Pre-Cambrian of the British isles Flashcards
Where do the majority of good pre-cambrian outcrops occur?
Scotland
How are the pre-cambrian deposits separated in scotland?
By a series of units (terranes) from the NW highlands to the southern highlands
What is the lithostatigraphical division of the UK in the precambrian?
Lewisian split into:
1. Scourian (older)
2. Laxfordian (yoiunger)
What are the typical Lewisian lithologies?
Foliated and banded gneiss
What is the history of the Lewisian?
Billion year time history so lots happened
With cross cutting dykes
What is the Hebridean terrane (scotland)?
Most north-west part of Scotland consisting of older basement lewisian and overlying Torridonian
What splits the Hebridean terrane and N.Highland terrane?
The Moine thrust
What are the lithologies of the N.Highland terrane?
Lewisian basement overlain by Moine
What happened in the Hebridean terrane between the Lewisian and the Torridonian?
Uplift and erosion in the Proterozoic
How many unconformities are there in the pre-cambrian?
3
Where are the unconformities in the precambrian sequence?
- Between Lewisian and Clachtoll
- Between Meall Dearg and Diabaig
- Between Diabaig and Applecross
What is the deposition of the diabaig evidence of?
Lacustrine deposition
What is lacustrine deposition?
finely layered fine grained sediments usually in lakes showing seasonal change in deposition
What event can be seen at similar horizons throughout the diabaig?
dropstones - thought to be due to a glacier coming into contact with water
What are dropstones?
are lumps found within the fine grained sediments
What additional layer is found overlying the Torridonian?
Eriboll (cambrian)
What is characteristic about Moine rocks?
Sheath fold which leads to eye structures in rock
What is the Grampian terrane?
Youngest in precambrian sequence
What rock age makes up the Grampian terrane?
Dalradian
What rock group in scotland holds evidence of multiple glaciations?
Argyll through tillite at its base
How many levels of evidence for glaciations is in the Dalradian Terrane?
3 with Port Askaig being the first
What are the 2 principle divisions of the Cambrian and pre-Cambrian in Anglesey?
Older- south stack - low grade metasediments
Younger- Gwna - sedimentary breccia
What is the rock boundary for the Cambrian / pre-Cambrian at Wrekin?
Cambrian sandstone and pre-Cambrian separated by unconformity
What evidence of volcanism in the late pre-Cambrian is there from Wrekin?
Rhyolites (effusive phase) and Granifiers
