Chapter 8

TP-EB rtt r-l

Chapter 8 Chapter 8

Chapter ⁸

Discussion and concluding ^remarks

With

the resurgence of interest in the Kanmantoo Group, new theories about the tectonic history

of the

region have been advanced (e.g. Jenkins, 1986; and

in

press; Clarke and Powell, ^1989;

Steinhardt,

in ^prep.).

Proofs

for the

various models are

difficult to obtain. In the

previous chapters,

the

magnetic response and structural interpretation was ptesented. Several subareas requiring

further attention

were

identified. On the

basis

of

th.e research carried

out for

this thesis, different models are reviewed using geophysical and geological

facts.

On many issues, geophysical interpretation may not directly contribute to the geological questions. The following section relates

to the

^{central issues}

^listed in

Section 1.3 and reviews current theories

of

the deposition and subsequent history of the Kanmantoo Group

in

a geophysical perspective.

The following discussion is based on the geophysical interpretation ^presented

in

the previous three chapters and on geological work carried out by other researchers.

This

chapter focuses on those aspects of the deposition, deformation and metamorphism of the sequence which are most controversial.

Of the

newer ideas suggested,

the

most interesting are

that the

Kanmantoo Group

is

al- lochthonous,

that part

of the Kanmantoo Group has had a different metamorphic history from

that ofthe

Adelaide Supergroup, and

that

the sequence represents a stack

ofthrust

sheets. These ideas have arisen

in

response

to

the need

to

explain the apparent thickness of the secluence and the high grade metamorphism of parts of the Kanmantoo Group ^(see section 1.3).

The discussion of the

important

geological issues is followed

by

the concluding remarks.

8.1 The development of the Kanmantoo Group

The

deposition and metamorphism of

the

Kanmantoo Group are discussed belorv. The major structures and the structural

history

were presented

in

the previous chapter.

8.1.1 Deposition Age of sediments

Kanmantoo Group metasediments are essentially non-fossiliferous (Section 1.2.1) and early ^re- searchers (e.g. Mawson and Sprigg, 1950) and recently Steinhardt

(in prep.)

have suggested

CHAPTER 8.

DISCUSSION ^,A.ND CONCLUDING REMARKS ¹⁰⁶

that the

sequence may be

partly or wholly

coeval

with the

Proterozoic Adelaide Supergroup.

However

*ork by

sprigg uoã

cu*purra

^{(1953), Daity} (1956), Campana and Horwitz (1956) and Gatehouse

et

^at-.

(ii ^p.ãrr)

among others discount

this theory.

^{Though there is ümited control} on the age of the Kanmantoo Group, a Cambrian, rather than

partly

Precambrian' ^age is more consistent

with

all the known geological data.

The

Carrickalinga Head Formation

is

considered

by Daily and Milnes (!972a) to be

the basal

formation

^of

the

Kanmu,ntoo

Group.

Few distinctive fossils have been found

within

this formation

but

compared

to

the other Kanmantoo Group formations,

it

^{is the} most fossiliferous.

Daily

(1g63) found the brachiopod,

Lingutella\n

^{the basal 10m of}

the

Carrickalinga Head For- mation at Carrickalinga Head, and

Daily

(1977) found the Lower Cambrian

trilobite

Redlichiain what

Daily

and

MilnÃ

(1973) believe to be lower Kanmantoo Group rocks on l{angaroo Island.

Gatehouså

et

^al.

(inpt"r.)

reported Hyolithiitae

at

Carrickalinga Head.

This

^places

the

age of deposition of the òarrickalinga Head Formation

in

the Early Palaeozoic. Worm tracks and ^casts urã

th"

only other fossils

to

have been found

in

the lower five formations'

Daily

and Milnes (1g71b) state

that

"where Lower Cambrian rocks are overlain by Kanman-

too

Group rocks

without structural

discordance, confotmity between

them

can be expected".

This is

supported

by

investigations

in the

following

regions:

Delamere

(Daily,

1963; Leslie, 1962),Red Creek and ^Sedan

Hill

sections (Gatehouse

et al.,in

^press) and

at

Carrickalinga ^Head

1Uuíátt"tow, lg7g).

^Note

that

on

the Truro

1 : 63 360 scale map (Coats and Thomson, ^1959),

the relation

has been interpreted

to

be unconf rmable though

Daily

and Milnes (1971b) sug- gest

that

a

fault

contact

is

more consistent

with

the evidence.

This is

supported

by

magnetic

interpretation

(Section

7.3). ^Fault

contacts between

the two

Cambrian Groups have been in- ferred near Myponga, ^east

of Australia

^{Plains and}

in the Mount Barker-Mt'

Torrens area by Mancktelow

lirizo¡.

The transitional/conformable relationship between the Carrickalinga Head Formation and the underlying Cambrian Normanville Group reinforces a Cambrian ^age

for

the

formation.

(There is

limited

palaeontological evidence

for

a middle

Early

Cambrian ^age for the Heatherdale Shale, Normanville Group, ^(Jago

et

a1.,,1984; 1986) which is conforrnably overlain

by the

Carrickalinga Head Formation).

The timing of Ordovician granite intrusions into Kanmantoo Group rocks ^{places a} lower

limit on the age.

Dasch

et

aI.

(LgTl)

^dated

^the

^granites and metasediments around the Ðncounter Bay area and determined an ^age

of

⁴⁸⁷

+

60 Ma

for

Kanmantoo Group metasediments'

In the type section, successive Kanmantoo Group units have been shown by Daily and Milnes (Lg12auoaiOZe; to be

in

apparently conformable contact which places the seclrence as awhole

i' tt

" ^Early

Palaeozoic and

younger.

