29 March 2011

Tillite in outwash

Posted by Callan Bentley

Hoo boy. This one made me yelp…

While on the glacial geology of western Pennsylvania trip last Saturday, we visited a gravel quarrying operation. The operators were extracting gravel from a glacial lake delta deposit, and it was full of glacial outwash — sediments washed out from the melting front of the Erie lobe of the Laurentide ice sheet.

We were there to look at the Pleistocene features, but several of us twitched uncontrollably, to be so close to so many interesting looking cobbles, and we were soon rock-hounding it, checking out what the glaciers had brought to this location from so far away. We found mylonites and diabases, and sandstones, and limestones, granites and gneisses, most bearing distinctive glacial scratches and facets.

But this one… this one took the cake:

Yes, that’s one rock: The image gives you two views. I scanned the front and back, and used Photoshop to turn the two scans into a single image (with, of course, my awesome LRRD scale pencil for scale). A much bigger version is here.

With its poor sorting, dark matrix, and mix of rounded and angular clasts (many of distinctive pink granite), that looks to me like the Gowganda Tillite, a Paleoproterozoic “Snowball Earth” deposit from central Ontario.My student Robin picked it up on the field trip, right next to the feet of my student Troy! (Troy is pissed that he didn’t spot it first, let me tell you!)

Get what’s so cool about it? This rock is made of  sediment that was deposited by a glacier in central Ontario ~2.5 billion years ago. Then that till (poorly sorted glacial sediment) was lithified into tillite, and then well nigh on 2.5 billion years later, a piece of it got plucked off by a Pleistocene glacier, and the cobble was carried south, across what would eventually be declared the U.S. border. It melted out of the ice in western Pennsylvania. A stream carried it into a lake at the margin of the glacier, where it was deposited amid an assortment of sediment that strongly resembled its own internal texture. There it remained, buried until unearthed by the excavations of the gravel company. Then we came along, and Robin picked it up: it’s a twice-glaciated rock!

Technically, the Pleistocene glacial deposit was outwash, rather than moraine (“till”), so it’s not a perfect match for the same depositional process acting at two times. But it was deposited twice by glaciers, glaciers whose ages are separated by half the age of the Earth. Neat, eh?