Engineering Geological Factors Affecting Slope Stability in Soft Brown

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ENGIIÍEERING GEOLOGICåI, FÀCTORS ÀFFECTING

SLOPE SÎÀBILIfY IN SOFT BROÍIN COÀI, DEPOSIÎS - A SOUTH ÀUSTRÀI,IÀIÜ E¡(ÀMPT,E

(voLrrME 1)

ANDREW GEORGE I(REMOR B.App.Sc., APP Geoloçry. (S.A.I.T) Grad,. Dip. CoaI Geology. (Newcastle)

t992

DEPÀRTMENT OF GEOLOGY ÀND GEOPHYSICS UNIVERSITY OF ADELÀIDE

Awor't]¡'tC ir-)lí-l'-;

THIS THESIS IS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY STATETTEIIT OF ORIGIIIÀI,ITY

This thesis contains no material which has been accepted for the award or any other degree or diploma in any university and, to the best of my knowled.ge and. belief , it contains no material- previously published or wri,tten by another person' except where due reference is made in the text.

Andrew Kremor. I

TÄBLE OF CONTSNTS I LIST OF FIGT]RES VI LIST OF TÀBLES XII ÀPPEIIDICES - VOLI]IÍE 2 XIII

ÂCKNOIVLEDGEMEITTS xIv

ÀBSTRÀCT XV Pacre No.

1.0 INTRODUCTION 1 1.1 Nature of engÍ.neeri-ng geological 2 aspects of soft brown coal nining areas I.2 The Lochiel Coal Deposit - Àn example 4 of a soft brown coal dePosit L.2.I Previous investigations 6

2.O GEOLOGICAI, SETÎINGS OF SOFT BROÍ|N COÀÉ I DEPOSITS - CLÀSSIFICÀTION OF TIIE S1 VIIICENT BASIIÚ 2.I Geotectonic and' d'epositionâI settings I of soft brown coal d'ePosits 2.2 Geotectonic setting of the Lochiel 10 CoaI DePosit 2.2.I Structure of the northern 11 part of the St Vincent Basin 2.2.2 Igneous activitY 16 2.2.3 Geotectonic controls and 18 Basin formation 2.2.4 SummarY 20

2.3 Depositional characteristics of coal 20 de¡rcsits on nidPlate continental margins 2.3.L EustacY and sedimentarY 2t sequences 2.3.2 SedimentarY environments 23 2.4 Palaeogeographical sett'ing of the 26 northern St Vincent Basin 2.4.L Sources of sediment 26 IT 2.4.2 Cainozoic hiatuses 28 2.4.3 Palaeoclimate 30 2.5 Stratigraphic success ion 30

2.5.L Tertiary - St. Vincent Basin 32 2.5.2 Tertiary Pirie Basin 37 2.5.3 Quaternary 38 2.6 Geotectonic and eustatic model for the 39 stratigraphic sequence of the northern St Vincent basin 2.7 Summary 43

3 . O IDENÎIFICÀTIOIT OF EIÜGIITEERING GEOLOGICÀL 45 FÀCTORS ÀFEECÎIIÚG SLOPE STABILIIY II5 TTIE LOCHIEL COAI, DEPCISIT 3.1 Generic modes of slope failure in weak 46 rock a¡rd soil 3.2 Mecha¡risms causing failure of slopes 49 in weak rock and soi-l 3.3 Review of slope stability analyses for 52 the Ëochiel CoaI DePosit 3.3.1 SummarY of stratigraPhY 52 3.3.2 SummarY of engineering 54 properties 3.3.3 Modes of fail-ure 56 3.3.4 Mechanisms causing failure 57 3.4 Review of engineering properties 60 3.4.1 Intact material strength 60 3.4.2 Physical components of shear 62 strength in fine grained soils 3.4.3 Residual angle of friction 64 versus mineralogrY and grain shape 3.5 Discontinuiti-es 69 3.5.1 Tectonic discontlnuities 69 3.5.2 SedimentarY vteak zones 70 III

3.6 Porewater movement 74

4. O METHODOLOGY ÀI{D DATÀ ÀVÀIT,ÀBILITY 78 4.I Data availability and limitations 79 4.2 Methodology 84 4.2.1 Principtes of geological 85 interpretation and modelling 4.2.2 Principles of engineering 94 geological interPretation and modelling 4.3 Stratigraphic nomenclature for the 95 Lochiel CoaI DePosit

5.O .âNALYSIS OF SEDIMEIITS OCCTIRRING BELOW TIIE 98 O\ÍERBI'RDEI{ 5.1 Bunbunga Sa¡rd. Formation - Na¡rtawarra 98 Sand llenber 5.1.1 LithologY and lateral 100 variatíon 5.L.2 Depositional environment 105 5.1.3 Engineering Properties LO7 5.2 Bunbunga Sand Formation - Darnleigh 108 Park Sand Ìlember 5.2.7 LithologY and lateral 108 variation 5.2.2 DePositional environments 111

5.3 Bunbunga Sand Formation - Condowie 113 SiIt Member 5.3.1 Litholog'Y and lateral 116 variation 5.3.2 Depositional environments 1-24 5.3.3 Properties of weak zones L29 5.4 Bumbunga Sand Fomation - Kooliata 131 CoaI Member 5.4.1 H zone 133 5.4.2 GH interseam sediments L42 IV

5 .4.3 G zone ]-44 5.4.4 FG interseam sediments L47

5 .4.5 F zone 150 5.4.6 Depos itional environments 151 5.4.7 Synthesis of geological 155 controls and engineering properties 5.5 Bunbunga Sand Formation - Slmthesis of 156 geological processes controlling the properties of weak zones and porewater movement

6.0 ÀNÀLYSIS OF OVERBITRDEN SEDII'ÍEI{TS 160 6.1 Stratigraphic nomenclature 160 6.2 Warrind.i SiIt Formation 163 6.2.1 Lithological d'escriPtion, r66 facies relationshì.Ps and depos itional Processes 6.2.2 SummarY of dePositional 193 processes 6.2.3 Engineering Properties t96 6.3 Tarella SiIt Fomation 202 6.3.1 LithologY and lateral 202 variation 6.3.2 SummarY of facies 272 relationshiPs and depos itional environments 6.4 Tectonic strrrctures in the TareIIa 2L5 Silt Fomation 6.4.I Data available 2r5 6.4.2 ExÈensi.onal structures 2L7 6.4.3 Shear structures 223 6.4.4 Origins of tectonic 230 structures v

6.5 Slmthesis of geological processes and 234 engineering properties for the TareIIa SiIt Formation

6.5. 1 Strength of the ÎarelIa Silt 234 Shear Zone 6.5.2 Porewater movement 23s

7 .O SUUTTARY OF ENGINEERING GEOLOGY OF THE 238 LOCHIEL COÄI, DEPOSIT 7.L Highwall Stability Design Sectors 239

8.0 SIJUIIÍARY ÀI{D COIÚCLUSIONS 249 8.1 Geological processes 25t 8.1.1 Bumbunga Sand Formation 25r a.2 Warrind.i SiIt and TareIIa SiIt 252 Formations 8.3 Key engineering geological factors 253 affecting slope stability 8.4 Relationship between engineering 256 geological factors affecting slope stability and tectonics, eustacy and depositional processes

REEERENCES 258 VT

LIST OF FTGURES

CHAPTER 1 TITLE PÀGE NO.

1.1 Locality plan of the studY area. 5 L.2 Summary of sequence of investigati-ons. 7

CHAPTER 2 2.L Spatial relationship of major depositional 10 eñvironments for coal bearing sediments associated with coastal settings. 2.2 Geotectonic setting of the St Vincent T2 Basin including location of major structural elements and of the studY area. 2.3 North-south section through the northern 13 part of the St Vincent Basin illustrating tfre structural and stratigraphic relationships. 2.4 Structural setting and elements of the 15 Lochiel Coal Deposit including' structure contours of the basin floor. 2.5 Illustration of the concePts and 22 terminology of sequence stratigraphy. 2.6 Eustatic sea leve1 curves and relative 24 change in coastal onlap for the Midd1e Eocene to Late Oligocene. 2.7 Depositional environments and vertical 25 profiles for midplate continental margin settings. 2.4 Palaeogeographical reconstruction of the 28 northern St Vincent Basin for the Oligocene and Eocene. 2.9 Stratigraphic nomenclature and correlation 31 of Formations in the northern St Vincent Basin. 2.ro Sediment relation diagram and inferred 40 depositional setting for the northern part of the St Vincent Basin.

CHÀPTER 3 3.1 Schematic representation of the 47 characteristics of the modes of failure for weak rock and soil. VII

3.2 Relationship between factors affecting 50 failure along a discontinuity. 3.3 Stratigraphy and lateral lithological 53 variation for the Lochiel CoaI Deposit. 3.4 Relationships between the key factors 57 affecting slope stability for the Lochiel Trial Pit site. 3.5 Radial and vertical drainage mechanisms 59 and their relationship with pumping bores. 3.6 Stratigraphic location of weak zones and 61 their relationship with rates of dewatering/depressurisation . 3.7 Relationship between residual friction 65 angle and clay fraction. 3.8 Relationship between residual friction 66 angle and plasticity index for the Lochiel Coal Deposit. 3.9 Summary of results of Àtterberg, xRD, total 67 organic carbon, clay mineralogY, moisture content and percentage clay fraction tests for the Tarella SiIt.

3 .10 Discontinuities associated with 7L tectonically formed shear zones. 3.11 Locations of weak zones in progradational 72 and aggradational sequences. 3.12 Summary of grain size analyses for sandy 76 sediments occurring within the Lochiel CoaI Deposit. 3.13 Summary of well test results. 77

CHÀPTER 4.O 4.7 Summary of number and types of data 80 collection sites. 4.2 Core and geophysical log response 91 i-nterpretation f lowpath. 4.3 Legend for stratigraphic symbols. 93

CHÀPTER 5.0 5.1 Thickness and lateral distribution of the 99 Nantawarra Sand Member. VIII

5.2 Nantawarra Sand Member Lateral variation 100 in lithology and vertical sequence' 5.3 Schematic representation of the 106 d.epositional setting for the Nantawarra Sand Member. 5.4 Thickness and Lateral distribution of the 109 Darnleigh Park Sand Member. 5.5 Darnleigh Park Sand Member - Geophysical 110 log resþonse and vertical lithological profile. 5.6 Thickness and. Iateral distribution of the LL4 Condowi-e SiIt Member. 5.7 Cond.owie Sitt Member - Geophysical log 115 response and vertical tithological profile.

5 .8a Thickness and' Iateral distribution of the r77 CN# unit - Condowie SiIt' Member. 118 5 .8b Thickness and lateral distribution of the CN2 unit - Condowie Silt Member' 119 5 .8c Thi.ckness and lateral distribution of the CN1 unit - Condowie Silt Member' 5.9 Condowie Silt Member - Thickness and 125 Iateral distribution of clean sand'

5 .10 Depositional setting for the Condowie Silt' L28

5 .11 Condowie Silt Member weak zones 130 Location of test sites and summary of results of strength testing. 5.L2 Thickness and. lateral distribution of the L32 Kooliata CoaI Member. 5.13a Kooliata Coal Member - Thickness of H 1-34 Zone coal and H Zone coal Partings' 5.13b Kooliata CoaI Member GH Interseam 135 sediments - lota1 thickness and facies d.istribution and thickness of clean sand' 5.13c Kooliata CoaI Member - Thickness of G 136 Zone coal and G Zone coal Partings' 5.13d Kooliata CoaI Member FG Interseam 1-37 sediments Tota1 thickness and facies distribution and thickness of clean sand' 5.13e Kooliata CoaI Member Thickness of F 138 Zone coal. IX

5.13f Detailed stratigraphy and lateral 139 relationships for the Kooliata CoaI Member.

5 .14 Kooliata CoaL Member - Schematic 153 representation of coal forming depositional environment.

5 .15 Mechanism controlling peat accumulation. 154 5.16 Kooliata Coal Zone - Location of test sites L57 and summary of results of strength testing. 5.17 Bumbunga Sand - Definition of allocycles 159 and autocycles within the stratigraphic profile.

CHÀPTER 6.0 6.1 Detailed stratigraphy, Iateral contÍnuity 161 and lithologry for warrindi Silt and Tarella SiIt Formations. 6.2 Thickness of the Warrindi SiIt Formation. 165 6.3a Summary of depositional phases and t67 vertical and lateral lithological relationships for the Warrindi SiIt Formati-on. 6.3b Geophysical 1og response for the 168 !{arrindl SiIt. 6.4 Synthesis of facies distribution, coastal 168 oñtap and tectonic movement associated with the southern source. 6.5 Warrindi Sitt Formation - Thickness of WÀl L72 and WA4. 6.6 wAl - Detailed lithological L74 description and vertical lithological profile for the Trial Pit site. 6.7 Warrindi Silt Formati-on - Thickness and r77 distribution of WA2. 6.8 wÀ5, wA7 and WAB - Ðetailed lithological r82 description and vertical profile for the Tria1 Pit site. 6.9 V{arrindi Sitt - Thickness of l{À9. 189 6. 10 Schematic representation of the 195 depositional setting for the t{arrindi Silt' Formation. x

6.tL Warrindi SiIt Formation - 20r Location of test sites and summary of results of strength testing. 6.12 Tarella SiIt Formation - Thickness and 203 Iateral distribution. 6.13 TareIla Silt Formation - geophysical log 204 response, Iithology and nomenclature. 6.L4 TL4 Member Thickness and lateral 205 distribution.

6. 15 TL3 Member Thickness and lateral 206 distribut,ion. 6. 16 1L2 Member - Thickness and lateral 204 distribution. 6.L7 TL1 Member - Thi-ckness and lat'eral 2ro distribution. 6. 18 TLl Member - Thickness and lateral 2LI distribution of the clean sand facies. 6. 19 Schematic representation of the 214 depositional- setting of the Tarella Sitt Formation. 6.20 Summary of the geometric relationships 2L8 betweeñ structuial features in the Tarella Silt and their relationship with the regional structures. 6.2L Relationships between tectonic structures 220 in the tarella SÍIt from mapping of the floor of the Trail Pit. 6.22 Lateral continuity and correlation of open 222 joints in the Trial Pit. 6.23 Tarella Silt. Shear Zone tYPes and 224 geometric relatÍonships of associated structures. 6.24 Tarella Sitt Shear Zone SummarY of 226 structural characteristics . 6.25 Model of stresses and the resultant 233 structures present at the Trial Pit site. 6.26 TareIla Silt Formation - 237 Location of test sites and summary of results of strength testing. XI

CHÀPTER 7.0 7.t Flowpath for defining highwalt stability 242 design regimes. 7.2 Thickness of overburden. 243 7.3a Percentage of clean sand in the overburden' 244

7 .3b Summary of t.he characteristics and 245 distriËution of clean sand facies occurring in the Lochiel Coal DePosit. 7.4 Location of near horizontal weak zones' 246 7.5 Location of highwall stability design 247 regimes. 7.6 Typical vertica I1 ithological Profiles and 248 tateral litholo gicat relationshiPs between Highwal 1Stability Regimes.

CHAPTER 8.0 8.1 Summary of geological factors 255 controiling the location and engineering properties of near horizontal shear zones' XII

LIST OF TÀBLES

PÀGE NO.

CHAPTER 2 2.I Plate-Tectonic subdivisions of the earth' 9

CHAPÎER 3

3.1 Summary of engi.neering properties-of 55 sed.imeñts occurring in the Lochiel Coal Deposit.

CHAPTER 6

6 1 Structural features in the TareIIa Silt' 2r7

6 2 Summary of the types of shear structures 223 in the Tarella SiIt. 6.3 Summary of estimates of vertical 236 permeabilities and coefficient of consolid.ation. XIII

VOLI]ME 2

ÀPPETYDTCES

ÀPPE¡úDIX 1. Photographs referred to in Vo1ume 1.

APPENDIX 2. Geophysical log response model-s for the Bumbunga Sand.

ÀPPEIYDIX 3. Geophysical and lithological correlation sections for the Bumbunga Sand.

ÀPPENDIX 4. Geophysical- and Iithotogical correlation sectións for the Tarella and Warrindi Silts.

ÀPPENDIX 5. Structural analYses for the Tarella Silt.

ÀPPEI{DIX 6. Borehole location plan. xrv

ÀCKNOWLEDGNTTEUTS

During the preparation of this thesis a number of people have provided assistance and support. The author gratefully acknowledges and_is thankful to Dr' vi. Càstin (Utti.t"t=ity of adelalde) and to Professor David Stapled.on (Ùniversity- of South Àustralia) for thg support and assistance proviãed during the research and in creating this thesis. Iir particular Di Costin for his guidance with the sed.imentologiãal aspects and Professor StapJ.edon for the geotechnical asPects. The efforts of Evette condon, who drafted most of the figures, are also appreciated and recognised as being a silnifiåanÈ help in-þrogressing this thesis to completion' staff of the coal Resources Department of ETSA also provided. support an¿ assistancè, especialfy Yr Àlistair fouir (Senioi-Geologist) an¿ Mr Mick O'Brien (Manager CoaI Resources). Finally, this thesj-s would not have been completed-without significant contribution by my wife Her efforts and saórifices in providing suþport, understanding-Sandra. and motivation werä outstañding- and certainly beyond what could normally be expected. XV

ÀBSTRÀCT

This geological study brings toget.her the areas of geology and. engineering as relaEed to highwall slopes in soft brown coal mines. The approach centres on the role of the engineering geologisE whose main objectives relat,e to the id.entification and. prediction of the key geological- factors affecting slope sE.abíIity. The Lochiel CoaI Deposit in SouE,h Aust.ralia is used as the example- The Lochiel CoaI Deposit is cLassified based on ; . geological setting - Midplate continental position with - depõsitional environmeñts controlled by the interaction of movement about tectonic structures and by eustatic sea level changes. The depositional environments examined are those characteristic of continental shelves, including swamps, marshes, estuaries, deltas, lagoons, barrier bars and fluvial. . engineering properties fine grained, saturated soils ánd weaÈ -rock characterised by early deformational structures. This study identified and analysed the key engineerrng geological factors characteristic of soft brown coal mining areas rather than those generic to other mines or excavations in weak rock and soil. Three formations typical of coal bearing sequences lfere d.ef ined and analyseã. The f luvio-deltaic Bumbunga Sand, the fan-delta and lagoonal Warrindi Silt and the estuarine Tarella Silt. The key factors affecting slope stability for the study area were identified as the strength and geometry of near horizontal- defects in the sediments. These defects are generally concordant with the bedding in the surrounding ãedimentê and continuous over large areas. Where present their strength and geometry largely determine both the geometry of Lfre slope ãs well as the direction of mining. The location and formation of these d.efects is related to the boundaries of significant cyclic depositional events. The d.efects are located at the base of coarsening upward cycles in close stratigraphic location to the top of the pievious cycIe. The óhalacteristics of the surrounding sediments êuggested. an aqueous depositional environment typical of lacustrine or estuarine basin infilI sequences. Detailed examination of the geologicaL structures associated with the major overburden defect (the Tarella Silt Shear Zone) indicátes that they are characteristic of shear defects. The stresses controlling their formation appear to be related. to both compaction and the tectonic tilting of the deposit area. Typically^ the defects have residual f riction angles as low as 'u" . Furthermore, given that cyclicity is characteristic of ,midplate continental margi-n XVI deposits, it is likeIy that such features are present in other si.milar areas. These d.efects are also a key factor affecting the rate at which the fine grained sediments can be depressurised. There are two asþects involved. Fi-rst, the shear plane itself acts as a ielatively impermeable barrier to vertical drainage and., second, tha stiuctures associated with the shear zorre act as vertical drains for the fine grained sed.iments. The result is that the fine grained silts above the shear zone behave as would a fractured rock maSS but the drainage of porewater vertically is restricted by the shear zone. The relationship between tectonics' eustacy and depositional Processes and ttreir effects on the keY faõtors affecting slope sta-bility The Sequence in the study area encompasses two maJor episodeJ of transg'ression/highstand,/regression. The first episode is represented by the fluvio-deltaic Bumbunga Sand ¡ the sediments associated with this pñase fine I and sand at the base to silt, clay and The accumulation of the coal was eustat and appears to coincide with a Highstand Systems Tract. The second transgressive episode superimposed -marginal marine cond.itions -on the arãa and is represented by the f an-d.elta and lagoonal Warrindi Silt and the estuari-ne Tarel-Ia Silt Formations. the sources of sediment and the sites of tectonic movement appear to be simitar for botr episodes. This provides the o-niqn" opportunity to contrast the effects of marine influence-ón the k"y factors of porewater movement and weak zones against a non-marine influence. The basin infill processes wt ich led to formation of the upward coarsening sequences containing weak zones were al-so simílar for Uottr èpisodes. Furthermore, there is no signi-ficant d.ifference between the two episodes in terms of geómetry, Location and strength of the weak zones. The ãnalysiË' also ind.icates that the location and charäcteristics of clean sand Iithofacies are similar for both episodes. The conclusion is that whilst eustacy controlled the regional depositional- setting and the condi-tions necessary foi coa] foimation, tectonics had the major control- over the depositional processes and the segmentation and chãracteristics of Iithofacies. The implication is that within the study area, tectonics rather than eustacy was the major geological control on engineering geological factors affecting tñe movement of porewater and the properties of weak zones. t_

CEÀPTER 1. O I¡WTRODI'CTIO¡I

This geologicat study brings together the areas of geology and engineering as related to highwall slopes in soft brown coal mines. The approach centres on the intermediate role of the engineering geologist whose main objectives relate to the provision of pred.ictive information on the key geological facÈors affecting slope stability'

The specific objectives are specified as follows

1 Dewelop models which characterise Ehe key engineering geological factors in terms of depositional and tectonic processes.

2 classify the study area in terms of its global tectonic and depositional settíng thus providing a basís for translating the results t'o oËher sofÈ brown coal mining sites.

3 Contribute to the underst.and.ing of the ínÈeraction of tectonics, eustacy and deposiÈionaI processes in t'he St VincenÈ Basin.

In addition, there are a number of undeveloped soft brown coal deposit,s within the wor}d. It is anticipated that the results of this study will íllusÈrate the benefits of the applicatíon of deÈaiIed engíneering geological interpretaEion to slope stabílity analysis and also provide a model for further research. 2- . Discussion of the objectives and of the approach used in this study Many factors affect the stability of open cut mine highwall slopes including mining, geomechanical, hyd.rogeological and geological. Whilst recognising the importance of integrating each of these, this study approaches the identification and prediction of key factors from a geological persPective and focuses on characterising them from a geological rather than engineering viewpoint.

It is generally accepted that geological understanding can provide the basis for pred.icting key engineering geological factors ahead of mining. However, the application of this principle is restricted because few models exist which provide the basis for linking geological characteristics with engineering properties, especially for deposits of soft brown coal.

The mod,els that do exist are generally based on geological d.escription of the materials. This tends to make the mod,eLs site specific and doesn't provide a good basis for applying them to other mining areas. To address this inadequacy the approach taken in this study is to first classify the d,eposit in terms of tectonic and depositional setting and, second, to identify and describe the particular geological Processes which have controlled the development of the key factors.

1.1 Nature of engineering geological aspects of soft brown coal mining areas

Brown coal mining takes place in sedimentary geological 3- environments and, generally, the overburden consists of saturated sediments with a strength between soil and weak rock. However, Soft brown coal deposits cover large areas (the study area covers some 1OO square kilometres) and often there is a significant variation in the engineering properties of these sed.iments. In the pre-mining stages the engineering geologist has two related' objectives stith respect to sloPe stability analysis. First, to identify the geologically related factors which affect slope stability and, second., to define and predict variations in the properties of these factors within t'he mining area ' This study focuses on these objectives.

The geological factors affecting slope stability are the result of processes which first deposited the sediments and then modified them after deposition. This study analyses both types of processes in order to develop engj-neeri-ng geological models. Furthermore, âS discussed earli-er, the models are defined. in a globa1 context to Provide a basis for translating the results of this study to other areas'

The benefits of Engineering Geological- Models to slope stability analysis

In recent years, slope stability studies have been increasingly focused. on the integration of geological, geomechanical, hyd.rogeotogicat and mining information (tlathews and Rosengren, 1986, Shields 1986, and Hawley and Stewart 1986). The development of engineering geological models is an important. cont.ribution to this trend with geological und.erstand.ing providing the integrative mechanism. The benefits provided by effective engineering -4 geological models include ; . the early recognition of potential problem areas allowing timely development of cost effective solutions, . the reduction of the economic and safety risk, . more cost-effective data gathering in the pre-mining stages.

L.2 Ihe fochiel CoaI Deposit - Àn exag¡t1e of a soft brown coal deposit

The Lochiel Coal- Deposit is located 130 km north of Àdelaide in the northern part of the Tertiary/Quaternary St. Vincent sed.imentary basin, South Australia (figure 1.1).

In the period. between 1975 and 1989, the Electricity Trust of South Àustratia (ETSÀ) evaluated a number of soft brown coal deposits as possible sources of fuel for future electricity generation. The Lochiel CoaI Deposit is one of these d.eposÍts and since discovery in L982, has been the subject of a series of íntensive geotechnical investigat.ions. The Author has been largely responsible for the data collection and interpretation of the engineering geological aspects of these investigations.

The sedÍments in the d.eposit are essentially f lat lying, undeformed. and pre-consolidated. They were deposited in fault controlled depressions in deltaic, marginal marine environments, oD a passive continental margin (Kremor, 1986). The coal is ranked. as soft Brown, class 15 (ISO) or Lignite B (ÀTSM), (Kremor and Springbett, 1989). 5-

-l r SOUTH Port Jomestown AUSTRALIA Pirie I

AOELAIOE AREA OF SPENCER INTEREST GULF

LOCHIEL Burro COAL DEPOSIT

Snow Clore Bute

Loch iel Kqdi no

mons [oktovo Port

:.'r, YORKE

PENINSULA 5tìrt Hwy

uriootpo

Gowte

GULF

5r ELAI

VI NCEN T StirtinI Muloowurtie Point

Murroy Bridge

0 't0 20 30 to 50km

SCALE 1:1,000,000

Figure 1.1 Locality plan of the study area -6 I.2.1 Previous investigations

At the time of its discovery, no specific information existed on the area although Stuart (1969, 1-9'70), ÀIIey (1969 , tg73), Harris (1966, L97L, 1980) and Meakin (1985) had reported on the stratigraphy and structure of surrounding areas. Subsurface informati.on on the stratigraphy south of the Lochiel area based on results of coal exploration drilling had been reported by Anderson (1950), Cooper (L976, ]-977) , Parkin (L952) , Hillwood (1961), Johnson (1960 , 1964), Meyer (79?6, 1980) and Springbett (1980). À brief geological description of the geology of a trial excavation in the Bowmans coal deposit some 30 km south of Lochiel was also reported by Springbett (1e81).

The formal st.ratigraphic nomenclature for the Lochiel area is defined by Kremor (in prep. ) on the basis of this thesis.

The work carried out for the mining studies is summarised in ETSA (1989), and a summary of the integration of geological analyses is described by Kremor and Muir, (198?). A summary of the sequence and scope of investigations undertaken for the Lochiel Coal Deposit is provided on Figure I.2. Year Ending Decernber Evaluation Activities 83 85 86 87 88 89 90 ù l. RcgionalExploration and initial discov- E ery. ã Ð 2. Delineatory Drilling on a one Km grid. E,| 3 3. Bulk coal sample site investigation and È initial geoæchnical evacuations of V458. ! 4. Bulk coal sample - lm diameter cores. r' È 5. Geotechnical and hydrogeological investi- ¡ ô gations and analysis. o

6. Geological interpretation and modclling bæed on core and geophysical logs. Development of initial geotechnical regimes. - I 7. 1984 field program - geotechnical s ll -J a. coring and sample collection at¡ s lt 'á b. laboratory æsting arraaaa I a c. clay scam coring ¡ l¡ | d. trial dcpressurisation r'¡r¡rrl¡r o e. sclf boring pressure meter L aa I geological A f. engineering modclling laart and interpretation. lt 8. Rcgional hydrogeological pump testing ltt and analysis. 9. Infill drilling, coal coring to 500m cent¡es lt¡ a. weak zone sampling aoaaaaaaaaola ltl

10. Trial Pi[ excavation ll a. sile selcction drilling & analysis ¡ inst¡umenlat¡on s b. site and installation of oaaaa pumping bores I ¡ c. excavation, pit mapping and sample I aaa 3 collection I f& d. laboratory testing oL¡r I e. analysis and exkapolation of results I l. Feasibility mine design reporting

Figure I.2 Summary of sequence of investigations - 8-

2.0 GEOI.OGIC.AI¡ SETTINGS OF SOFT BROTCN CO.âI¡ DEPOSITS CLÀSSIFICÀTION OF TEE ST VTNCEMI B.ÈSIN

Soft brown coals are defined by the ISO system as havíng a moísture contentr greater Èhan 50 percent. Coals of this rank have been formed early in the coalification process wit,h the fundamental coalif icaE,ion f actors of heat and pressure having had only a limited impact. In addition, although the geological processes associated with the d.eposition of soft brown coal d.eposits are essentially t'he same as those for higher ranked coal deposits, the sed.iments surrounding t,he coal sea¡ns in sof E brown coal deposits are like1y to have been only marginally affected by diagenetic factors. As a result the sediments are generalty classified. in the engineering sense in the range of soils to weak rocks.

2.L GeotecÈonl.c and depoeitlonal settlngs of EofU brow¡. coal depoeits

Large scale coal formation can only occur in actively srrJrsid,ing sedimentary basins wÍth plat,e tectonics being the major factor causing basin formation. Curray (1975) provided a classification model for the major plate-t,ect,onic settings and based on this model Ðiessel (1980) id.enÈified the sit,es cont,aining major deposits of coal. This mod.el is shown on Talcte 2.L. Three major caEegories of coal d.eposit, are ident,ified by Díessel (1980) as follows; 1. t"tidplate positions - Continental margin and interior 2. Rift zones - Divergent plate margins 9 3. Subduction zones Convergience continental collision - oceanic subduction

Table 2.L P1ate-Tectonic subdivisions of the earth. Sites of major coalfields are emphasised by heavy ruling (after Diessel' 1980).

PI,ÀTE EDGE CONTINENTÀL CRUSÎ OCE.AIIIC CRUST POSITION

MIDPI"ATE CONTINENTAL MIDPI,ATE ocEÀl{ POSITION CONTINENTÀL FLOOR IIÍ.ARGIN

NASCENT RIFT ZONE RIFT VÀLLEY CONTINENTAL SPREADING I,ÍÀRGIN RIDGE

CONTINENTAL SUBDUCTION ZONE ISLAND ÀRC SUBDUCTION COLLISION CONTINENTÀL AND TRENCH ZONE MOUNTÀIN R.,A,I'TGE I'ÍÀRGIN COMPLEX

TRANSFORM TRÀNSFORM FAULT TRÀNSFORM FÀULT ÀCTIVE PÀRT FÀULT CUTTING ÀCROSS TYPE CONTINENTÀL OF FRACTURE ZONE CONTINENT IÚÄRGIN ZONE

Sedimentation tends to be paralic in all but the Midplate continental interior settings which tend to be limnic.

Limnic deposits have no connection to the open sea and sed.imentation is generally associated with Iacustrine depositional processes. Falini (1965) has developed a number of models of depositional processes associated with this type of deposit. Àlthough limnic deposits of coal are common the paralic deposits comprise a large majority of economic coal deposits. For this reason, thi-s study focuses on paralic rather than limnic coal deposits.

Paralic deposits usually contain marine sequences within their straÈigraphic column. In paralic deposits coal seams are associated with coastal environments such as marshes, 10 coastal plains, deltas, lagoons and estuaries. Transgressi-on/regression of the sea, whether caused by tectonics or eustacy, leads to migration of depositional environments producing sometimes complex vertj-caI and Iateral distribution of sediment types. A spatial relationship of the major depositional environments for coal bearing sediments associated with coastal settings is provided by Horne et aI. (1978) and reproduced on Figure 2.1 (see Figure 2.7 for detailed depositional models).

2.2 Geotectonic setting of the Lochiel CoaI Deposit

Às shown on Figure 2.2, the Lochiel Coal Deposit occurs at the northern end of the St Vincent Basin and also correlates with sediments of the Pirie Basin to the north.

AREA INFTUENCED BY AREA INFLUENCED MARINE TO BRACKISH WATER 8Y FRESH WATE

A UPPER BAR-I BACK. LOWER TRANSITIONAI¡ DELÍA PLAIN. RIER I BAR,RIER DELTA PLAIN LOWER FLUVIAL I DETTA P A

SCATE ORTHOOUARTZITE l:l t------l SHALE ffi SANDSTONE GRAYWACKE METERS COAL ffi SANOSTONE I o t(lr.oÀrtlERs

Figure 2.L Spatial relationship of major depositional environments for coal bearing sediments associated with coastal settings (after Horne et al. 1978). -11 The St Vinceng Basin has been defined by Glaessner and ltlade pirie ( 1958) , Ludbrook (1980 ) and. Parkin (1969 ) and t.he Basin by Lindsay (in press). These two basins were previously considered to be separat,e but, based on interpretation and correlation of drilI cuEtings and geophysical logs, iÈ was concluded by Kremor (1986) that the two basins are contiguous.

As illustrated on Figure 2.2}-, both Ehe St Vincent and Pirie Basins overlie a structural boundary (Torrens Hinge Zone) which occurs between t,he stable Gawler Crat'on to the west and. the mobile Ad.elaide Geoslmcline/Oelamerían Fold Belt (Preiss, L98'l; Cooper, 1985) to the east' and south'

2.2.L Structure of the northern part of the St Vincent, Basin

The present margins of Lhe SÈ Vincen! Basin are bound by north-south E.rending and northerly converging faults (Figure 2.28). To the norÈh of PE lilakefield, these are named the Ardrossan Fault' (west) and the Redbank Fault (easE).

A nort,h-south elongate occurrence of ABC Range Quartzite, locat,ed. beÈween these margínal faults and known as t'he Nantawarra Hígh, is bound Eo t,he eagÈ by the Templet'on Fault and, t,o t,he south by the east-west trending l{hitwarta Fault. A detailed north-south cross-section shotfll in Figure 2.3 illust,rates the relat,ionship beEween the basemenÈ faults and. the sedimentary sequences of the northern St Vincent Basin. The stratigraphic nomenclature used on Figure 2.3 is díscussed later in sectíon 2.5. L2

À MÀJOR TECTONIC ELEMENTS OF CENTRÀL SOUTH ÀUSTRÀLIÀ

GAWLER CRATON

B ENLÀRGEMENT ADELAIDE PIRIE BASI GEOSYNCLINE r- -l áa J ( I 6 c ST. LOCH¡EL ô bs VINCENT DEPOSIT q, BASIN g b F a tautt __J Uh i tt{art SEE ENLARGEMENT J I J I $ U, A' I È _= a. è ø) w q. I

See Figure 2.3 t¿! I Y / J for Section À-A' € I lÙ o (/)ì DELAIDE ð ,\' I A) I I ( lv I b t \ $ \ c \ ô \ \ (-u*t"u t

KANGAROO ISLAND hatched area on B corresponds to the Nantawarra High o loo

K I LOM ETRES

Figure 2.2 Geotectonic setting of the St Vincent Basin including location of major structural elements and of the study area. A , À

SOUTH NORTH

l.r¡ront¡l ...1. ¡¡ ¡.t..¡ 0 raaa ¡aaa taa qa s LO(HIEL OEPOSIT

V.rtlc¡l icl. i rt 15 arr¡ata.a

BOWù,IANS DEPOSIT

Nantawarra High

r¡¡. F UJ

E¡f

aÆrú s rf)

Figure 2.3 North-south section through the northern part of the St Vincent Basin illustrating the structural and stratigraphic relationships (Modified after Kiemor and Springbett, 1989). - 1,4 Figures 2.g and 2.4 iltustrate the major slructural elements occurring within t,he Lochiel area. fivo basemenÈ fault orientations are defined, northeasE to southwes| and easL-nort.heast. to west-southwest.. A1Èhough no faults have been recognised within the sediments in Ehe area' these basement faulEs appear to have acted aS hinge zones about which very genEle monoclinal folding has occurred.

This is consistent with uhe findings of Stuart (1969). He concluded that within the Cainozoic sedimenEs on Èhe eastern side of the St Vincent basin, tect'onically formed strucÈures are confined to gent,le monoclinal folds with more complex structures generally associated with basement f ault.s.

Structure conEours of the floor of t,he basin indicate a gent.Ie (.10) slope towards Ehe west and northwesE reflecting a local basement tilting.

Ivlagnitude of fault movemenEs

The Ard.rossan FauIE (previously called t,he Kulpara Fault by Crawford., 1965 and A1ley, L96g) has displaced t'he Tertíary beds near Clint,on by about 650 m (Horwitz, L961) . Near I-,ochie1 this fault has a d.isplacement, of aE leasE 500 m (the elewat.ion d,if ference between the base of t'he Tertiary sediment,s and the Eop of the South Hununocks ranges) . A1ley (fgeg) estimated a displacement of some 330 m at' Sno\¡Itov'm t'o the north (see Figure L.L, page 5 for locat,ion). The I{hitwarÈa Fault, has been traced south past the Bowmans coal 15

ooomE 24o oæmE 245 @mE o N

oOu \ L

-7o L l- I ,¡ I f s25oooomtt

o\a 5o s

l \ No

+ f e24r æomN \ Ò

s' L H

g I

I ¡ 6

( $ / a(o â +

\ x^reaRoo ISLAND

1,,"'Î' irLtr!t¡!a

Figure 2.4 Structural setting and elements of the Lochiel Coal Deposit including structure contours of the basin floor. - 1,6 deposit, but is 1ost. nort.h of Snowtown (Figure 2.2) . The magnitude and sense of movement along this fault appears to be related to the interacE.ion of east-southeast trending fault,s which are located north of Pt Wakefietd (Figures 2.2 and 2.3) . To the south of these faults t,he down-thrown side ís t,o the wesE whereas to the north, the eastern side is down.

