Practical guide to MNRRA/metro-area bedrock geology

I can't believe I didn't put up a post like this earlier. Here is a thumbnail guide to the various bedrock formations exposed within MNRRA and, by extension, most of the Twin Cities metro. Further information can be found in Ojakangas and Matsch (1982) or Ojakangas (2009), if you'd prefer a nontechnical level of discourse, or Mossler (2008) if you want a technical overview. The maps published by Mossler and Tipping (2000) and Mossler (2013) (see previous post) are also useful.

The formations of interest are the Jordan Sandstone, Prairie du Chien Group, St. Peter Sandstone, Glenwood Formation, Platteville Formation, Decorah Shale, and Cummingsville Formation. We'll take it from the top, or rather, the bottom, going in ascending order from the oldest rocks that are exposed (the Jordan Sandstone; there are older rocks below it, but they aren't exposed within MNRRA or in the central Metro. You can find most of them along the St. Croix, though).

Jordan Sandstone

The Jordan Sandstone is the only Cambrian formation that can be seen in the MNRRA corridor, and it only shows up very low on the north side of the Mississippi near Hastings. Here, it shows up as a couple of tens of feet of orange-yellow rocks; when unweathered, it is gray, but here it's been stained by iron in the water. It is most easily seen from the observation areas near Lock & Dam 2, where you can also see the overlying Oneota Dolomite of the Prairie du Chien Group. As the name suggests, it is primarily composed of sandstone. It is typically about 85 to 100 ft (26 to 30 m) thick, but of course we don't see all of that in MNNRA. You might try the Saint Croix River valley for that.

The Jordan Sandstone is economically useful, like most of the formations of the area. If you've ever heard of the Jordan Aquifer, this is what they're talking about. It also makes good frac sand, which is where most of the interest in this formation has been lately. The Jordan Sandstone was deposited under shallow marine conditions during a regression (the sea is withdrawing), and includes tidal features. It is not known for its fossils. Most are marine invertebrate burrows, with some trilobites, brachiopods, and various extinct things that for the most part had a hard time making out of the Cambrian. The sparse fossils indicate a Sunwaptan age (don't worry too much about the technical names of the stages), putting it near but not at the end of the Cambrian, around 493 to 491 million years ago.

Orange rocks of the Jordan Sandstone below beige rocks of the Oneota Dolomite near Lock & Dam 2.

Prairie du Chien Group

The Prairie du Chien Group includes two formations deposited during the Early Ordovician, the Oneota Dolomite and Shakopee Formation. You have probably seen plenty of Oneota Dolomite without knowing it: "Kasota Stone" is a trade name. The Shakopee Formation is not as famous, but it is quarried for aggregate. Both are off-white formations composed primarily of dolomite, and they are quite durable, forming tall steep walls. If you walk near the trail edge at Spring Lake Park Reserve and look down, you'll see the steep walls there. Going downstream from St. Paul, the rocks first appear around Newport, and their bluffs bracket the river the rest of the way to the St. Croix. The best places to see the Prairie du Chien Group in MNRRA are Grey Cloud Island, Spring Lake Park Reserve, near Lock & Dam 2, and the roadcuts along U.S. Route 10 between Highway 61 and Prescott. There are excellent exposures of the Shakopee Dolomite here, but you have to be careful because of traffic.

The Oneota Dolomite is about 50 ft (15 m) thick in the metro, while the overlying Shakopee Formation typically reaches 80 to 100 ft (24 to 30 m), to as much as 180 ft (55 m), varying depending on how much erosion occurred before the St. Peter Sandstone was deposited. The Shakopee Formation differs from the Oneota Dolomite by being much sandier; the lower part is sandiest. Deposition of both formations occurred under shallow seas, probably carbonate banks with occasional pulses of sand. Neither formations have much in the way of fossils except for laminated structures such as stromatolites, made by microbes: a mound covered by photosynthetic microorganisms would inadvertently trap sediment, forcing the microbes to grow over the sediment layer, and starting the cycle again. Stromatolites and similar structures are most common today in harsh environments where grazing animals like certain snails cannot get at them, and the lack of other fossils in the Prairie du Chien Group suggests something similar was going on. Aside from stromatolites, snails are reputedly fairly common in isolated beds. Most body fossils have been converted to natural molds and casts.

