NMZ

SMZ

16

15

152520

35

45

15

20

10

10

MRF

0 2 km

UltrabasicLayered

Suite

Triassicsandstone

N

b

WesternGranite

MRF

MRF

Area of Fig. 3

TCAS

TCSM

TCAS

TCAS

TCSM

TCAS

LL

FL

LF

F

Laimhrig Shale Member (TCDL)

Allt Mor na h-Uamha Sandstone Member (TCAM)

Scresort Sandstone Member (TCAS)

Sgoir Mhor Sandstone Member (TCSM)

Unconformity

Fiachanis Gritty Sandstone Member (TCDF)

Torr

ido

n G

rou

p

(Pro

tero

zoic

)

Lewisian Gneiss (Archaean)

OnlyexposedwithinMRF

Rum

N0 50

km

Skye

Ardnamurchan

Mull

PalaeogeneIgneous Centres

a

10Bedding

Strike & Dip

Major Fault

Figure 1: a) Location map and b) outline geological map for the Isle of Rum (after Emeleus 1997). NMZ = Northern Marginal Zone; SMZ = Southern Mountains Zone; MRF = Main Ring Fault zone; LLF = Long Loch Fault.

The Southern Mountains Zone, Isle of Rum, Scotland: volcanic and sedimentary processesupon an uplifted and subsided magma chamber roof

by

E.P. Holohan, V.R. Troll, M. Errington, C.H. Donaldson, G. Nicoll & C.H. Emeleus

Geological Magazine

Appendix to:

Hypothetical palaeosurface

MRFSMRF

Present-dayexposure

level

NW SEDegassed acid magma squeezes up ring-faults to form ring-dyke intrusions and sills.

Sudden, late-stageexplosive degassing produces a fluidized aggregate of tuff and

country-rock fragments- "Intrusive Tuff"

1.2

1.0

0.8

0.6

0.4

0.2

- 0.8

- 0.6

- 0.4

- 0.2

0 km

SnGAns

Intense brecciation and abrasion of uplifted, tilted, and partly-subsided block

of country rock by escaping volatiles

3

2

1

F

FF

F

F

Palaeogene Rocks

Intrusive Tuffs(Am Mam-type Breccia)

Acid magma (Rhyodacite)

Explosion breccias(Coire Dubh-type breccias)

Lewisian Gneiss

Torr

idon

Gro

up

Diabaig Formation Grit

Diabaig Formation Shale

Applecross Formation

Precambrian Basement

Unconformity

F

Figure 2: Schematic representation of Hughes's (1960) model for the subterranean formation and emplacement of the Southern Mountains Zone breccias and rhyodacites. SMRF = Southern Mountains Ring Fracture of Hughes (1960); MRF = Main Ring Fault of Bailey (1945); SnG = Sgùrr nan Gillean Peak; Ans = Ainshval Peak.

sharp

Intrusive contactwith UltrabasicLayered Suite

RG

A

Coherent TCDL

Lower Rhyodacite Ignimbrite(s)

Clasts: TCDF?(80), TCDL(5), LG(5), Bs (5)

Clast diameter: 1-12 cm

Coarse sandstone withaligned pebbly layers

UpperRhyodacite Ignimbrite(s)

'Epiclastic' Sandstone

0 m

5

10

20

30

40

50

60

70

15

25

35

45

55

65

75

80

90

100

85

95

110

105

120

115

130

125

140

135

150

115

160

155

170

165

180

175

190

185

200

195

210

205

A'BouldersCobblesGravelSand

CS-MS

Inter-fingering

Ash lenses with rareaccetionary lapilli

An.Clasts: TCDL (95),

TCDF (5)Clast size: 1-6 cm

Clasts: TCDL, TCDF

sharp

grades Lithic tuff: strong clastalignment + NG

Fine massive layer

RG

NG, CA

(rare crinkledlaminations)

RG

NG

RG

RG

NG

Msv. An. - Sr.

NG

MS - CS

Local folding of fiamme (rheomorphic?)

Two fiamme types:1) Light-grey/cream2) Black

Interdigitating contacts

RG

Fine top, fiamme rare

Msv. Ground Layer

Msv.

Msv.

MS

CS

'Epiclastic' Sandstone

Clasts: TCDF, TCDL

Clasts: TCDF, TCDLCSSa. - Sr.

MSAn. - Sr.

Clast diameter: 1-12 cm

Clasts: TCDF, TCDLClast diameter: 1-12 cm

MS-CS

Basal fiammepoorly welded

Debris Fall / Scree?

Ground layer/Surge

Pumice concentrationzone

Pumice concentrationzone

Msv.

Some lithics & fine fiamme

Ash surge/fall

Ground Layer

Pumice concentrationzone

Debris FlowDeposit

Stream FlowDeposit

Debris FlowDeposits

Debris FlowDeposits

Lithology Contacts & SedimentaryFeatures

Interpretation

0 m

5

10

20

30

40

50

60

70

15

25

35

45

55

65

75

80

90

100

85

95

110

105

120

115

130

125

Tuff/sandstone with accretionary lapilli

Lithic tuff

Dark -grey weathering fiamme

Coherent TCDL

Light-greyweathering fiamme

NB: Upper ignimbrite sheet thickness is

a very conservativeestimate, as location of intrusive rhyodaciteboundary is uncertain

Dark- weathering fiamme

Poorly - moderately welded near base,

Lower Ignimbrite

Dark-green tuffaceous sandstone.

