Otway & Sorell Basins SEEBASE Project · • The basement geology of the Otway and Sorell Basins is dominated by the Kanmantoo Fold Belt. • Basin architecture is largely controlled

Steffen, Robertson & Kirsten (Australasia) Pty. Ltd., ABN 56 074 271 720, trading as SRK Consulting

1Otway & Sorell Basins SEEBASE™ Project

Suite 6, Deakin House50 Geils CourtPO Box 250Deakin West ACT 2600Australia

email: [email protected]: http://www.srk.com.auTel: +61 2 6283 4800Fax: +61 2 6283 4801

ENERGY SERVICES

Otway & Sorell BasinsSEEBASE™* Project

SRK Project Code: GA701

February 2002

Authors:Dr Jon TeasdaleDr Lynn Pryer

Dr Peter Stuart-SmithDr Karen Romine

Dr Tom LoutitDr Mike Etheridge

Dr Zhiqun Shi (Encom)Dr Clive Foss (Encom)

John VizyPhil Henley

Danielle Kyan

*SEEBASE™ = Structurally Enhanced view of Economic Basement

_______________ ________________Jon Teasdale Tom LoutitConsultant Geologist Technical Director

The conclusions and recommendations expressed in this material represent the opinions of the authors based on the data available tothem. The opinions and recommendations provided from this information are in response to a request from the client and no liability is accepted for commercial decisions or actions resulting from them.



Table of Contents

IntroductionExecutive Summary 3Recommendations 4Project Background, Aims, Why SRK? 5

Data Compilation & ProcessingDatasets, Project Area 6Aeromagnetics 7Enhancement Processing of Magnetics 8Geoscience Australia Gravity 9Geosat Satellite Gravity 10Digital Elevation Model 11Seismic Coverage, Wells Coverage 12Plate Tectonic Reconstructions 13Interpretation Confidence 14

Calibration & MethodologyCalibration of Potential Field Data, Why Basement? 15SABRE, Basement Character & Petroleum Systems 16

Basement Geology of the Otway and Sorell Basins 17Basement Terranes 18Basement Composition 19Basement Structure – Overview 20Basement-Involved Faults 21Deep Crustal Fracture Zones 22Proterozoic-Paleozoic Basement Deformation 23Neoproterozoic Basement Evolution 24Early Cambrian Basement Evolution 25Cambro-Ordovician Basement Evolution 26Early Silurian Basement Evolution 27Mid-Devonian Basement Evolution 28Early Carboniferous Basement Evolution 29E-W Basement Structures 30

Geological Evolution of the Otway and Sorell Basins 31Basin Phase 1: Late Jurassic Extension 32Basin Phase 2: Early Cretaceous Extension 35Basin Phase 3: Mid Cretaceous Inversion 37Basin Phase 4: Late Cretaceous Extension 38Basin Phase 5: Eocene Inversion 40Basin Phase 6: Miocene Inversion 41Tertiary Volcanics 42

SEEBASE™ Depth to Basement 43SEEBASE™ Methodology 44Model Magnetic Profiles 45Otway, Sorell and Bass Basins SEEBASE™ Model 1 46Otway, Sorell and Bass Basins SEEBASE™ Model 2 47Seismic depth to basement in Offshore Otway 483D Views of Otway and Sorell Basins Architecture 49

Basement Controls on Basin Architecture 50Summary 51References 52



Executive Summary

The key findings of this project are as follows:

• The basement geology of the Otway and Sorell Basins is dominated by the KanmantooFold Belt.

• Basin architecture is largely controlled by basement structures, composition, fabric and rheology.

• NW to N-S trending basement shear zones/terrane boundaries (mainly Delamerian)were a first-order control on basin evolution during the Mesozoic.

• Seven basin phases/tectonic events have shaped the basin during the Mesozoic and Tertiary.

• The present-day geometry of the Otway and Sorell Basins was established during 3 extensional Mesozoic basin phases:

1. Late Jurassic – Early Cretaceous NW-SE transtension (Casterton/Crayfish)

2. Early – Mid Cretaceous NE-SW extension (Eumeralla )

3. Late Cretaceous NNE-SSW extension (Sherbrook)

• Initial basin evolution occurred during NW-SE Late Jurassic transtension, forming linked pull-apart basins and oblique graben.

• At least 3 inversion “events” have variably influenced the Otway and Sorell Basins(Mid Cretaceous, Eocene, Miocene-Recent).

• Two SEEBASE™ models for the Otway, Sorell and Bass Basins shows basement topography, and can be used to map basin phase distribution, migration pathways and trap type/distribution. Further work on the deep crustal architecture of the offshore Otway is required to determine which of the two SEEBASE™ models is more appropriate.

*SEEBASE™ = Structurally Enhanced view of Economic Basement

This project was initiated by Geoscience Australia to provide an integrated regional interpretation of basement composition, structure and depth in the Otway and Sorell Basins, and investigate the effect of basement geology on basin evolution and petroleum systems. This project follows on from the Bass Basin SEEBASE™* Study complete by SRK in July 2001. SRK Consulting was contracted by GeoscienceAustralia in January 2002.

SRK’s approach primarily relies on the interpretation of magnetic and gravity data, calibrated with many other datasets including mapped geology, event histories, wells and seismic. SRK utilizes a “bottom-up”approach to basin analysis, starting with a rigorous understanding of basement geology. By integrating the plate-scale kinematic event history for the area of interest, an interpretation of the basin’s structural evolution can be mapped. Combined with a SEEBASE™* map of depth to basement, this data can be used to understand basin phase architecture and the petroleum systems developed within the basin.



Recommendations

• Extend the Otway-Sorell and Bass Basins studies to cover the entire Southern Margin of Australia to produce a template for explorers in the region. Such a coverage would be a powerful tool for planning and acquisition, designing exploration strategies, and understanding the geology of the margin.

• More detailed Otway SEEBASE™ study at permit scale with full seismic calibration. The SEEBASE™ model and reactivation history presented here is regional in scale and could be significantly refined by integrating additional seismic data, rock and property information withdetailed structural interpretation.

• Acquire new aeromagnetic and gravity data in poor data areas. This would help to resolve uncertainties in areas of poor data quality, including areas N and E of King Island (including TorquaySub-basin), and parts of the offshore Otway and Sorell. Such data would also enable interpretation of intrasedimentary features such as basin floor fans/deltas, basement-detached faults and volcanics.