The two formations of the Prairie du Chien Group were deposited during separate marine advances, probably separated by several million years. Mossler (2008) interpreted the Oneota Dolomite as dating to the Tremadocian (485 to 478 million years ago), and the Shakopee Formation as dating to the Arenigian (478 to 470 million years ago), and with the limited fossils it's unlikely that we'll get much better than these "early Early Ordovician" and "late Early Ordovician" dates. Periods of erosion occurred between the Jordan Sandstone and Oneota Dolomite, Oneota Dolomite and Shakopee Formation, and most extensively between the Shakopee Formation and St. Peter Sandstone.

The Shakopee Formation along U.S. Route 10 (we of the geological persuasion have a strange habit of stopping next to roads to look at rocks).

St. Peter Sandstone

The St. Peter Sandstone is exposed along the Mississippi River from Minneapolis to above Newport. It is difficult to exaggerate the purity of the sandstone: it is almost 99% quartz sand grains, medium to fine in size. The grains are rounded and frosted, and there is almost no mineral cement binding them, so, as has been discovered by generation after generation of bored children and teenagers, the St. Peter Sandstone is very easy to use as a natural canvas. It has long been an object of curiosity to geologists because the formation is essentially a sheet of pure quartz sand one hundred feet thick from Minnesota to Arkansas and Ohio. It is also of great interest to economic interests, who as you might suspect can think of a number of things to do with a hundred feet of pure quartz sand. The old Ford Plant in St. Paul was built there in part to take advantage of the sand for glassmaking. It has also been used for abrasives, as frac sand, and for other industrial uses. In the Twin Cities, it also forms caves, which have been used for storage, cheese and mushroom production, recreation, sewers, and other purposes. Because it erodes easily, it is best exposed where the Platteville Formation is present to cap it.

In the Twin Cities metro, the St. Peter Sandstone is relatively thick, up to 155 ft (48 m), thinning from north to south. You can see it exposed in many places along the bluffs and the vicinity, although it is difficult to find a clean face due to its talent for hosting graffiti. (It also has a tendency to turn an unhealthy shade of gray when exposed to the elements.) Even if it's not well-exposed, you can usually tell when you are near it because of the white, beach-like sand that has eroded from it. The formation is probably a combination of wind and marine processes, perhaps a coastal dune field that was submerged and reworked by rising seas, or a shallow marine setting receiving abundant wind-blown sand. Like the Jordan Sandstone, the St. Peter Sandstone has few fossils. Natural molds and casts of mollusks have been found in a handful of places near St. Paul. The St. Peter Sandstone does not lend itself to dating, but was probably deposited near the Middle–Late Ordovician boundary (think about 460 to 455 million years ago).

The St. Peter Sandstone beneath State Highway 5, near Fort Snelling; the formation was named from the bluffs near the fort, back when the Minnesota River was the St. Peter River.

Glenwood Formation

The Glenwood Formation is kind of an afterthought; it's only about 3 to 5 ft (1 to 1.5 m) thick in the metro, it's difficult to observe because it erodes so readily, it doesn't have any notable industrial uses, and it doesn't have much for fossils, except for microfossils. It is mostly good for keeping the Platteville Formation from resting directly on the St. Peter Sandstone. It is gray to brown in color and is mostly composed of shale, with sandy beds near the base. At Lock & Dam 1, one of the few places where it's easy to see, it seems to have a lower brownish section and a upper grayish section. It is a shallow marine formation deposited around 455 million years ago. At one point, it probably had a lot more fossils, but leaching and dolomitization are thought to have destroyed many of its fossils. Scolecodont worm jaws and conodont elements, both microfossils, are the most common fossils of this formation.

The Glenwood Formation, photographed at Lock & Dam 1, is the brown and gray beds between the chunky resistant beds of the Platteville Formation and the light-colored quartz sand of the St. Peter Sandstone.

Platteville Formation

I covered the Platteville Formation and its four members in more detail in a previous post. This formation consists of four members that vary in lithology, fossil content, thickness, and how they erode. The Pecatonica Member is 1 to 7 ft (0.3 to 2 m) thick, is composed of sandy dolomite, and erodes in chunky blocks. It is followed by the Mifflin Member, roughly 8.5 ft (2.6 m) of thin wavy dolomitic limestone beds that breaks up in enormous straight-sided blocks. Above it is the Hidden Falls Member, 3.5 to 5.5 ft (0.9 to 1.7 m) of shaly dolomite that is the most recessive of the four members. Finally, there is the Magnolia Member, about 8 ft (2.4 m) of resistant dolomite arranged in thick beds. Outside of geology and paleontology, rocks from this formation have sometimes been used for building, but Platteville Formation stone is not the best stone for that purpose. It is, however, an integral ingredient in many scenic areas around the metro, supporting the various waterfalls and high bluffs. The formation is exposed from Nicollet Island to Dayton's Bluff along the Mississippi.

