VOLCANIC SUCCESSIONS MODERN AND ANCIENT A geological approach to processes, products and successions R. A. F. CAS Department of Earth Sciences, Monash University J. V. WRIGHT Consultant, Sheffield, England iv ^^'«n London ALLEN & UNWIN Boston Sydney Wellington
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V O L C A N I C SUCCESSIONS
M O D E R N A N D A N C I E N T
A geological approach to processes, products and successions
R. A. F. CAS Department of Earth Sciences, Monash University
J . V . W R I G H T Consultant, Sheffield, England
i v
^^'«n London
A L L E N & U N W I N Boston Sydney Wellington
CONTENTS
?^IEFACE ix LIST OF TABLES xix
-' " / L
I H A P T E R O N E
miroduction to fades analysis ' zoleanic terrains 3
2.6 Strength 27 2.7 Fluid flow character .• ^ 27 2.8 Further reading - 30
C H A P T E R T H R E E
Volcaniclastic deposits: fragmentation and general characteristics 33
Initial statement 33 3.1 Introduction 33 3.2 Fragmentation due to magmatic
explosions 34 3.2.1 Explosive fragmentation from a
sealed, near-surface magma chamber or conduit 35
3.2.2 Explosive fragmentation of a vesiculating magma erupting from an open vent 36
3.3 Magma mixing as a means of triggering explosive eruptions 40
XI
xii CONTENTS
3.4.2 3.4.3
3.4.4
3.4.5
3.4 Phreatic or steam explosions and phreatomagmatic eruptions 3.4.1 Interaction with ground water
Interaction with surface water Lava flowing into water or over water-saturated sediment Pyroclastic flows moving into water or over water-saturated sediment Magma rising into a hydrothermal system
3.5 An introduction into the products of pyroclastic eruptions
4 W AciTt^tinnfirv laml i l 126 7.2 Fluidisation 1 79 PiM /\<. LI L l l v J l l c i l V l a j T i i i i 4 I) IGir ihpr rF^aHinQ" p,V I Ul l i i c i icauiiig, 126 7.3 Pyroclastic flow units and grading :
7.3.1 Thickness 86 87
( 11 A I ' l'ER SIX 7.3.2 Basal layers 7.3.3 Vertical grading
88 88
7.3.4 Gas segregation structures 90 i / ( ' ( / f I I I J/y 1 ULitlJLiV J l l l - l - U'C'jyL/oLvo
///( // ( / UpilUflb
129 7.3.5 Lateral grading 193 i / ( ' ( / f I I I J/y 1 ULitlJLiV J l l l - l - U'C'jyL/oLvo
6.4.3 Mechanisms and dynamics 148 7.8 Transportation and grain-support
i\ s Sub-nlinian I I l i e i l r l_/ 111 11U.1 1
151 processes in surges 205
(i () Ultranlinian 152 7.9 Depositional processes in surges 207 1 p , 1 F v_.' 1 1 1 n ^ l\ 1 \I u\c'AV\VAX\ 1) / V U l U C l i l l c l l l
153 7.10 Facies characteristics of surge deposits 209
(i S Surisevan and phreatoplinian 156 7.10.1 Geometry 209 | | | | 1 i P L I l l v P ^ y Cll 1 Ml * ^ AAA. -A* — 1
6.8.1 Surtseyan activity and deposits 157 7.10.2 Grainsize 209
6.8.2 Phreatoplinian activity and 7.10.3 Sorting 210
deposits 158 7.10.4 Shape and vesicularity 210
t-i X 3 Mechanisms 162 7.10.5 Composition 2̂ 11
1, 0 ni>;i'il silicic air-fall ash lavers I t / l y i c y l d l o l l i x - l e - £111 i c i i i c i i i i i i c i ^ A - i ' . '
163 7.10.6 Depositional structures 211
6.9.1 Whole-deposit grainsize 7.11 Surges compared with turbidity currents 217
populations 163 7.12 Pyroclastic surges and pyroclastic
0 .7 .Z occonuary i m c K c i i n i g ai iu flows - relationships 217
bimodalitv 164 7.13 Further reading 219
(. 10 Welded air-fall tuffs 165 6.10.1 Characteristics and examples 166 6.10.2 Conditions of formation 168 C H A P T E R E I G H T A in ̂ T h e r m a l facies model 172
C H A P T E R E I G H T
( i l l Further reading IP 1 I A vpivAAWA * — * ce
174 Ignimbrites and ignimbrite-forming eruptions 223
C H A P T E R SEVEN Initial statement ' 223
1 i.nichnrt and deoosition of subacrial 8.1 Enigma of ignimbrites 224
pyroclastic flows and surges 177 8.2 Occurrence, composition and size 225 pyroclastic flows and surges 8.3 Eruption sequence ana coiuimi c o i i d p » c 229
Iniiial statement .. • 177 8.4 Source vents 233
/ . I Subaerial pyroclastic flows as high 8.4.1 Linear fissure vents 233
particle concentration flows 177 8.4.2 Ring fissure vents 234
xiv CONTENTS
8.4.3 Vent system for the Fish Canyon Tuff 235
8.4.4 Central vents . , 237 8.5 Co-ignimbrite breccias 237 8.6 Co-ignimbrite ash falls 242 8.7 Depositional facies model 244
8.7.1 Bandelier tuffs and model 244 8.7.2 Rio Caliente and Taupo
8.11 Chemical analyses? 258 8.12 The great Taupo A D 186 eruption 260
8.12.1 Early air-fall phases 261 8.12.2 Taupo ultraplinian fall deposit 262 8.12.3 Taupo ignimbrite 264 8.12.4 Overview . 265
8.13 Further reading . , 265
C H A P T E R N I N E
Subaqueous pyroclastic flows and deep-sea ash layers • .