• Resolve deep crustal architecture in offshore Otway for thermal modelling . Currentinterpretations of deep seismic data could be significantly improved if integrated with isostaticallycorrected gravity data interpretations, modelling of gravity and shiptrack magnetic data, and a re-investigation of deep reflectors in seismic (current moho interpretations are too shallow – <4km thick continental crust!). This would help to resolve which of the two SEEBASE™ models presented here is more appropriate.

• Paleogeographic analysis and basin modeling to track accommodation space through time usingSEEBASE™ as a starting point . This would resolve the relative effects of the 3 extensional basin phases, and the extent and nature of any late Jurassic depocentres .

• This projects provides new base to investigate the stratigraphic evolution of the Otway and SorellBasins. A sequence stratigraphic study based on the structural framework and SEEBASE™ model presented here would provide new insights into the evolution of the basins petroleum systems.



Project Background• This project was initiated by Geoscience Australia and PIRSA as part of the

Southeast Basins Program to attract new explorers to the Otway and Sorell Basins by providing new insights into its geology and hence reduce exploration risk.

• SRK Consulting was contracted by Geoscience Australia in June 2001.

• This project was completed in 4 weeks by the SRK Energy Services team.

Project Aims• To provide an integrated regional interpretation of basement composition, structure

and depth in the Otway and Sorell Basins, utilizing available gravity, magnetic, seismic and other data.

• To investigate the effects of basement geology on basin evolution and petroleum systems in the Otway and Sorell Basins, focusing on structural evolution/reactivation, basin architecture and tectonic history.

• To provide an ArcView GIS containing all interpretive layers.

Why SRK?

• SRK Consulting is one of the world’s largest natural resource consultancies, with 22 offices in 5 continents.

• The SRK Energy Services group is based in Canberra, Australia. We are leaders in innovative, integrated geological interpretation of non-seismic and seismic data, principally magnetic and gravity data. We have worldwide experience in thepetroleum, minerals and coal sectors.

• SRK Energy Services has worldwide experience in basin analysis, and has pioneered many new techniques for rapidly evaluating the structural framework and tectonic evolution of all types of basins, based largely on geopotential field data. SRK utilizes a “bottom-up” approach to basin analysis, starting with a rigorous understanding of basement geology. By integrating the plate-scale kinematic event history for the area of interest, a interpretation of the basin’s structural evolution through time can be mapped. Combined with a SEEBASE™* map of depth to basement, this data can be used to understand basin phase distribution and petroleum systems. (*SEEBASE™ = Structurally Enhanced view of Economic Basement)



DatasetsThe following datasets were provided by Geoscience Australia for the Otway-Sorell SEEBASE™ project:

• Gravity (2001 Geoscience Australia 800m grid)

• Magnetics (Offshore Otway, Bass S)

• DEM (Auslig 9 sec)

• Seismic (data held in-house by Geoscience Australia )

• Well information

PIRSA provided the SA state 100m magnetic grid in addition to we ll and seismic information.

Various onshore magnetic datasets were also obtained (at no cost) from the Geological Survey of Victoria.

Public domain satellite gravity and bathymetry datasets were used in areas not covered by other datasets.

In addition, SRK integrated its extensive in-house knowledge of Australian geology, published literature, andplate tectonic reconstructions

Project Area



Magnetics

Aeromagnetic data measures variations in the Earth’s magnetic field caused by variations in the magnetic susceptibility of the underlying rocks. It provides information on the structure and composition of the magnetic basement. Most bodies within the basement have a distinctive magnetic signature which is characterized by the magnitude, heterogeneity and fabric of the magnetic signal. When calibrated with known geology, terranes canbe mapped under a cover of sedimentary rock and/or water.

The most important and accurate information provided by magnetic data is the structural fabric of the basement. Major basement structures can be interpreted from consistent dis continuities and/or pattern breaks in the magnetic fabric. Once the structures have been evaluated and combined with those interpreted from the gravity data, a model for the evolution of the basement and overlying basins can be developed.

At least 20 datasets have been stitched and/or mosaiced to construct the above image. All magnetic grids were imaged in ERMapper using a Hue-Saturation-Intensity colour model. Various enhancement filters were applied to resolve the geometry and structure of the basement at depth (see overleaf).

HSI image of Total Magnetic Intensity



Enhancement Processing of Magnetics

20km Automatic Gain Control (AGC)20km Automatic Gain Control (AGC)

First Vertical Derivative (FVD)First Vertical Derivative (FVD)

TMI Reduced to the Pole (RTP)TMI Reduced to the Pole (RTP)



Gravity data is a very important tool for interpreting basins. It maps subtle changes in the Earth’s gravitational field caused by variations in the density of the underlying rocks. Although the resolution of this dataset is low (7km spacing), it provides valuable information on the nature of the deeper parts of the crust and mantle beneath the basins. Important intra-basin elements often have an associated gravity signature indicating that each element is related to a deep basement structure.

In order to evaluate the source of the gravity signature, the data must be calibrated with known geology and/or geophysical models. Gravity images show density contrasts within the crust and upper mantle but the source of the contrast is not unique. Thus the nature of each anomaly must be distinguished in this calibration process.

For the Otway-Sorell Basin study, the Geoscience Australia 2001 National Gravity Grid was imaged in ERMapper using a Hue-Saturation-Intensity colour model. The new Geoscience Australia gravity grid is a compilation of onshore survey data, Geosat satellite gravity offshore, and marine shiptrack data.

HSI image of 2001 Geoscience Australia Gravity dataset (Bouguer onshore, Free Air offshore)

Geoscience Australia Gravity



Geosat Satellite Gravity

Free Air

BouguerBouguer corrected to eliminate the effects of bathymetry.Principal signal is long-wavelength change caused bymoho topography.

Signal dominated by bathymetry in areas with steep slopes.



Digital Elevation Model

Digital Elevation Models (DEM’s) often show the youngest structures, and any active geological structures.They are widely used for neotectonic analysis. The composition of eroding terrain controls its resis tance to weathering, hence DEM’s can be used to distinguish different compositional domains.

The Digital Elevation Model (DEM) for the project area shows the topography of Tasmania and southern Victoria, resulting from Cretaceous to Recent uplift. Notably the bathymetry of Bass Strait is very shallow and flat. The continental margin transects the Otway and Sorell Basins, with a steep shelf edge sloping down to the abyssal plain at ~4-5km depth.