In sumrnary, there is evidence for considerable tecEonic movement. along Ehe faults formÍng the margins of the northern St Vincent Basin and also along faults formíng the margins of the study area. However, faults are confined to the basement and do not appear to displace sediments wit,hin the basin.

Timing of fault movements

Cooper (1985), suggest,ed that the most active períod of faulting occurred during and inunediately after basin initíation in the Middle Eocene with another dístinct, period in the Pleistocene. In addition, âs reported by Se1by and Lindsay 1-982, and SEewart L972, paEEerns of sedimentation and recent seismicity indicate that episodes of Eectonic act.ivity have occurred from the Middle Eocene Èo present,.

2.2.2 Igneous act,ivity

Stuart ( 19 69 ) f irst recognised t,he zeol j-te clinoptilolite in the Blanche Point FormaE,ion. This mineral ís of ten regarded as indirect evidence for volcanic input,. ,Jones -L7 and Fitzgerald (1984, L987) , from chemical and mineralogical analyses of the formation regarded the non biogenic portion E,o be an accurmrlation of volcanic ash which settled through the waEer column. Furthermore, they found. t.hat passive continental margins world wide contaín a common mineralogy t]æified by the presence of not only clinoptilolite but also smectite and opal CT and t'hat explosive volcanic aclivity appears Eo hawe produced t'his mineralogy. ilones and. Fít,zgerald (198?) surmised t,hat the preservation d,emands limi¡,ed Eerrigenous J-nput and a lack of st,rong sea currents.

As descríbed by Cooper and Lindsay 0978) Èhe St Víncent Basin had restrict,ed. marine access throughout its geological history. AÈ Èhe same time it can be surmised that the marginal peat swamps would have actred as filters of clastic sed,iments as ind,icated by the nature of t'he coal seams ( see Chapter 5 .4) . It would appear t'hat the deposit,Íona1 cond,itions ín Èhe St Vincent Basín ürere conducive to preser'\¡e such ash deposíts.

Bror,'rn (Lg75) from analyses of samples from a borehole in t,he south part. of t.he SL VíncenÈ Basj-n found clinoptilolite and smectite to be present in t.he Sout'h lvfaslin Sands, Blanche Point FormaÈion, Chinaman GuIIy Formation and t'he Port Willunga Formation. If clinoptilolíte and smectite do represenÈ volcanic input then it ís apparent' that' volcanic activity could. have extended over a considerable time period. encompassÍng several eustatic and tectonic evenEs in Ehe sedimentation history of the basin- -18 The possible presence of volcaníc sediment's in the sequence has significance. $Ieak seams of bentonitíc clay formed from the chemical weathering of volcanic ash are conunon in mines from the western plaíns of North America, some of which are up to 1OOO lcn away from t,he volcanic source (I¡Iade, 1985) . Furthermore, these weak seams can have residual angles of frict,ion as Iow as 50 (Wade , !985) .

Volcanic activity is recorded in the Cainozoic in central and west,ern Victoria. Although these are some 400 km away it is possible that they could have been a source of volcanic ash.

The evidence for volcanic input into the basin is inconclusive, however, it is possible that such sediments are present.

2.2.3 Geotectonic cont,rols and basin formaÈ'ion

Rccording to Cand.e and. Mrltter (L982) there was a rapid increase in Èhe rate of separaÈion of Australia and Ant,artica at. al.ouu Ehe lvliddle Eocene. Cooper ( 1985) consid.ered. thaÈ the commencement of basin formation is related to this separation.

Bott, (1971) pointed out that the principle stress which acts on a divergent, (passive) continental margin is t,ension iniE,iated by E,he jr:xtaposition of thick continental crusE against. t,hin oceanic crust. The brittle continental crust respond.s to this tensÍon by normal faulting- -19 Whereas E,hese f aults Eend to align parallel to the contínental edge if associatred with t,he diwergent continental margin, the faults in the area of the St Vincent Basin are aligned perpendicular to Ehe edge. The conclusion is that whilst the separation of the two continents is coincident with basin formation in the Study area, the stress regime which controlled t'he structure cannot be directly relaÈed to the separation.

Basín formation

On the south-eastern margin Ehere are a number of smaller fault, bounded basins which, on t.heir southern ends, open inÈo the St vincent Basin (Figure 2.2). By contrast, the only such basín occurring on the wesEern margin is t'he Lochiel Basin. These smaller basins are bound by almost vertical normal fault,s, concordant with the E,rends in Ehe underlying basement.

It was considered by Campana (1954), Forbes (1'966) , and Cooper (1985) that these bounding faults formed along pre-existing lines of weakness in the underlying Precambrian/Cambrian basement rocks. Moreover, the Èensional stress field required to form Èhese faulÈs was considered by Cooper (1985) to be relat,ed to the relat,iwe difference in mobility between the Gaw1er Craton and Èhe Adelaide Geosyncline (refer to Figrrre 2.2A).

The stresses produced from this relat,ive difference in mobility appear t,o offer a plausible geotect,onic mechanism for basin formation. It, also ex¡llains Èhe prevalence of 20 the smaller basins along Èhe margin of t,he more mobile Adelaide Geosyncline.

2.2.4 Sununary

TecÈonic controls leading to the formation of the St Vincent. Basin are best, relat,ed to tension developed between two cratonised areas on a continental plate. It is also recognised t,hat the basin was in relatively close proximity to the divergent continenE,al margin formed by the separation of Aust,ralia and AnËartÍca (which could ocplain the preserlce of sediments of possi-ble volcanic origin in the sequence).

The f,ochiel CoaI DeposiU t,herefore appears to have formed on a midplate coatinental margia position in the sense of the classification model used by Diessel (rgeO) and illustrated on Table 2.L.

2 .3 DepoeLtl.onal characterLstLcs of coal deposLts oll ntdplaÈe cont,Lae¡.ta1 uargLns

Diessel (1980) prowided a sunrnary of t,he major depositional characterist,ics of coat deposits f ormed on midplat,e cont,inental margin positions. Of signif icance is t,hat such deposits are ì

tr. . invariably paralic with intercalat'ions of marine sediment,s. They belong Èo a group of depositional siLes near Èhe edge of a craton which are part of t'he continent.al shelf environment. n Furthermore, Dj-essel pointed out that rsuch regions oft.en have a setting intermediate between the craÈon and. other geot.ectonic domains, for instance, geoÐmclines. " -2L

In the case of the St Vincent Basin these relat'e to Èhe Gawler Craton and. the Adelaide Geosyncline respectively'

As a resulE. of the interact.ion of Èectonic movement of fault,s and eustatic movemenE of sea levels, midplate cont,inental margin sett,ings could. be expected Èo oscillate between the deposiUional environments U1çical of marginal marine settings d'iscussed in section 2 '3 '2 '

2.3 .1 Eust,acy and sedimentary sequences

The concept,s of sequence stratigraphy in relation to eustatic changes in sea level were first described Vaíl eE ,:-977). aI. (1g77a,b) and, Micthufr et al. Th.ese concepts are used to assist with t,he interpretat'íon and characÈerisat,ion of the vertical and laEeral distribution of the major formaÈions in t,he st vincent Basin.

Sangree and. Sneid.er (fgeZ) described Ehree basic t')rpes of eustatically controlled sequences (refer to Fig:ure 2.5) :

.Transgressive syst,ems Tractr (TST) - sediment,aEion associated wit.h eust,atic sea level rises ' During this period. shoreline posiEions t,ransgress landward. .Híghstand Systems Tract (HST) - As Ètre eust'aEic rise slows, regression begins as the sediment's entering Ehe basin prograde back across the shelf' .LowsÈand Systems Tract, (LST) - sedíment's related t'o low . poínt.s in sea level. Sedíment.ation is similar t'o HST. 22 Condensed Section (CS) thin accumulations of marine sediments relating to the peak of the transgression.

Figure 2.5 Illustration of the concepts and terminology of sequence stratigraphy (modified after Vail, Hardenbol and Todd, 1984).

Level Terminoloqv Relottve c honoe in coostol onloP Eustolic Seo Londword Seoword H h L Deltoic HIGHSTAND SYSTEM Morgino I TRACT morine Morine

TRANSGRESSIVE conde d SYSTEM section TRACI

LOWSTANO SYSTEM IRACT

SEDIMENT RELATION DIAGRAM

IGHSTAND J- 2 3 Bosement

LOWSTA TRANSGRESSIVE t/ o n e n sed Sec rion (morginol envirnoments ( Mo r I n e ) progrode uP sloPe I

Continentol Morgin

In ad.dition to providing the conceptual f ramework to interpret eustatically controlled sequences' Vail and Hardenbol (tg|g) and Vail and Mitchum (t979) developed' a model of actual eustatic sea leveLs for the Cenozoic and this is reproduced on Figure 2.6. Of significance are the multipte transgressive/regressive episodes which occurred during the Eocene and Oligocene. 23 2.3.2 Sed.imentarY environments

Typicald'epositionalenvironmentsformidplat'econtinental margin settings includ'e coastal plains and marshes ' Iagoonsrdeltasrbackswamps,estuariesand'peaÈswamps' Schematicmodelsofthesed'epositionalenvironmentsare reproduced on figure 2'7 '

Twobasicsequencesaredeveloped'throughtheinteraction of eustacy and' tectonics; regressive and transgressive'

DurÍngregressiveeustaticphases(representedbyHighstand Systemstractsed,iments)laterallycoexistíngsequenceS covering prograde down the palaeoslope, with Lhe coal seams in the sed.iments occurring closer to the sea' This results abroad'Sequencecomprisingbasalmarinesedimentsoverlain by coal bearing, deltaic and lacustrine sediments. For basins on midplate continental margins' Diessel (1980) considers this process to be analogous to the gradual silting uP of a lake.

phases The opposite is the case for transgressive eustatic marine (TST) where coals seams are overl'ain bY the sequences.

(It must, however' be recognised' that' the actual straÈigraphic sequence is modified by the rate at which sediments enter the basin and, the d'iscussion above relates to a constant rate of sediment input' ) 24

SEQUENCE STRATIGRAPHY EUSTATIC CURVES cø U COASTAL ONLAP l¡J > z<(Jc i l¡J O LANOWARO BASINWARD u= Of=z l¡J l¡JO(, = I'O o5 o Øû< 200 áo oo 50 0M F -ro.5- I ( -t2.5- zl¡J -t3'8 - -s (Jl¡J 3 o -r5' 5 - - r5.5 - - t7.5-

É, l¡J LONG TERM - TERM 20 - 2l- (, J=o

G - 26.5 t¡l - (L q_ l¡¡ I z o- -28.4- o ol¡J = .F{ o -30- 3o- r0 oÐo J æ, Fl'.{ o l¡l a .o oo J .qÊ 33 Ðo Ê -36- ¡t{ É. l¡l -37- OFI È 38- rd fL - Éo 3 \ rú c) 40 P{ -40.5 - ø

- I l¡J 42.5 - l¡l c¡ - 44- z o 45 l¡J() o = - 46'5- ) l¡¡ -4t.5 -

Figure 2.6 Eustatic sea level curves and relative change in coastal onlap for the Middle Eocene to Late Oligocene- (ùodified after Haq et aI' 1987) 25

w^va r¡^lN¡ Barrier I.AGOON t!tsiloal o:tta

t ^GooN SWAMP iloÂt cxÂÑNÉt IOA¡ CIIANN¡1 IHATE iaNoSfoN: n ar v tattoox (tolt-¡ió Ío^t ffi ottxæuÂallÍl s#aÀAx^l <¡olt-¡aol DAt E @ rro rro orttx rxeu AGOON St urrrroxr IARY CHANNEL

0l r trltutrry- bth 8.r SPTAY Lower

DeIta Ol!t.¡ 8àr

MUOS Ptain t.df5 lrlb0t.rt

cr.Yarr. Sqlat scAl.ES lrt.Ìdl ¡trlù¡t¡rt 8ÀR SANOS E.y Or t¡ Mtl¡¡! Pódal t. ììì, . .,-,*o flrexorror: : iltrtll lÈ¡strtrroxl þ' ctoss'"zo; !rrtr tæl lltuclutf ¡ & ttow ¡otrt e? ¡UlWllru<¡urlt

t:val SPI.AY NTÉFOISTRIBUIARY BAY ortflruañ

IX¡¡¡O5T¡IEUfASI CHANN EI. 8AY

5P( AY Transitional fNIEUIARY !AY Lower 5PLAY DeIta Plain

CHANN EL

TEVEE

POIN BAR t EvÉE

CHANNEL

PI.AIN BACXSWAMP (EVEE

IAR Upper POINT DeIta ct{^NNlt

LEVEE Plain I.AKE Fluvi-aI PTAIN

OepostrroNAL Mooeu VeRtrcRu PRoFTLE

Figure 2.7 Depositional environments and vertical prõfiles for mid.ptate continental margin Ëettings (diagrarns taken from Horne et al. L978, Éorne tólgarbrcrd,, Ferm and Horne 1979) ' -26 The implication f or sedimentation and, theref ore, t,he characteristics of the owerburden, is that t,he stratigraphíc profile will be determined largely by which of the t,ransgressive or regressive regimes was present during deposition. Sequences related to predominantly regressive phases (HST) are likely to have deltaic and lacustrine sediments in the overburden whereas transgressive sequences (TST) are likely to be dominated by finer grained shallow marine sequences.

2 4 Palaeogeograpbical setting of the norther'¡, St VLncent BagLn

From palynological ewidence both Ludbrook (1980) and Cooper (1985) considered t,hat sediment,ation in the St Vincent Basin conrnenced in the Middle Eocene. Daily etr aI. (L976) concluded Èhat, prior to t,he commencement of sediment.atj.on the area no!ìr covered by the basín was undergoing weathering and erosion.

2.4.L Sources of sedíment

The geographical studies by Alley (1969) are most informat,íve as to the nature of Ehe drainage system operating withín the norÈhern part of t,he basin and also t,o hiatuses in deposition in Èhis area.

A major conclusion of the work of Alley (L969) ís t,hat the Broughton Ríver, which ent,ers t,he area from the east, has maintained its middle and upper courses for most of t,he period between the Eocene to E,he presenE day. Furthermore, it was also concluded in subsequent st,udies (AIIey, 1973) -27 that for this period of time the ancient Broughton River ran down the eastern side of the Templeton Fault (i.e. to E,he east of the Lochiel area, refer to Figure 2.2) . AII-ey /1973 ) def ined these f luvial sedi-ments as the Snowtown Sands.

The SnowÈown Sands have been correlated stratigraphically by t.he author with the Tarella Silt and üIarrindi Silt in the l,ochiel area. More recent palynological analyses by Alley (pers. contrn., 1990) also support. this conclusion.

The implication is that the ancient Broughton River drained t,hrough the Lochiel area at least during parts of Ehe Oligocene. In addition, the conclusions reached in Chapter 5.0 suggest, that prior to this time it is like1y that draínage of the ancient, Broughton River was into the Pirie Basin to the north, the souEhern pat,h being blocked by an uplifted Nantawarra High.

Palaeocurrent analyses by Stuart (1970) on Uiddle to Late Eocene fluwiatile sediments occurring in the south of the study area indicate sources also originated from the west aL that time. this is consistent with the f indings of È,his thesis (Chapter 6.0) which índicate that, a major fluwial source entered from t,he west (at Ehe norLhern end of the I-,ochiel area) during the l-,ate Eocene and appears to have drained to the north, probably coalescing with the ancient Brought.on River. -28 Based on the discussion above, a palaeogeographical model for the area for the Late Eocene to Miocene is presented on Figure 2.8.

2.4.2 Cainozoic hiatuses

Alley (1969) defined three major erosional episodes based on a study of erosional surfaces at the northern end of the St Vincent Basin.

The oldest of these he named the Mt Herbert surface which was characterised by heavily ferruginised and weathered basement.

Figure 2.8 Palaeogeographical reconstruction of the northern St Vincent Basin for the Oli"gocene and Eocene.

Eocene 0lrcoceHe

n N a N 0

I \ I \, I *o*r*arn, \, I *o*-rar* I I u,""a*, \ I ,r. u,*ar*, \ I ,t \ \ LOCHIEL \ \ BASIN \ BASIN \ \ ! \ $ \ Ð \ Ð O \ o \ \ \ olo llîì Bo¡rmont outc.oo, I olo ao:rment outc.oor tll 't ttl I \ k¡lomrtrr3 liloftlre. f---l n¡c"nt scdím¡ntr I nccrnt s.d¡m.nr. t'l fl I - - Md¡or toullt - - Mo¡o? foull!

MAJOR SOURCE MINOR SOURCE -29 This surface was eroded. to form the Moorundie et'ch surface' Both these surfaces pred.ate Tertiary deposition and equate to the paleocene planation surface defined by DaíIy et aI' (Le76).

The second epísode called the Huddleston surface with its relat,ed etch, Ehe Yacka Moorund.ie surface, was considered Eo separate the oligocene/Eocene sed.iments from the Miocene sedíments in the Snowtown area. The Huddlest'on surface was considered to be a prolonged or intensive period of weat.hering and erosion. It. d.eweloped on a fairly level surface with the base levels of erosion being díctaEed by the level of the groundwaEer tab1e. Thís surface is characterised by the formation of a thin silcrete capping which covers highly kaolinised bedrock'

Hard,, silcrete bands have also been intersected in boreholes in the Merriton and crystal Brook areas at a stratigraphic location coincid.ing wit,h the top of the Tert,iary sediments, ETSA (1987) .

According to AlleY Ígeg) the third epísode corresponds to t,he present land surface and is catled the Condowie surface.

From a review of Alley (1969, L9'73) it' is not possible to d,istinguish any erosional/weathering periods in t'he

Bocene/oligocene . - 30 2.4.3 Palaeoclimate

Accord.ing to Cooper (1985), Cainozoic sedimentation in the St Vincent Basin reflect,s deposition under temperate climatic cond.ítions although t,he appearance of large foramínifera aÍrongst the IJate Eocene, Late Oligocene/øatly Miocene and Ivlidd.le Miocene faunas (Lindsay, L969; McGowran' L97g)isind.icativeofincreasedtropícalinfluence.

A tropical clÍmaÈe for the Late Eocene ís also supported by pataeobot.anical studies by christophel and Blackburn (1978), Kemp (1978), and. by the presence of widespread coal formation.

2.5 StratigraPhJ.c guccessLon

The study area is located between the SE Vincent Basin t'o E,he south and the Pirie Basin to t,he north. The stratigraphy of t,he northern part, of the sE vincenE Basin has been descríbed. previously by ilohnson (l-960), Hillwood (1951) , Harris (19?1) , Meyer (t976) , Cooper Í976 ' L977) pirie and. Stuart (1969, 1970) - The stratigraphy of Ehe basin ís d.escríbed by r,indsay (in press) '

A slultrnary of Èhe current sEratigraphíc nomenclaÈure for the Bowmans - Clinton, and Snowt'own areas is presented on Figure 2.g and includes a correlat,ion wít'h eusÈat'ic sea level cut¡/es of Haq et, aI. (1987) and with erosion surfaces def ined by Alley ßgeg , !9'13) . In addition, Figure 2 '3 on page 13 shows t,he retationship beEween sedíment's occurring at the norÈhern end of the St Vincent' Basin' 31

AGE EUSTATIC CURVES LOCATION SEA LEVELS Moslcn My Beoch - to Rising Falìing Lochiel Pt Wokefield Point Pt Willungo

t5 zlrl IJ Port () Willungo o = Port (D 20 3 o Willungo J Formotíon Port

Willungo 25

t- c)

t¡J z. (JUJ 30 Huddleston Torello o S¡It IJ Rogue Rogue o (D Worrindi Formotion Formotion o Chinomon J= sitt ly 35 Th ko Ruworung PtJ

Bumbungo onc o Tortochillo CI cr Sond Clinton f Muloowurtie Formotion Fm 40 Mt Herbert Nth Moslin Sond zlrl ul g c)o ît l¡J p 45

Meokin Horris il986) ( r985), Horris Cooper ( Alley ( r e88) (rs87) Stuort rs85) 50 ( r969 Kremor Cooper (¡s69), Porling (r973 il986) 0s85) ile70) il969)

Figure 2.9 Stratigraphic nomenclature and correlatior of format,ions in the northern St Vincent Basin. -32 2.5.L Tertiary - St VincenE Basin

Clint,on Formation ( f ormerly ref erred to as CIint'on CoaI Measures) palynological studies by Harrís Q966) assigned Èhe fluvio-deltaic brown coal bearing sand and clay in the Bor¡rmans area sout,h of Lochiel to the lvliddle to Upper Eocene Clinton Coal Measures.

Meakin (1985) and Harris (1987) divided the Clinton CoaI Measures in Èhis area into upper and lower seç[uences (spanning late Middle Eocene to Early Oligocene) based on the st,rat,igraphic díst.ribuEion of index palynomorphs. Meakin (1985) inferred a hiatus of short duration and correlated t.he upper sequence wit,h the chinaman Gully Format,ion and. the lower to the Blanche point' Format'ion'

More recenL sÈudies bY AIIeY (pers. colßn. 1990) also suggest Èhat, the Ctínton Formation in t,he Bo\,mans area is lJate Eocene t,o EarlY Oligocene.

A1ley Í969) and Harris (1970, 1985) both considered that' t,he Clinton CoaI lúIeasures become progressively younger towards the north. This t,rend is consist'ent as far south as the Vfíllunga embayment where Middle Eocene coal bearing sediments are present (souch l"faslin and North Maslin sand) .

Harris (1980) recognised, t,he close correlat,ion of sea level cycles in Ehe Eocene to t,he depositional architect'ure of the basin and. the occurrence of coal. Based on palynological evidence Harrís suggest,ed Èhat Èhe lateral -33 d.ist,ribution of coal d.eposits was related to the position of the shoreline which in turn was controlled by eustat,ic sea level changes.

A similar relaLionship is also enwisaged by Holgate (1985) for the extremely thick d.eposits of Tertiary age brovnr coal in the Latrobe Valley. Ho1gat,e (1985) suggested that these accumulated behind' a sequence of barrier bars'

AlLhough the formation of coal seams and major facies in the St Vincent Basin appear to be relat'ed to eustatic Sea Ievel changes, âs d.íscussed lat.er in Chapter 6'0, it is consid.ered that the Ehickness and 10ca1 facies relationships are more influenced by the local t'ectonic sÈructure.

AIso significant is t,hat ttrese d,eposius are both overlain by marginal marine sequences (ttre C}int'on CoaI lvleasures in the Borrùmans area is overlain by the marginal maríne Rogue g'ormat,ion) . This is also true of the Muloovnrrtie Formation, a correlative of Ehe clinton coal Measures occurring on t.he Yorke Peninsula, which is overlaín by Ehe marginal marine Throoka silts and, in Eurn by the marine

Rogue FormaÈion (sEuart. L969, 1970) '

Based on the concepts of sequence stratigraphy it is evident, t,hat Ehe Clint,on Formation and its correlatíves are represenÈative of a Transgressíwe Systems Tract' 34 Rogue Formation

Stuart, (1970) defined the Rogue Formation as being a mainly marine sequence consist.ing of quartz sand, sandstones and siliceous sandsÈones, siliceous and' arenaceous li:nestones, mudstones and clays. He assigned the Rogale Format'ion an Late Eocene to oligocene age based on pallmological dating of samples from the east coast of Yorke Peninsula. The formation is characterised by numerous facies changes and erosion has limited its dist'ribution.

From the d.escriptions provided by stuart e969, L97O) it ís Iikely that t,he Rogue Format,ion represent's marginal marine and d.eltaic sedimentation. Furthermore, the Rogale in the Bov¡mans area is represented by fine t'o medium grained sand with thinner intercalations of silty and carbonaceous sand'

IE is considered. t,hat these represent barrier bar and shoreface deposit,s. This is consistent with the barrier bar depositional environment at'tribut'ed by St'uart (1970) Co the Iat,erally eqgivalent, Quartoo Sand Member occurring at' the top of the Muloowurtie Format'ion fart,her Eo the south' stuart. (1970) also recognised t,hat t,he QuarÈoo Member was laterally equivalenU Eo t,he marine Port VincenÈ Limest'one (inctudes parÈs of the wíIlunga Formation and the Blanche Point ruarls) fart,her south. St'uart (19?o) hlæothesised t,hat t,he relationship beÈween the Muloowurt'ie-Quartoo Sand.-Port. WíIlunga represented a regression, standstill and t,hen a northerly transgression respectively. He -35 hlpothesísed that these Iateral facies relationships were tectonicalty controlled.

In addition, Harris (L985) on the basis of patynological evid.ence concluded tha! the Rogue and Port I'rlíIlunga Formatíons were also lateral equivalents' stuart (19?O) reported. that clay minerals occurring in the format.ion are mainly montmorillonites, mixed montmorillonit.es and. glauconite with variable amounts of illite. The zeolite clinopt,ilolite is also comrnon-

The Rogue Formation includes the Port ,JuIÍa Greensands Member, a thin green glauconitíc quartz sandst'one and arenaceous glauconitic mudstone (Stuare, L970) which cont,ains occasional opaque minerals and chert nodules' cooper og77, 1985) correlaÈed. t,his member to similar biostratigraphic levels fart.her south in the basin and ít is consid,ered to represent an erosional, transgressive phase of marginal marine sedíment'atíon'

Hillwood (1961) also identified similar sedíments in the Bor¡mans area and although Stuart Ígeg) proposed that Èhere was probably nog a dírect correlat,íon with those described on the Yorke Peninsula, it is possible t,hat these sedimenÈs represenE an equivalent Èransgressive phase of sedimentation. 36

Snowtown Sands

To the north and northeast of t,he Lochiel area' and on the basis of palynological interpretatíon by Harrís (tg6g,tg7o), Alley iG973) assigned the name snowtown sands to an interbed.ded sequence of Miocene f luvio- Iacustrine sands, clays and brown coals which overlay t'he clinton Formatíon. However, subsequent, analysis by Alley (pers contrn. L99O) suggest that t,hese sed.iments are Oligocene in âgê, correlat,ing "largeIy with the oligocene Proteadidites tuberculatus zone. I'

Furthermore, lithologíca1 and. geophysical log correlation, suggest.s that t,he snowt,ov,rl sands are laterally equivalent to the ltlarrindi and. Tarella Silts in the l-,ochiel area'

From vertical profile analysis of geophysical logs and interpretation of drill cuEtings and cores it appears Ehat Ehe Snowt,own Sands represent a mainly fluvial depositional environment. The contrnon occurrence of t'hin coal seams and upward coarsening cycles of silt and fine sand also ind.icate the lateral co-exist.ence of peats!,tamp and lacust,rine and crevasse splay environments '

Port !{illunga Formation

The Port Willunga Format.ion was def íned by t indsay (1967 , 1969) and is equivatent to t,he Port, Vincent Limestone described by Stuart' (1969, L97O) . It is a bryozoal Iimestone of Late Eocene to Middle Miocene age and Ehe distribution of the format.ion aÈ the northern end of the -37 basin coincid.es with and appears to be limited by maj or structural features.

In general, the Port T,rlillunga Format.ion thins towards the north.Stuart(1970)suggestedEhatthisthinningcanbe explained, by the interaction of t,hree controlling mechanisms; angular unconf ormity aÈ t'he top of the formation, lateral gradation down to the Rogue Formation and minor diastems within the formation.

Near Muloowurtie Point, Stuart (1959) described a transgressive and erosional quartz sand only L.2 m thick d.irect,ly overlying the Rogue Format'ion.

The occurrence of the erosional sands is contrnon in transgressíve marine sequences and the base is defined by swift. (1g5g) as the ravinement. surface. It may be related to the Ruwarung Member described elsewhere in the basin (cooper, Lg77, 1985; I-,indsay, L967, lg87; Mccowran, 1978) .

2.5.2 TertiarY - Pirie Basin

The Pirie Basin is defined by Lindsay (in press), and based on t,he interpretation of drillholes, Kremor (1986) has suggested a connect.íon wit'h the sequences in the Snowtown

and Lochiel areas (Figure 2.2) .

The Basin contains two major stratigraphic seç[uences' the Kanaka Beds and. the Melton Limestone. The Kanaka Beds consist of carbonaceous silt.sLone, shale and sand, wit'h minor lignite. The age of the Kanaka Beds has not been .38 accurately determined, however, Lindsay (in press) suggests a possible range from latest Eocene to Early Oligocene. It is considered, by the aut,hor that the the Kanaka Beds correlate with the Clinton Formation.

The contact between the Kanaka Beds and the Melton Limestone is characEerised by the presence of a t'hin sand unit representing the marine transgression which preceded the deposit,ion of the Melton Limestone (r,indsay, 1970) .

The Me1ton Limestone represent,s marine deposition associated. wiUh f ive t.ransgressive episodes bet'ween the Late Oligocene and the early Middle Míocene. The Melton Limestone is not present in Ehe Condowie Basin, nor have any sediments of Miocene age have been identified. It is apparent t,hat the for most of t,he Condowie gasin deposit'ion ceased between the lJate oligocene and Ehe Early Middle Miocene. However, ít is considered Èhat' if some sedímentation did occur, iÈ is most like1y Uhat' it would have been against the Ardrossan and Templeton Fau1ts.

2.s.3 Quat.ernary

Wit,hin the Lochiel deposit. t,he Tertiary sequence is overlain unconfonnably by t,he Hindmarsh Clay and by the Glpsum Hill Beds. These sed.iments were described in detail by Kremor (1989) but as Ehey were not, part of t,he coal forming deposítional environment, detailed analysis is beyond the scope of this study- 39

2.6 Geotectoaíc and, euEÈatíc model for t'he stratigraphlc sequence of the Northern St' VLnceo't Basio'

From the review of the sedímentary formations occurring withín the basin it is possible Eo develop a broad model of t,he laterally coexisting envíronments of deposition within thenort'hernStVincentBasin.Thismodelisshownon Figure2.!o.Themajordepositionalenvironmentsare consídered to be;

Formation' 1 Fluvío-dett,aic - Bumbunga Sand', Ctinton Muloowurtie Formation, North lvlasIin Sand' Chinaman Gully Formation.

Throoka 2 t"Iarginal marine - T¡Iarrindi silt, Tarella Silt, silts, Quartoo Sands, South lvlaslin Sands'

3 Marine-PortltlillungaFormation,Blanchepointl'larIs.

Greensands, 4 Transgressive destruct,ional units - Pt ,Julia Ruwarung Member of the Port willunga Formation,

Tortachilla Límest'one -

Figure 2.6 on page 24 and' Figure 2'g on page 31 also provide an indication of t.he ínteraction of eustat'ic sea revers with sedimentation patterns. There is a general ríse in sea level up to the Eocene/otigocene boundary after which there is a generat fall. Vlithin these major trends thereareanr-rmberofrelativelymínor transgressive/regressíve epísodes' SEDIMENT RELATION DIAGRAM BEACH LOCHIEL PT WAKEFIELD MULOOWURTIE POINT MASLEN I I I s N HIGHSTÀND SYSTEMS TRÀCT Huddleslon surfoce Porl Willungo Formollon Torello Silt Worrindi silr HS Bumbungo Pl Julio Sond Rogue Formolion Clinton Formolion Throoko Sond Blonche Point Formolion TRÀNSGRESSIVE SYSTEMS TRACT

Norlh Moslen Deposrt¡oHnL SETTTNGS Èo

U PPER BAR-I towEl DETTA PI. RIER I DELÍA PIAIN I

RoEue Fm' BuEbunEa gand warrlndl sllt Port wlllunga F!ì. Cllnton F¡t. fa!e114 sllt Nth l{asrln sande Íhrooka silÈs setting for Figure 2,tO Sediment relation diagram and inferred depositional Lhe northern part of Étre St vincent Basin' -47 Middle A Srutmary of the main features of the Late Eocene to oligocene depositional settings are discussed below' a) A nort.herly mígratíon of the fluwio-delt'aic sequences culminating in the LaEe Eocene to earliest oligocene.

(TST) These equate to Transgressive Systems Tract seçßrences.

IE is evident that from the conìmencement of basin formation in the Middle Eocene to the Late Eocene/Early Oligocene, eustatic rise in sea-level (Eransgression) was the major controlling process . NOtwithstand.ing this, a nr¡mber of smaller scaled regressive (HST) episodes also occurred during t,his t,ime. These HST episodes are characEerised-by containing accumulations of coal and discussed below'

b) A number of smaller scaled regressive episodes withín themajortransgressiveepisodeoftenrepresentedby accumulations of coa1.

This characterist,ic has important implications for both the naEure of basin subsidence and coal format'ion. As Diessel (1980) point.ed out coal can only accumulat,e when the depth of wat,er is within a range of no more Ehan a few metres ' Greater d,ept,hs tend to drown the vegetation whilst lower peat dept,hs lead to oxidauion and hence dest.ruction of the ' -42 The presence of thick coal seams such as those found in the Botvmans and. Lochiel deposits must, therefore, represent a balance of eustatic and E,ectonic controls. That is, the rat,e at which the sea Ievel ríses/falts is matched approxj.mately by t,ecEonic sr¡bsidence and peat' accumulation' Theoretically t,his musÈ occur atr least at Highstand (HS) points in the eustat,ic cYcles.

ItisevidentÈhaÈHighstandpointsarealikely stratigraphic locat,ion for coal formatíon and coal seams' At, this time coal accumulation can be terminated by eiEher of two processes.

which case uhe 1 First , by a continued È,ransgression in coalseamsareowerlainbyÈhemarginalmarineand marine seç[uences (as is the case for the Bowmans d.eposít) . A variation of t,his process occurs when cransgression is btocked by tectonic uplifu along the shoreline. In t.his case the peaEswamp is also drowned buÈ a lagoonal/estuarine environment forms (as is the case aÈ Lochiel, and f or t.he Maslen Beach/Port willunga sequences). Both of these variations equate t.o Highst,and systems tracÈ sequences '

2 Second',byprogradationofthefluvio-deltaic environments over the peatswaÍPs ' Again Highst'and sysEems t,ract sequences form, however, iû contrasE Eo the al¡ove there is little or no marine influence. -43 ¡ocene/oligocene 3 A híatus and regression at the Late boundaryfollowedbyatraf]'sgressivedestructional episode which reworked' the underlying sediments'

This episode covered the area up to t'he Nantawarra High In wit.h sediment,s of the marine Port vüillunga Format'iOn' this cont,exE the Port ilulia Greensands and chinaman Gully (cs) Format.ion are representative of a condensed seccion ' It is appears that this episod,e coincided wíuh hawing the shorelíne located in the Lochiel area'

2.7 Sumary

The st vincent Basin is ctassified on the basis of its plate tectonic sett,ing as representative of a midplate continental margin posiEion. The basin is bound by north-soutrr trending and. noruherly conwerging normar faults.Ïüithinthebasínasecond.seEoffault'swith easÈ-west Erends are colwnon. In t'he Cainozoic, the most' Iikely cause of movement along t,hese faults is related t'o different,ial movement between the stable Gawler Craton t'o the west and, t,he more mobile Adelaide Geoslmcline to the east,.

The midplate cont,inent,al margin sett,ing is a coÍtrnon sit'e of peat accumulation lead'íng to coal seams ' Tlfpically' the associated depositional environment's are marginal marine in cTraracter and include barrier bars' back-barrier environments, esuuaries, lagoons and deltas' -44 Two major controls on sedimentation patt,erns and lateral facies dist,ribut.ion are identified; tect'onic movement along basement faults and eusEatic sea level changes. The major Cainozoic formations can be interpret,ed on the basis of these cont.rols.

One major transgressive/regressive eusta[ic epísode is recognised in t.he strat,igraphic sequence in the northern SÈ Vincent Basin. This eusgatic episode resulted in deltaic and marginal maríne sedimenEs (Transgressive Systems Tracf (TST) deposit.s) being overlain by marine sediments and culminated in Ehe f ormat,ion of coal seams at both Ehe Lochiel and. Bowmans deposits. A standstilt (condensed Sect,ion (CS) ) and then regression (represented by Highstand System Tract (HST) sedimentation) appears Uo have followed the TsT. The cs and HST phases Ied to the est.ablíshment of marginaÌ marine depositional enviroilnents wit'hín the Lochiel area.

The controlling processes outlined above trave led to the sed.iment,ary seç[uence now presenÈ within the Lochíel Coal Oeposi|. In add.iE,ion, the def inition of these processes and Èhe classif icat,ion of Èhe study area within t'hem provides the basis for both translating the results of this sÈudy t.o other sof t, brown coal occurrences and for t'he detailed analysis of geological processes which follow in Chapt.ers 5 and 6. -45

3. O IDEìXÍIFICATION OF EI{IGINEERING GEOIJOGICAIJ FACTORS ÀFFECTII{G SLOPE STÀBILIIY IN TEE IJOCEIEI¡ COAIJ DEPOSIT

The purpose of t.his, ChaPt'er is Eo ídent.ifY the key engineering geological features af f ect.ing sloPe stability in the l-,ochiet CoaI DePosit,.

The analysís of slope stability necessarily identifies which geological factors have the greatest impact on slope stability. Therefore, results of slope stabilíty analysis underpin the approach used to identify the key factors to be analysed in this studY-

considering that the role of t,he engineeríng geologist' cent.res on provid.ing pred.ictive inf ormation on the engineering geological properÈies and. for Èhe geotechnical engineer to analyse slope behaviour iÈ is beyond the scope of t.his stud,y to undertake such slope stability analyses.

