1986, 1987). The syncline containing in the post-Emsian (Talent,

the Wentworth Group formed pre-Late Devonian interval

1963). This structure

time as the & structures

formed

at the same

in the Hotham

Group

because: (a) the c; structures have a similar orientation to the Middle Devonian folds in the Wentworth Group, (b) the r;; folds postdate the Fl folds which are truncated at the un~onfor~ty surface at the base of the Wentworth Group (Fig. 2), and (c) the & folds in the Mitchell River area have a consistent vergence with their location on the western limb of the syncline containing the Wentworth

Group.

Discussion

that locally parasitic F, folds face downwards to the southwest (Fig. lOa, section OP; Fig. 11). The eq folds die out rapidly up and down their axial surfaces and are both upward and downward facing (Fig. 8). Near the unconfor~ty with the overlying Wentworth Group the axial surface of

Ordovician Hotham Group either side of the Mitchell syncline are anomalous and appear spatially related to the folds in the overlying Wentworth Group. The following relationships need to

the F, synclinorium dips steeply to the east and is at a high angle to the unconformity (Fig. lOa, cross section MN). In this zone F, folds with contrasting o~entations are in contact along steep northwest trending faults that probably formed during the D, event.

be explained: (1) steepening of plunge and change in plunge direction from east-west to a southeast plunge direction of F, folds as the folded unconformity is approached from the west (Mitchell River area); (2) the changing orientation of F, fold axial surfaces from steep in the east to flat-

Changes

in the orientation

of early folds in the

104

105

Fig. 9. Lower hemisphere the four domains

equal area stereographic

shown on Fig. 5. Also plotted

projections

for the Sandy

are S, crenulation contour

cleavage,

intervals

Creek area. Stereograms 4

1 to 4 show bedding

fold axes and F3 axial surfaces

(AS,).

(.S,) for

All nets show

of 38.

lying in the west as the folded unconformity is approached from the east (Sandy Creek area). Plunge variations are generally attributed either to refolding or to large strains producing stretch-

tial relationships

ing in the

Simple modelling was undertaken to whether folding of the unconformity-overlying

X-direction

(Ramsay,

1967;

Ramsay,

1979). The latter explanation is not appropriate as the F, folds in the Mitchell River do not show evidence of higher strains than F, folds, which elsewhere have minima1 plunge variations. Thus the unusual plunges of the F, and Fj folds suggest refolding with the development of the D, deformation and folding of the unconformity-overlying Wentworth Group. A composite cross-section across the Mitchell syncline (Fig. 12a) shows the variation in the early-fold axial surface orientation in the Hotham Group relative to the syncline, and areas where the F7 folds are downwards facing. These variations, in particular the vergence relations between the Fl axial surfaces and the unconformity are identical with that expected for a folded angular unconformable sequence (Fig. 12~) given the ini-

unconformity

for the axial surfaces as shown

below

the

in Fig. 12b.

Modelling

quence

could

also explain

the observed

test se-

variations

in plunge of the F, folds. A foam-layer strip, with oblique lines inscribed to show the initial attitude of the early fold-axial surfaces beneath the unconformity, was used to simulate the observed geometry of the Mitchell syncline. Boundary conditions included: (1) the upper surface of the foam inscribed with the strike-line traces of the F, axial surfaces constitutes the unconformity surface; (2) the initial

attitude

of the F,

axial

surfaces

was

taken as 300 “/60 o NE based on F, axial surface orientation away from the Mitchell syncline; which gives (Y,)= 60 O and 6, = 60 o where LY” is the initial angle between the fold axis and the intersection of the inclined surface with the unconformity and 6,) is the initial dihedral angle

106

-53

w

a

I’,’

H.S. = V.S.

east of the folded angular unconformable

RS

Wentworth

m

Group of the Mitchell

-0

-100

southwestern part of the Sandy Creek area (see Fig. 8 for location). h. Sections for the eastern part of the Sandy Creek area.

