The Decorah Shale

So we've gotten through the St. Peter, Glenwood, and Platteville. It's high time that the collection is completed with the one and only Decorah Shale. The Decorah is a bit of a shy bird in the Twin Cities because it's not the most resistant unit in the world and is pretty well covered otherwise. It's often more easily seen by what it erodes into: a gray-green clay with bits of buff-colored heavily fossiliferous limestone. In the metro, it is best known from western St. Paul and just south of the Mississippi River in the Mendota Heights/Lilydale area, but I suspect it is more widely distributed than it is usually mapped. Along the Minneapolis side of the gorge, I've encountered characteristic gray-green clay and buff fossiliferous limestone fragments near or at the top of the bluffs in a few places, perhaps representing erosional outliers.

The slope photo strikes again!

The nomenclature of the Decorah Shale is the most tangled of the major Twin Cities units. Sloan (1987) has a useful chart showing its various escapades going back to the late 19th century. 19th century documents from the first incarnation of the Minnesota Geological Survey usually called it either the "Trenton shale" or "Black River shale", via extension of formation names from eastern states. The Decorah Shale name wasn't extended into Minnesota until 1916 (Sardeson 1916). In adjoining states (Illinois, Iowa, and Wisconsin), the Decorah has been divided into, in descending order, the Ion, Guttenberg, and Spechts Ferry members, and sometimes authors have attempted to extend these divisions into Minnesota, but so far this has been unsuccessful. The Decorah of Minnesota is just not as divisible. In other places, the Decorah is thinner and has more limestone beds, but in Minnesota it is almost all shale, and in the Twin Cities it is at its thickest. The homogeneity is because the Decorah of Minnesota was deposited closer to the coast, which ran from northeast to southwest across the state. Being so close the coast, there was a more reliable supply of sediment, and carbonate deposition didn't reach into the area as frequently. We can also toss in some subsidence of the Twin Cities Basin just to complicate things. In addition, the expansion of carbonate deposition that signals the end of the Decorah Shale reached Minnesota later than farther out in the basin. This makes the upper contact of the Decorah Shale time-transgressive. (This is just something that happens in basins. At a given locality, you may see nice layer-cake stratigraphy, but at a larger scale you will see gradual shifts in the kind of deposition and thus the kind of rocks.)

World's thickest Decorah Shale section, at the Brickyard in Lilydale.

Anyway, the Decorah Shale doesn't lend itself to division in Minnesota, or at least it didn't until recently. The Carimona Limestone was long considered the highest part of the Platteville Formation in Minnesota, but in 2008 the Minnesota Geological Survey recommended moving it to the overlying Decorah Shale, because the two interfinger and to make the Minnesota version of the formation better match up with the formation as it is designated in Illinois and Iowa (Mossler 2008). (This is such a Minnesotan reaction. We could have kept right on using the Franconia Formation, but instead we decided to go along with the other states and use the Tunnel City Group.) In terms of looking at the formation at a given location, it's kind of an awkward change; it's easier to go somewhere and say "limestone, shale" as opposed to "limestone, limestone that looks about the same but belongs with the overlying shale, shale", but there you go. This is a thin member, comparable in thickness to the various units of the underlying Platteville Formation at about 1 to 2 m (3 to 7 ft) thick.

The Carimona Limestone is interesting in its own right because it contains the Deicke K-bentonite, one of the handful of volcanic ash beds to have its own Wikipedia article. The Deicke represents one of the most massive volcanic eruptions in the geologic record (note for the map at the article that the eastern margin of the ash is actually the eastern margin of the continent then; the easternmost part of the continent would not be acquired until later). The date the article provides is a bit outdated; the most recent I've seen is 454.59 ± 0.56 Ma (Renne et al. 2010). To repeat from the Shadow Falls post: "It can be traced over millions of square km (Bergström et al. 2004), and the eruption that produced it may have let loose over 400 times the material of Mount St. Helens (Ojakangas 2009), roughly 270 cubic miles of ash (about 1,120 cubic km), or a cube about 6.5 miles (10.5 km) on a side (Huff and Kolata 1990). Today, it's only about 3 in (7 cm) thick in the Twin Cities area, but before compression it may have been 11 in (27 cm) thick (Dokken 1987). As you might suspect, introducing this much ash was not a great thing for the immobile shallow-water suspension-feeders of the early Late Ordovician, and there appears to have been a local mass-extinction at this time (Sardeson 1926; Sloan et al. 2005)." You can find "cut-ins" in the Carimona Limestone at several locations, which include the Deicke and shale beds heralding the appearance of the shaly part of the formation. The slope between the Platteville platform and the overlook at Shadow Falls is the easiest, but there are exposures up and down the gorge on the St. Paul side if you don't mind hitting the goat trails. I've seen some interesting differences in the fauna overlying the Carimona as well: in the area about a third of a mile north of the Ford Bridge, I've seen many chunks of bifoliate bryozoans in the first few m/ft of the shale, while thicker branching bryozoans seem to be more common in about the same interval south of the St. Thomas campus. Incidentally, the shaly part of the Decorah Shale has another bentonite, the Millbrig, found low in the unit.

The Carimona Limestone at Shadow Falls, from the guitar case down probably to about the base of the "steps". Note the two "cut-ins" representing finer, less resistant material.