A large amount of information is available on how slopes fail (the mode of failure) and on why they fail (mechanisms for failure). Based on a review of this existing information, the following sect,ion attempts t'o su¡rrnarise the key engineering geological factors which contribuEe t'o t.he st,ability of an excavated slope-

The fÍrst step is to est,ablish whaE are the key engineering geologícal features relating t.o t,he mode and mechanipm of failures in generic terms. 46 The second step is to clarify t,he nature of these features for the r,ochiel coar oeposit. this wíIl be accomplished using both interpretations made by the auLhor as well as by reviewing published ínformation on slope stability analyses f or t.he dePosit'.

3.1 GenerLc uodeg Of slope failure Ln weak rock and sofI'

Hoek and Bray í977) and, Ross -Brown {n,979 ) provided descriptionsofthemajorE]æesoffailure.Themajort)æes (plane' can be cat,egorised int,o those wít'h failure surfaces wedge, two-block and. cert,ain Elpes of círcular faitures) and t,hose wit'hout' faíture surfaces (Eoppling' ravelling and most circular failures) ' These modes of failure are illustrat,ed on Figure 3 - 1. a) Plane Failure. Failure occurs when a geological diScOntinuiEy strikes parallel to the slope face and. int'o the excavaÈion aÈ an angle greater t'han t'he angle of fríction' Variat'ionstot'hisoccurwhenatensioncrackforms, or when Ehe mean sliding plane is a combinat'ion of two joint seÈs which form a stepped pat'h' b) liledge tr'ailure. when two discontinuit.ies surike obliquely across the slopefaceand't'heírlíneofintersectiondaylight'sin theslopeface,thewed'geofrockresE'ingonthese disconEinuities will slide down the line of inEersection, provided that the inclinaEion of this line is great'er t'han t'he angle of friction' -47 Figure 3.1 Schematic representation of the characteristics of the modes of failure for weak rock and soil- (rnodified after Hoek and Bray, 1977).

ens ion crack slope in upper su rface face of slope

f i I ure surface PIane P1ane with associated tension crack

Wedge Circular

c) Circular FaíIure. When material is very weak, as in a soil slope, the failure wiIl be defined by a single discontinuity surface but witl tend to follow a circular path. À variation of this type of failure is the non-circular failure, where the failure surface tends to run parallel to a set of weakness planes. d) Two Block failure Occurs when an unstable wedge produces an acÈive force on a lower, second wedge which would otherwise be stable, resulting in failure of both wedges' -48 e) Toppling Failure. VÍhen blocks rot.ate about' their lowest contact edge. Thís mode of failure can occur when Lhere isa vertical or near vertical joint set- f) Ravelling Failure. I^Ihen materials deÈeríorate on exposure and break off in sma1l blocks.

It is recognised Èhat this sunmary of generic faíIure modes is not. exhaustíve and E,hat many failures acEually involve combinations of Èhose list,ed above. However, it is considered E,hat this sturunary reflects the current sEat'e of thinking for generic modes of slope failure.

Discontinuities in the sediments are crucial Eo all the g:eneríc modes of failure listed above (although to a lesser exÈent, for circular failures) - The conclusion is that dLecoutLauLtLes are a key geological facuor affecting slope st,ability. Furthermore, Ehe key engineering properÈies of discontinuities are locaÈion, geometry and sÈrength. 49

3.2 MechanLsms caueing failure of sJ.opes in weak rock and EoLls

The science of predictíng the behaviour of soils was founded. by Terzaghi in about 1918 (Stapledon, L982) - In doing so Terzaghi provided a ". . . framework that helps engineers to organise, interpret and evaluate experíence' I' (Peck, Lg62). This soil mechanics framework will be used to develop a simplified description of the mechanísms j-n causing slope f ailure. It is recognised t'hat pract'ice the analysis of the causes of slope failure is a complex exercise reqgiring the consideration of such factors aS time, slope geometry, movement of equipment, seismicity and ctimate, however, such detailed analyses are beyond Ehe scope of this sEudY.

Failures occur when the avaíIab1e shear strength is less than the shear stress (mobilising force) ' The shear stress required, to cause stid.íng can be calculated for a soil by using the following equation (Bolton, 1984) : -

T = c + o/ran p

These component,s are illustrat'ed on Figure 3 '2 '

IE is apparent from t,hís equation that for a part'icular soil or weak rock, the shear strength (residual friction angle and cohesíon for intact, materials and defecÈs) and the normal effect.ive Stress are the key factors in determining the sÈabitity of a slope- 50 Fígure 3.2 Relationship between the factors affecting failure along a discontinuity (modified after Hoek and BraY, L977).

Friction angìe Q F tn 6 o t- \L¡ I/| Norma I s tress o t- o c, Shear stress r ! a^î,

Cohes ion c

Normal stress o

the normal effective stress (oi is defined bY the following

equation ;

o ot u

where j = normal stress and u = pore water presSure

In soft brown coal mining areas the normal effective stress (and thus the shear stress) can be varied to reduce the probability of failure by reducing the porewater pressure through activities such as d.ewatering and depressurising' '51 Strear scresses can also be reduced by minimJ'sing any imbalance between horizont.al and vertical 10ads. IN pract,ice, this can be achieved' by excavating part of t,he overburden ahead of the highwall face (unloading or pre-st.ripping) or by the design of the slope geomefry'

As described. in section 3.1, Ehere are a number of modes of faiture each of which is characEerised by a particular failureplane.Essentiallythesefailureplanescan coincide with pre-exist,ing geological feat'ures such as shear planes, ot can develop within intact maLerial.

Further analysis of these or other methods of varying the normal effect,ive stress or shear stress is beyond Èhe scope of this t,hesis. However, for the purposes of this study it can be concluded that slope failure is controlled by ¡ . the normal effect'ive stress, . cohesion, . st,rengt,h of Ehe sediment, or of geological strucEures (discontinuities) within the sediment''

In addition, it, can be conclud.ed' thaÈ the methods avaílable Èo influence Lhese controlling factors are : . the raEe and. extent t,o which the porewater pressure can be reduced, . geometry and. st.ratigraphic locaÈ.ion of the geological structures. 52 3.3 Revlew of slope stalcility analyses for the tochiel CoaI Deposit

An extensive series of tests were undertaken to ident'ify the key geological features affecting slope stability for t,he Lochiel CoaI Deposit. The author was responsible f or t,he engineering geological inputr int,o t,he planning and design of these t,esUs and subsequently for the interpreEation of the results from a geologicat viewpoint. A discussÍon on these t,ests and. their timitations is provided in Chapter 4.

Specific slope stability anatyses were underLaken by Coffey

and Partners Pty Lt,d (1988a) .

3 .3.1 Summary of st.ratígraphy

The stratigraphy and laterat continuiEy of sediments wíthin the Lochiel Coal Oeposit is suûnarised on Figure 3.3. Three Cainozoic format,ions are present,, the Br:mbunga Sand, Warrindi Silt and ttre Tarella SilE. Each of these format,ions is analysed in deÈaiI ín Chapters 5 and 6 t'o identify and, define the processes which led to theír

particular charact.eristícs .

In ad.diÈion, the deposit is overlain by t,he Pleistrocene Hindmarsh Clay. Ilolrüever, because the Hindmarsh clay was noÈ. part of the depositional event associaÈed wíth coal 53

STRATIGRAPHY S N LITHOLOGY A LATERAL CONTINUITY

STRATIGRAPHY o EPOCH ? 9 G ! UJ t- o- È t! GYPSUM o HILL BEOS t- Holocene = o Pleistocene ro HINOMARSH Miocene il ililtt CLAY Oligocene 20 É. g Lote 30 t- TARELLA SILT É. lo t¡J 40 t- Middle WARRINOI SILT Eocene 50 il KOOLIATA ll COAL z. z. ll lt lt DARNLEIGH Ê. É. illttl cD(D PARK ilttlttt SAND == ll oo 80 ll tt l¡J É. coNoowlE È SILT

NANTAWARRA SAND

HORIZONTAL SCALE ROSEHAVEN oÆ CLAY kilometrea RANGE OUARTZITE

lithological Figure 3.3 Stratigraphy and' Iateral- variation for ïft"-iððniel coal Deposit '54 formation, it is not appropriate to include analysis of this unit, in this studY.

3.3.2 Summary of engineering propertíes

The properties of materials present in the L,ochiel coal Deposit, are surmnarised on Table 3 ' 1'

The properties of the st,ratígraphic FormaEions can be stated as fo110ws :

TareIIa Sitt - Firm to hard, highly carbonaceous clayeysiltoflowtohighliguidlimit.Moisture contenE greater than plastíc limit''

Vlarrind.i Silt - Firm to hard, clayey silt' sandy clayey síIu of low to high liquid limit' Moisture content' greater ttran plastic limit'

Burnbunga Sand : FzoneandHzorLe.stiffE'ohardcarbonaceous sand. Coal seams - Stiff to hard coal' Cond.owie SiIt - Firm to very stiff clayey silt' and silty clay of low plasticit'y and high liquid limit- Stratigraphy Moisture Content Dry Density Unconfined Cor pressive Intact Shear Strengths t å Strength (kPa) t'tean Lower Bound Mean Mean Mean c' (kPa) o' (Deg) c' (kea) o' (oeg (Range) (Range ) (Range) Tarella SiIt 57 1.0 244 65 26 33 22 (r2 1s6) (0.4 1.e) (6s 448)

Vùarrindi SiIt 30 1.6 255 4A 31 7 31 (16 - 81) (1.3 1.8) (67 s48)

F-Zone 51 1 240 (r2 130) (0.7 '_ 1.8) (60 380)

GH Interseam 46 1.1 246 (2 1s3) (0.5 1.4) (16s - 4oo) I (¡ (¡ H-Zone 58 0.9 317 I (2e - 88) (0.7 L.2) (1e8 - s13)

CoaI r26 0.6 400 t74 26 L22 26 (2L 186) (0.4 r.7 ) ( 134 67s )

Condowie Silt 46 r.2 28L 85 22 54 22 (24 - 103 ) (0.8 1.4) ( 104 37s )

TÀBLE 3.1 Summary of engineering properties of sediments occurring in the Lochiel CoaI Deposit (summarised from tests performed for ETSÀ by Coffey and Partners 1987). 56 3.3.3 Modes of failure

the Two modes of fai-Iure were analysed for highwatls in Lochiel cOaI Deposit, circular and' non-circular (plane failure).Non-circularfailureswerefoundtohavethe lowest factor of safety for a particular slope (Coffey and PartnersPtyLtd',1988a)and,therefore,non-circular failures are Èhe focus of this thesis'

Theengineeringgeologicalfactorsaffectingslope stability in the Lochiel coal Deposit are illustrated on Figureg.4.Thismodelwasd'eveloped'fromanalysisof failures within the Trial Pit (plates 3.1 and 3.2)'

The key factors affecting the mode of non-circular failures \{ere considered to be the

shear strength and' orientation of near horizontal weak zones, porewater pressure (effective stress) location and orientation of tensional joints'

Stratigraphic location of weak zones and porewater pressure

Figure 3.5 summarises the stratigraphic location of the weakzonesaswellaStheporewaterpressureprofi}e. Two typesofweakzonewereidentified'bothrelating to the sed.imentary horizons and. both being characterised by presence of a shear zone. 57 RelationshiPs between the key factors- Figure 3.4 ;¡;;tils - slabiritv ior the Lochiel Deposit Trial='r"1" Pit site'

Active wedge oring through intocl moleriol Depth (m) o- Gypsum Hill Beds

Piezometric Surfoce Hindmorsh Ê- in Torello Silt Possive Cloy wedge ¿- -Y---''-

TARELLA to- Active woler Prossure SILT in vorlicol extension tt joints providing lhrust tr. TARELLA SILT SHEAR ZO NE t5- c': O KPo WARRINDI SILT (WAg) 9', =g'

Wedge foilure ol Uplif t loe of pressure ocli voled Woter POle ìx e

3.3.4 Mechanisms causing failure

groundrdater The Lochiel Coa] DePosit is subject to artesian the effective stress' Pressures which tend to lower ad,verselY affecting mine slope stabilítY.

Itisconmonpracticeinmineswithhighporev'aterpressure toincreaseeffectivestressbyloweringthepore\{ater pressure. The method which is used. is to pump porewater fromboresatarategreaterthanitcanberecharged. This reduces the porewater pressure of the sequence wiÈh .58 the piezomet,ric surface usually taken down to t'he leve1 required to maintain a st'able slope '

In t,he Lochiel coal oeposit Èhe high porewater pressure necessitages depressurisat'ion of t'he fine grained sediments (silts and clayey sitts) and. dewaEering/depressurísation of the coarser graj-ned. units (sands and gravels).

The geological factors affecting t.he dewat.ering/ d.epressurising of sands are relatively weIl understood' The geological fact,ors relaÈe mainly Eo graín size and soil struct,ure.

On the other hand, it is not contrnon practice Eo atEempt' Eo d,epressurise f ine grained sediments such as silt's - In fact, conventional thinking at the Eime thaE the Lochiel coal oeposít, was being evaluated was Ehat' fine grained sediment,s, such as those occurring within the overburden at I-,ochiel, could. not, be economically depressurised wit'hin the time limits imposed by Èhe mine's coal schedule.

As discussed by coffey and. Part,ners Pty LEd (1988b) and illustrated on Figure 3.5, porewater pressure in fine grained sediment,s (sills) is controlled by two mechanisms;

well 1 Radial draínage towards È'he plxnp or drainage t,hrough aquif ers ( zones of relatively high permeability such as sands);

2 vercical d,rainage to an aquif er and then radial drainage Eowards the pump or drain' 59 Figure 3.5 Rad.ial and vertical drainage mechanisms and their relationship with pumping bores.

WELL schemotic representolion Hydro - CI o STRATIGRAPHY of types ond relotive imporlonce geologico I o droino Ie mechoni Chorocterislics I

Fine to groined Torello Silt 7; corÞonoceous --tr; sedimenls il !

Worrindi 20 Silt WARRINDI SILT AQUIFER Aquif er

E o o 30 z C)L U' :3oE -o) Fine 8= groined :¿ corbonoceous sediments ule (9 Condowie Sill Sheor Zone z Condowie Silt l (D :) NANTAWARRA æ SAND AQUIFER -5O Aquif er Nonloworro Sond

bosemcnl LEGEND

Rodiol droinoge

^I Verticol droinoge I (Size of orrow represents relotive hydroulic conductivity) 60 À number of tests \ùere und,ertaken within the Lochiel CoaI Deposit to examine the response of the fine grained sed.iments t,o d.epressurisation through pumping f rom wells. The effect of pumping on the porewater pressures within the sediments is illustrated on Figure 3.6.

It became apparent from these tests that the porewater pressure of the majority of the fine grained sediments could be l0wered within a reasonable timeframe. However, within the sequence there $rere several relatively impermeable zones which determined. the rate at which the sed.Íments could be d'epressurised ' Of particular significance is that the impermeable zones are the weak zones described above. As further evi-dence of the af fect of these weak zones on vertical drainage it was observed in the Trial Pit that water accumulated and run out from directly above these weak zones

3.4 Review of engineering properties

3.4.1 Intact material strength

shear strength is defined by the Mohr-coulomb Law as d.escrlbed in section 3.2, page 49. The fundamental components of shear strength are effective normal stress, cohesion and. friction angle. As described on page 50, effective stress parameters are a function of both normal stress and. porewater pressure. To evaluate what constitutes a significant effective stress parameter, shear strength may be viewed as related to two 6L

LOCATION OF POLISHED STRATIGRAPHY SHEAR ZONES PIEZOMETRIC PROFILE

to Torello Sill ll _-tt; u Torello Silt Sheor Zone Worrindi Sill 20

E Õ o - (JL 30 (n :3 E oE J ': O) e 8> èf :¿ o o

o .9 À ço N (9 + z Condowie Sill mognitude l æ Condowie Silt of pressure d) Sheor Zone reduction

=(D -50 Nonloworro Sond 60

oto zo 30 40 50 60 70 80 90 PIEZOMETRIC SURFACE - RL(m)

x Piezometer reoding (RL) prior to pumpinq o Piezometer reoding (RL) ofter 24O doys pumping

Figure 3.6 StratigraPhic location of weak zones and tlneir relationship with rates of dewater ing / dePressurisation' -62 basic components, physícaI and physio-chemical. For cohesíve soils (such as most fine grained clayey soils) Èhe physical componenÈS of shear Strengt,h arise primarily from t,he resistance to relative mowement of sliding of one particle over anolher and, to interlocking bet'ween particles (Young and Warkentin, L975). Rosenquist (1955) divided the interlocking phenomena into macroscopic and microscopic. l"Iacroscopic interlocking is important for granular ( i ' e ' sand.y) soils where appreciable movement' of granular particles normal to t.he failure plane is required.

Microscopic int,erlocking is crucial for fine grained (c1ay) soils where small movements can result in aIígnment of particles parallet to t,he failure plane. This study ís concerned mainly with the shear sErengt'h of fine graíned soils and Èhe following section concenEraEes on t,he aspects related to shear failure of these.

3 .4.2 Physical component,s of shear sErengt'h in f ine grained soils

For the purposes of examining t,he major facÈors affecting shear st,rengt.h, f ine graíned soils can be viewed aS consist,ing of two tl4)es of particles; discreÈe minerals and bond.ed set.s of discret,e minerals. ConcepEually, shear sÈrength of fine grained soils is corlprised of an int,eracÈion of particles through an absorbed particle layer (cohesion or physio-chemical interacÈion) and also the interaction directly between part'icles (physical interaction) . 63 Cohesion when two particles are brought into contact under stress, the close proximity of contacting areas results in adhesive or frictionaL resistance due to the electrical forces of attraction. However, in a clay-water system the interaction of clay particles and fabric units with water is such that there may be little actual particle-particle contact. In these soils, cohesion is a component of shear strength which arises mainly from the inÈeraction of clay particles through the layer of absorbed water and through diffuse layers of exchangeable cations'

Direct particle contacf

Duringshear,particlesandfabricunitsinthefai-lure plane become oriented parallel to each other. At shear, the soil particles are arranged. to give a minimum particle interference and. this is best accomplished by a parallel arrangement at or near the failure plane'

This concept is reinforced by stress-strain relationships d.eveloped after failure. Typically, the stress-strain relationship is characterised. by a peak stress required to cause failure and then once failure has occurred' a lesser stressisrequired'tocausemovementalongthefailure surface. The latter state is referred to as the residual shear strength (SkemPton, 1964) ' -64 In cases where the soil has previously been sheared t'he stress-d.eformation cur-ve may nog exhibiÈ a peak. This is the case f or Ehe shear zones occurring in the l-,ochiel Deposit.

3.4.3 Residual angle of friction vs mineralogy and grain shape uitchell (1975) found from tesEs relat'ing residual sÈrengt'h to part,icle shape that low residual friction angles were associat,ed with platy particles (clays) , \^Iith angular and rod-shaped. particles giving higher residual frictíon angles.

Lupíni et al-. (1981) investigated residual shear behaviour using thin section and electron microscope t'echniques and recognised three modes; turbulent, transition and sliding, depending mainly on the dominant particle shape and on Èhe mineralogy (coeffÍcíenU of interparticle friction) .

Ttrrbulent shear occurs with essentially granular soils and is charact,erised by relatively high residual strrength- The sliding mode is ty¡lica1 of clay soils and is characterised by low residual shear s¡rengt.h. The transit'íonal mode occurs in mixEures of clay and granular part'ícIes.

Skempton (1964) first suggested Ehat there was a general relationship between clay content and the resídual angle of friction. This relationship is presented on Figure 3.7 and illust,rates a gradual increase ín residual St,rength wit'h decreasing clay content. Figure 3.7 ReraÈionsrri-i ll.r""r residuar frict,ion angle and clay fraction (modified aft,er Lupini et al. 1981) .

uPPer r0l bounds 'g 30 Boundaries of other researchers o, findings from Lupini et al. cOr ( 1e81 ) o c .9 20 (J lower o bouriäs Lochiel Deposit P= I{eak Zones !tì 10 À, É.

0 0 20 ¿0 60 80 100

Ctoy f roction (% )

vaughan and lrlalbanke (L97s) , as reported in Lupini et ar. (1981), posLulated a discontinuous relationship between resíduar fricÈion angle and plasticity rndex (rp) (Figure

3.8) .

They postulaÈed Èhat the controrling factor was tÍkeIy t,o be related to the proportion of praty clay mínerals present and t,hat, this would correrat,e with rp for clays of simirar act.ivity. Bucher (L97s) also found a similar retat.ionshÍp between Ip and residual fricÈion angle and Seycek (19?g) after performing his own t.ests and sunrnarising E,he work of others reporÈed that there was a beÈt,er correration between resÍduaI friction angle and rp than any other parameLer. 66 Figure 3.8 Relationship between residual friction angle and plasticity index for the Lochiel CoaI oepoJit (modified after Coffey and Partners lees ) .

LEGENO 35" aX Torello Defecls - lorge diometer core o Torello Defecls * Worrindi Defects 30" o Condowie D efecls I Ronoe in Ø from 3o lo 8" slroin t- NOTE, 2 Normo-l slress ronge l3O lo l4OO kPo

-- Boundorv of other reseorchers findings from LUPINI S K INNER AE ONd VAUGIIAN P' R \ J. E , , ' 2 -t\ \\

-t\- o 150 o o Lorge Block Somple

c o 100 o Convenlionol Sheor Bo¡ tL

.I 5" E 'Ø o æ

0" o r0 20 30 40 50 Plosl¡city lndex lg "/"

In summary, the proportion of clay minerals and their mineralogy have a major affect on the residual shear strength and this relationship is likely to be reflected in the plasticity indices.

As iltustrated on Figures 3.7 and 3.8 these relationships are found to hold. for the ldeak zones occurring within the stud.y area. À summary of results of Àtterberg, total organic carbon, XRD and. moisture content tests for the Tarella Silt is provided on Figure 3.9. (obundooc.l MOISfURE 7. PERCENTAGE CLAY Ltouro LIMIT '¿ TOTAL ORGANIC CARSON 7. CLAYMINERALOGY o 50 5AMPL€ PLASTIC¡TY INOEX 7. PLASTIC INOEX 7. 30 50 50 too o 20 40 20 40 Kaoì in Quartz 7. CloY lrqcl¡oÂ

f . ?tl

. ¡¡a fp . l!O . at¿

. l7a .2¡¡

. ¿¡l

. ¡¡t . ?Ol . ?94

Ol . 2a7 TL3 { . ta? ¡al '.2r3

. 220 . 12! '?a¡

, ?5f . tat

.:22 TL4 . ¡!l . ¡ra ' !90 o, . aÓo ploô. Ilìite Undifferentiated

Figure 3.9 Summary of results of Àtterberg, XRD, total organic carbon, clay mineralogy, moisture content and percentage clay fraction tests for the Tarella Silt. 68 Directional ProPerties

Bishop et aI. (tg66) from extensive Eests on London clay concluded tha! bot,h peak and residual strengths were approximately consEant whether samples \¡Iere cut across or parallel to beddíng. However, Barden (L972) from microscopic examination of London CIay considered Èhat there was only a tendency Eo horizonÈal orientation so Bishop,s findings may not be applicable to clays wíth a strong tendency for preferred orient'ation'

ï8, could be reasonably ex¡lected that t'he peak strength of clays may var:y in strongly oriented samples, however, t,he residual sErengrÈh is an intrinsic property and thus (I97t) independ.ent of d.irect,ional propert'ies. ilarnes ' however, Eested samples of oxford clay and report'ed repeat,edly higher residual strength f or samples test'ed perpend.icular t,o dip direct,ion compared with those parallel. He suggested that part'icIe orientation resulting from depositional clraracE,eristics or Èhe consolidaÈion of Ehe formation has an important bearing on t'he path to the residual in any particular direcÈion.

Surrnary

The clay contenÈ, clay mineralogy and t'o a lesser extent' directional propertíes have a significant' -impact' on shear strength. The characteristics of each of these propert'ies can be relat,ed t,o geological processes and are, thus, wit,hin Ehe scope of this study. However, t,he primary object,ive of this thesis is relates to the modelling of the .69 keyfactorsaffect'ingslopestabilityratherthant'he actua]-propertiesofE'hesefactors.Inlinewiththisthe characterisat,ion of these properties is considered to be a minor or supporting objective of t'his st'udy'

3.5 DLEcoD,tfnuiÈies and weak zoneg

Discontinuities and' weak zones in soils can be related to two basic tlæes of geological processes

Tectonic, Sedimentary.

3 .5. 1 Tectonic discontinuit'ies

Tectonic díscont'inuit'ies are those produced by Èhe progressiwe deformat'ion of the int,act soil when subj ected to external stresses.

Specif ic stud.ies on t,ect'onic processes f orming soil stralcLures are limited. Based on analysis of the geomet'ry and Eree of defects in a stiff, overconsolidated siwalik clay, Fookes (1965) and, Fookes and' vlilson (1956) post'ulatred that their format,ion was cont,rorred by ford.ing due to basin subsidence, lead,ing t,o the development of shear stresses in the more comPetent' beds.

Similarly, Sullivan :-:ge2) in his study on the origin of defects in heavily overconsolidated clays and oil shales, concluded that soil macro-strucÈure is relat'ed t'o basin folding. 70

CharacterisÈics of tect'onic discontinuities

Skempton (1966) defined. a shear zone as consisting of singte or mu1t,ip}e discontinuit'ies along which a f inite shearing has taken p1ace. Tchalenko (1970) compared the morphology of different magnitudes of shear zones and Morgenstern and. Tchalenko (t967a,b) compared shear feaEures of experiment,ally deformed. kaolin wíth regard to the primary f alcric. Each of these sÈ'udies f ound Ehat both the same E,]æes of defect,s and similar geometric relationships existed.

The developmenE of shear st.resses leads t'o the progressive development of shear defects in the soíI. Based on direct shear and microscopic Èest obser:vat'ions, Morgienstern and Tchalenko íg76) described three stages of formation of

shear zones as shown on Figure 3 .1-0 -

3.5.2 Sedimentary weak zones

fteak zones are characgerised by having a relatively high clay cont,ent in contrast with the surrounding sediments' These clay-rich zones form from clay seÈtling from suspension in st,and.ing bodies of water. zurthermore, it has been sholrün by Staub and Cohen (1978) that^ clays flocculate rapidly when they come into contact wit'h hígh acid.ity waters which are characteristic of coal forming

depositi-onal environment.s . 7L Figure 3.10 Discontinuities associated with tectonically formed. shear ããtt"= (after Skemptonr1-966) '

/-t'

Stoge (l)

Stoge ( 2 )

Stogc (3 ) ---t2

I Riedet (R) Ihrust (P) - \ ¿a" ------á--- + - - - - DisPtocement (D) ¿ I R.,

The geometry of the clay seams is related to the geometry of the surface on which they v¡ere deposited and on the effects of differentia] compaction'

ClaySeamscantheoreticallyformaSconformablebeds within both prograd.ational and aggradational sequences or aSd'isconformablebedsdraped'overerosionalsurfaces. These depositional locations are illustrated on Figure

3 .11. 72

A PROGRADÀTIONÀL ENVIRONMENT Suboqueous Delto Ploin Levee islributory Chonçl

C revorse Sploy

Crevosse sploy inf illed with silt ond cloy of 8oy- fill Muds similor groin sirc to'surrounding boy- f ill muds. Fossible polishcd Poss ib le Distriþulory Moulh Bor Sonds Possible Cloy bosin conlocl Cloy Seoms LEGEND CLAY SEAM SAND COAL

SPLAY SILT A SANO rTr ROOTING

BAY- F[-L SILT/CLAY * CROSS- BEDS

B AGGRÀDÀTIONÀL ENVIRONMENT

Chonnel Poinl 8or

Lev ee'

Bockswomp

ffi.. C loy Seoms

Possti ble Cloy Seoms

Figure 3.11 Locations of vreak zones in progradational and aggradational sequences (modified after Ferm and Horn1L979). -73 Progradational clay seams

For progradational seç[uences such as crevasse splays and fan d.ettas or t,idat delt,as, clay deposition is most likely to occur either at, the waníng sEages of parÈicular depositional episodes ( i. e. atr Ëhe top of a sediment,ary cycle) or at locations distal from the sediment source, refer t,o Figure 3 .114. These two processes have implicaÈíons for the locat,ion and geomeËry of the clay seams with two types of clay seam beÍng possible.

FirsÈ, t,hin beds of clay deposited between tr,Io sediment'ary cycles. The lat,eral continuit.y would be largely determined by Èhe presence and locat,ion of the lake/swamp/marsh which covers the area.

Second, clay-rich sediments deposiUed aÈ distal locations within È,he basin. Because these f orm as part, of an act,ively prograding sequence, the location, thickness and extent are largely det,ermined by the nature of Ehe sediment source. Irlhen the sediment supply is const,ant in volume and mineralogy, then the progradíng sequence would form a zone of clay-rich sediments at, iUs base and this zone is like1y Eo be both laterally cont.inuous and extensíve-

Aggradational weak zones (refer t,o Figure 3.118)

Because deposiEion of clay relies on st,ill or a1mosE. st,ill wat,er, clay Seams in aggradational seçluences such aS those deposited. by fluvial processes rely on a mechanism which produces episodic or cyclic vtater movements. The most 74 striking example of this are sequences formed within the tidal range.

CIay seams are d.eposited on point, bars during high and low t,id.e and characteristrically f orm nbundles" of clay seams Separated by sand. A variation to these occurs when a meander channel is cuE off and is filled with fine grained sediment.s. Bot,h of these t]4)es of Sequence are contrnon to Èhe sequence occurring in the Athabasca Tarsands ín Canada

(PfacCallum et âI. , L979) .

For Ehe purposes of thís study t'he conclusion is that t'he geometry and lat,eral continuiÈy of clay seams are controlled by the depositional environment -

3.6 Porewater movemeat,

The key information required on t,he hydrogeology is on Ehe hyd,raulic properties of t,he sediments and Eheir variation within t,he deposit. For soils, the hydraulic properties are cont.rolled by grain size and soil sEructure/soil fabric. For soil stability analysis, particle size distribut,ions are used t,o given indicaÈions of permeability, capillarity and likely mat'erial behaviour.

A srumary of the grain size analysis and v'teIl test results for Ehe Lochiel CoaI Deposit are provided on Figures 3.L2 and 3.13. For sand and gravel soiIs, the shape of Ehe grading curve can be useful t,o det,ermine some aspects of engíneering behaviour. A Smooch curîue indicat,es a well graded. soil in engineering terms (poorly sorted in 75 geological terms) and hence unlikely to be subject' to red.uction in volume due to vibration or washing ouÈ of t'he finest fract,ion. If t,he soil ís to be used as a drainage layer then a uniform particle size distribution (well sorted.) would indicate highesÈ porosity and permeability.

Sedimentary soil deposits are likely' however' to show marked varíat,ion between horizontal and vert ical permeabitity due to stratification and grain fabríc. Lambe and. Irlhitman (rggg) and. Bolton ( 1984) considered that, the use of particle size d.istribut,ion for fine grained soils is quest.ionable because t.he chemical and mechaníca1 Ereatment' given for the analysis usually results in effecÈive particle sizes which are quite differenL to Ehose of natural soils.

Rowe (tg72) demonst,rat,ed by tests on silEy clays thaÈ soil structure can give in situ permeabilit,ies 100 - 1 000 times higher than remoulded. samples. He emphasised that' gradings and index tests alone are not indicative of engineering perfonnance.

As d.iscussed by PetÈijohn et aI. (L987) , there is nearly always a good correlation between effecÈive porosity and permeability wit,hin a facies buL not beÈween facies. rhis is because per:meability ís relatred t'o specific surface area so Èha¡ two facies of differing grain size may trave equívalent, effective porosity but Ehe flner facíes will hawe a lower permeabilitY. 76 with respect to porewater movemenÈ the conclusion is that it is essential to first understand the facies characteristics and relationships before interpreting the resul-ts of grain size analysis.

Figure 3.t2 Summary of grain size analyses for sandy sediments occurring within the Lochiel CoaI Deposit. (GH and FG interburden are discussed in section 5.4 and represent intercalations between the coal seams).

T iELL SILT or{ rr.f Eltutof x loo po Å åo & €o .: õ I !eo Et ts F

.5 I ü40 4

o 720 3¿o

À o w o Á ol -ol o o o roo rJAtRtNOt srLf cql{)orrã srLT tæ r Æ ro æ : .: a5eo o ts I !{) t ) I o Å o E20 C € c ffi l,. è ,s ry o o r-o roo .()¡ .Kt o rO roo F E T'TERAURD€i ÐLnÐ¡rrGA sa¡os fx) !ãe : Y. a a I o ¡ fl tsÍn F Ë.o s o E¿o

À À o o .)o o Fodiclt S¡rr (ro I Fo.ti< la S¡r 9ro) o.o@ o06 2.O 6(, o'oæ o.oc z.o Tra4smissivitY Hole No. StratigraPhy Aquifer Hydraulic Thickness ConductivitY (m'/daylm) (m) (m" /day/m')

7.0 v 572 Tarella SiIt - TL1 3.3 2.r 14.0 v 611 Tarella Silt - TL1 2.t 6 7 7.O v 533 ffarrindi SiIt - WAz 3.3 2.r 73.0 v 623 Warrindi Silt - wÀ10 2.3 32.0 3.0 7.5 v 437 f{arrindi Sitt - WÀ4 2.5 FG 0.7 7.t 5.0 v 627 Kooliata CoaI - I 2.3 31.0 -¡ v 620 Bumbumga Sand - NSz 13. s -J 20,0 t v 629 Bumbunga Sand - NSz 6.8 2.9 3.9 61 .0 v 61-4 Bumbunga Sand - NSl 15. B 7.0 v 613 Bumbunga Sand - NS3 3.0 2.3 V 6IL Darnleigh Park Sand 4.0 40.0 160 .0 13 .0 400 .0 v 640 Bumbunga Sand - NSl 30.0 160.0 v 630 Bumbunga Sand - NS1 20.o 8.0

test sites') Figure 3.13 summary of well test resurts (see Àppendix 6 for location of 78

CEAPTER 4. O METEODOI.OGY À}ID DÀTÀ AVÀITJÀBIIJIIY

The methodology applied to the analysís of Èhe I-,ochiel coal Oeposit relies on t,he int,egraEion of geologícal and engineering interpretation techniques. The interpretation is based on a nr¡mber of díf f ering data types and the Iimit,ations on these need to be reviewed prior to detailed analysis.

The analysis to follow is supported by a series of Tables and Figures. These are included in the text' when they are considered to be of direct support to the analysis-

There are, however, a number of figures which either provide more detail on the figures included in Èhe Eext or provide indirect support Lo the analysis. These figures are set. ouE in a series of appendices in volume II. These include;

Appendix 1 ehot,ographs referred to in Volume I '

Appendix 2 GeophysícaI log response models for the Bumbunga Sand Formation.

Appendix 3 Geophysical and lithologícal correlation sect,ions for the Burnbunga Sand Formation'

Appendix 4 Geophysical and, lit'hological correlation sect,ions for the vüarrindi and TareIIa Silt's '

Appendix 5 StrucÈural analyses for the Tarella Silt''

Appendix 6 Borehole location PIan- 79 4.L Data availabillty and limiÈatLons

Unconsolidated, sed.íment.ary deposits, such as Lochiel, are generally flat lying with few or no surface exposures. In these set,tings subsurf ace meE.Frods of obtaíning data are used. Four t)æes of data are available for t'he study of the Lochiel Coal DePosit : -

orillholes - cuttings and core, Geophysical downhole logs, Hydrogeological Èest sites, Triat excavations.

A su¡nmary of both the number and t)4)es of data collection sítes used in this study is presented on Figure 4-L with a detailed borehole location plan awailable in Appendix 6.

These d,ata t,)æes are the fundamental source of all information on the geological and engineering characteristics of the sediments. Furthermore, Ehese t)æes of informatíon are also generic Eo other soft, brown coal deposiEs.

However, it is important to realise that' even in the design st.age, the spacing of data points in Ehe deposit' will be relatively broad., especially when compared Eo t'he geometry of t,he operat,ing pits and of signif icant' engineering geological featsures. To compound this limiuation, the engineering and, hydrogeological test sites make up only a smalI percent.age of the to!a] nr:mber of data points ( f or 80

24! oooírE 24gooomE SumnarY of Data sites netres Legend I TyPe Number TotaI (apProx. )

300 A I PartiallY cored 26 o Fully cored 14 1200 tr welIs 29 I Water 23000 open hole sites .. 236 '1D Trial dePress. srte 4 1P l Trial pit síte 1 l BuIk coal core site B I tr

f o2soooomt't I o I I A I I tr^T'¡j / À .a I E

\ò' a¡e. o A ê A e" + A A AA A +r24ô ooornN A .'.pfP Oril. f9 \.eJ EltP A ò A AA A A .A

,o A I Ä l o A l LOCHIEL I I oB A

f ezeooæmH + +

SCALE

XMS I o 2 IXMS

Figure 4.I Summary of number and tYPes of data collection sites -81 I-,ochie1, fully cored geoEechnical holes make up less than 5t of the totat nr:mber of holes drilled) '

DriIIhoIes

Apart Êrom when a t'rial excavation is undertaken, d.rillholes are Ehe only means of Obtaining subsurface material for engineering Eesting' Drillcores of diametrer upto0.gmetrescanbeobtained.butgenerallydiametersof 0.053 and, 0.100 metres are recovered'

Apart from Lhe limit.ations concerning Ehe spacíng of data poinÈs, a significant limitat,ion wiuh having to rely on drillcores for samples in unconsolid'ated sediments is that partsofthesequenceareoftenmissedduetocorelosses. This factor can have an significanu impact especially if t.he lost sect,ion contains a significant feaEure sucÏr as a weak zone.

