Structural patterns in the Karinya Synclinal Subzone

TP-EB rtt r-l

CHAPTER 7. CHAPTER 7. MACROSCOPrc STRUCTURES 91

7.3 Structural patterns in the Karinya Synclinal Subzone

The Karinya

Synclinal Subzone lies

north

and east of the Intermediate Subzone

(ISZ)

and ^ex- tends

north of the

main

study

area

into

the Eudundø sheet

area.

The outcrop

of

Kanmantoo Group metasediments

in

the

northern

closure of

the Karinya

Syncline is

the

northernmost ^oc- currence of

the

Group. The eastern margin of the KRSZ is defined

to

be the western margin of

the EMZ. In

^{the south} and southwest, the KRSZ passes

into

the

ISZ.

The

major

structures in the KNSZ and KRSZ are synclines which have foided the Kanmantoo Group rocks. West of the

I{arinya

Synclinorium, Proterozoic rocks are found

within

the cores of anticlinal structures.

About a sixth of the main study

area

is

covered

by the KRSZ. CRA

aeromagnetic and aeroradiometric data are available over most of the area and

BMR

aeromagnetic data have ^been used

for the

remainder. As mentioned

in

Section 5.1, the rocks

in the

main study area are not confined to the Kanmantoo Group. The discussion of structures in the KRSZ has been extended

to

include structures

in

the contiguous

WMZ.

The magnetic characteristics

of this

Subzone are

very similar to that of the KNSZ.

There

is

no evidence

of

deep-seated magnetic sources. Magnetic property variations

in

the metasedi- ments of the Adelaide Supergroup, Normanville Group and Kanmantoo Groups result

in

Inear

to curvilinear

magnetic trends

which

dominate

the

magnetic

maps.

Amplitudes

of

anomalies are

typically low ald

less

than

a few hundreds

of

nanoTeslas,

the

exception being

the

strong anomalies (several hundreds of nanoTeslas) ^caused by the Ulupa Siltstone magnetic marker ^US-

T. Granitic

rocks (aplites, . . .

)

^{are found}

in the

southern closure of the

Karinya

Syncline. The

stratigraphy in

much

of the

area

is

uncertain and has

not

been updated

to

accommodate the revised classification of

Daily

and Milnes (7972a).

There

is a

marked contrast between

the

intense

and

complex magnetic character

of

the Intermediate Subzone and the subdued rnagnetic response of the KRSZ. Migmatites are common

in

the ISZ while absent

in

the

KRSZ.

Fold axes

in

the KRSZ trend approximately NS, while the magnetic trends

in

the ISZ are dominantly

NN\M. Abrupt

termination of magnetic units have been used

to infer

a

fault

contact between

the northern part of the ISZ

and

the KRSZ.

The western margin of the KRSZ may also be

fault

bounded.

The

regional

structure in the KRSZ

consists

of a

doubly-plunging syncline

(the

Karinya Synclinorium)

in

the east. To the west of the synclinorium, the

Truro Anticlinal

structure (new name) consists of a south plunging anticline

in

the

north

and a doubly plunging anticline

in

the soutlr. The fold axes of both these folds trend approximately NS. Both folds are part of the F*o¿n macroscopic folding event. The relation betrveen the Truro

Anticlinal

structure and the Karinya Syncl-ine

is uncertain; at

least

in part the

common

limb is faulted. The

Kanmantoo Group magnetic anomalies, CH2,

BP-KRS,

and

TC-KRS

are essentially

linear

except

for

occasional bull's-eye anomalies. These deviations may be due

to

refolding of early folds.

ÊI

!õ

iT 00

-100 0 500

LINE: 2850 EASTING: 323187 331838 NORTHING: 6191941

FE

(,ç

4000

2000

0@0

- 2000

..40@

250 500 750 Eq

E

Figure7.14:WestlimboftheKarinyaSyncline.

Anomalies:

CH2at6500,BP-KRSat7500and

TC-KRS between 8100 and 8700.

LINE: 3os0 EASTING:323382 3¡ì9495 NORTHING: 5198045 200 0

'100 0

00

Ê_c

=^o II

E Fc

(,

-100.0 t.m0

o.500

Eo E

0m0

0

æ00 3000

Figure 7.15: Magnetic model of the Karinya Syncline. Anomalies: US-KRS

at

5000 and 13700, CH2

at

6800.

CHAPTER 7.

