Gorter Et Al APPEA 2009 Permian West Australia

8/11/2019 Gorter Et Al APPEA 2009 Permian West Australia

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APPEA Journal 20093

THE PERMIANTRIASSIC BOUNDARY

IN WEST AUSTRALIA:

EVIDENCE FROM THE BONAPARTE

AND NORTHERN PERTH BASINS

EXPLORATION IMPLICATIONS

Lead author

John

Gorter

the presence of reworked sediments and paleontologic

material (both conodonts and spore-pollen) and to th

significance of geochemical shifts.

The age of the basal Kockatea Shale (northern Per

Basin) and the basal Mt Goodwin Sub-group (Bonapar

Basin) is reassessed using palaeontological data, au

mented by carbon isotopic measurements and geochem

cal analyses, supported by wireline log correlations an

seismic profiles. The stratigraphy of the latest Permia

to Early Triassic succession in the Bonaparte Basin also revised, as is the nomenclature for the Early Triass

Arranoo Member of the Kockatea Shale in the norther

Perth Basin. The Mt Goodwin Sub-group (new rank)

composed of the latest Permian Penguin Formation ove

lain by the Early Triassic Mairmull, Ascalon and Fishbu

formations (all new).

KEYWORDS

Permian-Triassic boundary, western Australia, northerPerth and Bonaparte basins, palaeontology, carbon istopes, well log correlations, 2D and 3D seismic, redefine

Mt Goodwin Sub-group and Penguin Formation, defineMairmull, Ascalon, Fishburn formations (new), ArranoMember, tectonics, eustacy, source rocks.

INTRODUCTION

The Early Triassic sedimentary succession is one of thmost tightly age-controlled intervals in the Phanerozosedimentary record. Recent advances in the radio-isotopmethods of dating of ashfall tuff zircons are producinreproducible results with error bar constraints of +/- 0Ma (Mundil et al, 2001; 2004; Mattinson, 2005; Galfetet al, 2008). In China a biostratigraphically (ammonoiand conodonts) well-constrained succession from the LaPermian (Changhsingian) through to the Middle Anisiaand containing literally hundreds of interbedded tuhorizons has been examined (Galfetti et al, 2008). Tab1 shows the revision of the age constraints in the EarTriassic from GTS 2004 (Gradstein et al, 2004) throughpreliminary revision by Ogg et al (2008) to the most rececompilation of Galfetti et al (2008). The time constrainon this Early Triassic interval demonstrate that the durtion of the Griesbachian Substage was only about 0.49 Mthe Dienerian about 0.53 Ma, the Smithian about 0.73 Mand the Spathian about 2.87 Ma. The short duration

J.D. Gorter1, R.S. Nicoll2, I. Metcalfe3, R.J. Willink4

and D. Ferdinando5

1Eni Australia40 Kings Park RoadWest Perth WA 60052Geoscience Australia

GPO Box 378Canberra ACT 26013

University of New EnglandSchool of Environmental and Rural ScienceEarth Studies Building C02

Armidale NSW 23514Origin Energy339 Coronation DriveMilton QLD 40645Murphy Australia OilLondon House, Level 1216 St Georges TerracePerth WA 6000

[email protected]@[email protected]@originenergy.com.au

[email protected]

ABSTRACT

Several sedimentary basins in west Australia contain

petroleum reservoirs of Late Permian or older age that

are overlain by thick shaly sequences (4002,000 m) that

have been assigned an Early Triassic age. The age of the

base of the Triassic shales has been, and continues to be,

contentious with strata being variously ascribed to the

latest Permian (Changhsingian Stage) or wholly within

the earliest Triassic (Induan Stage). In the Perth Basin

the Permian-Triassic boundary appears to be located

somewhere in the Hovea Member of the Kockatea Shale.

In the Bonaparte Basin, the boundary would appear to

be either in the uppermost Penguin Formation or at the

boundary between the Penguin and Mairmull formations.

The uncertainty of the boundary placement relates

to the interpretation of the sedimentological, biostrati-

graphic and geochemical record in individual sections and

basins. Major problems relate to the recognition, or even

the presence of unconformities, complications related to


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312APPEA Journal 2009

J.D. Gorter, R.S. Nicoll, I. Metcalfe, R.J. Willink and D. Ferdinando

these time intervals may well contribute to our inabilityto recognise sediments of the early part of this interval insome regions of the Western Australia margin.

Marine Triassic rocks are widespread in the Bonaparte,Browse, Canning, northern Carnarvon and Perth basins(Fig. 1). Many petroleum exploration wells have penetrat-ed Triassic strata and together with widespread seismicprofiling in the search for oil and gas resources, enable arobust lithostratigraphy to be established. The Early Tri-

assic sedimentary succession is predominantly composedof marine fine-grained clastic sediments and occasionalinterbedded carbonates, with total thickness ranging from4002,000 m, deposited over a time interval of 5.1 millionyears (Ogg et al, 2008). Most of the Early Triassic formationsidentified in the literature are based on lithostratigraphy,that is, correlated by electric log profiles and dominantrock types, with some biostratigraphy included by way ofpalynology. Unfortunately, most of the Australian Triassicspore-pollen zones are endemic to the region (Balme, 1969;1990; Balme and Foster, 1996; Dolby and Balme, 1976;Helby et al, 1987) and correlations to better-dated strata

elsewhere in the world is somewhat problematic. In someinstances, particularly in the upper part of the Triassic,dinoflagellates can be correlated outside of the Australianregion (e.g. Balme, 1969; 1990; Balme and Foster, 1996),and in even fewer instances conodonts and ammonoidsprove useful in correlation (e.g. Nicoll and Foster, 1998;Nicoll, 2002; Skwarko and Kummel, 1974).

In the Perth Basin, the basal Triassic shales are seals tounderlying Permian reservoirs (e.g. Cliff Head and Hoveafields) and also interpreted to be source rocks (Thomasand Barber, 2004), and minor oil and gas reservoirs arepresent in the regressive Early Triassic strata at the Don-gara oil and gas field (Arranoo Member). In the Bonaparte

Basin the basal Triassic shales form the ultimate seal forPermian reservoirs at the Prometheus, Rubicon, Fishburn,Petrel and Tern fields and include thin reservoir qualityand gas-bearing sandstones at Blacktip1 and Ascalon1A(Ascalon Formation, see below).

The primary correlation is by wireline log curve match-ing between petroleum exploration wells, supported byconodont and palynological control in shallow marine tocontinental strata. Inter basinal correlations are made us-ing the same methodology but more emphasis is placed onthe matching of chronostratigraphic surfaces supportedby the generally short-lived conodont faunas. In addition,carbon isotope profiles over the Permian-Triassic transitionprovide information on the base of the Triassic System

by means of correlation to well-dated extra-Australianmeasured sections controlled by detailed palaeontology(Galfetti et al, 2008; Mundil et al, 2001, 2003).

STRATIGRAPHIC NOMENCLATURE AND THETRIASSIC/PERMIAN BOUNDARY

Bonaparte Basin

The present lithostratigraphic nomenclature of Triassicrocks known from the subsurface across the Timor Sea, and

Table 1. Radiometric ages in million of years (Ma) of the base of Early Triassic Substages from different studies.

xmas0027.dgnScotese (2000)

60

30

0

30

60

Warm Temperate

Arid

Arid

Arid

Tropical

Perth Basin

Carnarvon BasinWarmTemperate

Bonaparte Basin

Figure 1. Locality and palaeogeographic map for the Early Triassicshowing continental distributions and inferred temperature regimes

(after Scotese, http://www.scotese.com/lpermcli.htm) and the

position of the Perth, Carnarvon and Bonaparte basins.

