Diseussion of unlt 6

3 CYCLES

2.7.3 Diseussion of unlt 6

Between Elder Range and Bunyeroo Gorge

unlt 6

comprises

a

^proxlmal

calcareous tempestite facies ^generated

by

frequent storm wave reworklng ln

an

environment supplied

with

abundant carbonate

mud.

To

the north

and east

there is a transition into a

more

distal

tempestite facies indicatinB ^less frequent and less intense storm influence suggestive

of

increasing water

depths.

The gradual

transition

from proximal

to distal without

evidence of steep slopes (except

locally

as discussed

ln

^chapter

³⁾

^suggests

^a

^ramp

morphology

for the shelf at this time. ^This will

^{be discussed}

in

^more

detail in

^chapters

³

^{and 6.}

Like units 5 to 7 at

Bunyeroo ^Gorge,

the laterally restricted

mixed

carbonate/siliciclastic facies

to the

south and west

of the study

^area

clearly

represents

a

proximal tempestite

facies.

However,

the very

obvious cycles remain

to

be

explained.

The cycles are

not readily

apparent

in

the adjacent

correlative

carbonate dominated sequence, and

there is

no ^positive evidence from sedimentary structures

that

^each cycle

is a

response

to

any

significant

shallowing

or

deepening

of the shelf.

^{Small depth changes may}

be responsible

for the thickening

and

thinning of

HCS sets

in the

upper

parts of

cyclqs,

but in

^general,

an

open middle

to inner shelf

sedimentary environment always below fairweather wave base

is indicated.

Hence, the cycles seem

to

be

largely

related

to

changes

in the rate of

supply of

detrital siliciclastics,

which ^probably had

a

westerly

deltaic

souree as

indicated

for

petrographically

similar

sandstones

in other units

and

underlying

formations.

Such periodic sand pulses could be generated by phases

of delta

progradation and

delta

lobe switching,

in

concert

with

basln subsidence. However

delta

lobes

usually

occur

in

thickening and shallowing upward sequences and would

not

be expected

to

be as

laterally

continuous or as uniform

in

thickness as

the

cycles described

here. Alternatively,

minor

fluctuations of relative

sea

level

along

a very

low gradient shoreline could

significantty

change

the point of

sediment

input

and hence

the

distribution

4l

of clastlc

sediment on

the shelf.

Such regular small alternatlons of sea

level

could be

driven by climatic

cycles

lf ^polar

lcecaps were present

at the

time.

.

^Such

ls the

case

in the

Eastern GuIf

of

Mexico, one

of

the worlds malor modern carbonate

shelves.

However

it is

^unusual

ln

currently

having a

domlnantly

siliciclastic inner shelf zone.

Doyle (1982) indicates

that

during

low

stands

of

sea

level, rivers

^provlde abundant

detrital

^sand

to the shelf, but during high

stands, no sand

is

added

to ^the

^{system and}

carbonate sediments- encroach onto

the

narrow remnant

siliciclastic

^shore

zone from

a(iacent

carbonate gen€ration areas, producing carbonatization of

the

underlying

sands,

The

net result is a cyclic

sequence

of

siliciclastics and

shelf

carbonates. ^'

Climatic cycles could have

a

more

direct

influence on

the input

of sediment

into the

basin

if they

are responsible

for the

generation of

alternate wet

and

dry periods.

During

dry

periods, when

river output ís

at

a

minimum, carbonate sedimentation dominates on

the shelf.

During wetter periods

river output greatly

inereases and

the delta

edge

rapidly

^progrades across

the inner shelf with the resultant

deposition

of

mixed carbonate/

siliciclastic sediments,

Climatic cycles would also ^probably

affect

the

ràtes of

carbonate

production.

_Such

_climatic

change can be

driven

^by Milankovitch cycles which are purturbations

of insolation

related to periodic changes

in the earth's orbital parameters. In the

Recent these have ^periods

of

21,000 (precession

of the

equinoxes), 41,000 (obliquity cycle), and 95,OOO, 123,000, and 413,000 years

(eccentricity

cycles)

(Imbrie, 1982; Olsen,

l936).

These periods seem

to

have remained constant

for

long ^periods

of

geological

time,

There

is

convincing evidence

that

Milankovitch

cyclicity

influenced Quaternary

glacial

cycles ^(Hays

et

al., 1976; Imbrie, 1982) and

a

number

of

authors ^(e.9. Barron

et al.,

1985;

Olsen, 1986) have suggested

their

influence on

the

geological record during

non-glacial

^periods, mainly through

the

generatlon

of alternating wet

^and

dry periods. If

Milankovitch ^cycles

are

responsible

for the unit

⁵

42

cyclicity

^(and

other

cycles

in the

Wonoka Formation), cycle thickness would

imply that

one

of the

longer perlod cycles had

the

dominant lnfluence on the

palaeoclimate. Of

these

the

95,000

year eccentriclty

cycle seems

to

be the most geologically

significant in

Phanerozolc sequenees. However, the

thickness

of the

^eycles

fn unit 5

(average ,8 m) would

imply a rather

high sedimentation

rate for a relatively

stable

shelf of

84 m/million ^years,

lf this

cycle were

responsible.

However,

it

should be noted

that there

are no

palaeontological

or

radiometrlc constraints on

the

sedimentation

rate

of

this

Precambrian sequence.

If the

413,000

year

cycle were responsible, the implied sedimentation

rate is

approximately 19 m/million

years.

The fact

that

cycles sometimes occur

in

^packets

of four is

^evidence

that the

95,000

year (or

_less

likely the

123,000

year)

cycle may

relate to the

main sediment cycles,

with a

weaker

overprinting

influence from

the

413,000 year cycle.

The Wonoka Formation was probably deposited

at

low palaeolatitudes

(inferred

_from palaeomagnetic

data of

McWilliams

&

McElhinny, 1980; and McWilliams,

lgSl)

and

thus the

wet

part of a climatic

cycle would be

expected

to

be associated

with the

^gradual development

of

deep weathering and

lateritization

on

land.

^Erosion

of this

material could be one

explanation

of the

reddened upper portions

to the

sandy

top of

most cycles, The reddened

tops are

also associated

with the

waning phase

of

sand input and

a

considerable decrease

in the rate of

sand supply as indicated

by

the increasing carbonate

content. This

^provides

the alternate

explanation that

a

decreased sedimentation

rate

allorved more complete oxidation

of

mafic

detrital

components such as biotite,

The much greater abundance

of

HCS

in the

sandstones could be explained

by rapid

sea

floor

cementation

of the

lnterbedded carbonates following depositional

events. As

discussed

later

^(2.9.2),

the

stylonodular texture present

in

^some

of

these carbonates was apparently

initiated

during very

early lithification.

Subsequent wave aetion would produce

the

intraclast horizons common

ln

these

beds. In

contrast, slow

lithlfieation

has allowed

43

conslderable storm reworklng

to

^occur

in the sands.

Tucker ^(1982b,c) suggested storm

disruption of incipient

subaqueous hardgrounds

to

explaln

the orlgin of lntraclastlc shelf

llmestones lnterbedded

with

^sandy

tempestltes

ln ^{the late}

Precambrian

of

southern Norway.

2.A

UNIT ⁶