MACROSCOPrc STRUCTURES ⁹⁴

7.9.1 ^{The KarinYa} SYnclinorium

The

major

structure

is

an oval structure elongated

in the

NS direction and clearly identifiable

from

magnetic maps. Drummond (1972) called

it

the

Karinya

Synclinorium while Mancktelow (1979) referred

to it

^as

the Karinya Syncline. Only lower

^s

outcrop

in

the core of the syncline.

All

three subunits of

the

^e

been recognized

by

Gatehouse et

al. (in ^press)' Normanvill

^e

eastern

limb

^{and also}

in the northwest.

Rocks belonging

to the

Adelaide Supergroup outcrop around

the northern

closure

and the

eastern

limb.

Several magnetic markers

(US-T'

CH1, CH2,

Bp-KRS, TC-KRS

^and

NG2)

have been used

to

deüneate

the structure.

Stratigraphic boundaries ^are uncertain and ^since geological information is available mainly on a regional ^scale, magnetic

interpretation

can provide the necessary control for

future

mapping'

The northern ^closure of the

major

syncline is best defined by the magnetic marker US-T in

the Ulupa Siltstone.

^{US-T can} be clearly followed

from

near

Dutton

^on

^the

^west

^{limb of}

^the

,yn.lin",

around

the

closure and then under

Murray

Basin cover east

of the

Milendella Fault

till

south of Red Creek. Figure 7.13 shows the relation between mapped outcrop

of

the Ulupa Siltstone and inferred Ulupa Siltstone magnetic anomaly'

North

of

Australia

Plains, linear magnetic anomalies, possibly

in

the Adelaide Supergroup, have been followed

on the

eastern

limb of the Syncline.

These anomalies

are

truncated by

the

post-tectonic intrusives

of the EMZ.

Kanmantoo Group rocks have also been mapped ^as forming a fold closure in the north (Coats and Thomson, ^1959; Drummond

,1972).

^{But magnetic}

uoo-J", within this

Group,

if

present

in

the

north,

could

not

be resolved from

BMR

data.

Several magnetic markers,

CHl,

CH2, BP-KRS, and TC-KRS were used to map the southern closure. The hinge of the Karinya Syncline is sharp and this is demonstrated by fold closures in

the

Talisker Calc-siltstone and Backstairs ^Passage Formation. Subsequent cross-faulting of the ümbs has added to the discontinuous nature of the magnetic markers, which are unlike the more continuous anomalies

typical of the KRSZ. Illuminated total

magnetic

field

images (and the detailed geology map produced by

Mills,

1964) have been

particularly

beneficial

to

the mapping of these

folds. Thisls

because on the images anomalies appeaï

to

be continuous and the effect of minor

faulting

is suPPressed.

Magnetic anomalies

in

the Carrickalinga Head Formation and Backstairs ^Passage Forma,tion are

better

developed on

the

western

limb than

on

the

eastern

limb.

^{The western}

^{limb of}

^the

Karinya

Syncline

is

shown

in

Figure 7.14. BP-KRS is folded

into

a syncline-anticline-syncline combination. The anticline is ^a

tight

fold and shearing may have taken place along the hinge or eastern

limb.

CH2 is similarly folded. CH1 appears on the southwest ^of the Karinya Syncline but is faulted

out

and does

not

appear

further north.

The anomaly

TC-KRS

generally follows the mapped Karinya Shale (considered

by

Coats and Thomson, 1959, ^as equivalent

to the

Talisker Calc-siltstone)

but

there are gaps of several kilometres

in

places. Bull's-eye anomalies on CII2, BP-KRS and TC-KRS may be the result of

tight

folding

or

the effect of several deformations.

In

the southern

part

of the Karinya Synclinorium, the Milendella Limestone Member (which is

within

the Blowhole Creek Siltstone Member of the Carrickalinga Head Formation

-

^Cooper,

lggg)

acts as an excellent

lithologic

marker and forms ridges on the western and eastern lirnbs.

The limestone has a

distinct

low radiometric signature and because

of

the contrast between

it

and the surrounding arkoses, siltstones and granitic tocks,

it

can be easily mapped from digital images of radiometric data (Plate

a).

The incompetent limestones become strongly deformed in

the anticlinal fold

closures and

this is

clearly seen

in

Plate

4.

The mapping

of the

Milendella Limestone Member from aeroradiometric data is another example of the complementary nature of aeromagnetic and aeroradiometric interpretation'