Stage Substage Gradstein et al (2004) Ogg et al (2008)

Anisian

Basal age 245+/-0.15 247.4

Spathian

Olenekian 248.1 249.6

Smithian

Basal age 249.7+/-0.7 251

Dienerian

Induan 250.3 251.5

Griesbachian

Basal age 251+/0.4 252.5

Permian


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APPEA Journal 20093

The Permian-Triassic boundary in west Australia: evidence from the Bonaparte and Northern Perth basinsexploration implicatio

outcrops in the southeastern Bonaparte Basin (Dickins etal, 1972; Mory, 1988, 1991), is illustrated in Figure 2. Cor-relation between wells relied on wireline log similarities.

The basal Triassic unit redefined by Mory (1988) wasthe Mt Goodwin Formation of the Kinmore Group, recog-nised regionally as a seismically bland package from thesoutheastern part of the Bonaparte Basin (e.g. Petrel field),

westwards across the Londonderry High (e.g. Osprey1area) and onto the Ashmore Platform (e.g. Sahul Shoals1area). The Mt Goodwin Formation was originally definedby Helby (1974) from Petrel1 well between 2,887 and3,464 m, where it consists of a series of interbedded shalesand siltstone with minor fine-grained sandstones.

Defining the base of the TriassicBonaparteBasin

The base of the Triassic in the Bonaparte Basin has gen-erally been considered to be at the base of the Mt GoodwinFormation (now Sub-group); however, in some wells there

is a shaly section similar to the Mt Goodwin Sub-groupbetween the undoubtedly Late Permian Tern Formationand the well-dated basal Triassic. This unit was called thePenguin Formation by Gorter (1998). Log correlations inthe southern Bonaparte Basin (e.g. Fig. 3) show that thereis an unconformable contact between Late Permian andEarly Triassic strata in the region of Fishburn1 and thisunconformable relationship is supported by seismic pro-files and is well illustrated by 3D seismic at Blacktip1,where well-defined dendritic systems are incised into thetop of the Tern Formation (Fig. 4). The two Permian lime-stones sequences seen in Petrel2 in Figure 3 are readily

correlated on seismic profiles across the Bonaparte Basand the erosion of the Late Permian Dombey Limestoneevident at Fishburn1. In this well there is a shaly intervbetween the eroded top of the Cape Hay Formation anthe base of the dated Mairmull Formation (new nameThis shaly sequence is assigned to the Penguin Formatioand contains theP. microcorpusOppel Zone at Fishburn

(Fig. 5). The major change in the carbon isotopic profileseen to occur at the base of theP. microcorpusOppel Zonin Fishburn1, but the most negative values lie at the baof theL. pellucidusOppel Zone (Fig. 5).

If the top of the Permian is defined as lying betweetheL. pellucidusOppelZone and theP. microcorpusOppeZone, then the most likely positioning of the basal Triassis at the lightest carbon isotopic horizon, i.e. the top of thPenguin Formation. The base of the Penguin Formationa clearly defined unconformity with the Tern Formatioand Dombey Formations absent by erosion. The base of thPenguin Formation was cored in Tern5. In the core thbase of the Penguin Formation lies below a granite pebb

lag described from about 2,545.2 m in the core above thTern Formation (Fig. 6). Palynological information frojust above the pebble lag indicated the lowerP. microcorpOppel Zone is present in a nearshore depositional enviroment (Purcell, 2008). The granite pebble horizon suggeslong distance transport from granitic terrain to the sout

The major isotopic break in Tern3 (data from Morant1996) also occurs below the base of the P. microcorpOppel Zone (Fig. 7). The isotopic shift in Tern3 is satoothed in pattern, with more negative isotopic valuincreasing up section (Fig. 7). Helby (1983, in the Ternwell completion report) placed the base of the Triassic

3247.dgn

Early Triassic

Anisian

Ladinian

S7

S6

S5

S4

S3

S2

S1

PollardFm

OspreyFm

CraneForm

ation

Anjo Sst Mbr

Ascalon FmFishburn Fm

Mairmull Fm

Penguin Fm

? ?

? ?

Ol1,2,3

In2In1

Lad3

Lad2Lad1

An4

An3

An2

UnnamedUnnamed

N. aldae

C. timorensis

N. pakistaniensis

S. ottii

S. quadrifidus

T. playfordii

K. saeptatusN. dieneri

Sandstone

ShaleLimestone

240

250

260

Epoch / StageTethyan

sequencesPalynology Sequences

Age(Ma) (Hardenbol

et al. 1998)

Baseconodont

zone(after Ogg & Nicoll,

in press)

Micro-plankton

Spores /pollen

(Mory, 1988)

West EasHigh Low

(Haq et al.1987)

CurrentStratigraphy Stratigraphy

West East

Timor Sea

Permian

(Changhsingian)

Longterm

Carnarvonsequences

(Gorter, 1994)

Whimbrel Sst Mbr CapeLondonde

rry

Fm

Osprey

MtGoodwin

Sub-group

Tern Fm

and older

Londonderr

y

Fm

Mt GoodwinFm

Figure 2.Stratigraphy of the Timor Sea region and Bonaparte Basin (modified from Gorter, 2008 unpublished poster).


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12600'

12800'

Perth

W.A.

N.T.

S.A.

QLD

VIC. T

AS.

N.S.W.

TIMOR

S

EA

Fishburn1

Tern3

0.0

0.1

0.2

0.6

-35

-30

-20

0

200

3500

3000

4000

500

Cores

100

Permian

1750

2000

2500

3000

2250

2750

Petrel2

0.0

0.8

L.pel

luc

idus

0.7

Jurassic

TopPearce

TopTorrens

CapeHay

0.4

0.3

0.5

-25

2900

2800

2700

2600

3400

3300

3200

3100

3900

3800

3700

3600

4100

4200

4300

0.1

0.30.40.5

0.2

0.6

Top

of

Cores

Triassic

Cores1-3

10Cores1-9

GR

DT(sec/m)

Palynology

T.

playfordii

K.

saeptatus

K.

saeptatus

Wey

landites

D.

stel

lata

TopPearc

eFormation

TopMtGo

odwin

Sub-group

CapeLondonderry

Depth

(m)

2500

5

8

9

"

@3192m

Depth

(m)

-35

-30

-20

-25

2100

2000

1900

1800

1700

1600

2200

2300

2400

2500

2600

Depth

(m)

0.0

0.1

0.2

0.6

-35

-30

-20

0.4

0.3

0.5

-25

?

Caved

TopFossilHead

PenguinFormation

Typesectionof

FishburnFormation

Typesectionof

MairmullFormation

LimestoneMarkers

EarliestTriassicisotopictrend

Transitionisotopictrend

LatestPermianisotopictrend

P.

samoi

lov

ich

ii

P.

microcorpus

upper

Stage5

c

upper

Stage5

b

l

ower

Stage

5b

l

ower

Stage

4

C.

torosa

upper

Stage5

a

Stage3

b

3

8

13

"

@2580m

L.pel

lucidus

OspreyFo

rmation

FishburnFormation

AscalonF

ormation

MairmullF

ormation

PenguinF

ormation

13

11

Carbon

Isotopes

0

200

500

Cores

100

GR

DT(sec/m)

Palynology

Carbon

Isotopes

0

200

500

Cores

100

GR

DT(sec/m)

Palynology

Carbon

Isotopes

TernForm

ation

TopDomb

eyFormation

CapeHay

Formation

TopTorrens/FossilHead

Formation

s

3363.dgn

Fishburn1

NB:Logsandgeochemical

datafromPetrel-2adjusted

downtocorrelatewith

Petrel-4corebased

palynologyandcarbon

isotopicvalues.

Datum:baseAscalonFormation

69.5km

65.1km

Petrel2 P

P5

.5

-

PP6

PP6

PP5

.4

-

PP6

PP5

.4

PP4

.3

.2

PP4

.3

.2

-

PP5

Approx

.

PP5-P

P6

i

nT

ern-4

Tern3

up

P.

microcorpus

l

o

P.

microcorpus

Approx

.

