29 November 2016

Islay’s Port Askaig tillite

Posted by Callan

Have a look at this:

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Pretty uninteresting rock at first glance – massive gray stuff.

But what about those pink bits? What are those?

Turns out those are pebbles and cobbles of granite. Here are a few more:

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These “outsized clasts” that ‘float’ in a deposit of much finer grained sediment qualify this rock as a diamictite.

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There’s more than one way to deposit this blend of clast sizes and make a diamictite (landslide, etc.), but the coolest way is for a glacier to melt and dump its sedimentary load as a pile of till. Later lithification sticks that till together into a new rock, a tillite.

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I think it’s fair to say that all tillites are diamictites, but not all diamictites are tillites.

The diamictites in these photographs are interpreted as tillites. They crop out along the eastern shore of the island of Islay in Scotland. The name of the unit is the Port Askaig Tillite.

Here’s the parking area for the ferry terminal at Port Askaig itself:

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The walls of the roadcut are all Port Askaig Tillite, though the exposures are not especially good there.

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The rock walls there are good, though. They are locally sourced!

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In addition to granitic clasts, carbonate clasts (weathering orange/tan) can also be seen:

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The exposures are better further south along the coast.

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Like many of my favorite diamictites, the Port Askaig Tillite is Neoproterozoic in age, and is cited as evidence of a “Snowball Earth” glaciation then.

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The Port Askaig Tillite is associated with a limestone unit (more on that later) that is interpreted by some workers as a “cap carbonate.”

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This was the only “exceptional” example of bedding that I saw — unfortunately, it was in a boulder loosed from its original in situ orientation.

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Many of the outcrops that are just out of reach of the surf look like this – decorated with a dozen varieties of lichen:

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Here are some field-based GigaPans of outcrops of the Port Askaig Tillite that I saw, as well as front and back views of two specimens I collected on Islay:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GIGAmacro by Callan Bentley

Link GIGAmacro by Callan Bentley

Link GIGAmacro by Callan Bentley

Link GIGAmacro by Callan Bentley