CHAPTER 8, DISCUSSION AND CONCLUDING REMARI{S 109

Mancktelow (1g7g) and

Boord

(1985) believe

that the

Backstairs Passage Formation and

Middleton

^Sandstone

we." d"porited in

shallow marine conditions while

the other units

^were deposited

in

^{generally deep}

^water conditions.

Gatehouse

et al. (in

^press) suggest

that

the Carrickalinga Head Fãrmati,on ^was deposited in deep water at Carrickalinga Head, but

in

shallow

water

conditions

on

^Kangaroo

^Island

^and

in the

Sedan

Hill section. They

suggest

that

the Campana Creek Member

ãt ttt"

Carrickalinga Head Formation represents a shallowing upwards

,"qoão." ^which is a

precursor

to the

shallow

water, high

energy deposits

of the

Backstairs Passage Formation which prograded acloss the present area of outcrop.

The

Normanville Group and Kanmantoo Group could ^represent

a

shelf-slope combination off a passive continental margin (von der Borch, 1980). The

term

"Adelaide

Rift"

^{was proposed}

by

uo'

der Borch

(op.

cit.)

but

again there is

little

evidence for

rifting.

Boord (1985) considers

that

the basin was a passive continental margin

but

actively subsiding'

The aeromagnetic map ^shows two major geophysical boundaries

to

the west and east of the

Mt. Lofty

Ranges. To the west, off the coastline of Yorke Peninsula, a wide magnetic anomaly has been interpreted

by Gunn

⁽¹⁹⁸⁴⁾ as representing

the

ancient continental ^edge

with

the Adelaide Geosyncline developing ^{as a}

rift

on

the

"shoulder" to the ^east. The anomaly has ^been interpreted

to

be

left-laterally

^{faulted and}

it

continues

riglit into the

centre

of

South Australia.

Alternatively, the

anomaly could represent moderately deep magnetic basement.

The

second

major

boundary is the

Murray

Magnetic High

(MMH)

^{and this is discussed}

in

Section 5.2.1.

Limits of the Kanmantoo GrouP

schists intersected,

in drill

^holes in the

MMH

are often very low grade (wegmann, 1980; Lewis, P., 1gg5) and cannot be definitely correlated

with

Kanmantoo

Group' ^Tertiary

rocks and ^Recent sediments obscure basement on

the Milang

and

Alerandrina

^{sheets and}

in the

l\tlurray Basin' Aeromagnetic

interpretation

over

the Milang

and Alerand'rina ^{sheets shows}

that the

synclinal structures continue to open southwards and

that

the southern

limit

of the KNSZ, and therefore of the Kanmantoo Group, is not ^known on Fleurieu Peninsula.

The

migmatites and meta-arkoses found

in the ISZ

are

likely to

belong

to the

Backstairs passage

Formation. ^Further

^east,

^the

^{complex magnetic} signatures

of the

igneous intrusives charaiteristic of the

EMZ

overshadow the magnetic effect of possible Kanmantoo Group rocks.

In

^a few ^areas to the east of

the

Truro sheet, linear, relatively smaller magnetic anomalies, which are different to the anomalies ^caused by the igneous intrusives, may be ^caused by metasediments' There

is no control on the

age

of

these possible metasediments'

The

Glenelg River beds in

Victoria

are considered

by wãnr ltoso)

^{and Cooper} and Grindlay (1982)

to

^be correlatives of

the

Kanmantoo

Group.

However

this

does

not imply that

the basin

in

which

the

I{anmantoo Group sediments was deposited extended

into Victoria'

Basement to the Kanmantoo GrouP

The Kanmantoo Group may ^have been deposited on attenuated Normanville Group or Adelaide Supergroup rocks. Altãrnatively, ^as Steinhardt (in

prep.)

and Clarke and Powell (1989) ^suggest, thá se[uence may be allochthonous and was

later thrust into its

present position.

Much

of the

argument

in

favour

of

an allochthon ^depends on an older age and a separate metamorphic

history.

As has been shown above,

the

basal

unit of

the secluence is

likely to

^be

cambrian.

The second

point will

be discussed

in

section 8.1.2.

CHAPTER 8.

DISCUSSIO¡Ù

AND CONCLUDING REMARIß

¹¹⁰

As the northern

closure

of the Karinya

Syncline shows (refer

to

Section 7.3.1), Adelaide Supergroup

and

Kanmantoo

Group

metasediments are folded together around

the

syncline.

The

important Ulupa

Siltstone magnetic marker can be followed under cover almost down to Red Creek east of the Palmer-Milendella

Fault. In

this region, Adelaide Supergroup rocks must

lie

under

the

Kanmantoo Group rocks.

In the

EMZ, the strong magnetic anomalies caused by near-surface intrusive rocks make

it difficult to

locate narrow,

linear

anomalies

wliich

may be caused by metasediments.

South

of this,

strong anomalies caused

by intrusive

rocks

of the Murray

Magnetic High obscure any analysis of the continuation of metasediment type anomaly

in

the legion to the east of

the

Palmer

Fault

and Milendella Fault.