P.

microcorpus

i

nT

ern-3

Figure3.CorrelationsectionbetweenFishburn1,Tern3andPetrel2acrossthePermian-Triassictransition.NotetheunconformablecontactatFishburn1isalsosupportedby

theseismicandwelllogcorrelationtooffsetwells,withtheDombeyandTernformationsbotherodedatthewellsite.


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APPEA Journal 20093


the base of the upperP. microcorpus Oppel Zone (Fig. 8).Detailed examination of the Permian-Triassic transition inTern3 shows a sharp boundary between the Tern Forma-tion and the overlying Penguin Formation indicated by thegamma ray (GR) curve (Fig. 8). Also shown in Figure 8 arethe palynofacies percentages of opaque organic matter,plant debris and acritarchs (after Helby, 1983, in Tern3well completion report). It is clear that over the transitioninterval marked by theP. microcorpusOppel Zone that thedecreasing amount of opaque organic debris parallels theupward negative shift of the carbon isotopic curve (Fig.8). Note that the carbon isotopic values were derived from

digitising a figure from Morante (1996) whereas the othercurves are derived from digital data in the Tern3 wellcompletion report; there may be a slight discrepancy indepth between these measurements.

In Petrel4 the boundary is not so clearly demarcatedbecause the isotopic break occurs above the 9 casing(data from Morante, 1996), with the cuttings-based isotopicvalues below the casing shoe having heavier isotopic values.These cuttings-derived values are not as isotopically heavyas from the cored Cape Hay Formation section below theDombey Formation limestone marker (with the notableexception of the sample from 3,600 m where over 93.5%

of the kerogen is from acanthomorph acritarchs) (Fig. 9The evidence from the southern Bonaparte Basin show

that the major unconformity lies between theP. microcorpOppel Zone and older Late Permian strata. The uncoformity is supported by 2D and 3D seismic data, well locorrelations and palynology. The carbon isotopic profilshow a negative inflexion at the base of the Penguin Fomation above relatively monotonous heavy isotopic valuthrough the Permian (except where complicated by venegative measurements equated to marine incursions). Thbase of the Triassic is marked by the most negative carboisotope values and the incoming of theL. pellucidusZon

Northern Perth Basin

The basal Triassic stratigraphic subdivisions of the PerBasin were established by Playford et al (1976) and thsummary published by Cockbain (1990) is essentially uchanged by subsequent publications, other than the reognition of the Hovea Member at the base of the KockatShale (Thomas and Barber, 2004) and the reinterpretatiothat at least some of the sandstone bodieslike the Dongra Sandstoneare of Late Permian age. Nonetheless, theis considerable debate about the nature of the Permia

Fig04.dgn

Dendriticpatterns

Dendriticpatterns

Top TernFormation

Blacktip1

0 4km

Figure 4. Dendritic channel-like feature near the top of the Tern Formation or base of the Penguin Formation near Blacktip1. Smal

channel forms are seen to the southeast of the well (note the major linear features shown in red are faults).


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Triassic transition (Fig. 10) and the precise placement ofthe boundary. Thomas and Barber (2004) contended thatdeposition was essentially continuous across the Permian-Triassic boundary and Metcalfe et al (2008) suggestedthat the lack of Early Triassic (Induan) conodont faunasfrom any locality in Western Australia probably indicatesa depositional break in the boundary interval.

In establishing the Hovea Member of the KockateaShale, Thomas and Barber (2004) extended the base ofthe Kockatea Shale into the Late Permian. They placedthe base of the Kockatea Shale into the upper part of theWuchiapingian Stage and included both theD. parvitholatoP. microcorpusOppel Zones in the Permian. There maybe some question regarding the recognition of theD. parvi-tholaOppel Zone in the base of the Hovea Member, but theinterpretation of aP. microcorpusOppel Zone in the baseof the Hovea Member is more certain (Thomas et al, 2004).

The Arranoo Member is the interbedded fine-grainedsandstone and siltstone facies in the upper Kockatea Shalein onshore wells in the Dongara gas field area (Gilchristand Holloway, 1983; Mory and Iasky, 1996: Crostella andBackhouse, 2000), and is overlain by a regressive unit, theWoodada Formation (Mory and Iasky, 1996). The ArranooMember contains theK. saeptatusOppel Zone.

Kockatea Shale

The Kockatea Shale crops out in the northernmost PerthBasin around the Northampton Complex and as far north as

2300

2500

2700

-30 -25

2100

1900

1800

3076.dgn

Hyland BaySub-group

Mt GoodwinSub-group

MetersMD

GR (API)

0 200 500 100 Palynology

(Morante 1996)

marine

marine

marine

Cuttings based

Jurassic

1900

2000

2100

2200

2300

2400

2500

2600

2700

2800

C.torosa

P.samoilovichii

L.pellucidus

P.microcorpus

upperStage5c

upperStage5b

upperStage5a

lowerStage5b

lowerStage4

Stage3b

Fishburn Formation

Ascalon Formation

Mairmull Formation

Penguin Formation

Cape Hay Formation

Torrens Formation

Fossil Head Formation

Pearce Formation

DT (sec/m)

2550

GR (API)

0 200

0 200

2540

2539

2550

2551

Digitised GR (API) Core

2542

2544

2546

2548

2552

2554

2542

2544

2546

2548

LogDepth

(m)

CoreDepth

(m)

2541

2543

2545

2547

2549

Penguin

Formation

Granite pebble lag

Shelly lag

Tern5

Tern

Formation

LowerP. microcorpus

LowerP. microcorpus

Fig06-3400.dgn

Figure 5. Carbon isotopic profile across the Permian-Triassic transition in Fishburn1 (isotopic data digitised from Morante, 1996).

Figure 6.Correlation of cores from Tern5 by means of a gamma ray

log of the cores to the downhole gamma ray logs (data from Tern5

well completion report, Santos, 1998a, b). The core description is

by N.M. Lemon in the completion report. Palynological dating by

Purcell (2008).


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APPEA Journal 20093


2350

2550

-30 -25 -20-35

3077.d

Mt Goodwi

Sub-group

Hyland BaySub-group

MetersMD

GR (API)

0 200 500 100

Palynology

(Helby WCR)

Cuttings-based

(Morante, 1996)

Dombey Formation

2100

2200

2300

2400

2500

2600

K.saeptatus

L.pellucidus

Weylandites

D.stellata

upperP.microcorpuslower P.microcorpus

Fishburn Formation

Ascalon Formation

Mairmull Formation

Penguin Formation

Tern Formation

Cape Hay Formation

DT (sec/m)

Figure 7. Carbon isotopic profile across the Permian-Triassic transition in Tern3 (isotopic data digitised from Morante, 1996). Note t

maximum negative shift at the base of the Mairmull Formation below the P. microcorpusOppel Zone (based on core data).

2350

2550

Depth(m)

3086.dgn

Palynofacies( Helby, WCR )

100

150

200

0 50 -3

5

-30

-25

-20

0

2400

2450

2500

Palynology( Helby, WCR )

Carbon Isotopes( Morante, 1996 )

90

10

20

30

40

50

60

70

80

L. pellucidus

Weylandites

K. saeptatus

GR(API)

% opaques

% plant debris

% acritrachs

-?

T R I A S S I C

L A T E P E R M I A N

A

B

DOMBEY FM

CAPE HAY FM

TERN FM

MAIRMULL

FM

PENGUIN

FM

upper P. microcorpus

lower P. microcorpus

Figure 8. Details of the Permian-Triassic transition in Tern3, showing the sharp boundary between the Tern Formation and the overlyi

Penguin Formation (left hand column), the carbon isotopic profile (after Morante, 1996), and the palynology in the right hand column (fro

Helby, 1983, in Tern3 well completion report). Also shown are palynofacies percentages of opaque organic matter, plant debris and acritarc

(after Helby, 1983, in Tern3 well completion report).