Offshore Seismic + Published Cross Sections

Seismic coverage in the onshore and near-shore parts of the Otway Basin is extensive, however imaging of basement is generally poor. Offshore coverage in both the Otway and Sorell Basins is provided by Geoscience Australia deep (16s) seismic data.

In this study the limited seismic data shown above have been used as a calibration tool for the depth to basement modeling and the structural interpretation (particularly timing of structural reactivation). In addition, numerous published cross sections (largely based on seismic) have been used for similar purposes.

Wells Coverage

Onshore and nearshore parts of the Otway Basin have good wells coverage. The offshore Otway and SorellBasins have very sparse well coverage. In this study, basement-penetrating wells were used to calibrate the SEEBASE™ model.



Plate Tectonic Reconstructions

Plate tectonic reconstructions, produced from the PaleoMap project (Ross & Scotese, 2000) and Muller (2000), were used in rate-azimuth analysis and the creation of plate tectonic movies. The reconstructionsprovided constraints on the interpretation of basement terranes and the timing and kinematics of tectonic events.

Ross & Scotese, 2000

Muller, 2000



Interpretation Confidence

Due to the large differences in data quality (particularly in the magnetic coverage) and the presence or absence of magnetic basement lithologies, the confidence levels in the interpretations presented here vary significantly. The above map shows where the interpretations are of relatively low confidence (orange). Aeromagnetic dataquality in these areas varies from poor to non-existent. Cream coloured areas are covered by reasonable quality gravity and magnetic data, but due to geological factors (e.g. magnetic Quaternary volcanics and non-magneticbasement lithologies) are more difficult to interpret. Green areas are covered by re latively recent, high resolution aeromagnetic surveys and good gravity data. See Appendix 1 for magnetic survey information.



Calibration of Potential Field DataCalibration is a critical process in any potential field interpretation.

In order to extract as much reliable geological information as possible from potential field data, it is critical to calibrate the data. This is done initially using mapped geology or basement well intersections combined with rock property data (e.g. magnetic susceptibility, density). Once identified, mapped geological units can be traced offshore or undersedimentary cover. Knowing the particular geological units provides information about basement composition and allows for much better constrained depth models from magnetic data.

Away from outcrop control, seismic data are integrated (when ava ilable) to furtherconstrain the development of a geological model. Basement penetration by wells and deep seismic data are particularly useful in constraining depth-to-basement estimates from theaeromagnetic data.

Why Basement?The basement of any basin provides the foundation onto which the sediments are deposited. The rheology and mechanical behaviour of the basement controls the geometry and rate of subsidence of the evolving basin. Basement rheology and mechanical behaviour aredetermined by its composition and structural fabric. Thus it is important to understand basement evolution prior to basin development.

Understanding basement structures allows models to be developed that can predict which structures will reactivate, and how they will move under an applied stress. Using plate tectonic reconstructions, the far- field stress state during past events can be estimated and akinematic reconstruction produced for each event. Basin sediments deform in response to movements in the basement and to gravity. Knowing how and when the basement moves provides a basis for predicting the most likely locations of depocentres and structures in the sediments.

Hence basement influences:

• basin phase architecture

• source-rock quality and distribution

• heat flow

• migration focusing, pathways and timing

• trap timing, distribution, type, integrity & size

• sediment supply and stratal geometry

• reservoir, seal quality & distribution



Systematic Approach to Basin Resource Evaluation

The methodology used to develop a comprehensive structural model relies on theintegration of all available geological information. Individual datasets alone can be ambiguous and when isolated often produce poorly constrained interpretations. Through integration, the model can be tightly constrained. Integration provides the means withwhich to calibrate each dataset to the other.

Basin Phase Architecture

Subsidence/Uplift

Climate

Eustasy

Paleogeography

Intraplate

DeformationStratal

GeometryPlate

Interactions

SourceSeal

ReservoirTiming,

Distribution &Character

Trap Timingand Style

Timing and/or focussingof Fluid Movement

Basementgrain, fabric &

structure

Basin Phase Architecture

• Scoping

– risk evaluation –•risk reduction per unit cost

• Calibration

• Interpretation• Documentation• Presentation

Choosing the “right” approach

S.A.B.R.E.™

Efficient and Effective Exploration

Old Data

New Technology

Good Geology

New Views in Old Basins

New Exploration and Acquisition Strategies

Bottom-up

Basement Character and Petroleum Systems



Basement Framework of SE Australia

The Otway and Sorell Basins overlie basement of the Kanmantoo Fold Belt.

The basement framework of SE Australia remains poorly understood, especially the “connection” between Tasmania and mainland Australia. The new model presented here is the latest SRK interpretation of available datasets. This model has largely been derived using a combination of the new 2001 Geoscience AustraliaNational Gravity Grid and magnetic data reprocessed during this project (datasets unavailable to previous workers).

Our interpretation shows that the western part of Tasmania is part of the greater Kanmantoo Fold Belt; a Cambro-Ordovician mobile belt containing Neoproterozoic -Ordovician sediments and volcanics. The eastern part of Tasmania is part of the Silurian-Carboniferous Lachlan Fold Belt. The Kanmantoo and Lachlan Fold Belts are separated by the Moyston-Tamar Fault Zone; a major, shallowly east-dipping structure. Four ?Neoproterozoic rigid crustal blocks have been identified, that have remained relatively undeformed duringPaleozoic tectonism. Deformation has been focussed between these blocks, causing complex, sinuous, structural geometries in the Bass Strait area.

The contrasting basement terranes and the structures within and between them were a first-order control on the evolution of the basins of SE Australia, including the Otway, Sorell, Bass and Gippsland Basins.

This report outlines the basement composition, structure, terranes and depth, and the influence these have onbasin evolution and character.

Lachlan Fold Belt

GawlerCraton

KanmantooFold Belt

AntarcticCraton

RobertsonBay Terrane

WilsonTerrane

Cambrian fold belt overlying attenuated Proterozoic curst

Palaeozoic fold belt overlying attenuated Proterozoic crustwith Cambrian oceanic crustalelements

Proterozoic craton

Rigid Neoproterozoic crustal block

“Kimban” Mobile Belt

SorellFault Zone

Governor Fault Zone

Moyston-Tamar Fault Zone

?



Basement Terranes

The above map shows key basement terranes and deep crustal fracture zones in SE Australia. The Otway and Sorell Basins overlie the Kanmantoo Fold Belt, which consists of Cambrian platform sequences, turbiditesand volcanics deposited on the extended Proterozoic margin of Gondwana. They were deformed during the Cambro-Ordovician Delamerian Orogeny, forming the Kanmantoo Fold Belt.

