PLATE 2.11

of

siliclclastics.

oolds

with

combined

radlal

and coneentrle

fabrics

[Ptate 2,10 ^(D)] have been found immediately above an eroded hardground developed

wlthin

^ooid

grainstones

at

lVllkawilltna

Gorge.

The hardground surface, below ^which ooids and

other particles

show evidence

of

dissolution and ^geopetal

collapse,

is

^encrusted

^with

^minute

stromatolite-like

structures described in

2.12.9. Calcrete-like

material

is

^also

^present. Of particular interest

is

the

^presence

of very weli

^preserved

fine

grained concentric ooids composed

of

dolomite

within

recrystallized limestones

at the

Black Range Spring section [827-BS1; Plate 2.10

(E)1.

Rare examples also occur

at

^Bunyeroo

Gorge,

2.1g.2.2 Discussion: Brick textures

^were

first

described bV

nþefto

and

Folk

(1976) and

later by

Tucker

tfSaS|

^{and Singh}

(1987).

These authors provide convincing petrographic and geochemical evidence

that this

texture

results

from

the

replacement

of original

aragonite

by calcite.

^Neomorphic

pseudospar replacing ooids

with or without relic

lamellae has been described

by

Rich ^(1982), Tucker (1985), Wilkinson

et al.

(1985) and Singh (1987), and

is

^also eonsidered

to indicate

replacement

of aragonite.

^{The wider} implications

of

Proterozoie aragonitic ooids

is

discussed

in

6.4,

The

radial

ooids from Wilkawillina Gorge suggest

an original

calcite mineralogy (e.g, Wilkinson

et aL, 1985). Their

association

with a

corroded

hard

^{ground and possible calcrete}

is

consistent

with

deposition

in

the

vadose zone and hence

the

term vadoid

(Peryt,

_IgSg) may be more appropriate

than ooid.

The

well

^preserved dolomitic ooids preserved

within

limestones were presumably composed

of

^dolomite

at the time of

limestone deposition and

it is

tempting

to

^suggest

that they

may have been formed as primary dolomite, as suggested

by

Tucker ⁽¹⁹⁸²⁾

for well

^preserved Precambrian dolomitic ooids from

the

Beck Springs Dolomite,

California,

^However

contemporaneous

or very early

diagenetìc replaeement can produce

well-

6l

preserved prlmary

fabrics ln

dolomite (Zenger, 1982), and

this

followed ^by reworking

Into a

calcareous envlronment

is

probably more lil<ely'

2.f

3.3

Stromatolites

of Unit lt

Several

different

forms

of

stromatolite. occur

withln unit

^I

I.

^{The most}

common form has been described

by

Walter

et al.

(1979), who compared

it

to Tungussia

julia

from

the

suspected age equivalent

Julie

Formation

of

the Amadeus Basin

in central Australia. In unit ¹¹

^{Tungussia cf}

.

T.

iulia

occurs

in

isolated spherical

to eliptical

bioherms IPlate

2.9

^(E,F)l which occasionally reach

I

m

in height

and several metres

in width.

Bioherms may

be closely spaeed,

or

many metres

apart,

and generally ^grow on hardground surfaces

within, or on top of cyeles.

The bioherms are composed

of

^a

dendritic

branching and coalesing

array of

closely spaced columns radiating from

a central core.

The columns, which may be elongate

in plan view,

are

inciined

near

the

edge

of the

bioherm, sometimes

to the point of

being

horizontal.

Stromatolites, which

are

probably referable

to

Tungussia occur near

the top of the

wonoka Formation

in a

carbonate

build-up

surrounding ^a syndepositional

diapir

along

the

C/NFZ boundary (see

3.7'1)'

Several other forms

of

stromatolite, including

large

domal and columnar

varieties,

are occasionally observed near

the top of unit

I

I

^(e.g.

at

Witkawillina Gorge

and Reaphook

Hill), but

have

not

been studied

in

detail.

Gehling (1982)

briefly

describes another form

of

stromatolite from

unit

11 along Chace

Range.

The author has examined Gehling's coLlected material,

but

has

not

observed

this

form

in the field. In

hand specimen this stromatolite consists

of

closely spaced

vertical,

and

frequently

linked columns

0.5 to 2

cm

in

width.

Minute

stromatolite-like structures are

found encrusting

a

hardground surface

within

ooid grainstones

at

Wilkawillina

Gorge.

Fragments

of

similar

material

are found as

intraclasts on top of

hardgrounds

at

Bunyeroo Gorge.

These

structures are black in

hand speclmen and have domal

to

branching

62

dtgitate

morphologies. The columns

are

one

to

several mllllnretres wlde and

up to

6mm

high,

Petrographlcally

they

conslst

of radial

arrays

of

fibrous

calcite,

long axes perpendicular

to

concentric accretlonary lamellae, and

dlsptaying chevron

twlnning patterns

lPlate 2,10

(C)1, _X-Ray

diffractlon indicates

the

^presence

of

minor carbonate

fluorapatite within

these struetures, although

this

has

not

been

identified in thin section.

Such

structures fall into the

minlstromatolite ^category

(0.2-10

_mm)

of

Hofmann (1986), who distinguishes

radial-fibrous

aggregates

that are of

dubious biogenicity, from forms composed

of

^granular elements,

that

are more

likely to

have been formed

by

microbial

activity.

Schmitt ⁽¹⁹⁸⁵⁾ has described

structures of similar

size and petrography as stromatolites from the Carboniferous

of Spain.

Those, however, were associated

with

algal

filaments. It is

concluded

that the

structures described here were

precipitated inorganicaily and probably represent

travertine that

was

deposited

in the

vadose zone, along

with the

associated radial-fibrous ooids.

2.13.4

Discussion

of units 9 and

¹¹

The cyclie sequence developed

within unit 1I is similar to the

^classic shallowing and coarsening upward cycles found

in

many ancient shallow water carbonate sequences (e.g. James, 1984; Tucker,

1985b).

The

well

developed oolite-capped cycles

in the

lower

part of the unit

resemble

the

^grainy

sequence

of

James (1984),

indicating relatively

low energy

conditions,

The Iower

micritic

and peloidal

portion of

^each cycle was deposited

in a

shallow

subtidal

(probably lagoonal)

environment. This

shallowed

up into

ooid shoal and beach environments,

with

repeated periods

of

exposure and associated cementation and erosion indicated

by

hardBrounds, tepees, and desiccation

cracks, Unit

11 bears many

simllarities to unit 9, but

cycles occur on ^a much smaller scale and

in

most cases probably

did not

reach

the level

of exposure.

The

rapid

thlckness and facies changes

at unit 1l level within

the 63