Transcript Lake District Mapping Vol. II

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
District
NW of Coniston
Geology of wider area
Map & stratigraphic column
Dark Siltstone (ds)
Pale Siltstone (ps)
Lever’s Water
Limestone & calcareous
siltstone (lcs)
Lithic-rich Tuff (lrt)
Volcaniclastic siltstone
and sandstone (vs)
Lapilli Tuff (lt)
More vs
Interbedded Siltstone and
Tuff (ist)
More vs
Coniston
Rhyolitic Welded Tuff
(mwt)
Andesitic Welded Tuff
(dwt)
Porphyritic Intrusion (pi)
Succession
700
600
500
lt
400
300
ps
200
100
ds
lcs
lrt
vs
pi
ist
vs
vs
mwt
Rhyolitic Welded Tuff (mwt)
700
600
500
lt
400
300
ps
200
100
ds
lcs
lrt
vs
pi
ist
vs
vs
mwt
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)
700
600
500
lt
400
300
ps
200
100
ds
lcs
lrt
vs
pi
ist
vs
vs
mwt
Volcanic
Volcaniclastic sandstone and
siltstone (vs)
• ~1km thickness in three separate units
• Siltstone – conglomerate
Interbedded siltstone and tuff (ist)
700
600
500
lt
400
300
ps
200
100
ds
lcs
lrt
vs
pi
ist
vs
vs
mwt
Volcanic
Interbedded siltstone and tuff (ist)
• ~200m thickness
• Dacitic welded lapilli tuffs
• And volcaniclastic siltstones
Lapilli Tuff (lt)
700
600
500
lt
400
300
ps
200
100
ds
lcs
lrt
vs
pi
ist
vs
vs
mwt
Volcanic
Lapilli Tuff (lt)
• ~50m thickness
• Only un-welded tuff in the succession
• dacitic
Lithic-rich tuff (lrt)
700
600
500
lt
400
300
ps
200
100
ds
lcs
lrt
vs
pi
ist
vs
vs
mwt
Volcanic
Lithic-rich tuff (lrt)
• ~400m thickness
• Dacitic welded
tuff
• Larger lithics
were rhyolitic
• Columnar
jointing
Unconformity between vs-lrt
Unconformity between vs-lrt
lrt
vs
Limestone & calcareous siltstone
(lcs)
700
600
500
lt
400
300
ps
200
100
ds
lcs
lrt
vs
pi
ist
vs
vs
mwt
Sediment
Limestone & calcareous siltstone
(lcs)
• ~150m thickness
• Ashgill (Ordovician)*
• Fossiliferous…
Pale siltstone (ps)
700
600
500
lt
400
300
ps
200
100
ds
lcs
lrt
vs
pi
ist
vs
vs
mwt
Sediment
Pale siltstone (ps)
• ~30m thickness
• Llandovery (Silurian)*
• Bioturbated…
Dark siltstone (ds)
700
600
500
lt
400
300
ps
200
100
ds
lcs
lrt
vs
pi
ist
vs
vs
mwt
Sediment
Dark siltstone (ds)
• >500m
• unfossiliferous
Porphyritic Intrusion (pi)
700
600
500
lt
400
300
ps
200
100
ds
lcs
lrt
vs
pi
ist
vs
vs
mwt
Intrusive Igneous
Porphyritic Intrusion (pi)
• ~250m
• Plagioclase phyric
• Interesting jointing patterns…
And finally…
Volcanic
Andesitic welded tuff (dwt)
• >200m
thickness
• Pyroxene
and
amphibole
rich
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)
Lever’s Water
Lever’s Water
Lever’s Water
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 cmscale
Structure –
faulting (small)
Small scale faulting
in volcaniclastic
sand/siltstone (vs)
• Fine grained
laminations
displaced on cmscale
67/
328
Structure - faulting (strike-slip?)
In Eastern corner of my area
Lever’s Water
Lever’s
Water
x
dwt
ds
ps
lcs
lrt
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?