The Platteville Formation was deposited in shallow marine settings, similar to a modern carbonate bank. Deposition occurred between about 455 and 454.6 million years ago. Marine invertebrates flourished, leaving behind innumerable shells. Brachiopods, snails, and bivalves are particularly common, although most fossils underwent dolomitization and are now represented by natural molds and casts.

The Platteville Formation capping Dayton's Bluff. It begins above the plant-covered level (the Glenwood Formation, hiding as usual). There is the thin blocky Pecatonica Member, the thick flat-sided Mifflin Member, the partially eroded Hidden Falls Member, and the blocky Magnolia Member just below the top.

Decorah Shale

The Decorah Shale, as the name suggests, is mostly shale, with thin beds of limestone interspersed throughout, and a couple of thin beds of bentonite, which began as volcanic ash. It has been used for brick-making (i.e. the Brickyard of Lilydale). It reaches its maximum thickness of about 90 ft (27.5 m) in Lilydale. The formation is present beneath many places of the central Twin Cities, but it is poorly exposed. True outcrops are only found in a few areas, where it looks like this:

The Decorah Shale in the middle clay pit at the Brickyard.

Usually what you get is something like this, where the rocks aren't visible but you can see that tell-tale gray-green clay, along with loose fossils and small thin fossiliferous chunks of limestone:

The Decorah Shale is a short distance below the eroded clay in the ravine at Shadow Falls Park.

The Decorah Shale includes a thin (up to about 7 ft/2 m) limestone unit at the base (the Carimona Member, formerly of the Platteville Formation). There is a bentonite bed known as the Deicke K-bentonite near the very base, a couple of inches thick. It is known throughout eastern North America and represents a massive volcanic eruption (one of the largest known) that occurred about 454.6 million years ago. Deposition of the Decorah Shale continued for a couple of million years ago, in shallow marine settings. Like the Platteville Formation, fossils are extremely abundant. However, things weren't completely rosy. The Deicke event resulted in a local mass extinction (it turns out that dumping about a foot of ash on small immobile bottom-dwelling organisms isn't really a good thing for them). Additionally, the small size of many of the fossils has promoted suggestions that something was limiting physical growth, perhaps oxygen stratification.

The most common fossils are chunks of bryozoans, brachiopod shells, and the columnals from crinoid rings. Also abundant are solitary corals, shells of snails, bivalves, and nautiloids, and fragments of trilobites.

Cummingsville Formation

I don't have any photos of the Cummingsville Formation, which in MNRRA and the Twin Cities is only present in a few areas near the Brickyard. It is predominantly limestone with some beds of shale, which has historically made it difficult to place the contact with the Decorah Shale (shale with some limestone). Again, it is a shallow marine formation. Up to 26 ft (8 m) may be present buried under Quaternary sediments, but only about 18 in (45 cm) is exposed at the Brickyard. It has abundant fossils, but the limestone makes it more difficult to extract them, as opposed to the fossils in the soft shale of the Decorah. The most characteristic fossil, at least in the lower part of the Cummingsville, is Fisherites (formerly Receptaculites), round structures a few inches across with intersecting double whorls on their upper surfaces, giving them the nickname "sunflower coral". What exactly Fisherites were has long been debated; at this time they are thought to have been a type of algae.

Quaternary sediments (not bedrock)

All that gravelly, sandy, silty, etc. stuff, covering up the interesting bits. If you're interested in glacial geology, Minnesota is a great place to start.

References:

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

Mossler, J. H. 2013. Bedrock geology of the Twin Cities ten-county metropolitan area, Minnesota. Minnesota Geological Survey, St. Paul, MN. Miscellaneous Map Series 194. Scale 1:125,000.

Mossler, J. H., and R. G. Tipping. 2000. Bedrock geology and structure of the seven-county Twin Cities metropolitan area, Minnesota. Minnesota Geological Survey, St. Paul, MN. Miscellaneous Map Series 104. Scale 1:125,000.

Ojakangas, R. W. 2009. Roadside geology of Minnesota. Mountain Press Publishing Company, Missoula, MT.

Ojakangas, R. W., and C. L. Matsch. 1982. Minnesota’s geology. University of Minnesota Press, Minneapolis, MN.