Sunday, November 15, 2020

How to complicate the Glenwood

Recently I've been doing some work with geological stratotypes in National Park Service areas. The naming of geological formations is not unlike naming a fossil species of organism (or living species for that matter), except the type locality is also the type "specimen", and you don't dig up the type locality and put it in a museum (although you could certainly take a core). Anyway, there are a few stratotypes located within Mississippi National River and Recreation Area. I've alluded a couple of times to the type locality of the St. Peter Sandstone being the bluff under Fort Snelling. This locality is no longer accessible due to protective measures, but the formation *is* exposed in the immediate vicinity. The type locality of the Hidden Falls Member of the Platteville is in Hidden Falls Park (Sloan 1956). Sloan didn't state specifically where, but I have a pretty good idea. Finally, there are two units named from the section exposed at Lock and Dam No. 1.

Beautiful exposure, even if you can't climb around on it.

The Twin Cities Basin is just a small part of the area where the St. Peter–Glenwood–Platteville–Decorah sequence is exposed, and across this area, the rocks differ and the names differ. Reconciling nomenclature across states is part of what Mossler (2008) was about. An example of outstate attention on the Twin Cities Basin is Templeton and Willman (1963), from the Illinois Geological Survey. This publication brings the Illinois nomenclature into the Twin Cities, via a section at Lock and Dam No. 1 (p. 226–227). If you're used to the local names, the Lock and Dam section requires a lot of translation. For example, the Platteville is considered a group, divided into three formations and nine members. The Templeton and Willman nomenclature has not taken off in Minnesota, which has preferred a simpler system, and I can't say I disagree. It seems like overkill to pack that many formal divisions into about 9 vertical meters (30 ft), many of which are not visually distinguishable at a distance of a few meters. Their divisions of the transition from the St. Peter to the Platteville are another matter.

Beginning with our old friend the Pecatonica and going down, Templeton and Willman (1963) divided the Lock and Dam rocks into the Chana Member and Hennepin Member of the Pecatonica Formation (Platteville Group), the Harmony Hill Member and Nokomis Member of the Glenwood Formation, and the St. Peter Sandstone. Two of these members were named from this section: the Hennepin Member and Nokomis Member. In turn:

My initial thoughts. Arrows point to the approximate center of the named unit, so I don't have to commit to contacts like the coward and cad I am. The scale boxes are *very* rough; I was able to put a scale bar on the outer wall, but this is some distance back, so prefer a larger number and don't take it too seriously.

...Or is this what Templeton and Willman intended? Might be easier if I could get closer.

The Chana Member, 28 cm thick (11 in), is basically what we recognize as the Pecatonica;

The Hennepin Member is 69 cm (27 in) thick, divided into three parts: 46 cm (18 in) of clay-rich greenish-gray limestone, and green shale, over 15 cm (6 in) of clay-rich greenish-gray slightly sandy limestone, over 8 cm (3 in) of brown dolomitic sandstone (Templeton and Willman 1963). This unit, or at least the upper part of it, can be easily distinguished visually beneath the blocky overlying rocks (pers. obs.; see also the photos at the end of this post—areas that look white are weathered). The other two parts I'm not as sure about. The Hennepin Member should be thicker than the Chana and the underlying Harmony Hill put together, but from my photos and observations, I see four distinct units over whitish sandstone. Three of them seem fairly similar in thickness: the classic Minnesota Pecatonica, a light-colored recessive unit, and a greenish-brown recessive unit. Below them is a thinner, even more recessive yellow-brown unit. My first thought was that the greenish recessive unit was the Harmony Hill Member, but if I put it in the Hennepin Member and assume that the 15 cm and 8 cm beds of Templeton and Willman (1963) are both included in it, we get something much closer to the overall relative thicknesses T&W found for the Hennepin versus the Chana. Plus, the underlying thin yellow-brown interval looks to be closer to their description of the Harmony Hill Member. The internal proportions are still out of whack, though (three roughly equal units in photos versus 28 cm over 46 cm over 15+8 cm in the publication). (In case you were wondering, there is a grand total of zero [0] photos of outcrops in T&W '63, which is regrettable. A single decent photo with a couple of arrows would have helped immensely.) At any rate, the Hennepin Member is more shaley than the description in T&W '63 implies, or, rather, is less carbonate-rich (Mossler 2008). The Minnesota Geological Survey regards it as the upper part of the Glenwood Formation, rather than the basal part of the Platteville (Mossler 2008);

The Harmony Hill Member is 23 cm (9 in) of yellow-green shale (Templeton and Willman 1963);

The Nokomis Member is also tricky. Templeton and Willman (1963) described it as 330 cm (110 in) thick, divided into 10 to 18 cm (4 to 7 in) of silty white sandstone, over 58 cm (23 in) of very silty, greenish-buff to red-brown, thin-bedded, partly ferruginous sandstone, over 224 cm (78 in) of very silty white to yellow-buff sandstone. Mossler (2008) noted that an interval of silty sand is commonly found in Minnesota between standard Glenwood and standard St. Peter, and this interval has frequently been included in the Glenwood. (The Glenwood is a bigger deal outside of Minnesota.) However, this interval is difficult to distinguish from standard St. Peter in well logs and natural gamma logs, so for practical purposes Mossler (2008) recommended including it in the St. Peter. It is also difficult to identify a difference just by looking at the outcrop from the vantage point of the landing. The best I can do is identify a couple of slope breaks that may indicate changes in mineralogy which are not otherwise apparent from color or bedding at that distance. A contact in the vicinity of the lower slope break would give a thickness in the vicinity of the quoted figure.

The breaks are easier to see in oblique view than straight on. Note also how plants like to colonize the Glenwood–upper Nokomis interval.


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

Sloan, R. E. 1956. Hidden Falls Member of Platteville Formation, Minnesota. Bulletin of the American Association of Petroleum Geologists 40(12):2955–2956.

Templeton, J. S., and H. B. Willman. 1963. Champlainian Series (Middle Ordovician) in Illinois. Illinois Geological Survey, Champaign, Illinois. Bulletin 89. [large file]

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