Divisions of the Platteville Formation

There comes a time when we must speak of the Platteville Formation in detail. If there wasn't a Platteville Formation, there would be no narrow gorge in the Twin Cities, no Minnehaha Falls or Hidden Falls, and most importantly no Saint Anthony Falls: if there's no Saint Anthony Falls, Minneapolis develops in a radically different way. No falls, no easy water power, no mills. The Twin Cities owe their existence to a 455 million year old carbonate bank and the vagaries of glacial and river erosion.

And today the Falls of St. Anthony are mostly artificial. Turns out that drilling holes through the Platteville into the St. Peter Sandstone near the edge of a cataract can be unsafe.

The Platteville Formation is found in several states; the USGS sees various usages in Minnesota, Wisconsin, Iowa, Illinois, Missouri, and Kansas. I can't speak to its use in other states, but in Minnesota, you've got the Platteville Formation as it is in the Twin Cities Basin, and as it is in the southeastern part of the state. Now's as good a time as any to tell you about the basin: the Twin Cities Basin is a geological downwarp that resembles an oblong oval pointing northeast-southwest, beneath much of the metro. It is associated with the billion-plus-year-old Midcontinent Rift zone: as the rift rifted and the thick band of igneous rocks cooled, contracted, and settled, folds and faults formed, creating basins. Occasionally, the faults and such were reactivated much later, which appears to account for the Twin Cities Basin (for more information, see Ojakangas [2009] [semitechnical] or Mossler [1972, 2008] [technical]): most of the basin's deepening seems to postdate the Platteville. In a structural basin, folds and faults cause an area to be depressed. Wait long enough for erosion to bevel the topography to a level surface, and you will notice something: if you compare two points, one inside the basin and one outside, that have the same elevation, the point outside of the basin will have older rocks at the surface compared to the point inside the basin. You can see this in geologic maps as well. The Twin Cities Basin is one of these eroded structural basins. Younger Upper Ordovician rocks are found at the surface in the central Twin Cities, but older Lower Ordovician rocks are found at the surface at comparable elevations farther out.

Anyway, the Platteville Formation of the Twin Cities is a very durable formation about 30 ft (9 m) thick. Even if you don't know what it looks like, you've probably noticed it if you're a local. It's the gray rock unit that juts out over the lighter-colored slopes (St. Peter Sandstone) on the bluffs.

If you've seen Minnehaha Falls, you've seen the Platteville Formation (or at least the lower part): it's what's holding up the falls.

Divisions of the Platteville Formation

The Platteville Formation is sometimes called the Platteville Limestone, but that doesn't really fit the formation in the Twin Cities. It is true that it began as limestone, but around here the limestone was mostly altered to dolomite after deposition. Incidentally, dolomitization has done a number on most Platteville fossils around here, replacing the original calcium carbonate with less faithful molds and casts in dolomite. The durability of the Platteville Formation is one of its most noticeable characteristics, but it is not made of equally stern material throughout. In the Twin Cities Basin, it has four subdivisions, also known as members. In ascending order, they are the Pecatonica, Mifflin, Hidden Falls, and Magnolia members. A fifth member, the Carimona Member, was formerly included at the top, but it is now assigned to the overlying Decorah Shale (Mossler 2008). The presence of several members has actually improved the overall durability of the formation: in order to erode the whole thing, each member must be dealt with. Importantly, joints (fractures without associated movement) formed in one member typically do not continue into other members, so for example freeze expansion of water in joints in the Magnolia does not affect the Mifflin. Outside of the basin, the distinctiveness of the upper members peters out until you're left with the Pecatonica and an upper everything else called the McGregor Member.

The four members are surprisingly easy to differentiate in the field, although it often helps if you can stand back when first trying to orient yourself geologically (you can get a better feel for relative bed thicknesses that way). Conveniently, they are also reasonably easy to pick out in old descriptions that predate their names (Stauffer and Thiel 1941, I'm looking at you – this document describes the Platteville Formation as the Glenwood, McGregor, and Spechts Ferry members. Long story short, the Glenwood is the Glenwood Formation, the Spechts Ferry is the Carimona, and the McGregor covers everything else). The following descriptions are based on Mossler (2008), the most recent extensive description of the Platteville Formation in Minnesota.

Know Your Twin Cities Formations, Dayton's Bluff edition. You may prefer to put the Magnolia/Hidden Falls contact a little lower, but you get the idea.