As noted, the shaly part of the Decorah Shale is not particularly divisible in Minnesota. However, it has sometimes been divided by fauna. I've mentioned the Sardeson beds a few times; these are Sardeson's faunal divisions of the Upper Ordovician rocks of Minnesota. He began by tying them to specific genera, which the Survey modified when they borrowed his system (well, not so much borrowed, but that's the 19th century for you). As the chart in Sloan (1987) shows, there were dueling systems using different names. Sardeson eventually scrapped the names and just used numbers. Beds 1 and 2 cover the Platteville. Bed 3 covers approximately the upper Platteville and lower Decorah, at least in the original, pre-transferred Carimona conception of the Platteville. Beds 4 and 5 cover most of the Decorah, and Bed 6 covers the upper Decorah and equivalent beds of the lower Cummingsville Formation, which grades out of the upper Decorah. These beds are sometimes encountered in stratigraphic and paleontological publications. The upper contact with the Cummingsville can be seen at the Brickyard, which coincidentally has the thickest known complete section of the shaly part of the Decorah, at about 27 m (90 ft) thick. The contact is arbitrary, which is understandable given that a thumbnail definition of the Decorah is a shale unit with some limestone, and the same for the Cummingsville is a limestone unit with some shale. The shaly part of the Decorah Shale thins quickly to the south, to around a quarter of its maximum as it heads out of state.

This image, from the plates to Sardeson (1916), shows the upper Decorah (the "shaly limestone" is probably part of the transition to the Cummingsville Limestone). Note the High Bridge in the background.

The Decorah Shale seems to represent a fine place where a brachiopod could raise up a family of little brachiopods, although this has been questioned: as you've probably noticed from other posts, the local fossils are generally small, and sometimes the shale beds are reported as poorly fossiliferous (although some people don't seem to have had any trouble getting specimens out of them). These suggest some sort of environmental limiting factor, perhaps oxygen stratification of the water column from freshwater runoff coming from the nearby coast (Simo et al. 2003). Some things that we *do* know: fossils are not abraded and there are many that are not broken; we do tend to get heavy concentrations of fossils in those thin limestone beds, which could be storm events; and the thickness of unlithified ooze at the sea bed was usually not enough to cover large nautiloid shells, which are frequently found incompletely preserved.

We should already be well-acquainted with our Decorah fossil friends by this point, but to reiterate a few things: brachiopod shells, bryozoan fragments, and crinoids columnals are by far the most common types of fossils, with snail shells and trilobite fragments in the next tier, and a host of other reasonably common fossils making up the rest of the assemblage (horn corals, bivalves, nautiloids, etc.). Trace fossils are not uncommon, but you do have to know what you're looking at. The Minnesota specialty in this area is the burrow Rauffella. Most of the fossils represent stationary filter-feeders, with a few things crawling around on the surface (snails and trilobites) or actively swimming (nautiloids). Fossils are generally either found on chunks of limestone or, if they are reasonably durable, eroded loose from the shale. The Decorah is a few million years older than the famous Late Ordovician rocks of Cincinnati, and the faunas are reasonably comparable, even if the Minnesota version is not quite as heralded.

Just your typical Decorah hash.

References:

Bergström, S. M., W. D. Huff, M. R. Saltzman, D. R. Kolata, and S. A. Leslie. 2004. The greatest volcanic ash falls in the Phanerozoic: trans-Atlantic relations of the Ordovician Millbrig and Kinnekulle K-bentonites. The Sedimentary Record 2(4):4–8.

Dokken, K. 1987. Trace fossils from Middle Ordovician Platteville Formation. Pages 191–196 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.

Huff, W. D., and D. R. Kolata. 1990. Correlation of the Ordovician Deicke and Millbrig K-bentonites between the Mississippi Valley and the southern Appalachians. AAPG Bulletin 74(11):1736–1747.

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

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

Renne, P. R., R. Mundil, G. Balco, K. Min, and K. R. Ludwig. 2010. Joint determination of 40K decay constants and 40Ar*/40K for the Fish Canyon sanidine standard, and improved accuracy for 40Ar/39Ar geochronology. Geochimica et Cosmochimica Acta 74:5349–5367.

Sardeson, F. W. 1916. Minneapolis–St. Paul folio. Folio 201. U.S. Geological Survey, Washington, D.C.

Sardeson, F. W. 1926. Pioneer re-population of devastated sea bottoms. Pan-American Geologist 46:273–288.

Simo, J. A. T., N. R. Emerson, C. W. Byers, and G. A. Ludvigson. 2003. Anatomy of an embayment in an Ordovician epeiric sea, Upper Mississippi Valley, USA. Geology 31(6):545–548.

Sloan, R. E. 1987. History of study of the Middle and Late Ordovician rocks of the Upper Mississippi Valley. Pages 3–6 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.

Sloan, R. E., M. D. Middleton, and G. F. Webers. 2005. Late Ordovician stratigraphy and paleontology of the Twin Cities Basin. Late Ordovician lithostratigraphy and biostratigraphy of the southern margin of the Twin Cities Basin. Pages 105–143 in L. Robinson, editor. Field trip guidebook for selected geology in Minnesota and Wisconsin. Minnesota Geological Survey, St. Paul, Minnesota. Guidebook Series 21.