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!

I wrote about the more northerly part of Interstate Park last year, following a visit to the basalt and potholes. The park is justifiably famous for its glacially sculpted Precambrian rocks. What is less well-known to the public is that there is a long history of paleontologic exploration in the St. Croix Valley, as far back as the 1840s (see Owen 1852 for some early mentions). Charles Walcott, in his quest for Cambrian fossils, designated the Cambrian rocks of the St. Croix as the Croixian series, representative of the Late Cambrian. The Croixian series encompasses three biostratigraphic divisions, from oldest to youngest Dresbachian, Franconian, and Trempealeauan. These were also stratigraphic units, but making a name do double-duty is frowned on these days, and the rocks have been divided into various formations (the Franconia is the last holdover, but see below). These stages are all named after locations in eastern Minnesota or western Wisconsin. The Cambrian rocks of Taylors Falls represent the Franconian stage, currently thought to date to between about 497 to 492.5 million years ago. To put that in perspective, that's about 600 million years younger than the basalt, and about 40 million years older than our old favorites the Decorah and Platteville. In other words, there's more time between the Cambrian sedimentary rocks and their basement than there is between the Cambrian rocks and us, and the amount of time between us and the deposition of the sediments that became the Badlands of South Dakota and Nebraska (about 37 million years old at the base) would fit with some room to spare between these Cambrian rocks and the Platteville. (Of course, what *really* fits between them is the St. Lawrence Formation, Jordan Sandstone, Prairie du Chien Group, St. Peter Sandstone, and Glenwood Formation.)

And also toads.

So, what was going on in those 600 million years? We can't say too much about what was happening, due to the absence of evidence. Presumably some significant part of that time was occupied in erosion of the basalt and any rocks, sedimentary or volcanic, that happened to have been above them. Because the Cambrian sandstones are quartz-rich, with lesser amounts of feldspar minerals and dolomite, there must have been source rocks somewhere in the region that weren't plain old basalt. When Cambrian seas came pounding, the basalts of Taylors Falls proved to be made of stern material. If you pull up the bedrock geology of Chisago County (found here; the rest of this county atlas, Setterholm 2010, can be found here), you'll see that the town and surrounding area are within an area mapped as "Pcf", the Precambrian-age Clam Falls Volcanics. All around this green blob are areas mapped as units starting with "C" with a line through it, meaning Cambrian rocks. It's a little simplistic to describe it as such, but that green blob was a large island of basaltic rock during the Cambrian. The geology was a little confusing to the first geologists; we see in Owen (1852) that they thought the volcanic rocks had come up through the sedimentary rocks.

Imagine this, only without the plants, and an ocean instead of land on the other side of the river. The rubble becomes the Mill Street Conglomerate (next paragraph).

As the sea arrived in the area, it would have battered at the basalt, producing rubble that was then pounded and rolled in the surf. The eventual product of this action was conglomerates, and one such example, the Mill Street Conglomerate, can be found in the park. The Mill Street Conglomerate is much beloved by regional geologists. It is a rare example of one of these initial conglomerates, and as such is a reasonably frequent subject of field trips. It is composed of rounded chunks of basalt up to a couple of feet (a little more than half a meter) across, with very fine sandstone as the matrix. (For the technically minded among us, it has been described as a facies of the Wonewoc Sandstone, or in some sources the Ironton Sandstone [Mossler 2008], although Setterholm [2010] included it in the lower part of the Lone Rock Formation. There's been some adjustment of terminology in the past decade; see Mossler 2008 for the gory details.) Although conglomerates are not known for their fossils, small fossils of brachiopods, trilobites, and snail-like mollusks are preserved in the matrix, and show the Late Cambrian age.

Sufficiently conglomeratic?

I'm standing where a tropical sea pounded volcanic rocks. Not bad for Minnesota!

Away from and above the conglomerate is the remains of the beach and shore, in the form of thick sandstone beds. The sandstone has traditionally been assigned to the Franconia Formation, but of late geologists are shying away from using the Franconia as a formation. (The problem is the Franconia is also used widely for biostratigraphy, which was once permissible but is now avoided, if for no other reason than to reduce confusion. Again, see Mossler 2008.) The Minnesota Geological Survey has imported the Tunnel City Group name from Wisconsin for these rocks. The sandstone in the park is divisible into two units, a less resistant lower unit and a thicker, more resistant overlying unit. Under the old terminology, these were the Birkmose Member and overlying Mazomanie Member of the Franconia Formation (Berg 1954 [found the ref; 2017/05/01]). Under the new terminology, these are called the Birkmose Member of the Lone Rock Formation and the Mazomanie Formation, both of the Tunnel City Group. Incidentally, the eye may interpret "Mazomanie" as "mah-ZOH-man-ee", but it is actually pronounced "may-zo-MAY-nee". The Birkmose tends to be finer-grained and is gray to green in color, while the Mazomanie is lighter colored and tends to be coarser-grained. There isn't a lot of Birkmose exposed in the park, so most of the namesake sandstone of the Sandstone Bluffs trail is Mazomanie.

Bedding in the Mazomanie.

Curtain Falls, currently dry. Much like the falls in the Twin Cities, a more durable bed (in this case part of the Mazomanie) is being undercut by erosion of softer underlying rocks.

Fossils have been found and described from up and down the valley, including the Taylors Falls area and outcrops that are probably within the park. Because most of the Cambrian formations in the area are sandstones, preservation can be so-so at best, and fossils can be very subtle and difficult to observe. Most body fossils pertain to trilobites, inarticulate brachiopods, and snail-like mollusks, but burrows and other trace fossils can be abundant. These rocks represent energetic, fluctuating environments, so you can guess that the animals probably had some durability and tolerance for changing conditions.

Inarticulate brachiopods in a rock from about a mile upstream from the park, near the dam.

References:

Berg, R. R. 1954. Franconia Formation of Minnesota and Wisconsin. Geological Society of America Bulletin 65(9):857–882. [added 2017/05/01]

Mossler, J. H. 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 65.

Owen, D. D. 1852. Report of a geological survey of Wisconsin, Iowa, and Minnesota; and incidentally of a portion of Nebraska Territory. Lippincott, Grambo & Co., Philadelphia, Pennsylvania. Available at http://archive.org/details/mobot31753000174885 (plates not included), https://archive.org/details/reportofgeologi00owen (full plates) or http://books.google.com/books?id=Y_ZYAAAAYAAJ.

Setterholm, D. R. 2010. Geologic atlas of Chisago County, Minnesota [Part A]. Minnesota Geological Survey, St. Paul, Minnesota. C-22. Scale 1:100,000 and 1:200,000.