St. Croix Cambrian trace fossils

Here's something simple: photos of Cambrian trace fossils in the St. Croix Valley. If you would like a refresher on the rocks in question, may I suggest this post? We've already seen some photos of Skolithos burrows in the Mazomanie, a sandstone unit. The finer-grained rocks have other kinds of trace fossils, which makes sense because they represent different environments than the Mazomanie. At the same places in Osceola where Skolithos are found in abundance in the Mazomanie, much different burrows are locally abundant in the overlying St. Lawrence Formation. In the upper part of this formation, where the beds are sandy and can be hard to tell from the Jordan Sandstone (Sardeson 1932), the burrows are much thicker and horizontal.

There are a few in these pieces of float, including just below the scale bar and in the block above and to the left of the central block. The squares in the bar are 1 cm, so the burrows are a bit larger, on the order of finger-sized.

Returning to the same area nearly two and a half years later, I came across these remnants of a disintegrated block. If you expand the photo, you'll see that the largest chunks are those with burrows.

Comparable burrows can be found lower in the formation, in more typical-looking gray-green blocks. I wasn't seeing body fossils, which are reputed to be there, but there were certainly plenty of trace fossils.

This is more like what the St. Lawrence is supposed to look like, and there's another horizontal burrow, a bit smaller than those in the photos above but pretty similar.

One of the finer-grained units intertonguing with the Mazomanie is the Tomah Member of the Lone Rock Formation, another part of the ex-Franconia Formation. The Tomah is the finest-grained part of the Lone Rock Formation and has a tendency to erode into angular chips, blocks, and chunks, usually hand-sized or smaller, often in pastel greens and oranges. Some of the beds are heavily marked by various kinds of trace fossils.

Several different sizes are apparent here.

This one's unusual for having eroded out as a substantial piece.

This chip has traces close to 1 mm in diameter near the top and a thicker trace several mm across near the center, with a "lobed" appearance that may be due to erosion.

The large straight burrow on this piece has a lumpy surface somewhat reminiscent of "corn cob" Ophiomorpha, but not as coarse.

A slice of pizza covered with grains of rice?

I could go on ad nauseum with trace fossil photos from the Tomah (you may already be there), so just one more for the road. As far as I know, nobody has published a detailed analysis of the trace fossils in the Tomah or St. Lawrence, although I can't rule out there being some dissertation or other piece of grey literature I haven't run across. There's certainly quite a lot of these fossils there, though!

This one is dominated by burrows a couple of mm in diameter. Note the long slender trace in the upper center

References

Sardeson, F. W. 1932. Fauna of the Jordan Sandstone. Pan-American Geologist 58(2):103–106.