Fig. 10. Cross sections of the folded Hotham Group immediately

b

syncline.

for the

0 m

100

200

300

a. Sections

T

Fig. 11. Shallowly

inclined

F, angular

folds in Sandy

fabric in pelite and overprinting

Creek (Area B) showing

of the shallow-dipping

parasitic

limb by subvertical

folding

of a well-developed

bedding-parallel

F3 folds. Ray Cas for scale.

between the plane and the unconformity surface, after Ramsay (1967, fig. 9-I) (3) the F3 folding direction was parallel to the side of the foam, and (4) the foam was flexed about F3 to reproduce as closely as possible the limb dips and geometry of the Mitchell syncline and other now eroded folds

turbidities of the Hotham Group. Varying the wavelength of the folds below the unconformity in the modelling will obviously change the width of the domains shown in Fig. 14 and vary the spatial

within the sub-Devonian angular unconformity. The results of the analogue modelling are shown

adjacent to the folded unconformity the zone of contact strain associated

in Figs. 13 and 14. The domains (Fig. 14) very closely match the observed relationships for fold plunge changes from the west side of the Mitchell syncline to the east side. The front of the foam, which corresponds to the F3 profile section with oblique views of the F, folds, shows that F, fold geometry will change (1) from upright horizontal to inclined steeply plunging (with some folds reclined) approaching the unconformity from the west (Area A), and (2) from recumbent to inclined

within the unconformity. In summary, both the fold axis and axial surface

plunging and/or upright plunging

reclined to upright horizontal to leaving the folded unconformity

on the east (Area B). Given the nature of the layering in the Hotham Group and that in the Wentworth Group, folds in the unconformity-overlying sandstones and conglomerates should have siginificantly greater wavelengths than those in the thinner bedded

distribution of the fold axis and axial surface reorientations. These effects will only be localised surface within with the folds

orientations in the foam analogue (Fig. 13) match very closely with the observed changes across the Mitchell syncline (compare with Figs. 2 and 12a). This suggests that these anomalous F, fold patterns are related to refolding and contact strain effects associated with folding of the unconformably overlying Wentworth Group now only preserved in the tight to isoclinal Mitchell syncline.

Significance of the Middle Devonian deformation

The isoclinal Mitchell Syncline indicates that the Middle Devonian deformation is related to strong E-W compression. A major contrast across the unnamed unconformity between the Went-

inclined steeply plunging Ft folds

(4

---+

I

I

P

upr;$t& F t

reclined to recumbent F1 folds

-I-

2km I

(b)

Fig.

12. Structural

between

4

relations

axial surfaces

ob~quely angular

inclined

in the model

a folded sequence geometry

with axial surfaces of the Mitchell

surfaces..

unconformable

shownin c. b. Undeformed

predicted overlying

to fit the observed

for folded and the folded

obliquely

a. Composite

Wentworth inclined

surfaces

Group.

regional

section

Note

involving

showing

the observed

an angular

the angular

relations

unconformable

match

relations with that

sequence

(0)

(\) inclined

syncline,

the un~nfo~ty

at 30 o to the unconformity. c. Angular unconformable sequence folded showing refolding of the early axial surfaces and their vergence relations with (modified

worth Group and the underlying Hotham Group is the intensity of the Middle Devonian deformation which has formed only open folds in the latter unit. To the southwest in the Freestone Creek area the Ordovician succession contains east-southeast-trending close to open Fl folds with weakly developed younger N-S structures. None of these N-S structures are easily recognized as Tabberabberan structures (Fergusson, 1987).

from Ramsay.

Farther

1967, fig. 9-16).

east in the Tambo

River area the Tab-

berabberan structures are open to broad upright folds that overprint the early E-W F, holds (Fergusson, 3987). In the Buchan area the Early Devonian volcanics and limestone have been shortened by 20% during the Tabberabberan event (VandenBerg, 1978). Thus the Wentworth Group appears to have been anomalously strongly affected by the Middle Devonian deformation.

110

Fig. 13. Photograph

of folded

surfaces

with the upper

depicts

the unconformity

foam-layer

analogue

and front surfaces surface

of the Mitchell

syncline,

of the foam ( a0 = 60 o and

with the angular

unconformable

sequence

Fig. 14. Diagram

moderate to steep SE plunge

based on the foam-layer spatial

variation

analogue of inclined

and shows plunge

variations

of the early folds.

subhorizontal plunge moderate NW plunge

in Fig. 13 showing to recumbent

removed

of a weak zone is that it formed from modification of a half-graben which was active during sedimentation of the Wentworth Group.

Presumably the Wentworth Group occupied a mechanically weak zone in the Middle Devonian crust. A possible explanation for the development

subhorizontal plunge

with inscribed intersection traces of early-fold axial 8, = 60 O, after Ramsay, 1967, fig. 9-l). The upper surface

domains

Ft geometries

of plunge

variation

across the modelled

for F, folds (cf. Fig. 6) and the syncline.