Questions regarding

the

acceptance

of tire

stratigraphy proposed by

Daily

and Milnes (1972a) have been raised by Jenkins

(in

press) and Steinhardt (in

nr"n.)

^and

^will

^{be discussed}

ⁱⁿ

a subsequent section. Obviously, while

the

Carrickalinga ^Head Formation

is

Cambrian,

if

the formations above are

not

conformable,

they

could be older than

the

Carrickalinga Head Formation.

In the

Williamstown region,

Ofler

(1966) has recorded three deformations

in

the Adelaide Supergroup, of which the later two affected the Kanmantoo Group rocks. This is again consistent

with

a younger ^age

for

the Kanmantoo Group ^compared

to

the Adelaide Supergroup.

Stratigraphy

The

classification developed

by Daily

and Milnes

(I972a)

and applied

by

Mancktelow (1979) has been used throughout

in the

correlation

of

magnetic anomalies

with

stratigraphic units'

The

classiflcation

is

based

entirely on lithology. The most

common lithologies are ^atkoses,

CHAPTER 8.

DISCUSSION A¡üD CONCLUDIAIG REMARKS

greywackes, and

pelitic

units and these may be found

in

any

unit

and are

not

distinctive of any

individual

^formation.

Based

on similar

lithologies,

the

^{sequence can}

be

simplified

from the original

eight for- mations

into four

subunits:

KGl -

Carrickalinga Head Formation, KG2

-

Backstairs ^Passage Formation, KG3

-

Talisker Calc-siltstone, Tapanappa, Tunkalilla, Balquhidder and Petrel ^Cove Formation and KG4

-

^Middleton Sandstone. This subdivision is convenient

for

analyzing ^pos- sible repetitions

in

the sequence.

It

should not taken

to

suggest

that

the classification proposed

by Daily

and

Milnes (I972a)

should

be revised. Subunit KG1 is

characterized

by

siltstones and occasional argillaceous and pure limestones; KG2 and KG4 are dominated

by

arkosic sand- stones; and KG3 as a whole consists of a sequence of siltstones and greywackes intercalated with siltstones rich

in iron

sulphides.

The subdivision defined

in

the coastal section has not been applied far

inland.

Going inland

from the south

coast,

the thick

^sequence

of

greywackes and

pelitic

rocks can

no

longer be subdivided

into the

Tunkalilla, Balquhidder and Petrel ^Cove Formation and has instead been mapped as Tapanappa Formation (Mancktelow, 1979).

This

could be due

to

non-exposure of

the

younger

units or may indicate that the stratigraphy

deflned was

too

complex and

that

subdivision

into

these formations

is not warranted.

So KG3 would consist

of two

formations (Talisker Calc-siltstone and Tapanappa Formation) rather

than five.

Note

that

failure

to

map individual ^younger units on the Monarúo sheet may be due to the fact

that

few detailed mapping studies have been undertaken subsequent

to

the

Daily

and Milnes' (I972a) classification.

There is no difference between the ^arkoses found

in

the Backstairs ^Passage Formation and the Tapanappa

Formation.

These formations cannot be distinguished

in

regions of arkose outcrop unless the intermediate Talisker Calc-siltstone ^can be mapped (Mancktelow, 1979). Stratigraphy of large areas

of the

Tepko and Angasúon ^sheets

is

therefore

uncertain.

The sandstones of the Middleton Sandstone and the Backstairs ^Passage Formation are also similar

but

the epidote rich segregations characteristic of the former have not been reported

for

the

latter.

The Kanmantoo Group has been described by

Daily

and Milnes (7972a) to be a conformable sequence of

units

as described

in

the

type

section along

the

south coast of Fleurieu Peninsula.

The two main problems

with

accepting

this

are the enormous inferred thickness of the ^sequence and the repetition of lithologies both

within

and across

unit

boundaries. Current theories (Jenk-

ins, in

pïess; Steinhardt,

in prep.)

suggest

that the

thickness has been greatly augmented by

thrusting.

Jenkins

(op. cit.)

has drawn attention

to the

appearance

of

carbonaceous phyllites

at

the base of

the

Tunkalilla, Balquhidder and Petrel Cove Formation which he suggests could represent

thrust

surfaces. Similarly, Steinhardt

(in prep.)

gives examples of a number of units

which

could have acted as detachment surfaces

during thrusting: the

marble

at

Macclesfleld which lies below

the

Kanmantoo Group,

the

Taiisker Calc-siltstone below the Tapanappa For- mation and the various sulphide-rich units

in KG3.

However, no field data have been produced

to

support

this

model.

Steinhardt

(op. cit.)

^{goes on}

^to

^suggest

that the

Tapanappa

f'ormation

may represent re- peated Carrickalinga Head

Formation.

There are several differences betrveen these

two

for-

mations.

Though

their

geochemistry

is similar

(Table 2.7), fossils found

in the

Carrickalinga Head Formation have

not

been found elsewhere,

iron

sulphides which appeal

regulally in

the Tapanappa Formation

first

appear

in the

^Kanmantoo

Group

sequence

in the

Talisker Calc- siltstone and

the

argillaceous and pure limestone found

in the

Carrickalinga Head Formation have

not

been reported

for the

Tapanappa

Formation. The

subdivision

of the

Carrickalinga Head Formation

into

three members on the south coast has been repeated

by

Gatehouse eú ¿l'

(in

press)

at

Sedan

Hill.

These differences support

Daily

and

Milnes'

(I972a) classification of the two as separate formations.

107

CHAPTER

8. DISCUSilON AND CONCLUDING

^{REMARKS 108}