Arcola Bluffs Day Use Area

It's been a good while since our last St. Croix post, hasn't it? I wanted to let you in on a fun place I recently visited for the first time; Arcola Bluffs Day Use Area. This site is not widely known; there's this article (with much more artistic photos than my own), an NPS cultural landscape assessment from 2018 that weighs in at 204 mb (NPS 2018; absolutely worth it if you want a thorough understanding of the site, and also includes a section on Fairy Falls), and then some short pieces here and there, and that's about it. Visiting it, though, you'll discover great geology, views of the river and the historic Soo Line High Bridge, forest and prairie settings, and some evocative ruins.

Practical guide to St. Croix Valley sedimentary formations

Now that I've seen a fair amount of the Cambrian rocks of St. Croix National Scenic Riverway and the St. Croix Valley, it seems like a good time to set them out as was done for the MNRRA formations. This time around, we'll go to the base of the Cambrian sequence in Minnesota/Wisconsin and work our way up to where the sequence overlaps with the MNRRA rocks. One day I'll have to get into southeastern Minnesota and complete the Paleozoic sequence with the rest of the Ordovician and the Devonian.

As with the MNRRA formations, we're covering a fairly narrow span of time. The Cambrian formations were all deposited between about 500 to 491 million years ago based on biostratigraphic correlations. This includes some unconformities. One other note: I'm working from the Minnesota side of the St. Croix River, and I'm most familiar with the Minnesota names. Mossler (2008) harmonized the stratigraphic nomenclature of Minnesota's Paleozoic rocks with the schemes used in neighboring states, but there is still one difference: the Minnesota Geological Survey uses lithological terms in formation names, while the Wisconsin Geological & Natural History Survey doesn't. The upshot is slightly different names. For example, the units called the Jordan Sandstone and Oneota Dolomite on the Minnesota side of the river are called the Jordan Formation and Oneota Formation on the Wisconsin side. There isn't really a practical difference; the names just look different. In ascending order, the rock units we're most concerned with are the Mount Simon Sandstone, Eau Claire Formation, Wonewoc Sandstone, Tunnel City Group, St. Lawrence Formation, and Jordan Sandstone.

The limitations of the layer cake

To be perfectly honest, we use simplifications for practically everything. Atoms don't really look like bunches of colorful spheres surrounded by smaller spheres orbiting them. The Earth is an oblate spheroid, which is close to but not quite a sphere. The planets of the Solar System don't have nice circular orbits centered on the center of the sun, lying in a flat plane. The need to simplify complex topics is obvious, both on the grounds of providing what someone needs to know to do something, and tailoring material to what someone can understand. There's a simple version for grade school kids, a more complex version for undergrads, and so on, until you're working professionally, where you've got very detailed models which are still abstractions, only closer (hopefully) to reality. One of these simplifications in geology is "geological formations as layer cakes", where formations maintain their thicknesses and are easily distinguished. The layer cake abstraction is most useful at a local level, in settings where deposition wasn't switching back and forth between different processes and sediment sources. For example, the Ordovician rocks of the Twin Cities fit pretty well. However, the cake starts getting funny-looking as you head into southeastern Minnesota. The photo below is of the Sogn roadcut, where some familiar rocks are exposed.

No, I don't know how to pronounce "Sogn".

At Sogn, though, what we would know as the lower half of the Platteville Formation is absent. Instead, the deposition of the Glenwood Formation persisted much longer (Sloan et al. 1987). Similarly, the Decorah Shale is at its thickest at the Brickyard in Lilydale, but going southeast, the upper part is replaced by the Cummingsville Formation. We can get these shifts in deposition from a number of causes. Sometimes you're looking at the boundary between two different modes of deposition shifting over time (such as a shoreline prograding or regressing). Sometimes there is a tectonic component, such as a basin subsiding. Sometimes the source of sediment changes or runs out.

Here's a more advanced example: the interval long known as the Franconia Formation and now known as the Tunnel City Group is divided into four parts in the St. Croix Valley. These are the Mazomanie Formation and three members of the Lone Rock Formation, from oldest to youngest the Birkmose, Tomah, and Reno members. We've met the Mazomanie Formation before; it's a quartz-rich light-colored very-fine- to medium-grained sandstone with abundant burrows and various forms of cross-bedding. (This of course is also a simplification, boiling down the essence of a rock unit that was deposited across some hundreds of thousands of years over parts of two states.) The Lone Rock Formation is a finer-grained, darker, wormier unit. The Birkmose Member is a greenish-gray very-fine to fine-grained sandstone, with a lot of feldspar and glauconite grains (glauconite being a green mineral that likes to form on marine bottoms with little sedimentation). The Tomah Member is a brownish-gray feldspar-rich siltstone and very-fine-grained sandstone with thin interbeds of gray-green shale. Finally, the Reno Member is similar to the Birkmose Member, but somewhat finer-grained and with better defined sedimentary structures. The Mazomanie Formation is a lateral equivalent to most of the Lone Rock Formation. While the Lone Rock Formation was deposited in an offshore setting centered in Minnesota, the Mazomanie was deposited under shallower conditions, and its sediment came from topographic highs to the north and east in central Wisconsin. The two formations intertongue over a wide geographic and vertical range. If you trace the zone of intertonguing, you're seeing deposition fluctuating over time, as pulses of uplift and erosion on the Wisconsin highs sent sand to the south and west. It doesn't look much like a layer cake, at least not a competent example. There are at least three major Mazomanie tongues, plus who-knows-what going on between Franconia and Marine-on-St. Croix. The Tomah seems to go quietly, but the Reno is engaged in some kind of geological close-quarters combat with the Mazomanie.