Almost a1t of the holes dríIled are vertical thus límit'ing the effectiveness of obt.aining daÈa on steeply dipping tectonic defecÈs. A possible solution Eo this is to drill inclined. holes, hovùever, as t,he t,ect'onic strucEures in soft sedimentary environment,s are difficult Eo distinguish even inoçosedfaces,angleholeswillprovidelitt'Ie addit,ional data, especially if Èhey are not oriented'

Geophysical borehole logs

Geophysicalboreholelogsareused'exEensiwelyin the petrolerrrn ind.ustry f or the quancítative analysis of -82 material propertíes. Some geophysical logs are also used to characterise physical rock propert'ies (Mathewson and Cato, 1986). However, the application of quantitative interpretation technigues to engineering geo1ogícal aspects of soils is largety impractical because : -

of the nature of the properEies beíng measured. Most defects cannot be d.eÈected from the tools currently available. In ideal conditions (where there is a contrast in maE,erial t]æes) t,he tools current'Iy available can detect features down to 50 ntrn thick. In the Lochiel deposit, the sub-horizontal defects are mostly less than 10 ¡run thick, with little cont,rast in material tr)æe.

of the strength of both the intact soils and the defects are in general so low as to be out of the normal operating range of the sonic Èools'

d.rilling ín unconsolidated sediments is normally done by rotary mud techniques. The technique cuts and flushes t,he soils oft.en resulting in holes of uneven d.iameter and mudcake t.hickness. This ef fect is further compounded when drilling takes place in areas of high art,esian pressure, such as at Lochiel, because E.he add,itives which are included to control the f10w of water can invad.e the formatíon Eo give inaccurate tool responses. 83 These problems can be overcome by modifying the drilling technique and enhancing the capabilities of the logging tools and their Processing. However, these solutions are relatively expensive and rarely employed.

Notwithstand.ing these limitaLions, and because geophysical logs are normalty avaitabte for every driIlhoIe, the geophysical logs can be a prime source of data for geological stratigraphic interpretation'

1o minimise the Iimitations d'escribed above, a suite of logs have been used. as the basis for interpretation.

A set of typical log signatures are defined for the various sed.iment.ary facíes. These signatures are based primarily on the responses of the gamma-gamma density, neutron and natural gamma tools and. vtere derived from detailed correlation of geophysical log responses to I fully cored d.rillholes (approximately ?oo metres of core). Detailed core to geophysical log correlation sheets are not incLud'ed in this thesis but are stored by CoaI Resources Department,

ETSA.

The geophysical signatures provide the means to interpret geophysically logged open d,rillholes. This interpretation forms the primary basis for correlating and mapping the sedimentary sequence around the deposit' 84 Hydrogeological test sites

Four detaited. tests vtere und.ertaken ( including the trial excavation site) to obtain d.ata on the rate and extent to which the porewater pressure could be reduced. These tests involved. a pumping bore and. a number of piezometers set in each of the major stratigraphic units'

sites must be serected so as to be representative of the total d.eposit. This can be difficult to achieve especially earlyint'heexplorationand'investigationprogranme becauseofthelackofinformationavailablefor interpretation.

Trial excavation (refer to P1ate 4'L)

the Lochiel Trial Pit provid'ed the opportunity for a 3-dimensional vi-ew of the relationship between sedimentary and structural features.

A trÍal excavation also has the advantages of being able to demonstrate the actual behaviour of the slope and allows fordetailedana}ysisofthefailuremechanism.

The major Iimitation of a trial excavation is that it provid'esdataononlyonesite.othertimitations concern the geometry and' timing of the excavation'

4.2 Methodology

Thepurposeofinterpretingdataistoproducepredictive 85 geologicalmodelswhichdescribethekeyfact,orsaffecting slope st,ability- These mod'eIs are then used by t,he geotechnical engineer to analyse the behaviour of t'he key fact'orswhensubjectedtomininginducedstresses. The purpose of this sect'ion ís to :

describe the interpretive meÈhods in current use' d.escribe E,he t')æes of models produced' def ine t,he límitations of the methods and models '

FortheinterpreEationtobeeffective,theelementsofthe model must have significance to the context of Èhe f or t'he obj ectives of the study ' The key f acEors geological,geomechanicalandhydrogeologicalaspecEshave wílI been d.escribed previously and the following discussion and t'he examine how werl the methods of inEerpretat,ion models address Ehese factors'

and 4.2 .L erinciples of geological interpret'at'ion modelling

of Geological int.erpretat,ion relies on an underst'anding geologicalprocessesandtheoutcomesoftheseprocesses, nfindíng whatrs or in Ehe words of stapledon o97g) out' downt'herebyunderstand.inghowitgottheren.FurEher,he geological statred t,hat nwithout (an¡ understand'ing of Ehe environment'weareunliketyt'oaskt'heríght'geological quest,ions, that is the geological questions which are relevanE tro t,he geological environment' and the proposed operation. r' - 86 lt is considered' that' t,hese views captrure t'he esgence of geological interpretation. In sedimentary deposit's, such as Lochiel, the processes are broadly cont,rolled by the depositional environment's present aE the t'ime of sedimentation and. by the processes which deform Ehe sediments subsequent to deposit'ion'

A principle around which thís thesis is develoPed is t'hat an underst,anding of t'hese processes wilI f acilitat'e the predíct,ion of t.he key fact,ors affecting slope st'ability'

Key aspects of t'he geological interpreÈation process

The geological int,erpretat,ion process involves a seç[uence ofiterativest'epsduringwhichEhelevelofdeEailín understanding is progressively enhanced. The recognition of depositional facies relies on the ex¡lerience of the analyst, and in part'icular his or her knowledge of precedent,s. Numerous accounts of sedimentary facies ín coal bearing environments currently exist and these can be used Eo provide additional detail'

ïlowever, because of the variations in the controls affeccing the sedimentary processes' t'hese precedent's can of t'he only acE as a guide to the deLailed nature sedíments

and Irltren examining a deposiL from an engineering vierupoinE, part,icularlyforslopest'abilítyst'udies,itiscommon'for these det,aíIed' features to be t'he key aspecÈs ' Hence' although the E,raditional met,hod's employed by Èhe -87 environmental analyst can provide valuable informat'ion, it is unlikely to provid.e the key informat'ion unless the site specific controls on t,he process forming the features are understood. a) Facies classification

The design of the facies classífication syst.em is crit.ical for successful inÈ,erpretaEion, and was stated by Lindholm (198?) asrrone of the most difficult aspects of f aci-es analysis...rr.

I_,indholm (1987) and I¡IaIker (1984) emphasised that both the d.egree and scale of subdivision is governed by the obj ect,ives of Ehe study. Furt,her, the classif icatíon system should also be simple enough to allow easy understand,ing and use. In practrical t.erms Andert'on (1985) considered E,his to mean E,haE the nuÍìber facies be kept to a minimum- The only caveat, is EhaE t,he facies def ined adequately encompass all rock t)æes present in the study

area (Ivfiall , L984) .

And.erton (1985) and. Lindholm (1987) considered that the classification syst,em be designed in a hierarchial manner. Furthermore, Lhe classification criteria select'ed should allow objective descript,ion and measurement Eo enable use by successive workers.

b) Cyclic sediment,aÈion - Classification - 88 sed.imentation within the overall vertical seç[uence at Lochiel is characterised by cyclic seç[uences wit'h both rapid and. grad.ational changes between cycles ' clastic cycles may be d.ivid.ed. Ínto three t)rpes on the basis of grain size. These are upward fining, upward coarsening and homogeneous cycles. As described by lvliall ( 1984) , these can be caused by a variety of sediment'ary, climatic and tectonic mechanisms-

be As discussed by Beerbor¡rer (1964) ' these mechanisms may divid.ed, into E,wo E)æes, those related to 1. the depositional system, aut'ocyclic, 2. an external cause, allocYclic-

Generalty, the autocyclic mechanisms are related to sedimentation mechanisms such as channel swiUching, Ehe building and. breaking of bars and banks, shoreline migration, channel arnrlsion and delt'a building'

Allocyclic mectranisms includ.e t'ectonics, sediment supply, palaeoslop€, sea level changes and climate, and are large scale conE,rols which effect the whole of the basin aE one t,ime.

Thus a sedimentary seç[uence may be divided intro Ewo scales of cycles. Fírst the large scale cyctes which were initíated, and. terminaÈed. by altocyclÍc mechanisms and seeond, smaller scale cycles which are relat'ed to

autocyclíc mechanisms . -89

Miall ( 1984) related t,hese mectranisms to deposÍtional systems analysis and, facies analysis respectively. Each allocycle is made up of one or many autocycles and in terms of deposit.ional environmenÈ, the allocycle represent's a sediment,ary system while the autocycle represents the numerous subenvironments making up that sysÈem'

These concepts are a useful adjunct' to the concepfs d.iscussed. in Chapter 3 and. Eoget'her wiII be applied to Èhe interpretation of t,he Lochiel sequence - c) StratigraPhic correlation

Generally, two Processes are involved in buitding sedimenÈary seç[uences, verÈ'ica1 accretion and Iateral aggradation. vertical accretion occurs in sEaÈic systens where sedimentation is uniform, developing strat'igraphic zones with homogeneous proper¿ies and a ta.bular geomef'ry. Correlation of E,hese zones is relativety easy (the G Seam of the tochiel Coal Deposit is an example).

Lateral aggrad.at,ion occurs where deposit'ion is related Èo migraÈing envirorunents which may build up or ouÈ or both' The distribution of environments is controlled by inÈerna1 or ext.ernal (autocyclic or allocyclic) mechanisms leading t,o variable geometry and rapid lateral changes of facies' Det,ailed corretatíon in migrating syst'ems is t'hus more

dif f icult t,han f or aggradational sysEems - - 90 v{ithin both of E.hese tlpes of sequences Iithological bound.aries correspond to Eíme horizons. trilhere bot'h processes are acuing aE Èhe same time ín laterally equivalent deposiÈionaI environments, such as fluvial syst,ems and, Iakes, díst,incÈ lateral facies changes occur' Correlation across these bound'aríes is dífficulE' because of marked, liE,hological changes buE equivalent facies can be conf id.ently correlated if contaíned bet'ween known time horizons.

at In E,he Lochiel area such Eime horizons may be ident'ified the top or base of wertical accreEion deposits such as coal seams. This provides a means of correlating and int,erpreting complex facies assocíat'ions '

d) Correlat,ion Procedures

Correlat,ion of sed.iment,s was based on geophysical log responses. The interpret,at'ion f Iow paÈh as shown in Figure 4.2 consists of an initial calibration phase in which geophysical responses were mat,ched with drillcore t'o obt.ain a seE, of geophysical signatures for the sediment's '

using these signatures as a basis, all open holes were Ehen interpret,ed manually for tiEhology and depositional facies' Lithological columns were Ehen placed on orEhogonal cross- sect,ions encompassing all holes in t'he area and t'he major 1 i t hol ogy / geoPhYs i cal log responses representing changes correlated. At t.hat st,age, the sequen'ce \rlas d,ivided ínto members which were then analysed in detail in terms of depositional system and facies' 91' Figure 4.2 Core and geoPhYsical log resPonse interpretàtion flow Path.

DRILLING

COREHOLES OPEN HOLES Geophysical loggmg and Calibration of - 1500 mcres - 23,000 me¡es tools (approx) (approx)

Detailed Correlation ofcore to geoPhYsical log responses

,J,

Geophysical resPonse Interpretarion of oPen signatures defined holes for lithologY

Cor¡eladon of vertical lithological profiles on onhogonal cfoss-secnons

Interpretation of deposiúonal facies t

Relationships with engineering propcrties

e) Geological d.escription and representation

sed,iments are generally described under the following grain f abric general head,ings of sedi-mentary structures ' ' trace fossils, particle morpholog-y, mineralogy' grain size' colour, fossils. '92 The description and, representat'ion of sediments under these headings serves two PurPoses :

It documengs in det.ail sediments at' a particular site, Èhus prowiding a basis for reinterpretat.ion or comparison with engineering test data'

Provides a basis for environmental interpreEation Èo altow prediction of geological properties elsewhere.

Interpretat,ion is based on Èhe way in which various facies are associated and on E,he geometry and internal characÈerisËics of the sediment's.

Analysis of the d.epositional environment is done by comparison with modern and ancient models'

Isopach, st,ructure conEour and rat,io maps adequat'ety define the geometry and orient.ation of facies while also enabling easy comparison with ot,her modern or ancient' models.

For comparison of internal characÈeristics, graphic logs are used. The mosE, corunonly uSed columns are those which graphically represent, sedirnenÈary st,ructures and variaÈion in grain size with one of more ot,her features (Mia1l , L984; Collinson, L978; Lindho1m, 1987) -

Anderton (1985) d.iscussed Èhe use of graphic logs for presentíng geological information and suggest'ed that Ehe use of stand,ardised symbols tended to I'resulE in only a sma1l part, of the available sed'imentological det'aiI being 93 recorded on the log". He went on to suggest that "Iogs should be as detailed and realistic as the ability of the drawer will aIlow" and that this could be accomplished by using symbols which accurately depict the relevant features.

In this study, standardised symbols are used but these symbols are desi-gned in such a way so as not to lose detail. This type of approach is required in this study to allow detailed correlation of engineering test data and facies. The legend is presented on Figure 4.3.

Figure 4.3 Legend for stratigraphic symbols

VERTICAL PROFILE SEDIMENTARY STRUCTURE

Anasfomosed bedding

lD rSA ND-r U E cl- + ltt ==ø(u Þ -"<. o Ripple cnossbeddinq .>- ¡J =ËËEEÈvr>5tsucn -

~~Trough crossbedding~~

i nterbedded / [amina ted Bioturbation d"\{ fining upward o o Ptant fraqments o o o o o o -gradationat contact 0 o o o 0 o o o oo o oo [arge & smal[) 0 o o o o o o o o o o o and roots sfi ÂÂÀa Breccia AÂA AAAô

~~CARBONACEOUS (-D SEt]IMENTS and <ÊD Clay ctasts <Ð fincreasinq up) pyrite nodules qÐ@~~

~~XXXXXXXXX C O NTACTS l'lineralised bands XXXXXXXXX SHARP - planar -.irregular _ undulose _æ, -94~~

~~4.2.2 principles of engineering geological interpretaEíon and modelling~~

~~Geotechnical inÈ.erpretation aims t,o assign values of engineering properties Eo Ehe geological models'~~

The traditional practice is t,o d.ivide the deposit into secEors or regimes in which simílar geot'echnical conditions exist(Irlad.e,1985;Kremor,1988)'Thed'efinit'ionofEhe regimes is based solely on the results of analyses direct'ed t.oward. specif ic questions. These corunonly relate t'o slope stabj-Iity, dewatering/depressurisation and spoilpile

~~stability (Kremor, L988) .~~

This approach can be d,efícient if it, presenEs only the average cond.itíons f or each regime without giving any indication of the relat,ionship between regimes. This study ad.dresses this issue by providing a detailed basis f or understanding the relationship beEween eaclr regime based on geological understanding. -95 4.3 StratigraPhic noneuclature for tb,e Lochiel Coal Deposit

The sedimenÈs which comprise t,he Btrmbunga sand Formation correlate with the Clinton Formation described by llarris (1966, lgTt) in t,he st. vincent Basin to t'he south and also to the informally named Kanaka Beds of Ehe Pirie Basin to the north (Lindsay, ir Press).

The Br:mbunga sand represents the initial phase of basin infilling in Èhe Lochiel area. The sediments utere derived from surrounding uplifted basement and transporÈed to the depositional sit.e by fluvial chan¡rels originating from the nort.hwest.

~~The character of sedimencation farttrer north indicaues that' several f ans were present during t,hat time wit'h the channels coalescing Èo form a longitrudinal system-~~

Regional correlat,ion of the ctrannel f acies suggesÈs Ehat the flow was sout.h to north, wit'h trhe t'hickest accumulation of fluvial channet sed,iment.s occurring in the narrovú neck of Ehe basin near Crïsta1 Brook. Channel facies sediment's in Ehe Br:rnbunga Sand are confined to the north and cent'ral areas and do not extend sout'h- This is also true for the area Eo the east. of the Nant'awarra High where borehole inf ormat,ion indicat'e t'he exístence of a basement high overlain onIY bY Hindmarsh ClaY. -96 The implication is that the sediments north of the Nantawarra High block were deposiEed. in a basin which was separaÈ.e from the st vincent Basin-

The alternatiwe is that there was a conE,inuit'y at the time of deposition but subsequent uplift of the Nantawarra High has led to erosion. Uplif E, is known t'o have occurred because the elevation difference beEween t'he coal seams ín the Bowmans d,eposit, and t,hose in Lochiel is in Èhe order of abouE 1oo metres (Kremor and Springbett', 1989) ' Any sediments which may have overlain the NanEalâtarra High could have been eroded. However, ghe interpreted limits of successive uníts (from the base of t'he sequence upwards) Iie progressively further to the souuh, overlying the Nanuawarra High.

In ad.dition, the Nantawarra High in known to be a source of sediment, E,hroughout t.he IJaUe Eocene-O1ígocene. The conclusion is tha¡. uplifÈ and erosion are not the cause of the present distribut'ion-

To the norÈh, Èhere appears to be a conl'ect'ion with the pirie Basin (Kremor, !986) . Alt'hough this Basin is not well def ined, I-,indsay (in press) considers the sandy, carbonaceous and, coal bearing Kanaka Beds t'o be IJat'e Eocene to Early Oligocene in age and can be correlated with t'he Bumbunga Sand in t,he Lochiel area.

There is strong evidence to suggese È'haE the sedímenÈs north of E,he Nant,awarra High were deposit,ed in a discret'e basin, separate from the St Vincent Basin to t'he sout'h' -97 The channel facies can be correlated north past' crystal Brook and it is consid.ered t,hat it is like1y that' the basal sedÍments are cont.inuous wit,h the Kanaka Beds in t'he pirie Basin t,o t,he north. there are, however, other considerations relating to the d.efinition of Ehe strat.igraphic nomenclat'ure. There is evidence which ind.ÍcaÈes that during t'he deposit,ion of the sed,iments of the Warrindi and TareIIa Silt Formations there was a connection with Èhe St VincenÈ Basin t'o the souÈh' However, âs discussed in chapt.er 5. 0, Ehese dif f er in lithology from their sÈratigraphic equivalenls in the St Víncent basin.

~~St.aines ( l-985) suggesÈed t'he establishmenu of new sErat.igraphic nomenclat'ure only where it will aid the geological interpretation of Èhe area-~~

To promotre a clearer und.erst,anding of Ehe strat'igraphy and sed,imentology iC is proposed Ehat' t'he Tert'iary sequences occurrj-ng beEween t,he Nant,awarra High and Cryst'al Brook and between the Ard.rossan FaulÈ and the Redbank FaulÈ be assigned, Èo a separat,e basin, Ehe snowtown gasin. Accordingly, the basal, Irat,e Eocene sediments are to be assigned t,o the Br.unbunga Sand Formation which conÈai¡rs the Nant,awarra Sand, Condowie SíIt,, Darnleígh Sand and Kooliata CoaI Ivtenibers (Figure 3.3). 98

~~CEAPTER 5.0 ANALYSIS OF SEDrIITÀITS OCCT'RRIIIG BEI.OTC TEE~~

~~O\TERBI'RDEN~~

The aim of this Section is t'o anatyse t'tre Burnbunga Sand Format,ion t,o; . describe the lithology and lat,eral variat'ion of t'he sediments, . identify Ehe major factors conUrolling sedimentation and develop a model which will assisÈ in interpreting t,hese controls, . define the relationship beLween Ehe key geological features af f ecting slope st,abilit'y and characterise these feaEures in terms of their engineering properties.

~~The strat,igraphy is sununarísed on Figure 3.3 (page 53) .~~

~~5.1 Bunbunga Saad Fomatioa - ¡[aEtawarra Sand UeDber~~

The Nant,awarra Sand. MeÍiber unconformably overlies boÈh trhe Rosehaven Clay and. ghe ABC Range Quart,zit'e - The member ranges in thickness up Èo 65 m in the n'orÈh$test' and laps out t,o the souEh and east (Figure 5-1) ' To the wesE it abuts t,he Ardrossan Fault, and. is lat'erally continuous wit'h f luvial and lacust,rine sequeflces of similar character to the norÈh.

~~Lithologically t,he formation scÌribits a gradaLional lateral change from fining upward, and sÈacked cycles of clean quart,z sand. (NS1 graided SEream F'acies) in the northlrest' 99~~

~~24o 24õmmE N \ I I l~~

~~I +525oooomN /~~

~~/ I~~

~~s2ea æomt't + TRIAL PIT f~~

~~I I I / + + f s24oooomN~~

~~SCALE~~

~~KMS I o 2 3KMS~~

Figure 5.1 Thickness and' lateral distribution of'the Nantawarra Sand Member 100 through interbedded clean sand (NS2, Floodplain Channel Facies ) and carbonaceous sand (NS3, Distat Fl_oodplain Facies) in the central- areas and carbonaceous silty sand in the south and souÈheast (Figure 5.2 and Àppendix 2 for detailed geophysical log response models).

~~Figure 5.2 Nantawarra Sand Member - Lateral variation in Iithology and vertical sequence.~~

~~LATERAL LITHOLOG]CAL VARIATION ANALYSIS VERTICAL PROFILE~~

~~Broided Streom Distol Floodploin v568 V57t 597 594 598~~

~~NSI NS2 NS3 GRAIN SIZE DISTRIEUTION~~

~~5.1.1 Lithology and lateral variation~~

~~Northwestern area (NS1 facies)~~

To the northwest of the area, the Nantawarra Sand Member comprises a relatively thick sequence of stacked and truncated fining upward, coarse to fine grained clean sand { ûF 4 - 101 a' oLts and. gravel cycles interbedded with fine sand' c sand. and sand,y coal - The clean sand within thi s faci generatty poorly sortred and' subangular' ranging in grain size from medium sand to fine gravel '

Individual sand. zones range up to 10 m in t'hickness and are composed of cycles up to 2 m thick' Bedding within the sand and gravel zones is ind.istinct, probably due to t'he homogeneous nature of t,he sediments being deposited, but' occasionally individual beds are outlined by fine carbonaceous matE,er. crossbedding with dips of up to 35o have been observed within the zones'

Both E,he individuat cycles and the sand' zones t'end Èo fine upward.s in grain size and, characÈeristically have sharp, irregrrlar and erosional lower contacts with large clay galls and wood pieces lying on ttre boundary (P1ate 5'1) '

As shown by Plate 5.2, the fining upward cycles are often truncated at Ehe Eops of the cycles by overlying cycles Eo form st,acked. sequences of clean sand. Vlhere t'he t'op of the cyclesarenottruncated,t'heycontinuetogradeupward inE,o a sequence of interbed.ded f ine clean sand, carbonaceous silt and dirtY coal'

The carbonaceous zones are up to 18 metres thick and often d.ist,inct coarsening upward cycles up t'o a metre t'hick are present within the sequence. These comprise silt'y coal or clayey, highly carbonaceous silt' at the base and become Iess carbonaceous and more sandy upward. Towards t'he top, the cycles are ctraracEerised by indist'inct parallel ' L02 laminations of fine to very fíne, clean white sand (refer to Plat,e 5.3) wít'h an occasional thin bed of fine sand at t,he Eop. Generally these coarsening up cycles are bioturbaE,ed. and rooted. and characterístically have sharp, planar upper and lower boundaries '

There is a strong trend characterised. by a rapid increase in grain size towards the wesE corresponding to the Iocation of the Ard.rossan FauIE. prillholes locaÈed close cobble Eo t.his f auLE, int.ersect a sequence dominated by sized particles.

~~The charact.erisE,ics of NS1 appear t'o correlaEe wít'h those d.escribed by Coleman and. Prior OSAZ) as being typical of braid.ed stream dePosits.~~

The trend toward.s a rapid increase in grainsize towards the west indicates a clrange in gradient t'owards the Ardrossan Fau1t. It is common f or braided st.ream deposit's to be laterally equivalent. to alluvial fans with the extent of t,he lat,ter being controlled targely by the gradient '

~~It is considered thaE NS1 represents an alluvial fan/braided stream depositional environment.~~

~~Cent,ral area (NS2 facíes)~~

In t,he central part of the area, t.he fining upward cycles grad,e lat,erally int,o a Sequence of interbedded clean sand and silty carbonaceous sand. .103 The clean sand present in this ínterbedded zorLe is usually Iess than 2 metres thick and rarely do individual cycles stack t,o f orm thícker units. The sand is f ine grained, moderately well sort,ed and generally fines upward into carbonaceous sand comprising an abundance of wood fragmencs in a slightly carbonaceous, siltry sand matrix'

~~Bedding is rarely obserr¡ed, being destroyed by intensive bioturbation and rooting. Occasionally, Ehe carbonaceous sand.s grade into dirty, woody coal seams up Eo 1.0 metres thick.~~

The features described for the NS2 facies are charact.eristic of lacustrine fill environments as described by Coleman and. prior (!982) and Horne et al- (1978) or as fan deltas (alluvial fans ent,ering bodies of water) as described by WescoE,t and Etheridge (1980) - The clean sands are consid.ered. to represent distributary channels entering a swamp environmenu.

~~SoutheasEern area (NS3 facies)~~

Farther to the souÈheast, E,he Nant,awarra Sand Member grades int,o a carbonaceous sand facies. A number of carbonaceous sand lit.hologíes are present.. lvIany of these lithologies are present. in other parts of the seç[uence and an abbreviated nomenclature is developed to assist wit'h the description of these ot,her Parts. - 104 a) rnferior coal facies (SA5)

A large proport,ion of the Nant,awarra Sand Member in the southeast.ern par¿ of the area consists of an abundance of wood fragment,s of waryíng síze in a slighlly carbonaceous, silLy sand matríx. This is equivalent to t'he SA5 lithology presenÈ in many of the overlying members aS well as being comrnon in the Nantawarra Sand Member. Bedding is rarely observed in Èhese beds, being disrupted by intensive burrowing and root systems. Depending on Ehe amounts of ptant fragments, t,he malerial may grade into coal but only thin seans hawe been obserwed.

~~b) sand wit,h a fine grained carbonaceous mat'rjrc (sA3) (refer t,o Plate 5.4)~~

This È]æe of sand is of ten present, both within the Nant,awarra sand Member and. in overlying members. !{ithin the former it consist,s of ind,isUinctly bedded sand wit'h a fine t.o very fine carbonaceous matrix. The carbonaceous matrj-:< forms a cement for the sand grains and depending on the proportion of carbonaceous mat,rix, the sands vary from uncemented. (designated sA2 E)æe sand) to weakly cemented (sA3) . The sand comprises very well rounded, poorly t'o moderat,ely weII sortred quart,z varying from fine Eo coarse grained.

~~c) Bioturbatred fine sand facies (sA6)~~

Occasionally, thin beds of fine to very fine grained, cIean, white sand occur within the carbonaceous Sequence' These zones are usually strongly bioturbaled and . 105 indistínctly laminat,ed. with light brown silt' or silty sands. The laminations ofEen becorne more obvious towards Ehe base of the zone and dips range up to 3Oo' CharacEerist,ically E.he upper contract is sharp and irregular while the lower contact is gradational, usually with a clean sand zone. d) Non-carbonaceous silt

Occasionally Ehin zones of partially indurated non-carbonaceous, light,-brown to cream silt and sandy silt occur aS inE.erbeds within the carbonaceous sequence- These zones are structureless apart from being ext'ensively rooted. Generatly these zones have sharp, irregiular upper boundaries and. grad,ational basal contacts. They appear t'o represenL soil horizons.

~~The NS3 facíes represents the díst'al equivalent to the fan~~

~~delta environment, (NS2) .~~

~~5.t.2 Depositional environment. - Nanta!Ûarra sand Member~~

~~Based on t,he characteristics of t,he sediments described above a palaeogeographic reconsE.ruction of the area is~~

~~present,ed on Figure 5.3 . 106 Figure 5.3 Schematic rePresentation of the depositional for Ñantawarra Sand Member "åitittg~~

~~ENAD€D SOURCE AREA CHAttlltEL~~

~~FAIV OELTA~~

¡É

~~ç2 Ç ìf Ç* ÇÇ Ç IF a .5ta lÉ + s a ,l a e È s ù\ t~~

are indicated by The drill hole locaÈions for Figure 5'2 the numbers. L07 The lateral Iithologícal variat.ions are considered t,o represent a deposit,ional setrting characEerised bY the lateral associat.ion of ;

~~STIIAMP - Ns3 facies~~

~~FAII DEÍ,TA - NS2 facies~~

~~BRAIÐED STREJAI\,I - NS]- facies~~

~~AIJLT'VIAL FA}I - NS1 facies~~

~~It is apparent. from the thickness distribuEion, grain size and f acíes dist,ribut,ion t.hat Lhe source of sediments originated from an uplifted area to the northwest..~~

~~Regional correlation underÈ,aken by Kremor (1986) also suggested that, the braid.ed streams coalesced and formed part, of a major fluvial system which drained Èo the north int,o the Pirie Basin.~~

~~5.1.3 Engineering ProPertíes~~

~~The Nantawarra Sand. Member is the major su.b-coal aquifer in t,he area. Particle size analyses are presented on Figure~~

~~3 .t2, paçfe 76.~~

The NSI facies exhibit a significantly higher hydraulic conduct.ivity t,han the NS2 and NS3 facies (Figure 3.13, Pâ9e 77). Thís relates to t,he grealer proportion of clean sand and also t,he trend of increasing grain size towards the source of Ehe alluvial fan/farr delta- - 108 - 5.2 BurbuDgaSaudFo¡mation.DaralelghParksaadMember

~~The Darnleigh Park sand Member is present only at the northern end of [,ake Brrmbunga. IL has an averagie thickness of about 13 meEres, and attains a maximum thickness of~~

~~24 met.res (Figure 5.4) .~~

~~The lateral and vertical relationship between the Darnleigh Park Sand and t,he Nantawarra Sand Members is illust'rated in Appendices 2 and 3.~~

~~The oarnleigh Park sand. Member is laEerally equivalent to the NanÈawarra sand Member t,o the north and erodes into Ehe upperpartsoft'heCond'owiesilt'MembertotheSout'h.~~

The member is considered to represent continued alluwial/delta fan sedimentation but is assígned a separate formal n¿Lme on the basis of its stratigraphic and areal locat.ion and because of it,s importance Èo t,he hydrogeological regime -

~~5.2.L LiEhology and lateral variation~~

~~The Darnleigh Park Sand Member is described from core from V571, (Appendix 3) and t.he vertical lithological profile and geophysical log response ís sunrnarised on Figure 5'5' 109~~

~~24. N I \ -l~~

~~/ +6250ooomN / I I I I / I~~

~~ò s' ! s"Q I Ql e24a æomN TRIAL PIT f + ¿¿l-~~

~~ar o\~~

~~I I I LOCHIEL I / /, f + fszeommn~~

~~SCALE~~

~~KMS I 2 3KMS~~

~~Figure 5.4 Thickness and lateral distribution of the Darnleigh Park Sand Member. 110 Figure 5.5 Darnleigh Park Sand Member Geophysical-I!9 responsã and vertical lithological profile'~~

~~GEOPHYSICAL LOG RESPONSE LITHOLOGICAL STRATIGRAPHY LOG~~

~~ãà= -id us - I ñ ê I ilttttt~~

~~ô 70- H SEAM z o F~~

~~o=É. L OARNLEIGH ô z. PARK SANO 80- U' MEMBER (o~~

~~=d¡ =l @~~

~~coNoowtE SLT 9()- MEMEER~~

~~2ro 240 270 300 330 360 39() 420 90 NEUTRON CPS e92022-Z HIGH R€SOLUTION DENSITY GM/CC~~

OveralL, the sequence is upward fining from poorly sorted, subangular, clean, coarse sand/fine gravel at the base to silt and, clay at the top. within the lower part of the sequence there exists a number of truncated/erosional upward fining cycles.

~~Trough crossbedding is indistinct due to the monomineralic nature but occasionally beds are outlined by fine detrital carbonaceous matter. Large (> 0.1 metre diameter) wood~~

pieces are also present. À O . 5 metre thick bed of medi-um grained. sand. with 1Og clay galls is present towards the toP of the sequence. The galls are 5 mm in diameter, rounded, elongate (formed, by compaction) are composed. of cream and fawn silty clay. - 111 The upper part of t,he sequence comprises int,erbedded dark grey-brown, bioturbaÈed, fine to medium grained sand and moderat,ely carbonaceous silt. The beds are generally discont.inuous and. part,Iy anastomosing with crossbed dips up t.o 150.

~~These beds grade from light brown to cream moEt'Ied clayey siltandasiltyclayintotheoverlyíngtheHzone.This cont.act is charact,erised by intensiwe bioturbaÈion and rooting (Plate 5.5).~~

The oarnleigh Park sand l"Iember appears to have eroded laterally intro the cond.owie silt. Member, however, to the north it merges wit,h the Nantawarra sand' Member where iÈ becomes indistinguishable on geophysical log response

~~(Appendix 2) .~~

The characÈerist,ics of the Darnleigh Park Sand Memeber are similar to those of the Nantawarra Sand MeÍiber and it is considered that the two members are represenÈative of a braided sÈream environment' However' because Èhe Darnleigh Park sand Member represenÈ,s a single episode of deposiE.ion ít is possible to analyse in great,er det'ail t'he features of the braíded st,reaÍi sysEem in Ehe Study area'

~~5.2.2 Depositional environment's~~

~~The Ðarnleigh Park sand Member has t'he following definitive charact.erist,ics~~

~~1 upward fining graín size and sedimenÈary features.~~

~~2 IJatera1 consistency of cyclic paEterns ' - Lt2~~

~~3 Erosional, scoured base.~~

~~4 cross bedded sands.~~

~~5 InÈraclasÈ, gravel and wood fragments at the base of cycles.~~

~~6 Sands represent 80t of the cYc1e-~~

~~7 Sharp lateral contact.~~

~~These charact,eristícs are similar t,o the sequences descríbed by cant (L982) and. Miall (1978) including :~~

~~A. Channel Floor - trough cross beds and intraclast st'rewn scour surfaces developed in coarse sandstone. sed.iments are poorly sort,ed and internal stratification is poorly defined.~~

B In channel - trough cross beds in slighElY finer sandstone wit.h intercalations of planar crossbedded sandstone. c Bar top - interbedded rippled sandsÈone, mudst'one and low angle to horizontally laminaEed sandst'one'

~~Int,erpretaÈion of the vertical profile on the basis of these depositional sites is included on Figure 5.5~~

~~vertical and laE,eral deposítional environmenE relationships~~

Braided rivers are lat,erally unstable and migrate leawing sheet or wedge shaped deposits of channel and bar complexes, preserving only minor amounts of flood plain material (Schoelle and Spearing, !982) . CharacÈeristically - 113 braided stream deposiEs may be divided ínt'o two scales of sand bod.ies, Ehe largest of Ehese representing the migration of the river system it,self and the smaller representing lateral migratíon of channels within the

~~sysf em.~~

cant, (rgez) drew a hlpothecical picture of st,acking of f luvia] f acies Seç[uences, with ghe maj or f eaEures being major erosional surfaces associated with migration of the entire river sysEem and Eruncation surfaces assocíaEed with channel migrat,ion within the rj-ver sysuem'

The geophysical lithofacies secÈion (Appendix 3) illustrat,es these f eaÈures within t,he Nantawarra Sand Member and the Darnleigh Park Sand MeÍiber. The section also indícaEes that the Darnleigh Park sand Member represent,s an erosional phase of sedimentation.

~~5.3 Br¡nbr¡¡ga sand FornatLon - coudowle sLlt' lfember~~

The condowie silt, Member conformably overlies the Nantawarra Sand Member and, is t'runcaued Eo the nort'h of t'he area by t,he erosional Darnleigh Park sand Member - The Cond,owie Silt, Member laps onto basement on the eastern and souEhern margins and t,ermina!,es against t'he Ardrossan Fault' on Èhe western side.

~~Over mosE of Ehe area Ehe member has a Ehickness of approximaEely 22 melres and. alt'ains a maximum t'hickness of~~

~~30 meÈ.res in Ehe south (Figiure 5.6) . IT4~~

~~24o oæmE 245mmE / A N I s \ èo I øY I L l l l I I~~

~~/ f ezSoooomH I~~

~~I ß I I~~

~~ñò a¡\ 6 QI 246 @omN + TRIAL PIT/) . fe~~

*ar Ò

~~H I LOCH EL~~

~~f e 24oooomN~~

~~SCALE~~

~~KMS I o ? 3KMS~~

Figure 5.6 Thickness and lateral distribution of the Condowie SiIt Member. - 115 The Condowie Silt Member has been divided into three submembers named CN1, (top), CNz and CN3. The definition of the these submembers is based on the Iithological and geophysical log response changes which occur in the vert|cal sequence in.the centre of the area (figure 5.7).

~~Figure 5.7 Condowie Silt Member - Geophysical log response and verticaL lithological profile~~

~~GEOPHYSICAL LOG RESPONSE LITHOLOGICAL STRATIGRAPHY LOG~~

~~ü ¡: I ñ c I o trrlrlt~~

~~60-~~

~~oz. É t- ul rô =É. U o L f- J ô Ø z 70- UJ ú, = zo (9 o z. (J l (D =, =fD~~

~~80-~~

~~to 20 30 40 50 60 70 80 GAMMA API t.5 t.6 t.7 t.8 HIGH RESOLUfION OENSITY GM/cc rB5 2ro 235 zao zBâ 3to 335 NEUTRON CPS~~

From the central area, the geophysical signatures have been correlated to the margins of the deposit utilising Iithological descriptions from cored hoLes where available. 116 These correlations have attempted to folIow entire depositional systems and as such transgress facies boundaries. This tyPe of correlation procedure has inherent difficulties and relies on trend recognitj-on and lithostratigraphic distinction between and wiÈhin cycles.