The Paleozoic Lachlan Fold Belt consists of ~N-S trending zones which overlie attenuated Proterozoic crust with Cambrian oceanic crustal elements. The fabric of the Lachlan is controlled largely by inherited crustal elements formed during Cambrian extension and contraction of the Proterozoic continental margin. Structures with each terrane are generally “thin-skinned”, with major basement-involved, “thick-skinned” structures forming terrane boundaries.

The boundary between the Kanmantoo and Lachlan Fold Belts is the Moyston-Tamar Fault Zone, which has provided a first order control on the development of the Bass Basin.

Terrane boundaries have also provided a first-order control on the location and geometry of the Otway and Sorell Basins. The “Kimban” Front (i.e. the interpreted margin of the Paleoproterozoic Kimban mobile belt of the Gawler Craton in South Australia) has controlled (i) the margin of the Kanmantoo Fold Belt, (ii) the outer margin of the Otway Basin, and (iii) the locus of Late Cretaceous/Tertiary breakup between Australia and Antarctica. Intra-Kanmantoo Fold Belt structures have also significantly influenced the Otway and SorellBasins. The northern margin of the Otway and Gippsland Basins is defined by somewhat enigmatic E-Wtrending deep basement (?mantle) structures which possibly evolved during the Permian.

KANMANTOO FOLD BELT

Tabberabbera–

Mallacoota

MathinnaBendigo

Stawell

Oceanic Crust

Moyston-Tamar Fault Zone

L A C H L A N F O L D B E L T

Transition Zone

MelbourneBlock

Sorell Fault Zone

Otway BasinOtway Basin

SorellSorellBasinBasin

“Kimban” Front



The basement beneath the Otway and Sorell Basins is composed of Cambro-Ordovician marine clasticsediments and volcanics of the Kanmantoo Group. The Kanmantoo Group was variably deformed and metamorphosed during the Late Cambrian-Early Ordovician Delamerian Orogeny. Several phases ofmagmatism resulted in numerous syn- to post-orogenic granite intrusions (which may have implications for heat flow in the overlying basins). Data is insufficient to map the offshore distribution of these plutons.

Basement Composition

Granites

mid to Upper Devonian

Lower Devonian

Siluro-Devonian

Lower Silurian

Late Cambrian post-orogenic

Late Cambrian syn-orogenic

(Meta)sediments

Late Devonian-Carbsediments

Late Devonian volcs

Early Devonian volcs +sediments

Early Devonian clastic seds

Siluro-Devonian sediments

Ordovician metamorphics

Ord-Early Silurian sediments

Late Cambrian-Ord sediments

Cambrian greenstone belts

Early-Mid Cambrian sediments

Latest Neoprot sediments

Neoproterozoic (meta)sediments

Oceanic Crust

Highly attenuated continental crust



Basement Structure - OverviewBasement structures are key reactivation zones during basin formation. The following basement structures have been interpreted during this project:

• Faults/shear zones

• Fabric/grain/foliation

• Deep crustal fracture zones

• Transfer/accommodation zones

These structures have been interpreted using the following data sources:

• Mapped faults

• Magnetic anomalies & discontinuities

• Gravity anomalies & discontinuities

• DEM trends & breaks

• Seismic basement-involved faults

The history of the structures is quantified using the following criteria and calibration:

• Structural superposition

• Age of strata displaced

• Relationship to intrusive bodies

• Consistency of fault kinematics to regional paleo-stress regimes and plate movements

• Correspondence to: mapped structures, known movement history

In the GIS, the faults are all attributed by:

• Source (magnetics, gravity, DEM, map etc)

• Orientation

• Displacement

• Basement character (involved or detached)

• Initiation age

• Reactivation history & displacement



Basement-Involved Faults

All interpreted basement-involved faults in the Otway and Sorell Basins (colours represent initiation age). Allfaults have been attributed according to their reactivation history, data sources, orientation, displacement etc.

Neoproterozoic

Early Cambrian

Cambro-Ordovician

Early Silurian

Mid Devonian

Early Carboniferous

Paleo-Mesoproterozoic

?Permian

Late Jurassic

Early-mid Cretaceous

Initiation Ages:



MOYSTON

- TAMAR FAULT ZONE

Deep Crustal Fracture Zones

Deep crustal fracture zones are seldom directly mappable in basins. They are deep seated (possibly mantle -derived), ancient zones of crustal weakness that directly or indirectly influence the subsequent developmentof structures and basins. They are often repeatedly reactivated. Often they coincide with terrane boundaries.They are represented in map form as polygons, in order to take account of their width and dip (e.g. the Moyston-Tamar Fault Zone dips shallowly to the NE and transects the entire crust).

The Otway and Sorell Basins are bounded and transected by several deep crustal fracture zones which have exerted a first-order control on basin location and geometry. Of particular note is the Southern Australian Fracture Zone which is described in more detail on page 31.

SorellFAU

LT ZON

ESOUTHERN AUSTRALIAN FRACTURE ZONE



Proterozoic Basement Deformation

Basement terranes in SE Australia have undergone a complex history spanning the Proterozoic to the late Paleozoic. The entire area is interpreted to be underlain by variably extended Paleo-Mesoproterozoiccontinental crust. Large scale structures which evolved during extension of the Proterozoic crust have exerted significant influences on the subsequent structural evolution of the Kanmantoo and Lachlan Fold Belts.

Both the Kanmantoo and Lachlan Fold Belts were formed during the evolution of the eastern Australian active margin during the late Neoproterozoic to late Paleozoic. Seven major tectonic “events” have been identified (in addition to a complexProterozoic basement evolution):

1. Neoproterozoic Extension 5. Late Silurian-early Devonian Deformation

2. Early Cambrian Extension 6. Mid-Devonian Deformation

3. Cambro-Ordovician Orogeny 7. Early Carboniferous Deformation

4. Early Silurian Deformation

These events are outlined in the subsequent pages.



Neoproterozoic Basement Evolution

Intracratonic rifting in Rodinia was initiated at ~800Ma by rapid NE-SW extension resulting in the NW trending Gairdner Dyke Swarm in South Australia. Subsequent extension between North America and Australia exploited these structures (some of which constitute the Tasman Line), resulting in the deposition of thick Neoproterozoic sedimentary sequences in a belt that included the Adelaide Geosyncline, what is now western Tasmania and parts of the Ross Orogen in Antarctica.