Again in the

Red Creek area, Carrickalinga Head Formation overlies Normanville Group,

the

contact

is

gradational and

the Truro

Volcanics

may

extend

into the

Carrickalinga Head Formation (Gatehouse, pers. comm.).

All

this is consistent with the Kanmantoo Group overlying Normanville Group/Adelaide Supergroup

in this

region and

of

having done so

at the time

of deposition.

Sediments

of the

Carrickalinga Head Formation occur on

both

sides

of the

anticlinorium

core.

There

is a

difference

in

facies: deeper

water at

Carrickalinga Head

(whicli

lies west of

Mt.

Compass

Inlier)

and shallow water on Kangaroo Island (Moore, 1983). Gatehouse et al. (in press) interpreted the formation

in the

Sedan

Hill

section as shallow water facies

-

^{they were}

able

to

apply

the

same three-fold subdivision

of

the formation

at

Sedan

Hill

corresponding to

that

exposed on the south ^coast.

In the Mt.

Barker Creek area,

the

contact between

the

Adelaide Supergroup and

the

Kan- mantoo Group must be low-angle and the two appear

to

have been folded and metamorphosed together.

Liu

and Fleming (1989) have been studying amphibolites

in the

Palmer-Tungkillo and sur- rounding regions. They have noted three generations of amphibolites: pre-D1, syn- to post-D1, and a

third

generation of undeformed amphibolite dykes intruded after the close of the Delame- rian Orogeny. They deduced from the geochemistry

that

the dykes have oceanic basalt affinities and

that the

basic magmas which produced these dykes may have been derived from elevated mantle under the Kanmantoo basin.

The evidence from gravity and seismic data is inconclusive. Magnetic maps provide no direct clues regarding the present basement to the Kanmantoo Group. Indirectly,

if

magnetic basement exists,

it must be

more

than

several kilometres

deep. The

thickness

of low to

moderately magnetic metasediments

in

the Kanmantoo Synclinorium is probably several kilornetres.

8.L.2 Metamorphism

Previous researchers (e.g.

Tate,

1879; Jenkins, 1986; Clarke and Powell, 1989) have found

it

incongruous

that the "structurally

highest sequence", i.e.

the

Kanmantoo Group, should have been metamorphosed to higher grades than the Adelaide Supergroup. Offier and Fleming (t968) have shown

that the

metamorphism was of

the

Buchan style

- ^{high T, low P.}

Undoubtedly, as Figure 1.3

in

Chapter 1 shows,

the

migmatite zone

is

confined

to

the metasediments of the Kanmantoo Group.

The Kanmantoo Group ^is

likely

^(see earlier discussion on stratigraphy) to be stratigraphically higher

than the

Adelaide Supergroup

- ^{the tetm} "structural

^highest sequence"

is

ambiguous.

CHAPTER

8.

DISCUSSrcN

AND

CONCLUDING REMARKS ¹¹¹

If

the metamorphism were due

to

depth of crustal

burial

alone, then the difference

in

maximum metamorphic grade between the Adelaide Supergroup and the Kanmantoo Group might be used

to

suggest

that the

Kanmantoo Group was emplaced as a

hot

allochthon (Clarke and Powell, 1g8g). However

the P-T

conditions reflect

a

severely perturbed thermal regime well

in

excess

of

that

expected for

the

conductive heating of tectonically thickened crust (Sandiford ^eú

al.,in

press). Clarke and Powell (1989) also ^agree

that

the perturbed geotherm is inconsistent

with

a

iectonically thickened crust alone. Thus

it

is not necessary to explain the higher metamorphism of the Kanmantoo Group rocks by a separate metamorphic history

(the

"allochthonous" theory)

or by

ascribing an older ^age

to

the rocks'

Within

the CMZ, the Palmer, Monarto and

Mt.

Kitchener Granite, the Rathjen and Tanunda Creek Gneiss, and

the

Reedy Creek Granodiorite

form

one

group; and the

^{Encounter Bay}

Granites and the granites on Kangaroo Island form another. There is ^a well-defined relationship between the spatial distribution of the

flrst

group of granites and the boundaries of the migmatite zone

but the

metasediments

into which they

have

intruded

have

not

developed discernable contact aureoles (Mancktelow, 1979). Magnetic trends and radiometric highs also bear a strong preferred orientation

to

the

NNW

trend of the migmatite zone which lies

within

the G2 gravity

corridor(O'Driscoll,

1983). Of

thesecondgrollpofgranites,thereisastrongspatialassociation

between

the

andalusite-staurolite zone boundary and

the

contact between

the

Bncounter Bay Granites and the country

rock.

The metamorphism around

Victor

Harbor appears to be directly related.

to

the intrusion of the Encounter Bay Granites (Mancktelow, 1979).

