The sedimentary record is a really important part of the geologists' toolkit in understanding how Earth has changed over time. But it is far from complete, with variable preservation of rocks and some large gaps in the record. But these gaps can be useful in themrelves for understanding geological history. Here on Kerrera there is a classic example of an angular unconformity, where the older beds have been deformed prior to being exposed (by erosion) and overlain by younger undeformed, horizontally bedded sediments.
In this case there are about 300 million years of geological time missing - the underlying beds are ~750 Million year old Precambrian slates and limestones (see last weeks' OMG for a detailed description of the folding in them) (with no fossils in them) while the conglometares and sandstones which sit on top of them are Devonian (around 380 Ma, with fish fossils).
It takes majior geological upheaval to remove over 300 million years of geological record - that upheaval was the Caledonian mountain building event (orogeny) which turned the Cambrian and Ordivician marine sediments into a Himalayan-scale mountain chain arouynd 450-400 million years ago. A further 100 million years of erosion and plate tectonics brought the mountains low enough for deposition to re-commence in the Devonian. All that history is evident in the unconformity here.
See also:
Extreme folding of sediments Isle of Kerrera -
• Extreme Folding of sed...
Double unconformity at Loch Assynt -
• Double unconformity at...
6 сен 2024
