CHAPTER 7. MACROSCOPrc STNUCTURES 86

TP-EB rtt r-l

CHAPTER 7. CHAPTER 7. MACROSCOPrc STNUCTURES 86

with stratigraphy.

The radiometrically

but not

^always magnetically

distinct

lithologies cleally delineate the Macclesfield Syncline. The different rock types include sandstone, shale, siltstone, limestone, dolomite, marble and quartzite (Belperio, 1985).

This

area

is

a good example of the complementary nature

of

magnetic and radiometric

information

and

the

advantages obtained through their

joint

acquisition and integration.

Magnetic anomalies, US-MS1 and US-MS2, ate caused

by

magnetic minerals

in

andalusite schists near the base of

the

Ulupa Siltstone. They can be followed

for

50

km

around the major folds. US-MS2 is the upper

unit

and US-MSI the lower

unit

and the difference in

their

magnetic properties is evidenced by ^changes around the folds. The relative

ductility

of the Ulupa Siltstone

iith ,".p".t to the

^{quartzite above}

it

has resulted

in

the development of parasitic folds on the shared

limb

(diagram

A in Figure 7.1).

These

folds

are several hundreds

of

metres across.

The

fault

which offsets US-MS1 and US-MS2 on the western

limb

might represent the effect of shearing along one such

fold.

Along the western and eastern limbs, the amplitude of ^{US-MS1 is} of the order of

200-400nT.

On the shared

limb,

anomaly

US-MSIis

negative

(-300nT)

though anomaly US-MS2

is positive. Around the

^closure

of the

Strathalbyn

Anticline, the

contimrity

of US-MSI

and US-MS2

is

disrupted

by a

series

of faults. On the

eastern

limb,

US-MS1 ^is continuous

but

US-MS2 less ^so.

A

^quartzite

unit

crops

out

sporadically along the eastern

limb

(quartzite at

Mt.

Barker) and more continuously along the western

limb

(quartzite at Macclesfreld). BP-SA,

in

^{the Backstairs}

passage Formation,

is found

along

the

eastern and shared

limbs. The

disappearailce

of

BP-

SA and the quartzite

around

the

closure

of the Strathalbyn Anticline

^are

^probably

^related.

Ofler

(1963) interpreted the outcrop

in this

area

to

indicate

that

the quartzite

at

Macclesfield had changed facies

into

a meta-arkose and was continuous around the fold closures, i'e.

that

the quartziteãnd Bp-SA were continuous. However, Mancktelow (1979) interpreted the meta-ar'l<ose as being

in

the Backstairs ^Passage Formation. The anomaly BP-SA is interpreted

to

be

in

the basal member of the Backstairs ^Passage Formation. The termination of the quartzite

unit

and the anomaly, BP-SA, is consid.ered

to

be indicative of a

fault

^{(see below)'}

TC-MS has been mapped around the fold closures of the major syncline and anticline, though

Ofler

(1963)

in

his mapping shows

that

the

pyrite

bands are ^seen only

within the

syncline and appear

to

continue as an andalusite schist around

the

closure of

the anticline.

On the western

ti-t, 1C-US thins out

reappearing as

a

negative anomaly as

part of the

Channel Anomaly (see

below).

On the eastern

limb

there

is

a

major

break between TC-MS and TC-\MKS

in

the

iinpot

^{area. Kleeman and} Skinner (1g5g) have mapped the eastern and western

pyrite

bands and found

that in

the

vicinity

of

Tinpot

the

pvrite

bands are

not

continuous and

thin

out.

A

modeiled magnetic proflle is shown

in

Figure 7.4. The effect of magnetic basement is ^seen

in the

west

but

disappeais

to the east.

Modelled susceptibilities (order

of

400

x

10-5

SI)

of

the magnetic Kanmantoo Group rocks are

in agre

^rops'

Mode[ãd

susceptibilities

(-

⁴⁰⁰⁰

x

10-5

sI) for

^than

measured magnetic susceptibilities and

this can

^(Sec-

tion 2.3.1). Th" mugn"tic

dips of the

units

are generally

to

the east,

indicating that

the axiai limbs are slightly overturned.

Macclesfield Fault

Marlow (1925) has inferred the ^presence of two faults

in

the

vicinity

of the Macclesfleld Syncline area: the Macclesfield Fault which forms the contact between the quartzite at Macclesfield (taken to be precambrian) _and the Cambrian rocks, and the

fault

between the Normanville Group and the Kanmantoo Group. Both faults ^have been validated through magnetic interpretation though

CHAPTER

7,

MACROSCOPrc STRUCTURES ⁸⁷

the

interpretation

^is

slightly

different from

that

of Marlow's (1975). Both faults were probably

thrusts

considering

the

large apparent movement and

the

orientation which

is

subparallel

to

bedding.