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3300

3500

3700

-30 -25

3078.dgn

Mt GoodwinSub-group

Hyland BaySub-group

MetersMD

GR (API)

0 200 500 100

Palynology

(Foster et al. 1997)

Top Pearce

Cuttings and

core-based

marine

(Morante 1996)

K.saeptatus

Coresfrom3563.6m

3100

3200

3300

3400

3500

3600

3700

3800

3900

PP5.5 -PP6

PP6

PP5.4 -PP6

PP5.4

PP4.3.2 -PP5

PP4.3.2

Approx.PP5-PP6

inTern-4

DST 4

958"casing958"

casing

Ascalon Formation

Mairmull Formation

Penguin Formation

Tern Formation

Dombey Formation

Cape Hay Formation

Fishburn Formation

DT (sec/m)

Approx. P.microcorpus

inTern-3

Figure 9. Carbon isotopic profile across the Permian-Triassic transition in Petrel4 (isotopic data from Morante, 1996; Foster et al, 1997)

indicating no marked negative shift at the base of the Penguin Formation in this well, possibly because of complications resulting from the

placement of the 9 casing. Note the highly negative marine spike in the Permian section caused by the abundance of spinose acritarchs

and the marked negative isotope excursion at the base of the Mairmull Formation.

3249.dgn

S8

S7

S6

S5

S4

S3S2

S1

? ?

Kockatea

Shale

? ?

? ? ?

Ol1,2,3

In2

Car3

Car2

Car1

In1

Lad3

Lad2Lad1

An4

An3

An2

Unnamed

Unnamed

N. aldae

C. timorensis

N. pakistaniensis

P. polygnathiformis

S. ottii

S. quadrifidus

T. playfordii

K. saeptatusN. dieneri

Early Triassic

Anisian

Ladinian

Carnian

240

250

260

Epoch / StageTethyan

sequencesPalynology Sequences

Age(Ma) (Hardenbol

et al. 1998)

Baseconodont

zone(after Ogg & Nicoll,

in press)

Micro-plankton

Spores /pollen West EastHigh Low

(Haq et al.1987)

CurrentStratigraphy Stratigraphy

West East

Perth / Abrolhos (GSWA Report 75)

Permian(Changhsingian)

Longterm

Carnarvonsequences

(Gorter, 1994)

Sandstone

Limestone

KockateaFacies

Beekeeper /Dongara

KockateaShale

(Condensedsection)

ArranooMember

Unnamed Lst marker

LesueurFormation

LesueurFormation

Woodada Formation WoodadaFormation

BookaraSandstone

WittecarraSandstone

Indoonmember/

ArranooMember

Hovea Sapropelicinterval

Hovea Inertinitic intervaland older Permian

Figure 10. Stratigraphy of the northern Perth Basin (from Gorter et al, 2008).


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APPEA Journal 20093


Kalbarri in the southern Carnarvon Basin (Mory and Iasky,1996). Ammonoids indicate that the base of the KockateaShale in the subsurface in BMR Beagle Ridge10 is EarlyTriassic (Griesbachian), and Smithian at the top (Dickinsand McTavish, 1963; Balme, 1990; Gorter et al, 1995; Fosteret al, 1997). North of Beagle Ridge10, at Mt Minchin, ammo-noids from the lower part of the formation are of Smithian(late Early Triassic) age demonstrating that the Kockatea

Shale is time transgressive onto the Northampton Block(Playford et al, 1976). Outcrop in the north of the basin isgenerally sparse and weathered, and detailed measuredsections are non existentthe type section is 12 m thickaccording to Mory and Iasky (1996)so that well logs anddrill cuttings are the major source of lithostratigraphicand biostratigraphic information.

The Kockatea Shale is the only Triassic formation in thenorthern Perth Basin with established ties to internationalbiostratigraphy including conodonts and ammonoids, butmost of the non-marine Triassic in the basin relies on longranging spores and pollens for age dating. The KockateaShale contains the K. saeptatusOppel Zone, sometimes

sub-divided into theP. samoilovichii zone and the basalL.pellucidus zone. Conodonts from the basal limestone areusually dated as Smithian by theN. dieneri-N. pakistanensiszones. Carbon isotopic profiles have been established forthe Permian-Triassic transition in Woodada2 (Gorter etal, 1995; Foster et al, 1997), several Dongara area wells(Andrew et al, 1999; Morante et al, 1999), and Hovea3(Thomas and Barber, 2004; Thomas et al, 2004). Moranteet al (1994) and Morante (1996) provided some additionalinformation on BMR Beagle Ridge10A, but unfortunatelyinsufficient to enable a vertical profile to be drawn.

The Dongara oil and gas field is the largest hydrocar-bon pool in the northern Perth Basin. Reservoirs includeseveral named Permian sandstones that are sealed, andsourced by the overlying Early Triassic Kockatea Shale. AtDongara1 the basal part of the Kockatea Shale includes aninertinitic interval overlain by a sapropel-rich unit calledthe Hovea Member (Thomas et al, 2004). The exact age ofthe inertinitic unit is still in dispute (see above) but thesapropelic interval in the upper part of the Hovea Memberis clearly near-basal Triassic in age as it contains distinc-tive bivalves, conodonts and ammonoids that are of thesame age as well-dated Early Triassic sections elsewherein Gondwana. The sapropelic interval and the overlyingKockatea Shale proper contain theK. saeptatusOppel Zone(Fig. 11). The informally named limestone marker, whichoverlies the sapropelic unit, dated by the occurrence in

several wells of theN. pakistanensisconodont fauna (e.g.Corybas1 in Metcalfe et al, 2008), is included in the HoveaMember by Thomas et al (2004).

Defining the base of the TriassicPerth Basin

Detailed palaeontological and organic geochemical in-formation derived from cores from Dongara4 is illustratedin Figure 12. The Rock-Eval T

max, hydrogen index (HI) and

production index (PI) measurements are from in-house Enidata measured by Geotech (West, 2000). The GR (API) and

sonic (DT) logs are from digitised electric log data frothe Western Australian Department of Mines and Petrleum website. The SC GR (API) is a gamma ray log ruover slabbed core by CoreLab in 1998 for British-Borne(now Eni). The ranges of the various fossils in the corare from Dr Peter Jones (Australian National Universitpers. comm., for the ostracod Carinaknightina discarinataRSN (new conodont identifications), and McTavish (197

for the N. dieneri zone conodonts. The identification Claraia sp was by Dr Mac Dickins (deceased, ex Bureaof Mineral Resources, pers. comm.) and JDG (in houidentifications). The position of the limestone marker derived from the sonic log correlation to nearby wells ancuttings described from the Dongara4 mud log. Cores wematched to the down hole logs via the core gamma log fcores 1 to 3, and are matched at the top of core 4 based othe reported rounded pebble lag (Backhouse, 1996) anthe gamma ray curve. The interpreted trends for the latePermian (blue box) and Early Triassic (pink box) carboisotopes follows the Hovea3 data (Thomas and Barbe2004; Thomas et al, 2004) as shown in Figure 11.

The Dongara4 Rock-Eval data derived from the coresection (cores 1 to 3) show the contained organic mattis immature (also shown by vitrinite reflectivity values


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The very fine-grained sandstones of the informallynamed Indoon member in Indoon1 (Fig. 14) can be cor-related from Indoon1 to East Lake Logue1 and into someof the Woodada gas field wells, but sandstones are absentin other field wells like Woodada2, where the stratigraphicequivalent may be represented by calcareous shales de-posited on the pre-existing, pre-Triassic topographic high(e.g. Gorter and Davies, 1999, fig. 13). In the Dongara areathe Indoon member may merge with the Arranoo Member(both occur in a section containing theK. saeptatusOppelZone), but further work is required to validate this.