In the centre of the deposit, very distinct cyclic sequences are developed, allowing for confident definition of submembers. At Èhe southern end of the deposit the distinction between cycles is poor and a lower level of confidence should be placed on interpretations.

~~Lithofacies and geophysical log resPonse models are presented in Àppendix 2.~~

The CNl and. CN2 submembers each consist of a single cycle whereas CN3 consists of multiple cycles. À11 three submembers foIlow the same thickness trends of being thicker in the south and then thinning to the north (figure

~~5.8a,b &c) .~~

~~The CN3 submember also thickens again at the northern end of the deposit.~~

~~5.3.1 Litholog:y and lateral variation~~

Each of CNl,2 & 3 d.isplays a similar lateral lithologic variation and each can be divided into either a sitty or a sandy facies. the silty facies occur in the north and central areas while the laterally equivatent sandy facies are present in the south. LL7

~~24ooæmE 24ó oæmE / N I I I I l l I I I o I f s25oooomN~~

~~\A I I I / I~~

~~+624! æomN~~

~~q, ò\~~

~~I LOCH EL I (- / / ,4 -----\~~

~~f s24oooomN~~

~~SCALÉ~~

~~KMS I o 2 3KMS~~

~~Figure 5.8a Thickness and lateral distribution of the CN3 unit - Condowie SiIt Member. 118 -~~

~~24oomE ?46M8 / N I \ I I l l I I~~

~~f ezSoooomt't~~

~~/ /~~

~~ò N s¡Q s \0^l + f szao æomtt~~

~~Cr o\~~

~~I LOCHIEL I /~~

~~+ R f e24oooomt't~~

~~SCALE~~

~~KMS I o 2 SXMS~~

~~Figure 5.8b thickness and lateral distribution of the CN2 unit - Condowie Silt Member. 119 I 24o oæmE 246mmE N I I I~~

~~I I f s2SoooomN I I I I I / I~~

~~+ {s2+r æomt't~~

~~ù ù\~~

~~I LOCHIEL I I I / + t f s24o9oomN~~

~~SCALE~~

~~KMS I o ? 3XMS~~

Figure 5.8c Thickness and lateral distribution of the CN1 unit - Condowie Silt Member. - ]-20 In vert,ical profíle Ct{1 and CN2 share similar lithologic characÈeristics. BoÈ,h are síngle Sedimentary cycles so will be described toget,her whereas CN3 comprises multiple cycles and. is d,escribed separately. In comparison wit'h the CN2 and CN1 cycles, CN3 cycles are t,hinner, contain more sand and are lat,eral}y more wariable.

~~CN3 lithology and lateral variation~~

CI{3 conformably overlies the NanÈa!üarra Sand MeÍiber and consist,s of mu1t.iple coarsening upward sedimentary cycles. CN3 reaches a maximum thickness of about L6 meÈres in t'he south, and t,hins t,o less than 10 metres in Ehe central area

~~( Figure 5 . 8a) .~~

~~Toward.s the north Ct.I3 appears to grade laterally into Ëhe upper seclions of t,he Nant,awarra Sand Member (Appendix 2, )~~

In Èhe central area, CN3 comprises upward coarsening cycles int,erbedded wit,h clean sand, carbonaceous sand and coaI. The coarsening upward cycles are up Èo four metres thick and. consist of a basal clayey, carbonaceous silt which grades upwards into very fine grained and laminated silty sand or sandy silt.

Bedding within t.hese cycles is oft,en indistinct and rarely present, in the lower port,ion of cycles, where present bedding is less than 1Oo. Interbedded. wit.h Èhese coarseníng upward cycles are zones, generally less than two meEres thick, of fine grained clean sand, carbonaceous sand and coal. L2L

These tlpes of vertical prof iles are conìmon l-n alIuvial/deltaic fan envj-ronments and can be attributed to either crevasses splays (Galloway and Hobday, 1983; Davies, 1980), or to lacust,rine delt'a fítIs (Coleman and Prior,

~~1982) .~~

Crevasse splay deposits are generally small in areal dist,ribution (maximum of several square kilomet'res, Galloway and. Hobday, 1983) and require a nearby fluvial source. ThÍs is Ínconsistent with the area covered by Èhe CN3 silty facies and fur¡her, none of the drillholes in the area have intersecLed erosional channel sand in this facies.

However, the size and lateral continuity of beds is consistent with lacustrine delta fill deposits and this mod.el is also supported by t,he radial locaEion of marginal facies laterally equivalent Eo E,he carbonaceous sand swamp facies of CN3 and t.he braided stream f acies of the Nant,awarra Sand Member.

To the South and wesÈ CI.I3 grades int.o a carbonaceous sand facíes (SA5 litholog¡1r) . This facies t,hickens towards Ehe margins and is considered Eo represent small deltas which were fed from smal1 streams originat.ing from t'ectonically uplifted basement. The high proport,ion of carbonaceous matter in Ehe sediment,s indicat,es that these deltas were heavily vegetated. - L22 CN1, CN2 lithology and lateral variation a) silty facies cN11, CN21 - liÈhological descríption

~~The silty facies are present over the cent,ral and northern parts of the area with t,he silty facies of CTI1 extending' furt,her south than CN2 (Figures 5.8b & c) .~~

The silÈy facies has a characteristic verÈical liEhological seçluence which is easily dis¡,inguished f rom geophysical Iogs, in particular, t,he density response, (Figure 5 - 8, Appendix 1, Figure 3).

The seçluence coarsens upwards and grades from a highly carbonaceous silg at Èhe base to a fine Eo very fine graíned laminated sand aÈ t,he top. Úütrere CN1 and CN2 are superimposed, they are in sharp cont,acÈ (P1ate 5.5).

The silt at the base of the cycles is conunonly clayey and highty carbonaceous, brown to dark brown and often cont'ains sub-horizontal, slickensided shear planes and st,riated subvertical joints. Thin, irregrrlar horizonÈal clay seams are also presenE. in this basal zone.

~~The highly carbonaceous silt zone is generally less t'han one metre Ehick and passes vertically inÈo a gradaÈional silt zone which is up Eo six met,res E.hick.~~

The grad,ational silt. zone ís charact,erised by a decrease in clay and carbonaceous conÈent. and an increase in discontinuous fine to very fine grained sand lamínations L23 upwards The sand lamínat,ions are usually bedded at less _o E.nan 5

~~Near vertical joint,s are conunon in this zone and towards Ehe Eop of the zorLe t,hese are somet'imes infilled witn fine sand from overlying beds (Plate 5.7) -~~

As t,he number of fine sand laminations increases, t'he Sequence gradually passes into a thin zorae of indistinct'Iy laminated fíne to very fine grained sand and silty sand' This zone is conunonly rooted and bioturbaËed with bedding dips up to too.

~~The contact with the overlying H zone is grradaLional at the south end of the deposit, and sharp in the central and northern parE,s. b) Sand facies - CN12, CN22~~

Towards t,he soutfl, the SiIt f acies CN11 and CN21 grade Iaterally into sand facies. The lat,eral gradat,ional zorte is between 500 met,res and. 2 OOO meÈres. Lithologically the lateral gradation is charact,erised by t'he basal part' of t'he silt facies becoming progressively more sandy. This sand f acies t,híckens in a souÈ,herly direction, gradually

~~replacing the silt facies (Appendíx 2-) -~~

The sand facies is variable in graín size, consisEing of fine grained, poorly sorued, subangrrlar silty sand cont,aining up t,o 5Ot plant, fragmenÈs. The planE fragmenÈs ]-24 consist of organic debris such as twigs and leaves, as well as large wood fragments (SA5).

~~The thickness of clean sand is shown on Figure 5 9 and indicat.es sediment input from sources to the norEh, SOIIEh and west.~~

~~5.3.2 DePositional environments~~

~~The lateral and vertícaf variations and associat'ions obserwed within the Condowie Silt Member may be sunrnarised as consisting of:~~

~~1 vert,ical coarsening upward cycles,~~

~~2 lateral coarsening Eowards the margins,~~

~~3 lateral association with fluvial sediments'~~

~~The Condowie Silt Member is associated with the alluviaI/delta fan sediments of t,he Nantawarra sand Member and represen¡s d.eposition ín a lacusÈrine delEa fill environment.~~

~~Lacustrine depositional environment's~~

FouchandDean(Lg82)consideredt'hatit'isdifficultand poEenÈia1ly mísleading to propose a set of physical and biological crit,eria that, uniqgely idengify specific Iacustrine environments because of the dynamic naEure of these deposit.ional sit,es. They considered t'hat' tecÈonics, bat,hymet,ry and waEer chemíslry to be the mosÈ import'ant' mechanisms controlling sedimentation. r25

~~246mmE I 24o ommE N I lI I I l l I I I I I +625oooomN I I I I I / I~~

~~ò a s, ì\ a ê ql + C + 524ð ooomN~~

~~q, o\~~

~~l o I~~

~~I LOCHIEL I~~

~~f e 24oooomH + +~~

~~SCALE 2 3KMS KMS I o~~

Figure 5.9 Condowie Silt - thi'ckness and lateral ái=tri¡otion of clean sand' - L26 Picard and High (tg72a,b, 1981) also recognj-se t'he variability of lacustrine sediment's and did not attempt t'o def ine a generalised model on the dist'ribution and occurrence of sediment t)æes. They also considered that climate was a major factor cont.rolling sedimentat'ion.

In the deeper parts of the lake basins, sedimentation is almost ent,irely f rom suspension and. E,here is a distal fining of sed.imenÈ away from Èhe Source. However' Picard and High (1981) noted trhat deposition in t'he cent're of the

lake : . may not, develoP bedding, . can be cont,inuous and develop bedding, . may develop bedding but bedding may be destrroyed by organisms (Reading (19?8) considered that because of this, bedd.ing lamination will only be preserved in reducing condíEions). The result, ís t,hat, the sediments may be finely laminated or non-bedded.

Picard and High (1981) and. T'wenhofel (1932) considered t'he ideal lacust,rine pattern to consisE of an outer belt of relatively coarse grained. shore deposits which gradat,ionally become finer Eowards the centre'

~~This reflect,s the delt,aic, progradational process which is~~

~~contrnon for lacustrine deposics as described by Reading (1e78).~~

As lakes are u]E,imat.e]y f il1ed, regressive processes d.ominaEe and t,he ideal lacustrine sequences grades upward ' 1,27 from fine to coarse grained sediments. This corresponds Èo a lateral migrat,ion of environment, from fíne sediments occurring below wave base int,o coarser shore and on-shore sediment,s.

The transition from nearshore Eo offshore is generallY abrupt and if luave action is limited, the nearshore to offshore phases can consist of organic, fine grained materials. visher (1955) suggested thaE takes are filled by deposition of fÍne materials over t,he entire basin floor and thus exLensive deposits of below wave base sedíment' will occur and. shore environ¡nents will be restricted and irregular'

~~Sunrnary of d.epositional environment,s and controlling processes~~

Cvlt3 represent,s a t,ransit.ional phase of sedimentatíon in which Ehe deposit,ional environment changed from a f loodplain dist,ributory environment to an essent'ially lacustrine environment. As such CN3 consisEs of numerous interbed.s of mat,erials from each of Èhese environment's'

These have been cat,egorised int,o t,hree depositional facíes, fan delta sand, lacust,rine silt and marginal forest/swamp carbonaceous sand. A schemat,ic representation of t'he deposiUional seÈÈíng for the Condowie Silt MeÍiber is presented on Figure 5.10.

~~Cftt and CNZ represenÈ d.eposiÈion during a relat'ive1y stable tectonic perioü. This allowed. for ext,ensive areas of L28 -~~

~~BRAIDED~~

~~CHAIVAIEL~~

~~OELTA~~

~~Lakc~~

~~o a aÇ 0 a çÇo O o~~

~~FAI'l DELTA t~~

~~Figure 5.10 Depositional setting for the condowie silt Member . t29 lacust,rine sediments Èo be d.eposiued over most of the area but wit,h a marginal forest/swamp facies being present in the sout,h.~~

These sedimerits represent cyclic phases of lacustrine delta fill. During this sed.imentalion episode, the northweslern source had, Iittle inf luence on t.he deposit'ional patterns ' The major source of sediment was from a fan delEa to Èhe sout.h but with sediment,s also entering from the western margin.

The gradat,ion between lacustrine and fan delta sediment's is charact,erised in vertical profile by a gradual increase in sand. from Èhe base upwards. This Erend is consistent with prograding sequences which t1pically coarsen upward, Galloway and Hobday (1983).

~~5.3.3 ProPert,ies of weak zones~~

The sitty facies of the condowie silt, Member is a firm to very stiff, clayey silt and silty clay of low plast'icicy and. high liguid limit. A sumnary of the intact propert'ies is presented on Table 3.1 (page 55) with t,he location of t.est. sites and resulEs of strengt'h tresting shown on Figure 5.11.

The weak zones are characgerised by polished parÈÍngs and are strat.igraphically located at Ehe base of coarsenl-ng upward tacustrine cycles. This st'rat,igraphic locaE'ion coincides wit,h a relaUively high clay content and is related t,o deposit.ion in the dist.al part of t,he lake. 130

~~llçqo. a V597 trÊ{ole cc.¡¡¡ ârur¡ER ¡1.4. o€Fn 8€¡.o.,9.FC c¡ 6~~

~~C.O O.Pa. 0o.il. 0O.ll'5'~~

~~6 2alOOilX +-rRlAl- Pll~~

~~av59aFm-l I o''t'l~~

~~LECENO~~

X v572 81,81 - 81.9!¡ . v597 62-05 - 62.2. GElggbrl- o V59t 66.7¡ - 6ó.90. .tê vt9t 75.1 - 7J.48¡ A v:gr 75.57 - tt.óO¡ + vt9E 58.óJ - 5t.81¡ !E un umru E c'¡orh øì [5' !ô É !ilt¡ FJr,o c'. olÈ 6" lO' U

~~Residual Friction Angle~~

iloRMAL STRES¡¡ (rÈl Figure 5.11 Condowie SiIt Member weak zones Location of test sites and summary of results of strength testing (testing performed by Coffey and Partners Pty Ltd, 1988). r-3 1 This tlæe of weak zorLe is of E,he ProgradaEional t)t'Pe discussed in secÈi-on 3.5.2-

From d.rillcores alone it, is not possible to conclude on the lateral continuíty of Ehese weak zones. However, their straE,igraphic posit.ion and relationship with CN2 and CN2 implies thaÈ their distribution coincides with Lhe silty lacustrine facies.

~~5.4 Br¡rbu.uga Sand Fomat,LOn - KoolLata coal !Íeuber~~

The Kooliat,a CoaI Member conf ormably overlies bot'h the Darnleigh Park Sand Member and the Condowie Silt Member' The coal zone has a cumulaËive thickness of about 15 metres over mos! of the area but the Èhickness increases towards

~~the margins (Figure 5.1-2) .~~

This trend reflects a change from silU and coal in the cenEre, to sand at the margins. Figure 5-13a, b, c' d and e show the latreral facies retationshíps and t'hickness for all units within Lhe xooliat'a Coal Member.

~~The coal zone taps onto basement on the sout'hern and eastern margíns, is continuous with Br.unbunga Sand Formation equivalent,s t.o Èhe nort,h and in fault'ed con|acÈ with basemenE to the west.~~

The Kooliata CoaI Member correlates with Ehe coal bearing horizon in t,he Bowmans deposit. alEhough Harris (1987) concluded from palynological dat,ing t'haÈ trhe coals ín Èhe Bowmans and Ctinton areas were slightly older. The KooliaÈa trt q}J. + x É 3 F,l 2 o v P ro N o a)r m r- Ir UI cl lá z XH Þ o!f t- OP. rfl HO prÞP'¡í ( Éo Nñ Þ, U¡ a I ote N o N oÊ, ÊrÞ t-o x ¡ -______0 Ep ø + to I oÊ, +'\ 3 ãrt --\-¿¿J n (,P tto N o r-1 \) O r-l p, H I

~~p, o ( P. u o rf Fl ttP. É rf Ð P. o~~

~~o Fn vrf o + + + N N Nu -6 ô o o o o o 8 o o o 3 3 3z z z .133~~

Coal Member represents a major coal forming episode in the Lochiel area wit,h coal formagíon being periodically ínterrupÈed. by delt.aic sedimentation entering the area from the west, northwest and southeast.

Three correlatable coal bearing zones separated by well defined interbeds of sand, silt and clay have been defined. The coal zones are named. H, G, and F wilh the respecliwe interseam sed.iments of GH and. FG (refer to Figure 5.13f ) ' Vertical and lat,eral lithological variations and geophysical 1og response models are presented in Appendix 2 and. a geophysícal correlat'ion section in Appendix 3'

~~5.4.L H Zone~~

~~The H zone conformably overlies the Condowie SilL Member in the south and the Darnleigh Park Sand Member to the north.~~

It ranges up t.o 13 - 5 metres thick in the south where dominated by a sandy facies but over Ehe rnajorít'y of the area, it mainÈains an averag:e thickness of about' 6 metres (Figure 5.13a). The thickness distribution reflects Ehe t,rends of C'Iv1 and C¡{2.

~~Lithology and laÈeral facies dÍstribution~~

The H zone can be divided into a clean coal faci-es and a carbonaceous sand facies (SA5) - These two facies are laterally gradationaL and the variation in vertical profile is represented in ApPendix 3. N N

~~I I~~

~~(,J~~

~~a I~~

~~I (,P È + + l¡út "iÍ¿ ¡ I a~~

~~I LG|ÊLNl I locxrer$ i I H SEÀ¡{ COÀT / H SEÀM COÀI PÀRTINGS / + + + +~~

SÂLE LEGEND SCALE LEGENO sstolzlKss o hhrrd æbælh So:mnr J--\ ht.rr.d pobmlbw 80¡alml --> dirdbdErhor ffi mld Émd EEa Fdl m,¡ødmnt wca Cdúa k¡ mlraa Cdùa h dlr.a Figure 5.13a Kooliata CoaI Member - thickness of H Zone coal and H Zone coal Parting. N Nj a ï

~~LÆ|JSTRNÊ o gny Ctq~~

~~I Mtu (,F o ul~~

~~I tfra~~

~~MAAGNAL FOREST Cqtu%~~

~~LMELN LocHrEL Nl GH INTERSEÀ}Í FÀCIES DISTRIBUTION ÀND THICKNESS OF GH INTERSEÀM CLEÀN SAND TOTÀ¡ THTCKNESS + + +.¿..@d + +~~

SCALE LEGENO SCÂLE LE6ENO û5ræ o | ¡ tnl.rdæþætbw eo.mnr hl.rtd úúælbr Eo.¡ml :!x¡5 _^. I drGrh dd bcdtb 0l _-->v mgtdtut gca > tuir Eliú¡ g'ca Fdr CøMa 6 frlaa¡ Cdúr h ûlraa Figure 5.13b Kooliata coal Member GH Interseam sediments Total thickness and facies distribution and thickness of clean sand. e N Nj

~~-r T I I I~~

~~I (,P O Oì ol "s +:o t+os I Ð~~

~~\-1._~~

~~o a \) rocrer$l LocxrEL Nl~~

~~G SEÀM COAI G SEÀ.M COÀl- PÀRTINGS~~

~~+ +~~

SCÂLE LEG€NO SCÂLE L€GENO I !¡r5 sllol?5x¡¡ hr.rrd Fbælbr eorønr I hf.rr.d FloælbÉ eormnr fl > ftlú adidôl sa. Fdrit mi¡ ¡.dimnl ffia Fcdl Cdlúa h frtaat cútua h mtr.a Figure 5.13c Kooliata CoaI Member - thickness of G Zone coal and G Zone coal parting.

~~138~~

~~3b ódóJnE I 24!oæmE N I \ I I I l I I I o~~

~~f e25oooomN o~~

~~o I~~

~~rh[[a f czcc ooomlt~~

~~ù o\~~

~~I LocHrEL 1 ' I I /~~

~~+ l- f c24oømN~~

~~SCALE~~

~~KMS I o I 2~~

~~Figure 5.13e Kooliata CoaI Member - thickness of F Zone coal (no partings occur in the F seam). s N WÀRRINDI SILT~~

~~(,P to~~

~~(otDotdtt stLT HEHBER~~

~~0 z 3 I~~

~~SCALE lN km~~

~~Figure 5.13f Detailed StratigraphY and lateral relationships for f legend. the Kooliata CoaI Member. Ref er E.o Appendíx 2 or ]-40 a) Coal facies~~

The coal facies is composed of. carbonaceous material wit'h less than 3ot ash on a dry basis. The coal atEains a of t'he mancirmrm Èhickness of 5.5 metres at' Ehe norÈhern end lake. In Ehis area the coal is in sharp cont,act wit'h bot'h t,he condowie silt Member and the Darnleigh Park sand Member. Elsewhere, Ehe coal grades down into a carbonaceous, silty sand (SA5) of Lhe H zone' The upper contactoftheHsealnisgenerallysharpandthetop0.5m of coal is of Ëen extensively burrowed with t'he burrows infitled with fine whit,e sand (Plate 5.8) .

The coal in the H seam consíst,s of large woody fragments ín a fine grained matrix and ofEen vertical root sEructures are obsenred in the core. only minor partings of carbonaceous, sandy silt are presenÈ wit,hin the seam. The part,ings generally thicken Eowards Èhe north and northwest and grade int,o fining upward cycles of delÈ.a fan channel sand. Partíngs of highly carbonaceous, fine grained sand.y silt with thin discontinuous beds of silty ctay also occur in t,he cent.ral areas. These appear Eo be isolat,ed from Èhe marginal sediments '

~~b) Sand facies~~

The coal seam grades taterally into a sandy coal facies (SA5), which is equivalent in litrho1ogy to the sand facies of the Condowie Silt Member and also the carbonaceous sand facies of the Nantawarra Sand Member. It' consísEs of large L4L plant fragments, includ'ing roots, in a fine t'o medium grained, poorly sort'ed sand mat'rix' on the basis of trends in t,he wert,ical profile, the H zolae may be divid.ed into t,hree laterally gradatíonal areas, see eppendix 3. These t,rends correspond to changes in the relative proportions of sand and carbonaceous matEer and have been id.entified from ínterpretaEion of Ehe apparent density logs.

more 1 At the southern end Ehe vertical profile becomes carbonaceous downward and thin, dírty coal seams may be present. In t,he central and south-cent'raI area, the vertical sequence shows Iittle or no variat'ion in vertical profile and, no coal occurs in this area.

~~more 2 Fart,her norÈh, t,he vertical prof ile becomes carbonaceous upward and. grades into Ehe H seam coal.~~

~~the 3 In a relat,ively small area at, t,he northern end of Iake, the sequence consists almost entirely of H seam coaI.~~

These variat.ions are considered. to be the result' of the int,eraction of sediment, sources, primarily from t'he south and west. The increase in carbonaceous cont'ent upward reflects a waning of t,he source whilst the decrease upwards indicaEes an increase in sedimen! input. Given ÈhaÈ Èhese Uwo source areas are subjec¡, Uo Ehe same climatic cont'rols it, can be concluded t,hat Èhe varíaÈions relate t'o variat.ions in tect,onic movement, about the source areas. L42

~~5 .4 .2 GH int,erseam sedimenEs~~

~~The GH interburden conformably overlies the H zone with a sharp conformable cont,acE., the Ehickness distribution is presented on Figure 5.13b-~~

The unit reaches a ma>cimum thickness of 6.3 metres to the northwest but awerages about 2.5 metres elsewhere. The GH interseam sediments wedge ouÈ t'o the south, and are laL,erally eqr-rivalent to Bumbunga Sand Formation equivalents to the north.

~~Lithology and lat,eraI facies distribution~~

The lateral facies distribution and thickness of clean sand is presented on Fígure 5.13b. Ower most of Èhe area the GH interseam sediments consist, of a silty facies but a linear, easE,-west trending sand body is presentr in the centre of the area. South of t,his sand body the GH grades lat'erally ínto a SA5 facies which is indístinguÍshable from Èhe H zone. a) silty facies

The vertical sequence of E.he silty facies ís consistenL over Èhe area. It, may be divided in!,o Ewo parts, a basal section of sandy silt and an upper section consisting of a Ëbarsening upward cYcle. L43 The basat section consists of about 0.8 meLres of Iight grey t.o brown, very fine grained. silty sand or sandy silt. rt is int,ensely bioturbated with many of the burrows ext,ending int,o the top of the H seam ( Plates 5.8 and 5.9a,b).

Often roots are present within È.he zorLe and aÈ V653 a large sÈrrrnp was intersect,ed.. The presence of these stumps and the bioturbation suggest, a drowning of the peatswamp by relatively shallow water.

~~Overlying t,he basal section with a sharp contact are 2.5 metres of fine grained sediments forming a coarsening upward cyc1e.~~

This cycle begins with a dark brown, slightly carbonaceous síIty clay and grades upwards to a laminated, fine to very fine grained, mícaceous, si]ty, fawn sand and carbonaceous brownsilt'(Plate5.10).oftenat'hinlayeroffine grained, whiE.e sand is present at the top of Èhe cycle - This sand layer is of t.en penet'rated by smalI rhizomes originat,ing from the G seam coal above but' no large rooEs are present.

~~Of particular interest, is the presence of a polished shear zone at or near t,he contactr between the two lithologies ' This shear zone is discussed in section 5.4.7 '~~

~~The characteristics of these sed.iment.s are consist'ent with the those of lacustrine environmenÈs as discussed previously. L44 b) Sandy facies~~

In the central part of the area the GH int'erseam consists of a basal section of lighÈ brown to fawn, fine Èo medium grained, moderately sorted, subangular sand with rare well rounded coarse grains. This sand grades upward into a laminaÈed sect,ion consist,ing of light brown fine sand and med.ium brown, slightly carbonaceous silt. The con't'act with the overlying G seam is usually sharp. This sand body is elongate in an east,-northeastr direct'ion and appears t'o originate from t.he western margin-

~~The lateral gradation between the clean sand facies and the silEy facies is characterised by a progressively higher proportion of silt and clay Eowards the north'~~

~~5.4.3 G Zone~~

The G seam is present. over most of t,he area and has an average thickness of abouÈ 6.5 meÈres (Figure 5.13c). The seam t,hins Eowards the margins and is split by a parting at the southern end,. In t,hís area the G seam is divided into the G (Iower seam) and G1 (upper seam). As wiUh the other seams iE, grades into carbonaceous silty sand (SA5) at the margins. L45 Lithology and lateral facies variation a) G Seam Coal

A number of coal lithotlpes are present in the G Seam with earlhy, f ibrous anfl wood.y t)T)es coÍlmon. The vertical profile generally cont.aíns wood.y coals at the base with the f ibrous and earthy t)æes occurring higher in the sequen'ce.

ChrisEophel (1983) concluded from exarnination of a limited number of samples of plant macrofossils from the southern part of the area that the coal represenEs a sr^Iampy environment with few emergent trees-

~~From a microscopic examination of Èhe coa], Springbett (pers. con'ûn. 1989) f ound a high proportion of deUrital plant matter and suggested È,hat^ the seam is allocht'honous.~~

This is supported by t,he absence of a well developed seat earÈh and large tree roots at the base of the coal. Springbet.t also mapped the distribution of coal lit'hoE]æes. In general he found a high degree of 1aÈeral consistency and, was able to correlaÈe specj-fic lithot)æes for several kilomet,res.

At, boÈh the southern and northern part,s of the deposit' the c seam is split by part,ings (Figure S. r+c) . These partings thicken toward,s the margins of the area forming a wedge shape. L46 The part,ings to the south consist.s of carbonaceous sandY silt which gradationallY becomes more sandy towards the margins. b) G1 seam coal

~~The Gl seam is present at the southern end of the area' It has an ovoid shape with the coal gradually thickening to a ma:cimum of 3 . 0 metres at the centre -~~

The coal is generally f ibrous and woody wit'h small fragments of charcoal common. Towards E,he margins of t'he seam, Ehe coal appears t,o be fractured giving it' a friable texture. The coal is also characterised by inclusíons of fíne sand. and. silt as pods or discontinuous beds near the contacts, especially t.owards Ehe margins-

The upper contact is sharp and as wit,h the F seaIn, a fusain-rich zorLe is present at Ehe Eop of t'he G1 Seam' The lower contact is also sharp and is sometrimes inclíned at up to 2Oo. A fusain-rich band is also ofÈen present directly below the G1 seam.

~~Structure~~

A feature of E,he G Seam aE, the V653 bulk sample sit'e is Ehe presence of near vertical extensíon joints (Plat'es 5'11 and 5.L2). These joints are planar wíth very irregular sides' Many extend t,hrough the entire thickness of the G sean. - L47 The joints are infilled with either a very fine coal mass or a fine Èo medium grained light brown sand and appear to be parallel. However, is was not possible !o orient t'he core samples So the orient,a!.ion of these j oint,s could not be determined.

~~No such extensional j oints were present at the t,ria1 pitr siE,e.~~

~~5.4.4 FG Interseam sediments~~

~~The sedÍments between G zone and F zone have an average thickness of abou! 1.2 metres butr thicken radially t'owards Ehe margins (Figure 5.13d). These sediments were intersected in the Tría1 PiE,.~~

~~LiÈhology and lat,eraI facies variat,ion~~

The distribution of facies wiÈhin the FG interseam is presented on Figure 5.L4. Four facies are presenE, clean sand; anastomosing sand and coal; and tr¡üo silty carbonaceous sands, one north and t.he other south of the clean sand facies.

~~a) C1ean sand facies~~

The sand body is elongate in an east,-wesÈ direcfion, Èhinning to the easEern, norlhern and southern margins and continuous Eo the Ardrossan Fault to t,he west. - L48 vJhere Lhe sequence is t,hickest, it comprises L.hree parts.

1 The basal part consists of 0.35 metres of dark brown, moderately carbonaceous, medium grained sand equivalent to t,he SA3 E)æe. This sand has a sharp, planar contact with the underlying G seam coal and is gradational upwards. Bedding in the basal part is indist,inct, or absenÈ.

2 The basal zorae is overlain by 2.5 metres of clean, white medium to coarse, moderat.ely well sort,ed, rounded to subangular quartz sand which becomes finer grained upwards. The sand is bedded. at 15o in the lower secÈ.íon and 3Oo towards the top. Bedding is characterised by varíaEions in grain size between beds and rounded pyrite nodules up tro 20 rffn in diameter are contnon.

3 The clean sand facies grades into a transitional zone consisting of int,erbeds of silty, sandy coal and medium grained, ripple crossbedded sand with beds dipping aÈ 3oo. rn the trj-al pit, which is locaÈed on the margin of the sand body, the interseam sediment's consist of 0.6 metres of SA2 t)æe sand.

The association with the Ardrossan Fau1t, t.he geometry of the sand body and the lat,eral facies relat,ionships suggest' that the FG inËerseam represenÈs a delÈa fan depositional environment,. Furt,hermore, the cont,rols discussed for t,he cH interseam sediment.s appear also to apply t.o FG. - 1,49 b) Anastomosing sand and coal facies

~~The clean sand facies grades into a broad zone of anast.omosing beds of clean medÍum graíned sand and sandy coaL (refer Èo Plate 5.13).~~

This zone averages 1.0 metre thick and thins and becomes Iess sandy E,owards Ehe norEh. The anastomosing sand and coal facies grade verEíca1ty into the overlying F seam coal and is in sharp contact with the underlying G seam coal.

~~Strucfure~~

At. the v653 bulk sampte siE,e the anastomosing Êacies $tas int,ersected by near vertical extensional joints (Plate 5.14) , These joints were infilled wit,h a very fine grained coal and are continuous with similar joint,s in the G seam.

~~It is apparent t,hat at least two tensional episodes caused the f ormat,ion of these j oinÈs . The f irst, being characterised by coal infíIled joinÈs and the second by~~

~~sand infilled joints (PIaÈe 5.L2) .~~

~~c) Silty facies - nort,h~~

FarÈher t,o Èhe north, the anastomosing facíes becomes increasingly carbonaceous and silty and at the northern end of the area it consists of light, brown, indistinctly laminat.ed clayey silt with some charcoal f ragrnent.s. In a smalI area to the northeast the FG inEerburden Èhickens and consists ,gf laminated sitt and fine grained sand. - 150 d) Silty facies south

To the south, the clean sand facies grades laterally into a silty carbonaceous sand. f acies. The FG inE,erseam sediment's in this area were also influenced by sources to the east and south. The sedimenÈs consist predominantly of fine graíned, well sorted, si1ty, carbonaceous, ç[uartz sand of the SA3 E]æe.

Laterally Èhe lithology grades through the 5A6 t]æe towards the Clean sand facies and into a SA3 to the souE.h and east. The sedimenEs generally contain some fusain fragments at the top of the seçluence.

~~5 .4.5 F Zone~~

~~The F zone averages about 5 met,res in thickness over mosE of the area (figure 5.13e). As with the H and G seams Ehe F Seam thins radially towards the margins where it grades inE,o a SA5 facies.~~

~~I-,ithology and distribution of sediment tlæes a) F seam~~

The F seam is present, in Ehe central and northern part of the area. The coal waries considerably in thickness wiUh a 1.5 met.re vertical change over a 4 . 0 metre horizontal distance being obserwed at Lhe V634 large dÍamet,er site. The seam thíckness averages 2.0 metres but ranges beEween

~~2 .3 met.res and 8. 0 metres. r_51~~

The coal is fine grained and earthy or fibrous wit'h a layer of hard, black and. brittle charred coal, (fusain), often present. at the Eop of the seam. Fusain is formed from a nr¡mber of processes but, forest and peat fires are the most coÍtrnon mechanisms, ICCP ( 19?1) . It appears t'haE this mechanism caused t,he considerable variation in the thickness of coal in seam F.

~~The upper contacE with the overlying Ï{arrindi Silt is usually sharp and. in addition t,o the fusain zorre it' is also ofEen }ined. with a thin band of fine grained clean sand.~~

~~5.4.6 oePosiÈional environments~~

~~The Kooliata CoaI Member is made up of Ewo basic facies associatíons: (1) Coal zone facies assocíation (2) Interseam sediment facies associat'ion~~

~~The depositional Set,tings for each of these associations are illusEra!.ed on Figure 5.14. In addition a geophysical and Iit,hologica1 mod.el for Èhe Kooliata Coal Member is presented in APPendix 2.~~

The continuity of t,he coal zoîe has enabled the definition of t,he relatively thin inÈ,erbeds which on the current' drillíng spacing would ot,herwise have been impossible. The definition of Ehese int,erbeds has also enabled a reliable appraisal of the nat,ure of lateral facíes variatíons and L52 trends for fan deltas in the l,ochíeI area and possibly also for the St, Vincent, Basin.

~~Coal Facies~~

The controlling facÈor as to wheÈher a backswamp area will become a peatswamp or an open lake depends on the positíon of the groundwat.er tal¡Ie (Diessel , A98O) . PIanE growth and pea! accumulation can only occur in a limiCed range of lvater dept.hs. A lack of water cover will enable aerobic decay processes t,o quickly d.isintegrat,e any plant, debris, while deep water witl not allow plant' growth and only transported carbonaceous material will accumulate.

~~Once the peatswamp is established, t'he passage of sediments inÈo the area is rest,ricted. by E.he sediment t,rap of vegetaE,ion.~~

All coal seams in the area have a characterist'ic facies assocÍaEion consist,ing of a coal and a carbonaceous sand facíes. This association is indicative of a sediment t'rap mechanism as illust.rated on Figure 5.15.

~~b) Int,erseam sediment,s~~

BoÈh the GH and FG int,erseam sedimenÈ,s contain elongiaÈe, east-west trending bod,ies of clean sand. These lithologíes are considered to represenÈ a fan delta entering t,he area from an uplif t,ed, area E.o the west. Of parÈicular note is Èhe coíncidence of the trend of t'he facies with an inferred basement structure. IE. is probable t'haE movement' along ERAIDED STREAM -CHANNEL

~~.lf oa +~~

~~QA a çQ q, a aAQo 0 a I Ç P(¡ (, Q ,n, t I DELTA~~

~~ic representation of coal forming Figure 5.14 Kooliata CoaI Member - Schemat a"po= itional environment' 154 Figure 5.15 Mechanism controlling peat accumulation~~

~~cGtonoceor/g COAL SEAM FACIES oorbonooeous sdld foci€s ã sond focics~~

~~= INPUT~~

~~FAOES assocraTloìls~~

~~BASIN SIJBSIDENCE~~

~~closl¡c sediment MINERALOGY distf¡but¡on~~

~~DEPOS]TIONAL fiwiol/dsil!¡c--- v€gt€totion trop Peot occumulolim vegelotkìn lroP ---flwiol/delto¡c ---- PROCESSES~~

~~this fault (north side down) led to drowning of the peatswamp (H seam and. G seam) and its replacement with a shallow lake.~~

The }ateral and vertical facies distribution are consistent with the infilling of this lake. Two stages of infilling are recognised for the GH Interseam sed'íments. The first stage was associated. with the fan detta to the west and the second stage with sources to the north and south. The progradation of the latter produced the characteristic upward coarsening sequence. - 155

~~5.4.7 synÈhesis of geological cont,rols and engineering properties~~

~~Fact.ors affect,ing porewater movement~~

Èhe G sseam coal was 5 /day/m with a Transmissivity of ^3 of 7 .L /d^y/^2 , relatively high hyd.raulic conductíwíty ^3 refer to Figure 3.13 (page 77.) . Based on the relatively t,he uniform grain síze recorded from core logging it is expect,ed Èhat, similar properties are likely where Èhe clean sand facies is present. However, as only one pump t,est was compleÈed it is not possible to draw direct conclusions as to the laEeral variat,ions in hydraulic propertíes.