The Lower Platteville Formation

The Pecatonica Member is a yellowish-gray blocky unit composed of sandy dolomite. It is 1 to 7 ft (0.3 to 2 m) thick in Minnesota, in my experience on the thin end of that range in the Twin Cities (closer to 2 ft/0.6 m). It forms fairly thick beds and is not particularly noted for its fossils. Its blocky beds and overall thinness contrast strongly with the thin wavy beds of the much thicker Mifflin Member. Mossler cited a thickness of about 8.5 ft (2.6 m) in the Twin Cities, but many exposures in the cities are thicker, up to as much as 11 to 12 ft (3.3 to 3.7 m). Bed thicknesses are inch-scale. It is susceptible to making regular, right-angle joints, and these planes of weakness cause it to break apart on flat surfaces as if cut by saws, often with massive chunks up to the size of cars. Fossils include bryozoans, brachiopods, trilobites, ostracodes, burrows, and fragments of echinoderms.

The lower Platteville Formation near Minnehaha Falls: a foot+ of blocky Pecatonica and several feet of thin-bedded Mifflin.

 

The Upper Platteville Formation

The Hidden Falls Member is a silty dolomitic unit, 3.5 to 5.5 ft (0.9 to 1.7 m) thick. No points for guessing the location that gave it its name. The silty fraction makes it more vulnerable to erosion than the other members, so it is more difficult to observe. For example, the outcrops along the Fort Snelling State Trail between Minnehaha Park and Fort Snelling have a gap where the Hidden Falls Member should be exposed; instead, coming from the north you see the upper Platteville, ride or walk for a bit without outcrops, and then see the Mifflin Member. You generally need the Magnolia Member present to keep the Hidden Falls Member around. Where this condition is satisfied, for example at the bluffs near Lock and Dam 1, or at Dayton's Bluff, the Hidden Falls Member typically looks like a pasty gray weathered strip between the thicker and more resistant Mifflin and Magnolia members. The Hidden Falls Member is noted for its patchy fossil distribution. One famous locality, the former Johnson Street Quarry, yielded diverse echinoderms including sea stars, crinoids, cystoids, and edrioasteroids. There's now a shopping center there.

The Magnolia Member caps the whole thing. It is a thick-bedded dolomite about 8 ft (2.4 m) thick, noted for the occasional shell bed. You can see these in cross-section on outcrops sometimes; look for layers of "eyelashes", which are shells. More typically you'll see shell beds on the upper or lower surfaces of blocks that have fallen from outcrops. These will be crowded with brachiopods, snails, and clams and other bivalves (remember Keating?) Put the four members together, and you have the blocky sandy relatively thin Pecatonica at the bottom, 8+ feet of thin/wavy-bedded Mifflin with flat walls, the relatively thin and pasty gray shaly Hidden Falls Member, and finally 8+ feet of thick-bedded Magnolia at the top.

Along the Winchell Trail south of the Short Line Bridge, the Magnolia Member broods over the gray shaly Hidden Falls Member, with some Mifflin visible as well (the darker stuff with the flat sides and thin beds).

Depositional Environment

I've gone on quite a while already, but it would be remiss of me to leave you without some commentary on what the depositional setting was like. Ojakangas and Matsch (1982) compared the Platteville Formation to the Bahama Banks, a tropical marine carbonate platform. Deposition was apparently not particularly exciting. Thin shale layers (called partings) between thicker carbonate beds probably represent storms (Sloan 1972), while more abundant shale (like the Hidden Falls Member) can be tied to uplift of nearby sediment sources (Webers 1972).

[original post Jan. 19, 2014; some thicknesses and the depositional setting revised Apr. 7, 2014]

References:

Mossler, J. H. 1972. Paleozoic structure and stratigraphy of the Twin Cities region. Pages 485–497 in P. K. Sims and G. B. Morey, editors. Geology of Minnesota: a centennial volume. Minnesota Geological Survey, St. Paul, MN.

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

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.

Sloan, R. E. 1972. Notes on the Platteville Formation, southeastern Minnesota. Pages 43–53 in G. F. Webers and G. S. Austin, editors. Field trip guidebook for Paleozoic and Mesozoic rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, MN. Guidebook 4.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, MN. Bulletin 29.

Webers, G. F. 1972. Paleoecology of the Cambrian and Ordovician strata of Minnesota. Pages 474–484 in P. K. Sims and G. B. Morey, editors. Geology of Minnesota: a centennial volume. Minnesota Geological Survey, St. Paul, MN.