DISCUSSIONGorter (1994) interpreted regressive-transgressive

events in the Triassic strata of the offshore CarnarvonBasin (Fig. 1) by correlating palynological zones with thesea level curve of Haq et al (1988). These transgressiveand regressive events are correlated to sea level eventsdeduced for the Triassic succession in Tethys as determinedby Hardenbol et al (1998) in Figure 2. The Induan 1 and 2sea level events are here equated with a sea level fall atthe base of the Ascalon Formation (new name) betweensequences 1 and 2 of Gorter (1994), and the sea level fall

between sequences 2 and 3 is correlated with the Olen-kian 3 event of Hardenbol et al (1998). In the Early andMiddle Triassic, Ogg (in Gradstein et al, 2004) noted majorregressions in the Induan (In1), middle Olenkian (Ol1) andnear the top of the Olenkian (Ol4) and the late Ladinian(Lad3). In Ogg et al (2008) this was changed to limit themajor regressions to the Late Permian (late Changhsingian(end Perm)), the late Olenekian (Ol4) and the late Ladin-ian (Lad 3). The Ol4 event of Ogg et al (2008) probablycoincides with the Olenekian 3 event of Hardenbol et al(1998), but given the widespread nature of the regressiveevent at the base Ascalon Formation in the Bonaparte

Basin, probably reflected in the incoming of the Indoonmember in the northern Perth Basin, it is surprising thatit is not recognised by Ogg et al (2008). In Ogg et al (2008)peak transgressive events are shown to have occurred inthe mid to late Olenkian (Neospathodus pingdingshanensisZone) and at about the AnisianLadinian boundary level(topNevadites secedensisZone).

Isotope stratigraphy

High-resolution carbon isotope measurements frommany stratigraphic sections in south China have demon-

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rvithola

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CORE GR (API) GR (API)

in situ

Evidence ofmigrated or

hydrocarbons.

Inertinitic

Sapropelic

DONGARASANDSTONE

from Rock-Evalbasic data

Unconformity ?

maxT possiblyeffected byhydrocarbons:Temperaturevalues too low.

digitised fromThomas et al.(2004, fig. 2)

Figure 11. Rock-Eval pyrolysis digital data from Hovea3 plotted against Rock-Eval pyrolysis measurements digitised from Thomas et al

(2004, fig. 2). The overlay of the original hydrogen index (HI) values in red diamonds by the digitised values of the HI (green diamonds) from

Thomas et al (2004, fig. 2) demonstrates that the digitised 13C values for kerogen is on depth; however, the unpublished production index

data (PI) show that the section with lower isotopic values contain either in situ generated or migrated hydrocarbons, suggesting the Rock-

Eval Tmax

values reported are probably too lowthe probable break in maturity of the contained kerogen at about 1,981 m (dashed line)

could indicate an unconformity.


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APPEA Journal 20093


strated that the pronounced carbon isotopic excursion atthe Permian-Triassic boundary is not an isolated event butmerely the first in a series of large fluctuations that contin-ued throughout the Early Triassic before ending abruptlyearly in the Middle Triassic. Composite carbon isotopiccurves (13C

carb) for the well-dated Meishan locality and

other sites in China (e.g. Krull et al, 2004, fig. 8; Payne andKump, 2007; Galfetti et al, 2007, 2008), show that the light-est carbon isotopic content (i.e. the most negative values)occurs in bed 25 at Meishan in the upper Changsingian.In marine sections, such as at Meishan, the negative car-bon isotopic excursion and the Permian-Triassic boundarymay be separated by a few centimetres whereas in non-marine sections several tens or even hundreds of metresmay separate these events, and the boundary can not beplaced with any certainty within the transitional palyno-flora or vertebrate fauna (Metcalfe et al, in press). ThePermian-Triassic boundary in the Bonaparte and northernPerth basins lies within a siliciclastic-dominated succes-sion that, on the present palaeontological dating, appearto have been deposited rather rapidlyconsistent with

a stretched carbon isotopic profile (e.g. in FishburnFig. 5 and Tern3, Fig. 7). The abrupt break in the carboisotopic curve shown by Hovea3 (Fig. 11) is consistewith extremely rapid sedimentation over the inertinitand sapropelic interval boundary or a sedimentary hiatu

Payne and Kump (2007, fig. 1), demonstrated that least four marked swings in isotopic values occur in thEarly Triassic in the Great Bank of the Nanpanjiang Bas(e.g. Lehrmann et al, 2003), an isolated Permo-Triasscarbonate build-up at Guizhou in southern China (Fi15). Ogg et al (2008) have shown preliminary revised stagand substage boundary ages (our Table 1) and these havbeen further revised by Galfetti et al (2008) and used redraft Figure 15 contrasting carbon isotopic curves froGuizhou (Payne and Kump, 2007) and Guangxi (Galfetet al, 2007). Galfetti et al (2007, fig. 2) indicate the monegative isotopic excursion in the Early to Middle Triassoccurs in the early Smithian at the base of theFlemingitrursiradiatusbeds in the Jinya/Waili composite measuresection in northwestern Guangxi in southern China, aproximately equivalent to theN. dieneri-N. waageni co

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Cores

Pal

ynol

ogy

K.saeptatus

P.

sinuosus

or

younger

Limestonemarker

Core1

Core2

C3

Lost

core

Barren/Permian

Roundedpebblelag

? reworkedPermian

Claraia

C. discarinataLatest Permianisotopic trend

Earliest Triassiisotopic trend

N. dienerizone

DT (sec/m)

C4

Figure 12.Dongara4 Rock-Eval Tmax, hydrogen index (HI) and production index (PI) from in house data by Geotech (West, 2000). G

(API) and DT logs are from digitised data in DoIR website fi les. SC GR (API) is a gamma ray log run over slabbed core by CoreLab in 1998 f

British-Borneo (now Eni). The ranges of the various fossils in the core are after Peter Jones (pers. comm., for the Carinaknightina discarinat

Robert Nicoll (pers. comm.) and McTavish (1973) for the N. dieneri zone conodonts; Claraiasp from Mac Dickins (pers. comm.) and Gort

(in house); and the limestone marker from sonic log correlation to nearby wells and cuttings described in the Dongara4 mud log. Cor

are matched to the down hole logs via the core gamma log for cores 1 to 3, and are matched at the top of core 4 based on the reporte

rounded pebble lag (Backhouse, 1996) and the gamma ray curve. The carbon isotopic compositions are from Morante (1996) and Moranet al (1999). The trends for the latest Permian (blue box) and Early Triassic (pink box) carbon isotopes follows Hovea3 data (Thomas a

Barber, 2004; Thomas et al, 2004) as shown in Figure 11.


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Claraia

C.

discarinata

Megaspores

MO

NDARRA

DONGARA

MT

HORNER

Co

rybas1

Dongara4

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saeptatus

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parvithola

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saeptatus

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jolfensis

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IA

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th

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beenshiftedtothe

lefttohighlight

comparison

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Iso

topes

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Isotopes

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Isotopes

CORE

GR

CO

RE

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R

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3/ST1

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migratedor

insitu

hydrocarbons.

DT(

sec/m)

P.

sinuosus

or

younger

N.

dienerizone

N.

pakistanensis

Figure1

3.Wirelinelogcorrelatio

nbetweenHovea3/ST1,Dongara4and

Corybas1acrossthePermian-Triassicboundary.Thiscorrelationindicatesthat

theHovea3boundary

islikelytobeerosionalandtheon

lapoftheTriassicovertheHovea3areaoccurredlaterthanatDongara4andCorybas1.


13/26

APPEA Journal 20093


odont zones (composite 13Ccarb

curve reproduced in Fig.15). Foster et al (1997) had earlier demonstrated that thecarbon isotopic content of kerogen in the Triassic couldbe artificially heavy because of the reworking of Permianwoody material with heavy isotopic values. In this isolatedbank, influx of recycled Permian wood is unlikely and theisotopic swings must be caused by other means, perhapssea level fluctuations (see Haq et al (1988) and Hardenbolet al (1998) for eustatic variations in the Early Triassic),or the episodic and catastrophic release of methane fromhydrates (e.g. Hesselbo et al, 2000; de Wit et al, 2002;Retallack and Krull, 2006).