UpperRhyodacite Ignimbrite(s)

B'

B

BouldersCobblesGravelSand

MS, CAgrades

CS. An. Scree/Debris Fall deposits?

Ground layer

Surge and/or stream-flow deposits

Clasts: TCDL (95), TCDF (5)

RG

Fiamme fade outNG

Ash-surge

Pumice concentrationzone

RG Sharp

MS. An. - Sr.

CS. An. - Sr.

MS - CS. An. - Sr.

Clasts: TCDF? (90), TCDL (5), LG (5)Clast diameter:

1-40 cm

Debris Flowdeposits

NG

Lithology Contacts & SedimentaryFeatures

Interpretation

140

135

150

145

160

155

Upper parts of ignimbrite

sheet not logged in detail

Surge and/or stream-flow deposits

Clasts: TCDF (90),TCDL (5), LG (5)Clast diameter:

1-90 cm

BouldersCobblesGravelSand

Intruded by rhyodacite

NG

C

Coherent TCDF

Lahaar?

0 m

5

10

20

30

40

50

60

70

15

25

35

45

55

65

75

80

90

100

85

95

C'

NG

NG Normal graded lithic tuff lenses with sharp bases

Elutriation pipe(fossil fumarole)

Normal graded fiamme with basal lithic tuff

Lithology Contacts & SedimentaryFeatures

Interpretation

Surge?

Ground layer(s)

Polymict lithic tuff with accretionary lapilli.

Clasts: TCDF, LG, Bas

MS

Debris Flow&

Debris Fall deposits

Ignimbrite

An - Sr.

NG

MS - CS. An. - Sr.

Lithic trail perpendicular to undisturbed fiame

Lewisian Gneiss (LG)

TCDF

TCDL

TCAM Msv MassiveNG Normal GradingRG Reverse GradingCA Clast AlignmentMS Matrix SupportCS Clast Support

Sedimentary features

Bas (5)

Other Lithic Clast types (%)

Basalt/Dolerite

Rn RoundedSr Sub-roundedSa Sub-angularAn Angular

Note: Log thicknesses arecorrected for slope and dip

Clast roundedness

Palaeogene rocks Precambrian Basement

Fine - coarse, pink arkose +/- siltstone

Laminated grey silt, shale, and fine sandstone

Fine - coarse, grey sandstones +/- siltstone

Mesobreccia

Extrusive Rhyodacite

Mafic Cone Sheet

Lithic Tuff

Tuffaceous Sandstone

Sandstone

RDP (5) Rhyodacite

TA (80) Arkose sandstone(TCAM/TCAS)

Fiamme in rhyodacite

Large fiamme, often in'swarms'

Ash lenses in rhyodacite

Figure 4: Measured logs of the volcano-sedimentary successions preserved: A) on Beinn nan Stac; B) in Sandy Corrie; and C) in Forgotten Corrie. See Figure 3 for lines of logs.

Msv

Msv

Msv

Msv

NGRG

CA

Msv

CS

CS

MS

MS

Msv

CS - MS

Msv

MS

Msv

Msv

Msv

MS

Msv

MS

Gap In Exposure

Upper contact against overlying Torridonian (fault?)

Clasts: TA(90),LG(10)

Clasts: LG(100)

LG(95), TA(5)

Sr - An

Sr - An

TA(95), LG(5)

An

TA(50), LG(50)

Sr - An

TA(70), LG(30)

MS Sr - Sa

TA(30),LG(30), Bas(30)

An - Sa

TA(50), Bas(50)

TA(90), LG(10)Sa

Sr - An

Sr - Sa

TA(98), Bas(2)

Sr - An

TA(95), LG(2),Bas(2)

Sr - An

Sr - An

TA(95), LG(5), RDP

Several largeblocks

(0.5 -1.5m diameter)

LG(65), TA(35)

RG

NG

Clast size: 1-65 cmAve: 10-12 cm

NG

Gritty sand lensMsv

Msv Pebbly sandstone Stream Flow depositNG

NG

NG

MS - CSRG

NG

TA(90), LG(10)

SaMS - CSRG

NG

RG

TA(95), Bas(5)

MS

NG

Gritty sand lensMsv

TA(98)

CS - MS

RG

NG

NG

NG

RG

NG

Lithic Tuff - NG

TA(80), LG(15),TCDL (5)

0 m

5

10

20

30

40

50

60

70

15

25

35

45

55

65

75

80

90

100

85

95

110

105

120

115

130

125

140

135

150

115

160

155

170

165

180

175

190

185

200

195

210

205

D'BouldersCobblesGravelSand

Lithology Contacts & SedimentaryFeatures

Interpretation

Debris Flow deposits

from arkose- dominated source area

Stream Flow deposit

Stream Flow deposit

Coherent Gneiss

Debris Fall / Scree?

