19 August 2011

Glaciofluvial outcrop annotated

Posted by Callan Bentley

On Thursday I posted a photo of an outcrop along Route 49 (Looking Glass Hill Road), north of East Glacier, Montana. I asked you what you thought of it. We had some good comments as a result, of which Howard Allen came the closest to what I wanted to convey.

Here’s an annotated version (click through for big version) to illustrate the discussion below:

Here’s what I think happened here:

These are Pleistocene glacial outwash sediments – some of which suggest calm water (lake) conditions, some of which suggest more energetic depositional currents due to their large grain size (pebbles, gravel, cobbles). The source of the sediments is the Rocky Mountains to the west, where Belt Supergroup strata (Mesoproterozoic) are exposed. The most obvious clasts are the red and green argillites. There are some honey-colored limestone clasts in there, too.

I think “up” is to the right, based on the piercing into soft silt by dropstones (as labeled), as well as flame structures pooching up into the gravel layers, and some wide arcuate cross-beds which are concave to the right. I labeled one set. There’s another tangential set of cross beds to the left of the uppermost thick gravel layer, near the top of the outcrop. Again, concave to the right.  Having sedimentologists say up is to the left instead worries me, but I’d ask them to take another look. Some of the commenters cued in to the same details that I was noticing. Regardless of my interpretation of primary structures, I feel pretty good about the story I’ve woven from this outcrop. The younging direction isn’t a critical component of my tale. (But I still think it’s to the right.)

Key take-away message:

These strata are young, and barely lithified – only to the point of semi-coherence. I broke off big pieces of silt”stone” (crumbly and powdery/gritty to the touch) by hand. Even though they’re crazy young, they’re still bolt upright (regardless of whether up is to the left or to the right). How did they get that way? There hasn’t been an orogeny in western Montana in many tens of millions of years, and certainly not in the past several tens of thousands. So I think what happened is that lake of still water formed on the margin of the glacier, and in those calm waters, these strata were laid down as alternating silt (settling from “glacial milk” suspended in the water column) and gravel (little outwash deltas advancing into the lake, then avulsing elsewhere and dying out).  Icebergs floated out other sediment and dropped it in. Perhaps all the gravel is iceberg-rafted debris, in fact, and then I don’t need the deltaic hypothesis at all. Then again, the lens-like pinching out of the gravel strata suggest to me some sort of channel scoured into pre-deposited silt. Anyhow, to go from this original horizontality to the vertical state we find them in now, I imagine that the glacier re-advanced, and nosed into the outwash deposits in front of its snout. Those strata got folded and faulted (in a soft-sediment sort of way) and some of them were pushed to vertical orientations as a result. The glacier was acting like a bulldozer; the sediments behaving like a miniature accretionary wedge. Then the glacier melted away, leaving its sedimentary offspring in this awkward position.

That’s my story. Whaddya think?