Major NW linear structures in western Tasmania and western Victoria are interpreted to have been initiated during this Neoproterozoic extension. Therefore the Moyston-Tamar Fault Zone probably dates back to the early Neoproterozoic. Later Neoproterozoic extension was oriented more E-W, resulting in N-S trending extensional structures in the Kanmantoo Fold Belt.



Early Cambrian Basement Evolution

Early Cambrian extension in eastern Australia was caused by ongoing rifting and eventual breakup between Australia & North America along the Tasman Line. In the early Cambrian this extension was oriented ~NW -SE. Most of the extension was accommodated to the east of the Tasman Line on structures beneath the Lachlan Fold Belt.

In the Kanmantoo Fold Belt, Early Cambrian extension in ~NE trending rifts resulted in the deposition of thick sedimentary sequences (e.g. Kanmantoo Group turbidites and volcanics (including the Mt ReedVolcanics in Tasmania). Similar Cambrian sequences may underlie large parts of the Lachlan Fold Belt.

The structures and variation in crustal thickness produced during the Neoproterozoic and early Cambrian extension events define the principal basement architecture that has significantly controlled the location and nature of all subsequent Palaeozoic and Mesozoic contractional and extensional events in SE Australia.



Cambro-Ordovician Basement Evolution

The Cambro-Ordovician Delamerian Orogeny (~520-480Ma) caused extensive ~WNW-directed deformation in the Adelaide and Kanmantoo Fold Belts, and presumably in basement beneath parts of the Lachlan Fold Belt. Deformation was accompanied by extensive syn- to post-orogenic plutonism. Obducted Cambrianoceanic crustal elements and island arc fragments (preserved in the Victorian greenstone belts), together with Cambrian passive margin sediments deposited on the extended Proterozoic craton margin, were deformed and variably metamorphosed at this time. After initial deformation, post-collisional medium- to high-K calc -alkaline andesitic volcanic arcs formed on the active continental margin. Thick Late Cambrian-Ordovicianquartz-rich turbidite deposits overlie the arc sequences in parts of the Lachlan Fold Belt.

Delamerian structures underlying the Otway and Sorell Basins include N-S thrusts and NW trending sinistraltranspressional faults and shear zones. These structures have significantly influenced Mesozoic basin evolution.



Early Silurian Basement Evolution

The Early Silurian (Benambran) deformation was a protracted, contractional, widely distributed, event that took place between 450 and 430 Ma. Structures associated with the deformation include widespread thrusts, local nappe development and upright folds. East-west fold axes and thrust kinematics reflect mostly N-S to NW-SE shortening. The focus of deformation, metamorphism and granitic plutonism was further to the NE in NSW, however extensive deformation, metamorphism and mineralisation (e.g. Bendigo-Ballarat) occurred in a narrow belt extending from eastern Tasmania to the Stawell and Bendigo Zones of the Lachlan Fold Belt.These structures were critical to the evolution of the Bass Basin.



Mid-Devonian Basement Evolution

Widespread folding and faulting occurred during late Early to mid Devonian times throughout most of the Lachlan Fold Belt and the eastern-most parts of the Kanmantoo Fold Belt. The (Tabberabberan) deformation, with an indicated NNE- to NE-directed principal stress (eg. Glen, 1990) was a short-lived event between 390 and 380 Ma (Vandenberg, 1999). The deformation followed an episode of Early Devonian rifting, felsicvolcanism and granite intrusion, and was associated with contraction and foreland thrust loading to the west of a volcanic arc developed on an oblique convergent paleo-Pacific margin (Veevers, 2000).

Curvilinear Tabberabberan meridional-trending folds and reverse faults form the dominant structures of the basement in the Bass Strait area. This zone wraps around a more rigid ?Proterozoic crustal block (the Melbourne Block) covered by a continuous sequence of Ordovician to early Mid-Devonian sediments. The margins of the underlying crustal block have largely focused deformation, in particular, the Mid -Devonianevent which was the first to affect much of the area. East of the Melbourne zone, in both onshore and offshore areas, reverse faulting was primarily linked to reactivated margins of Early Devonian rift margins



Early Carboniferous Basement Evolution

During the Early Carboniferous (Kanimblan) deformation event, renewed lateral compression in eastern Australia was associated with an adjacent magmatic arc, located at the convergent paleo-Pacific margin of the Lachlan Fold Belt (Collins & Vernon, 1992). The deformation, at about 345 Ma (Veevers, 2000), resulted in the regional formation of new conjugate strike-slip faults and some reverse reactivation of N-trending Early Silurian and Siluro-Devonian faults. In the Bass Strait region, major NE trending dextral strike slip faults formed. The strike -slip zones terminate in places against N-trending reverse faults that displace Upper Devonian sediments.



E-W Basement Structures [Southern Australian Fracture Zone]

The Southern Australian margin is transected by a set of relatively straight ~E-W trending structures. The structures are continuous, deep-seated, long-lived, and controlled the locus of separation from Antarctica. Thegenesis of these E-W structures is poorly understood. Some of them date back to the Paleoproterozoic (e.g.Polda Trough), and those in Eastern Australia probably post-date tectonismin the Lachlan Fold Belt (but pre -date Mesozoic extension); hence may have evolved in the Permian. The structures have been repeatedly reactivated, and constitute a first-order control on the basins of Southern Australia, including the Otway.

We propose the term Southern Australian Fracture Zone to describe this set of E-W structures.

Such continuous, continental-scale structures have long been recognised but not understood. Recent work suggests that they may form in the crust above density contrasts (ie compositional boundaries, possibly structures) in the mantle lithosphere, hence are long lived and largely unaffected by crustal deformationprocesses.



Basin EvolutionThe present-day geometry of the Otway and Sorell Basins is the result of the superposition of 7 tectonic “events” or basin phases spanning the Jurassic to Recent. These events/basin phases represent the effects of the breakup of Gondwana on the southern margin of Australia.

The following chart details the tectonic history of the Otway and Sorell Basins:

Stresses operating during each basin phase caused reactivation of basement structures and reactive fabrics, as well as the development of new structures. Understanding the kinematics of each tectonic event allows a predictive model for structural reactivation to be applied to the interpreted faults from fault history data calibrated with geological observations (e.g. seismic, maps), event maps for each basin phase have been constructed. The following pages show our interpretation of the structural evolution of the Otway and SorellBasins and surrounds.