~~eroperties of weak zones~~

A weak zone characterised by a polished shear plane was id.entif ied within the GH interseam sediment's - The lateral and. straE,igraphic location t,ogether wit'h a srunnary of the resid.ual shear strength is presented on Figure 5.16.

As with Ehe Condowie Sí]t Member weak zones, the GH weak zorre occurs aÈ the boundary of È!ùo dist.inct sedimentary cyctes. In particular the weak zone occurs at t'he base of a coarsening upward cycle which was formed by lacustrine infill processes.

The higher shear strength results for V680 and V683 related to shear along intact material and t'he presence of surface irregularit,ies respect,ively. DiscounÈing these tluo t'ésutt,3, t,he remaining shear plane result's show : - 156 uniformly zeÊo cohesion, frict,ion angle increasing from wesE to east

~~5.5 Bumbunga Saad Fomation - slmtheELs of geologLcal procesgeE coatrolll,ng the propertLeE of weak zones aD'd porewater uoveneat~~

In t,he Lochiel area, the verÈical seçFlence consisEs of one f ining upward allocycIe (Figure 5.1?) . From t,he base, t'his cycle is comprised of braided sÈrearn-fan delEa-Iacustrine- peatsvramp depositional environments.

~~Sediments for the Bumbunga Sand Formation were supplied f rom maj or sources of sediment which originaÈ'ed from Ehe nort,hwest,, west and south.~~

For t,he Nantawarra Sand Member, the deposit'ional æcis runs paralle1 to Ehe Ardrossan Fault, with the depocentre located against Èhis fault in the northwest, of the area. This St,rong1y suggest,s that the Ardrossan Fault had a major control on sedímentation and, in t,he sense of BeerbOwer (L964) and McLean and ilerzykiewicz (1978) , cont,rolled the allocyclicity of the NanÈawarra Sand Member-

The Nantawarra Sand Member comprises t,hree laterally equivalent, and gradational facies. The gradat'ion between each of Ehese is across a northeast-southwest trending æcis and, t,hese Èwo axes coincíde with two inferred faulE lines. Furthermore, t,hese inferred faul¡ lines also coincíde with major facÍes changes for boEh E,he Condowie SilE Member and t,he KooliaÈa Coat Member and mark ttre southern limit of Lhe r57

~~llrgq a) V5'97 æREHot€ cor!ån a NUMB€n~~

~~51.6ñ OEPNI SE¡¡W C.SH'E C¡80 @HEtot tt rP€ ú'22o Á¡rct€ oF FÊtcîoil ô C?llOæOnN gCAR ZOfl€ oAfA Wm 80t c¡-aY sgal 0Â1À wtrt{orJÏ u~~

~~v75¿t 5Õ3il cro ø.þ~~

~~624ffiñx s!)-rRAL Prl~~

~~LECElflt~~

X V680 - 43.55¡ I . V664 - 4ó.80r A vea¡ - 50.52¡ I v684 - 43.q'¡ * Vl2l - 5ó.05 - 56.25¡ @þmñ{ + v154 - 56.18 - 56.i8¡ o V659c- 50.95 - 5l.t5r Æ ruxo c''lzrPo ø''zo'ao A v6sl"- 41.gs - 41.22. :t g H b * + G tffiffi u c'.otPo 6''9n ¡l ø T

~~)+ Residual Friction Angìe~~

~~NORMAL STRESS (TPO¡~~

~~Figure 5.16 KooliaÈa Coal Member - Location of test, sit,es and sumrnary of results of strengt,h testing (testing performed by Coffey and Partners Pty Ltd, Rept,., 1988) . 158~~

~~Darnleigh Park Sand Member. On this basis iÈ' is considered t.hat t,hese structures provided Lhe maj or autocyclic controls for the Br:mbunga Sand Formation-~~

~~Effects of depositional controls on Èhe propert'ies of weak zones and porewater movement-~~

For Èhe Bumbunga Sand Format,ion it, is concluded t'hat t,ectonics was the major geological control. Movement along the Ardrossan Fault conÈrolled t,he E,iming, rat'e and Iocation of sedimenE. input, whilst t,he east-west t'rending basemenE faults appear t,o have controlled Ehe distribution of these sedimenEs.

The depositional models presenged on Figures 5.3, 5.10' 5.14 provide Èhe basis for predictíng the disÈribut'ion of aquifer zones. In general they coincide with Ehe braided stream and disÈribut.ory part of the fan delEa environmenEs.

l¡Ieak zones occur at Ehe boundaries bet'ween dístincE depositional episodes. Lithologically they occur at t'he base of coarsening uputard cycles and these cycles have been shown t,o represent, lake infill processes and a lacustrine d,epositional environment. They are considered Eo represenÈ prograd.ational weak zones as discussed in secLion 3.5.2

~~(page 70) . 159~~

~~ALLOCYCLE AUTOCYCLES~~

~~G SEAM -50 KOOLIATA COAL~~

~~H SEAM~~

~~-60~~

~~DARNLEIGH PARK SAND~~

~~-70 cNl CNzCONDOWIE SILT~~

~~cN3 -80~~

~~-90~~

~~= .= -roo 'ãc o .Þ I î,ø o 3 -ilo CLa olr .g üC,~~

~~REGIONAL LOCAL -r20 TECTONISM A TECTONISM EUSTACY~~

~~Figure 5.L'7 Br:mbunga Sand Formation oefinit,ion of allocycles and autocycles within Èhe stratigraphic Profile. - 150~~

~~CEAPTER 6. O .â¡TÀIJYSIS OF O\TERBI'RDEÀI SEDI}ÍEIÍTS~~

The purpose of this Section is ¡o analyse the overburden sediments, however, âs discussed. in chap¡er 2 the analysis is confined. to t,hose format,ions which are associaÈed with coal deposition. The aim is to : . describe the lithology and lat,eral variation of the sediment,s, . ident.ify the major factors controlling sedímentation and develop models which assist in interpret'ing Ehese controls, . define the retaEionship between Ehe key geological feaEures(identifiedearlierasdiscontínuit,iesand porewater movemenE) affecting slope stability and characterise these features ín terms of their

~~engineering ProPert ies .~~

~~The st,ratigraphy is sununarised on Figrure 3.3 (page 53) . The' overburden sequence comprises two formations, t'he Warrind.i SiIE Format,ion at Èhe base overlain by the Tarella~~

~~SiIt FormaE,ion (Fígure 6.1) -~~

~~6. 1 StratLgraPbLc nøenclature~~

Pallmological analysis by Harris í9e2, L983, 19e6) gives the warrindi and, Tarella SiIt, Formations an Oligocene-Miocene age. Formations which f all inEo t'his E,ime range are the Rogue Formation (south, SÈuart', L970) and. Èhe Snowtown Sand (north, A11ey, 1973) ' Howewer' neit,her of Ehese two provid.es a good lit,hological correlation. SOUTH NORTH

~~EROSION~~

~~Upper TLI d ello Ploin TL2~~

TARELLA SILT TL3 Lower TL4 dello Es luo ri ne Boy fill ploin wAlo Dello fronl/ WAg Morsh Delto Fon dello 5 bor WARRINDI SILT WA3 wA4 wA8 Logoon Boy / logoonol dello wA7 Tido I Logoon - wA2 suboqueous WA6 HIATUS Fon Dello P KOOLIATA COAL SURFACE Ot F Peot swomp (F.G,H seoms) Locustrine dello (FG,GH interseom)

~~BUMBUNGA~~

~~SAND CONDOWIE SILT Broided NS3 Locu s lrine Slreom Fon Dello NS2 De lo /crevosse sp oys NSI NS3 NANTAWARRA SAND Dislol dello~~

~~Bosemenl~~

~~SCALE lOm~~

lhm for warrindi Figure 6.1 Detailed stratigraphy, Iateral continuity and lithology SiIt and larellá SiIt Formations. - L62 The Rogue Formation contains a significant proportion of calcareous (biogenic) and glauconitic sediment's; none of these are presenÈ in either the TareIIa or Ìrlarrindi sitt Formations.

The Snowtown Sands appear to represent sandy and carbonaceous, fluwial delta plain sediments (AIIey, 19'73) , however, more recent analysis by Alley (fggO) suggested Ehat, there is also an estuarÍne influence. The lrlarrindi and. Tarella SilE Formations represent predomínantly fan delta, delta front and estuaríne enwironments, but, while the estuarine influence tends to favour an association with the Snowt.own Sands, lithologically E,hey are quiEe distinct.

~~On the basis of the discussion abowe iE is, therefore, appropriate t,o assign both the Vlarrindi and Tarella Silts formation status.~~

Before conrnencing the descriptíon of sediments it should be not,ed t,hat sometime af ter the deposition of sediments of the l{arrindi and Tarella SilÈ Formations, and prior to Èhe deposition of the overlying Hindmarsh C1ay, the area under.went, a period. of uplifÈ, ÈiItíng and erosion. This erosion is E,hought, Eo partially account for Ehe t,hinning of t,he Warrindi Silt Formation on the southern and eastern margins and also Lhe thinning of Ehe upper part of Ehe Tarella si1t, Format,ion. (rt should be noted that in attempting to clarify Ehe strat.ígraphic nomenclaÈure it was apparenE that E,he awailabte palynological evidence was insufficient to provide a relial¡Ie basis on which to base

~~regional correlations. ) 163 6.2 learrLndi Sf lt Formatíon~~

The Warrindi SilÈ Formation overlíes the Kooliata CoaI Member. IE is the most complex formation in the area in Eerms of lithotogy and. facies relationships and is been assigned an Oligocene to Miocene age by Harris, (1-982, 1983

~~and 1986) .~~

The naEure of Èhe contact between the fooliata Coal Member and. the T¡Iarrind.i Silt, Format,ion appears Uo be variable. The top of Ehe Kooliata is characterised by the presence of a thin zoÍae of fusain-rich sediments. However, Ehe F seam has this fusain zorle only aE the very north of its occurrence, the remainder of the seam having a sharp, apparently conformable contacu wíth the overlying Warrindi si1r.

The presence of this oxidised zone suggests thaE after the deposition of t.he Kootiata Coal Member there was a hiatus over mosE of the area, but to the south sedimentation appears to have been continuous.

~~Figure 2.9 (page ¡r) ind.icates a eusE.at'ic fa1l in sea level aE, the boundary between the Early and LaEe Oligocene. The hiaÈ.us described above may correlaEe with this sea level change.~~

~~StraEigraphic correlation by Kremor and Springbet't (1989) also suggested a correlat.ion wiÈh t'he Rogiue Formation occurring to the Sout,h in the Bo!'rmans area (refer to Figure~~

~~2.3 page 13) . L64~~

It, is probable that, Èhe lrlarrindi Silt FormaÈion represents a period of sedimentation coinciding wiUhin t.he maj or eust,aÈic transgression discussed in sect,ion 2.6 (page 39). Moreover, Ehe l{arrindi Silt Formation woul-d appear to represent or be 1aEera1ly equivalent to a E,ransgressive systems tract, in Èhe sense of Sangree and Sneider (1987)

~~(refer Èo rigure 2.5 and section 2.3.1 page 2a) -~~

Elsewhere in the SE Vincent Basin the equivalenÈ of the Br:mbunga Sand, FormaÈion, t,he Clinton Formation is overlain by E.he glauconite-rich Pt ilulia Greensand, which Harris (pers. conun. 1989) interpreÈed to be a condensed secEion in the sense of Sangree and Sneider (1987).

~~In general, the !{arrindi Sí1t, is abou! 4 metres thick in the cenÈre of Ehe area bu! thickens radially as t'he fíne grained facies grade into more sandy facies Eowards t,he margins (Figure 6.2) .~~

The I,{arrindi Silt FormaEion atÈains a rnÐcimr¡m thickness of over 20 metres in the southern part, buE, this area has been subject, t,o erosion and t,he original thickness was greaEer. As wít,h the underlying f ormat.ions, the !{arrindi Silt Format.ion laps onto basement in the southern and eastern margins, is lat,erally cont,inuous Eo t,he north and trrrncated by the Ardrossan FaulE to t,he wesÈ. 165

~~24o oæm€ 246mmE / N H I 5 I H~~

~~I I I L I I / / _.1f- ! f s2SoooomN~~

~~I \X-r--^ / + I L~~

~~ùò os qs + { !z+o ooomlr o4tanuen~~

~~q, t o t E J H~~

~~oz oU' I E, LOCHIEL H i I~~

~~+ :'f sz+oo*.N~~

~~SCALE KMS I o 2 3KMS~~

~~Figure 6.2 Thickness of the warrindi silt Formation. 166 6.2.L Lithological description, facies relationships and depositional processes~~

~~The l{arrind.i SiIt Formation has been divided int'o f ive gradational depositional phases based on a detailed correlation of all drillholes in the area (refer to Figure~~

~~6.3a,b and c) .~~

Phases 2 and. 4 represenË active periods of sedímentation associaÈed with local tectonic uplift, of surrounding source areas whitst phases 1,3 and 5 represent relatiwely quiet depositional epísodes beE,ween phases 2 and 4. Each depositional phase comprises a number of laterally equivalent facies. Figure 6.3a is a cross-section illustrating the vertical and lateral relationships bethleen t,he depositional phases while Figure 6.4 sunrnarises these relationships

The lithofacies for each phase are defined on the basis of geophysical log response (refer to Figure 6.3b). The fine grained f acies associated wit,h phases 2 and 4 are characterised by a relatively high natural gaÍma log response reflecting the higher proportion of clay within E,he sediment,s.

~~The fine grained facies are ltIA6 for phase L, VÍAT for phase 3 and I{45 and VIA9 for Phase 5.~~

~~All facies grade laterally either directly or Èhrough other facies int,o t.he !'IA3 facies in t,he cenEre of the area. v672 v72s s N TARELLA SILT v576 v736 v743 v577 v703 v696 v662 v66t wA5 wato~~

~~wA8 Phose 4~~

~~wa3 Phose 3~~

~~WA7 Phose 2 wA3 WA7 Phose I WAI fu-a-*-ln -- FG ts wA2 {Oì~~

~~o I Kms SCALE~~

Figure 6.3a Summary of depositional phases and vertical and lateral lithological relationships for the llarrindi SiIt. North-South section (refer to Appendix 6 for detailed location of this section). 661 577 Scole (m) o- 736

~~696 WAIO --? wAe wA7 o- wA7 WA7-3 wA2~~

~~G FG - FG ?o- G G P ct\ @~~

~~o 40 ao t20 i i ' i NqlurqlGommoAPI~~

~~30- f t5 7 9~~

~~H¡gh Rosolul¡on Dmsily G)¿/CC o40æ120 o{mtæ Nolurol Gommq API r---r----ll-_i Nolu¡ol Gommo API o 40 80 t20 l'...... U Noturol Gommo APf f ¡ 5 t7 I~~

~~H¡gh Rcsolul¡on Ocnsily High Resolul¡on D.nsily f t5 7 t9 40- GM/CC GM/CC H¡gh R€solul¡on oens¡lt GM/CC~~

~~Figure 6.3b Geophysical Iog response for the lrlarrindi Sitt. Refer to Figure 6.3a for lithological interpretation. Tectontc Cooslol uplifl of Onlop s N~~

~~Rsng Fo ng~~

~~ts Extenl of Eocene / Oligocene sediments oì TLI \o 4 wAlo (+WA2) TL2 3 TL3/4 wA8 wA7 2 WA2 wAr 3 Phose I Sondy focies KOOLIATA COAL~~

~~Bosed on Bosed on position FAN DELTA SHORELINE SUBAQUEOUS Sediment of shoreline input~~

Figure 6.4 Synthesis of facies distribution, coastal onlap and tectonic mòvement associated with the southern source. - r_70 Three major source directions are recognised for the I¡Iarrindi Silt Formation, from the north and northr^resÈ' several points from the west and t,he third from t'he south and southeast (see Figure 2.8, page 28). As with the Bumbunga Sand Formation, the relative influence of each of these varied with tíme as did the specific entry point of the supply channels:

The sources to the northwest and west were associated with small streams originating from areas uplifted by Ehe Ardrossan Fault. These sources, although intermittenE, provided. a supply of sediment throughout all depositional phases and are now represented by the UIA4 facies (delta fans). The WA4 facies is hence laterally equivalent to and gradational with facies from each of the four depositional phases.

~~LiEhology and lateral variat,ion~~

~~Phase 1 sediment.ation - !iIA6 lagoonal facies~~

~~wA6~~

WA6 occurs j-n a rest,ricEed area ín t,he central part of the d,eposit.. IE overlies the F zolae and F seam with a sharp contact which is conrnonly lined with fine grained light brown silty sand. Vü46 averages 1.5 metres in thickness and its lateral distributlon closely follows E.hat of the F seam

(refer to Figure 5.13e, page 138) . - t'll V[46 consists of moderately to highly carbonaceous (sometimes reaching coal quali¡y), dark brown sílt which has undergone extensive brecciation. Breccia blocks are generally less than 1OO ntrn in width and fractures are comfnonly infÍlled with fine silty sand. As depicted on Fígure 6.4, iE. is considered t,hat phase l coincided with a eust,at,íc rise in sea level which resulEed in the formation of a lagoon. This rise in sea level Ìllould have also t,erminated peat formation by drowning the peatswamp (refer to the d.iscussion in sect,ion 2.6 page 39). The brecciation represents a period of exposure and desiccaÈion at the end of the phase t associated with a faIl in Ehe sea leve1. This evenE marked t,he comrnencement of phase 2.

~~Phase 2 sedímentation - V'IA1 and IìIA2 delÈa front and fan delÈa facies.~~

~~T¡IA1 delEa fronÈ clean sand f acies.~~

VlAl occurs as an elongaÈe east-northeasÈ t.rending body of clean sand reaching a maxinnrm thickness of 3.5 m (Figure 6.5). In the Tria1 PiE, Ehe WA1 member averages 3.1 metres in thickness an{ the main lithology is well sorÈed, rounded,, medium grained,, light, brown to whit'e, biot'urbaÈed, quarE,z sand, (Plate 6.1). A deEailed vertical profile is presented on Fígure 6.6. The basal part, of WA1 is comprised. of a 0.3 met,re bed of f ine t'o medium grained clean sand (PlaEe 6.2) . Bedding in this sand is horizontal although some ripple crossbeds are presenE aÈ Èhe base. This bed. overlies the F zone with a sharp irregular cont,acE t72

~~245 ooomE N ( I ? I I I I I I I uuô I I +€25oooomN I \ I~~

~~a WA4 / OtuJ I~~

~~+ f ezcc ooorN Ò~~

~~l I I LOCH EL / / / + + f sz+ooæmN~~

~~SCALE~~

~~KMS I o ? SKMS~~

Figure 6.5 Warrindi Silt Formation - Thickness of wAl and WÀ4 Members. - L73 marked. by a change in carbonaceous content, (P1ate 6.3). Bed.s equivalent in lithology, distríbuEion and t'hickness also occur in E,he F zone and FG int,erburden in the Trial Pit. IÈ is consid.ered, that all t,hree are representaÈive of the same depositional processes. A feature of these sedÍmenÈs in each of the FG, F zone and T,IIA1 ís thaE they each conEain E,hin, latera11y conÈinuous carbonaceous Sand beds wiE,hin the clean sand (Plate 6-4) - These carbonaceous beds are charact,erised by irregular, slumped lower conÈacts and sharp upper cont,acËs. They are thought Eo represenE quiet, periods where sed.ímengs are deposited from suspension in a stand.ing bod.y of waÈer. In the F zoLTe and FG interseam, E,his facies is characterised by occasional logs along the base aligned in a northeasÈ-southwesÈ orientat'ion and by smalt channel infill sEructures. No logs are present' in the bed in VIA]- in the Trial Pit.

The association of these clean sand beds wiCh a major delt'a fan (VlA2) (originatíng from Ehe sout.heast as indicaEed by Iateral lit,hological correlaEion and by the orientation of the logs in Ehe F zone) and t,he presence of smaI1 channels indicat,e E,hat Ehese sediments represent deposit'ion E1çical of sheet floods as described by Galloway and Hobday (1983) and Banerjee (1989). Furthermore, the presence and character of the carbonaceous beds indicates that Èhe sheeÈ floods entered a stand.ing body of water (probably a lake for the F zone and FG interburden, and as díscussed lat'er, a lagoon for WA1). L74

~~GRAIN SIZE~~

~~E^ È -!ond- DEPOSITIONÀL ãs a ç¡lEgtrrll LITHOLOGICAL DESCRIPTION FÀCIES~~

~~59 Reworked DeIta Fl to vhlt¿ vlth r¡rc dlscontlnuou¡ Front 4 SAN0, llghf b¡oYn B sorted. Sand noduhs, vrll Facies~~

~~SAN0, tlghl brovn, flnlng upvard, rootcd rÌ lop'~~

~~58 SAN0, llqht brovn, co¡rsenlng uprrrd, nod:r¡h sortlnq.¡lpPl! closs D.os.r ÍoP.~~

~~SAN0. tlqht grry to vhlle, v¿ll sorted, mo¡c cirbonacrous tovard toP. tovrrds thr top' SAH0, flnlng upvard cyclc, claycy Sheet SAN0, madlum grcy lo nadlun brovn, carbonactous' Flood~~

57 SANO, llght brovn, vcll sort¿d vlth lrrgc planr tros¡ brds. Inter- Distributary f¡¡ Peat Swamp oz N SAN0, lnrcgulrr & anrslonlslng bcddlng. lladlur brov-n c¡rbon¡ccous s¡nd & vhlt¿ ¡¡nd. h lftnY brgc Phnl frrgnlnts. Sheet flood sAiln. llqht brovn. vrll roundrd, v¿ll sortrd vlth rnãny largc vood frrgrcnts ¡t b¡s¡. 56

~~SANO. lrr¿oul¡r & rnrstornlslnq bcddlng. ll.dlur brovn clrùon¡ctous s¡nd ó vnlfc s¡nd. llrn¡ lrrg: frrgrcnls- Phnt Sheet o l¡¡ llght brovn, vrll sortrd, sll}y. FIood SAt{0, Facies~~

~~SAt¡0, n¿dlun broYn, rrrbonrccous, sllty 55 Ed Peat Êt Swamp Ø Facies o l!ÂL d¡rk brovn, roodY.~~

~~Figure 6.6 wA1 - detaiLed. lithotogical description and vertical Iithological þrofile for the Trial Pit site. - t75~~

The basal section of the WA1 is characterised by a number of beds similar to the sheet flood beds discussed above' In add.ition, the beds in the basal section commonly contain small fining upward cycles (Plat,e 6.5) which Galloway and Hobday (1983) and Banerjee (1989) also considered characÈeristic of sheet flood processes. Thís conclusion is supported by the presence of mud chips and carbonaceous det,rital rnat'Èer within the sand beds (P1ate 5'5) ' Mccowen (1971), from a study of the Gum HoIIow fan delEa in Texas, obsen¡ed t.hat mud chips are a feature of sheet f lood deposit,ional Processes .

In the middle and upper parts of ?lA1 both f ining and coarsening upward. cycles are present and carbonaceous sand beds are very rare. In order tro identify the depositional set,ting of the middle and upper parts of wA1, it is useful to consider its locat,ion beÈween delÈa fan (T^IA2) to the south and a lagoon (WA3) to t,he north. This location suggestrs a lagoonal shoreline environment.

In Ehese Set,tings, t1ùo processes conÈrol sedimentation' FirsÈ, wave reworked delEa front, sands t,ransgress Ehe delta front, and second, subsidence causes lagoonal development on the landward side. ffith conÈinued subsidence, !Ùave reworking of Èhe delt,a front, ceases, and t'he lagoon eventually ext,ends over the abandoned delta and delta front sands. As m¡ds and silCs are deposited first', Ehe result is a drape of mud and silt on tTre conEact. - L76 Galloway and Hobday (1983) considered that t,he geometries of river dominat,ed lacusÈrine deltas are conrnonly elongate or lobat,e. However, t,he geometry of !{41 is arcuaÈ,e suggest,ing Èhat Ehe delta was Èide or wave dominat,ed, or E,hat it represenEs a modif ied distal delÈa, or a combínaÈion of both.

It, is also important to recognise Ehat the northeast-southwest trendíng structural feature described in Chapter 2 (refer to Figure 2.4 page 15) coincides ín both location and orientation È,o thaE of t,he I{41 facies. As with Lhe FG and GH clean sand facies it appears that this basement. sEructure was a major facEor conÈrolling the lateral location of Èhese sand facies.

IIIAT overlíes WA1 with a very sharp, planar cont,acE lined by a thín, díscontinuous clay drape (P1at'es 6 -6 and 6 -7) . According to Galloway and Hobday (1983), Galloway (1986) and Penland and SuÈer 6gAZ) such contacts En)ify abandoned delt.as.

~~wA2 fan delta silty carbonaceous sand and coal.~~

To t,he sout,h V{41 grades int,o Ehe Vü42 facies . WA2 attains a maximum Èhickness of over 20 meEres at the Southern margin and thins to È,he nort,h and westr forming a wedge shaped body (Fig-ures 6.3a,b, 6.4 and 6.7) . VÍ42 consists of moderately t,o highly carbonaceous f ine to medium grained, poorly sorEed, silly sand (S43,4,5) with thin interbeds of fine to mediu¡n grained clean sand and woody coal. The gradational zone beEween WAt and TIA2 was intersecEed in the Trial Pit. L77

~~24o o@mE 245 ooomE N \~~

~~l I~~

~~+s25oæomN I~~

~~+ f eâcc ooorn~~

~~PIT~~

~~.a ò\~~

~~I LOCHIEL \IH I~~

~~+ H f szeommN~~

~~SCALE~~

~~KMS I o 2 3KMS~~

Figure 6.7 !{arrindi Silt Formation - Thickness and distribution of the WA2 Member. - ]-78 The carbonaceous material occurs as a fine grained matrix buÈ many discontinuous and thin ( < 10 rn¡n) f ibrous coal bands are also present. The fine carbonaceous matter appears to hawe par¡ially cementred Èhe sand. Bedding is often ouE,lined by carbonaceous maE,ter wit,h dips of between 15o - 2Oo comrnon and. basal bedding congacts are generally sharp and irregular.

WA2 is considered Lo represent, a fan delta which entered the area from Ehe south. Fan deltas are defined by V{escott and. Etheridge (1980), Holmes (1965) and Pettijohn et al- (1987) as alluvial fans E,hat enter a st,anding body of water such as a lake or sea from an adjacenÈ highland (in this case t,he Nantawarra High) .

AlEhough fan deltas hawe been recognised and documented for almosÈ Ehree decades, few attempts have been made at' categorising them. Wescott, and Etheridge (1980) provided a sumrnary based. on recenE and ancient, deposits on the basís of Eectonic, geologic and geomorphíc settings while Hayes and Michel (L982) examined fan delEas in a macrot'idal setting and recognised fiwe t)T)es based on sediment supply and. E.he inf luence of waves. Of Ehe examples discussed by Hayes and Michel (L982), the marginal coasÈ of southwesÈ Texas, and t,he GuIf of Arabia and Red Sea coasEs appear t'o be analogous tro Ehe VlA2 depositional environment.

The occurrence of fan deltas was considered by PetEijohn etr al. (1987) to be an indicat,or of ÈecÈonic act,ivity. This is consistent, wit,h the fan delta associat,ion at Lochiel as Èhe alluwial fan rmrst, have originated from the Nantawarra - t79 Hígh which is known to be f ault. bounded and tect,onically mobile.

Further, Alley (L969) recognised the presence of a well developed erosion surface cut across the sandstone and quartzite on Èhe southern parE of the NanLawarra High which would have provided a 1ocal source of sediment.

~~A charact.eristic feature of the I,{42 sediments is the high carbonaceous content.~~

A clue Eo the origin of this carbonaceous mat,ter is its f ine graíned nature. None of t,he core intersections of carbonaceous sand contain any large intact plant material even though such mat,erial is comrnon in the coaI. This suggests thaÈ the fine carbonaceous materíal is allochthonous.

The question arises as Eo the nature of the source of Ehj-s carbonaceous material. There are Ewo possibilities. FirsE, it could be derived from vegetaÈion growing higher on the delÈa or, second, it, could be derived from reworking of peaÈ which had accumulat,ed prior Èo Ehis phase of sedimentat,ion. rf the maEeríal was derived from vegetation (and as indicated by Lhe large wood pieces in the coal this vegetaEion would hawe included at least some large t,rees) then t,he minimum EransporÈ distance lr¡as probably less Èhan 1 kilometre (the distance Èo vegetated parts of Ehe delta). - 180 It is considered, most unlíkely that complet,e disintegration of the plant mauter could be achieved in thaÈ distance-

On t,he other hand, there must. hawe been a ready source of peat, and. other carbonaceous lithologies derived from uplift of E.he Bumbunga Sand Format,ion along the st,ructure ( s) cont,rolling the formation of the fan de1Ea. Based on the discussion above iU is considered that reworking of carbonaceous sediments is the most likely source of the det,rital carbonaceous mat,t,er conÈained in the sediments of wA2.

As indícated on Figrrre 6.3b by the natural ganma log response (borehole 66L) as well as from observations in the Trail Pit and from core, WA2 is in sharp cont,act with Ehe overlying IrIAT indicating a rapid change in Ehe depositional regime. This charactreristic is considered to reflect, a change in Èhe sedimenEary environmenE, not' greaEly influenced by tecEonism.

~~.i¡ra=r.izt=1 Dhraa 2 aazl .i r^r.l ^- T¡Ìr.t {-^ arrht.iÆ='l t=¡iac~~

~~wA7 inEert,idal to subÈidal sandy sitEy c1ay.~~

The wA? lithofacies is lateral1y continuous with we: to t'he north and overlies both I{41 and wA2 to the south (Figure 6.3a). As shown on Figure 6.3b the geophysical log response of wA7 is characE,erised by a ganrna peak of bet'ween 40 and 1-20 API uniEs. This characÈeristic prowídes the basis for det,ailed correlation over a large area even though it is only beÈween 1 Èo 3 metres thick. - 181 At the trial pit, site, I{47 is 1.7 met.res t.hick and overlies T¡IA1 with a very sharp, planar irregular cont.acÈ which is lined. by a thin, disconE,inuous clay drape (Plates 6.6 and

~~6.7) .~~

~~As shown on Plates 6 .6 and 6 .7 and on Fj-gure 6 .8, I{47 comprises two inlensely bioturbated EIT)es of sedimenEs; 1. sandy fining upward cycles and 2. brecciaEed carbonaceous siIts.~~

The fining upward sand beds are no more trhan 0.2 metres t,hick. They consist of fine sand at, Ehe base grading up inÈ.o flaser bedded sand and clay (as described by Reineck and lrlunderlich, 1968) . Somelimes t,hese beds also have a thín, brecciated carbonaceous silt aE the Eop. According to Clifton (L982) flaser bedding constituges one of the most characteristic feaEures of Èidally deposited sand.

The brecciaEed, carbonaceous silt, beds are generally less Èhan o.2o meEres thick (P1ate 6.7) . r,ithologically t'hey comprise brecciaEed, angplar blocks of carbonaceous silC surrounded by fine grained sand. These beds probably represenE, mudcracks developed from periodic Ðq)osure of lagoonal silts.

~~The association of flaser bedded sand and desiccated lagoonal silts indicaEes an intertidal E,o subEidal depositional environmenÈ .~~

~~The upper half of WA7 has rare breccia layers and consisÈs prímarily of grey silgy clay with gradational interbeds of r82~~

~~GRAIN SIZE d(J~~

~~5 z I -Sd- B .t6o E o- DEPOSITIONÀL o E å J o E o o FÀCIES E¡ 5 o ;i¡:Eittrtll LtTl-loLoGlcAL oESCRIPTIOî{~~

dark b¡ovh, c¡rboneceous, (lat!t, oßt.slon¡l coacscnlng up cytlcs vlth rlppla rross Ol bcddcd sand rt loP. Subtidal '< .- SILT. lnt¿rb¿dd¿d co.rsrnlng up cyths'LlghÌ brorn Estuarine B bloturbahd ¡t b¡sc to derk brovn -grrtn Facies 65 -

lrrrgulrr con]acts. --SILT. blolurb¡tcd. d¡¡k brovn, t¡rbon¡ccous !n bccornlng norr sandY 1o îoP. 4 - Intertidal B SANO. fl¡hl brovn-vhlle' nan¡ snall plrnt Facies fiagnrn}r & lnlcnscly bloturb¡lcd' 64 SAN0, lnt¿rbcddad & tros¡ bcddcd t¡rbon¡c¿ous _-/- slllY, vhltr rrnd' ----l¡!g vhlt:, qredlng to cerbon¡c¿ous ¿l bas¿'

~~54N0, tlghl grcy vlth nrany srnall planl fragncnts'~~

~~63 SAtl0, tlqht brovn to favn, sllghlly r¡cbon¡ceous' lndlsllnrt cross brddlng. Distributary @ Channel B Facies~~

SAN0. Ught brovn to nadlun brovn, nora ------::.¡ibon...ou¡ lo top & bas¿'CLAY 0RAPE ¡t b¡sr. l.S¡¡!, tlght broYn to llght grey, vcry stlghtly rirbon¡c¿ous ¡t b¡¡c. 62 fu!. - tlght grct Ìo llght brovn, poorly sortcd' SANO. lloht brovn, b¿dd¿d, lrrcaular E dlscontlnuos diape rt basr.

~~-J 5lLT, mcdlum graY.~~

~~.J sANo. tloht brovn, slltY.~~

~~poorly sortrd, subrngular, tlght grc¡' -{lAN0, ,t- -N!!, PoorlY sortcd, toarscnlng u9' 6¡ hlghlY claYeY, nrdlun grrY. Intertidal SILT, to subtidal F hlghtY~~

~~SAN0, llght grr¡-brovn, cby drrpc rt top'~~

SAtt0. tlnlng ug, clrYaY rl toP. .-.-!.| ntdlun brovn, (¡rbon¡clous' 60 --J!!!., flnlng uP. clrY drrPc rl toP' .-l!!!, llght brovn-grc¡, rrors brddcd rt top' Figure 6.8 wÀ5, WA7 and. WÀ8 - Detailed tithologica-l desåription and. verticaL profile for the Trial Pit site. - 183 poorly Sorted,, Subangular, coarse grained sand and fine gravel. The rarity of brecciaÈed beds and the generally f iner and. more homogeneous nature of the sedi-ments indica!,es a progressive increase in the depth of water.

The coarse grained beds occur in thin upward fining cycles- Their presence in this part of the sequence raises the quest.ion as to the source of these coarse sediments. As described later, coarse graíned, angular sediment's occur in the sediments associat,ed with Phase 4 sedimentaEion. It is consid.ered that their appearance in IrIAT marks Èhe beginníng on local t,ect,onism which led to Phase 4. The f ining upward nature ind.icates a sheet flood process of deposit,ion.

~~!rIA7 grades into l{A,3 towards the north. phase 4 sedimenEaÈion - T¡IAB and IìIAIO distributary channel and fan delta.~~

~~wA8 distribuÈary channel silty carbonaceous sand.~~

üIAB is presenÈ in Ehe Sout,h-cent,ral part of the area. It' reaches a mancimln thickness of 10 metres. The member thins Eo the west, and north where iE. grades vert'icalIy and laEerally inEo VlA5. To Ehe south and east' it grades int'o wA10.

On Èhe basis of lilhotogy and sedimentary structure' T{AB in the trial excavation can be divided into two depositional facíes : 1. Sheet, flood sand, 2. Díst,ributary channel sand, - 1"84 1. Sheet, flood sand facies (refer Eo PIaEe 6.8) This facies is about 0.7 m Ehick and consists of int,erbedded f ining upward cycles containing mott,1ed, intensely bioÈurbaÈed, Iight grey-brown welI sorted fine sand at Ehe base and grading E,o medíum brown carbonaceous lamínated silty sand at the top. An inÈerbed of coarse, angular sand similar Eo those in the underlying WA7 is also present.

2. Fan delta distribuEary sand (refer t,o Plate 6.9) This facies is about 1.1 metres Ehick and consists of fine graíned, well sorfed, slightly carbonaceous silty sand with interbeds of fine grained clean sand. Abandoned channel ínfilIs, trough crossbed sets and finer grained overbank deposíts were also obserr¡ed in E,he Trial Pit. ( see Plate 6.9). The contac! between the sheet flood sand and the dj.st,ribuÈary sand is very sharp and irregular and líned with a very thin (r to 2 millimetres) clay drape.

?fAB grades Eo t,he easE, and souEh into coarse sand and grawel of wA]-O. Elsewhere in the deposit, wAB also consisE,s of int,erbedded carbonaceous sand and clean sand, however, f arEher Lo t,he west and south iE becomes considerably more carbonaceous.

This is a progressiwe change and reflecEs a decrease in the wave processes away from t,he shoreline. Thin horizons of well rounded quartzite pebbles and cobbles were inÈersecËed in core from v458 (refer to Appendix 6 for location) and these are considered to represent shoreline reworking of the delta fan sediments. - 185

~~I,{A1O f an delta clean sand and gravel.~~

VüA1O is located on the souEhern and eastern margins of the study area. This lithofacies reaches a maximum thickness of 12 metres on the east,ern margin and Ehins Eo Ehe west and north. The Eop of IlIAlO is eroded on the margins and the thickness trend is partly in response Lo this- The description of this member is from cuttings descriptions and interpreÈ,ation of geophysical logs.

From geophysical logs, TiIAIO has a distinct coarsening upward wertícat profile. This coincides with carbonaceous silty sand at the base of the sequence and a gradation to interbed.ded gravels and pebbles and cobbles of quartzite and coarse sand at, the top.

