30 July 2022

Well-preserved mudcracks in Belt argillite, Glacier National Park, Montana

Posted by Callan Bentley

I spent last week in western Montana, teaching my annual “Geology of Glacier National Park” for Montana State University’s Master’s of Science in Science Education program. As usual, it was a fulfilling and enriching experience.

There are many great things about Glacier’s geology, but a perennial favorite for me is the abundance of truly ancient primary sedimentary structures, like these mud cracks:

Originally formed during the Mesoproterozoic, these delicate patterns speak of a very shallow Belt Sea, where mud deposited at high tide or in the wet season was then exposed to the air during low tide, or the dry season, inducing desiccation and contraction. The example above shows the resulting mud cracks filled with more mud, but they can be filled with sand, too, as this inverted block shows:

In cross-section, the mud cracks make V-like shapes where younger layers “bite” down into their predecessors. Here’s an example in green (chlorite-rich argillite of the Appekunny Formation) and another in red (hematite-rich argillite of the Grinnell Formation):

My favorite new example of this phenomenon is one I’ve walked by a dozen times before without noticing. It’s on the trail up to Grinnell Glacier, at the spot just before the waterfall-on-the-trail, where the route was closed this year due to a hazardous snowfield crossing. Forced to have my lunch there instead of atop the stromatolites up top, I was forced to examine my new surroundings. I appreciated this one especially:

Here, though the mud was deposited under oxidizing conditions (=red), later reducing fluids moved through the sediment (or sedimentary rock), altering blotchy portions of it (=pale green). I love the “palimpsest” overlap between the oxidation/reduction contrast and the pattern of mud cracks. Whether you’re a geochemist or sedimentologist, there’s a lot to love in this slab!