Schematic colorized version of St. Croix diagram in Berg (1951, 1954), with information from Quaschnick (1959) taken into account for northern Tomah extent. Thick black lines are reasonably certain stratigraphic contacts, thin black lines are inferred, red line are biostratigraphic boundaries, and brown vertical lines show the extent of the measured sections (with the locations identified below the lines). With Berg's Woodhill Member removed (Ironton Sandstone Member of the Wonewoc Sandstone), the Tunnel City Group here is around 52 m (170 ft thick). The rocks continue for a long way south of Afton, but there aren't any good outcrops for a while.

The concept of a simple planar formational contact is in itself a simplification. Sometime you get a nice flat contact between two units. Sometimes you get a contact with vertical relief, because the underlying formation was eroded into hills and valleys before the overlying unit was deposited. Sometimes the contact is arbitrary, because the lower rock type grades into the upper rock type. Sometimes the contact is arbitrary because the two units meet over a zone of alternating beds, due to the two types of deposition switching from time to time. This last kind is what we're seeing here between the Mazomanie and the Lone Rock formations, and if we could see through the ground to get a full picture of what is going on from Taylors Falls from Afton, the contacts would probably look "fuzzy" due to smaller and smaller-scale interbedding.

Finally, I've mentioned a few times how the Franconia Formation was problematic because of mixing rocks with biostratigraphy. Back in the day, people tried to define subunits based on trilobites. Berg (1951, 1954) pointed out that the zones don't actually follow the rocks. When your biostratigraphic formations don't correspond to rock types, it makes it a real pain to try to map. In addition, you have to have both a paleontologist who can identify the relevant species, and well-preserved examples of those species in the rocks you are studying. (Of course, it gets even worse if some significant number of the species you are dealing with are actually minute variations on a single species, but who would ever do that to you?) The red lines in the diagram show that the trilobite zones skew upward going north in the St. Croix Valley. This is not entirely surprising, when you get down to it: the Lone Rock Formation is notable for its glauconite content, which as mentioned is a sign of low sedimentation rate. The Mazomanie Formation lacks glauconite. I'm going to guess that the Mazomanie had a greater rate of sedimentation than the Lone Rock, which would naturally cause the zone boundaries to skew higher where there is more Mazomanie deposition.

References

Berg, R. R. 1951. The Franconia Formation of Minnesota and Wisconsin. Dissertation. University of Minnesota, Minneapolis, Minnesota.

Berg, R. R. 1954. Franconia Formation of Minnesota and Wisconsin. Geological Society of America Bulletin 65(9):857–881.

Quaschnick, R. K. 1959. The geology of the Marine quadrangle and the Falls Creek area. Thesis. University of Minnesota, Minneapolis, Minnesota.

Sloan, R. E., D. R. Kolata, B. J. Witzke, and G. A. Ludvigson. 1987. Description of major outcrops in Minnesota and Iowa. Pages 197–231 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.

Further adventures in the Mazomanie

One of the projects I'm working on concerns the paleontology and geology of Saint Croix National Scenic Riverway, so I've been doing some location scouting to get a feel for the geology. It's not quite as simple as MNRRA, even though I'm still only dealing with a handful of formations and there's not much structural geology to contend with. The main issue is finding access to outcrops. Other complications include sparser fossils and all of these darn Cambrian cratonic sheet sandstones that look about the same.

Cambrian island-hopping at Taylors Falls

Taylors Falls in the St. Croix Valley of eastern Minnesota presents an unusual geologic snapshot along a Cambrian coast. Within the south part of Interstate State Park, for an investment of a little more than a mile of walking round-trip, you can go from ancient basaltic islands, to the lithified rubble surrounding them, to the flanking sandy beach. Geologists love to try to paint verbal pictures of vanished settings, but rarely do the modern outcrops cooperate so nicely. Thank you to the park staff for suggesting an interpretative walk!