Key stratigraphic packages associated with the above basin phases include:

BASIN PHASE 1: Casterton Formation, Crayfish Group

BASIN PHASE 2: Eumeralla Formation

BASIN PHASE 3: ?

BASIN PHASE 4: Sherbrook Group

BASIN PHASE 5: Wangerrip Group

BASIN PHASE 6: ?Nirranda Group

BASIN PHASE 7: Heytesbury Group

LATE

LATE

EARLY

EARLY

MIDDLE

PALEOCENE

EOCENE

MIOCENE

OLIGOCENE

PLIOCENEPLEISTOCENEQUATERNARY

150

200

100

0

50

BASIN PHASE 1

BASIN PHASE 2BASIN PHASE 3

BASIN PHASE 4

BASIN PHASE 5

BASIN PHASE 6

Late Jurassic Extension -Sinistral strike-slip, pull-apart basins in Otway & Sorell.Graben formation in Robe Trough, ColacTrough & Torquay Sub-basin.

Early Cretaceous Extension - Rifting inOtway & Sorell Basins.

Mid-Cretaceous Inversion

Late Cretaceous Extension + Sag

Early Eocene Inversion

Eocene-Miocene Platformal Sag

BASIN PHASE 7 Miocene-Recent Inversion



Basin Phase 1: Late Jurassic Extension

Late Jurassic to Early Cretaceous (~?160-120Ma) NW-SE extension marks the last episode of the final break-up of Gondwana. The extension was initially focussed in western Australia, and propagated across the southern margin of the continent to the Bass Strait region. A series of linked E-W trending basement structures can be traced from the southern Perth Basin, through the Great Australian Bight and stepping south to the northern margin of the Gippsland and Otway Basins. These structures (collectively termed the Southern Australian Fracture Zone - SAFZ) provided the key “boundary condition” for development of the southern Australian margin basins (eg. Polda Basin and Robe Trough).

These ?Precambrian E-trending faults underwent oblique left-lateral normal reactivation and generated a set of major NW-trending left-lateral relay zones separating the principal depocentres in the Basins. Major sinistralstrike-slip deformation occurred on the SW Ceduna Accommodation Zone, which partitioned significant extension to the west and relatively minor extension to the east forming relatively narrow lacustrine basins in the Otway, Sorell, ?Bass and Gippsland Basins. The SW Ceduna Accommodation Zone exploited pre-existingbasement structures (the boundary between the Kimban Mobile Belt and Kanmantoo Fold Belt).

This extensional basin phase has remained poorly understood in the Otway Basin, since evidence for it is largely overprinted by later Cretaceous rifting. Sinistral strike-slip movement along NW trending, pre-existingbasement structures (e.g. SW Ceduna Accom Zone and parallel structures to the E) resulted in complex riftgeometries which are described overleaf.

SWCeduna

Accommodation Zone



115 Ma

Antarctica

Australia

SWCeduna

Accommodation Zone


?

Pull-apart basins

The kinematics and geometry of early rifting in the Otway Basin has remained poorly understood. The recognition that extension was probably oriented NW-SE until ~120Ma is the key to understanding this basin phase. Extension occurred via:

(i) sinistral strike slip movement on pre-existing NW trending basement structures (e.g. the SW CedunaAccommodation Zone);

(ii) sinistral transtension on E-W trending basement structures (e.g. Southern Australian Fracture Zone);

forming complex rift geometries. During this basin phase, the Casterton Formation and Crayfish Group were deposited.

The Bass Basin may have been initiated at this time as a sinistral pull-apart on a NW trending structure.




SWCeduna

Accomm

odation Zone

Pull-apart basins

In the Otway and Sorelll Basins, late Jurassic-early Cretaceous NW-SE extension was focussed by parts of the E-W trending Southern Australian Fracture Zone (SAFZ [see previous page], shown in blue above and by pink arrows overleaf). Extension was accommodated via:

(i) oblique left-lateral extension on pre-existing E-W trending basement structures (i.e. parts of the SAFZ, e.g. northern margin of Robe and Colac Troughs) forming complex half graben;

(ii) new NE trending normal faults forming NE trending half graben (e.g. Otway Ranges - Torquay Sub-basinarea, parts of Robe Trough);

(iii) pull-apart basins in the stepover between sections of the Southern Australian Fracture Zone (e.g. Penolaand Ardonachie Troughs), caused by sinistral strike-slip movement on pre-existing NW trending basement structures in the Kanmantoo Fold Belt (the basins form in the stepovers in the basement structures).

(iv) pull-apart basins on NW trending sinistral strike-slip structures (e.g. parts of Sorell Basin, basins within the SW Ceduna Accommodation Zone).



Basin Phase 2: Early Cretaceous Rifting

Early Cretaceous (~120-100Ma) NE-SW extension in Great Australian Bight, Otway, Bass and GippslandBasins is indicated by the development of major NW-trending growth faults separated by NE-trendingtransfer zones. Both sets of structures follow older basement structures which evolved during theNeoproterozoic and Paleozoic. Plate tectonic reconstructions (e.g. Muller, 2000) do not show any significant movement between Australia and Antarctica until after 115Ma when Antarctica moves southeastward relative to Australia. It is therefore probable that any NE-extension would most likely to have occurred prior to 115Ma.

The switch in extension direction from NW-directed to NE-directed in the Early Cretaceous resulted in a change of roles for the faults in the basins of SE Australia; the NE structures acting as transfer faults and the NW structures acting as normal faults.

115 Ma

Antarctica

Australia

SWCeduna

Accommodation Zone



Basin Phase 2: Early Cretaceous Extension

Early Cretaceous (~120-100Ma) NE-SW directed extension was focussed along the a ~NW trending fault zone including the Tartawup, Mussel and Sorell Faults (see pink arrows above). Only minor rifting occurred inboard of this fault zone, and was accompanied by widespread regional sag, resulting in the deposition of the Eumeralla Formation. The amount of rifting outboard of the major fault zone is not well known since the age of sediment fill is unconstrained. Major extension also occurred in the Bass Basin at this time.

Extension in the Otway and Sorell Basins was localised by pre-existing basement structures, principally ~NW trending faults in the Kanmantoo Fold Belt.

Since the Eumeralla Formation was probably deposited during an entirely different kinematic regime/basinphase to the older units in the Otway Group, it may warrant separation from the Otway Group.