A

fault

^{has been} proposed between the Normanville Group and the Kanmantoo Group in the region of the Macclesfielã Syncline. The ^sequence of Adelaide Supergroup rocks and Normanville

Grlop

rocks on

the

western

limb is

appreciably different

from that

^on

the

shared and eastern

limbs (Marlow, 1975). On the

^western

limb,

Normanville Group

units

disappear

or thin

out as

they

approach

the

Macclesfield Syncline while Kanmantoo Group

units thin out

away from

the fold closure. This

may indicate

a fault

contact between

the

Normanville Group and ^the Kanmantoo

Group.

Steinhardt

(in prep.)

^suggests

that

the marble

at

^Paris Creek acted as ^a detachment surface. However this theory is discounted by the discovery

by

Sprigg

(in

prep.) of

intact

Archaeocyathidsin the marble (Preiss, pers' comm')'

The quartzite at

Macclesfield

is

seen

to

strike

into

^the magnetic anomaly BP-SA and this formed the basis of Offier,s (1963) argument

that

there had been ^a facies change from quartzite

into

meta-arkose.

Marlow

(1975) reported an increase

in the

thickness

of the ^quartzite

^cor-

responding

to the

change

in

facies

and a

divergence

of

bedding-cleavage relationships

in

the quartzite at Macclesfield from the western lirnb

to

the hinge of the Strathalbyn

Anticline.

How- ever,

if Bp-SA

has been correctly interpreted

to

be

in

the Backstairs ^Passage Formation (this follows the mapping of Mancktelow (1979), and the interpretation of BP-SA ^being the continu- ation of Bp-Vt/KS

I ,""

Section 7.2.2) then there must be ^a

fault

contact between the quartzite and

the

Backstairs ^Passage Formation ^as

it

^is

^not

^possible

^to

^account

^for

overlying beds strik- ing

into

underlying beds

by

an unconformity. The Macclesfield Fault is

likely to

represent the extension of

the

Nairne Fault which has been mapped

by

Toteff (1977) on the western

limb

of the Kanmantoo ^SYncline.

Thrust above Talisker Calc-siltstone?

Along the eastern

limb

of the Strathalbyn Anticline, magnetic horizons

within

the Talisker Calc- siltstáne give rise

to

characteristic magnetic anomalies, TC-MS and

TC-WKS

(Section 6'1'1)'

TC-WKS is

essentially

the

northern continuation

of

TC-MS except

for a major

disruption in magnetic trends

in

^the

^vicinity

^of

^Tinpot.

In this

region, magnetic anomalies

trend NNW

^contrary

to the

mapped NS

to NE

trends

within

the Backstair, Èurrug" Formation (trends traced from aerial photographs by Mancktelow,

lg7g).

The anomaly TC-WKS is made up of a number of magnetic horizons of which there are

two main ones.

These can be related

to the two main pyrite bands. The

^{western band had} been assigned

to the

Talisker Calc-siltstone and

the

eastern

to the

Tapanappa Formation by Thomson"(1g6gb), though Kleeman and Skinner (1959) interpreted

both pyrite

bands ^as being

part

of

theNairne pyritã

_{Formation (the old} name for what Mancktelow (1979) identifies ^as the Nairne

pyrite

facies of the Talisker Calc-siltstone)'

The

similarity in

magnetic response between

the two

and

the

suggestion

from the

contour map

that in

the

vicinity

ãf

Tinpot, TC-WKS

narrows and the western

unit

appears to fold into the eastern

unit, ¡as

been interpreted to indicate

that

the simple model indicated on geologic.al maps may be incorrect. Instead, ^one feasible interpretation is

that

the western and eastern units ur"'"qoi1ru,1"nt, and the repetition caused by an early

thrust

during which the pyrite schists acted as

a

detachment surface. The

NNW

trending anomalies between

the

northern end

of

TC-MS and the southern end of TC-\MKS could be due

to

dykes parallel and similar

to

the dyke swarm along the

Mt.

Beevor Shear Zone (Pain, 1968).

2981 00

2881 00

6095700 6089400

NORTHING (metres)

Figure 7.6: lvlagnctic intcrprctation of the _Channel Anomaly showrr superimposed on contours of the vertical rna,gnetic gradient.

G

_q)

r-

0)

E

CI

z

tr

U)

LU

O Õ

-ì

O

O

O

0

o

ô

\_-

=4

-

---1

r

o

Þ

O ô

r

--\

!2,

I

I

I

I I

ì'-

^I

{\ ì)

\r

t\

Þ \\

{

.(=

J (

/,\

r:)

\ 1¡ ^I

(

()

,

/-ì

I

L

<)