...arkose- dominated and gneiss- dominated

source areas

Debris Flow/Fall deposits...

...alternating from...

Ignimbrites and

Debris Flow deposits

Debris Flow deposits

D

Figure 5: Measured log of the Base of Sgùrr nan Gillean volcano-sedimentary succession. See Figure 3 for lines of logs. See Figure 4 for key.

c d

a b

Figure 6: Sedimentary features in Coire Dubh-type breccias. (a) Normally graded top of middle mesobreccia unit on Beinn nan Stac [NM 3977 9400]. Breccia passes up into coarse sandstone that is in turn overlain by lithic-rich rhyodacite. Note the variously sub-rounded to sub-angular clast shapes. Maximum clast diameter is ~ 20 cm. Clasts are predominantly coarse to fine, often laminated, grey sandstone (Fiachanis Gritty Sandstone Member?); (b) Contacts between a metre-thick, lens-shaped, arkose-rich mesobreccia bed (dashed white outline) and gneiss-rich beds (above & below) in the Base of Sgùrr nan Gillean succession [NM 3810 9246]. Note 35 cm long hammer (circled) for scale. Beds dip to NW; (c) Gritty/pebbly lenses (offset by small fault) in coarse, poorly-sorted sandstone in the Base of Sgùrr nan Gillean succession [NM 3781 9255]. The notebook is 15 cm in length. The sandstone dips NW and lies just below a concordant rhyodacite sheet and above arkose-dominated mesobreccias; (d) Accretionary lapilli in coarse tuffaceous sandstone just below the Peak of Sgùrr nan Gillean rhyodacite sheet [NM 3819 9276]. The pen-top points to a ~ 8 mm diameter lapillus with multiple concentric zones.

Fiamme foliation

a

b

c

W B

W

B

Figure 7: Ignimbritic features in rhyodacite sheets. a) Streaky (moderately-welded) fiamme in the upper Beinn nan Stac rhyodacite sheet [NM 3966 9403]; Note white-weathering (W) and black-weathering (B) fiamme types. Note 35 cm long hammer for scale; b) Planar and wavy bed-forms in ash and crystal tuffs (surge deposits?) below the Peak of Sgùrr nan Gillean ignimbrite sheet [NM 3783 9330]. Hammer shaft is 25 cm long; c) Discordant 'lithic lapilli' band in bedded rhyodacite, Forgotten Corrie [NM 3840 9429]. Little or no deflection of fiamme at the band implies it is pre- or syn-compaction. It is thus interpreted as a 'fossil fumarole', characteristic of high-temperature ignimbrites.

1000

900

800

700

600

500

400

300

200

100

0 m OD

- 100

- 200

- 300

- 400

- 500

- 600

- 700

- 800

- 900

F

F

F

F

F

Uplifted, outward-tilted & folded basement.

Palaeotopography (unconformity) produced by erosion

of uplifted basement.

Basement thrust over Palaeogene sedimentary succession (resurgence related?).

Sheets of extrusive rhyodacite (ignimbrite) underlain by, but also interbedded with, sheets of sedimentary

breccia (debris flow deposits). Eruptions associated with onset of incremental caldera collapse.

Steeply-inclined, fault-related conduit for non-fragmented & fragmented rhyodacite (feeding ignimbrite-forming eruptions).

Downthrown Loch

Dubh an Sgoir block.

Intrusion of Layered Ultrabasic Suite (gabbros & peridotites).

Basal sedimentary breccias & sandstones (debris flow & stream flow

deposits), commonly of local derivation, deposited on uplifted basement.

Basement blocks downfaulted inside previously uplifted blocks - structural evidence for caldera collapse. Am Mam-type breccia intruded.

X Y

3

4

5

6

7

9

8

2

1

1

22

4

5

5

6

7

8

9

RRe

RRi

XAM

RRi

RRi

RRe

RRi RR

eRRe

Sgurr nan GilleanSgurr nan Goibhrean

RRe

RRe

2

2

3

3

7

3

ZCD

Coire Dubh-type brecciasand sandstones (ZCD)

Extrusive Rhyodacite (RRe)

Unconformity

Palaeogene Volcanic & Sedimentary RocksProterozoic Rocks

Torr

idon

Gro

up

Fiachanis Grit Member (TCDF)

Laimhrig Shale Member (TCDL)

Allt na hUamha Member (TCAM)

Scresort Member (TCAS)

Unconformity

Lewisian Gneiss (F)F

Applecross

Form

ationD

iabaigF

ormation

Palaeogene Intrusive Rocks

Phase 2 Ultrabasic & Hybrid Plutonics (U)

Intrusive Contact

Pha

se 1

sili

cic

intr

usiv

e R

ocks

Am Mam-type Breccia (XAM)

Intrusive Rhyodacite (RRi)

Archaean Rocks

Figure 9: Interpretative cross-section through the Southern Mountains Zone. There is no vertical exaggeration. See Figure 3 for line of section (X-Y). Relationships around Sgùrr nan Gillean Peak are projected from the SW onto the plane of section. Numbers 1 to 9 list in approximate chronological order the main geological features/events, as reconstructed from this study.