Basin Phase 3: Mid-Cretaceous Inversion

Mid-Cretaceous (~100-90Ma) compression caused extensive inversion and uplift in the Otway and StrzeleckiRanges (up to 3km), and (very) minor inversion in the remaining parts of the Otway and Gippsland Basins.The cores of the resulting major anticlines have an associated positive gravity anomaly. Inversion of both Late Jurassic growth faults and Early Cretaceous accommodation structures reflects a ~NW principal stress direction. Extensive uplift occurred in northern Tasmania, Bass Strait and onshore Victoria at this time (as evidenced by Fission Track data – Dumitru et al, 1991; O’Sullivan, 1994).



Basin Phase 4: Late Cretaceous Extension

This basin phase occurred during the final phases of breakup between Australia, Antarctica and the Lord Howe Rise. Extension was complex, and involved:

(i) ENE directed extension in the Late Cretaceous (~90-80Ma) between Australia and the proto-Lord Howe Rise terminated with break-up and seafloor-spreading in the Tasman Sea. The oldest age of Tasman oceaniccrust interpreted is ~80Ma (Geoscience Australia timescale) or 83Ma (Muller’s reconstructions). ExtensiveLate Cretaceous volcanismat the main rift margins ( ie. to the east of the Bass Basin) was associated with the latest stages of extension and the onset of sea floor spreading.

(ii) NNW directed extension between Australia and Antarctica along the southern margin, culminating in sea floor spreading in the Eocene (~45Ma). This extension was accomp anied by significant subsidence along the entire southern margin.

The Otway and Sorell Basins were affected by both of these extensional stress regimes, resulting in ~NNE directed extension.

90 Ma

Tasman Sea

Lord

How

e R

ise

?

?

Highly attenuatedContinental crust



Basin Phase 4: Late Cretaceous Extension

In the Otway and Sorell Basins, NNE directed Late Cretaceous extension reactivated pre-existing NW trending basement structures of the Kanmantoo Fold Belt. Extension was largely focussed along and outboard from the present-day continental shelf edge (dotted yellow line above), and on the Tartawup-Mussel Fault Zone (pink dotted line above). Very little extension is recorded east of the Sorell Fault Zone (green dotted line). Late Cretaceous extension spanned the early Turonian to late Maastrichtian in the Otway Basin (~83-65Ma), and resulted in the deposition of the Sherbrook Group (up to 5km in the Voluta Trough, more in the Outer Otway). Only minor extension occurred inboard of the Tartawup-Mussel Fault Zone.

Geoscience Australia deep seismic data and interpretations do not show much evidence for large scale uppercrustal extension in the outer, deep water Otway Basin. Therefore the major subsidence accompanying Late Cretaceous extension may have been accommodated in the lower crust and/or mantle lithosphere.

Significant extension also occurred in the Durroon Sub-basin of the Bass Basin at this time. This extension was oriented ~ENE-WSW, and was more directly related to the opening of the Tasman Sea.



Basin Phase 5: Eocene Inversion + Sag

Onset of sea floor spreading between Australia and Antarctica began at ~45Ma. The change from rifting to drifting caused a stress reversal in the basins along the Southern Margin, from ~N-S extension to ~N-Scompression, probably caused by the onset of ridge-push forces. The resulting compression caused minor inversion of some pre-existing structures in the Otway and Gippsland Basins, as evidenced in the seismic. In all liklihood more than one inverion “event” occurred, as structures responded to subtle changes in thespreading rate and direction. These “events” are likely to be unevenly distributed along the southern margin.

Ongoing sedimentation occurred throughout the Tertiary in the Otway and Sorell Basins, largely due to thermal subsidence that occurred in response to continental breakup. The Wangerrip and Nirranda Groupswere deposited during this basin phase.



Basin Phase 6: Miocene-Recent Inversion + Sag

During the Miocene-Recent, ~ENE directed intraplate stresses have reactivated “weak” basement structures in the Adelaide Fold Belt, Otway Ranges and Strzelecki Ranges. This compression led to uplift which formed the present-day topography. Uplift continues today, as evidenced by recent seismicity. Up to 1km of uplift has occurred on major structures. Minor inversion in the Otway and Sorell Basins may have occurred at this time.



Tertiary – Quaternary Volcanics

Volcanic activity in SE Australia has been fairly continuous throughout the Tertiary to Recent, with peaks in the Paleocene-Eocene (~55Ma, shown above in turquoise) and Plio-Pleistocene (shown above in green). This volcanism has been cause by the Australian plate moving over mantle hotspots, and continues today in the Mt Gambier region of South Australia. The offshore extent of Tertiary – Quaternary volcanics in offshore areasis relatively poorly constrained due to the poor quality aeromagnetic data on which the interpretation relies.



SEEBASE™ (Structurally Enhanced View of Economic Basement)

Depth to Basement

SEEBASE™ is much more than just another magnetic depth-to-basement model. It is the culmination of a number of calibration and integration steps:

• Integrated structural/kinematic interpretation

• Geophysical modeling

• Seismic & well calibration

• Integration of tectonic events & responses

SEEBASE™ is a qualitative model of economic basement topography that is consistent with the structural evolution of the basin. SEEBASE™ defines basin architecture, and is a predictive model for exploration. It is a key base for understanding basin phase geometry/distribution and petroleum systems. As new data are acquired that allows more precise calibration, SEEBASE™ can be updated to reflect all new information.

SEEBASE™ provides a foundation for petroleum systems evaluation, including play element distribution (source/reservoir/seal), migration pathways, zones of structural complexity, trap distribution, trap type & integrity, paleogeography, oil vs. gas distribution etc.

What is SEEBASE™?



