The Brew and Hammer: Ancient shorelines - sedimentary structures of the Belt Supergoup of Montana

Its been a while since I had time to post anything and after several beer can posts, its time to do some geology. A recent vacation to northwestern Montana gave me the opportunity to observe the exposures of the Belt Supergroup. Just like the entertainment world, we geologists have our own supergroups, although ours aren't defined by popularity! First off, a few definitions for my non-geologist readers-- a supergroup is a thick sequence of sedimentary rock that occurs over a very large area and contains numerous formations (a formation is a type of rock that can be recognized and mapped over at least a few miles of horizontal distance) that are all the same general age and part of the same sedimentary sequence (no large gaps in age).

The Garden Wall, Logan Pass, Glacier National Park, MT

The Belt Supergroup may actually be more like the mega-rockstar -- the rocks of the Belt cover most of western Montana (including what are popularly called the Rocky Mountains) and much of northern Idaho and eastern Washington. While this isn't really such a large area, the thickness of the Belt Supergroup is estimated to be as much as 60,000 ft (almost 12 miles)! Its an estimated thickness because the whole area of the Belt is buckled and faulted into numerous mountain ranges, including the MacDonald range of Glacier National Park, and nowhere is any more than 5,000 vertical feet of the group exposed for viewing. That means geologists have spent years examining outcrops of rock and measuring/mapping the exposed sections to come up a good description of the Belt Supergroup.

Belt rocks near Logan Pass, Glacier National Park, MT

What makes the Belt Supergroup special (besides the fact that Belt rocks make up the incredible scenery of Glacier National Park) is their age. The Belt Supergroup rocks are dated between 1.4 and 1.5 billion years old - and are among the oldest exposed sedimentary rocks in North America. To put that age in perspective, the oldest exposed sedimentary rocks (limestone, shale, siltstone. sandstone, and occassionally salt or gypsum) east of the Rocky Mountains are between 500 and 600 million years old - a full 700 to 800 million years younger than the Belt rocks.

Ripple Marks with mud cracks, near Libby, MT

The other special characteristic of the Belt rocks is that very little of the 12-mile thick pile was deposited into deep water. The sediments washed off an ancient landmass to the south and west into a shallow sea that was probably like the modern Caspian Sea. The waters ranged between a few feet and a few hundred feet deep for 200 million years as the seafloor slowly sank at the same rate as the sediments accumulated. The sediments filled the basin as fast as the bottom sank into the earth's crust, without completely filling and forming completely dry land. The closest thing to dry land in the Belt basin was broad mudflats that occassionally dried out in the sun. How do we know this? Simple- just like after a rainfall today, the wet mud dried and cracked, leaving curled up edges on the cracks, and a distinct polygonal pattern on the ground surface.

Mud cracks, Glacier National Park, MT

The other sedimentary structure common in the Belt rocks are ripple marks from either wind driven currents on mud flats or from wave currents in shallow seawater.

Enjoy the pictures. Time to enjoy a good brew. Tonight's is thanks to my friend Kenny, who visited family in PA this fall and brought back a sampler pack of Stegmaier Brewing products--the one I'm tasting tonight is the India Pale Ale. As their label says "Zum wohl" -- to health!!