~~Based on E.he facies distributíon and geomelry, Ehe source appears Eo have been from the east and Ë,o a lesser extent to Èhe sout.h.~~

~~Phase 5 sedimentation - WaS and IiIAg intertidal to subtidal facíes~~

~~wA5 intertidal flaser and lent,icular bedded sand.~~

WAs is 1.0 m t,hick and overlies !iIAB wilh a sharp, erosional contact (P1ates 6.10, 6.LL and 6.L2). As indicated on Figure 6.4 it, is considered that the change in depositional environment f rom VIAS/üIA1O to VtAs/I'IA9 represents another period of sea leve1 rise and/or decrease in tecEonísm. - 186

The base of IrIA5 is marked by a t,hin ( < 10 nrn) bed of medium to coarse grained, sub-angular white and brown carbonaceous sand. This is followed by a sequence of silty carbonaceous sand and white fíne grained well sorted sand which diplay flaser, lenticular and ïra\ry bedding (plat,e 6.L2) .

Small erosional channels were also obsen¡ed within !{45. These were generally infilled with carbonaceous, fine grained sand and silu and often lined with a very Ehin silty clay drape (Plate 6.13). rE is apparent from these features, particularly Èhe bioturbaEion, t,hat t,his liE,hof acies represenEs an intertidal deposit,ional enwironments which was cut by t.idaI channels.

It is considered that this sand unit formed when Ehe shoreline began reworking t,he v[48/!{A10 delt,a fan and marks the beginning of a major Eransgressive episode which led to Ehe depositi-on of sediments of the Tarella Silt Format,ion.

Elsewhere in the deposit, equivalent sedimenÈs conLain clay drapes. f.n Y723 (refer to Appendix 6 for location), the clay beds occurred as drapes on fine grained sand beds. One of these clay drapes was concordant wiÈh bedding whíIe the oÈher had a dip of 1Oo. The clay beds vtere up to 20 nrn thick and. the uppermost, bed r¡ras obsen¡ed to cont,ain a polished shear plane. In v360, Ehe clay bed also contained a shear plane but was parallel to Èhe surrounditg, horizontally bedded - 1,87

~~carbonaceous silt. These clay drapes are thought Eo represent the basal lining of channel infil1s.~~

The intertidal sand is overlain by 0.2 m of lighL brown, fine grained sand (P1ates 6.1-4 and 6.15) in the Trial Pit area. A number of phases of sediment,aÈion and erosion are recognised in t,his bed. One major phase is characterised by planar and shallow angle crossbedding consisting of alternaE,ing beds of carbonaceous and non-carbonaceous sand and by a thin zone of carbonaceous sand aÈ E,he base. This thin carbonaceous sand at the base is considered to represenE, deposition inE,o a tidal lagoon.

A number of subsequent depositíonal phases reworked the sediments in small erosional channels. The size of these is indicated by the channel infilt strucÈ.ures which average about 0.15 m in dept.h and 0.4 m in width.

~~The Iítholog¡¡ and geometry of Ehese beds suggest a suorm redeposiE,ed sand or a crevasse splay enE,ering into a dístributary bay.~~

~~wA9 subtidal carbonaceous clayey silt.~~

!iIA9 at,t,ains a maximum Èhickness of 2.5 meLres and aÈt,ains a relaÈively consEanE thickness of about 2-0 meÈres (Figure 6.9). This facies occurs at the top of the I¡farrindi Silt Formatíon (Plate 6.L6) - The conÈ,act between E,he overlying Tarella Si1È Formation is sharp and planar and coincides with the Tarella SiIt Formation shear zone discussed laEer in sect,ion 6.4.3 . Towards the sout,h the - 188 vlAg is composed of highly carbonaceous siIts, interbedded with thin clay seams (Plate 6.L7) and clay shear zones, non-carbonaceous silt beds, thin carbonaceous sand beds and flaser and lenticular bedded sand (P1ate 6.18).

The carbonaceous material in Ehe silt is very fine grained and dispersed evenly throughout. Pet,rographic analysis indicates that, most of E,hís mat,erial is fine grained and broken suggesÈing transportation and that. a high proportion of the exine coal groups is present, (see page 191).

!{ithin this silt, Èhin seams of clay up to 40 ntrn thick occur and these generally dip at less than 50 (Plare 6.L7) . The clay seams are of tt.to tlæes: as part, of small scale coarsening upward cycles and as discrete beds wÍE,h no apparent relationship with surrounding beds.

These Ewo È]T)es of clay seams lvere presenE in a 900 nrn diameter core taken at, site v634 (refer to Appendix 6 for Iocat,ion). The discrete clay seam consisted of 3 nrn of lighÈ brown cIay, sIíckensided on one side and discontinuous because of numerous intersecting sedimenÈary faulEs. Displacement in Ehese fault,s !'ras of t,he order of 50 nun. These Ehin clay seams characteristically have dips of less than 50.

The second tlæe consists of a relatively thick zorTe (up to 100¡mn) of silty clay occurring at the base of a coarsening upwards cycle. IE is common for Ehe clay-rich base of these zones to be sheared. 189

~~240 oæmE 24a ooomE / U N I \ t, I l I I I I I + f s25oooomtt / I I I I / I \, o~~

~~+ f e Zlr m.tl~~

~~l ) I LOCHIEL .'; '~~

~~+ + f ez+ooæ.tl~~

~~SCALE~~

~~XMS I o 2 SXMS~~

~~Figure 6.9 Warrindi Silt Format.ion - thickness of WA9 - 190~~

At the trial Pit site, I^IA9 overlies IÄtAs wíth a sharp conÈ,act - AÈ Èhis location üIA9 is about 1 metre Èhick and consists of interbedded coarsening upward cycles comprising highly carbonaceous silt at the base and grading to silty f ine sand at t,he top. These sands are charact,erised by lenticular and flaser bedding (P1aÈe 6.18).

~~Cycles var1r from 10 Ím to 20 nrn in Ehickness. Occasional, small erosional distributary channels lvere also presenE.~~

In the trial PiE, area t,he upper 0.4 metres of l¡IA9 is comprised of highly carbonaceous, black sí18 or coal. From Ehe base upwards, this liEholog¡¿ gradually becomes less carbonaceous and more clayey and grades verÈicaI1y into the overlying TL4. No bedding or lamination was visible but elongat,e íntraformational clay peI1eÈs or clast,s are present. in the gradaÈional zorLe (Plate 6.19). The Tarella SilE Formation shear zotte is locaÈed on Ehe boundary beÈween t,he ?larrindi SilE and E,he Tarella Si1t, FormaÈions and is the subject of det.ailed analysis t,o follow in sect,ion 6.4.3.

The feaEures of vtAg suggest that at least in Ehe initía1 st,ages of sedimenÈation Ehe depositional sit,e was subject to Èida1 influence. However, the absence of sand at Ehe top of t,he sequence indicat,es a cessat,ion of input, of sand to Ehe depositional sit,e. From the presence of allocht,honous carbonaceous maEter it is likeIy Ehat, t,he environment graded from intertidal to subcidal. The ínclusion of clay pelleEs within E,he upper parts of vlA9 - 191 indicate Ehat there were erosional channels flowíng through this subtidal environment.

~~Petrology~~

The highly carbonaceous black silt or coal was examined to determine the nature of the organíc const'ituents. A number of representative samples were examined using mícroscopic reflectance and fluorescent, t,echniques (P1at'es 6.20a,b) .

~~The samples contained a high proportíon of liptodetrinite indicating that much of Ehe organic matt'er was transported prior to deposition under anaerobic conditions.~~

~~The presence of Botryococcus related telalginite and phyt,oplankton indicate deposition in an estuarine environment.~~

~~SedimenÈation in the centre and western margins of the area~~

~~wA3 lagoonal clay and siltY claY~~

In Ehe cenÈra} part of the area, the Warrindi SilU FormaÈion consist,s of about 4 meÈres of s1íghE,1y to non-carbonaceous silt (Figure 6-3) - A laterally continuous band. of interbedded highly carbonaceous silt and carbonaceous fine grained. sand is present in the middle of the Sequence. This sand is generally less Ehan one metre thick and. of E,en cont,ains coal breccias and thin clay beds. The silt above is generally s1ighE,ly carbonaceous, brown and bioturbaÈed and. has a sharp contacL with Èhe owerlying - r92 The contact with the overlying I^IA9 is generally gradational but owerlíes IìIA6 with a sharp contacÈ.

~~VÍ44 fan delta dístributary channel sand~~

~~This facíes is present on the $testern margín of t,he deposit and from t,he thickness disE,ribution appears to hawe been related to movements along Ehe Ardrossan Fault (Figure 5.s).~~

I{44 ís comprised of subrounded, moderately sort'ed, non- carbonaceous and medium grained sands. These occur ín discrete cycles up Lo 2.5 metre Ehick. In the fulIy cored hole V582 (refer Eo Appendix 6 for location) only the basal Section of these sand beds !ùere recowered but from Èhese and from int,erpret,ation of geophysical logs, it appears that they are part of upward coarsening cycles.

These cycles contrnence as a series of interbedded, carbonaceous, bioÈurbated, sandy and clayey silt,s usually with gradational boundaries foltowed by poorly sorted, subrounded, non-carbonaceous sands. The upper-most cycle in V582 was charact,erised by a carbonaceous and silty fine to medium grained sand.

The Sand zones can be correlated over large distances and the cumulative thickness tends Èo increase towards Lhe margins ín response to an increase in Ehe number of sand zones present in E,he seç[uence.

The limited amount of core awailable adds difficulty Èo the - 193 interpretation of the underlying members, the associat,ion of sandy units adjacent to faults in the Lochiel area ís diagnostic of fan deltas.

To support Èhis, the geophysical logs and limited core strongty suggesf t,he sand occurs in upward coarsening cycles. These are wíde1y accepted as one indicaEor of progradational environments t14lica1 of deltas.

~~6.2.2 Sununary of deposiuional processes~~

The st,rat,igraphic sequence in Ehe üIarrindi Sílt, Formation represents a complex interaction of bot,h tectonic and eustatic cont,rols and. depositional processes. IE is apparent that, a sea level change at Èhe end, of t,he Late Eocene led to encroachment of the sea from Ehe souÈh. This produced. a deposítional setting characterised by marginal marine deposítional environments (Figure 6. 10) .

The maj or characterisEics of t,his setting include a lagoon/estuary ín t,he central part, of the area surrounded by fan delt,as. The major source of sediment for these fan delt,as was from t,he south and southeasÈ, from tTre west and, Uo a lesser extent, from the norÈh. The inÈeraction of t,hese deposiUional environmenE,s produced a complex vertical and lateral facies association.

~~Deposit,ional archit,ecEure~~

~~A slmthesís of t,he facies distribution with Ehe maj or deposítional conËrols of Eectonics and eustacy is províded L94~~

Five phases of sedimentaÈion are recognised for the I{arrindi Silt Formation. These phases resulted in a Iat,eral shift of depositional environment and produced the facies distribution obse:¡¡ed in t,he the study area.

The posit,ion of the shoreline can be ínterpreted from Èhe location of t,he intertidal depositional environmenEs and the t,ectonic movement,s for the southern source are interpreE,ed. from the st,ratígraphic disEribution of fan delta sands and gravels. 3 aÇÇ a * Ç COASTAL 9 ALLWUL l¿ + PLAIN * Ç D€TTA * \ ë Çç + ìr f{À4 ìF Ç * * !È \ Tidally influenced wÀ4 Lagoon P wÀ3 (¡\o

~~+ ìt4 ( + + 5 + MAffiH7 TIOAL FLATS *- Ç *. l"a 2 Ç 0 t æLTA + ç Ç o~~

~~of the depositional Figure 6.10 Schematic representation ;;ïü;-iot tñ" warrindi sitt Formation' - L96 6.2.3 Engineering properties~~

A Sunrnary of Ehe intact engineering properties of the IVarríndi Silt Formation is presenEed on Table 3.t, page 55. These properties re1aEe mainly to the WA3 and !{45 members- In general, iE can be concluded from Tab}e 3.1 that Ehe propert,ies of the Ï{arrindi Silt Formation are similar to the Taretla Silt, Format,ion.

~~Geological factors affect,in€f porewater movement~~

~~Hydraulic conductivities and grain size distribution for Ehe üIarrindi Silt, FormaEion are presented on Figures 3 -1,2 and 3.13 (pages 76 and 77 respectively).~~

The high hyd.raulic conducgivity recorded for V623 is attributed to it,s locaÈ,ion in Èhe WA10 lithof acies. WA10 is characterised by an inÈerbedded Sequence of coarse grained sand and gravel and as suctr would be elq)ected t'o have a relatívely high hydraulic conductivity-

It ís apparent that E,he locaÈ,ion and properties of aquifers are primarily controlled by Eectonics. Movement on faults at, the margins of t,he area hawe produced several d.ifferent sources of sedimenE which lvere acÈive inE,ermittently prod.ucing a series of stacked and IateralIy variable sand aquífers.

In contrast t,o the Br:mbunga Sand Formation, the l¡Iarrindi Silt Formation represents deposiUion in a marginal marine depositional environmenÈ. XotwithsÈanding this, the - ]-97 essentíatly the same, fan deltas entering a central body of water, and it is apparent that the eustatic sea level rise had litt1e effect on these processes.

~~The conclusion is that for the study area, tecEonics was the major factor affecting the location, distribution and properties of Èhe aquifers.~~

~~Geological factors affecting VIeak Zones~~

~~A nr¡rnber of weak zones characterised by thin Seams of clay are presenÈ within the lrlarrindi Silt Formation and the depositional and stra|igraphic association of these is shown on Figure 6.11.~~

~~These clay seams have been identified in association with; a)rntertidal lit,hofacies (wl's, vrAT) b) Subaqueous lithof acies (v{43 , vfA9 )~~

The clay seams present within the study area can be att,ributed Eo two geological processes; 1. Tidally influenced distributary channels A model for the location of clay seams in Eidally influenced channels is presented on Figure 3.11.

~~Clay drapes can form on the crossbed sets, being deposited at either high or ebb Eide, or as linings of abandoned channels. Only Ehe lat.ter has been obserwed within the study area. -198-~~

~~2 Onlap of aqueous deposit.ional environments during regressive periods.~~

~~A prime site for the deposition of fine grained sediments and clay is at the distal end of d.istributary channels. The processes are similar Eo those described for the clay zone occurring within CN1~~

~~and. CN2 (refer to Figiure 3.11-, page 72) -~~

~~Engineering propert,íes~~

As presenled on Figure 6.11, the test results for the few clay seams f rom the lrlarrind.i Silt Formation indicated relatively high resid.ual shear strengt,h with a mean of 15o' and cohesion of 30 kPa. Onty 5 samples were tested from a toÈal of 4 sites and it is considered that it. is not possible to draw any conclusions as to lateral variation-

~~Petrology of clay seams~~

~~A sample of core taken from V723 (see Appendix 6 for Iocation) and containing a sheared clay sean was examíned using microscopic Eechniques (P]ates 6.2La, b, c, d, e and f).~~

Some experimentation $ras required regardíng impregnation t,echníques because t,he samples were very friable, wet and water sensitive. The samples !ùere impregnated using t'vapour exchangen techniques by the Wait,e Research Institute, Diwision of soils in Adelaide- - L99 The sample examined has five dist'inct beds;

~~1 top bed. of fine grained sand with distinctly angular detrital grains and intergranular carbonaceous material,~~

~~2. Thin (<1 rffn) lamination of carbonaceous clay,~~

~~3 - 30 nrn bed of weakly carbonaceous silt,~~

~~4 10 ntrn bed of highly carbonaceous clay which includes a polished shear Plane,~~

~~5 f ine grained angular sand wiE,h intergranular carbonaceous material as for bed (1-)~~

~~The top (1) and botlom (5) beds are of similar sediment and comprise a very loosely packed aggregate of angrrlar sand grains moderat,ely to well sort,ed with an average size of~~

0 .15nrn. Carbonaceous material is colwnon as an inÈergranular matrix and in the E,hicker, lower bed is oft,en more abundant than Ehe quartz grains. ltinor flakes of clay are also presen!, in the lower bed and there is an apparent primary porosiUy forming up to 3Ot of this lithology'

A gently wa\ry lamínation of highly carbonaceous clay Q) separates the upper most fine sand from Ehe underlying middle silt bed (3). 200 The middle silt bed (3) is a fairly homogeneous massive sediment, composed of a random compact aggregate of angular quartz silt grains, and, minor fine detrital muscovite, with a sr:l¡ordinaÈe matrix of very slightJ-y carbonaceous c1ay.

Minor extremely fine discrete graíns of carbonaceous materíal and ctay occur sporadically along the bedding. Small poorly defined mud. clasts (carbonaceous clay mixed with silt) occur in t,he lower portion of this bed-

The highly carbonaceous clay bed (4) consists almost entirety of carbonaceous clay with a trace of dispersed silt material. Throughout most of bed 4 the clays have a common orientation probably reflect,ing their primary beddíng orientation and compaction. lwo shear zones are conÈained in bed (4) and Ehese appear to have the same mineralogy as thaÈ of Ehe host bed. ttithin Ehese zones the clays have a turbulenE orientaEion. The Shear zones are less t,han 1 ntrn thick and paralle1 t'o bedding. 20L

~~tlme, a v67r 5¡tOm Ct 15 a)V597 roRo{ol¡ c¡u-aiailuMæR o.l5o~~

~~!6ñ OEPB 8E¡¡!fl SJMCE 80 coHÉstotr til rPd 220 ar¡q¡ oF FRrctìil ô €25OæOñ¡ r€ÁR zoflE üta wffi 80x cl-ÀY sÉax 0a1a wrHoJt æx~~

~~C. !O C.('.~~

~~I 624.øOnfl I 4-rRAl pn~~

~~1440 X ttil - r¡.9t - i¡.ot~~

~~a vtt{ - ¡t.æ - 2l.tb l+ vd -:a.& - Ja.th a¿¡toGt~~

~~UFP€R æUtÐ C'¡JA¡po 6'.2ã5o~~

~~gE 6 !¡ ,-G Ø E * bffi Bo{Jto Í Ci Ortu øi tOP ø * ^ Residual Friction Angle~~

~~NORMAL srRESS ( rPo)~~

~~Figure 6.11 l{arrindi SiIt, Formation - Location of test sites and summary of results of strength testing (testing performed by Coffey and Partners Pty Ltd, 1988). 202 6.3 Tarella StIt Fo¡mation~~

The Tarella Silt FormaÈion owerlies the lrlarrindi Silt Formation wit,h a sharp contact. The formation attains a maximum thickness of 25 m in the north-cent,ral part' of the area and overlaps on the Sout,hern and eastern margins

~~(Figure 6 -L2) .~~

The formation has been divided into four lithofacies based on lit,hology and. geophysical log response. A t1çica1 vertical profile illustrating these subdivisions is presented in Figure 6.1-3. The top of Ehe Tarella Silt Formation represents a surface of weaEhering and erosion

~~(PIaÈe 6.22) .~~

~~6.3.1 Líthology and lateral variaEion~~

~~TL,4~~

TIJ4 owerlies I¡IA9 with a gradat,ional conEact. This conEact coincides with the location of t,he Tarella SíIt Formation Shear Zorre díscussed IaÈer in sectíon 6.4.3. AS shown on Figure 6. 13 and Append.ix 4, TL,4 grades vertically into the overtying TIr3. TL4 is distinguished from TL3 a-bowe and WA9 below by having a relatively low carbonaceous cont'enE, (higher apparenÈ density response) and relatiwely high clay content (higher natural gamma response).

~~The material consists of a medium brown, moderately carbonaceous silt, with rare remains of arenaceous foraminifera dispersed throughout (Plate 6.23a and b). 203~~

~~24o@mE N I I I L I I I~~

~~/ I I I / / ,j j.~~

~~cz+amr -r¡ + TRIAL PIT f~~

~~o ò\~~

~~I LOCHIEL~~

~~+ + fezeommH~~

~~SCALE~~

~~KMS I o 2 3XMS~~

~~Figure 6.I2 Tarella SiIt Formation - thickness and lateral distribution. - 204 Figure 6.13 TareIla Silt Formation - geophysical log response, lithology and nomenclature.~~

~~GEOPHYSICAL LOG RESPONSE~~

~~STRATIGRAPHY DEPOSITIONAL 20 ENVIRONMENT fL2~~

~~TARELLA ESTUARINE TL3 SILT~~

~~TARE¡J.Â SILT TL4 SHEÁR ZONE IYA9 SUBIIOAL~~

~~WARRINOI wa3 LA6OONAL SILT~~

~~wa6 ESTUARINE~~

~~F XOOLIATA COAL PEATSWAMP ZONE~~

~~Nolurol API o oo~~

~~oo 2o0 Neulron - Naltron gm/cc CPS~~

vague bedding is the only sedimentary structure ín the sequence. TL4 waries from zero tlrickness in Èhe south to a mæcifium of 6.5 m in the northwest. The thickness variations of Ehis sr:bunit are complementary to those of Ehe overlying tr,3 unit (Figures 6.7.4 and 6.15).

~~TL3~~

TL3 waries in thi-ckness from zero in the south and south-east, up to 10.5 metres in the central area, Ehinning again farther norÈh (Figure 6.15). rt represenEs a gradaÈional zone consisting of mod.erat,ely carbonaceous, 205 t 1o dôófü€ / N I I H

~~/ + 625oooomN 5 / ? < J / J l¡J / É. h- I : lJ- / L o / t- Js~~

~~+ TRIAL4 PIT f e z+o ooonll *o o I~~

~~I I uoëirrer.,$.~~

~~+ + f ozeommtt~~

~~SCALE KMS I o ? SKMS~~

~~Figure 6.14 TL4 Thickness and lateral distribution. 206 I 24o oæmE 24!@mE N I \ I I~~

~~I t-:-~~

~~f izsoooott't 5i J J l¡J (E É I t- / è I J~~

~~ó. ò~~

~~I ""ù , as I + f o24o ooomN I \e, I o I \~~

~~+ + fealommtt~~

~~SCALE~~

~~Kts I o 2 3XMS~~

Figure 6.15 TL3 Thickness and lateral distribution. 207 mediun brown silt at Ehe base to interbedded sIightlY carbonaceous silu and fine grained sand at the top. A charact,eristic f eature is the presence of arenaceous foraminifera (Lindsay, pers. contrn. , 1-98'7) which íncrease in number up Èhe sequence.

~~The upper 2 metres characterísticalty comprises flaser and lenticular bedded fine grained sand and carbonaceous clayey silt.~~

~~TL2~~

~~to 4.? metres of carbonaceous silt (Figure 5.16). It' is similar in lithotogy and cycle t)æe to the underlying TIJ3.~~

The base of E,he cycle consists of dark brown, highly carbonaceous silt. the silu becomes gradationally less carbonaceous towards the Èop until the upper 0.5 metres is essentially non-carbonaceous grey silt.

As the silt becomes less carbonaceous upwards, the proportion of fine to very fine grained white and light brown discontinuous sand interbeds increases. These beds are irregular both in thickness and geometry. Generally t,hey are less than 5 nrn and. dip at 50 to 1Oo wiÈh mæcimum dips up Eo 15o. They are Èlpica1 of the f laser and Ienticular bedded sediments. -208-

~~24o@mE 245 ooomE / N I \ I~~

~~I 4 / q25oæomN 5 + / L / I I~~

~~/ /~~

~~+ TRIAL f czao oootnt't~~

~~ù 9ù o\~~

~~9( l ,rñ I I LOCHIEL É.ßos\oNAu I / + f czeoooo.tt~~

~~SCALE~~

~~tûts I 0 z 3KMS~~

Figure 6.16 fL2 Thickness and lateral distribution. - 209 Some discontinuous medium brown silty clay seams up to 20 nun thick also occur within the flaser and lenticular bedded sediments. These clay seams generally occur as lenses buE occasionally the lenses overlap Eo form contiguous beds.

~~TLz grades into the overlying TL1.~~

~~TIr]-~~

TL1 is considered to represen! the last phase of Tertíary sedimentation in the area and also to represent the same depositional enwironment as the underlying Tarella Silt members (section 6.3.2). The top of TL1 ís weathered and eroded.

Tl1 reaches a maximum thickness of 8.1 metres (Figure 6-!7) on Ehe northl¡¡estern side of the area and thins gradat,j-onally to the east and sout,h. This thickness trend is thought to reflect both the weathering and erosion and a depositional trend.

~~TL1 is comprised of fluvial sand and overbank and lacustrine silt and clay (refer to Figure 3.1L Page 72) .~~

The fluvial sand is up to 5 metres thíck (Figure 6.18) and comprises fining upward mediun to fine grained, moderately well sorted, subrounded and non carbonaceous sand. The lêquence becomes more silty towards the top and is occasionally owerlain by Ehin (<0.3 m.) coal seams- 2LO

~~zao oæí€ m€ N / I \ I I L I I I t{ I~~

~~+€25oæomN H I I~~

~~/ I~~

~~ÍARELLA SILT + o( TRIAL¿ PII f czeo ooornlt~~

~~e, o\~~

~~I I I LOCHIEL /~~

~~+ fc24omnil~~

~~SCALE KMS I o 2 SXMS~~

~~Figure 6.77 TLl Thickness and lateral distribution. - zLt~~

~~24o oæmE 246mmE / I N I I I I I I I I a~~

~~Ç f s25oooomN / I I I I / I~~

~~ñ'""e QI + f szaa wtw~~

~~.q, ò\~~

~~I I I LOCHIEL~~

~~+ + f eZloooort't~~

~~SCALE~~

~~KME I o 2 3KMS~~

~~Figure 6.18 TL1 - Thickness and distribution of the clean sand facies. - 2L2 bedd,ed light grey or cream sandy and clayey silt representing crevasse splay and overbank sediments.~~

The fluvial and overbank and lacusÈrine facies are overlain by a zone of brecciated síIty c1ay. This zotae is up to 4 metres thick and comprises angular to subangular blocks of green, gîêy, yeIlow and brown clay in a light grey silty clay mat,rix (Plate 6.24). The blocks vary in size from 5 rnm to greaÈer t,han 100 nrn and oft,en appear Lo be weat,hered, having carbonaceous centres which gradually become less carbonaceous towards the edges. The breccia zone is associated with thin coal seams and where t,hese are interbedded the breccia is extensively rooUed. This breccia is considered to represent elq)osures and partial desiccatíon of lacustrine sediments.

~~6.3.2 Sununary of facies relalionships and deposiE,ional environments~~

The Taretla Sil-t Formation represenEs deposítion in a maínly subaqueous, estuarine and back-barríer lagoon environmenÈ (Figure 6.19). Three distinct phases of sedímentation are recognised; TI'3/4, TT'2 and TLl.

TT'3/4 and TI.2 comprise coarsening upward sequences with highly clayey, carbonaceous sedíments at the base grading to discontinuous inÈerbeds of carbonaceous clayey silt and silty fine grained sand at the Eop. These seçnrences are considered to represent estuary/lagoon infills grading from aqueous at the base (carbonaceous silt) to subaqueous and intertidat at the top (flaser and lent,icular bedded). the sediment,s exhibit a similar lateral dist,ribution to Ehe ' 2r3 organic shales described. by Piaseckí et al. (1990). They found that hypolimnic sites l¡tere characterised by higher carbonaceous content (TL3) whilst t'he epilimnic sites were generatly less carbonaceous (refer to Figure 3.9, page 67 for Total Organic Carbon (TOC) content of TL3 and TL4).

~~As discussed prewiously, TL1 represents fluwial and lacustrine depositional environments -~~

Of particular noEe is that the characteristics of the upward. coarsening seçF,rences in the Tarella Silt Formation are essentially the same as ttrose for the Warrindí SiIt Format,ion, GH inEerseam and Condowie Silt Member-

It, is apparent that the deposit,ional processes for each of these was equivalent, being controlled by tectonic movement along basement faulÈs. However, t,he nature of the depositional environments changed from lacustrine for the GH intersem and. Cond.owie Silt Member to marginal marine/estuarine for the Tarella SiIE Formation reflecting the influence of eustacy. Fígure 6.1,9 sctremat,ic representation of the depositíonal setting of the Tarella SiIt Formation. - 2r5 6.4 Tectonic strr¡ctureE in tbe Tarella Silt FormatLon

In contrast to the underlying sedimenEs, the Tarella SiIt Formation ís characterised by the presence of a nr:mber of Eectonic structures. As discontinuities were identified in Chapter 4 as being a key factor affecting slope stabítity the purpose of t,his section ís to : . descríbe the tectonic structures and Lheir interrelationships, . define the geological factors whích conÈ.rol their location, dist,ribuEion, geometry and strength.

An understanding of the structural geology provides the basis for interpreting the properties of the defects present in the maEerials. However, before proceeding with t,he interpreEation it is necessary to def ine the data available and the procedures best suited for analysis.

~~6 .4.1 Data avaílable~~

~~Data on tectonic st,ructures is derived from two sources, core from drillholes and from the Tria1 Pit,.~~

The core informaEíon was limited mainly to subhorizontal defect,s, whilst E,he tríal excavat,ion provided the opportunity to examine in deEail the characEeristics of all def ect,s occurring at the lrial Pit, sit,e.

~~At Èhe Trial Pit site, structural data was collected by three methods : - 2L6~~

~~1 Line mapping of cut faces.~~

A tape was run between two survey points and any structural feature which inÈersected the tape was measured and. described on a structural logging form for inclusion into a computer data base. orthogonal lines were used to eliminat,e geometric bias (Priest, 198s).

~~2 Floor mapping. Plans of joint geomet'ry and tIæe were prepared using E,ape and comPass.~~

3 Detaited shear zorLe Struclurat lines. TTùo orthogonal suli¡eys of the surface structure of the Tarella Silt Formation Shear Zorre were carried out. Measuremeri.ts of all associated shear surfaces were sun/eyed at poinEs wit,h a spacing of about 0.5 meEres.

~~TecLonic structures in Èhe Tarella SiIt Formation~~

A nr:mber of tlpes of slructures ürere recognised in the Tarella Silt Formation. Charact,eristics of t'hese are sununarised on Table 6.1 and disc-ussed in the following section. The strucEures have been classífíed into è*t,Þnsional and shear and eactr of these SÈI:ucÈures was ñrrther categorised on the basis of morphological characEeristics and orientaÈion. 2r7 Table 6.1 Structural features in the Tarella Silt

GENETIC MORPHOLOGIC ORIENTATION TYPE TYPE Extension . Mineralised . Pyrite Near vertical . Sand Non mineralised Near vertical Shear Polished Near vertical Polished, slickensided, Near vertical Ribbed PoIished, continuous Near horizontaL

~~6.4.2 Extensional structures~~

~~Pyrite infilled joints~~

These occur as vertical Lo sub-vertical discontinuities infilled with pyrite/marcasite (plates 6.25, 6.26 and 6.27). pyrite is present as continuous and rough mineralisation ranging in thickness up to 10 nm and averaging 5 mm. Àtthough there is considerable variation in the vertical extent of these joints they are commonly continuous from the top of TL3 to the shear zot:.e at the base of TL4.

~~Stereographic projection of poles to these defects indicate a mean strike of O25" (rigure 6-20), N i I r2z t-38 !¡rl ï~~

SAND INPILI.ED PYRITE/MÀIqSITE INPIT¿ED JOINÎS JOTNTS NuEb€! of æÀluêoenÈs, l 27g Nu[bcr of æÂlureûênEs - IA2 ì o STEREOGRÀPHTC PROJECTION FOR STRUCTURES I OCCURRTNG I L TN THE TÀRELLÀ SILT N F @ I 1-8å I + L2z

~~ORTENTATION OF MAJOR L H STRUCTURES I OCCURRTNG L2z I I IN THE BÀSEMENT I I I I ( +~~

ÀLL OgEN JOINÎ5 ÀLL SåEÀ.R JOIIIÎS NuD.ba! of ùaarurcoqt¡ r ¡¿la Nu¡bar ol DGrsur.Entr - 1Il Figure 6.2O Summary of the geometric relationships between structural features in the Tarella SiIt and their relationship with the regional structures. - 2r9 Floor mapping shows that the j oints are cont.inuous and

~~curved (Figure 6.2L) . ft is this curwature which leads Eo the broad scatter exhibited by the stereograPhic proj ection.~~

~~Normalised spacings for large continuous joints were analysed with a lognormal dístribution providing the best fit (Appendix 5). This anatysis gawe a mean spacing of~~

~~2 .9 met,res.~~

~~The pyrit,e inf illed j oints of t,en grade upward into sand infilled joint.s and down into shear joints.~~

In the trial pit pyrite infilled joinÈs also occur below t,he Tarella Silt Formation in üI49. These joínts are sma1I, gradíng upward int,o joínts with shear features and terminat,e against the Tarella SiIt Shear Zone. Towards the base, these joints terminate against IIIA5.

~~Sand infilted joints~~

Stereographic projections of poles indicate a wide scat,ter of orientations wit,h weak preferred orientation strike trends at, O3Oo, O95o and L2Oo (Figalre 6.20) . The joints occur as continuous vertical to near vertical discontinuities infilled wíth fine Eo very fine grained silty sand (P1at,e 6.28 and 6.29) . The infíI1ing is thíckest at the top of TL3 (maximum thickness 30 nun), wedging out dov¡nward. 220

ì\

~~SCALE~~

~~METRES~~

~~LEGEND DISCONTINUITY TYPE~~

~~sheor Polished I Slickens¡ded P Undifferentioîed co extention Minerolised - So¡rd - Pyrite PY ¡,lon minerolised~~

Figure 6.2t Relationships between Èect.onic structures in Ehe Tarella Silt Formation from mapping of an intermediate floor of the Trial Pit. - 22L The sand. infilled joints have irregular, rough and curwed surfaces at the top grading Eo planar at the base- The Iower parts of the sand infilled joínEs comrnonly grade into pyritic joint,s. Less freguently, the sand infilling grades into silEy c1ay.

The sand infilled extensional joinÈs are laterally cont,inuous for up to 5 lrl, terminating agaínst oÈher sand infilled joint,s or wedging out into t,he formation. Rarely, the joint.s cont,inue down t.o the Tarella Silt Shear Zone buÈ in atI cases terminaE,e agaínst t,his shear zone- enalyses of spacings of sand infilled joints indicate a lognormal daÈa distribuE,ion with a mean spacíng of 2.7 metres (Append,ix 5) . MeasuremenÈ and observatíons of these joint,s in Èhe Pit indicates a bímodal spacing. The joints occur in sets stit,h the se¡s hawing a mean spacing of 2.7 meÈres. These sets are about, 1 metre wide and contain a number of closely spaced sand infilled joinÈs.

~~Non-mineralised, exÈensional joints~~

These joint.s occur as vertical t,o near vertical, open defects which are continuous vert,ically between t'he top of TL3 and the shear zorle. Lat,erally E,hese features have been mapped for distances of at least 30 metres and a diagranrnatic represenÈa¡,ion of their occurrence is presented on Fig:ure 6.22. The joinEs have curved to planar surfaces (P1ate 6.30) and often hawe rib structures (as d.efined. by Hodgson, Lg6L) near t,heir terminaÈion againsÈ the Tarella Silt Shear zone (Plat'es 6 - 31 and 6.32) - 222 Figure 6.22 Lateral continuity and correlation of open joints in the lriat Pit

~~N .[~~

~~LEf¡ENO~~

~~opln Jo¡nt hon mlncr¡llsrd cxtenslonl [onllnultt --lnfÚrrd- torrrlellon b¡scd on cnd uall rapping ðnd i¡t¿rrcdl¡tc frca orpplng~~

Stereographic projections of normals to these features índ.icate a strong alignment of strike trend at 310" with relatively minor sets occurring at O9O" and 22O" (Figures 6.2O and 6.22). These minor sets were not observed consistently over the trial pit area and are laterally discontinuous. Statistical analyses of the spacing between these joints indicates a lognormal distribution with a mean

~~joint spacing of 2.4 metres (eppendix 5) - - 223~~

~~6 .4.3 Shear st.ruct,ures~~

A sufrrnarlf of the Elpes of shear joints occurring in the Trial pit is presented on Table 6.2. The dipping joints generally terminate againsÈ the Tarella Silt Shear Zone E,hough some cont,inue through and are offset by t'his zone.

~~Tal¡le 6.2 Surwnary of the t14>es of shear strucÈures in the Tarella Silt FormaEion~~

~~Genetic Range of Average Preferred Morphology Type Dip Dip SÈrike Trend Shear Near 0" Polished, striated slickensided~~

~~Shear 00 30" 15" 310 " Polished, slickensided 60" 80" 70" 310 " Polished, ribbed, slickensided 80" 90" 310 " PoIished, slickensided~~

~~Terminology used is accord.ing to skempton (1966), Morgenstern and Tchalenko (L967 ) and Tchalenko (1970).~~

~~Tarella Silt Shear Zone (Dip near Oo)~~

The Tarella Silt Shear Zone has been studied to define the nature of the: . stratigraphic location . regional and locat structural dip . surface features . rel-ationships with other structurat discontinuities . sense of movement. - 224 a) Stratigraphic location and description

The shear zone is stratigraphically located at the base of E,he Tarella Sí1È, FormaÈion and coincides with the clay-rich base of a coarsening up cycle as indicated by the natural ganrna geophysical response (Figure 6.13).

The morphology of Ehe Shear ZoI;,e appears to be tl4gical of those described elsewhere, consistíng of a cont,inuous principal shear plane surrounded by a number of smaller, discontinuous shear slices (Plates 6.33-6.35 and Figure

~~6.23) .~~

~~Figure 6.23 Tarella Silt Shear Zone types and geometric relationships of associated structures~~

~~PFINC¡PLE SHÉÂR PI-ANE~~

~~-1-~~

_ BASE OF ÎARELLA StLl Gffiic - - - - - LEGEND æan o€scRrFrror{ S€"ôlf (oppronrmþ) \ [email protected]€ Cd tG¡d s^ Sod hfllh ¿- atsi4l.ô diffiti¡¡ty ã Pyiil. fird t Opqi/mintlll¡E --- lrüû¡tÊ¡ disti!¡ty .lra Þ Poli.à.d 4y' eyn atw dbdlir¡ry Pg 6 PoLlh.d hd.l¡ñ - 225 The thickness of the shear zol:re (principal shear plus slices) ín Èhe deposit area ranges between a few míllimetres where only the principal shear surface is present up to 50 nrn where several shear slices are present.