Australian carbon isotopic curves from 13Corg

overthe Late Permian to Early Triassic transition (based ondata from Morante, 1995, etc.) have been published byMorante et al (1994) and Morante (1996) for Tern3, Foster

at al. (1997) for Petrel4, and Morante (1996) and Retalack and Krull (2006) for Fishburn1 (see Fig. 5). Whicarbon isotopic curves based on the carbon isotope valufrom organic-derived carbon (total organic componen13C

org

) are not strictly comparable to carbon isotopvalues derived from carbonate minerals (13C

carb) (e

Kump and Arthur, 1999; Retallack and Krull, 2006, fig. Arthur, 2008), as both presumably reflect the isotopic composition of the global atmosphere, it is assumed that thnegative-positive shifts in both measures move in tandemas shown by de Wit et al (2002) across the palynologicaldefined Permian-Triassic boundary from five basins the interior of the former Gondwana Supercontinent. addition, as pointed out by Giddings and Wallace (200913C

carb values derived from basinal carbonate faci

are generally lighter than proximal facies with chang

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Probably Permian

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Early Triassic toLatestPermian

DT (sec/m)1400 200

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40

K.saeptatus

Palynology(Balme,

Ingram, WCR)

Jurassic

LondonderryEquivalent

OspreyEquivalent

Arranoo

Microfossils(Ingram, WCR)

50%

100%

Acritarchs

"mangroves"

Lycopods

Lycopods

Conifers

Preferre

dcorre

latio

n

Eneabba

7" @

2109m

3093.

3300

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3700

- 30 - 25

Alternativecorrelation

Conodonts inBeekeeper1APP4.2 to APP5age range

AratrisporitesSp

p.

Densoisporitesn

ejburgi

Triplexisporitesplayfordi

Falcisporitesaus

tralis

2100

2200

-3 0 -25 - 20-35

WoodadaFormation

Lesueur

Formation

BeekeeperFormation

CarynginiaFormation

~2400 km

INDOON1 PETREL4

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Cape Hay

Plover

Top Triassic

Datum :

Palynology

(Foster et al. 1997)

PP5.5 -PP6

PP6

PP5.4 -PP6

PP5.4

PP4.3.2 -PP5

PP4.3.2

Top Hyland BaySub-Group

Top Dombey

Top Pearce

Cores from 3563.6m

Base Ascalon

Ascalon Formation

Top Mt GoodwinSub-group

OspreyFormatio

FishburnFormatio

MairmullFormatio

KB = 25m, WD = 95m, TD = 3975m

Elf Aquitaine, 1988

TroughtoGroup

Cape LondonderryType Section2471-2887min Petrel-1

?

Approx.PP5-PP6inTern-4

589 "

@

3417m

Possiblefault

589 "

@

846m

DST 4

Reduviasporonites?

(Morante 1996)

S N

MD

Meters

Carbon isotopicdata in house.

Cuttings show well bottomedin Pearce limestone

Malita FormationType Section2229-2471min Petrel-1

Penguin Formatio

DT (sec/m)

Approx. T.playfordiiinPetrel-1

Approx. K.saeptatus

inPetrel-1

Approx. P.microcorpusinTern-3

Approx.P.arcoensisinPetrel-2

'Indoonmember'

Figure 14. Shallowing event (Indoon member) in the Kockatea Shale at Indoon1 recognised from the gamma ray and sonic log profi

and by a decrease in the spinose acritarch content of the palynofacies (from Indoon1 well completion report). The correlation betwe

Indoon1 in the northern Perth Basin and Petrel4 in the southeastern Bonaparte Basin shows close similarities in formation architectu

supported by palynological information and carbon isotopic measurements across the Permian-Triassic boundary.


14/26



of 811 in age equivalent facies. Similar gradients canoccur in proximal siliciclastic facies, where much of thecontained organic matter may be derived from land plantsresulting in heavier carbon isotopic values, compared todistal settings where the organic matter is more likely tohave been derived from marine organisms, with lighterisotopic values.

The Early Triassic Kockatea Shale in the northern Perth

Basin has a distinctive log facies and contains theK. saep-tatus Oppel Zone. The last appearance of the species K.saeptatusis at the top of theB. buurensis-S. millericonodontzone, i.e. it is no younger than topmost Smithian (Ogg and

Nicoll, 2007) in the Arctic. The lower part of the formationcontains theN. dieneritoN. pakistanensisconodonts in andbelow a limy interval (i.e. the limestone marker) seen inmost northern Perth Basin wells. The overlap of conodontranges reported from Corybas1 by Metcalfe et al (2008)indicates an early Smithian age for this carbonate interval.Both species have their last appearance near the top of theB. buurensis-S. millericonodont zone, i.e. latest Smithian.

These early Smithian limestones lie below a slightlysandy interval, seen in the Indoon1 and Woodada fieldwells, in theK. saeptatus Oppel Zone that is interpretedto be the equivalent of the Ascalon Formation in the

-2 -1 0 1 2 3 4 5 6 7 8

252.5

247.88

251.48

dieneri

Stabilis

ation

MiddleTriassic

EarlyTria

ssic

Anisian

Smithian

Spathian

Age(Ma)

252.01

Gries-

bachian

250.75

-2 -1 0 1 2 3 4 5

waageni-milleri?

Dien-

erian

3082.dgn

13C

PTBexcursion

Changsin

gian

Late

Permian

Time scale from Galfetti et al. (2008) 13C

Figure 15. Composite carbon isotopic (13C) compositional changes measured across the Permian-Triassic boundary in an isolated

deep marine carbonate bank at Guizhou in southern China (after Payne and Krump, 2007, fig. 1) should be relatively free from reworked

terrestrial input which may bias the isotopic measurements. Also shown for comparison is the composite carbon isotopic (13Ccarb

) curve

from northwestern Guangxi (Galfetti et al, 2007). Both curves are fitted to the Galfetti et al (2008) timescale at the substage boundaries.


15/26

APPEA Journal 20093


Timor Sea wells (e.g. Fig. 14). In Fishburn1, the AscalonFormation, while itself not analysed for carbon isotopiccomposition, appears to be part of a continuum of carbonisotopic values. Values are initially increasingly negative,then switch upward through the Mairmull Formation toincreasingly positive values below the Ascalon Formation(Fig. 5). Above the Ascalon Formation in the FishburnFormation (new name), the isotopic values again become

increasingly negative upwards through the formation tobe the lightest values in the well, immediately below theunconformity at the base of the Jurassic. If the entiresedimentary section across the Permo-Triassic boundaryis present in the Bonaparte Basin wells shown in Figure 3,and assuming the sampling interval is narrow enough torecord all possible perturbations in carbon isotopic values,the lightest carbon isotopic value can be correlated withbed 25 at Meishan, and is located in the latest Permian, orat a younger light isotopic inflexion, for example near theGriesbachian-Dienerian boundary. If the former correla-tion is correct, the incoming ofL. pellucidusoccurs in thelatest Permian, or if the latter in the latest Griesbachian

or earliest Dienerian (cf. Figs 5 and 15).

Unconformities

The most negative carbon isotope values equate to thelower part of the Kockatea Shale in the northern PerthBasin and the lowermost Mairmull Formation or topmostpart of the preserved Fishburn Formation in the BonaparteBasin (Fig. 16a). The sharp break to the negative at about1,981 m in the Hovea3 core is of a comparable magnitude.The lack of any slope between the negative Smithian val-ues and the positive Permian values suggest that sectionis missing in the cored section at Hovea3 and is entirely

consistent with the Late Permian age of the P. microcor-pusOppel Zone in the positive isotopic interval, and thehighly negative isotopic part of the K. saeptatusOppelZone interval. Similar conclusions can be drawn from acomparison of the carbon isotopic curves and palynologybetween Hovea3 and Tern3 (Fig. 16b).