Mancktelow

(op. cit.)

suggested

two

explanations

for the relation

between

the first

gloup

of

granites and the

migmatite

zone: either the increased heat flow

in

the region produced the

gr-rit"r, or,

alternatively, the granites supplied the energy which caused the metamorphism of ihe sediments. The

first

is consistent

with

the lack of distinct contact aureoles

but

is incomplete

in that the primary heat

^source

is unknown. The

second explanation

is

consistent

with

the

interpretation

by Sandiford et aI.

(h

^press)

that

additional heat sources are recluired

to

explain

the

conditions

of

metamorphism.

The direct

correlation between the location

of

granites and

granite

gneisses and

the location of the

high-grade

belt

indicates

that the

simplest solution

-igþt

^be

^that

^the positioning of the granite intrusions is the cause of the high grade rocks being apparently confined

to the

Kanmantoo Group.

The

Palmer and Monarto Granite have been interpreted

by

Mancktelow

(op.

ciÚ.)

to

have

intruded

post-D1 (i.e.

after the

development

of the

Kanmantoo and

Monarto

Synclìne). But evidence óf

e.rly

thermal

activity

in the Central Magnetic ^{Zone has} been documented (Encounter

Bay Granites:

Milnes

et

al¿.,1977;

partial

melting

in

the migmatite zone, Fleming and White, 1g84; base

metal

mineralization

in the

Kanmantoo

Mine area:

Seccombe

et al.,

1985; ^pre- tectonic amphibolites:

Liu

and Fleming, 1989).

Apart from the

possible subsurface extension of the Rathjen Gneiss and a subsurface granite

in

the Dawesley area, deep magnetic sources are

not

apparent

in the

magnetic data, therefore precluding

the

existence

of

subsurface magnetic

graniies.

Other igneous

activity

includes the Woodside dyke swatm (Pain, 1968) and the only known volcanics

in

the

Mt. Lofty

Ranges,

the

Truro Volcanics which are intercalated

with

the Heatherdale Shale. Whether these igneous bodies would provide a sufficient quantity of heat to sustain the metamorphism is

not

known.

Madigan (1988) advocates

that

metamorphism commenced

with

the intrusion of granites into the sediments and

that

metamorphism continued while deformation produced an early stretching

lineation.

Mancktelow (1979) ^suggests

that

^peak metamorphic temperatures coincided

with

^D1

deformation. Prior to the intrusion of the

post-Delamerian intrusives

in the EMZ, the

rocks were cooling.

CHAPTER 8.

DISCUSSION A¡ÙD CONCLUDI¡\TG

REMARI(S Lt2

8.2 Concluding remarks

The geological problem of studying the Kanmantoo Group is typical of ^some terrains: the area occupied is large, lithological marker horizons are rare and outcrop

is

often weathered and in some areas

non-eústent.

Though much work has been done, there are gaps

in the

continuity and also considerable variation

in

the quality of available information.

The acquisition of high-resolution aeromagnetic data ^has made

it

^{possible to} study the entire area independent

of the nature of the

overburden.

It

was

thus

possible

to

present

a

unified interpretation of the rocks based on the difference in their magnetic properties. The stratigraphy and structure

of the

area have been reexamined

from a

magnetic perspective. Rock property studies, aeroradiometric interpretation and the results of previous geological investigations ^have been used

to

constrain and improve the aeromagnetic interpretation.

The advantages of reprocessing geophysical data and of using different presentation formats cannot be over-emphasized. The applications of aetomagnetic data have been extended beyond

that of

being

a

reconnaissance

tool

and an

aid to

geological

mapping.

There

is a

wealth

of

information inherent

in the

data set which makes aeromagnetic

interpretation

relevant

to

the study

of

subtle and varied geological problems. The extraction

of this

information

is

critically dependent on

the

processing techniques used. The transformation of the

interpretation into

^a geologically intelligible model requires the study of the relation between the magnetic properties of rocks and

their

geological evolution.

The

interpretation

of a geological terrain

is

an on-going process. As new data are collected

and

new ideas emerge, previous models

will be updated,

tested and

improved.

^{Basic rock} property data, the results of magnetic modelling and the

information

on the maps included in this thesis should provide more lasting factual

information.

Anomalous areas have been targeted where detailed geological investigations might reveal evidence crucial

to

an understanding of the early history of the Kanmantoo Group.

It

is hoped

that

the maps and interpretation presented here

will

provide a basis for

future

research.

Detailed data Regional data Flown for CRA Pacific

Bxpl.

CRA SADME

BMR

Year 1979 1979 1980 1978

Digital magnetic data

Yes Yes Yes No Yes

Digital radiometric data

Yes Yes Yes No

nominal ground clearance (metres)

B() 80 80 752 150

nominal flight line spacing (metres)

300 150 300 1609 1500

nominal sample spacing (metres)

25 25 25 continuous

recording 55

flight

line

direction BW BW EW BW E\M

Area

Mt. Lofty

Ranges

Echunga Murray Bridge

Mt.

Lofty Ranges, Kangaroo Island

Adelaide- Renmark

Table

4.1:

Aeromagnetic survey details.