111

and B.E. Hobbs,

Conclusions

of folding Refolding

of early

folds below

due to subsequent

folding

overlying

is responsible

fold

sequence

trends

Lachlan anomalous in the

in parts

Fold

Belt

changes

of the unconformity-

of erogenic of

belts.

southeastern

Hotham

below a folded unconformity,

Group

In

the

of Fi folds

of the

unconformably overlying sequence (Emsian Wentworth Group). This elastic sequence is now only preserved in an isolated isoclinal syncline within the folded unconformity (Mitchell syncline). The major deformation in the Ordovician Hotham Group consists of shallowly punging E-W to NW-SE upright tight to close Fi folds. West of the Mitchell syncline these F, folds swing in trend from easterly to northwesterly and become steeply to moderately plunging. East of the syncline the the Mitchell

syncline

to near recumbent

is approached.

ing the diagrams, thank

the referees

Folding

Bolger,

P.F.,

out at the Department

Monash

University

Victoria.

Pam HerWe

substantial

G.J.,

and

1976.

and the Palaeozoic Velez Rubio,

was

Research Grants. N. and I.R. Stewart prothe field. Initial thesis students Mark Sloan, Birnie in the Tabby Drs. R.A.F. Cas

Silurian

im-

stratigraphy

Creek-Benambra

Proc. R. Sot. Victoria,

Borradaile,

rocks

and

area, northeast

94: 35-47.

Structural

SE Spain.

facing

(Shackleton’s

of the Malaguide

Rule)

Complex

Proc. K. Ned. Akad.

near

Wet., Ser. B,

79(5): 330-336. Fergusson,

C.L., 1987. Early

in the Lachlan tions for terrane E.C. Leitch

translations

C.L.,

sequence,

1987.

Multiple

(Editors),

CL..

Gray,

in the Lachlan

Geology,

14: 519-522. C.L.,

Gray,

D.R.

and

Proc. Linn. Sot.

in the Palaeozoic

metamorphics

Victoria.

Geol. Sot. Aust.,

Fieldtrip

Guide

J.G.,

fold-thrust

Australia. V.J..

zones

1987. and

re-

of central-eastern

Spec. Group

Tect. Struct.

Geol.

No. 2.

1967. Folding

Graw-Hill,

1986. Overthrust

Morand,

gional

and

Fracturing

of Rocks.

Mc-

New York.

D.M.,

progressive

1979.

Analysis

deformation.

of rotation

of folds

during

Geol. Sot. Am. Bull., 90: 732-738.

J.A. 1963. The Devonian

of the Mitchell

and Went-

rivers. Vict. Geol. Surv. Mem.. 124.

VandenBerg, Victoria. Wilson,

Geodyn.

fold belt, southeastern

terranes,

worth

In: Accre-

of the Ordovician

Victoria.

Tectonostratigraphic

Talent,

Union,

D.R. and Cas, R.A.F.,

terranes

Ramsay,

Terrane

109: 293-309.

Fergusson,

Ramsay,

implica-

Gondwanaland.

folding

River, eastern

deformation

Australia:

Belts. Am. Geophys.

Tambo

N.S.W.,

back-arc

in eastern

and E.C. Scheibner

Orogenic

Fergusson,

Palaeozoic

Fold Belt, southeastern

Ser., 19: 39-56.

of

and

in the Wombat

tion and

funded by ARGS grant E83-315675 (principal investigators Drs. R.A.F. Cas and D.R. Gray) and Monash University Special Murphy, S. Winter, R. Cas vided valuable assistance in work by Monash Honours Fiona McCall and Andrew berabbera district supervised

and

for suggesting

1982. Ordovician

structure

Fergusson.

Sciences,

Muir

of the manuscript.

References

of

Acknowledgements

Earth

Gelt for draft-

to the manuscript.

as

the unconformably overlying sequence during the Middle Devonian E-W compressional event produced the isoclinal syncline in the Wentworth Group and open downwardand upward-facing F3 folds in the Hotham Group.

This work was carried

drafts

for

of this pro-

Draga

and June

for typing

the impetus

to refold-

strain effects due to folding

Fi folds change from upright

components

provements

immediately

are related

the field and laboratory

We thank

modelling

with D. Dur-

provided

mansen

Australia

about

sequences

ney and G. McDermott ject respectively.

for anomalous

in the orientation

Ordovician

ing and contact

unconformities

and discussions

unconformable

A.H.M.,

C.J.L.

development Australia.

1978. The Tasman

Tectonophysics and

de Hedouville,

P., 1985.

in the Late Ordovician J. Struct.

Fold Belt System

in

48: 267-297.

Geol., 7: 401-408.

Early

of northeast

cleavage Victoria,