Initial statement 9.1 Introduction
Types of subaqueous pyroclastic flow 9.2.1 Subaqueous pyroclastic flow
deposits 9.2.2 Ash turbidites
Hot subaqueous pyroclastic flows and subaqueous welding of ignimbrites Submarine eruption of pyroclastic flows? A model for the passage of pyroclastic flows into subaqueous environments Deep-sea ash layers Subaqueous base surges? , Further reading
9.2
9.3
9.4
9.5
9.6 9.7 9.8
269
269 269 270
271 275
276
284
285 286 290 290
C H A P T E R T E N
Epiclastic processes in volcanic terrains
Initial statement 10.1 Introduction 10.2 Importance of erosion and sediment
transport in volcanic terrains 10.3 Epiclastic sediment transport
10.3.1 Sediment transport not dependent on an interstitial medium
10.3.2 Sediment transport involving ice as an essential interstitial medium
10.3.3 Sediment transport involving water as an essential interstitial medium
10.3.4 Sediment transport in which air is an essential interstitial medium
10.4 Further reading
C H A P T E R E L E V E N
Crystal-rich volcaniclastics -pyroclastic or epiclastic?
Initial statement 11.1 Introduction
Three types of ash and tuff Possible fragmentation and transportation modes for crystal-rich volcaniclastic deposits Factors influencing high crystal concentrations 11.4.1 Eruption of highly
13.11 Intra- or subglacial volcanoes 13.12 Further reading
C H A P T E R F O U R T E E N
Facies models for ancient volcanic successions
Initial statement 14.1 14.2
14.3
13.2
Introduction Facies geometry and facies -stratigraphic relationships: factors affecting them in ancient successions Factors affecting original lithological characteristics and depositional structures 14.3.1 Polyphase hydrothermal
deformation and alteration Recognition of pumice in the rock record Facies as diagnostic indicators of palaeoenvironments and palaeoenvironmental conditions A suggested approach to facies analysis Facies models - what they represent and their uses Facies models for volcanic successions
Intraplate oceanic volcanism 452 Intraplate continental volcanism 452 Continental rift volcanism 453 15.6.1 Narrow linear rift zones 453 15.6.2 Broad continental rift zones 455 Young island arc volcanism associated with oceanic trench subduction zones Microcontinental arc volcanism associated with oceanic trench subduction zones
15.9 Continental margin arc volcanism associated with oceanic trench subduction zones
15.10 Igneous rock-types as indicators of basement
15.11 Volcanism related to regional tectonic regimes and local stress field conditions 462
15.12 Igneous rocks as palaeostress indicators in the crust and lithosphere
15.13 An approach to evaluating the tectonic context of ancient successions
15.14 Further reading
435 A P P E N D I X 1
423 424
425 426
456
458
460
460
465
466 467
Methods used in studying modem pyroclastic deposits
1.1 Physical analysis 1.1.1 Thickness 1.1.2 Maximum grainsize 1.1.3 Grainsize distribution 1.1.4 Proportions of components 1.1.5 Crystal content of pumice 1.1.6 Density and porosity
1.2 Stratigraphic analysis
A P P E N D I X 11
Grainsize-textural classes of volcaniclastic rocks, some possible origins, and suggested diagnostic characteristics