Support for absence of the earliest Triassic in westernAustralian basins is forthcoming from the fossil record:there are no unequivocally Griesbachian conodonts, am-monites or shelly fossils known from any basin on thewestern Australian margin. This is not to say that olderTriassic strata may not be present in undrilled basinalareas. The oldest Triassic rocks on the western Australianmargin, based on isotopic curve matching, are most likely

in the Bonaparte Basin where the latest GriesbachianP.microcorpusOppel Zone occurs in the Penguin Formation,which from well log correlations (Fig. 3) and seismic datais unconformable on the Tern Formation. Furthermore, inTern5 the base of the Penguin Formation with the lowerP.microcorpusOppel Zone (Purcell, 2008) is a granite pebblelag overlying the Tern Formation (Fig. 6). In the Blacktip1region, 3D seismic data indicates channelling of the topof the Tern Formation (Fig. 17). In the Perth Basin, thereis no conclusive evidence from dipmeter or seismic datafor an erosional break between the sapropelic and inerti-

nitic intervals of the Hovea Member in either Hovea3 Corybas1, but in other wells such as Cliff Head4, theis a clearly defined unconformity between the sapropelinterval and the older Permian based on palynology (Pucell, 2006). In this well the contact is shale-on-shale anthere is no obvious evidence of the unconformity such facies change, weathering, angularity or lag, but there a colour change and a marked palynological break (Ear

Triassic over Artinskian). A similar shale-on-shale contawith no discernible evidence of hiatus apart from palnology change and a negative carbon isotopic excursiooccurs in Woodada2 (Gorter et al, 1995; Foster et al, 1997

From a comparison of the isotopic curves (e.g. Paynand Kump (2007) and Galfetti et al (2007) in Fig. 16), can be argued that the base of the Kockatea Shale (iabove theP. microcorpusOppel Zone) is no older than laest Griesbachian and there is a substantial time gap the order of a million yearswith or without significaerosionbetween the inertinitic interval of the HoveMember and the sapropelic interval at the base of thKockatea Shale. This is clearly demonstrated at Dongara

(Fig. 12) where the unconformity occurs between the saropelic lowermost Kockatea Shale (upper Hovea Membof Thomas et al, 2004, fig. 2) and the underlying inertinitpart of the Hovea Member (Fig. 11).

Throughout the Bonaparte Basin, the base of the AscaloFormation demonstrates a widespread downward shift facies probably coinciding with the Induan 1/2 lowstan(Fig. 2). The sandy lithofacies in the proximal parts of thBonaparte Basin shales out distally until there is very littsandstone present in the system. The Indoon and Arranmembers in the northern Perth Basin may be a similacoeval lowstand deposit.

Depositional rates and palaeoclimateIn passing, the new constraints on the ages of the low

shaly units of the earliest Triassic suggests that the seeral hundred metres of the Mt Goodwin Sub-group wedeposited in basin depocentres in as little as 11.5 millioyears (averaged from Table 1), during which time there walso a marine regression and regional erosion at the baof the Ascalon Formation. This implies a depositional raof ~>500 m/million years for depocentres where seisminformation suggests up to 2 km of Mt Goodwin Sub-grouaccumulation (after compaction)a phenomenal depsitional rate for what is essentially a shallow marine paralic siltstone unitand implies a vast supply of fin

grained detritus in the hinterland that was transported inthe shallow sea. There are no known fossil river systemof basal Triassic or latest Permian age in the hinterlanof the Bonaparte Basin that may have distributed thweathered material, leaving open the possibility that mucof the finer detritus was dispersed by wind. Possibly thweathered detritus was from far afield, for example froa later Permian cool temperature desert that was locateon the craton away from the preserved Late Permian mrine strata (e.g. the Hyland Bay Sub-group contains cowater limestones and was, in the Bonaparte Basin area


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2250

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GR CarbonIsotopes

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upperStage 5c

P. microcorpus

Depth

(m)

Unconformity ?

-32

-30

-28

-26

-24

-22

COREGR CarbonIsotopes

1970

1980

1990

1995

1985

1975

1965Palynology

K. saeptatus

P. microcorpus

D. parvithola

100

150

200

0 50

Depth

(m)

2300

Possible correlation

based on carbonisotopic valuesand palynology

Earliest Triassicisotopic trend

Latest Permianisotopic trend

Unconformity ?

-32

-30

-28

-26

-24

-22

3156.dgn

2350

100

150

200

0 50

-35

-30

-25

-20

2450

L. pellucidus

Weylandites

K. saeptatus

TRIASSIC

A

B

MAIRMULL

FM

PENGUIN

FM

upperP. microcorpus

lowerP. microcorpus

COREGR

CarbonIsotopes

CarbonIsotopes

1970

1980

1990

1995

1985

1975

1965

TERNFM

GR Palynology

Palynology

K. saeptatus

P. microcorpus

D. parvithola

Depth

(m)

100

150

200

0 50

Depth

(m)

2400

MAIRMULL

FM

PENGUIN

FM

CAPEHAY

FM

LATEPERMIAN

TRIASSIC

Fishburn1

Hovea3

A

Hovea3

Tern3B

LATE

PERMIAN

Figure 16a and b.Comparisons between the carbon isotopic composition of kerogens in Hovea3 (data from Thomas et al, 2004) and

Fishburn1 (Fig. 16a) and Hovea3 and Tern3 (Fig. 16b) (data from Morante et al, 1994; Morante, 1996). If the carbon isotopic values are

directly comparable, the preserved P. microcorpusOppel Zone isotopic values (i.e. < -24 13C) in Hovea3 equates to the lower P. microcorpus

Oppel Zone in Tern3, supporting the suggestion that most of the P. microcorpusOppel Zone is eroded at Hovea3.


17/26


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the late Early Permian (Artinskian-Kungurian) CarynginiaFormation contain ice-rafted debris reflecting renewedsubsidence but continuing seasonally cool climates withcoarse debris ice rafted into a marine embayment. Just(2003) interpreted a cool water, shallow ramp origin for thecarbonates of the Beekeeper Formation (Fig. 14), whichcontains microfossils indicative of Wordian-Wuchiapingianage (David Haig, University of Western Australia, pers.

comm. 2009), about 254268 Ma. The overlying WaginaSandstone contains coal and so is probably of latest Perm-ian age (deposited before the earliest Triassic coal gap,Retallack and Krull, 2006), probably from the Capitanianto the Wuchiapingian. Tupper et al (1994) interpreted acool but not cold environment of deposition. Given thecircumstantial evidence for cool to cold climate aroundthe northern Perth Basin during the later Permian it wouldbe expected that a large amount of glacially derived andweathered material may have been available on the cratonto fill any nearby depocentre.

In a major review of zircon provenances, Veevers et al(2005) concluded that there was uplift along the Darling

Fault to the east of the northern Perth Basin at about255 Ma (approximately the Wuchiapingian-Changhsingianboundary), with drainage to the southeast and east awayfrom the subsiding northern Perth Basin. Thus, the inputof craton-derived material to the latest Permian strata ofthe northern Perth Basin was limitedthe Hovea Memberinertinitic interval may have been deposited at this time.With transgression in the earliest Triassic and the amelio-ration of the climate (Fig. 1), the sapropelic interval of theHovea Member was laid down in a marine environmentrelatively free of influx of weathered material from theuplifted rift shoulders.

The economic implications of depositing such a largethickness of shale in such a short period of time needsto be addressed. Note that the present measured thick-nesses of these shaly units from wells and seismic data isafter compaction and dewatering. Intuitively, this rapiddeposition should push any underlying source rocks rap-idly through the oil generation window implying that inthe depocentres at least, oil generation probably initiatedduring the Triassic possibly prior to trap formation relatedto the Middle Triassic to Early Jurassic Fitzroy Movement(Colwell and Kennard, 1996).

CONCLUSIONS

This study has shown that a combination of palaeontol-

ogy, carbon isotopic curves, wireline log correlations andseismic profiles allows inter-basinal correlation of lithofa-cies across the Permian-Triassic boundary in two basins atopposite ends of Western Australiathe Bonaparte andnorthern Perth basins.