~~The lateral extent of the Tarella Silt Shear Zone ís shown on Figure 6.24. This ínterpretatíon is based on the stratigraphic locatíon and. intersections in cored holes.~~

~~In the Tria1 Pit area, and from the inEerpretation of core, there was a gradation in the thickness of the shear zone from t,he northwesÈern side of the piE to the eastern and southern sides.~~

On the !ùestern side of the Trial Pit the shear zone consist,ed of three shear ptanes with the shear zone being 40 nrn thick whereas on the easEern and southern sides Ehe Shear ZorLe contaÍns only a single shear plane (equiwalent to the principal shear plane) Iess Ehan 1 nun in thickness. For V709, Ehe shear ptane was indistinct. Obserrzations on the shear zone within t,he TríaI Pit, conf irm Lhis trend though the minimum thickness of the shear zorLe was 10 rÛn and consisLed of two shear planes (principal plus one

~~slice) .~~

These obsen¡at,íons and measuremenEs suggest that Ehe shear zone is gradat,ionally thinner to the sout,h and east. This corresponds to a decrease in the Shear character of the zone (fewer slices, indisEínct, principal shear plane) and is possibly related to the marginal location of the pit with respect E,o Tr,4. 226

~~DETAILED SHEAR ZONE STRUCTURE ALONG E-W AND 246 oærnE N-S SURVEY LINES w E~~

~~I E.L. N (m) I I REGIONAL STRUCTURE OF THE 66 00- \ I TÀRELLA SILT SHEAR ZONE I Ê6{rÞ s.~~

~~I 65.900- l ) _---< I S N N I HORIZOôTÏAL SC¡LE E.L 2l (m) I 66 00 I e25oæomlt VERTIC¡L EXÁGGERÂÎOÎ{ f I'S 66 000-~~

~~o. to~~

~~65.900- E~~

I L.EGENO aEogË çþrml(Þh.¡rzæ (ìry-E) O.æ' (s-N) o.7? I Où.ffiioñ Þd¡f. AF¡c!. ñilrM d¡Þ O.93' Priñdpl. .h.r pl@ I AlGirtd diqtialy R.loliÊ diG¡on ol ffit / h.ltliEf .t¡¡ñhily PlT s1ÍE ÍRIAT, e f,OCation surv ey 1 I ì \ f ezeawrn See enlargement + l0 S!.la h ¡.l.as LEGIND lcnl.ur hlrrv¡l 0.1 ¡ 019 È dip ¿lrrtloi ol rh!¡r -0.95't02tó' n.htb¡hþ b.lY.!n I thr¡t.

~~LOCHIEL as'~~

~~DIRECTION OF MOVEMENT 55' )~~

~~_ ÀS IIIDICÀTED BY r-< I STRIÀTIONS ON THE rs' PRINCIPAL SHEÀR PLÀNE / \ + u- TARTLLA SIL'T sHEÀR ZONT - STRUTTURT OF PRINCIPAL OISPLAITTIEN'I 5HTAR.~~

~~Figure 6.24 Tarella SiIt Shear Zone summary of structural characteristics . - 227 b) Structural dip~~

As measured by detailed survey, the structure of the Tarella Silt Shear Zone in the frial Pit area is shown on Figure 6.24. The maximum díp is .93o and this is confirmed from two detailed section lines. This dip is consistent with the regional dip derived from interpretation and sunreying of all drillholes. c) Surface features

The shear planes are highly polished with only indistinct striat.ions. In order to determine Ehe nat'ure of undulation and. asperities, measurements were Eaken at a spacing of about 0.5 m along orthogonal 1ines. These lines are plotted and presented on Figure 6.24.

The average maximum dip of the principal shear zoÍLe is 0.600 and. 0.720 to the west and north respectively. From the line mapping it v¡as found that the principal shear plane of the Tarella SiIt Shear Zone exhibited an undulation with average variation in dip angle for the south-north líne being O.3o using a 0.5 m sample inÈe:¡¡al. d) Relationships with other stnlctural discon¡inuigies

The oLher discontinuiEies which are present in È'he Tarella Silt Formation are considered t,o be direcÈIy related to Lhe shear stresses and sLraíns leading to the formation of the shear zone. All Ehe extensional joinEs (pyritic, sand, open) terminate against this zone, t\gugh some shear joints - 228 were observed to displace the principal shear plane by up

~~to 10 rÍTn, within the shear zotte -~~

The shear zone forms a contínuous, clay-rich layer within the stratigraphic seç[uence. GroundwaÈer has been obserwed to accumulate above this layer in the end walls of t'he pit indícating t,hat the shear zoÍae acÈs as a relatively impermeaJrle layer between t'he overlying joínt'ed carbonaceous silE and the underlying sandy sílt'

~~e) Sense of movement~~

In order to evaluate the direcEion of movement, measurements v¡ere made on st,riaÈions occurring on the principal shear plane at a number of locations ín Ehe pit' TT|Io distinct and, consistent orientations were present towards 274o and 243o as shown on Figure 6.24. The 2740 set is the better develoPed-

~~Taretla Silt Formation - slickensided and polished shear joints (Reid,el shears dipping at between 0" and 3oo)~~

These joints originate on the principal shear zorae and are continuous upward for up to 4 metres' They become progressively less distinct upward and generally terminate on vert,ical or near vertical shear joints'

Laterally EheY are disruPted bY inÈersecting open joints but individual shear joints were obsen¡ed to cont'inue for at least 5 metres. 229 These joints are considered to be equivalent to the Riedel shears of Skempton (1966). They have a strong preferential orient,ation with an average dip and dip direction of 15o towards 31Oo (Figure 6.2O) .

~~Tarella Silt Format,ion - slickensided and poIíshed shear joints dipping between 600 and 80o~~

~~Shear j oint,s of Ehis geometry occur in Esto stratigraphic horizons, at t,he top of TL3 and at the base of TL3 ' a) Top of TL3~~

~~Shear joints of this orientat,ion are rare. They occur as striated and slickensided planes contínuous for less than 2 m vertically and less than ¿ m latera11y (P1at,e 6.36) . b) Base of TL3~~

~~Ttrese joints occur as ribbed and slickensided planar features which have been observed to continue for up to 10 metres Iaterally.~~

Vert.ically they are discontinuous, terminaÈing against the Tarella Silt Shear Zone at the base and against' other steeply dipping shear joints upward. Rib features on the surface of these joints (P1ate 6.37 ) are indicat'ive of lateral movement along' these planes, probably in association wíth movemenEs across the Tarella SiIt' Shear zone. - 230 As indicated on Figure 6.20(d) these shear defects have a preferential dip and dip direction of 70o t,oward 310o. However, striations on the principal shear plane indicate a direction of movement towards 2740. It is likely that Èhis r¡ras probably the latest of a number of movement's along the principal shear plane with the changes in stress directions resulting in the formation of E,he rib st'ructures.

~~Tarella Sitt Forma!ion - polished and slickensided shear joints dipping at bet'ween 8Ooand 90o~~

These are the most colwnon È]æes of shear joint in the Tarella Silt Formation. They are 1aE,era1ly conEinuous and grade verticalty upward for up to 4 met,res. Often the upper part of Ehese joints are infilled wich pyrite.

These joints hawe been obserwed Èo intersect and offset the Tarella Silt, Shear Zorre by up Uo 10 ÍEn (Figures 6.23 and 6.24) . iloints of Ehis tllge are considered Èo be equivalent to t,he conjugate Riedels of Skempton (1966) .

~~6.4.4 Origins of Eectonic structures~~

~~The Eect,onic f eatures obsen¡ed in the Tarella silÈ FormaEíon can be divided into t,hose associated with extensional stresses and Ehose associat,ed with shear sÈresses.~~

It is apparenE from the facies distrj-bution in Èhe study area and from observations by Stuart, Í969) that Ehe sediments in Èhe St Vincent Basin drape over normal faults in E,he underlying basement (Figure 6.25) . It is generally - 23]- accepted that these normal faults have been active intermittently since basin formation in the Middle Eocene to present.

Extension along the major principal stress axis would be conducive Èo the presence of these normal faults- This tension is considered to be related to Ehe relatiwe movement of Ehe more' mobile Adelaide Geosyncline in relaLion to the stable Gawler Craton-

~~At leasE Ehree sets of extensional joints are ídentified based on the Elæe of infilling material (sand, pyrite and open) .~~

Gramberg (1966) deweloped a joint classification model and proposed thaÈ when ubríCÈIetr materials are subject' to low confining pressures a:cial fracÈures are developed paralle1 t.o Ehe compressive sEress direction. The general character of Ehe extensional joint sets suggests t,hat Ehey were formed under direcÈ t,ensile s¡ress aÈ' low confining pressure.

Two major orienEaLions are identified for the extensional joints in t.he t,ria] pi¡ area. FirsE, a NNE-SSW trending seÈ of pyrite infilled joints (Figure 6.20(a)) and, second, a NNW-SSE seE of open joint,s (Figure 6.20 (c)). The open joints are obse:¡¡ed to intersect Ehe pyrite infilted joints and on this basis are considered to be younger. The sand infilled joints appear Eo be relaÈed to the pyrite ínfilled j oints . - 232 It is apparent from both the orient,ation of the two sets of extensional joints and the relationship with each other that there have been two different, compressive episodes.

The first of these compressive episodes had the major stress arcis aligned NNE-SSÏÍ and Ehe second episode a NN!{- SSE a:cis . These two Erends paralIel the strike of major basement st,ructures in t,he st,udy area. In addiEion, from the orientation of Èhe shear features in the area it is apparent Èhat shear sEresses were related primarily to the first compressive episode. There is also a secondary set of shear features parallel to the extension feat,ures associated with the second episode (refer to Figure

~~6.20 (d) ) .~~

~~This is supported by measurement of striations on the principal shear plane in the Tarella Sílt Shear zol:e which indícate two directions of mowement on the shear plane.~~

~~Fígure 6.25 presents a possible model for the formation of st.ructural features presenE in the Lochiel area. This model is based on the first of Ehe compressive episodes.~~

~~NotwithsEandíng Ehe affects of tectonic movemenÈ on the formation of structures, there was also likeIy t,o be a secondary sEress fietd associated with drainage and compaction of the sediments. 233~~

~~locolion of the lriol pit NW (see enlogement SE below)~~

~~less compelenl more compelenl lenslon ------+ Exl ension # BASEMENT E xlension NORMAL FAULTS~~

~~Co esston~~

~~Torello sill pyrile ond sond infilled exlension joints~~

~~Tension TOP OF WAg Tension~~

~~WARRIND| SILT BUMBUNGA SAND~~

~~BASEMENT To rello silt Sheor toints~~

~~Compression~~

~~Figure 6.25 Model of stresses and the resultant structures present at the Trial Pit site - 234 6.5 SlmthesJ.E of, geological proceEaeE and engineerLng propert,ies for tbe Tarella StIt Fo¡mation~~

~~6.5.1 Strengt'h of the Tarella SiIt Shear Zone~~

For the purposes of determining the properties of t'he Tarella Silt Shear z)ne, a major program of sampling and geotechnical laboratory ÈesEing hlas undert'aken' Most sam¡l1es were recovered from drillcores taken in the study area but some samples were also collected wiUhin the Trial PiE,.

. Testing procedure and timitat'ions The t.est,s associated with Ehe Tarella SilE Shear Zone were carried.outbyCoffeyandPartnersPtyl,td.strength testing of t,he shear zone l¡tas by the direct shear method and.t'het'estingproced.ureisdet'ailedinCoffeyand

~~Partners Pty Lt,d (1988) .~~

A number of aspects inf luence t,he accuracy of results obtained from d.irect shear tests including; . size of Èhe test sample. A number of different' sizes of samples were t,ested including 100nun, 900nrn and 500rnm' . accuracy of the test, procedure- From deuailed mapping of t.he trial piE exposures it was concluded that' the Tarella SitE Shear zone conÈained a principal shear plane surrounded by a number of other associated shears.ItisanticipaE'edthaEÈhepeakstrength resultsareaffectedbythealignment'ofthis principalshearplanewíthinÈheshearapparatus. (Refer to Plate 6.38 for example of how 500nun samples were collected in the Trial Pit.) - 23s orientation of the shear plane. As discussed in Section 3.4.3 page 64 there is tikely to be a difference in shear strength based on whether the shear Eest is performed across t,he dip d.irect,ion or along the dip direction. An attempt was made to assess the signif icance of t,his aspect on samples taken ín the t,rial excavation however results were inconclusive-

~~Test results~~

~~A srunnary of the location and results of t,he direcE shear t,ests undert,aken on the Tarella Silt Shear Zone is presented on Figure 6.26.~~

~~No lat,eraI trends in strength variation are recognised and it is considered that any sma1l variation which may be present is clouded by the accuracy of Ëhe Ëest results-~~

~~6.5.2 Porewat,er movemenÈ~~

A synÈhesis of the effects of pumpíng on the piezometric profile for E,he triat pit area and the major geological features is presented on Figrrre 3.5 (page 59). A number of Iithof acies wit,hin the seç$rence aE E,he Èrial piE sit'e developed near vertical porewaËer pressure profiles indicating a relatively high vertical permeabiliEy. Vlithin

Ehe coal bearing sequence these included the : . Tarella Silt Formation al¡owe the shear zoÍ:.:e - probably due to the presence of vert.ical open joint's and vertical sand infíI1ed joints - 236 . l{arrind.i Silt Formation, F zorLe and FG inEerseam Sediments - these are mosgly sand and are therefore Iikely t,o be free draining. The exception is vtAT which is silty bu¡ this líthofacíes is characterised by intensiwe biolurbagion which would provide drainage pat,hs. Also of significance are the relatively impermeable zones coinciding wieh the : . TareIIa Silt Shear Zone - The slrear zorte t,runcates the vert,ical extension struclures above it and in Ehe piÈ free water rata out from this zone down the face (note the signifícant slep in piezomet,ric profile across the

shear zone illusÈrat,ed on Figure 3.5) . . G seam coal - In t,he trial pitr area the G seam is relat,iwely Slructureless unlike Ín other areas of the deposit where the joint,s indicate E,hat there has been at least 2 períods of extensional deformation- . Bound.ary betrween t,he Condowie Silt Member units CN1 and CN2 - This is t,he sit,e of a clay-rich shear zone-

As indicated on Table 6.3, vertical penneabilities for Ehe Tarella SilE Shear Zone (TrJSz) are extremely low- This is probalrly due to t,he alignment, of platy minerals along Èhe shear plane. Table 6.3 Summary of estimates of vertical permeabilities and coefficient of consolidation (after Coffey and Partners Pty Ltd in ETSÀ 88/L3).

~~Stratigraphy Vertical Vertical Permeability Coefficient of Consolidation cm/sec x 10-8 m2/year~~

~~cN1/CNz 3 135 G seam coal 5 90 TL Shear Zone o.25 I TL above shear 2000 950 237~~

39'!6 C¡lO ¿'[á' 4soñ LEG€NO OV57l c's!6'rlt' o vs97 60âeiol-E dIAn a xuMm !l.tñ oEffi E€r¡{ 3¡Fa€ C¡ 6O COHeSIOI lfl lPd ô d.22' ffil¡ oF mrcf,d¡ 625OæOótl 9€Æ ZO.€ 0a1A wtlH æX cLÂY oafa *ffltorJÏ &x -Ä¡¡ va72 3¡t{ ñ C. 16 t. la' lHñ C'O ø.2Ç

~~avñr !Sgr c.20 6r tz~~

~~fããEä-l I 6.æl E*il avT'lrúzm8:8. v604~~

~~I I I I v634 I I I I I I I I 6 245øOñ¡ I av7-26 I I I a2,4Ã C¡O ,. to IN IAIAL ñf ov749~~

~~718 l2r.añ I lc.o I lø.p.¡ |~~

LECEND X v735 - 17.to - 17.678 O vTlr - 52.90¡ . v7l8 - 19.60 - l9.9OE (Ð v748c- 25.ó - 26.0t A vrts - 16.9Bm O v712 - 29. 18 - 29.1¡ I v720 - 21.64m o v7¿l - 21.O9 - 21.29û o lt725-22.8-22.9n + v757c- 16.25 - 46.55r It v126-25.9-26,078 El v5e7 - 12.77 - 72.87ù vt32-52.9-53.0n v v604 - 31.65 - 11.75¡ + q¡c¡fEÍb¡L A vit9 - 22.84 - 21.14ú + v634 - block 6ãûpl. saople o 0 vtró - 29.51 - 29.1h o v6l4 - block À ! vó34 - block daople 6 a vr43 - 3r.05 - 37.l0r O t!IA t-E * Ø upPaEdJôo C.2orFc l,.r" Ê d + ul + I o LOWÉR Bd.,l\o Ø tra c'.orPo 6r. o 50 Residual Friction Angìe

~~NOßMAL sTREss (kPo )~~

Figure 6.26 Tarella Silt Formation Location of test sites and summary of results of strength testing (testing performed by Coffey and Partners Pty Ltd, 1988). - 238 CEAPTER 7.0 SII!ûMARY OF EÑGINEERING GEOIJOGY OF TEE I..OCEIEI. CO.ãJi DEPOSIT

~~This section sunrnarises the engineering geological characEeristics of the Lochiel CoaI oeposit'~~

The aim is to categorise the d.eposit int,o discreEe regimes which could provide Èhe basís for slope stability analyses' A flowpath for defining regimes is provided on Figure 7 -L- The regimes are d.ef ined based on díf ferences in the following: 1. Intact maLerial characteristics- These are primarily the thickness of the various sed.iment t)æes (see chapEer 5.0) and the thickness of

overburden (Figure 7.2) . 2. Hydrogeological characteristics. The criteria used are Ehe percenEage and characEeristics of clean sand facies in the overburden (Figure 7.3a and b) and the thickness of the Tarella Silt Formation (Figure 6 -L2) . These criteria are appropriate because ÈogeÈher they control the rate at which Èhe sediments can be dewaEered/depressurised' For the Tarella SilE, Format'ion an arbitrary t'hickness cut,-off of 5 metres is used- 3. Near trorizontal weak zone characteristics' In the overburden these occur in the Tarella SilE Formation and GH int,erseam whilsE below the coal seam(s)theyarepresentwit,hintheCondowiesilt

~~Member (Figure 7.4) .~~

~~The regiimes are confined to Ehe area within the 6:1 strip rat,io (cubic metres of overburden per ton¡te of coal). - 239~~

~~7.L Descri.Pt,l,on of regLnes~~

~~The locations of the Regimes are d.ef ined on Figure 7 .5 ' Figure 7.6 also shows the lateral stratigraphic relationships between the Regimes.~~

Regime 1 Located at Èhe northern end of t'he deposit'. The depth to the base of coal averages ?0m, with the sediments comprising predominantly silty facies of the lrlarrindí silt and. Tarella SíIt Formations.

~~The Tarel-Ia silt FOrmation contains the shear zone and also the TL1 aqr:ifer-~~

Regime 2 Occurs in the north-central and eastern sections of the deposit. The overburden comprises mainly silt' and clay and has the lowest percentage of sand within the deposit. The boundaries are defined approximately by the 0.5m clean sand isopach but also by the 70m owerburd.en Ehickness isopach to the north. Depth to the base of coal averag:es 60 mefres '

~~The Tarella Silt Shear Zone is present within this regime. 240~~

Regime 3 Is tocated in the central-vÙestern area of the deposit' Its boundaries are defined on t,he basis of a marked increase in depth to the base of coal' this increase in dept,h is associated wit,h the regional tilting of the basin. The sediments are mainly silts and clays and include aquif ers in E,he rÍarrindi sitt Formation and'KooliataCoalMember.ThedepthÈoÈhebaseof coal averages about 65 metres.

~~The Tarella Silt shear zorLe is present withín this regime as is the weak zorle in the GI{ int,erseam.~~

~~The wA4 aquifer is also present with the thickness of clean sand increasing to the wesE. In addition, the TL1 aquifer is Present.~~

Regime 4 occurs in the cent,ral and southeastern sections of the d'eposit'.It,isanareawitharelativelyuniform depth Eo coal and the souÈ.hern boundary is defined by E,he lat,eral contact, beEween the Tarella Silt FormaEion and the sandy facies of the Trlarrindi silt Formation' The depth E,o Ehe base of coal is about' 50 metres and the overburden cofÌ.tains a mixÈure of sandy and silty facies of the l{arrind.i silt Formation and t'he silty Tarella Silt Formation. The Trial Pit is l0cated in this regime.

~~This Regime contains the Tarella Sílt Shear zone. 24L within this Regime aquifers include GH, FG, VÍ41, T¡IA4 and vÍ42.~~

Regime 5 Is locat,ed in the south of Èhe deposit. It is the only regime which contaíns no Tarella Silt FormaEion- The owerburden comprises mainly silu and sand. The dept,h to E,he base of coal is of the order of 35 metres. 242

~~ENGINEERING GEOLOGICAL INTERPRETATION~~

~~Thickness of clean sand in overbu¡den~~

~~> 1.0 metres < 1.0 meres~~

~~Thickness of REGIME 2 Tarella Silt; Presence of sheals~~

~~< 5.0 metres No shea¡ zone > 5.0 metres, Shear zone pfesent REGIME 5~~

~~Thickness of Overburden~~

~~> 34 metres < 34 metres~~

~~GEOGRAPHIC REGIME 4 LOCATION~~

~~V/est North~~

~~REGIME 3 REGIME 1~~

~~Figure 7.t Flowpath for defining highwaLl stability design regimes. 243~~

~~24o oflnrrf ?4r o@rnf Ji @omF I N I I I I I c l I I I I o +625oooomN I (ìÀ I ( I I I I I / I~~

~~\ò ale c' ^\ a + {s24a æoml't~~

ô\

~~I LOCHIEL I /~~

~~+ + f s24ooæmN~~

~~SCALE~~

~~KMS I o 2 3KMS~~

~~Figure 7.2 Thickness of overburden 244~~

~~24ooæmE 245 ooomE / N I \ I I I l I I I I I f e25oooomN~~

~~DISCONTINUOUS I ANo~~

~~o + f c24o æomN~~

~~o o\~~

~~0 o l~~

~~I LOCHIEL I I / + f s24oooomN~~

~~SCALE 3KMS KMS I o 2~~

Figure 7.3a Percentage of clean sand in the overburden o I uI É. o LATERAL AOUIFER UJ (L È Lrl STRATIGRAPHY CONTINUITY DISTRIBUTION CHARACTERISTICS OF AQU]FER SAND DEPOSITIONAL AVERAGE LATERAL É ENVIRONMENT GEOMËTRY THICKNESS VARIABILITY z SURFICIAL SEDIMENTS s N EL E Holocene locustrinè tobulor l'5 high lrl sl m) 80 ' k l Pleistocene o 70 estuorine shoestring 2 very high TARELL ,, t, tt chonnel ,t It ,, silt) tt ,, tt ó0 Oligocene tt ,t ta estuorine-chonnel, vorioble l-lO hish )r ,t , woshovers 50 . deltoic wedge 3 moderote locustrine shoreline elongote 2 moderote -? KOOLIATA COAL (0 (cool, silt) sond, shoreline elongote l'5 moderote locustrine -@ E 30 coNDowlE ,t t, t, It F tt tt I (r SILT 20 - meonder belt tobulor lO low f luviol l¡J Lote silt, tt ,t ,, t, @ N) t- Eocene tt t, ,t t, ,, .@Þ. È f ine sond ) t, tt t, ,t It ..@ crèvosse sploy wedge O'5 high (¡ locuslrine tt tt ,, tt tt t, 10 I NANTAWARRA SAND 0 fluviol system tobulor l5 low -crevosse sploys (sond, silty sond, cool) :-r -meonder belts f luviol 20

~~CAMBRIAN undif f erentioted cloy, PRECAMBRIAN quortzite siltstone~~

~~Figure 7.3b Summary of characteristics and distribution of clean sand facies occurring in the Lochiel CoaI Deposit. 246~~

~~36 óóó'äE 24offiE 24rffiE / N I I I I l l I I I / / f e2SoooomN~~

~~fL2~~

~~o73t~~

~~o757~~

~~0726 ûS f ceeo æom'~~

~~74lO f,S' 743 7490 o7420 74fJ0 7no~~

~~I LOCHIEL~~

~~/* + f e zeoooomtt~~

~~SCALE~~

~~KMS I o 2 5Kf,lS~~

~~Figure 7.4 Location of near horizontal weak zones 247~~

~~I I I I I I I o I / I I I 2 I Q,3 o~~

~~o 4 AO~~

~~I o LOCHTEL Nl 5~~

~~LEGEND SCALE fT Basement o 1 2 34 Fault a Trial depresr urisation site KILOMETRES O Geotechnical fully cored hole~~

~~Figure 7 .5 Location of highwall stability design regimes. REGIT.ÍE 1 î:l REGII.{E 2~~

~~rRIÀL PIT REGII{E 4 REGII,ÍE 3 REGIUE SITE ul ..-¡.1.t .útñ.a o ,{ Ð a, A~~

~~-to~~

~~-20~~

~~ÍARELLA SILT -30~~

~~WARRINDI SILT~~

~~-40 G SEAM~~

~~H SEAM -50~~

~~-6() N È @~~

~~CONOOWIE SILT -70~~

~~-EO~~

~~-9()~~

~~Figure 7.6 - too Typical vertical lithological profiles and fãteral lithological relationships between Highwall Stability Regines - 249~~

~~CEAPTER 8.0 SUMMAR:r AND CONCLUSIONS~~

As stated in the rntroduction (Section 1'0' page 1) the primary aim of thís thesis is to bring togeEher the related of areas of geology and engineering within the context highwall slope st'ability analysis for soft brown coal mínes.Theapproachhascentredon.theinuermediauerole oftheengj.neerínggeologistwhosemaínobjectivesrelate totheprovisionofpred'ictiveinformationonthekey facEors affecting slope stability'

~~follows The specific object'iwes were specified on page 1 as~~

~~1 DevelopmodelswhichcharacEeriseEhekeyengineering geological factors in terms of depositional and~~

~~Eect,onic Processes -~~

~~of its global 2 Classify the study area in t'erms tect'onicanddepositionalsettingt'husprowidinga basisfort'ranslatingtheresultstroothersoft'brown coal miníng sites.~~

~~3 Contributetotheund.erstand.ingoft'heinteractionof tectonics, eust'acy and d'epositional processes in the St Víncent Basin-~~

~~The l,ochiel Coal Deposit, which was used as the example, as can be classified in geological and' engineering terms~~

follows : - 250 geologicalsettíng-midplatecontrinent'alpositionwj.th deposit.ional environmenEs controlled by the interactionofmovemenEabouttectonicsE.ralcturesand by eustatic sea level changes' The depositional environmentsexaminedareEhosecharacterist'icof cont,inental shelves, including swamps' marshes' estuaries, d'eltas, lagoons, barrier bars and f luvial '

~~engineering properties - fine grained' saturated soils and weak rock characterised by early deformationar structures.~~

~~Lhe Thís classification provid.es Ehe basis for translating resultsobtainedEosimilarminingareaselsewhereinthe world.~~

The intention of the strudy r^Ias t,o analyse in detail key fact,ors which are characteristic of soft brown coal mining or areas rather than those generic to ot'her mines excawat,ionsinweakrockandsoil.ThesÈudyfocusedon analysing and' modelling t'he geological processes cont'rollingt'hekeyengineeringgeotogicalfact,orsof defect,streagthaadgeøetryandporewatermovenent'

TheapproachtakentoanalyseE'hesefactorswastoexamine t,herelat,ionshipbetweent'hekeyengineeríngproperties of relat,ing uo these fact,ors and Ehe geological properties the sediments- DaEa included drillholes' geophysical borehole logs, core, and Triat Pit' mapping for Ehe geologicald'ataand'testsoncoreand'Pitsamplesforthe engineeríng data. 25L 8.1 GeologJ-cal Procesaes

~~The sedimentary seçluence in soft brown coal mines on' midplate cont,inent.al margins is controlled by an interaction between t,he affectrs of eustacy and tectonics'~~

Tectonic movement largely controls the arnount of sediment' input, int,o Lhe area whilst eustacy controls the position of t,he shore face and therefore the specifíc depositional environment. The Lochiel Coal Deposit is an example of this tlpe of interaction.

8.1.1 Bumbunga Sand Formation fniUial basin formation commenced in the Middle Eocene after a period of weathering and erosion. The major cont,rolling struclures were two north-souÈh trending normal faults, the Ardrossan to Èhe west and the Whitwarta to the east.. These formed a half graben whose floor was t'ilted to the northwest.

Initial sed.imentatior¡. was fluvial with the major source of sed.imenE, being f rom t,he west,. Regional correlat'ion of this fluvial system indicates thaE the palaeoslope !Ías ínclined toward.s the north, flowing inEo the Pirie Basin. These fluvíaI sediments graded into lacustrine silEs and clays to the south.

These silts and clays are assigned to the Condowie Sitt Member of the Bufribunga sand FormaÈion. A characteristic feature of these sed.íments is the strongly cyclic, upward -252 coarsening nature. This is considered to represent a sed.imentation response to intermittent tectonic movements along the major faults. No evíd.ence of marine influence tras been distingrrished f or these sediments.

Towards the end of this basal deposit'ional epÍsode there is strongevidencethatseveralothersourcesof sediment became active. These emanated from the Ardrossan Fault, to the west and atso from the Nantaün^Iarra High to Ehe sout.h.

In add.it,ion, Eowards the end of thís depositional episode peatswamps formed and lead to the accumulaÈion of the three major coal seams. These seams are separaEed by inEerseam sed.iments originating from smaII delEas which entered the area from the wesÈ.

~~The Bumbunga sand Formation represent,s a tlpical basin fitl sequence culminating in the formatrion of thick coal sealns, the Kooliata CoaI Member-~~

~~8.2 WarrLnd,L StlÈ and Tarella StIt FomaÈions~~

The Br:mbunga sand Formation is overlain by a paralic sequence of carbonaceous silus and fine sands with fewer interbeds of non-carbonaceous sediments. DeposiEion spans the l¡ate Eocene to the Early Oligocene and iS Characterised by a series of major Eransgressive/tegtessive episodes' In broad terms the deposit,ional controls included : - 253

~~maj or sources of sed'iment' f rom the north' sout.heast and. to a lesser extent the west,~~

~~continued tectonic mowement on the north-south trending faults but with signíficant influence from movement about east-west trending faults,~~

~~a migrating shoreline characterised by micro tidal ranges,~~

~~a largely stationary body of water in the centre of the area but whose depth waried over time'~~

~~Sed.iments are tlpical for cont,inental margin coal settings and relat,e to shoreface, barrier bar, lagoonal, estuarine and deltaic environments subject, to tidal influence.~~

~~8.3 Key engLaeering geologfcal featsureE affectJ-ag slope etalrtlttY~~

~~The key features affect'ing slope stability in soft brown coal mines on midplate continentat margins hawe been identified as:~~

~~Defect strength and geometry, factors affecting porewater movement'~~

The analysis has demonstrated that whilst each of these f acE,ors is of importance to slope stability in the study - 254 area, it is the near horizontal shear zones which are the single most imPortant feature.

These defects are generally concordant wit'h the bedding in the surround.ing sediments and continuous over large areas. vühere present their strength and geomet,ry are Iikely influence significantly Èhe design of the highwall and the direction of mining.

As shown in this study and illust,rated on Figure 8.1, the locat,ion and. formation of t,hese defects is related to the bound.aries of signif icant cyclic depositional event's ' The d.efects are locat,ed at the base of coarsening upward cycles in close stratigraphic location to the Eop of the previous cycle. fn part,icular the naÈure of the surrounding sediments suggests an aqueous depositional environment typicat of a lacust,rine or esguarine basin infill sequence'

DeEailed examination of the defect's associated with the Tarella Silt shear Zone indicaCes that Lhey are characteristic of shear defects. The sÈresses controlling their format,ion appear to be related to both compaction and the tectonic tilting of the deposit area. T:fpically the defecÈs have residual friction angles as low as 7o -

Furthermore, given that cyclicity is characteristic of midplate cont,inental margin deposits, it' is tikely t,hat such features are present elsewhere in similar areas- ZONES GEOLOGICAL AND GEOPHYSICAL CHARACTERISTICS OF SHEAR

~~DEPOSITIONAL CYCLES PROCESS NATURAL GAMMA DENSITY ENVIRONMENT~~

~~API gm/cc I 20 50 70 90 t4 1.6~~

~~inf itl Boy~~

~~I z\r zv -v -v ^-SHEAR É. ^., N) r¡pple zone Tidol flots (¡ul -¡ I äE l- l I infill $;13õol F LOIô(l,l À! I .n ,ü lrl z\, ry -\- SHEAR PLANE- ^, ^, ^, morsh Suprolidol - f lols~~

Figure 8.1 Summary of geological factors controlling the location and ãngineäring-properties of near horizontal shear zones. - 256 the rate These defects are also a major factor determining at which the fine grained sediments can be depressurised' There are two aspects invotved' First' the sheared materialactsasarelativelyimpermeablebarrierto vert,ical drainage and, second' some of the extension verEical structures associated wiÈh the shear zorLe act as drainsforthefinegrainedsediments.Theresultisthat like a the fine grained silts above the shear zorLe behave fract.uredrockmassbutthedrainageofporewater vertically is restricted by the shear zone'

~~and 8.4 The relatLousbip betweeu Èectonics' eustacy deposr.tr.oaat proce'ae' aud engi.neering geological factors affectLag slope stalcfltty~~

~~Inessencethesequenceinthest'udyareaencompasst'wo major phases of transgression/highstand/regression'~~

a TST The first phase produced an allocycle equating Eo (NantawarraSand,DarnleighParkSandandCondowiesilt. Members) and HST (Koo1iat'a Coal tvtember) sedimentary sequence.I,ithologicallythesedimentsassociatedwith Lhisphasefineupwardsfromgravelandsandatthebaseto siIt,, claY and coal at the toP'

~~and The formation of the coal was eustat'icalIy controlled setting equivalent t'o a higtrstand with the depositional the south charact,erised by a barrier bar syst,em located to of the studY area.~~

IìIithíntheoverallsystem,tectonicswasthemainfactor in cont'rolling the seg:mentation of 1íLhof acies and - 25'l particular the clean sand Iithofacies. Tectonic episodes alsoprod'ucedthefinegrainedcoarseningupwardcycles which contain weak zones'

ThesecondEransgressiveepisodesuperimposed.marginal time' the marine conditions on the area' During this SourcesofsedimentandthesitesofEectonicmovement appeartobesimilartothefirstepisode.Thisprovides of marine the unique opportunity to contrast the effects influenceonthekeyfactorsofporewatermovementandweak zones agaínst a non-marine influence'

autocyclic The results of the analysis ind'iCaEe that the depositionalprocessesforthesecondepisoderemained essentiallythesameasthoseofthepreviousepisode.In particutar, the location and. characteristics of clean sand Iithofaciesandthecoarseningupwardscyclescontaining weak zones are similar for both episodes'

the regional The conclusion is that whilsE eusLacy Controls depositionalsettingandthecond'it,ionsnecessaryforcoal cont'rol over the f ormation, tectonics ís the maj or and depositíonal processes and the sesfment'ation is that characterístics of lit'hofacies' The implication withinthestudyarea'tectonicsratherthaneustacywas themajorgeologicalfactoraffectingthesubsequent weak zones' movement of porewater and' the properties of A1ley, N.F., Lg6g. The cainozoic history of the mid north of south Australia. M.À. thesis, uni. of Adelaide'

~~(Unpublished) .~~

~~Alley, N.F., Lg73. Landsurface development in the mid north of south Àustralia. Trans. Roy. Soc. S. Àust., VoI. 97, P. L-L7.~~

~~ÀIley, N.F., 1990. Personal communication' Senior Geologist, south Àustralian Dept. of Mines and Energy.~~

~~Ànderson, E., 1950. Whitwarta COaI Occurrence. south Àustralian Dept. of Mines and Energy Min' Rev', ÀdeIaide, No. 90, P. 1-47-1-51-.~~

~~Ànderton, R., 1985. Clastic facies mOdels and facies analysis. In Brenchley, P.J. and WiIliams, B.P.J.,(eds.)Sedimentology.Blackweltscientific~~

~~Pubtications, P. 3t-47 .~~

Banerjee, I., 1989. Tidal sheet origin Of the transgressive Basal Colorado Sandstone (albian): a subsurface study of the cessford. Field, south Àlberta. ButI. Canadian Petroleum GeoI., Vol. 37, No. 1, p' t-Ll '

Barden, L., L972. The relation of soil structure to engineering geology of clay soil' Jour' Eng' Geol' VoI. 5, P. 85-102. - 259 Beerbower,,J.R.,t964'cyclothemsandcyclicdepositional mechanismsinalluvíalplainsedimentatíon.In Merriam,D.F-,(ed')Symposiumoncyclicsediment'ation' 1' p' 3t-42' Kansas GeoI. Surv' BuIl'' vol' 169' No'

~~Bishop, 4.W., Webb, D'L' and Lewin' P'I' ' t966' ashford Und.isturbed samples of Lond'on CIay from t'he contrnon shaft': Strengtrh and effectiwe stress relat,ionship-Geotechníque'VoI'15'p'1-31'~~

~~Bolton, M., 1984. A Guide Èo Soit Mechanics' MacMillan Pr:blishers Ltd- London'~~

~~margins Bott, M.H.P., LgT1 Evotution of young continental andformationofshelfbasins.Tect'onophysics,vol. L!, p. 319-329.~~

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