PLATE 2.4

3 CYCLES

PLATE 2.4 PLATE 2.4

il'

t

i.;

considerable amount

of

carbonate

is still ^present. Llke unit 5 there ls

^a

transition

from proxlmal tempestite facies

to

more

distal

facles

to

the

north

and east

of the

Brachlna and Bunyeroo Gorge region.

2.9

UNIT ⁷

2.9.1

Lithofacies

of Unit

⁷

In the

CFZ and SFZ

unit 7 is

^generally

about I00

m

in

^{thickness and is}

characterized

by

grey-green argillaceous and

silty micritic

limestone, and

lesser

red

sandy and glauconitic

limestone,

^There

is

^generally

^very little

interbedded

shale,

The limestones

are

usually. somewhat

purer than

those of lower

units.

Planar lamination, small-scale and ^{normal HCS} [Plate

2.5 (A-

E)1, climbing wave

ripple

lamination [Plate

2.5

^{(F,G)1, polygonal}

interference

ripples,

and various forms

of soft

sediment deformation are the most common sedimentary

structures,

Stylonodular bedding (see 2.9.2) is abundant and

frequently

destroys,

or partially

destroys

the

original sedimentary

fabric.

Intraformational conglomerates,

either

as continuous beds

or

comprising

pot

_and

gutter

_moulds

are

common and increase in abundance towards

the top of the unit

lPlate

2.6 (C,F)],

_The abundance of

pot

_and

gutter structures

also increases tou'ards

the top of the unit,

^and

gutters

become more sinuous and develop bifurcations

in

upper beds.

Unlike

units 5

and

6

which show

significant lateral

facies changes,

unit 7 is very

consistent

in

facies and thickness across much

of the

study

area.

However

at

OId Wirrealpa

a

more

distal

facies,

i.e.

thin-bedded calcareous tempestites interbedded

with

shales,

is

^present

in the

lower

part.

Very

rapid

facies changes

at this level

along

the

C/NFZ boundary are described

in 3,4.

The

first

^evidence

of very

shallow deposition

(within

_the

photic

zone)

in the

Wonoka Formation occurs

in the

upper few metres

of unit 7 ln ^the

form

of a

widespread stromatolite bearing horizon described ln

2.9.4.

46

2.9.2

Stylonodular Llmestones

Stylonodular beddlng (Flugel, _1982,

pg. 93) ls a

secondary

structure

ln which

the

rock

is divided lnto

small ovold nodules

of

mllllmetre to

centimetre size

by a

series

of horizontally

anastomosing

stylolitic

sheets

[Plate

2.7

^(A,B)1, Stylonodular llmestones oçcur

in units 5 to 7,

and in

thelr lateral

equivalents

further north,

However,

it is

most abundant ln

unit 7,

where

it

^{may sometimes}

^{affect up} to

80%

of

grey-green limestones ln

the unit, often

completely destroying

the original fabrfc, Alternately,

the

original fabric

may

still

^be

^visible

^as

^the larger stylolites

selectively following bedding planes,

or outline

clasts

ln

intraformational

conglomerates. Stylonodular

textures

have

not

been observed

in ^the

^red

Iimestones.

Indlvidual

nodular limestone beds

often

show

a

reverse nodule ^size grading [Plate

2.5 (H);2.6 (A,B)].

_{The base}

of the

bed

is often free

of nodular

texture. A short

distance above

the

base

the first

nodules appear,

but are

small and

indistinct at first,

increasing

in

^{size and prominence}

towards

the top of the

bed, where

the

nodular

texture is

sharply truncated, despite an

identical overlying lithology. Individual

beds range from

a

few centimetres

to

over

0.5

m

in thickness.

Occasionally

the

upper surface

of

^a stylonodular limestone bed shows evidence

of

eonsiderable erosion which may

produce overhangs and

nirþcles that

could

not

have supported themselves unless

the

limestone was

at least partially lithified at the time of

erosion [Plate

2,6 (E)].

_The erosion surface

often cuts

around

individual

nodules [Plate

2,6

^(D,E)1. These observations suggest

that

nodule formation was

initiated

on

the

sea

floor shortly after

deposition and

prior to

erosion.

Nodule formation was probably related

to

subaqueous hardground formation between storm

events.

The

early

formed nodules were

later

enhanced by

stylolite

formation

during

subsequent compaction and dlagenesis,

2.9.3 Unit 7

Stromatolite Horizon

Stromatolltes have been observed

in the

uppermost beds

of unit 7 at

the

46

PI-A'I'E 2

. E;

FIELD PITOTOGRAPHS: UNIT 7

A

HCS

in

50 cm

red limestone.

Note

flat

basal laminae,

central

hummock¡¡ zone and upper

flat

laminae, eharacteristie

of the

idealized HCS sequence

(Dott &

Bourgeois. 1982, 1983;

lValker

et al.,

1983; see

Fig. 1.4),

The angle

of

truncation

is

e.xadurated

by the

oblique nature

of the photograph.

A

wave rippled

top is

absent from

this

example,

but is

^present

on

the top of

some HCS beds

in this interval,

Small-scale

HCS

is

^present

in

underlying

beds, This

example

is

from the Beltana area,

B

Portion

of a thick (2

m) red limestone

with

amalgamated

eharacter.

The base

of the

hammer rests on

a lenticular

red shale horizon which subdivides

the bed,

^The

first-order

boundaries between separate e\.'ents are more prominent than

the

seeond-order hummock¡' truncations (best seen

at

top

right),

due

to

selective

stylolitization.

Wilkau'illina Gorge

s ection.

C

Small-scale HCS

in a 7

cm bed

of

grey-green

silty

limestone

in line with the

base

of the

hammer, \Vavelength is

approximately

40 cm.

Weathering

hiehliehts siltier

laminae and

the

stylonodular

texture at the top

and bottom

of

the

nl.rnfna¡nn}l ôñ^ ìn +Lrn l^-^ ^l-^"a +L'^ Uno ¡'^l D-^^L:-- p¡rvLv6¡ aPrr, 4¡ru ¡tl L¡rç ¡ç¡lù 4vvv ç Lttç ltLJ vçu, uf 4uJlIltd

Gorge section.

D

Smail-scale HCS der¡eloped

in a

12 cm bed

of silty

limestone below

the

hammer. Waveiength

is

approximatel¡' 30

cm.

Note

the

nodular

top of this bed.

Brachina Gorge section.

E

Smali-scale HCS

in silty

limestone dispiaying

a

relarir¡ely steep angle

of truncation

^(approx. 25o) uncharacteristic of larger forms

of

HCS. Brachina Gorge section.

F Verticall¡'

climbing wa\¡e

ripple

laminae

in

^green limestone.

Such bed forms

are a very

common occurrenee

in units 5

and ⁷ Top surfaces

of

such beds generail¡' display

low

amplitude pol)'gonal interference

ripples, or longitudinai

wave ripples.

like

those

in

Plate

2,4 (H).

_Mayo Gorge secrion.

c Vertically

climbing wave

ripple

iaminae capped by

stylonodular

limestone.

Brachina Gorge section.

H

Large-scale styionodular

texture

developed

in

^green

silty limestone,

Itiote

the

coarsening upward tendene¡'. and the stromatolitic appearence

of

some

nodules.

Such examples have been mistaken

for

stromatolites

in the past,

Warraweena section.

Hammer

is

32 cm long