SEEBASE™ Methodology

1. Depth models to magnetic basement sources, obtained from profiles across selected anomalies

2. Attribution of source type to depth estimates (require top-basement sources)

3. Identification of major basement-involved faults

4. Integration of event/response history

5. Integration of gravity modeling & interp (if available)

6. Incorporation of refraction/seismic/well data (if available)

7. Intelligent contouring of “top basement” depth estimates

8. Grid construction using CPS-3

9. 2D and 3D image processing in ERMapper

deepcrustalsources

sediments

sediments &volcanics

top-basement

mafic dykes

Seismic Depth to Basement

Magnetic Sources

Calibration Example



Magnetic Profiles & Depth Points

Modeled Profiles & Modeled Bodies

ModelVision™ outputs

Intrabasinal Volcanics

Top Basement Bodies



Otway, Sorell and Bass Basins SEEBASE™ Model 1

Depth (m)

Tasmania

Kangaroo Is

Melbourne

Bass BasinBass Basin

Torquay Sub-basin

Penola Trough

Robe Trough

Sorell Basin

Voluta Trough

Deep Otway Basin

Outer Otway High

Beachport Trough

Ardonachie Trough

Penola Trough

SEEBASE™ modelling in the Otway and Sorell Basins was very successful in areas of good magnetic and gravity data quality combined with basement rocks containing magnetic units with minimal density contrasts (i.e. gravity data then reflects sediment thickness). The Onshore Otway from the Robe Trough to the Ardonachie Trough falls into this category. In such areas the SEEBASE™ model is probably accurate to ±10%. Elsewhere, especially in areas where no magnetic data is available, SEEBASE™ is likely to be less accurate, however we are confident that the overall basin shape is likely to be well represented. In areas where both gravity and magnetic data are poor, and there are few other constraints, SEEBASE™ construction purely relies on seismic interpretations, gravity “shape” and internal consistency with the nearest available constrained area. Most of the offshore areas above fall into this category.

Due to the poor constraints in the deep offshore Otway Basin, two SEEBASE™ models were developed during this project to account for radically different possible interpretations of basement (see p48). Model 1 (above) uses the relatively shallow offshore basement picks of Moore et al (2000), Palmowski et al (2001) and Norvick (pers. comm. 2001).



Otway, Sorell and Bass Basins SEEBASE™ Model 2

Depth (m)

Tasmania

Kangaroo Is

Melbourne


Torquay Sub-basin

Penola Trough

Robe Trough

Sorell Basin

Voluta Trough

Deep Otway Basin

Outer Otway High

Beachport Trough

Ardonachie Trough

Penola Trough

Model 2 (above) uses the relatively deep basement picks as outlined by the Victorian Geological Survey (PESA News 2001) and PIRSA (P Boult pers. comm. 2001). Model 2 nearly doubles the volume of the deep Otway Basin. This model may be more consistent with deep well penetrations at Breaksea Reef 1 and Copa 1which only penetrated ~Turonian sediments at ~4km depth, potentially implying thick Early-Mid Cretaceous sediments below.

Clearly more work is needed to resolve which SEEBASE™ model is more appropriate. Such work would require a study of the deep crustal architecture of the Otway Basin involving gravity profile modelling,analysis of refraction/velocity data, and re-interpretation of reprocessed deep seismic data. Acquisition of offshore aeromagnetic data would also facilitate further depth modelling.

Examples of the two different ways of interpreting top-basement in the deep offshore Otway seismic data area shown overleaf.



Seismic Depth to Basement in Offshore Otway

Line 137-03

Line 137-09

Shallow Pick (approx)Shallow Pick (approx)

Deep Pick (approx)Deep Pick (approx)

Shallow Pick (approx)Shallow Pick (approx)

Deep Pick (approx)Deep Pick (approx)

There are two possible interpretations of top-basement in the deep offshore Otway Basin:



Otway, Sorell and Bass Basins SEEBASE™

N


SorellSorell BasinBasin

TorquayTorquay SubSub--BasinBasin

Outer Otway Basin

Outer Otway Basin

Outer Otway High

Outer Otway High

PenolaPenola TroughTrough

Robe TroughRobe Trough

TasmaniaTasmania

King IsKing Is

The architecture of the Otway, Sorell and Bass Basins reflects the complex overprinting of 3 extensionalbasin phases, and at least 3 inversion “events”. Significant features evident in the Otway/Sorell/BassSEEBASE™ include:

• All basins dominated by NW trending half graben geometries

• Otway & Sorell Basins contain 3 distinct zones:

(i) An inner zone dominated by deep half graben and linear basement highs;

(ii) A central zone of deep, relatively flat basement;

(iii) An outer basement high.

• Torquay, Colac and Otway Ranges (i.e. Otway Basin E of Sorell Fault Zone) exhibit distinctly different basin geometry dominated by NE and E-W inverted half-graben.

• Bass Basin is significantly deeper than other basins and surrounded by basement highs

ColacColac TroughTrough

Otway RangesOtway Ranges

SorellSorell Fault ZoneFault Zone

Oceanic CrustOceanic Crust

Model 1



N EE--W Southern AustralianW Southern AustralianFracture ZoneFracture Zone

SorellSorellFault ZoneFault Zone

MoystonMoyston –– TamarTamarFault ZoneFault Zone

““KimbanKimban””FrontFront

DelamerianDelamerian StructuresStructuresinin KanmantooKanmantoo Fold BeltFold Belt

Basement Controls on Basin Architecture



Summary

• The basement geology of the Otway and Sorell Basins is dominated by the KanmantooFold Belt.

• Seven basin phases/tectonic events have shaped the Otway and Sorell Basins during the Mesozoic and Tertiary.

• The present-day geometry of the Otway and Sorell Basins was established during 3 extensional Mesozoic basin phases:

1. Late Jurassic – Early Cretaceous NW-SE transtension (Casterton/Crayfish)

2. Early – Mid Cretaceous NE-SW extension (Eumeralla )

3. Late Cretaceous NNE-SSW extension (Sherbrook)

• Initial basin evolution occurred during NW -SE Late Jurassic transtension, forming linked pull-apart basins and oblique graben.

• At least 3 inversion “events” have variably influenced the Otway and Sorell Basins(Mid Cretaceous, Eocene, Miocene-Recent).

• Basin architecture is largely controlled by basement structures, composition, fabric and rheology.

• NW to N-S trending basement shear zones/terrane boundaries (mainly Delamerian)were a first-order control on basin evolution during the Mesozoic.

• A SEEBASE™ model for the Otway, Sorell and Bass Basins shows basement topography, and can be used to map basin phase distribution, mig ration pathways and trap type/distribution.

The Otway-Sorell Basins SEEBASE™ study has shown that basement geology (particularly structure) is a first order control on basin architecture. Reactivation of basement structures can be used to explain the structural evolution of the Otway and Sorell Basins during the Mesozoic. The key findings of this study are as follows:



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Documents

Otway & Sorell Basins SEEBASE Project · • The basement geology of the Otway and Sorell Basins is dominated by the Kanmantoo Fold Belt. • Basin architecture is largely controlled