February 22. Cambrian Stratigraphy of western Montana

When I was a student at Indiana University’s geology field course, out here in Montana, we learned the stratigraphic section.

The Cambrian part is Flathead-Wolsey-Meagher-Park-Pilgrim. The Flathead is the oldest layer of the Cambrian out here, and I hope you aren’t surprised to learn that it’s a clean quartz sandstone like the Tapeats in the Grand Canyon and the Posdam back east. Like them, the Flathead sandstone sits above a profound unconformity, a break in the rock record, and the rocks below it are Precambrian in age, hundreds of millions of years older than the Flathead. It’s pinkish, like the Potsdam, because of some iron oxide cement, and it has little round green grains in it in places – we’ll talk about them tomorrow – but mostly, it’s just nice sandstone.

Trilobite Bathyuriscus formosis, Cambrian Meagher formation, Montana.
Photo by Stephen W. Henderson, used by permission.

The stratigraphic section here in Montana is a lot like the Cambrian section in the Grand Canyon. Above the Tapeats sandstone in the Grand Canyon we have the Bright Angel Shale, followed by the Muav limestone. Here in Montana, the Flathead sandstone is followed by the Wolsey Shale, then the Meagher Limestone. Then the Park Shale, and then the Pilgrim formation, limestones and dolomites.

The seas came in, the seas came out…. Alternating shale and limestone might mean that, but there are other ways to make it happen. I’m planning to have a conversation with an expert on Cambrian stratigraphy in a week or so – we might be in the Ordovician by then, but if we are we’ll just think back on the Cambrian when that conversation happens.

From the point of view of someone mapping geologic layers, the importance of the sequence – Flathead, Wolsey, Meagher, Park, Pilgrim – is that it’s really the best way, sometimes the only way, to be sure which rock or rocks you might be looking at. In western Montana, there’s another pinkish quartz sandstone called the Quadrant – a chunk of it looks an awful lot like a chunk of the Flathead, to the point that it’s virtually impossible to tell them apart in the field. But the Quadrant is Pennsylvanian in age, around 280 million years old, rather than around 500 million years for the Flathead.  If you look at the rocks below the Quadrant, you won’t find the Precambrian unless there’s some complicated structural thing going on, like faulting. And if you look above, you won’t find the precise sequence of the Wolsey, a specific kind of shale, the Meagher, a limestone with distinctive characteristics, the Park shale, and then the Pilgrim formation. It’s that sequence that’s like a fingerprint that tells you you’re in the Cambrian, even if the individual chunks of rock can’t tell you that for sure.

—Richard I. Gibson

Trilobite Bathyuriscus formosis, Cambrian Meagher formation, Montana. Photo by Stephen W. Henderson, used by permission.