Lake District Mapping Vol. II

Leo M L Ritchie

Strategic

• ~102km area covered

• Access– Youth Hostel at one end of my area

• Topography– 80m to 650m elevation– Poor sediment outcrops

• Weather– Mentally unprepared for sun in UK

Where I mapped?

In the

Lake

DistrictNW of Coniston

Geology of wider area

Mapping Area

Andesitic Welded Tuff (dwt)

Rhyolitic Welded Tuff (mwt)

Porphyritic Intrusion (pi)

More vs

Interbedded Siltstone and Tuff (ist)

More vs

Lapilli Tuff (lt)

Volcaniclastic siltstone and sandstone (vs)

Lithic-rich Tuff (lrt)

Limestone & calcareous siltstone (lcs)

Pale Siltstone (ps)

Dark Siltstone (ds)

Map & stratigraphic column

N

Lever’s Water

Coniston

Succession

200

300

400

100

500

600

700

ds

ps lcs

lrtvs

vs

vspi ist mwt

lt

Rhyolitic Welded Tuff (mwt)

200

300

400

100

500

600

700

ds

ps lcs

lrtvs

vs

vspi ist mwt

lt

Volcanic

Rhyolitic Welded Tuff (mwt)

• Oldest unit• Quartz rich• homogeneous• >200m• Caradoc (Ordovician)*

*Millward D. et al Geology of the Ambleside district. Memoir for 1:50 000 Geological Sheet 38 (England & Wales) British Geological

Survey H.M.S.O. 2000 2nd Session Sheet 38

Volcaniclastic sandstone and siltstone (vs)

200

300

400

100

500

600

700

ds

ps lcs

lrtvs

vs

vspi ist mwt

lt

Volcanic

Volcaniclastic sandstone and siltstone (vs)

• ~1km thickness in three separate units

• Siltstone – conglomerate

Interbedded siltstone and tuff (ist)

200

300

400

100

500

600

700

ds

ps lcs

lrtvs

vs

vspi ist mwt

lt

Volcanic

Interbedded siltstone and tuff (ist)

• ~200m thickness

• Dacitic welded lapilli tuffs

• And volcaniclastic siltstones

Lapilli Tuff (lt)

200

300

400

100

500

600

700

ds

ps lcs

lrtvs

vs

vspi ist mwt

lt

Volcanic

Lapilli Tuff (lt)

• ~50m thickness

• Only un-welded tuff in the succession

• dacitic

Lithic-rich tuff (lrt)

200

300

400

100

500

600

700

ds

ps lcs

lrtvs

vs

vspi ist mwt

lt

Volcanic

Lithic-rich tuff (lrt)

• ~400m thickness• Dacitic welded

tuff• Larger lithics

were rhyolitic• Columnar

jointing

Unconformity between vs-lrt

Unconformity between vs-lrt

U/C

lrt

vs

Limestone & calcareous siltstone (lcs)

200

300

400

100

500

600

700

ds

ps lcs

lrtvs

vs

vspi ist mwt

lt

Sediment

Limestone & calcareous siltstone (lcs)

• ~150m thickness

• Ashgill (Ordovician)*

• Fossiliferous…

Pale siltstone (ps)

200

300

400

100

500

600

700

ds

ps lcs

lrtvs

vs

vspi ist mwt

lt

Sediment

Pale siltstone (ps)

• ~30m thickness

• Llandovery (Silurian)*

• Bioturbated…

Dark siltstone (ds)

200

300

400

100

500

600

700

ds

ps lcs

lrtvs

vs

vspi ist mwt

lt

Sediment

Dark siltstone (ds)

• >500m

• unfossiliferous

Porphyritic Intrusion (pi)

200

300

400

100

500

600

700

ds

ps lcs

lrtvs

vs

vspi ist mwt

lt

Intrusive Igneous

Porphyritic Intrusion (pi)

• ~250m

• Plagioclase phyric

• Interesting jointing patterns…

And finally…Volcanic

Andesitic welded tuff (dwt)• >200m

thickness

• Pyroxene and amphibole rich

N

Lever’s Water

Structure

• Predominant cleavage throughout units• Faulting (big & small)

Structure - cleavage

A predominant cleavage was apparent in most of my units

Structure – faulting (big)

Normal faulting in lithic rich tuff (lrt)

N

Lever’s Water

N

Lever’s Water

N

Lever’s Water

N

Lever’s Water

72/108

Structure – faulting (small)

Small scale faulting in volcaniclastic sand/siltstone (vs)

Structure – faulting (small)

Small scale faulting in volcaniclastic sand/siltstone (vs)

• Fine grained laminations displaced on cm-scale

Structure – faulting (small)

Small scale faulting in volcaniclastic sand/siltstone (vs)

• Fine grained laminations displaced on cm-scale 67/

328

Structure - faulting (strike-slip?)

In Eastern corner of my area

N

Lever’s Water

N

Lever’s Water

x

ds

pslcs

lrt

dwt

Interpretation

1. Eruption of tuffs and deposition of volcaniclastic sediments

2. Deposition of shallow then deeper marine sediments

3. Strike slip faulting – bringing andesitic welded tuff southward

4. Compression causes cleavage

Problems

• Does compression or strike-slip faulting take place first?

• Why does pi intrudes within rock types? And when did it intrude.

Plans

• Find out more about tuffs and intrusions from thin sections

• Provinence study of two vs units

• Investigate fossils more closely to refine ideas about age

Any Questions?