We spent some time with the Mazomanie Formation last year, at Interstate State Park. The Mazomanie was once a member of the late lamented Franconia Formation, but has been upgraded from a member to a full formation, in the Tunnel City Group. The Franconia/Tunnel City rocks are divided into several formations and members. The Mazomanie Formation is the uppermost formation, found above (and intertonguing with) the Lone Rock Formation. As you go east and north, the Mazomanie becomes thicker and thicker until it has taken most of the Tunnel City Group interval in central Wisconsin. This transition is in progress in the St. Croix Valley, although there is significant intertonguing of upper Lone Rock Formation and lower Mazomanie Formation in places like northeastern Washington County (Mossler 2008). (Remind me sometime to write about the practical limits of the "layer-cake geology" of introductory classes.) If you were to see the two formations together, the quickest ways to distinguish them are by color and grain size: the Lone Rock Formation is often greenish to grayish, due to its glauconite content, and includes siltstone and shale with its fine-grained to very-fine-grained sandstone, whereas the Mazomanie Formation is light gray to yellowish in color and is pretty much sandstone, often coarser than Lone Rock Formation sandstone (Mossler 2008). Unfortunately, the basic description of the Mazomanie is roughly the same description one might use for the Jordan Sandstone, and they can be mistaken for the other (Runkel and Boerboom 2010).
That's enough exposition for now; let's get to some photos. The Mazomanie shows up frequently from about Stillwater to Taylors Falls. Aside from Interstate State Park, a couple of places where it's easy to see are at Cascade Falls in Osceola and in the Boomsite/Lookout Point area on the north side of Stillwater. On the Wisconsin side of the river, the geologic map coverage isn't quite as extensive as on the Minnesota side, but I'm comfortable in calling the rock at the falls the Mazomanie as opposed to the Jordan, which can be seen on Eagle Bluff just on the other side of the road (incidentally, if you go to Osceola to see Cascade Falls, it's worth it to continue following the trail up the bluff; the view of the St. Croix Valley from the top is worth the walk).
|Cascade Falls, cascading.|
The rocks at the falls are a combination of case-hardened and, for lack of a better word, soggy, which blurs the cross-bedding. All that moisture's gotta go somewhere.
(ooh, that was painful)
Another outstanding feature is the trace fossils. Some levels of the Mazomanie are laced with invertebrate burrows, typically the simple vertical burrows known as Skolithos (a common feature of Cambrian sandstones). They tend to stand out because the burrow fill is more consolidated than the surrounding rock, and may be differently colored as well, due to mineral infiltration. The Mazomanie isn't much for body fossils, but it has plenty of Skolithos.
|Here at Cascade Falls, the burrows have a darker color than the sandstone.|
Heading south to the Boomsite, the rocks aren't quite as moist, so the bedding is more sharply defined. The Mazomanie has some nice cross-bedding, as we saw at Interstate State Park.
|Cross-bedding in relief|
|Looks like we've got some herringbone cross-bedding in the center, which is evidence of a tidal setting.|
We also run into Skolithos again, although not quite so bold at this site.
|About the same color as the sandstone this time, but again more resistant, so they stick out slightly.|
Mossler, J. H. 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 65.
Runkel, A. C., and T. J. Boerboom. 2010. Geologic atlas of Chisago County, Minnesota [Part A; Plate 2—bedrock geology]. Minnesota Geological Survey, St. Paul, Minnesota. C-22. Scale 1:100,000 and 1:200,000.