We conclude that the main Latest Permian unconfor-mity in the Perth Basin lies above theP. microcorpusOppelZone, and in the Bonaparte Basin, the main unconformitylies below the P. microcorpus Oppel Zone, unless the P.microcorpusOppel Zone is time transgressive.

Implications from this conclusion include:

The tectonic history of the Permian-Triassic boundary

differs between the Bonaparte and northern Perth ba-sins;

The apparent correlation of the Ascalon Formation and

the Indoon member lowstand units in the lower part oftheK. saeptatus Oppel Zone implies a possible eustaticconnection between the northern Perth and Bonapartebasins; and,

The different tectonic history may partly explain thelack of an equivalent to the excellent oil and gas sourcein the sapropelic unit of the basal Kockatea Shale (up-per part of the Hovea Member) of the northern PerthBasin in the Bonaparte Basin.

ACKNOWLEDGEMENTS

We would like to thank the following colleagues for dataand/or discussion: Ric Morante, Arthur Mory (GeologicalSurvey of Western Australia), Robyn Purcell (P&R PurcellGeological Consultants), Geoff Wood (Santos), and ourreferee Roger Hocking (GSWA).

In addition, the following Eni personnel contributed tothis study: Shane McGilligan and Sandra Ball (drafting)and Kon Kostas who provided the 3D time slices.

We would like to acknowledge Eni for permission topublish (JDG) and ARC Energy for access to the northernPerth Basin well data (DF).

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APPENDIX: REVISED STRATIGRAPHIC NO-MENCLATURE

The stratigraphy of the Permo-Triassic Kinmore Group(Mory, 1988, 1991) is revised with the elevation of the MtGoodwin Formation to Sub-group rank and the recogni-tion of four subdivisionsthree newto the sub-group.

Mt Goodwin Sub-group (new)

The Mt Goodwin Formation, the upper unit of the Kin-more Group of Mory (1988) was informally sub-dividedinto the Lower and Upper units (Gorter, 1998; Gorter etal, 1998). As the Ascalon Formation is here proposed fora mappable unit in the Mt Goodwin Formation overlyingan unconformity, the unit is raised to Sub-group status inthe Kinmore Group. The Kinmore Group is thus made up

of a lower Fossil Head Formation, the middle Hyland BaySub-group and an upper Mt Goodwin Sub-group. The MtGoodwin Sub-group is composed of four units: the PenguinFormation (base), the Mairmull Formation, the AscalonFormation and the Fishburn Formation (top). The base ofthe Mt Goodwin Sub-group (i.e. the base of the PenguinFormation) is defined as an unconformity overlying olderPermian strata. The top of the Mt Goodwin Sub-group (i.e.the top of the Fishburn Formation) is an erosional surfacebelow the Osprey Formation in the eastern Bonaparte Ba-sin, and the Sahul Group in the western Bonaparte andnorthern Browse basins.

Correlations are made using wireline log signatures,palynology and limited micropalaeontology supplemented

by seismic profiles. Carbon isotopic values from Fishburn1,Petrel4 and Tern3 have been used to constrain the BaseTriassic-Top Permian contact.

Penguin Formation (additional information)

Definition and type sectionThe formation was for-mally defined by Gorter (1998) for a medium grey to darkgrey-brown claystone in Tern3 between (2,4002,449 m).

AgeThe Penguin Formation contains theP. microcorpusOppel Zone, considered by Helby et al (1987) to be latestPermian probably Changsingian.

Depositional environmentIn cores from Tern4 and 5and sidewall cores from Tern3, the finely laminated shales

contain low percentages of spinose acritarchs, particularlyin the lower part (Tern3, Fig. 5), and exhibit little, ifany, bioturbation. The lithology and palynofacies indicateoxygen-poor depositional environment. Jeff Wood (Santos,pers. com., 2007) suggested a lacustrinal facies.

Stratigraphic relationshipsUnconformably overliesthe Late Permian Hyland Bay Sub-group at Fishburn1,and appears to conformably underlie the Mairmull For-mation (Fig. 5).

Distribution and thicknessThe Penguin Formation ispresent throughout the southern Bonaparte Basin. It maybe absent at Troubadour1 and appears to be missing at

Kelp Deep1 on the Sahul Platform (Gorter and Davies,1999, fig. 5), where the Mt Goodwin Sub-group onlaps theTroubadour High.

SynonymsThe unit was previously included in the MtGoodwin Formation based on its lithological similarity.Helby (1983) in the Tern3 well completion report calledtheP. microcorpus-bearing interval the A and B shales andsuggested an affinity with the Hyland Bay Formation.

Mairmull Formation (new name)

Definition and type sectionThe Mairmull Formation,named after Mairmull Creek near the Triassic outcrop of MtGoodwin southeast of Wadeye, Northern Territory, wherethe unit unconformably overlies the Late Permian HylandBay Sub-group (of Gorter, 1998), and is unconformablebelow the Ascalon Formation (new name, see below). Theunit consists predominantly of claystone and siltstone andcoarser sediments are rare. The type section is designatedin Fishburn1 between 2,121 m and 2,320 m (Fig. 5).

AgeEarly Triassic based on its position in the

Kraeuselisporites saeptatus Oppel Zone of Dolby andBalme (1976). In the Perth Basin theK. saeptatusOppelZone is associated with theN. dieneritoN. waageniconodontzones suggesting correlation with the Dienerian to lateSmithian sub stages. The transition between the Permianand the Triassic may be marked by the largest negativeshift in carbon isotopic values as seen in Fishburn1 andTern3 (Figs 5 and 7).

Depositional environmentThe Mairmull Formationwas deposited in shallow water conditions as shown bythe occurrence ofLingulaand conchostracans in Petrel1,the presence of spinose acritarchs, and the greenish andsometimes reddish colouration of the sediment. While thegamma ray log profile is generally monotonous, in several

wells the lower beds are slightly more radioactive than theupper part of the formation, and there is a gradual increasein the proportion of thin sandstone beds in the uppermostinterval below the base of the Ascalon Formation. The car-bon isotopic profile at Fishburn1 mirrors the gamma rayprofile suggesting an initial transgressive portion of theformation shown by increasing upward negative isotopicvalues followed by a sustained upward trend to heaviercarbon isotope values (Fig. 5). While unproven in this well,the changes in the isotopic composition probably reflectthe proportion of spinose acritarchs present in the forma-tion, the larger percentage in the transgressive strata anda lessening proportion of these saline indicators in theinterpreted regressive part of the unit (e.g. see Gorter et

al, 1995; Foster et al, 1997).Stratigraphic relationshipsWhere the Penguin For-

mation is absent, the Mairmull Formation overlies uncon-formably the Late Permian Hyland Bay Sub-group. TheMairmull Formation unconformably underlies the AscalonFormation.

Distribution and thicknessThe Mairmull Formation ispresent throughout the Bonaparte Basin. It may be absentat Troubadour1 on the Sahul Platform where the Mt Good-win Sub-group onlaps the Troubadour High. The formationis recognised in Echuca Shoal1 in the eastern BrowseBasin where it is intruded or contains basaltic volcanics.


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SynonymsThe only synonym for the Mairmull Forma-tion is the Lower Mt Goodwin Formation of Gorter, 1998;Gorter et al (1998), now superseded.

Ascalon Formation (new name)

Definition and type sectionThe Ascalon Formation isa prominent, widespread, sandstone/siltstone unit roughly

located in the middle of the Early Triassic Mt GoodwinSub-group in the southern part of the Bonaparte Basin. InAscalon1A, the formation is described from cuttings asvery fine to fine-grained very well sorted sandstone, withtraces of silt and clay minerals, carbonaceous specks andpoor to fair visual and inferred intergranular porosity. Thelogs suggest the unit is tight, but a total gas show of 11.31%associated with a drilling break and the appearance ofunconsolidated or less cemented sandstone in the

Documents

Gorter Et Al APPEA 2009 Permian West Australia