21 December 2016
Remember the diamictite I featured here a few weeks ago, from Islay? It was the one that might be a Snowball Earth diamictite. Well, if you follow Snowball Earth science at all, you’ll doubtless be aware that the glaciogenic sediments are characteristically overlain by “cap” carbonates. There’s a stratigraphic successor to the Port Askaig Tillite, too – it’s called the Bonahaven Dolomite.
Unlike what you might expect for a cap carbonate, however, it doesn’t have crystal fans or giant wave ripples or “tube-stone” (a.k.a. “plumb-bob”) stromatolites – all of which are interpreted as evidence of rapid sedimentation or hyperwarm climate from a carbon-heavy atmosphere in the aftermath of the Snowball. Instead, the Bonahaven Dolomite looks a lot like other Proterozoic carbonates that I’ve known and loved.
One thing that makes it worth visiting regardless is its profusion of lovely stromatolites. Let’s examine a few today, all spotted along the shore southeast of the Bunnahabhain Distillery, conveniently enough:
And of course, I’d be remiss if I didn’t offer two outcrop GigaPans of these lovely microbial mats:
Link 1.46 Gpx GigaPan by Callan Bentley
Link 0.81 Gpx GigaPan by Callan Bentley